Pharmacology Ch7.pdf

Adrenergic Antagonists I. OVERVIEW The adrenergic antagonists (also called blockers or sympatholytic agents) bind to adrenoceptors but do not trigger the usual receptor-mediated intracellular effects. These drugs act by either reversibly or irreversibly attaching to the receptor, thus preventing its...

Adrenergic Antagonists I. OVERVIEW The adrenergic antagonists (also called blockers or sympatholytic agents) bind to adrenoceptors but do not trigger the usual receptor-mediated intracellular effects. These drugs act by either reversibly or irreversibly attaching to the receptor, thus preventing its activation by endogenous catecholamines. Like the agonists, the adrenergic antagonists are classified according to their relative affinities for α or β receptors in the peripheral nervous system. These drugs will interfere with the functions of the sympathetic nervous system. Numerous adrenergic antagonists have important roles in clinical medicine, primarily to treat diseases associated with the cardiovascular system. [Note: Antagonists that block dopamine receptors are most important in the central nervous system (CNS) and are, therefore, considered in that section (see p. 161).] The receptor-blocking drugs discussed in this chapter are summarized in Figure 7.1. II. α-ADRENERGIC BLOCKING AGENTS Drugs that block α adrenoceptors profoundly affect blood pressure. Because normal sympathetic control of the vasculature occurs in large part through agonist actions on α-adrenergic receptors, blockade of these receptors reduces the sympathetic tone of the blood vessels, resulting in decreased peripheral vascular resistance. This induces a reflex tachycardia resulting from the lowered blood pressure. The magnitude of the response depends on the sympathetic tone of the individual when the agent is given. Effects are more profound in an individual who is standing and less in a person who is supine. Hypovolemic patients will also have a more marked response as well. [Note: β receptors, including β1 adrenoceptors on the heart, are not affected by α blockade.] The α-adrenergic blocking agents, phenoxybenzamine and phentolamine, have limited clinical applications. 7 α BLOCKERS Alfuzosin UROXATRAL Doxazosin CARDURA Phenoxybenzamine DIBENZYLINE Phentolamine REGITINE Prazosin MINIPRESS Tamsulosin FLOMAX Terazosin HYTRIN Yohimbine YOCON β BLOCKERS Acebutolol SECTRAL Atenolol TENORMIN Betaxolol BETOPTIC-S, KERLONE Bisoprolol ZEBETA Carteolol CARTROL Carvedilol COREG, COREG CR Esmolol BREVIBLOC Labetalol TRANDATE Metoprolol LOPRESSOR, TOPROL-XL Nadolol CORGARD Nebivolol BYSTOLIC Penbutolol LEVATOL Pindolol VISKEN Propranolol INDERAL LA, INNOPRAN XL Timolol BETIMOL, ISTALOL, TIMOPTIC DRUGS AFFECTING NEUROTRANSMITTER UPTAKE OR RELEASE Guanethidine ISMELIN Reserpine SERPASIL A. Phenoxybenzamine Phenoxybenzamine [fen-ox-ee-BEN-za-meen] is nonselective, linking covalently to both α1- and α2-receptors (Figure 7.2). The block is irreversible and noncompetitive, and the only mechanism the body has for overcoming the block is to synthesize new adrenoceptors, which requires a day or longer. Therefore, the actions of phenoxybenzamine last about 24 hours after a single administration. After the drug is injected, a delay of a few hours occurs before α blockade develops. Figure 7.1 Summary of blocking agents and drugs affecting neurotransmitter uptake or release. 88 7. Adrenergic Antagonists 1. Actions: a. Cardiovascular effects: By blocking α receptors, phenoxybenzamine prevents vasoconstriction of peripheral blood vessels by endogenous catecholamines. The decreased peripheral resistance provokes a reflex tachycardia. Furthermore, the ability to block presynaptic inhibitory α2 receptors in the heart can contribute to an increased cardiac output. [Note: These receptors, when blocked, will result in more norepinephrine release, which stimulates β receptors on the heart, increasing cardiac output.] Thus, the drug has been unsuccessful in maintaining lowered blood pressure in hypertension, and its use has been discontinued for this purpose. Phenoxybenzamine Covalent bond Rapid Effector cell membrane Covalently inactivated α1 -adrenoceptor Figure 7.2 Covalent inactivation of α1 adrenoceptor by phenoxybenzamine. α-Adrenergic blockers have no effect on the actions of isoproterenol, which is a pure β agonist. Catecholamine Untreated control 200 Isoproterenol (mm Hg) 0 200 Epinephrine 0 200 Norepinephrine 0 Pretreatment with an αblocker Pretreatment with a βblocker α-Adrenergic blockers reverse the vasoconstrictive action of epinephrine. Figure 7.3 Summary of effects of adrenergic blockers on the changes in blood pressure induced by isoproterenol, epinephrine, and norepinephrine. b. Epinephrine reversal: All α-adrenergic blockers reverse the α-agonist actions of epinephrine. For example, the vasoconstrictive action of epinephrine is interrupted, but vasodilation of other vascular beds caused by stimulation of β receptors is not blocked. Therefore, in the presence of phenoxybenzamine, the systemic blood pressure decreases in response to epinephrine (Figure 7.3). [Note: The actions of norepinephrine are not reversed, but are diminished because norepinephrine lacks significant β-agonist action on the vasculature.] Phenoxybenzamine has no effect on the actions of isoproterenol, which is a pure β agonist (see Figure 7.3). 2. Therapeutic uses: Phenoxybenzamine is used in the treatment of pheochromocytoma, a catecholamine-secreting tumor of cells derived from the adrenal medulla. Prior to surgical removal of the tumor, patients are treated with phenoxybenzamine to preclude the hypertensive crisis that can result from manipulation of the tissue. This drug is also useful in the chronic management of these tumors, particularly when the catecholamine-secreting cells are diffuse and, therefore, inoperable. Phenoxybenzamine is sometimes effective in treating Raynaud disease, frostbite, and acrocyanosis. Autonomic hyperreflexia, which predisposes paraplegic patients to strokes, can be managed with phenoxybenzamine. 3. Adverse effects: Phenoxybenzamine can cause postural hypotension, nasal stuffiness, nausea, and vomiting. It may inhibit ejaculation. It also may induce reflex tachycardia, which is mediated by the baroreceptor reflex. Phenoxybenzamine is contraindicated in patients with decreased coronary perfusion. B. Phentolamine In contrast to phenoxybenzamine, phentolamine [fen-TOLE-a-meen] produces a competitive block of α1 and α2 receptors. This drug’s action lasts for approximately 4 hours after a single administration. Like phenoxybenzamine, it produces postural hypotension and causes epinephrine reversal. Phentolamine-induced reflex cardiac stimulation and tachycardia are mediated by the baroreceptor reflex and by blocking the α2 receptors of the cardiac sympathetic nerves. The drug can also trigger arrhythmias and anginal pain, and phentolamine is contraindicated in patients with decreased coronary perfusion. Phentolamine is used for the short-term management of pheochromocytoma. It is also used locally to prevent dermal necrosis and extravasation due to norepinephrine administration as well as being used to treat hypertensive crisis due to abrupt withdrawal of clonidine and from ingesting tyramine-contain- II. α-Adrenergic Blocking Agents 89 ing foods in patients taking nonselective monoamine oxidase inhibitors. Phentolamine is now rarely used for the treatment of impotence (it can be injected intracavernosally to produce vasodilation of penile arteries). C. Prazosin, terazosin, doxazosin, tamsulosin, and alfuzosin Prazosin [PRAY-zoe-sin], terazosin [ter-AY-zoe-sin], doxazosin [dox-AYzoe-sin], and tamsulosin [tam-SUE-loh-sin] are selective competitive blockers of the α1 receptor. In contrast to phenoxybenzamine and phentolamine, the first three drugs are useful in the treatment of hypertension. Tamsulosin and alfuzosin [al-FYOO-zoe-sin] are indicated for the treatment of benign prostatic hypertrophy (also known as benign prostatic hyperplasia, or BPH). Metabolism leads to inactive products that are excreted in urine except for those of doxazosin, which appear in feces. Doxazosin is the longest acting of these drugs. 1. Cardiovascular effects: All of these agents decrease peripheral vascular resistance and lower arterial blood pressure by causing the relaxation of both arterial and venous smooth muscle. Tamsulosin has the least effect on blood pressure. These drugs, unlike phenoxybenzamine and phentolamine, cause minimal changes in cardiac output, renal blood flow, and the glomerular filtration rate. 2. Therapeutic uses: Individuals with elevated blood pressure who have been treated with one of these drugs do not become tolerant to its action. However, the first dose of these drugs produces an exaggerated orthostatic hypotensive response (Figure 7.4) that can result in syncope (fainting). This action, termed a “first-dose” effect, may be minimized by adjusting the first dose to one-third or onefourth of the normal dose and by giving the drug at bedtime. These drugs improve lipid profiles and glucose metabolism in hypertensive patients. Prazosin and others are used to treat congestive heart failure. By dilating both arteries and veins, these agents decrease preload and afterload, leading to an increase in cardiac output and a reduction in pulmonary congestion. Unlike β blockers, these agents have not been found to prolong life in patients with heart failure. The α1-receptor antagonists have been used as an alternative to surgery in patients with symptomatic BPH (Figure 7.5). Blockade of the α receptors decreases tone in the smooth muscle of the bladder neck and prostate and improves urine flow. Tamsulosin is an inhibitor (with some selectivity) of the α1A receptors found on the smooth muscle of the prostate. This selectivity accounts for tamsulosin’s relatively minimal effect on blood pressure and its use in BPH, though dizziness (orthostasis) may rarely occur. [Note: Finasteride and dutasteride α1-ADRENERGIC ANTAGONISTS Figure 7.4 First dose of α1 receptor blocker may produce an orthostatic hypotensive response that can result in syncope (fainting). 5 α-REDUCTASE INHIBITORS Decrease in prostate size No Yes Peak onset 2–4 weeks 6–12 months Decrease in PSA No Yes Sexual dysfunction + ++ Hypotensive effects ++ – Commonly used drugs Tamsulosin and alfuzosin Finasteride and dutasteride Figure 7.5 Comparisons of treatments for benign prostatic hyperplasia. PSA = Prostate specific antigen. 90 7. Adrenergic Antagonists BP Orthostatic hypotension Tachycardia Vertigo inhibit 5α-reductase, preventing the conversion of testosterone to dihydrotestosterone. These drugs are approved for the treatment of BPH by reducing prostate volume in selected patients (see p. 329).] 3. Adverse effects: α1 Blockers may cause dizziness, a lack of energy, nasal congestion, headache, drowsiness, and orthostatic hypotension (although to a lesser degree than that observed with phenoxybenzamine and phentolamine). These agents do not trigger reflex tachycardia to the same extent as the nonselective α-receptor blockers. An additive antihypertensive effect occurs when prazosin is given with either a diuretic or a β blocker, thereby necessitating a reduction in its dose. Due to a tendency to retain sodium (Na+) and fluid, prazosin is frequently used along with a diuretic. These drugs do not affect male sexual function as severely as phenoxybenzamine and phentolamine. However, by blocking α receptors in the ejaculatory ducts and impairing smooth muscle contraction, inhibition of ejaculation and retrograde ejaculation have been reported. Tamsulosin has a caution about “floppy iris syndrome,” a condition in which the iris billows in response to intraoperative eye surgery. Figure 7.6 summarizes some adverse effects observed with α blockers. D. Yohimbine Sexual dysfunction Figure 7.6 Some adverse effects commonly observed with nonselective α-adrenergic blocking agents. Esmolol 10 min Acebutolol 3–4 hr Pindolol 3–4 hr Metoprolol 3–4 hr Propranolol 4–6 hr Timolol 4–6 hr Labetalol 4–6 hr Carvedilol 7–10 hr Nebivolol 10–30 hr Nadolol 14–24 hr Yohimbine [yo-HIM-bean] is a selective competitive α2 blocker. It is found as a component of the bark of the yohimbe tree and is sometimes used as a sexual stimulant. [Efficacy of yohimbine for the treatment of impotence has never been clearly demonstrated.] Yohimbine works at the level of the CNS to increase sympathetic outflow to the periphery. It directly blocks α2 receptors and has been used to relieve vasoconstriction associated with Raynaud disease. Yohimbine is contraindicated in CNS and cardiovascular conditions because it is a CNS and cardiovascular stimulant. III. β-ADRENERGIC BLOCKING AGENTS All the clinically available β blockers are competitive antagonists. Nonselective β blockers act at both β1 and β2 receptors, whereas cardioselective β antagonists primarily block β1 receptors [Note: There are no clinically useful β2 antagonists.] These drugs also differ in intrinsic sympathomimetic activity, in CNS effects, blockade of sympathetic receptors, vasodilation, and in pharmacokinetics (Figure 7.7). Although all β blockers lower blood pressure in hypertension, they do not induce postural hypotension, because the α adrenoceptors remain functional. Therefore, normal sympathetic control of the vasculature is maintained. β blockers are also effective in treating angina, cardiac arrhythmias, myocardial infarction, congestive heart failure, hyperthyroidism, and glaucoma as well as serving in the prophylaxis of migraine headaches. [Note: The names of all β blockers end in “-olol” except for labetalol and carvedilol.] A. Propranolol: A nonselective β antagonist Propranolol [proe-PRAN-oh-lole] is the prototype β-adrenergic antagonist and blocks both β1 and β2 receptors with equal affinity. Sustainedrelease preparations for once-a-day dosing are available. 1. Actions: Figure 7.7 Elimination half-lives for some β blockers. a. Cardiovascular: Propranolol diminishes cardiac output, having both negative inotropic and chronotropic effects (Figure 7.8). It directly depresses sinoatrial and atrioventricular activity. III. β-Adrenergic Blocking Agents The resulting bradycardia usually limits the dose of the drug. During exercise or stress, when the sympathetic nervous system is activated, β blockers will attenuate the expected increase in heart rate. Cardiac output, work, and oxygen consumption are decreased by a blockade of β1 receptors, and these effects are useful in the treatment of angina (see p. 222). The β blockers are effective in attenuating supraventricular cardiac arrhythmias, but generally are not effective against ventricular arrhythmias (except those induced by exercise). At high doses, propranolol may cause a membrane-stabilizing effect on the heart, but this effect is insignificant if the drug is given at therapeutic doses. b. Peripheral vasoconstriction: Blockade of β receptors prevents β2-mediated vasodilation (see Figure 7.8). The reduction in cardiac output leads to decreased blood pressure. This hypotension triggers a reflex peripheral vasoconstriction that is reflected in reduced blood flow to the periphery. In patients with hypertension, total peripheral resistance returns to normal or decreases with long term use of propranolol. On balance, there is a gradual reduction of both systolic and diastolic blood pressures in hypertensive patients. No postural hypotension occurs, because the α1-adrenergic receptors that control vascular resistance are unaffected. 91 Propranolol β2 Bronchoconstriction Propranolol β2 Reflex peripheral vasoconstriction c. Bronchoconstriction: Blocking β2 receptors in the lungs of susceptible patients causes contraction of the bronchiolar smooth muscle (see Figure 7.8). This can precipitate a respiratory crisis in patients with chronic obstructive pulmonary disease (COPD) or asthma. Therefore, β blockers, particularly, nonselective ones, are contraindicated in patients with COPD or asthma. d. Increased Na+ retention: Reduced blood pressure causes a decrease in renal perfusion, resulting in an increase in Na+ retention and plasma volume (see Figure 7.8). In some cases, this compensatory response tends to elevate the blood pressure. For these patients, β blockers are often combined with a diuretic to prevent Na+ retention. e. Disturbances in glucose metabolism: β Blockade leads to decreased glycogenolysis and decreased glucagon secretion. Therefore, if a patient with type 1 (formerly insulin-dependent) diabetes is to be given propranolol, very careful monitoring of blood glucose is essential, because pronounced hypoglycemia may occur after insulin injection. β Blockers also attenuate the normal physiologic response to hypoglycemia. f. Blocked action of isoproterenol: All β blockers, including propranolol, have the ability to block the actions of isoproterenol on the cardiovascular system. Thus, in the presence of a β blocker, isoproterenol does not produce either the typical cardiac stimulation or reductions in mean arterial pressure and diastolic pressure (see Figure 7.3). [Note: In the presence of a β blocker, epinephrine no longer lowers diastolic blood pressure or stimulates the heart, but its vasoconstrictive action (mediated by α receptors) remains unimpaired. The actions of norepinephrine on the cardiovascular system are mediated primarily by α receptors and are, therefore, unaffected.] Na+ Increased sodium retention Propranolol Rate Force β1 Decreased cardiac output Propranolol Acebutolol Atenolol Metoprolol Nebivolol Figure 7.8 Actions of propranolol and other β blockers. 92 7. Adrenergic Antagonists 2. Pharmacokinetics: After oral administration, propranolol is almost completely absorbed because it is highly lipophilic. It is subject to first-pass effect, and only about 25 percent of an administered dose reaches the circulation. The volume of distribution of orally administered propranolol is quite large (4 liters/Kg), and the drug readily crosses the blood-brain barrier. Propranolol is extensively metabolized, and most metabolites are excreted in the urine. 3. Therapeutic effects: a. Hypertension: Propranolol does not reduce blood pressure in people with normal blood pressure. Propranolol lowers blood pressure in hypertension by several different mechanisms of action. Decreased cardiac output is the primary mechanism, but inhibition of renin release from the kidney, decrease in total peripheral resistance with long term use, and decreased sympathetic outflow from the CNS also contribute to propranolol’s antihypertensive effects (see p. 233). b. Migraine: Propranolol is also effective in reducing migraine episodes when used prophylactically (see p. 556). β Blockers are valuable in the treatment of chronic migraine, because these agents decrease the incidence and severity of the attacks. [Note: During an attack, sumatriptan is used, as well as other drugs.] c. Hyperthyroidism: Propranolol and other β blockers are effective in blunting the widespread sympathetic stimulation that occurs in hyperthyroidism. In acute hyperthyroidism (thyroid storm), β blockers may be lifesaving in protecting against serious cardiac arrhythmias. d. Angina pectoris: Propranolol decreases the oxygen requirement of heart muscle and, therefore, is effective in reducing the chest pain on exertion that is common in angina. Propranolol is, thus, useful in the chronic management of stable angina but not for acute treatment. Tolerance to moderate exercise is increased, and this is measurable by improvement in the electrocardiogram. However, treatment with propranolol does not allow strenuous physical exercise such as tennis. e. Myocardial infarction: Propranolol and other β blockers have a protective effect on the myocardium. Thus, patients who have had one myocardial infarction appear to be protected against a second heart attack by prophylactic use of β blockers. In addition, administration of a β blocker immediately following a myocardial infarction reduces infarct size and hastens recovery. The mechanism for these effects may be a blocking of the actions of circulating catecholamines, which would increase the oxygen demand in an already ischemic heart muscle. Propranolol also reduces the incidence of sudden arrhythmic death after myocardial infarction. 4. Adverse effects: a. Bronchoconstriction: Propranolol has a serious and potentially lethal side effect when administered to a patient with asthma (Figure 7.9). An immediate contraction of the bronchiolar smooth III. β-Adrenergic Blocking Agents muscle prevents air from entering the lungs. Death by asphyxiation has been reported for patients with asthma whom were inadvertently administered the drug. Therefore, propranolol must never be used in treating any individual with COPD or asthma. b. Arrhythmias: Treatment with β blockers must never be stopped quickly because of the risk of precipitating cardiac arrhythmias, which may be severe. The β blockers must be tapered off gradually for at least a few weeks. Long-term treatment with a β antagonist leads to up-regulation of the β receptor. On suspension of therapy, the increased receptors can worsen angina or hypertension. c. Sexual impairment: Because sexual function in the male occurs through α-adrenergic activation, β blockers do not affect normal ejaculation or the internal bladder sphincter function. On the other hand, some men do complain of impaired sexual activity. The reasons for this are not clear and may be independent of β-receptor blockade. d. Metabolic disturbances: β Blockade leads to decreased glycogenolysis and decreased glucagon secretion. Fasting hypoglycemia may occur. In addition, β blockers can prevent the counterregulatory effects of catecholamines during hypoglycemia. The perception of symptoms such as tremor, tachycardia, and nervousness are blunted. [Note: Cardioselective β blockers are preferred in treating asthma patients who use insulin (see β1-selective antagonists).] A major role of β receptors is to mobilize energy molecules such as free fatty acids. [Note: Lipases in fat cells are activated, leading to the metabolism of triglycerides into free fatty acids.] Patients administered nonselective β blockers have increased low-density lipoprotein (“bad” cholesterol), increased triglycerides, and reduced high-density lipoprotein (“good” cholesterol). On the other hand, the serum lipid profile in dyslipidemia patients improves with the use of β1-selective antagonists such as metoprolol. e. CNS effects: Propranolol has numerous CNS-mediated effects, including depression, dizziness, lethargy, fatigue, weakness, visual disturbances, hallucinations, short-term memory loss, emotional lability, vivid dreams (including nightmares), decreased performance, and depression manifested by insomnia. f. Drug interactions: Drugs that interfere with, or inhibit, the metabolism of propranolol, such as cimetidine, fluoxetine, paroxetine, and ritonavir, may potentiate its antihypertensive effects. Conversely, those that stimulate or induce its metabolism, such as barbiturates, phenytoin, and rifampin, can decrease its effects. B. Timolol and nadolol: Nonselective β antagonists Timolol [TIM-o-lole] and nadolol [NAH-doh-lole] also block β1- and β2adrenoceptors and are more potent than propranolol. Nadolol has a very long duration of action (see Figure 7.7). Timolol reduces the production of aqueous humor in the eye. It is used topically in the treatment of chronic open-angle glaucoma and, occasionally, for systemic treatment of hypertension. 93 Fatigue Bronchoconstriction Sexual dysfunction Arrhythmias (upon abrupt withdrawal) Figure 7.9 Adverse effects commonly observed in individuals treated with propranolol. 94 7. Adrenergic Antagonists CLASS OF DRUG DRUG NAMES MECHANISM OF ACTION β-Adrenergic antagonists Betaxolol, carteolol, levobunolol, (topical) metipranolol, timolol SIDE EFFECTS Decrease of aqueous humor production Ocular irritation; contraindicated in patients with asthma, obstructive airway disease, bradycardia, and congestive heart failure. α-Adrenergic agonists (topical) Apraclonidine, brimonidine Decrease of aqueous humor production and increase of aqueous outflow Red eye and ocular irritation, allergic reactions, malaise, and headache. Cholinergic agonists (topical) Pilocarpine, carbachol Increase of aqueous outflow Eye or brow pain, increased myopia, and decreased vision. Prostaglandin-like analogues (topical) Latanoprost, travoprost, bimatoprost Increase of aqueous humor outflow Red eye and ocular irritation, increased iris pigmentation, and excessive hair growth of eye lashes. Carbonic anhydrase inhibitors (topical and systemic) Dorzolamide and brinzolamide (topical), acetazolamide and methazolamide (oral) Decrease of aqueous humor production Transient myopia, nausea, diarrhea, loss of appetite and taste, and renal stones (oral drugs). Figure 7.10 Classes of drugs used to treat glaucoma. 1. Treatment of glaucoma: β Blockers, such as topically applied timolol, betaxolol, or carteolol are effective in diminishing intraocular pressure in glaucoma. This occurs by decreasing the secretion of aqueous humor by the ciliary body. Many patients with glaucoma have been maintained with these drugs for years. These drugs neither affect the ability of the eye to focus for near vision nor change pupil size, as do the cholinergic drugs. When administered to the eye, the onset is about 30 minutes, and the effects last for 12 to 24 hours. However, in an acute attack of glaucoma, pilocarpine is still the drug of choice. The β blockers are only used to treat this disease chronically. Other agents used in the treatment of glaucoma are summarized in Figure 7.10 C. Acebutolol, atenolol, metoprolol, bisoprolol, betaxolol, nebivolol, and esmolol: Selective β1 antagonists Drugs that preferentially block the β1 receptors have been developed to eliminate the unwanted bronchoconstrictor effect (β2 effect) of propranolol seen among asthma patients. Cardioselective β blockers, such as acebutolol [a-se-BYOO-toe-lole], atenolol [a-TEN-oh-lole], and metoprolol [me-TOE-proe-lole], antagonize β1 receptors at doses 50- to 100-fold less than those required to block β2 receptors. This cardioselectivity is, thus, most pronounced at low doses and is lost at high doses. [Note: Acebutolol has some intrinsic agonist activity.] 1. Actions: These drugs lower blood pressure in hypertension and increase exercise tolerance in angina (see Figure 7.8). Esmolol [EZ-moelole] has a very short lifetime (see Figure 7.7) due to metabolism of an ester linkage. It is only given intravenously if required during surgery or diagnostic procedures (for example, cystoscopy). In contrast to propranolol, the cardiospecific blockers have relatively little effect on pulmonary function, peripheral resistance, and carbohydrate metabolism. Nevertheless, asthma patients treated with these agents must III. β-Adrenergic Blocking Agents be carefully monitored to make certain that respiratory activity is not compromised. Nebivolol also has vasodilator properties mediated by nitric oxide. 2. Therapeutic use in hypertension: The cardioselective β blockers are useful in hypertensive patients with impaired pulmonary function. Because these drugs have less effect on peripheral vascular β2 receptors, coldness of extremities, a common side effect of β-blocker therapy, is less frequent. Cardioselective β blockers are useful in diabetic hypertensive patients who are receiving insulin or oral hypoglycemic agents. D. Pindolol and acebutolol: Antagonists with partial agonist activity 1. Actions: a. Cardiovascular: Acebutolol (β1-selective antagonist) and pindolol (nonselective β blocker) [PIN-doe-lole] are not pure antagonists. Instead, they have the ability to weakly stimulate both β1 and β2 receptors (Figure 7.11) and are said to have intrinsic sympathomimetic activity (ISA). These partial agonists stimulate the β receptor to which they are bound, yet they inhibit stimulation by the more potent endogenous catecholamines, epinephrine and norepinephrine. The result of these opposing actions is a muchdiminished effect on cardiac rate and cardiac output compared to that of β blockers without ISA. b. Decreased metabolic effects: Blockers with ISA minimize the disturbances of lipid and carbohydrate metabolism that are seen with other β blockers. For example, these agents do not decrease plasma HDL levels. 2. Therapeutic use in hypertension: β blockers with ISA are effective in hypertensive patients with moderate bradycardia, because a further decrease in heart rate is less pronounced with these drugs. Carbohydrate metabolism is less affected with acebutolol and pindolol than it is with propranolol, making those agents valuable in the treatment of diabetic patients. [Note: The β blockers with ISA are not used as antiarrhythmic agents due to their partial agonist effect.] Figure 7.12 summarizes some of the indications for β blockers. 95 A Agonists (for example, epinephrine) β1 and β2 Receptor β1 and β2 Receptors activated CELLULAR EFFECTS B Antagonists (for example, propranolol) Epinephrine β1 and β2 Receptors blocked but not activated C Partial agonists (for example, pindolol and acebutolol) E. Labetalol and carvedilol: Antagonists of both α and β adrenoceptors 1. Actions: Labetalol [lah-BET-a-lole] and carvedilol [CAR-ve-dil-ol] are β blockers with concurrent α1-blocking actions that produce peripheral vasodilation, thereby reducing blood pressure. They contrast with the other β blockers that produce peripheral vasoconstriction, and these agents are, therefore, useful in treating hypertensive patients for whom increased peripheral vascular resistance is undesirable. They do not alter serum lipid or blood glucose levels. Carvedilol also decreases lipid peroxidation and vascular wall thickening, effects that have benefit in heart failure. 2. Therapeutic use in hypertension and heart failure: Labetalol is useful for treating the elderly or black hypertensive patient in whom increased peripheral vascular resistance is undesirable. [Note: In general, black hypertensive patients are not well controlled with β block- β1 and β2 Receptors partially activated but unable to respond to more potent catecholamines DECREASED CELLULAR EFFECTS Figure 7.11 Comparison of agonists, antagonists, and partial agonists of β adrenoceptors. 96 7. Adrenergic Antagonists Hypertension Propranolol, metoprolol, and other β blockers reduce cardiac output and renin secretion. Glaucoma Timolol and other β blockers reduce secretion of aqueous humor. Migraine Propranolol provides a prophylactic effect. ers.] Labetalol may be employed as an alternative to methyldopa in the treatment of pregnancy-induced hypertension. Intravenous labetalol is also used to treat hypertensive emergencies, because it can rapidly lower blood pressure (see p. 240). Acute administration of β blockers can trigger congestive heart failure or worsen the condition. However, large clinical trials have shown clinical benefits of carvedilol as well as metoprolol and bisoprolol in patients with stable chronic heart failure. These agents have also been shown to reduce mortality and hospitalization in this population. Carvedilol also used to prevent cardiovascular mortalities in patients with heart failure. 3. Adverse effects: Orthostatic hypotension and dizziness are associated with α1 blockade. Figure 7.13 summarizes the receptor specificities and uses of the β-adrenergic antagonists. IV. DRUGS AFFECTING NEUROTRANSMITTER RELEASE OR UPTAKE As noted on p. 127 some agonists, such as amphetamine and tyramine, do not act directly on the adrenoceptor. Instead, they exert their effects indirectly on the adrenergic neuron by causing the release of neurotransmitters from storage vesicles. Similarly, some agents act on the adrenergic neuron, either to interfere with neurotransmitter release or to alter the uptake of the neurotransmitter into the adrenergic nerve. However, due to the advent of newer and more effective agents with fewer side effects, these agents are seldom used therapeutically. These agents are included in this chapter due to their unique mechanisms of action and historical value. A. Reserpine Thyrotoxicosis Propranolol reduces cardiac rate and potential for arrhythmias. Arrhythmia prophylaxis after myocardial infarction Propranolol and metoprolol reduce cardiac output and renin secretion. Supraventricular tachycardias Propranolol and esmolol slow AV conduction velocity. Angina pectoris Propranolol, nadolol, and other β blockers reduce cardiac rate and force. Figure 7.12 Some clinical applications of β blockers. AV = atrioventricular. Reserpine [re-SER-peen], a plant alkaloid, blocks the Mg2+/adenosine triphosphate–dependent transport of biogenic amines, norepinephrine, dopamine, and serotonin from the cytoplasm into storage vesicles in the adrenergic nerves of all body tissues. This causes the ultimate depletion of biogenic amines. Sympathetic function, in general, is impaired because of decreased release of norepinephrine. The drug has a slow onset, a long duration of action, and effects that persist for many days after discontinuation. B. Guanethidine Guanethidine [gwahn-ETH-i-deen] blocks the release of stored norepinephrine as well as displaces norepinephrine from storage vesicles (thus producing a transient increase in blood pressure). This leads to gradual depletion of norepinephrine in nerve endings except for those in the CNS. Guanethidine commonly causes orthostatic hypotension and interferes with male sexual function. Supersensitivity to norepinephrine due to depletion of the amine can result in hypertensive crisis in patients with pheochromocytoma. C. Cocaine Cocaine [KOE-kane] is a widely available and highly addictive drug. The primary mechanism of action underlying the central and peripheral effects of cocaine is blockade of reuptake of the monoamines (norepinephrine, serotonin, and dopamine) into the presynaptic terminals from which these neurotransmitters are released (Figure 10.6). This blockade is caused by cocaine binding to the monoaminergic reuptake IV. Drugs Affecting Neurotransmitter Release Or Uptake DRUG RECEPTOR SPECFICITY 97 THERAPEUTIC USES Propranolol β1, β2 Hypertension Migraine Hyperthyroidism Angina pectoris Myocardial infarction Nadolol β1, β2 Hypertension Timolol β1, β2 Glaucoma, hypertension Acebutolol 1 Atenolol Esmolol Metoprolol β1 Hypertension Nebivolol β1, NO Hypertension Pindolol 1 β1, β2 Hypertension Carvedilol Labetalol α1, β1, β2 Hypertension Congestive heart failure Figure 7.13 Summary of β-adrenergic antagonists. NO = nitric oxide. 1Acebutolol and pindolol are partial agonists. transporters and, thus, potentiates and prolongs the CNS and peripheral actions of these monoamines. In particular, the prolongation of dopaminergic effects in the brain’s pleasure system (limbic system) produces the intense euphoria that cocaine initially causes. Chronic intake of cocaine depletes dopamine. This depletion triggers the vicious cycle of craving for cocaine that temporarily relieves severe depression. See p. 126 for a more complete discussion of the actions of cocaine.