Brody's Human Pharmacology: With STUDENT CONSULT

Chapter 24 Vasodilators and Nitric Oxide Synthase

MAJOR DRUG CLASSES

Nitrovasodilators

K+ channel activators

Phosphodiesterase type 5 inhibitors

Therapeutic Overview

Ischemic heart disease is characterized by angina pectoris, chest pain that arises generally midsternally but also may radiate along the inner portion of one or both arms, or to the back. Vasodilators, specifically the nitrates, are mainstays in management. There are several different types of angina, depending on whether the disease is of atherosclerotic origin, the result of coronary artery spasm, or both. Angina may also be classified according to whether the pain is exertional or occurs more frequently at rest. However, irrespective of its type, the purpose of drug intervention is to bring about vasodilation of the coronary arteries, redistribution of blood flow in the heart, and/or a reduction in cardiac O2 demand. Vasodilators, such as nitrates, provide no permanent beneficial effect on the underlying pathological condition but afford temporary symptomatic relief.

Vasodilators have important uses in management of coronary artery disease, hypertension, and congestive heart failure (CHF). Some modest success in preventing vasospasm or peripheral vascular disease has also been achieved. These drugs also play a minor role in lowering blood pressure to reduce bleeding in a surgical field. They are also increasingly popular for treatment of male impotence.

A summary of the uses of these compounds is provided in the Therapeutic Overview Box.

Therapeutic Overview

Clinical Problem

Goal of Drug Intervention

Hypertension

Decrease blood pressure

Congestive heart failure

Increase cardiac output and decrease O2 consumption

Coronary artery insufficiency

Increase effective flow through coronary arteries and decrease O2 consumption by the heart

Peripheral vascular disease

Increase blood flow to the ischemic area

Hemostasis

Slow bleeding into surgical field

Impotence

Increased erectile function

Abbreviations

ACE

Angiotensin-converting enzyme

cAMP

Cyclic adenosine monophosphate

cGMP

Cyclic guanosine monophosphate

CHF

Congestive heart failure

NO

Nitric oxide

PDE

Phosphodiesterase

Mechanisms of Action

Vasodilators act at different sites in the cascade of events that couple excitation of vascular smooth muscle to contraction (Table 24-1). Thus, to understand the mechanisms of action of these agents and their uses, it is critical to be familiar with the processes involved in the contraction of smooth muscle cells.

TABLE 24–1 Mechanisms, Sites of Action, and Uses of Selected Vasodilator Drugs

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Vascular Smooth Muscle Cell Contraction and Relaxation

Smooth muscle contraction is ultimately regulated by intracellular Ca++ concentrations. Excitation-contraction coupling occurs by several mechanisms. Depolarization of vascular smooth muscle cell membranes allows Ca++ entry through voltage-gated channels. When these channels open, Ca++ flows into the cell down its concentration gradient (Fig. 24-1). Activation of receptors for certain vasoconstrictor substances can also open Ca++ channels. In addition to elevating intracellular Ca++ by opening channels, receptor activation can also increase intracellular Ca++ by activating phospholipase C, which hydrolyzes phosphatidylinositol 4,5-bisphosphate to diacylglycerol and inositol 1,4,5-trisphosphate, both of which contribute to contraction (see Chapters 1 and Chapters 9). Inositol trisphosphate releases Ca++ from intracellular stores, whereas diacylglycerol activates protein kinase C, an enzyme that phosphorylates several substrates involved in the contractile response. When Ca++ enters the smooth muscle cell, it combines with calmodulin, and the Ca++-calmodulin complex activates myosin light-chain kinase, which in turn phosphorylates the myosin light chain, promoting the interaction of myosin and actin and cross-bridge formation, leading to contraction. Because Ca++-channel antagonists block or limit the entry of Ca++ through voltage-gated channels, these drugs dilate blood vessels that have some endogenous degree of vasoconstrictor tone, or limit vasoconstriction caused by endogenous or exogenous vasoactive stimulants (see Chapter 20).

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FIGURE 24–1 Mechanisms of contraction of vascular smooth muscle cells and its inhibition by Ca++-channel blockers. Increases in intracellular Ca++ can occur by Ca++ entry through channels opened by a change in membrane potential, by receptor activation, or by Ca++ release from sarcoplasmic reticulum (SR), an event triggered by inositol 1,4,5 trisphosphate (IP3). IP3 is formed by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by phospholipase C (PLC). Ca++ interacts with calmodulin (CM), which activates myosin light-chain kinase (MLCK). The latter phosphorylates myosin, which interacts with actin, resulting in contraction. Ca++-channel blockers act by limiting Ca++ entry through membrane channels.

Increases in cyclic adenosine monophosphate (cAMP) also lead to smooth muscle relaxation. Increased cAMP activates cAMP-dependent protein kinase A, which phosphorylates several proteins, leading to decreased intracellular Ca++ as a consequence of reduced influx, enhanced uptake into the sarcoplasmic reticulum, and/or enhanced extrusion through the cell membrane (Fig. 24-2, A). Myosin light-chain kinase may also be phosphorylated, leading to enzyme inactivation and inhibition of contraction. Because adrenergic β receptor agonists such as isoproterenol activate adenylyl cyclase and increase cAMP, these agents lead to relaxation of vascular smooth muscle. Similarly, drugs that inhibit phosphodiesterases (PDEs), which metabolize cAMP and cyclic guanosine monophosphate (cGMP), promote smooth muscle relaxation. Drugs such as papaverine may act by this mechanism.

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FIGURE 24–2 Mechanisms for relaxation of vascular smooth muscle cells. A, β Adrenergic receptor agonists cause relaxation by stimulating formation of cAMP, which activates protein kinase (PK) and decreases intracellular Ca++. This occurs by activating Ca++pumps in the sarcoplasmic reticulum membrane or cell membrane to either sequester Ca++ or pump it from the cell. B, Nitrovasodilators activate soluble guanylyl cyclase by releasing NO or related compounds, leading to an increase in cGMP and activation of cGMP-dependent protein kinase. NO may influence contractility by limiting Ca++ entry through channels or by directly decreasing the sensitivity of contractile proteins to Ca++. Type 5 PDE inhibitors, such as sildenafil, inhibit cGMP breakdown in cells where they are expressed (such as penile corpus cavernosum), and potentiate its vasodilatory actions. C, K+-channel activators, such as minoxidil, increase K+ conductance, which hyperpolarizes the cell, causing relaxation.image-hyperpolarization, image-depolarization.

Nitrovasodilators

Nitrovasodilators are organic nitrates that provide a source of nitric oxide (NO), which activates a soluble guanylyl cyclase in vascular smooth muscle, causing an increase in intracellular cGMP, which activates a cGMP-dependent protein kinase (see Fig. 24-2, B). This kinase leads to the phosphorylation of proteins, which results in smooth muscle relaxation. Although the cellular mechanisms involved are not entirely clear, they may include decreased entry of Ca++ through membrane channels, inhibition of phosphatidylinositol hydrolysis, stimulation of Ca++ pumps to extrude or sequester Ca++, and decreased sensitivity of contractile proteins to Ca++.

NO is one of the most important vasodilator factors formed in and released from the endothelial cells of blood vessels. Endothelial cells line all vessels of the body and release factors that affect both the contractile state and growth of smooth muscle cells. NO, a short-lived radical, is formed from L-arginine by a class of enzymes known as NO synthases. Two isoforms of this enzyme are particularly important with respect to vascular biology. The “constitutive” form is present in endothelium under normal physiological conditions, and its activity is dependent upon the concentration of Ca++-calmodulin. There is also an “inducible” form of NO synthase expressed in smooth muscle in response to trauma or pathological stimuli, such as invading bacteria. The activity of this isoform does not depend on intracellular Ca++-calmodulin concentrations and is not easily regulated. In severe septicemia, NO generated by this enzyme can cause harmful hypotension due to vasodilation. In all cases NO-induced vasodilation is associated with elevated levels of cGMP.

NO may be the final common mediator for several vascular smooth muscle relaxants. In addition to nitrovasodilators, which may form NO or a related molecule, some endogenous agents that cause vasodilation do so in whole or in part by releasing NO from endothelial cells. Included among these are bradykinin, histamine, adenosine triphosphate, adenosine diphosphate, substance P, and acetylcholine (Fig. 24-3). Because the endothelium is an important structure for communicating between the blood and the vascular media, it has the potential to be an important target for vasodilator therapy.

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FIGURE 24–3 Endothelium-dependent relaxation produced by vasodilators. These substances act on endothelial cells at their respective receptors to release NO. The latter diffuses into the vascular smooth muscle cell, increases soluble guanylyl cyclase activity and cGMP concentration, and promotes relaxation. L-Arginine is converted to NO by NO synthases.

K+ Channel Activators

Agents such as minoxidil cause vasodilation by activating K+ channels in vascular smooth muscle. The increased K+ conductance results in hyperpolarization of the cell membrane and relaxation (see Fig. 24-2, C). The hyperpolarizing effect also counteracts stimulants that act by depolarization, promoting Ca++ entry.

Phosphodiesterase Type 5 Inhibitors

Sildenafil, tadalafil, and vardenafil are selective inhibitors of cGMP-specific PDE type 5, which is found in high concentrations in the penile corpus cavernosum and is responsible for the degradation of cGMP. NO release during sexual stimulation activates guanylyl cyclase, resulting in increased levels of cGMP, which causes smooth muscle relaxation in the corpus cavernosum, allowing the inflow of blood. The PDE5 inhibitors prevent the catabolism of cGMP, thereby prolonging its actions (see Fig. 24-2, B). These drugs have no direct relaxant effect on isolated human corpus cavernosum and, at recommended doses, have no effect in the absence of sexual stimulation. Rather, they cause specific vasodilation in the presence of appropriate sexual stimulation and have become increasingly popular for treating erectile dysfunction in men.

Other Agents

Although hydralazine was one of the first antihypertensive drugs, its mechanism of action has not been definitively elucidated. It is a vasodilator with effects on arterioles and not venules. Hydralazine has been shown to decrease intracellular Ca++ concentrations, and recent evidence suggests that this action may be due to its ability to inhibit the inositol 1,4,5-trisphosphate-induced release of Ca++ from the sarcoplasmic reticulum in vascular smooth muscle cells (see Fig. 24-1). Additional sites of action have also been proposed (see Table 24-1).

Pharmacokinetics

Selected pharmacokinetic parameter values for the nitrovasodilators are summarized in Table 24-2. Organic nitrates are almost completely absorbed from the gastrointestinal tract and fairly completely absorbed from the buccal mucosa. After sublingual administration, peak plasma concentrations are achieved in 1 to 2 minutes. Absorption is much slower with topical ointments and transdermal patches, and plasma concentrations attained with transdermal preparations are lower and more variable than those obtained with ointments. The nitrates are metabolized in the liver by glutathione nitrate reductase (e.g., nitroglycerin is rapidly converted to inorganic nitrite and to denitrated metabolites). Isosorbide dinitrate is also metabolized by hepatic glutathione reductase and converted to inactive products and to an active metabolite, 5-isosorbide mononitrate, which may account for its longer duration of antianginal activity.

TABLE 24–2 Pharmacokinetic Parameters of Nitrovasodilators

Drug

Route of Administration

Remarks

Nitroglycerin

Sublingual

Onset 2-4 min, duration 30-60 min depending on patient activity, minimal first-pass effect, all organic nitrates metabolized by liver

Oral

Onset 10-20 min, duration 2-3 hrs, significant first-pass effect

IV

Immediate onset, used to maintain stable blood concentration

Transdermal

Discs or patches: slower onset, 10-18 hrs variable duration; ointment less variable, duration 20-24 hrs, for nocturnal angina

Aerosol

Rapid onset, difficult to control

Isosorbide dinitrate*

Sublingual

Similar in onset to nitroglycerin, longer duration (2-4 hrs)

Oral

Onset 10-20 min, duration 4-8 hrs

Erythrityl tetranitrate

Sublingual

Onset 3-5 min, duration 1-2 hrs

Pentaerythritol tetranitrate

Oral

Onset 15-30 min, duration 4-8 hrs

IV; intravenous

* Active metabolite; oral preparations: onset varies with dose, and duration depends on extent of first-pass metabolism.

Sublingual nitroglycerin is the mainstay of therapy in anginal attacks and is also used prophylactically. It is rapid in onset and inexpensive. Sublingual isosorbide dinitrate is also available and has a longer duration of action than nitroglycerin. The nitroglycerin aerosol spray appears to be as effective as the sublingual tablets. Transdermal patches are not as effective as the oral, timed-release preparations, largely because of variable absorption through the skin. As a result of tolerance, transdermal patches left in place for 24 hours are ultimately ineffective for treatment of angina, even if the dose is increased. However, patches that deliver 10 mg or more nitroglycerin can be effective, if the patches are removed for a 10-to 12-hour period daily.

The PDE5 inhibitors are given orally and are rapidly absorbed with an onset of action within approximately 30 minutes. Sildenafil and vardenafil undergo extensive first-pass metabolism and have half-lives of 3 to 5 hours, whereas the t1/2 of tadalafil is approximately 17 hours. All of these compounds are highly protein bound in the plasma and are metabolized in the liver primarily by CYP3A4.

Hydralazine is well absorbed after oral administration and undergoes extensive first-pass metabolism via acetylation. As a consequence, responses vary within the population depending on the ability of individuals to acetylate hydralazine. The t1/2 of hydralazine is 2 to 3 hours, but the vasodilatory effects are longer.

Relationship of Mechanisms of Action to Clinical Response

Nitrovasodilators and Angina Pectoris

The goal of therapy in coronary artery disease is to reduce pain and increase the patient’s exercise tolerance. This can be accomplished by administration of organic nitrates, the prototype of which isnitroglycerin. Organic nitrates are the mainstay of antianginal therapy, used effectively for this purpose for approximately 100 years.

The pharmacological properties of the organic nitrates that make them useful depend on the underlying cause of the angina. If pain is associated with atherosclerosis, the chief benefit arises from actions on the peripheral circulation and not on coronary vessels. Nitrates produce vasodilation of the venous vasculature. Dilation of venous capacitance vessels diminishes venous return to the heart, reducing ventricular volume and pressure. This decreases ventricular wall tension, a major contributor to the O2 demands of the heart (Fig. 24-4). Thus, by decreasing preload on the heart, O2 needs diminish, and demand is consistent with supply.

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FIGURE 24–4 Mechanism of vasodilator action in the therapy of CHF. The four main determinants of cardiac function act to determine the myocardial O2 demand. Nitrates decrease preload and afterload but do not affect contractile force, thereby decreasing O2 demand. Heart rate may increase slightly as a result of the baroreceptor reflex.

Other consequences of nitroglycerin administration also contribute to its beneficial effect in angina. For example, nitrates cause relaxation of resistance vessels of the arterial circulation. This decreases afterload placed on the heart, or the impedance against which the heart must pump. Reducing afterload decreases O2 demands of the heart, just as reducing preload does. The nitrate effect on resistance vessels generally requires somewhat higher concentrations than those needed for venodilation.

Another beneficial feature of organic nitrate actions in angina pectoris is redistribution of blood flow to the subendocardial regions of the heart, which are especially vulnerable to ischemia. Perfusion of the subendocardial region occurs most prominently during early diastole. Later in diastole, as the ventricle fills, subendocardial arteries are constricted because of pressure in the ventricles, with the subsequent decrease in perfusion of these arteries. By decreasing preload, nitrates reduce ventricular filling pressure and increase the time available for endocardial perfusion.

In management of angina pectoris caused by coronary artery spasm, the organic nitrates, in addition to effects described previously, are useful because they can dilate constricted coronary vessels. Nitrates are available in many dosage forms, including sublingual, transdermal, and longer acting oral preparations. The choice of nitrate preparation depends on the necessity for a rapid onset or a longer duration of action.

Other Drugs for Angina Pectoris

Additional drugs used for the treatment of angina pectoris are adrenergic β receptor antagonists and Ca++ channel-blocking drugs (see Chapters 11 and 20). The beneficial effect of β receptor blockers in angina is their ability to decrease O2 demands of the heart. These drugs decrease heart rate and ventricular contractile force (see Chapter 11). Heart rate and contractile force, together with ventricular wall tension, are major determinants of myocardial O2 demand (see Fig. 24-4). In addition, chronic therapy with β-blockers reduces blood pressure. Thus these drugs also decrease afterload. Unlike organic nitrates, β-blockers are not used to terminate an acute attack of angina pectoris but rather to increase exercise tolerance of the patient and to reduce the frequency of anginal attacks.

Vasodilators and Congestive Heart Failure

Recent studies indicate that vasodilator therapy is extremely effective in treatment of CHF. Drugs used more frequently for treating CHF are those that increase the force of cardiac contraction (see Chapter 23) or minimize Na+ and H2O retention (see Chapter 21). Cardiac glycosides affect only two of several determinants of cardiac function (e.g., contractility and rate).

Among the major mechanisms by which vasodilators increase cardiac performance are afterload reduction, preload reduction, and the resulting increased left ventricular diastolic compliance. Afterload reduction, by use of other vasodilators and high concentrations of nitrates, is accomplished by dilating arterioles and thereby decreasing systemic vascular resistance. This increases cardiac output and tissue perfusion. Venodilators, including low doses of nitrates, predominantly decrease preload, reducing systemic and pulmonary venous pressures. Ventricular volume is also affected by the decreasing preload. The venodilators do not increase the force of contraction, and the heart rate is generally unchanged, so that the work of the heart remains the same. The overall effect, therefore, is a reduction in myocardial O2 consumption and demand on the heart. The vasodilator drugs may also improve left ventricular diastolic performance by shifting the diastolic pressure-volume curve to the left (i.e., to pump the same volume at a lower pressure). This shift also moves the ventricular function curve to the left, demonstrating an improvement in left ventricular performance.

Patients who are refractory to cardiac glycosides frequently do well if treated with vasodilators. Among the direct-acting vasodilators used to treat CHF are the nitrates, hydralazine, minoxidil, and sodium nitroprusside. Angiotensin-converting enzyme (ACE) inhibitors such as captopril enalapril, and lisinopril (see Chapter 20 and 23) are also of proven effectiveness, as is the adrenergic α1 receptor antagonist prazosin. Long-term treatment with hydralazine alone is only minimally effective in treatment of CHF, but the combination of hydralazine with a nitrate, isosorbide dinitrate, effectively decreases mortality. Minoxidil is also generally not very effective when used alone.

Vasodilators and Peripheral Vascular Disease

Peripheral vascular diseases are either vasospastic or occlusive. In Raynaud’s disease, a vasospastic disorder, blood flow to the extremities is reduced as a result of a reversible vasoconstriction. Therefore vasodilators may be helpful to these patients by dilating the blood vessels of the skin. Direct-acting vasodilators, adrenergic α1 receptor blockers, Ca++ channel-blocking drugs, prostaglandins, adrenergic β receptor agonists, and ACE inhibitors are used to treat Raynaud’s phenomenon. Nitroglycerin ointment may be helpful as an adjunctive agent in Raynaud’s disease. Other nonspecific vasodilators, such as cyclandelate, papaverine, ethaverine, and nicotinyl tartrate, have been used but are of questionable efficacy.

Vasodilators are of limited usefulness in occlusive disease when organic obstruction is significant, and they do not generally improve flow to either skeletal muscle or skin. In some instances vasodilator therapy may actually be harmful when blood is shunted away from diseased areas.

Vasodilators and Hemostasis

Vasodilators may be used as aids during surgical procedures. They can be used to provide a more satisfactory surgical field, to minimize large blood volume losses, and to improve cardiac performance by reducing preload or afterload.

Phosphodiesterase Type 5 Inhibitors and Impotence

A new and evolving use of direct vasodilators is in treatment of erectile dysfunction in men. The use of PDE5 inhibitors, including sildenafil, tadalafil and vardenafil, has dramatically altered therapeutic treatment for this disorder. Parasympathetic nerves innervating blood vessels of the penile corpus cavernosum release NO when activated during sexual activity. Sildenafil and related drugs potentiate the action of NO on cGMP by blocking cGMP hydrolysis, resulting in prolonged dilation of cavernosal blood vessels, thereby enhancing erection.

Pharmacovigilance: Side-Effects, Clinical Problems, and Toxicity

Although vasodilators have useful therapeutic actions, they are not without problems. One major problem is the “steal” phenomenon. Some data indicate that use of vasodilator drugs to promote blood flow to ischemic or diseased tissue is limited. It appears that the small blood vessels around the ischemic area are already significantly dilated; therefore vasodilators may do little to enhance flow in this region. However, in normal nonischemic areas, where small vessels are not dilated, there is increased blood flow. By shunting blood to these areas, vasodilators may actually be reducing flow to the ischemic region.

Another concern is that by decreasing peripheral vascular resistance, vasodilators cause reflex activation of the sympathetic nervous system (see Chapter 11). Enhanced sympathetic activity can lead to unwanted cardiac effects. The release of renin from juxtaglomerular cells is also enhanced by reflex sympathetic nerve stimulation caused by vasodilators. To counteract this action, adrenergic β receptor antagonists are frequently administered in conjunction with direct-acting vasodilators.

Another problem is the potential of these drugs to cause dilation of other nonvascular smooth muscles. Although uncommon, there are circumstances in which this is clinically significant, as in treatment of hypertension associated with the toxemia of pregnancy. In this case vasodilators might interrupt labor by relaxing uterine smooth muscle.

Major side effects of vasodilators are summarized in the Clinical Problems Box.

Nitrovasodilators

As with all vasodilators, orthostatic hypotension and tachycardia are adverse effects of nitrate therapy. Vascular headache is quite common but rapidly disappears on continued use. Tolerance to the vascular effects of nitrates does occur; however, this is not of great clinical significance, except possibly in treatment of chronic CHF. Cross-tolerance exists between nitroglycerin and other nitrate esters, but this and other nitrate tolerance can be reduced by only a short period of nitrate abstention. Orthostatic hypotension can be minimized by careful adjustment of dose and by having the patient avoid the upright position when taking rapid-acting preparations. Physical dependence has been observed in munitions workers exposed continuously to very high concentrations of nitrates. In these individuals withdrawal from the industrial environment may result in angina. This phenomenon is not observed in patients normally taking therapeutic doses of nitrates but can occur in individuals who have been taking large doses for a long time.

Nitrates can be reduced to nitrites, which in turn can oxidize the ferrous iron of hemoglobin, converting it to methemoglobin. The latter reduces O2 delivery to tissues. Methemoglobinemia is not a problem with normal nitrate therapy but may be observed in accidental poisoning or overdose.

Phosphodiesterase Type 5 Inhibitors

Type 5 PDE inhibitors can cause abnormalities in color vision, although this is most common with sildenafil.

CLINICAL PROBLEMS

Vasodilators in General

Orthostatic hypotension, tachycardia

Nitrate Vasodilators

Headache tolerance

Hydralazine

Lupus-like effect

Sodium Nitroprusside

Thiocyanate accumulation

Minoxidil

Na+ retention

Hypertrichosis

Sildenafil

Hypotension and reflex stimulation of the heart

Problems with color vision

Inhibition of PDE5 in other tissues, such as esophageal smooth muscle, can result in a reduced tone of the esophageal sphincter and increased gastroesophageal reflux, as well as dyspepsia. In addition, as a consequence of PDE type 5 inhibition in the brain, these compounds have been reported to result in emotional, neurological, and psychological side effects. They also have the potential to cause hypotension.

New Horizons

Development of new vasodilators with greater specificity remains an important goal. One area with particular promise is the pharmacology of the vascular endothelium. The endothelium has a crucial function in regulation of both the contractile state and growth of vascular smooth muscle cells. It also has antithrombotic properties that inhibit adhesion and aggregation of blood cells. The important outcome of these effects is that the microvasculature is perfused without obstruction, whereas blood flow is regulated locally.

The most important approach has been to develop drugs that interact with the NO signaling cascade of the endothelium. Potential targets include:

• Drugs that activate NO synthase.

• Gene therapy to influence expression of NO synthase or other protein mediators.

• Drugs that prolong the vasodilator properties of NO.

Several other products of endothelial cells, called endothelins, have been isolated. The most prominent and well studied is a 21-amino acid peptide, endothelin-1.

TRADE NAMES

(In addition to generic and fixed-combination preparations, the following trade-named materials are some of the important compounds available in the United States.)

Nitrovasodilators and Related Compounds

Nitroglycerin ointment (Nitrol)

Nitroglycerin sublingual (Nitro-Bid, Nitrospan, Nitrolingual)

Isosorbide dinitrate (Isordil, Sorbitrate, Dilatrate)

Sodium nitroprusside (Nitropress)

Hydralazine (Apresoline)

K+ Channel Activator

Minoxidil (Lotinen)

PDE Type 5 Inhibitors

Sildenafil (Viagra)

Tadalafil (Cialis)

Vardenafil (Levitra)

This compound is released in response to physiological challenges, such as hypoxia or stress, or by endogenous hormones, such as angiotensin. Endothelin-1 initially dilates smooth muscle but subsequently produces an intense, long-lasting vasoconstriction. Two types of endothelin receptors have been characterized (ETA and ETB). Current evidence indicates that endothelin does not act as a circulating hormone but rather as an autocrine or paracrine substance. Abnormally high levels may play a pathogenic role in some forms of vasospasm. Antagonists of endothelin receptors have been developed, and their therapeutic potential is being investigated.

Finally, recent studies have demonstrated that NO is balanced by superoxide anion generated in the vascular wall. Superoxide anion is the product of several reactions, but the principal ones in the vasculature are NADPH oxidase, xanthine oxidase, uncoupled NO synthase, and cyclooxygenase. In several vascular diseases (atherosclerosis, hypertension, etc.), the production of superoxide in blood vessel walls is increased. The reaction between NO and superoxide is so fast that it effectively removes the vasodilator action of NO. Drugs that block production of superoxide anion may be useful in vasodilator therapy that focuses on NO.

FURTHER READING

Toda N, Okamura T. The pharmacology of nitric oxide in the peripheral nervous system of blood vessels. Pharmacol Rev. 2003;55:271-324.

Vanhoutte PM. Endothelial control of vasomotor function: From health to coronary disease. Circ J. 2003;67:572-575.

Webb RC. Smooth muscle contraction and relaxation. Adv Physiol Educ. 2003;27:201-206.

SELF-ASSESSMENT QUESTIONS

1. Which of the following vasodilators has specific actions only on arterioles?

A. Hydralazine

B. Nitroglycerin

C. Sodium nitroprusside

D. Sildenafil

E. Captopril

2. In the vascular smooth muscle cell:

A. Depolarization of the membrane allows Ca++ entry via voltage-operated channels.

B. Inositol trisphosphate, a product of phospholipase C activation, releases intracellular Ca++ from storage sites.

C. Ca++ combines with calmodulin to activate myosin light-chain kinase.

D. A and B are correct.

E. All are correct.

3. Activation of K+ channels in vascular smooth muscle mediates the vasodilatory effects of which of the following agents?

A. Sildenafil

B. Hydralazine

C. Sodium nitroprusside

D. Minoxidil

E. Captopril

4. The ability of the PDE type 5 inhibitors such as sildenafil to promote blood flow depends on:

A. Sexual stimulation, which activates levels of cGMP.

B. Ongoing activation of parasympathetic activity innervating blood vessels of the corpus cavernosum.

C. Activation of Ca++ uptake by the sarcoplasmic reticulum.

D. Activation of K+ channels promoting hyperpolarization.

E. None of the above.