Case Study

During a routine check, a 45-year-old man was found to have high blood pressure (165/100 mm Hg). Blood pressure remained high on two follow-up visits. His physician initially prescribed hydrochlorothiazide, a diuretic commonly used to treat hypertension. Although his blood pressure was reduced by hydrochlorothiazide, it remained at a hypertensive level (145/95 mm Hg) and he was referred to the university hypertension clinic. Your evaluation reveals that the patient has elevated plasma renin activity and aldosterone concentration. Hydrochlorothiazide is therefore replaced with enalapril, an angiotensin-converting enzyme (ACE) inhibitor. Enalapril lowers the blood pressure to almost normotensive levels. However, after several weeks on the new drug, the patient returns complaining of a persistent cough. In addition, some signs of angioedema are detected. How does enalapril lower blood pressure? Why does it occasionally cause coughing and angioedema? What other drugs could be used to inhibit renin secretion or suppress the renin-angiotensin system, and decrease blood pressure, without the adverse effects of enalapril?

Vasoactive Peptides: Introduction

Peptides are used by most tissues for cell-to-cell communication. As noted in Chapters 6 and 21, they play important roles in the autonomic and central nervous systems. Several peptides exert important direct effects on vascular and other smooth muscles. These peptides include vasoconstrictors (angiotensin II, vasopressin, endothelins, neuropeptide Y, and urotensin) and vasodilators (bradykinin and related kinins, natriuretic peptides, vasoactive intestinal peptide, substance P, neurotensin, calcitonin gene-related peptide, and adrenomedullin). This chapter focuses on the smooth muscle actions of the peptides.

Angiotensin

Biosynthesis of Angiotensin

The pathway for the formation and metabolism of angiotensin II (Ang II) is summarized in Figure 17–1. The principal steps include enzymatic cleavage of angiotensin I (Ang I) from angiotensinogen by renin, conversion of Ang I to Ang II by converting enzyme, and degradation of Ang II by several peptidases.

Renin & Factors Controlling Renin Secretion

Renin is an aspartyl protease that specifically catalyzes the hydrolytic release of the decapeptide Ang I from angiotensinogen. It is synthesized as a preprohormone that is processed to prorenin, which is inactive, and then to active renin, a glycoprotein consisting of 340 amino acids.

Renin in the circulation originates in the kidneys. Enzymes with renin-like activity are present in several extrarenal tissues, including blood vessels, uterus, salivary glands, and adrenal cortex, but no physiologic role for these enzymes has been established. Within the kidney, renin is synthesized and stored in the juxtaglomerular apparatus of the nephron. Specialized granular cells called juxtaglomerular cells are the site of synthesis, storage, and release of renin. The macula densa is a specialized segment of the nephron that is closely associated with the vascular components of the juxtaglomerular apparatus. The vascular and tubular components of the juxtaglomerular apparatus, including the juxtaglomerular cells, are innervated by noradrenergic neurons.

The rate at which renin is secreted by the kidney is the primary determinant of activity of the renin-angiotensin system. Active renin is released by exocytosis immediately upon stimulation of the juxtaglomerular apparatus. Prorenin is released constitutively, usually at a rate higher than that of active renin, thus accounting for the fact that prorenin can constitute 80–90% of the total renin in the circulation. The significance of circulating prorenin is discussed at the end of this section. Active renin secretion is controlled by a variety of factors, including a renal vascular receptor, the macula densa, the sympathetic nervous system, and Ang II.

Renal Vascular Receptor

The renal vascular receptor functions as a stretch receptor, with decreased stretch leading to increased renin release and vice versa. The receptor is apparently located in the afferent arteriole, possibly in the juxtaglomerular cells. Stretch-induced changes in renin release are mediated by changes in Ca²⁺ concentration in the juxtaglomerular cells.

Macula Densa

The macula densa contains a different receptor mechanism that is sensitive to changes in the rate of delivery of sodium or chloride to the distal tubule. Decreases in distal delivery result in stimulation of renin secretion and vice versa. Potential candidates for signal transmission between the macula densa and the juxtaglomerular cells include adenosine, prostaglandins, and nitric oxide.

Sympathetic Nervous System

Maneuvers that increase renal nerve activity cause stimulation of renin secretion, whereas renal denervation results in suppression of renin secretion. Norepinephrine stimulates renin secretion by a direct action on the juxtaglomerular cells. In humans, this effect is mediated by ₁ adrenoceptors.

Circulating epinephrine and norepinephrine may act via the same mechanisms as the norepinephrine released locally from the renal sympathetic nerves, but there is evidence that a major component of the renin secretory response to circulating catecholamines is mediated by extrarenal receptors.

Angiotensin

Ang II inhibits renin secretion. The inhibition, which results from a direct action of the peptide on the juxtaglomerular cells, forms the basis of a short-loop negative feedback mechanism controlling renin secretion. Interruption of this feedback with inhibitors of the renin-angiotensin system (see below) results in stimulation of renin secretion.

Intracellular Signaling Pathways

Research suggests that secretion of renin by the juxtaglomerular cells is controlled by the interplay among three intracellular messengers, cAMP, cGMP, and free cytosolic Ca²⁺ concentration (Figure 17–2). cAMP appears to play the major role. Most, if not all, maneuvers that increase cAMP levels including activation of adenylyl cyclase, inhibition of cAMP phosphodiesterase, and administration of cAMP analogs increase renin secretion. cGMP and Ca²⁺ appear to alter renin secretion indirectly by changing cAMP levels.

Pharmacologic Alteration of Renin Release

The release of renin is altered by a wide variety of pharmacologic agents. Renin release is stimulated by vasodilators (hydralazine, minoxidil, nitroprusside), -adrenoceptor agonists, -adrenoceptor antagonists, phosphodiesterase inhibitors (eg, theophylline, milrinone, rolipram), and most diuretics and anesthetics. This stimulation can be accounted for by the control mechanisms just described. Drugs that inhibit renin release are discussed below.

Angiotensinogen

Angiotensinogen is the circulating protein substrate from which renin cleaves Ang I. It is synthesized in the liver. Human angiotensinogen is a glycoprotein with a molecular weight of approximately 57,000. The 14 amino acids at the amino terminal of the molecule are shown in Figure 17–1. In humans, the concentration of angiotensinogen in the circulation is less than the K_m of the renin-angiotensinogen reaction and is therefore an important determinant of the rate of formation of angiotensin.

The production of angiotensinogen is increased by corticosteroids, estrogens, thyroid hormones, and Ang II. It is also elevated during pregnancy and in women taking estrogen-containing oral contraceptives. The increased plasma angiotensinogen concentration is thought to contribute to the hypertension that may occur in these situations.

Angiotensin I

Although Ang I contains the peptide sequences necessary for all of the actions of the renin-angiotensin system, it has little or no biologic activity. Instead, it must be converted to Ang II by converting enzyme (Figure 17–1). Ang I may also be acted on by plasma or tissue aminopeptidases to form [des-Asp¹]angiotensin I; this in turn is converted to [des-Asp¹]angiotensin II (commonly known as angiotensin III) by converting enzyme.

Converting Enzyme (ACE, Peptidyl Dipeptidase, Kininase II)

Converting enzyme is a dipeptidyl carboxypeptidase that catalyzes the cleavage of dipeptides from the carboxyl terminal of certain peptides. Its most important substrates are Ang I, which it converts to Ang II, and bradykinin, which it inactivates (see below). It also cleaves enkephalins and substance P, but the physiologic significance of these effects has not been established. The action of converting enzyme is prevented by a penultimate prolyl residue, and Ang II is therefore not hydrolyzed by converting enzyme. Converting enzyme is distributed widely in the body. In most tissues, converting enzyme is located on the luminal surface of vascular endothelial cells and is thus in close contact with the circulation.

A homolog of converting enzyme known as ACE2 was recently found to be highly expressed in vascular endothelial cells of the kidneys, heart, and testes. Unlike converting enzyme, ACE2 has only one active site and functions as a carboxypeptidase rather than a dipeptidyl carboxypeptidase. It removes a single amino acid from the C-terminal of Ang I forming angiotensin (1-9) which is inactive but is converted to angiotensin (1-7) by ACE. ACE2 also degrades Ang II to angiotensin (1-7). Angiotensin (1-7), which has vasodilator activity, may serve to counteract the vasoconstrictor activity of Ang II. ACE2 also differs from ACE in that it does not hydrolyze bradykinin and is not inhibited by converting enzyme inhibitors (see below). Thus, the enzyme more closely resembles an angiotensinase than a converting enzyme.

ACE2 has been implicated in cardiovascular and renal disease, diabetes, pregnancy, and lung disease. Interestingly, it serves as a receptor for coronaviruses including the virus that causes severe acute respiratory syndrome.

Angiotensinase

Ang II, which has a plasma half-life of 15–60 seconds, is removed rapidly from the circulation by a variety of peptidases collectively referred to as angiotensinase. It is metabolized during passage through most vascular beds (a notable exception being the lung). Most metabolites of Ang II are biologically inactive, but the initial product of aminopeptidase action—[des-Asp¹]angiotensin II—retains considerable biologic activity.

Actions of Angiotensin II

Ang II exerts important actions at vascular smooth muscle, adrenal cortex, kidney, heart, and brain. Through these actions, the renin-angiotensin system plays a key role in the regulation of fluid and electrolyte balance and arterial blood pressure. Excessive activity of the renin-angiotensin system can result in hypertension and disorders of fluid and electrolyte homeostasis.

Blood Pressure

Ang II is a very potent pressor agent—on a molar basis, approximately 40 times more potent than norepinephrine. The pressor response to intravenous Ang II is rapid in onset (10–15 seconds) and sustained during long-term infusions. A large component of the pressor response is due to direct contraction of vascular—especially arteriolar—smooth muscle. In addition, however, Ang II can also increase blood pressure through actions on the brain and autonomic nervous system. The pressor response to angiotensin is usually accompanied by little or no reflex bradycardia because the peptide acts on the brain to reset the baroreceptor reflex control of heart rate to a higher pressure.

Ang II also interacts with the autonomic nervous system. It stimulates autonomic ganglia, increases the release of epinephrine and norepinephrine from the adrenal medulla, and—what is most important—facilitates sympathetic transmission by an action at adrenergic nerve terminals. The latter effect involves both increased release and reduced reuptake of norepinephrine. Ang II also has a less important direct positive inotropic action on the heart.

Adrenal Cortex

Ang II acts directly on the zona glomerulosa of the adrenal cortex to stimulate aldosterone synthesis and release. At higher concentrations, Ang II also stimulates glucocorticoid synthesis.

Kidney

Ang II acts on the kidney to cause renal vasoconstriction, increase proximal tubular sodium reabsorption, and inhibit the secretion of renin.

Central Nervous System

In addition to its central effects on blood pressure, Ang II acts on the central nervous system to stimulate drinking (dipsogenic effect) and increase the secretion of vasopressin and adrenocorticotropic hormone (ACTH). The physiologic significance of the effects of Ang II on drinking and pituitary hormone secretion is not known.

Cell Growth

Ang II is mitogenic for vascular and cardiac muscle cells and may contribute to the development of cardiovascular hypertrophy. It also exerts a variety of important effects on the vascular endothelium. Indeed, overactivity of the renin-angiotensin system has been implicated as one of the most significant factors in the development of hypertensive vascular disease. Considerable evidence now indicates that ACE inhibitors and Ang II receptor antagonists (see below) slow or prevent morphologic changes (remodeling) following myocardial infarction that would otherwise lead to heart failure.

Angiotensin Receptors & Mechanism of Action

Ang II receptors are widely distributed in the body. Like the receptors for other peptide hormones, Ang II receptors are located on the plasma membrane of target cells, and this permits rapid onset of the various actions of Ang II.

Two distinct subtypes of Ang II receptors, termed AT₁ and AT₂, have been identified on the basis of their differential affinity for antagonists, and their sensitivity to sulfhydryl-reducing agents. AT₁ receptors have a high affinity for losartan and a low affinity for PD 123177 (an experimental nonpeptide antagonist), whereas AT₂ receptors have a high affinity for PD 123177 and a low affinity for losartan. Ang II and saralasin (see below) bind equally to both subtypes. The relative proportion of the two subtypes varies from tissue to tissue: AT₁ receptors predominate in vascular smooth muscle.

Most of the known actions of Ang II are mediated by the AT₁ receptor, a G_q protein-coupled receptor. Binding of Ang II to AT₁ receptors in vascular smooth muscle results in activation of phospholipase C and generation of inositol trisphosphate and diacylglycerol (see Chapter 2). These events, which occur within seconds, result in smooth muscle contraction.

The stimulation of vascular and cardiac growth by Ang II is mediated by other pathways, probably receptor and nonreceptor tyrosine kinases such as the Janus tyrosine kinase Jak2 and increased transcription of specific genes (see Chapter 2).

The AT₂ receptor has a structure and affinity for Ang II similar to those of the AT₁ receptor. In contrast, however, stimulation of AT₂ receptors causes vasodilation that may serve to counteract the vasoconstriction resulting from AT₁ receptor stimulation. AT₂ receptor-mediated vasodilation appears to be nitric oxide (NO)-dependent and may involve the bradykinin B₂ receptor-NO-cGMP pathway.

AT₂ receptors are present at high density in all tissues during fetal development, but they are much less abundant in the adult where they are expressed at high concentration only in the adrenal medulla, reproductive tissues, vascular endothelium, and parts of the brain. AT₂ receptors are up-regulated in pathologic conditions including heart failure and myocardial infarction. The functions of the AT₂ receptor appear to include fetal tissue development, inhibition of growth and proliferation, cell differentiation, apoptosis, and vasodilation.

Inhibition of the Renin-Angiotensin System

In view of the importance of the renin-angiotensin system in cardiovascular disease, considerable effort has been directed to developing drugs that inhibit the system. A wide variety of agents that block the formation or action of Ang II is now available. Some of these drugs block renin secretion, but the newer ones inhibit the conversion of Ang I to Ang II, block angiotensin AT₁ receptors, or inhibit the enzymatic action of renin.

Drugs that Block Renin Secretion

Several drugs that interfere with the sympathetic nervous system inhibit the secretion of renin. Examples are clonidine and propranolol. Clonidine inhibits renin secretion by causing a centrally mediated reduction in renal sympathetic nerve activity, and it may also exert a direct intrarenal action. Propranolol and other -adrenoceptor–blocking drugs act by blocking the intrarenal and extrarenal receptors involved in the neural control of renin secretion.

Angiotensin-Converting Enzyme Inhibitors

An important class of orally active ACE inhibitors, directed against the active site of ACE, is now extensively used. Captopril and enalapril are examples of the many potent ACE inhibitors that are available. These drugs differ in their structure and pharmacokinetics, but in clinical use, they are interchangeable. ACE inhibitors decrease systemic vascular resistance without increasing heart rate, and they promote natriuresis. As described in Chapters 11 and 13, they are effective in the treatment of hypertension, decrease morbidity and mortality in heart failure and left ventricular dysfunction after myocardial infarction, and delay the progression of diabetic nephropathy.

ACE inhibitors not only block the conversion of Ang I to Ang II but also inhibit the degradation of other substances, including bradykinin, substance P, and enkephalins. The action of ACE inhibitors to inhibit bradykinin metabolism contributes significantly to their hypotensive action (see Figure 11–5) and is apparently responsible for some adverse side effects, including cough and angioedema.

Angiotensin Receptor Blockers

Potent peptide antagonists of the action of Ang II are available. The best-known of these is the partial agonist, saralasin. Saralasin lowers blood pressure in hypertensive patients but may elicit pressor responses, particularly when circulating Ang II levels are low. Because it must be administered intravenously, saralasin is used only for investigation of renin-dependent hypertension and other hyperreninemic states.

The non peptide Ang II receptor blockers (ARBs) are of much greater interest. Losartan, valsartan, eprosartan, irbesartan, candesartan, olmesartan, and telmisartan are orally active, potent, and specific competitive antagonists of angiotensin AT₁ receptors. The efficacy of these drugs in hypertension is similar to that of ACE inhibitors, but they are associated with a lower incidence of cough. Like ACE inhibitors, ARBs slow the progression of diabetic nephropathy. The antagonists are also effective in the treatment of heart failure and provide a useful alternative when ACE inhibitors are not well tolerated. Like ACE inhibitors, they are well tolerated but should not be used by patients with nondiabetic renal disease or in pregnancy.

The current ARBs are selective for the AT₁ receptor. Since prolonged treatment with the drugs disinhibits renin secretion and increases circulating Ang II levels, there may be increased stimulation of AT₂ receptors. This may be significant in view of the evidence that activation of the AT₂ receptor causes vasodilation and other beneficial effects. AT₂ receptor antagonists such as PD 123177 are available for research but have no clinical applications at this time.

The clinical benefits of ARBs are similar to those of ACE inhibitors, and it is not clear if one group of drugs has significant advantages over the other. Combination therapy with both an ACE inhibitor and an ARB has a number of potential advantages and is currently being investigated.

Renin Inhibitors

Cleavage of angiotensinogen by renin (Figure 17–1) is the rate-limiting step in the formation of Ang II and thus represents a logical target for inhibition of the renin-angiotensin system. Drugs that inhibit renin have been available for many years but have been limited by low potency, poor bioavailability, and short duration of action. However, a new class of nonpeptide, low-molecular weight, orally active inhibitors has recently been developed.

Aliskiren is the most advanced of these and the first to be approved for the treatment of hypertension. In healthy subjects, aliskiren produces a dose-dependent reduction in plasma renin activity and Ang I and II and aldosterone concentrations. In patients with hypertension, many of whom have elevated plasma renin levels, aliskiren suppresses plasma renin activity and causes dose-related reductions in blood pressure similar to those produced by ACE inhibitors (Figure 17–3). The safety and tolerability of aliskiren appear to be comparable to angiotensin antagonists and placebo.

Inhibition of the renin-angiotensin system with ACE inhibitors or ARBs may be incomplete because the drugs disrupt the negative feedback action of Ang II on renin secretion and thereby increase plasma renin activity. Other antihypertensive drugs, notably hydrochlorothiazide and other diuretics, also increase plasma renin activity. Aliskiren not only decreases baseline plasma renin activity in hypertensive subjects but also eliminates the rise produced by ACE inhibitors, ARBs, and diuretics and thereby results in a greater antihypertensive effect (Figure 17–3). Renin inhibition has thus proved to be an important new approach to the treatment of hypertension.

Prorenin Receptors

For many years, prorenin was considered to be an inactive precursor of renin, with no function of its own. Thus the observation noted above in the section on renin that prorenin circulates at high levels was surprising. Recently, however, a receptor that specifically binds prorenin has been identified. Since it also binds active renin, the receptor is referred to as the "(pro)renin" receptor.

The receptor is a 350-amino acid protein with a single transmembrane domain. When prorenin binds to the receptor it undergoes a conformational change and becomes fully active. The catalytic activity of active renin also increases further when it binds to the (pro)renin receptor. The activated prorenin and renin interact with circulating angiotensinogen to form angiotensin (Figure 17–1). However, binding of prorenin to the receptor also activates intracellular signaling pathways that differ depending on the cell type. For example, in mesangial and vascular smooth muscle cells, prorenin binding activates MAP kinases and expression of profibrotic molecules. Thus, elevated prorenin levels (as occur, for example, in diabetes mellitus) could produce a variety of adverse effects via both angiotensin-dependent and independent pathways.

It is clear that this novel receptor could be important in cardiovascular disease, but recent observations suggest that its functions may extend much further. There is considerable interest in developing drugs to block the (pro)renin receptor.

Kinins

Biosynthesis of Kinins

Kinins are potent vasodilator peptides formed enzymatically by the action of enzymes known as kallikreins or kininogenases acting on protein substrates called kininogens. The kallikrein-kinin system has several features in common with the renin-angiotensin system.

Kallikreins

Kallikreins are present in plasma and in several tissues, including the kidneys, pancreas, intestine, sweat glands, and salivary glands. Plasma prekallikrein can be activated to kallikrein by trypsin, Hageman factor, and possibly kallikrein itself. In general, the biochemical properties of tissue kallikreins are different from those of plasma kallikreins. Kallikreins can convert prorenin to active renin, but the physiologic significance of this action has not been established.

Kininogens

Kininogens—the precursors of kinins and substrates of kallikreins—are present in plasma, lymph, and interstitial fluid. Two kininogens are known to be present in plasma: a low-molecular-weight form (LMW kininogen) and a high-molecular-weight form (HMW kininogen). About 15–20% of the total plasma kininogen is in the HMW form. It is thought that LMW kininogen crosses capillary walls and serves as the substrate for tissue kallikreins, whereas HMW kininogen is confined to the bloodstream and serves as the substrate for plasma kallikrein.

Formation of Kinins in Plasma & Tissues

The pathway for the formation and metabolism of kinins is shown in Figure 17–4. Three kinins have been identified in mammals: bradykinin, lysylbradykinin (also known as kallidin), and methionyllysylbradykinin. Each contains bradykinin in its structure.

Each kinin is formed from a kininogen by the action of a different enzyme. Bradykinin is released by plasma kallikrein, lysylbradykinin by tissue kallikrein, and methionyllysylbradykinin by pepsin and pepsin-like enzymes. The three kinins have been found in plasma and urine. Bradykinin is the predominant kinin in plasma, whereas lysylbradykinin is the major urinary form.

Physiologic & Pathologic Effects of Kinins

Effects on the Cardiovascular System

Kinins produce marked vasodilation in several vascular beds, including the heart, kidney, intestine, skeletal muscle, and liver. In this respect, kinins are approximately 10 times more potent on a molar basis than histamine. The vasodilation may result from a direct inhibitory effect of kinins on arteriolar smooth muscle or may be mediated by the release of nitric oxide or vasodilator prostaglandins such as PGE2 and PGI2 . In contrast, the predominant effect of kinins on veins is contraction; again, this may result from direct stimulation of venous smooth muscle or from the release of venoconstrictor prostaglandins such as PGF₂. Kinins also produce contraction of most visceral smooth muscle.

When injected intravenously, kinins produce a rapid but brief fall in blood pressure that is due to their arteriolar vasodilator action. Intravenous infusions of the peptide fail to produce a sustained decrease in blood pressure; prolonged hypotension can only be produced by progressively increasing the rate of infusion. The rapid reversibility of the hypotensive response to kinins is due primarily to reflex increases in heart rate, myocardial contractility, and cardiac output. In some species, bradykinin produces a biphasic change in blood pressure—an initial hypotensive response followed by an increase above the preinjection level. The increase in blood pressure may be due to a reflex activation of the sympathetic nervous system, but under some conditions, bradykinin can directly release catecholamines from the adrenal medulla and stimulate sympathetic ganglia. Bradykinin also increases blood pressure when injected into the central nervous system, but the physiologic significance of this effect is not clear, since it is unlikely that kinins cross the blood-brain barrier. (Note, however, that bradykinin can increase the permeability of the blood-brain barrier to some other substances.) Kinins have no consistent effect on sympathetic or parasympathetic nerve endings.

The arteriolar dilation produced by kinins causes an increase in pressure and flow in the capillary bed, thus favoring efflux of fluid from blood to tissues. This effect may be facilitated by increased capillary permeability resulting from contraction of endothelial cells and widening of intercellular junctions, and by increased venous pressure secondary to constriction of veins. As a result of these changes, water and solutes pass from the blood to the extracellular fluid, lymph flow increases, and edema may result.

The role that endogenous kinins play in the regulation of blood pressure is not clear. They do not appear to participate in the control of blood pressure under resting conditions but may play a role in postexercise hypotension.

Effects on Endocrine & Exocrine Glands

As noted earlier, prekallikreins and kallikreins are present in several glands, including the pancreas, kidney, intestine, salivary glands, and sweat glands, and they can be released into the secretory fluids of these glands. The function of the enzymes in these tissues is not known. The enzymes (or active kinins) may diffuse from the organs to the blood and act as local modulators of blood flow. Since kinins have such marked effects on smooth muscle, they may also modulate the tone of salivary and pancreatic ducts and help regulate gastrointestinal motility. Kinins also influence the transepithelial transport of water, electrolytes, glucose, and amino acids, and may regulate the transport of these substances in the gastrointestinal tract and kidney. Finally, kallikreins may play a role in the physiologic activation of various prohormones, including proinsulin and prorenin.

Role in Inflammation

Bradykinin has long been known to produce the four classic symptoms of inflammation—redness, local heat, swelling, and pain. Kinins are rapidly generated after tissue injury and play a pivotal role in the development and maintenance of these inflammatory processes.

Effects on Sensory Nerves

Kinins are potent pain-producing substances when applied to a blister base or injected intradermally. They elicit pain by stimulating nociceptive afferents in the skin and viscera.

Other Effects

There is evidence that bradykinin may play a beneficial, protective role in certain cardiovascular diseases and ischemic stroke-induced brain injury. On the other hand, it has been implicated in cancer and some central nervous system diseases.

Kinin Receptors & Mechanisms of Action

The biologic actions of kinins are mediated by specific receptors located on the membranes of the target tissues. Two types of kinin receptors, termed B₁ and B₂, have been defined based on the rank orders of agonist potencies. (Note that B here stands for bradykinin, not for -adrenoceptor.) Bradykinin displays the highest affinity in most B₂ receptor systems, followed by lys-bradykinin and then by met-lys-bradykinin. One exception is the B₂ receptor that mediates contraction of venous smooth muscle; this appears to be most sensitive to lys-bradykinin. Recent evidence suggests the existence of two B₂-receptor subtypes, which have been termed B_2A and B_2B.

B₁ receptors appear to have a very limited distribution in mammalian tissues and have few known functional roles. Studies with knockout mice that lack functional B₁ receptors suggest that these receptors participate in the inflammatory response and may also be important in long-lasting kinin effects such as collagen synthesis and cell multiplication. By contrast, B₂ receptors have a widespread distribution that is consistent with the multitude of biologic effects that are mediated by this receptor type. B₂ receptors are G protein–coupled and agonist binding sets in motion multiple signal transduction events, including calcium mobilization, chloride transport, formation of nitric oxide, and activation of phospholipase C, phospholipase A₂, and adenylyl cyclase.

Metabolism of Kinins

Kinins are metabolized rapidly (half-life < 15 seconds) by nonspecific exopeptidases or endopeptidases, commonly referred to as kininases. Two plasma kininases have been well characterized. Kininase I, apparently synthesized in the liver, is a carboxypeptidase that releases the carboxyl terminal arginine residue. Kininase II is present in plasma and vascular endothelial cells throughout the body. It is identical to angiotensin-converting enzyme (ACE, peptidyl dipeptidase), discussed above. Kininase II inactivates kinins by cleaving the carboxyl terminal dipeptide phenylalanyl-arginine. Like angiotensin I, bradykinin is almost completely hydrolyzed during a single passage through the pulmonary vascular bed.

Drugs Affecting the Kallikrein-Kinin System

Drugs that modify the activity of the kallikrein-kinin system are available, though none are in wide clinical use. Considerable effort has been directed toward developing kinin receptor antagonists, since such drugs have considerable therapeutic potential as anti-inflammatory and antinociceptive agents. Competitive antagonists of both B₁ and B₂ receptors are available for research use. Examples of B₁ receptor antagonists are the peptides [Leu⁸-des-Arg⁹]bradykinin and Lys[Leu⁸-des Arg⁹]bradykinin. The first B₂ receptor antagonists to be discovered were also peptide derivatives of bradykinin. These first-generation antagonists were used extensively in animal studies of kinin receptor pharmacology. However, their half-life is short, and they are almost inactive on the human B₂ receptor.

Icatibant is a second-generation B₂ receptor antagonist. It is orally active, potent and selective, has a long duration of action (> 60 minutes), and displays high B₂-receptor affinity in humans and all other species in which it has been tested. Icatibant has been used extensively in animal studies to block exogenous and endogenous bradykinin and in human studies to evaluate the role of kinins in pain, hyperalgesia, and inflammation. It shows promise for use in the treatment of hereditary angioedema and pain.

Recently, a third generation of B₂-receptor antagonists was developed; examples are FR 173657, FR 172357, and NPC 18884. These antagonists block both human and animal B₂ receptors and are orally active. They have been reported to inhibit bradykinin-induced bronchoconstriction in guinea pigs, carrageenin-induced inflammatory responses in rats, and capsaicin-induced nociception in mice. These antagonists have promise for the treatment of inflammatory pain in humans.

SSR240612 is a new, potent, and orally active selective antagonist of B₁ receptors in humans and several animal species. It exhibits analgesic and anti-inflammatory activities in mice and rats and is currently in preclinical development for the treatment of inflammatory and neurogenic pain.

The synthesis of kinins can be inhibited with the kallikrein inhibitor aprotinin. Actions of kinins mediated by prostaglandin generation can be blocked nonspecifically with inhibitors of prostaglandin synthesis such as aspirin. Conversely, the actions of kinins can be enhanced with ACE inhibitors, which block the degradation of the peptides. Indeed, as noted above, inhibition of bradykinin metabolism by ACE inhibitors contributes significantly to their antihypertensive action.

Selective B₂ agonists are under study and have been shown to be effective in some animal models of human cardiovascular disease. These drugs have potential for the treatment of hypertension, myocardial hypertrophy, and other diseases.

Vasopressin

Vasopressin (antidiuretic hormone, ADH) plays an important role in the long-term control of blood pressure through its action on the kidney to increase water reabsorption. This and other aspects of the physiology of vasopressin are discussed in Chapters 15 and 37 and will not be reviewed here.

Vasopressin also plays an important role in the short-term regulation of arterial pressure by its vasoconstrictor action. It increases total peripheral resistance when infused in doses less than those required to produce maximum urine concentration. Such doses do not normally increase arterial pressure because the vasopressor activity of the peptide is buffered by a reflex decrease in cardiac output. When the influence of this reflex is removed, eg, in shock, pressor sensitivity to vasopressin is greatly increased. Pressor sensitivity to vasopressin is also enhanced in patients with idiopathic orthostatic hypotension. Higher doses of vasopressin increase blood pressure even when baroreceptor reflexes are intact.

Vasopressin Receptors & Antagonists

Three subtypes of vasopressin G protein-coupled receptors have been identified. V_1a receptors mediate the vasoconstrictor action of vasopressin; V _1b receptors potentiate the release of ACTH by pituitary corticotropes; and V ₂ receptors mediate the antidiuretic action. V_1a effects are mediated by activation of phospholipase C, formation of inositol trisphosphate, and increased intracellular calcium concentration. V₂ effects are mediated by activation of adenylyl cyclase.

Vasopressin-like peptides selective for either vasoconstrictor or antidiuretic activity have been synthesized. The most specific V₁ vasoconstrictor agonist synthesized to date is [Phe², Ile³, Orn⁸]vasotocin. Selective V₂ antidiuretic analogs include 1-deamino[D-Arg⁸]arginine vasopressin (dDAVP) and 1-deamino[Val⁴,D-Arg⁸]arginine vasopressin (dVDAVP).

During the past decade, vasopressin has proved beneficial in the treatment of vasodilatory shock states, at least in part by virtue of its V₁ agonist activity. Terlipressin (triglycyl lysine vasopressin), a synthetic vasopressin analog that is converted to lysine vasopressin in the body, is also effective. It may have advantages over vasopressin because it is more selective for V₁ receptors and has a longer half-life.

Antagonists of the vasoconstrictor action of vasopressin are also available. The peptide antagonist (1-[-mercapto-,-cyclopentamethylenepropionic acid]-2-[O -methyl]tyrosine) arginine vasopressin also has antioxytocic activity but does not antagonize the antidiuretic action of vasopressin. Recently, nonpeptide, orally active V_1a receptor antagonists have been discovered, an example being relcovaptan.

The vasopressor antagonists of vasopressin have been particularly useful in revealing the important role that vasopressin plays in blood pressure regulation in situations such as dehydration and hemorrhage. They have potential for the treatment of hypertension and heart failure. To date, most studies have focused on heart failure and promising results have been obtained with V₂ antagonists. However, V_1a antagonists also have potential, and conivaptan (YM087), a drug with both V_1a and V₂ antagonist effects, has been approved for treatment of hyponatremia (see Chapter 15).

Natriuretic Peptides

Synthesis & Structure

The atria and other tissues of mammals contain a family of peptides with natriuretic, diuretic, vasorelaxant, and other properties. The family includes atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). The peptides share a common 17-amino-acid disulfide ring with variable C- and N-terminals (Figure 17–5). A fourth peptide, urodilatin, has the same structure as ANP with an extension of four amino acids at the N-terminal.

ANP is derived from the carboxyl terminal end of a common precursor termed preproANP. ANP is synthesized primarily in cardiac atrial cells, but it is also synthesized in ventricular myocardium, by neurons in the central and peripheral nervous systems, and in the lungs.

The most important stimulus to the release of ANP from the heart is atrial stretch via mechanosensitive ion channels. ANP release is also increased by volume expansion, changing from the standing to the supine position, and exercise. ANP release can also be increased by sympathetic stimulation via _1A-adrenoceptors, endothelins via the ET_A-receptor subtype (see below), glucocorticoids, and vasopressin. Plasma ANP concentration increases in various pathologic states, including heart failure, primary aldosteronism, chronic renal failure, and inappropriate ADH secretion syndrome.

Administration of ANP produces prompt and marked increases in sodium excretion and urine flow. Glomerular filtration rate increases, with little or no change in renal blood flow, so that the filtration fraction increases. The ANP-induced natriuresis is due to both the increase in glomerular filtration rate and a decrease in proximal tubular sodium reabsorption. ANP also inhibits the secretion of renin, aldosterone, and vasopressin; these changes may also increase sodium and water excretion. Finally, ANP causes vasodilation and decreases arterial blood pressure. Suppression of ANP production or blockade of its action impairs the natriuretic response to volume expansion, and increases blood pressure.

BNP was originally isolated from porcine brain but, like ANP, it is synthesized primarily in the heart. It exists in two forms, having either 26 or 32 amino acids (Figure 17–5). Like ANP, the release of BNP appears to be volume-related; indeed, the two peptides may be cosecreted. BNP exhibits natriuretic, diuretic, and hypotensive activities similar to those of ANP but circulates at a lower concentration.

CNP consists of 22 amino acids (Figure 17–5). It is located predominantly in the central nervous system but is also present in several other tissues including the vascular endothelium, kidneys, and intestine. It has not been found in significant concentrations in the circulation. CNP has less natriuretic and diuretic activity than ANP and BNP but is a potent vasodilator and may play a role in the regulation of peripheral resistance.

Urodilatin is synthesized in the distal tubules of the kidneys by alternative processing of the ANP precursor. It elicits potent natriuresis and diuresis, and thus functions as a paracrine regulator of sodium and water excretion. It also relaxes vascular smooth muscle.

Pharmacodynamics & Pharmacokinetics

The biologic actions of the natriuretic peptides are mediated through association with specific high-affinity receptors located on the surface of the target cells. Three receptor subtypes termed ANP _A , ANP _B , and ANP _C have been identified. The ANP_A receptor consists of a 120 kDa membrane-spanning protein with enzymatic activity associated with its intracellular domain. Its primary ligands are ANP and BNP. The ANP_B receptor is similar in structure to the ANP_A receptor, but its primary ligand appears to be CNP. The ANP_A and ANP_B receptors, but not the ANP_C receptor, are guanylyl cyclase enzymes.

The natriuretic peptides have a short half-life in the circulation. They are metabolized in the kidneys, liver, and lungs by the neutral endopeptidase NEP 24.11. Inhibition of this endopeptidase results in increases in circulating levels of the natriuretic peptides, natriuresis, and diuresis. The peptides are also removed from the circulation by binding to ANP_C receptors in the vascular endothelium. This receptor binds the natriuretic peptides with equal affinity. The receptor and bound peptide are internalized, the peptide is degraded enzymatically, and the receptor is returned to the cell surface.

Administration of BNP as nesiritide (see Chapter 13) in patients with severe heart failure increases sodium excretion and improves hemodynamics. However, the peptide has to be given by constant intravenous infusion and has caused fatal renal damage. Ularitide, the synthetic form of urodilatin, has beneficial renal and cardiovascular effects in patients with decompensated heart failure or cirrhosis with sodium retention. It also has to be administered by intravenous infusion. A more promising approach may be the use of drugs that inhibit the neutral endopeptidase responsible for the breakdown of ANP. This is discussed below under Vasopeptidase Inhibitors. Patients with heart failure have high plasma levels of ANP and BNP; the latter has emerged as a diagnostic and prognostic marker in this condition.

Vasopeptidase Inhibitors

Vasopeptidase inhibitors are a new class of cardiovascular drugs that inhibit two metalloprotease enzymes, NEP 24.11 and ACE. They thus simultaneously increase the levels of natriuretic peptides and decrease the formation of Ang II. As a result, they enhance vasodilation, reduce vasoconstriction, and increase sodium excretion, in turn reducing peripheral vascular resistance and blood pressure.

Recently developed vasopeptidase inhibitors include omapatrilat, sampatrilat, and fasidotrilat. Omapatrilat, which has received the most attention, lowers blood pressure in animal models of hypertension as well as in hypertensive patients, and improves cardiac function in patients with heart failure. Unfortunately, omapatrilat causes a significant incidence of angioedema in addition to cough and dizziness and has not been approved for clinical use.

Endothelins

The endothelium is the source of a variety of substances with vasodilator (PGI2 and nitric oxide) and vasoconstrictor activities. The latter include the endothelin family, potent vasoconstrictor peptides that were first isolated from aortic endothelial cells.

Biosynthesis, Structure, & Clearance

Three isoforms of endothelin have been identified: the originally described endothelin, ET-1, and two similar peptides, ET-2 and ET-3. Each isoform is a product of a different gene and is synthesized as a prepro form that is processed to a propeptide and then to the mature peptide. Processing to the mature peptides occurs through the action of endothelin-converting enzyme. Each endothelin is a 21-amino-acid peptide containing two disulfide bridges. The structure of ET-1 is shown in Figure 17–6.

Endothelins are widely distributed in the body. ET-1 is the predominant endothelin secreted by the vascular endothelium. It is also produced by neurons and astrocytes in the central nervous system and in endometrial, renal mesangial, Sertoli, breast epithelial, and other cells. ET-2 is produced predominantly in the kidneys and intestine, whereas ET-3 is found in highest concentration in the brain but is also present in the gastrointestinal tract, lungs, and kidneys. Endothelins are present in the blood but in low concentration; they apparently act locally in a paracrine or autocrine fashion rather than as circulating hormones.

The expression of the ET-1 gene is increased by growth factors and cytokines, including transforming growth factor- (TGF-) and interleukin 1 (IL-1), vasoactive substances including Ang II and vasopressin, and mechanical stress. Expression is inhibited by nitric oxide, prostacyclin, and ANP.

Clearance of endothelins from the circulation is rapid and involves both enzymatic degradation by NEP 24.11 and clearance by the ET_B receptor.

Actions

Endothelins exert widespread actions in the body. In particular, they cause dose-dependent vasoconstriction in most vascular beds. Intravenous administration of ET-1 causes a rapid and transient decrease in arterial blood pressure followed by a prolonged increase. The depressor response results from release of prostacyclin and nitric oxide from the vascular endothelium, whereas the pressor response is due to direct contraction of vascular smooth muscle. Endothelins also exert direct positive inotropic and chronotropic actions on the heart and are potent coronary vasoconstrictors. They act on the kidneys to cause vasoconstriction and decrease glomerular filtration rate and sodium and water excretion. In the respiratory system, they cause potent contraction of tracheal and bronchial smooth muscle. Endothelins interact with several endocrine systems, increasing the secretion of renin, aldosterone, vasopressin, and ANP. They exert a variety of actions on the central and peripheral nervous systems, the gastrointestinal system, the liver, the urinary tract, the male and female reproductive systems, the eye, the skeletal system, and the skin. Finally, ET-1 is a potent mitogen for vascular smooth muscle cells, cardiac myocytes, and glomerular mesangial cells.

Endothelin receptors are widespread in the body. Two endothelin receptor subtypes, termed ET _A and ET _B , have been cloned and sequenced. ET_A receptors have a high affinity for ET-1 and a low affinity for ET-3 and are located on smooth muscle cells, where they mediate vasoconstriction (Figure 17–7). ET_B receptors have approximately equal affinities for ET-1 and ET-3 and are primarily located on vascular endothelial cells, where they mediate release of PGI2 and nitric oxide. Some ET_B receptors are also present on smooth muscle cells and mediate vasoconstriction. Both receptor subtypes belong to the G protein-coupled seven-transmembrane domain family of receptors.

The signal transduction mechanisms triggered by binding of ET-1 to its vascular receptors include stimulation of phospholipase C, formation of inositol trisphosphate, and release of calcium from the endoplasmic reticulum, which results in vasoconstriction. Conversely, stimulation of PGI2 and nitric oxide synthesis results in decreased intracellular calcium concentration and vasodilation.

Inhibitors of Endothelin Synthesis & Action

The endothelin system can be blocked with receptor antagonists and drugs that block endothelin-converting enzyme. Endothelin ET_A or ET_B receptors can be blocked selectively, or both can be blocked with nonselective ET_A-ET_B antagonists.

Bosentan is a nonselective receptor blocker. It is active orally, and blocks both the initial transient depressor (ET_B) and the prolonged pressor (ET_A) responses to intravenous endothelin. Many orally active endothelin receptor antagonists with increased selectivity have been developed and are available for research use. Examples include the selective ET_A antagonists sitaxsentan and ambrisentan.

The formation of endothelins can be blocked by inhibiting endothelin-converting enzyme with phosphoramidon. Phosphoramidon is not specific for endothelin-converting enzyme, but several more selective inhibitors are now available for research. Although the therapeutic potential of these drugs appeared similar to that of the endothelin receptor antagonists (see below), their use has been eclipsed by endothelin antagonists.

Physiologic & Pathologic Roles of Endothelin: Effects of Endothelin Antagonists

Systemic administration of endothelin receptor antagonists or endothelin-converting enzyme inhibitors causes vasodilation and decreases arterial pressure in humans and experimental animals. Intra-arterial administration of the drugs also causes slow-onset forearm vasodilation in humans. These observations provide evidence that the endothelin system participates in the regulation of vascular tone, even under resting conditions. The activity of the system is higher in males than in females. It increases with age, an effect that can be counteracted by regular aerobic exercise.

Increased production of ET-1 has been implicated in a variety of cardiovascular diseases, including hypertension, cardiac hypertrophy, heart failure, atherosclerosis, coronary artery disease, and myocardial infarction. ET-1 also participates in pulmonary diseases, including asthma and pulmonary hypertension, as well as in several renal diseases.

Endothelin antagonists have considerable potential for the treatment of these diseases. Indeed, endothelin antagonism with bosentan, sitaxsentan, and ambrisentan has proved to be a moderately effective and generally well-tolerated treatment for patients with pulmonary arterial hypertension. Other promising targets for these drugs are resistant hypertension, chronic renal disease, connective tissue disease, and subarachnoid hemorrhage. On the other hand, clinical trials of the drugs in the treatment of congestive heart failure have been disappointing.

Endothelin antagonists occasionally cause systemic hypotension, increased heart rate, facial flushing or edema, and headaches. Potential gastrointestinal effects include nausea, vomiting, and constipation. Because of their teratogenic effects, endothelin antagonists are contraindicated in pregnancy. Bosentan has been associated with fatal hepatotoxicity, and patients taking this drug must have monthly liver function tests. Negative pregnancy test results are required for women of child-bearing age to take this drug.

Vasoactive Intestinal Peptide

Vasoactive intestinal peptide (VIP) is a 28-amino-acid peptide that belongs to the glucagon-secretin family of peptides. VIP is widely distributed in the central and peripheral nervous systems, where it functions as one of the major peptide neurotransmitters. It is present in cholinergic presynaptic neurons in the central nervous system, and in peripheral peptidergic neurons innervating diverse tissues including the heart, lungs, gastrointestinal and urogenital tracts, skin, eyes, ovaries, and thyroid gland. Many blood vessels are innervated by VIP neurons. Although VIP is present in blood, where it undergoes rapid degradation, it does not appear to function as a hormone. VIP participates in a wide variety of biologic functions including metabolic processes, secretion of endocrine and exocrine glands, cell differentiation, smooth muscle relaxation, and the immune response.

VIP exerts significant effects on the cardiovascular system. It produces marked vasodilation in most vascular beds and in this regard is more potent on a molar basis than acetylcholine. In the heart, VIP causes coronary vasodilation and exerts positive inotropic and chronotropic effects. It may thus participate in the regulation of coronary blood flow, cardiac contraction, and heart rate.

The effects of VIP are mediated by G protein-coupled receptors; two subtypes, VPAC1 and VPAC2, have been cloned from human tissues. Both subtypes are widely distributed in the central nervous system and in the heart, blood vessels, and other tissues. VIP has a high affinity for both receptor subtypes. Binding of VIP to its receptors results in activation of adenylyl cyclase and formation of cAMP, which is responsible for the vasodilation and many other effects of the peptide. Other actions may be mediated by inositol trisphosphate synthesis and calcium mobilization.

Selective VPAC1 and VPAC2 receptor agonists, as well as nonselective agonists, are now available for research use. These drugs have potential as therapeutic agents for cardiovascular, pulmonary, gastrointestinal, and nervous system diseases. They may also be effective in treating various inflammatory diseases and diabetes. Indeed, some VIP derivatives are currently undergoing preclinical and clinical testing for the treatment of type 2 diabetes and chronic obstructive pulmonary disease. Unfortunately, their use is currently limited by several issues including poor oral availability, rapid metabolism in the blood, and hypotension. VIP receptor antagonists are also being developed.

Substance P

Substance P belongs to the tachykinin family of peptides, which share the common carboxyl terminal sequence Phe-X-Gly-Leu-Met. Other members of this family are neurokinin A and neurokinin B. Substance P is an undecapeptide, while neurokinins A and B are decapeptides.

Substance P is present in the central nervous system, where it is a neurotransmitter (see Chapter 21), and in the gastrointestinal tract, where it may play a role as a transmitter in the enteric nervous system and as a local hormone (see Chapter 6).

Substance P is the most important member of the tachykinin family. It exerts a variety of incompletely understood central actions that implicate the peptide in behavior, anxiety, depression, nausea, and emesis. It is a potent arteriolar vasodilator, producing marked hypotension in humans and several animal species. The vasodilation is mediated by release of nitric oxide from the endothelium. Conversely, substance P causes contraction of venous, intestinal, and bronchial smooth muscle. It also stimulates secretion by the salivary glands and causes diuresis and natriuresis by the kidneys.

The actions of substance P and neurokinins A and B are mediated by three G protein-coupled tachykinin receptors designated NK ₁ , NK ₂ , and NK ₃ . Substance P is the preferred ligand for the NK₁ receptor, the predominant tachykinin receptor in the human brain. However, neurokinins A and B also possess considerable affinity for this receptor. In humans, most of the central and peripheral effects of substance P are mediated by NK₁ receptors. All three receptor subtypes are coupled to inositol trisphosphate synthesis and calcium mobilization.

Several nonpeptide NK₁ receptor antagonists have been developed. These compounds are highly selective and orally active, and enter the brain. Recent clinical trials have shown that these antagonists may be useful in treating depression and other disorders and in preventing chemotherapy-induced emesis. The first of these to be approved for the prevention of chemotherapy-induced nausea and vomiting is aprepitant (see Chapter 62).

Neurotensin

Neurotensin (NT) is a tridecapeptide that was first isolated from the central nervous system but subsequently was found to be present in the gastrointestinal tract and in the circulation. It is synthesized as part of a larger precursor that also contains neuromedin N, a six-amino-acid NT-like peptide.

In the brain, processing of the precursor leads primarily to the formation of NT and neuromedin N; these are released together from nerve endings. In the gut, processing leads mainly to the formation of NT and a larger peptide that contains the neuromedin N sequence at the carboxyl terminal. Both peptides are secreted into the circulation after ingestion of food. Most of the activity of NT is mediated by the last six amino acids, NT(8-13).

Like many other neuropeptides, NT serves a dual function as a neurotransmitter or neuromodulator in the central nervous system and as a local hormone in the periphery. When administered centrally, NT exerts potent effects including hypothermia, antinociception, and modulation of dopamine neurotransmission. When administered into the peripheral circulation, it causes vasodilation, hypotension, increased vascular permeability, increased secretion of several anterior pituitary hormones, hyperglycemia, inhibition of gastric acid and pepsin secretion, and inhibition of gastric motility. It also exerts effects on the immune system.

In the central nervous system, there are close associations between NT and dopamine systems, and NT may be involved in clinical disorders involving dopamine pathways such as schizophrenia, Parkinson's disease, and drug abuse. Consistent with this, it has been shown that central administration of NT produces effects in rodents similar to those produced by antipsychotic drugs.

Three subtypes of NT receptors, designated NT ₁ ,NT ₂ , and NT ₃ , have been cloned. NT₁ and NT₂ receptors belong to the G protein-coupled superfamily with seven transmembrane domains; the NT₃ receptor is a single transmembrane domain protein that belongs to a family of sorting proteins.

NT agonists that cross the blood-brain barrier have been developed. They are all peptide analogs of NT(8-13) and include PD149163, NT66L, NT67L, NT69L, and NT77L. These research drugs may have potential as therapeutic agents for diseases such as schizophrenia and Parkinson's disease. They may also aid in smoking cessation and weight loss.

NT receptors can be blocked with the nonpeptide antagonists SR142948A and meclinertant (SR48692). SR142948A is a potent antagonist of the hypothermia and analgesia produced by centrally administered NT. It also blocks the cardiovascular effects of systemic NT.

Calcitonin Gene–Related Peptide

Calcitonin gene–related peptide (CGRP) is a member of the calcitonin family of peptides, which also includes calcitonin, adrenomedullin, and amylin. CGRP consists of 37 amino acids and displays approximately 30% structural homology with salmon calcitonin.

Like calcitonin, CGRP is present in large quantities in the C cells of the thyroid gland. It is also distributed widely in the central and peripheral nervous systems, in the cardiovascular system, the gastrointestinal tract, and the urogenital system. CGRP is found with substance P (see above) in some of these regions and with acetylcholine in others.

When CGRP is injected into the central nervous system, it produces a variety of effects, including hypertension and suppression of feeding. When injected into the systemic circulation, the peptide causes hypotension and tachycardia. The hypotensive action of CGRP results from the potent vasodilator action of the peptide; indeed, CGRP is the most potent vasodilator yet discovered. It dilates multiple vascular beds, but the coronary circulation is particularly sensitive.

The actions of CGRP are mediated by two 7-transmembrane receptors named CGRP₁ and CGRP₂. Peptide and nonpeptide antagonists of these receptors have been developed. Of the nonpeptide antagonists now available, the best characterized is BIBN4096BS, which has a high affinity and specificity for the human CGRP receptor.

Evidence is accumulating that release of CGRP from trigeminal nerves plays a central role in the pathophysiology of migraine. The peptide is released during migraine attacks, and successful treatment of migraine with a selective serotonin agonist normalizes cranial CGRP levels. BIBN4096BS has recently been shown to be an effective, well-tolerated treatment for migraine.

Adrenomedullin

Adrenomedullin (AM) was first discovered in human adrenal medullary pheochromocytoma tissue. It is a 52-amino acid peptide with a six-amino-acid ring and a C-terminal amidation sequence. Like CGRP, AM is a member of the calcitonin family of peptides.

AM is widely distributed in the body. The highest concentrations are found in the adrenal glands, hypothalamus, and anterior pituitary, but high levels are also present in the kidneys, lungs, cardiovascular system, and gastrointestinal tract. AM in plasma apparently originates in the heart and vasculature.

In animals, AM dilates resistance vessels in the kidney, brain, lung, hind limbs, and mesentery, resulting in a marked, long-lasting hypotension. The hypotension in turn causes reflex increases in heart rate and cardiac output. These responses also occur during intravenous infusion of the peptide in healthy human subjects. AM also acts on the kidneys to increase sodium excretion, and it exerts several endocrine effects including inhibition of aldosterone and insulin secretion. It acts on the central nervous system to increase sympathetic outflow.

The diverse actions of AM are mediated by the 7-transmembrane G protein-coupled calcitonin receptor-like receptor (CRLR) which co-assembles with subtypes 2 and 3 of a family of receptor-activity-modifying proteins (RAMPs), thus forming a receptor-coreceptor system. Binding of AM to CRLR activates G_s and triggers cAMP formation in vascular smooth muscle cells, and increases nitric oxide production in endothelial cells. Other signaling pathways are also involved.

Circulating AM levels increase during intense exercise. They also increase in a number of pathologic states, including essential hypertension, cardiac and renal failure, and septic shock. The roles of AM in these states remain to be defined, but it is currently thought that the peptide functions as a physiologic antagonist of the actions of vasoconstrictors including ET-1 and Ang II. By virtue of these actions, AM may protect against cardiovascular overload and injury, and AM may be beneficial in the treatment of some cardiovascular diseases.

Neuropeptide Y

Neuropeptide Y (NPY) is a member of the family that also includes peptide YY and pancreatic polypeptide. Each peptide consists of 36 amino acids.

NPY is one of the most abundant neuropeptides in both the central and peripheral nervous systems. In the sympathetic nervous system, NPY is frequently localized in noradrenergic neurons and apparently functions both as a vasoconstrictor and as a cotransmitter with norepinephrine. Peptide YY and pancreatic polypeptide are both gut endocrine peptides.

NPY produces a variety of central nervous system effects, including increased feeding (it is one of the most potent orexigenic molecules in the brain), hypotension, hypothermia, respiratory depression, and activation of the hypothalamic-pituitary-adrenal axis. Other effects include vasoconstriction of cerebral blood vessels, positive chronotropic and inotropic actions on the heart, and hypertension. The peptide is a potent renal vasoconstrictor and suppresses renin secretion, but can cause diuresis and natriuresis. Prejunctional neuronal actions include inhibition of transmitter release from sympathetic and parasympathetic nerves. Vascular actions include direct vasoconstriction, potentiation of the action of vasoconstrictors, and inhibition of the action of vasodilators.

These diverse effects are mediated by multiple receptors designated Y ₁ through Y ₆ . All receptors except Y₃ have been cloned and shown to be G protein-coupled receptors linked to mobilization of Ca²⁺ and inhibition of adenylyl cyclase. Y₁ and Y₂ receptors are of major importance in the cardiovascular and other peripheral effects of the peptide. Y₄ receptors have a high affinity for pancreatic polypeptide and may be a receptor for the pancreatic peptide rather than for NPY. Y₅ receptors are found mainly in the central nervous system and may be involved in the control of food intake. They also mediate the activation of the hypothalamic-pituitary-adrenal axis by NPY. Y₆ receptors do not appear to contribute significantly to the physiologic effects of NPY in humans.

Selective nonpeptide NPY receptor antagonists are now available for research. The first nonpeptide Y₁ receptor antagonist, BIBP3226, is also the most thoroughly studied. It has a short half-life in vivo. In animal models, it blocks the vasoconstrictor and pressor responses to NPY. Structurally related Y₁ antagonists include BIB03304 and H409/22, which has been tested in humans. SR120107A and SR120819A are orally active Y₁ antagonists and have a long duration of action. BIIE0246 is the first nonpeptide antagonist selective for the Y₂ receptor.

These drugs have been useful in analyzing the role of NPY in cardiovascular regulation. It now appears that the peptide is not important in the regulation of hemodynamics under normal resting conditions, but may be of increased importance in cardiovascular disorders including hypertension and heart failure. Other studies have implicated NPY in feeding disorders, seizures, anxiety, and diabetes, and Y₁ and Y₅ receptor antagonists have potential as antiobesity agents.

Urotensin

Urotensin II (U-II) was originally identified in fish, but isoforms are now known to be present in mammalian species including the human, mouse, rat and pig. Human U-II is an 11-amino acid peptide. Major sites of U-II expression in humans include the brain, spinal cord, and kidneys. U-II is also present in plasma, and the kidneys may be a major source of this circulating peptide.

In vitro, U-II is a potent constrictor of vascular smooth muscle; its activity depends on the type of blood vessel and the species from which it was obtained. Vasoconstriction occurs primarily in arterial vessels, where U-II can be more potent than endothelin 1, making it the most potent known vasoconstrictor. However, under some conditions, U-II may cause vasodilation. In vivo, U-II has complex hemodynamic effects, the most prominent being regional vasoconstriction and cardiac depression. In some ways, these effects resemble those produced by ET-1. Nevertheless, the role of the peptide in the normal regulation of vascular tone and blood pressure in humans appears to be minor.

The actions of U-II are mediated by a G protein-coupled receptor referred to as the UT receptor. UT receptors are widely distributed in the brain, spinal cord, heart, vascular smooth muscle, skeletal muscle, and pancreas. Some effects of the peptide including vasoconstriction are mediated by the phospholipase C, IP₃-DAG signal transduction pathway.

Modifications of the disulfide bridge of U-II have yielded UT-receptor antagonists. A nonpeptide antagonist, palosuran, has also been developed.

Although U-II appears to play only a minor role in health, evidence is accumulating that it is involved in cardiovascular and other diseases. In particular, it has been reported that plasma U-II levels are increased in hypertension, heart failure, diabetes mellitus, and renal failure. In addition, the first study using a UT receptor antagonist in humans suggests that palosuran may benefit diabetic patients with renal disease.

Summary: Drugs That Interact with Vasoactive Peptide Systems

References

Angiotensin

Kinins

Vasopressin

Natriuretic Peptides

Vasopeptidase Inhibitors

Endothelins

Vasoactive Intestinal Peptide

Substance P

Neurotensin

Calcitonin Gene–Related Peptide

Adrenomedullin

Neuropeptide Y

Urotensin