Brody's Human Pharmacology: With STUDENT CONSULT
Chapter 11 Drugs Affecting the Sympathetic Nervous System
MAJOR DRUG CLASSES
Adrenergic receptor agonists
α receptor agonists
(selective and nonselective)
β receptor agonists
(selective and nonselective)
Adrenergic receptor antagonists
α receptor antagonists
(selective and nonselective)
β receptor antagonists
(selective and nonselective)
Catecholamine synthesis inhibitors
The sympathetic nervous system is an energy-expending system that has an ergotrophic function. Stimulation of this system leads to the “flight, fright, or fight” response characterized by increased heart rate, blood pressure, and respiration, an increased blood flow to skeletal muscles, and mydriasis. Almost all postganglionic sympathetic neurons release norepinephrine (NE) as their neurotransmitter to alter the activity of the effector organs. A minor exception is the small number of anatomically sympathetic neurons projecting to sweat glands and a few blood vessels that release acetylcholine. NE released from sympathetic neurons activates adrenergic receptors on exocrine glands, smooth muscle, and cardiac muscle to produce sympathetic responses.
Activation of sympathetic outflow also causes secretion of epinephrine (Epi) and NE into the blood from the adrenal medulla. Epi and NE are catecholamines and outside the United States are called adrenaline and noradrenaline, respectively, from which the adjectives “adrenergic” and “noradrenergic” are derived.
Drugs that facilitate or mimic the actions of the sympathetic nervous system are called sympathomimetics, adrenomimetics, or adrenergic agonists. Sympathomimetics constrict most arterioles and veins and can be used locally to reduce bleeding, slow diffusion of drugs such as local anesthetics, decongest mucous membranes, and reduce formation of aqueous humor to lower intraocular pressure in glaucoma. Systemic administration of sympathomimetics increases peripheral vascular resistance and mean arterial blood pressure and can increase blood pressure in hypotensive states, including neurogenic shock. By increasing blood pressure, sympathomimetics cause a reflex slowing of heart rate, which can be used to treat paroxysmal atrial tachycardia.
Aromatic l-amino acid decarboxylase
Central nervous system
Epi and certain other sympathomimetics strongly stimulate cardiac muscle and are used to treat cardiogenic shock. These drugs also relax various nonvascular smooth muscles, including bronchial and uterine smooth muscle, and are useful in treating bronchospasm and delaying delivery during premature labor. Drugs mimicking Epi or NE also contract the radial muscle in the iris, causing pupillary dilation (mydriasis) and facilitating eye examinations.
Drugs that block or reduce the actions of Epi or NE are called sympatholytics or adrenergic antagonists. Drugs that decrease sympathetic activity at vascular smooth muscle are used to treat essential hypertension and hypertensive emergencies as well as benign prostatic hyperplasia. Drugs that reduce the actions of NE and Epi on cardiac muscle are used to treat cardiac dysrhythmias, angina pectoris, and other cardiac disorders such as postmyocardial infarction. These drugs are also used in treating migraines, glaucoma, and essential tremor.
Due to the widespread distribution of sympathetic nerves throughout the body and the many different types of adrenergic receptors, drugs that modify the actions of sympathetic neurons produce many different clinically important responses. It is also important to keep in mind that as a consequence of the anatomical and functional diversity within the sympathetic nervous system, many drugs affecting this system will also have undesirable side effects.
The Therapeutic Overview Box presents a summary of the primary uses of different classes of compounds that affect the sympathetic nervous system.
Decrease formation of aqueous humor in glaucoma
Neurogenic shock, cardiogenic shock
Paroxysmal atrial tachycardia
Decrease diffusion of local anesthetics
Essential hypertension, hypertensive emergencies
Benign prostatic hyperplasia
Cardiac dysrhythmias, angina pectoris, postmyocardial infarction
Mechanisms of Action
The biochemistry and physiology of the autonomic nervous system, including a discussion of adrenergic receptors, is presented in Chapter 9. Detailed information on receptors and signaling pathways involved are in Chapter 1. This section covers topics that pertain specifically to noradrenergic neurotransmission and its modulation by drugs.
The neurochemical steps that mediate noradrenergic neurotransmission are summarized in Figure 11-1. The first step involves the transport of tyrosine into the neuron followed by its conversion to 4-dihydroxy-phenylalanine (l-DOPA) by the rate-limiting enzyme tyrosine hydroxylase. l-DOPA is rapidly decarboxylated to dopamine (DA) by aromatic l-amino acid decarboxylase (ALAAD), also known as DOPA decarboxylase. In dopaminergic neurons within the central nervous system (CNS), this is the last step in the synthetic process, and DA is released as a neurotransmitter. Noradrenergic neurons in the CNS and peripheral sympathetic nervous system contain an additional enzyme, DA β-hydroxylase, which converts DA to NE. This enzyme is located in synaptic vesicles, so DA must be actively transported into these vesicles before conversion to NE.
FIGURE 11–1 Prejunctional and postjunctional sites of action of drugs that modify noradrenergic transmission at a sympathetic neuroeffector junction. l-Tyrosine is actively transported into the neuron, where it is converted to l-DOP A by tyrosine hydroxylase (TH). This reaction is followed by the action of aromatic l-amino acid decarboxylase (ALAAD) to convert l-DOPA to DA, which is actively transported into synaptic vesicles, where it is converted by DA β-hydroxylase (DBH) to norepinephrine (NE). The arrival of an action potential at the varicosity causes Ca++ influx, which promotes the exocytotic release of NE. After release, NE can activate postjunctional α1, α2, or β receptors or prejunctional α2 receptors. Activation of prejunctional α2 receptors inhibits the further release of NE. The action of NE is terminated by transport back into the neuron by high-affinity uptake (Uptake 1), where it can be repackaged into synaptic vesicles or metabolized by monoamine oxidase (MAO) to inactive products. NE is also removed by diffusion and transport into the postjunctional cell (Uptake 2), where it is metabolized to normetanephrine by catechol-O-methyltransferase (COMT). Sites at which drugs act to enhance or mimic noradrenergic transmission are identified by numbers in circles.
Drugs that enhance or mimic noradrenergic transmission:
1. Facilitate release (e.g., amphetamine)
2. Block reuptake (e.g., cocaine)
3. Receptor agonists (e.g., phenylephrine)
Drugs that reduce noradrenergic transmission:
4. Inhibit synthesis (e.g., 4a, α-methyltyrosine; 4b, carbidopa; 4c, disulfiram)
5. Disrupt vesicular transport and storage (e.g., reserpine)
6. Inhibit release (e.g., guanethidine)
7. Receptor antagonists (e.g., phentolamine)
When the action potential depolarizes the nerve terminal, voltage-gated Ca++ channels open, allowing Ca++ to enter the neuron and cause the vesicles to release NE. At the neuroeffector junction, NE binds to adrenergic receptor subtypes on both the presynaptic and postsynaptic membranes, with different organs containing different receptor subtypes. Activation of α2-“autoreceptors” on the presynaptic neuronal membrane causes feedback inhibition and reduces further NE release. The signaling mechanisms occurring after adrenergic receptor activation are discussed in Chapter 9.
After NE has activated its receptors, its action is terminated primarily by a high-affinity reuptake system (termed “uptake 1” in Fig. 11-1), which transports NE back into the neuron for repackaging and eventual re-release. A smaller fraction of the NE in the synapse diffuses away from the receptors and can be taken up by a lower affinity extraneuronal process (termed “uptake 2” in Fig. 11-1). Some of the NE taken back into the sympathetic neurons by reuptake can be oxidatively deaminated by monoamine oxidase (MAO) located on the external mitochondrial membrane. The biologically inactive deaminated metabolites enter the circulation and are excreted in the urine. The NE that is transported into the postjunctional cell is O-methylated by catechol-O-methyltransferase (COMT) to normetanephrine. The high-affinity reuptake of NE into presynaptic terminals is the major mechanism that terminates its actions, although COMT and MAO also play important roles in metabolizing circulating NE and Epi and some exogenously administered sympathomimetic amines.
In addition to the synthesis and release of NE by sympathetic neurons, NE is also synthesized in and released from chromaffin cells in the adrenal medulla. These cells usually contain another enzyme, phenylethanolamine-N-methyltransferase, which converts NE to Epi. Chromaffin cells are innervated by sympathetic preganglionic cholinergic neurons and release catecholamines into the blood, where they are transported to various organs to act on adrenergic receptors. Circulating catecholamines, whether administered as drugs or released from the adrenal medulla, are also removed by high-affinity uptake into sympathetic nerves or metabolized, as discussed.
Drugs modify the activity of sympathetic neurons by increasing or decreasing the noradrenergic signal (see Fig. 11-1). Sympathomimetics may mimic noradrenergic transmission by acting directly on postsynaptic adrenergic receptors (e.g., phenylephrine), by facilitating NE release (e.g., amphetamine) or by blocking neuronal reuptake (e.g., cocaine). Amphetamine and cocaine have intense effects on the CNS and are drugs of abuse with severe addiction liability (Chapter 37).
Sympatholytics reduce noradrenergic transmission by inhibiting synthesis (e.g., α-methyltyrosine), disrupting vesicular storage (e.g., reserpine), inhibiting release (e.g., guanethidine), or directly blocking receptors (e.g., phentolamine).
Activation of Adrenergic Receptors
Over the last half century it has become clear that the actions of NE and Epi are mediated through multiple adrenergic receptor subtypes (see Chapter 9). We now know that there are nine different subtypes of adrenergic receptors, each encoded by separate genes, which are grouped into three major families (α1, α2, β), each containing three different members. Of these, only four (α1, α2, β1, and β2) are currently important in clinical pharmacology; therefore they are the major focus of this chapter. Both NE and Epi activate most adrenergic receptors with similar, but not identical, potencies, although NE is much less potent than Epi at the β2-subtype. Isoproterenol (ISO) is a synthetic analog of NE and Epi and selectively activates only β receptors. Differences in the pharmacological profiles of different adrenergic receptors are illustrated in Figure 11-2, where dose-response curves for the actions of these catecholamines on different tissues are depicted.
FIGURE 11–2 Dose-response curves (arbitrary scales) show relative potencies of three catecholamines in experimental muscle preparations. Changes in the force of contraction or relaxation for each muscle is shown after addition of progressively increasing concentrations of each catecholamine. NE, Norepinephrine; Epi, epinephrine; ISO, isoproterenol.
Adrenergic receptor activation increases the contraction of cardiac muscle and induces smooth muscle to either contract or relax, depending on the receptor subtype(s) present. Contraction of arterial strips is mediated by α1 receptors, where the relative potencies of the three catecholamines are Epi ≥ NE >>> ISO. Relaxation of bronchial smooth muscle is mediated by β2 receptors, with the relative potencies being ISO > Epi >>> NE. Contraction of cardiac muscle is mediated by β1 receptors, with the relative potencies being ISO > Epi = NE. The presence of different receptor subtypes in these three tissues is further demonstrated in Figure 11-3 by examining the effects of selective antagonists. Phentolamine, a competitive antagonist at α1 receptors, causes a parallel shift to the right of NE-induced contractions of arterial strips (see Fig. 11-3, A) but does not affect the other two responses. Propranolol, a competitive antagonist at both β1 and β2 receptors, causes a parallel shift to the right of responses mediated by bronchial β2 receptors (see Fig. 11-3, B) and cardiac β1 receptors (see Fig. 11-3, C), without affecting the α1 receptor response.
FIGURE 11–3 Changes in force of contraction or relaxation (arbitrary scale) of different tissues hung in separate tissue baths after addition of increasing concentrations of catecholamine in absence and presence of a fixed concentration of an α (phentolamine) or β (propranolol) receptor blocking drug. NE, Norepinephrine; ISO, isoproterenol.
Adrenergic α2 receptors are found on platelets, in the CNS, and postsynaptically in several other peripheral tissues (blood vessels, pancreas, and enteric cholinergic neurons). As mentioned, activation of α2 receptors on the terminals of sympathetic neurons reduces NE release. These receptors are also found both presynaptically and postsynaptically on neurons in brain, and activation of these receptors reduces central sympathetic outflow.
Direct-acting adrenergic receptor agonists mimic some of the effects of sympathetic nervous system activation by binding to and activating specific receptor subtypes (see Fig. 11-1, site 3). For example, as mentioned, ISO selectively activates β receptors, phenylephrine selectively activates α1 receptors, and clonidine selectively activates α2 receptors. Similarly, dobutamine and albuterol are relatively specific agonists at β1 and β2 receptors, respectively. The structures of some clinically important adrenergic receptor agonists are shown in Figure 11-4.
FIGURE 11–4 Structures of some direct-acting sympathomimetic agonists. Asterisk indicates asymmetrical carbon.
Indirect-acting sympathomimetics do not activate receptors directly but facilitate the release of NE from sympathetic neurons or block high-affinity reuptake. Thus they act presynaptically to facilitate adrenergic neurotransmission. Amphetamine and related drugs produce their sympathomimetic effects by facilitating NE release (see Fig. 11-1, site 1); the effects of these drugs in the CNS are described inChapter 37. On the other hand, cocaine and tricyclic antidepressants, such as desipramine, exert their sympathomimetic effects by blocking high-affinity reuptake (see Fig. 11-1, site 2). The structure and characteristics of cocaine are discussed in Chapter 37, with similar information for tricyclic antidepressants given in Chapter 30.
Because neuronal reuptake, and not metabolism, is the primary mechanism for terminating the actions of NE and Epi, it is not surprising that drugs that inhibit the metabolism of these amines show little or no sympathomimetic actions. On the other hand, inhibitors of MAO (e.g., pargyline) or COMT (e.g., tolcapone) can enhance the actions of other synthetic sympathomimetic amines that are substrates for these enzymes, with some important toxicological implications. For example, the actions of tyramine, a sympathomimetic amine present in a variety of foods, are greatly enhanced in patients treated with MAO inhibitors (see Chapter 30), and the pharmacokinetics of l-DOPA, used for the treatment of Parkinson’s disease, are increased when administered concurrently with a COMT inhibitor (see Chapter 28).
An indirect-acting sympathomimetic amine that releases NE must penetrate the noradrenergic neuron before it can act. Nonpolar, lipid-soluble drugs such as amphetamine can diffuse across neuronal membranes, whereas polar, water-soluble compounds such as tyramine must rely on high-affinity uptake by the NE transporter. Thus drugs that inhibit the NE transporter will reduce or block the effects of tyramine but will enhance the effects of Epi, which acts directly on the adrenergic receptors. Cocaine also causes a prompt increase in the response to Epi by blocking the reuptake system, whereas reserpine disrupts amine transport from the cytoplasm into synaptic vesicles (see Fig. 11-1, site 5). Therefore reserpine has only small effects on responses to direct-acting sympathomimetics.
Inhibition of Synthesis, Storage, or Release of NE
Catecholamine synthesis can be disrupted at several steps, but effective in vivo blockade is obtained only when tyrosine hydroxylase, the enzyme catalyzing the first and rate-limiting step, is inhibited (seeFig. 11-1, site 4a). This can be produced clinically with α-methyltyrosine (metyrosine). Several compounds can inhibit other biosynthetic enzymes (e.g., DOPA decarboxylase is inhibited by carbidopa and DA β-hydroxylase is inhibited by disulfiram (see Fig. 11-1, sites 4b and 4c, respectively). Although these drugs do not effectively block endogenous catecholamine synthesis when administered, they do have clinical utility by affecting other systems. By virtue of its ability to inhibit peripheral DOPA decarboxylase, carbidopa is used in combination with l-DOPA for patients with Parkinson’s disease, to increase the amount of l-DOPA available to the CNS to enhance DA synthesis (see Chapter 28). Disulfiram is used in treating chronic alcoholism (see Chapter 32) because it blocks aldehyde dehydrogenase; however, its ability to inhibit catecholamine synthesis leads to significant adverse effects such as hypotension.
Disruption of vesicular storage also modifies noradrenergic transmission (see Fig. 11-1, site 5). As discussed, reserpine disrupts the ability of the synaptic vesicles to transport and store DA and NE. Adrenergic responses can also be impeded by inhibiting NE release (see Fig. 11-1, site 6). Drugs such as bretylium and guanethidine, which accumulate in noradrenergic nerve terminals, prevent NE release.
Adrenergic Receptor Antagonists
Many adrenergic receptor antagonists exert different subtype-selective effects (see Fig. 11-1, site 7). The early antagonists such as phenoxybenzamine and propranolol blocked α or β receptors, respectively, whereas drugs are now available that selectively block α1 (prazosin), α2 (yohimbine), or β1 (metoprolol) receptors. These selective antagonists have important therapeutic advantages over the original broad-spectrum adrenergic receptor blockers and are used for various indications, such as the control of blood pressure (see Chapter 20).
Detailed pharmacokinetics are not known for many of these drugs in humans. Some known pharmacokinetic parameters for major drugs are summarized in Table 11-1.
TABLE 11–1 Pharmacokinetic Parameters
Relationship of Mechanisms of Action to Clinical Response
Direct and Reflex Cardiovascular Actions of Adrenergic Agents
The sympathetic nervous system plays an important role in regulating the cardiovascular system; thus adrenergic drugs have pronounced effects on this system. These drugs alter the rate and force of contraction of the heart and the tone of blood vessels (and, consequently, blood pressure) through activation of adrenergic receptors on cardiac and vascular smooth muscle cells. Compensatory reflex adjustments occur as a result of these responses, and these reflexes must be considered to understand the overall actions of adrenergic drugs on the heart and blood vessels.
Mean arterial blood pressure does not fluctuate widely because of feedback mechanisms that evoke compensatory responses to maintain homeostasis (Fig. 11-5). Baroreceptors are stretch receptors located in the walls of the heart and blood vessels that are activated by distention of the blood vessels. Increased blood pressure increases impulse traffic in afferent baroreceptor neurons that project to vasomotor centers in the medulla. Impulses generated in the baroreceptors inhibit the tonic discharge of sympathetic neurons projecting to the heart and blood vessels and activate vagal fibers projecting to the heart. When phenylephrine, which contracts vascular smooth muscle, is administered, peripheral resistance and blood pressure increase (Fig. 11-6). In turn, this increase in pressure increases afferent baroreceptor neuronal activity, thereby reducing sympathetic nerve activity and increasing vagal nerve activity. Consequently, heart rate decreases (bradycardia). Drugs such as histamine, which relax vascular smooth muscle, decrease blood pressure, reducing impulse traffic in afferent buffer neurons. Consequently, sympathetic nerve activity increases and vagal nerve activity decreases, resulting in an increased heart rate (tachycardia).
FIGURE 11–5 Baroreceptor control of blood pressure and heart rate. SNS, Sympathetic nervous system; PNS, parasympathetic nervous system.
FIGURE 11–6 Responses to intravenous injections of drugs that cause vasoconstriction (phenylephrine) or vasodilation (histamine) by acting directly on vascular smooth muscle.
In summary, drugs causing vasoconstriction secondarily cause reflex slowing of the heart, whereas drugs causing vasodilation produce reflex tachycardia. Thus the actions of adrenergic drugs on the cardiovascular system include both the direct actions of the drug on effector organs and compensatory reflex actions.
Epi is the prototype of direct-acting sympathomimetic drugs because it activates all known adrenergic receptor subtypes. The effects of direct-acting sympathomimetic drugs on selected tissues and organ systems are discussed in this section, with the effects of the prototype Epi first, followed by those of more selective sympathomimetics as compared with the prototype.
By activating cardiac β1 receptors, Epi alters the strength, rate, and rhythm of cardiac contractions; these actions may be either desirable or dangerous. Epi increases the force of contraction (positive inotropic effect) by activating β1 receptors on myocardial cells and increases the rate of contraction (positive chronotropic effect) by activating β1 receptors on pacemaker cells in the sinoatrial node. Epi also accelerates the rate of myocardial relaxation so that systole is shortened relatively more than diastole. Thus, while Epi is exerting its effects, the fraction of time spent in diastole is increased, which allows for increased filling of the heart. The combination of an increased diastolic filling time, more forceful ejection of blood, and increased rates of contraction and relaxation of the heart results in increased cardiac output. The initial increase in heart rate after administration of Epi may be followed by slowing of the heart (bradycardia) caused by reflex activation of the vagus.
Epi also activates conducting tissues, thereby increasing conduction velocity and reducing the refractory period in the atrioventricular node, the bundle of His, Purkinje fibers, and ventricular muscle. These changes, and the activation of latent pacemaker cells, may lead to alterations in the rhythm of the heart. Large doses of Epi may cause tachycardia, premature ventricular systole, and possibly fibrillation; these effects are more likely to occur in hearts that are diseased or have been sensitized by halogenated hydrocarbon anesthetics (see Chapter 35).
Vascular Smooth Muscle
The effects of Epi on smooth muscle cells in different organs depend on the type of adrenergic receptors present. Vascular smooth muscle is regulated primarily by α1 or β2 receptors, depending on the location of the vascular bed. Epi is a powerful vasoconstrictor in some beds; it activates α1 receptors to cause smooth muscle cells to contract in precapillary resistance vessels (arterioles) in skin, mucosa, kidney, and veins. At low doses Epi also activates β2 receptors, causing relaxation of vascular smooth muscle in skeletal muscle. Thus Epi increases blood flow in skeletal muscle but reduces flow in skin and kidney. Moderate doses of Epi increase systolic pressure while reducing diastolic pressure, which results primarily from activation of β2 receptors in blood vessels in skeletal muscles. Moderate doses of NE increase both systolic and diastolic blood pressure because it does not activate β2 receptors in blood vessels at these concentrations.
Systemic administration of Epi alters cerebral and coronary blood flow, but the changes do not result primarily from its direct actions on vascular smooth muscle in the brain. Rather, changes in cerebral blood flow reflect changes in systemic blood pressure, and increased coronary blood flow results from a greater duration of diastole and production of vasodilator metabolites (e.g., adenosine) secondary to the increased work of the heart.
Other Smooth Muscle
Epi is a potent bronchodilator, relaxing bronchial smooth muscle by activating β2 receptors. It dramatically reduces responses to endogenous bronchoconstrictors and can be lifesaving in acute asthmatic attacks (see Chapter 16). Epi also relaxes smooth muscle in other organs through β2 receptors. It reduces the frequency and amplitude of gastrointestinal (GI) contractions, decreases the tone and contractions of the pregnant uterus, and relaxes the detrusor muscle of the urinary bladder. However, it causes the smooth muscle of the prostate and splenic capsule and of GI and urinary sphincters to contract by activating α1 receptors. Epi can foster urinary retention by relaxing the detrusor muscle and contracting the trigone and sphincter of the urinary bladder.
The radial pupillary dilator muscle of the iris contains α1 receptors and contracts in response to activation of sympathetic neurons, causing mydriasis. Because Epi is a highly polar molecule, it does not penetrate the cornea readily when instilled into the conjunctival sac. Mydriasis occurs when less polar, more lipid-soluble α receptor agonists (e.g., phenylephrine) are applied. If Epi is instilled, however, intraocular pressure is lowered, possibly because the agent reduces formation of aqueous humor by the ciliary bodies, although this mechanism is not well understood.
Because Epi and all other catecholamines are polar and cannot penetrate the blood-brain barrier, systemic administration of these amines has no direct cerebral action. Nevertheless, systemic administration of Epi can cause anxiety, restlessness, and headache, possibly through secondary reflexes.
Epi exerts many metabolic effects, some of which are the result of its action on the secretion of insulin and glucagon. The predominant action of Epi on islet cells of the pancreas is inhibition of insulin secretion from pancreatic β cells through activation of α2 receptors and stimulation of glucagon secretion from pancreatic α cells through activation of β2 receptors.
The major metabolic effects of Epi are increased circulating concentrations of glucose, lactic acid, and free fatty acids. In humans these effects are attributable to the activation of β receptors at liver, skeletal muscle, heart, and adipose cells (Fig. 11-7). Adrenergic β receptor activation results in Gs protein-mediated activation of adenylyl cyclase. The resulting increase in cyclic adenosine monophosphate (cAMP) leads to a series of phosphorylation events, culminating in activation of phosphorylase and lipase. Lipase catalyzes breakdown of triglycerides in fat to free fatty acids. The characteristic “calorigenic action” of Epi, which is reflected in a 20% to 30% increase in O2 consumption, is caused partly by the breakdown of triglycerides in brown adipose tissue and subsequent oxidation of the resulting fatty acids. In liver, phosphorylase catalyzes the breakdown of glycogen to glucose. In muscle, glycogenolysis and glycolysis produce lactic acid, which is released into the blood. Release of glucose from the liver is accompanied by the efflux of K+, so that Epi induces hyperglycemia and a brief period of hyperkalemia due to activation of hepatic α adrenergic receptors. The hyperkalemia is followed by a more pronounced hypokalemia, as the K+ released from the liver is taken up by skeletal muscle as a result of activation of muscle β2 adrenergic receptors.
FIGURE 11–7 Mechanisms by which epinephrine (and other β receptor agonists) exerts metabolic effects in adipose, liver, heart, and skeletal muscle cells.
Secretion of sweat from glands located on the palms of the hands and forehead is increased during psychological stress, and this effect is mediated by α1 receptors. When administered systemically, Epi does not activate these glands, but secretion of sweat in these areas can be induced if Epi is injected locally. Epi, by acting on β1 receptors, also causes the release of renin from the juxtaglomerular apparatus in the kidney.
Other Direct-Acting Sympathomimetics
Most clinically useful direct-acting sympathomimetics differ from Epi because they selectively activate specific adrenergic receptor subtypes. The properties of some of these compounds are compared with those of Epi in the following text.
NE has a relatively low potency at β2 receptors; thus clinically relevant doses of NE stimulate only α1, α2, and β1 receptors. NE produces vasoconstriction only in vascular beds, therefore increasing diastolic blood pressure. Because total peripheral resistance increases, the reflex slowing of heart rate produced by NE is more pronounced than that produced by Epi. NE does not usually relax bronchial smooth muscle, and metabolic responses such as hyperglycemia are much less pronounced than those produced by Epi, because they primarily involve β2 receptor activation.
Phenylephrine and methoxamine are selective α1 receptor agonists and differ from NE because they do not activate α2 or β1 receptors and therefore do not stimulate the heart. These drugs increase total peripheral resistance by causing vasoconstriction in most vascular beds. Consequently, they produce a reflex slowing of the heart that can be blocked by atropine. These drugs, which are less potent but longer acting than NE, have been used to treat hypotension and shock. Phenylephrine is also used in topical preparations as a mydriatic and nasal decongestant.
ISO is a potent agonist at all β receptor subtypes and differs from Epi because it does not activate α receptors. It reduces total peripheral resistance through β2 receptors, resulting in a considerable reduction in diastolic blood pressure. It has a strong stimulatory effect on the heart; tachycardia results from a combined direct action on β1 receptors and a reflex action caused by the hypotension. Like Epi, it relaxes bronchial smooth muscle and induces metabolic effects. Clinically, ISO may be used to relieve bronchoconstriction; however, the cardiac side effects resulting from its β1 receptor agonist property can be troublesome. Accordingly, agonists that are relatively specific for β2 receptors have been developed.
Metaproterenol, terbutaline, albuterol, bitolterol, salmeterol, and ritodrine are relatively specific agonists at β2 receptors. Because these drugs are less potent at β1 receptors, they have less tendency to stimulate the heart. Nevertheless, their selectivity for β2 receptors is not absolute, and at higher doses these drugs stimulate the heart directly. These drugs also differ from ISO because they are effective orally and have a longer duration of action. Selective β2 receptor agonists relax vascular smooth muscle in skeletal muscle and smooth muscle in bronchi and uterus. Although the pharmacological properties of all β2 receptor agonists are similar, ritodrine is marketed as a tocolytic agent; that is, it relaxes uterine smooth muscle and thereby arrests premature labor. All other drugs are marketed for the treatment of bronchospasm and bronchial asthma (see Chapter 16). By activating β2 receptors, these drugs cause bronchodilation and may inhibit the release of inflammatory and bronchoconstrictor mediators (histamine, leukotrienes, prostaglandins) from mast cells in the lungs. The compounds are most effective when delivered by inhalation, which results in the least systemic adverse effects (tachycardia, skeletal muscle tremor). When used orally, selective β2 receptor agonists have an advantage over ephedrine (see later discussion) because they lack CNS stimulant properties.
DA and dobutamine are relatively specific for β1 receptors and are used to stimulate the heart. DA is an endogenous catecholamine with important actions as a neurotransmitter in the brain (see Chapter 27). When administered by intravenous infusion, DA produces a positive inotropic action on the heart by directly stimulating β1 receptors, and indirectly, by releasing NE. DA relaxes smooth muscle in some vascular beds, specifically in the kidney, increasing glomerular filtration rate, Na+ excretion, and urinary output. The latter effect is mediated by DA D1 receptors in the renal vasculature that can be blocked by many antipsychotic drugs (see Chapter 29). DA is also administered by intravenous infusion for treatment of shock, resulting from myocardial infarction, or trauma.
Dobutamine is a relatively specific β1 receptor agonist that also increases myocardial contractility without greatly altering total peripheral resistance. It has less effect on heart rate than ISO because it does not produce reflex tachycardia. Dobutamine is also administered by intravenous infusion to treat acute cardiac failure.
The sympathomimetic actions of some drugs stem from their ability to cause the release of NE from sympathetic neurons or block the neuronal reuptake of NE. Some of these drugs (e.g., amphetamine, cocaine) have noticeable CNS stimulant actions (see Chapter 37).
Tyramine is present in a variety of foods (e.g., ripened cheese, fermented sausage, wines) and is also formed in the liver and GI tract by the decarboxylation of tyrosine. Tyramine is an indirect-acting agent that is taken up by sympathetic nerve terminals and displaces NE from vesicles into the synaptic cleft via reverse transport of the NE uptake 1 transporter (not by exocytosis). Normally, significant quantities of tyramine are not found in blood or tissues, because tyramine is rapidly metabolized by MAO in the GI tract, liver and other tissues including sympathetic neurons. However, in patients treated with MAO inhibitors for depression, increased circulating concentrations of tyramine may be achieved, particularly after the ingestion foods containing large concentrations of tyramine, leading to the massive release of NE and a severe hypertensive response. Thus patients treated with MAO inhibitors should avoid eating foods containing tyramine (see Chapter 30).
Ephedrine and amphetamine are related chemically to Epi (Fig. 11-8) but exert their sympathomimetic effects mainly by facilitating the release of NE. Ephedrine is a mixture of four isomers: D- and l-ephedrine and d- and l-pseudoephedrine. l-Ephedrine is the most potent, but the racemic mixture is often used, as is d-pseudoephedrine. Ephedrine is not metabolized by COMT or MAO and thus has a prolonged duration of action. Ephedrine exerts a direct action on β2 receptors and has some limited usefulness as a bronchodilator. It also readily crosses the blood-brain barrier. Ephedrine was widely used as a dietary supplement for its stimulant and appetite-suppressant properties (see Chapter 7) but has now been removed from the market in the United States because of increasing reports of adverse effects.
FIGURE 11–8 Structures of some indirect-acting sympathomimetics compared with epinephrine. Asterisk indicates asymmetrical carbon.
Pseudoephedrine has fewer central stimulant actions than ephedrine and is widely available as a component of over-the-counter preparations used as a decongestant for relief of upper respiratory tract conditions that accompany the common cold. Its use has been subject to legal restrictions because it can be chemically modified to yield an abused substance (methamphetamine). It is often combined with analgesics, anticholinergics, antihistaminics, and caffeine. Pseudoephedrine is thought to act as a decongestant both by indirectly releasing NE and by activating α1 adrenergic receptors, constricting nasal blood vessels.
Methamphetamine and amphetamine exist as d- and l-optical isomers. On peripheral sympathetic neurons, d- and l-amphetamine are equipotent, but in the CNS, the d-isomer is three to four times more potent than the l-isomer. Amphetamine is used therapeutically only for its central stimulant action (see Chapter 37), and only the d-isomer is used to minimize peripheral sympathomimetic actions. Amphetamine facilitates the release and blocks the reuptake of NE. However, the central stimulant actions of amphetamine appear to result from its ability to cause DA release (see Chapter 37). Cocaine is structurally distinct but has similar central stimulant and sympathomimetic actions. It also blocks the neuronal reuptake of NE and DA; however, unlike amphetamine, it does not facilitate neurotransmitter release.
α Receptor Antagonists
Phentolamine is a prototypical competitive antagonist at α receptors, meaning that blockade can be surmounted by increasing agonist concentration (Fig. 11-9). Phenoxybenzamine, on the other hand, binds covalently to α receptors and produces an irreversible blockade that cannot be overcome by addition of more agonist (see Fig. 11-9).
FIGURE 11–9 Comparison of the effects of a reversible (phentolamine) and an irreversible (phenoxybenzamine) inhibitor of α receptors. Changes in the force of contraction of arterial strips were recorded after addition of increasing concentrations of NE in the absence and in the presence of low and high concentrations of phentolamine and low and high concentrations of phenoxybenzamine. The broken lines are larger doses of the antagonist.
Phenoxybenzamine and phentolamine have similar affinities for α1 and α2 receptors. They cause vasodilation by blocking sympathetic activation of blood vessels, an effect proportional to the degree of sympathetic tone. Adrenergic α receptor antagonists cause only small decreases in recumbent blood pressure but can produce a sharp decrease during the compensatory vasoconstriction that occurs on standing because reflex sympathetic control of capacitance vessels is lost. This can result in orthostatic or postural hypotension accompanied by reflex tachycardia, although tolerance to this effect occurs with repeated use.
The effects of α receptor blockade on mean blood pressure and heart rate in response to NE and Epi are illustrated in Figure 11-10. The intravenous injection of NE and Epi produces a brief increase in blood pressure by activating α1 receptors in blood vessels. A reflex decrease in sympathetic and increase in vagal tone to the heart occur (see Fig. 11-5), but reflex bradycardia may be masked by direct activation of cardiac β1 receptors. In the case of NE, bradycardia is often seen because systolic and diastolic pressure both increase. Because of such complex and opposing actions, the effects of these compounds on heart rate can vary. Blockade of α receptors with phentolamine lowers blood pressure but is accompanied by a reflex increase in heart rate. NE now has little effect on blood pressure and does not cause reflex bradycardia, whereas heart rate is increased by stimulation of cardiac β1 receptors. When Epi is administered, the former pressor response is converted to a strong depressor response because vasodilatation resulting from β2 receptor activation is unmasked. Epi now causes pronounced tachycardia by direct activation of cardiac β1 receptors as well as a reflex tachycardia after the blood pressure decrease. Thus, when α receptors are blocked, the actions of Epi resemble those of the pure β receptor agonist ISO.
FIGURE 11–10 Effects of intravenous injections of NE and Epi on mean blood pressure and heart rate before and after blockade of α receptors by phentolamine. (Note: the heart rate data [red line] appears above the blood pressure on the right part of the graph (beyond the dashed line).
The tachycardia that occurs after administration of α receptor antagonists is attributable in part to blockade of presynaptic α2 receptors on sympathetic neurons (Fig. 11-11). Activation of these receptors inhibits NE release, and this feedback inhibition is disrupted when α2 receptors are blocked. This has little consequence when the postsynaptic receptors are α1, because the α receptor antagonists mentioned also block these receptors. In the heart, however, the postsynaptic receptors are β1. Thus effects of sympathetic activation are enhanced when α2 receptors are blocked, increasing NE release and enhancing reflex tachycardia (see Fig. 11-11, B).
FIGURE 11–11 Comparison of the actions of phentolamine (α1 and α2 receptor antagonist) and prazosin (α1 receptor antagonist) at noradrenergic neuroeffector junctions in cardiac muscle (β1 receptors) and vascular smooth muscle (α1 receptors).
Prazosin, terazosin, doxazosin, tamsulosin, and alfuzosin are selective α1 receptor antagonists with similar pharmacological profiles but some differences in pharmacokinetics. By blocking α1 receptors in arterioles and veins, these drugs reduce peripheral vascular resistance and lower blood pressure; accordingly, they are used in treating hypertension. Selective α1 receptor antagonists cause less tachycardia than nonselective α receptor antagonists, because the α2 receptors, which reduce NE release, are not blocked by these drugs (see Fig. 11-11, C). Adrenergic α1 receptor antagonists also relax smooth muscle in the prostate and bladder neck and thereby relieve urinary retention in benign prostatic hyperplasia. Tamsulosin, terazosin, and alfuzosin are commonly prescribed for this condition.
The major side effects of α receptor antagonists are related to a reduced sympathetic tone at α receptors. These effects include orthostatic hypotension, tachycardia (less common with selective α1 receptor antagonists), inhibition of ejaculation, and nasal congestion. Other adverse effects are not related to their ability to block α receptors; for example, phentolamine stimulates the GI tract, causing abdominal pain and diarrhea.
β Receptor Antagonists
Propranolol is the prototypical nonselective β receptor antagonist. Newer compounds differ primarily in their duration of action and subtype selectivity. Propranolol is a competitive antagonist at both β1 and β2 receptors. Like all adrenergic receptor blocking drugs, its pharmacological effects depend on the activity of the sympatho-adrenal system. When impulse traffic in sympathetic neurons and circulating concentrations of NE and Epi are high (e.g., during exercise), the effects of the drugs are more pronounced. The most profound effects of propranolol are on the cardiovascular system. Propranolol blocks the positive chronotropic and inotropic responses to β receptor agonists and to sympathetic activation. It reduces the rate and contractility of the heart at rest, but these effects are more dramatic during exercise. The drug may precipitate acute failure in an uncompensated heart. Propranolol, but not some other β receptor antagonists, also has a direct membrane-stabilizing action (local anesthetic action), which may contribute to its cardiac antiarrhythmic effect.
When administered acutely, propranolol does not greatly affect blood flow because vascular β receptors are not tonically activated, although compensatory reflexes cause slightly increased peripheral resistance. Propranolol administered over the long term is an effective antihypertensive agent. How it lowers blood pressure is not completely understood, but it is believed to result from several actions, including reduced cardiac output, reduced release of renin from the juxtaglomerular apparatus, and possibly some actions in the CNS.
Propranolol also blocks the metabolic actions of β receptor agonists and activation of the sympatho-adrenal system. It inhibits increases in plasma free fatty acids and glucose from lipolysis in fat and glycogenolysis in liver, heart, and skeletal muscle. This can pose a problem for diabetics by augmenting insulin-induced hypoglycemia. However, selective β1 receptor antagonists are less likely to augment insulin-induced hypoglycemia. Adrenergic β receptor antagonists also reduce the premonitory tachycardia associated with insulin-induced hypoglycemia, so diabetic patients taking them must learn to recognize sweating (induced by activation of cholinergic sympathetic neurons) as a symptom of low blood glucose.
Propranolol causes few serious side effects in healthy people but can do so in patients with various diseases; heart failure is the main threat. Propranolol is usually contraindicated in patients with sinus bradycardia, partial heart block, or compensated congestive heart failure. Sudden withdrawal of propranolol from patients who have received this drug over the long term can cause “withdrawal symptoms” such as angina, tachycardia, and dysrhythmias. Rebound hypertension may occur in such patients taking propranolol to control blood pressure. These withdrawal symptoms probably result from the development of β receptor supersensitivity and can be minimized by gradually reducing the dose of the drug.
The ability of propranolol to increase airway resistance is of little clinical importance in healthy people but can be very hazardous in patients with obstructive pulmonary disease or asthma. Selective β1receptor antagonists (see later discussion) should be used in these patients, although even these drugs should be used with caution, because they are not completely inactive at β2 receptors.
Nadolol is a nonselective β receptor-blocking drug that is less lipid soluble than propranolol and less likely to cause CNS effects. It has a significantly longer duration of action than most other β receptor antagonists. Timolol is another nonselective β receptor antagonist administered orally for treating hypertension and angina pectoris or as an ophthalmic preparation for treating glaucoma.
Carteolol, pindolol, and penbutolol are nonselective β receptor antagonists that have modest intrinsic sympathomimetic properties. These drugs cause less slowing of resting heart rate and fewer abnormalities of serum lipid concentrations than other β receptor antagonists, and it has been suggested that they also produce less up regulation of β receptors. These drugs generally cause less severe withdrawal symptoms and are used to treat hypertension.
Labetalol and carvedilol are competitive antagonists of α1, β1, and β2 receptors. Consequently, they have hemodynamic effects similar to those of a combination of propranolol (β1 and β2 receptor blockade) and prazosin (α1 receptor blockade). Unfortunately, side effects are also similar to those of both drugs (e.g., orthostatic hypotension, nasal congestion, bronchospasm). However, these drugs are potent antihypertensive agents.
At low doses, acebutolol, atenolol, metoprolol, and esmolol are more selective in blocking β1 receptors on cardiac muscle than in blocking β2 receptors on bronchiolar smooth muscle. They are less likely than nonselective β receptor antagonists to increase bronchoconstriction in patients with asthma. These β1 receptor antagonists are useful in treating hypertension and angina pectoris. Esmolol is a selective β1 receptor antagonist that is rapidly metabolized by esterases in red blood cells and has a very short t1/2. It is used for the emergency treatment of sinus tachycardia and atrial flutter or fibrillation.
Drugs that Interfere with Sympathetic Neuronal Function
Drugs that disrupt the synthesis, storage, or release of NE have been used primarily in the treatment of hypertension; however, they are no longer widely used. Guanethidine is the prototype of this class of drugs, which impairs release of NE from postganglionic sympathetic neurons. Related drugs include guanadrel, bretylium, and reserpine, which depletes sympathetic neurons of stored NE. These drugs reduce sympathetic tone in a relatively nonspecific manner, causing substantial side effects, including reduced blood pressure, heart rate, and cardiac output and increased GI motility and diarrhea. Because of their side effects and the availability of better drugs, these drugs are no longer widely used.
Drugs that Reduce Central Sympathetic Outflow
The activity of peripheral sympathetic neurons is regulated in a complex manner by neuronal systems located in the lower brainstem. These central neurons, in turn, are regulated in part by α2 receptors. Clonidine is the prototype α2 receptor agonist. It is lipid soluble and penetrates the blood-brain barrier to activate α2 receptors in the medulla, resulting in diminished sympathetic outflow. Clonidine lowers blood pressure by reducing total peripheral resistance, heart rate, and cardiac output. It does not interfere with baroreceptor reflexes and does not produce noticeable orthostatic hypotension. Accordingly, clonidine lowers blood pressure in patients with moderate to severe hypertension and produces less orthostatic hypotension than drugs acting in the periphery. However, side effects of clonidine may include dry mouth, sedation, dizziness, nightmares, anxiety, and mental depression. Various signs and symptoms related to sympathetic nervous system overactivity (hypertension, tachycardia, sweating) may occur after withdrawal of long-term clonidine therapy; thus the dose of clonidine should be reduced gradually. Clonidine is also used to ameliorate signs and symptoms associated with increased activity of the sympathetic nervous system that accompany withdrawal from long-term opioid use.
Guanabenz and guanfacine are other α2 receptor agonists that act centrally to inhibit sympathetic tone with a relative sparing of cardiovascular reflexes. Guanfacine is longer acting, less likely to reduce cardiac output, and less sedating than clonidine.
Methyldopa is an analog of the catecholamine precursor l-DOPA that is transported into noradrenergic neurons, where it is converted to α-methyl-NE (Fig. 11-12). α-Methyl-NE partially displaces NE in synaptic vesicles, where it is released in place of NE. This compound selectively activates α2 receptors to cause hypotensive actions, which appear to be attributable to α-methyl-NE in the brain not in the periphery.
FIGURE 11–12 Metabolism of α-methyldopa in central noradrenergic nerve terminals.
Drugs that Inhibit Catecholamine Synthesis
Metyrosine (α-methyltyrosine) inhibits tyrosine hydroxylase in the brain, periphery, and adrenal medulla, thereby reducing tissue stores of DA, NE, and Epi. Metyrosine is used in the treatment of patients with pheochromocytoma not amenable to surgery. Its most prevalent side effect is sedation.
Carbidopa, a hydrazine derivative of methyldopa, inhibits ALAAD. Unlike methyldopa, carbidopa does not penetrate the blood-brain barrier and therefore has no effect in the CNS. Because ALAAD is ubiquitous, is present in excess, and does not control the rate-limiting step in catecholamine synthesis, clinical doses of carbidopa have no appreciable effect on the endogenous synthesis of NE in sympathetic neurons. They do, however, reduce the conversion of exogenously administered l-DOPA to DA outside the brain, rendering carbidopa useful for the adjunctive treatment of Parkinson’s disease (see Chapter 28).
Pharmacovigilance: Side Effects, Clinical Problems, and Toxicity
Major clinical problems associated with the use of these compounds are summarized in the Clinical Problems Box.
Drugs that Interfere with Sympathetic Nervous System Neuronal Function
Block NE release and storage
Increased GI activity
Decreased central sympathetic outflow
Na+ and H2O retention
Drugs that Block Adrenergic Receptors
α Adrenergic receptor antagonists
Impairment of ejaculation
Na+ and H2O retention
β Adrenergic receptor antagonists
Heart failure in patients with cardiac disease
Increased airway resistance
Fatigue and depression
The selective agonists and antagonists for adrenergic α1, α2, β1, and β2 receptor subtypes have led to fewer and less severe side effects than those used previously, and several additional adrenergic receptor subtypes and new drugs with greater selectivity continue to be tested for their therapeutic potential. Pharmacogenomic studies indicate that the β2 receptor polymorphisms occur in association with asthma severity. Such genetic polymorphisms may also contribute to variability in responses to β receptor antagonists. Human genotyping indicates that polymorphic differences in the genes encoding both α and β receptors of human populations are present based on ethnic or national origin. These genetic variations include changes in expression at transcriptional or translational levels, modification of coupling to heterotrimeric G-proteins, which can result in gain or loss in function, and altered susceptibility to down regulation.
MAJOR DRUGS AND 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.)
Epinephrine* (Medihaler, EpiPen)
Selective α1 receptor agonists
Selective β1 receptor agonists
Selective β2 receptor agonists
Metaproterenol (Alupent; Metaprel)
Terbutaline (Brethine, Brethaire)
Nonselective indirect-acting agonists
Dextroamphetamine (Dexedrine, Dextrostat)
Nonselective α receptor antagonists
Selective α1 receptor antagonists
Nonselective β receptor antagonists
Carteolol (Cartrol, Ocupress)
Propranolol (Inderal, Innopran, Pronol)
Timolol (Blocadren, Timoptic)
Selective β1 receptor antagonists
Betaxolol (Kerlone, Betoptic)
Metoprolol (Lopressor, Toprol XL)
Combined α1 and β receptor antagonists
Labetalol (Normodyne, Trandate)
Agents that reduce central sympathetic outflow
Clonidine (Catapres, Duraclon)
* In the United Kingdom the drug name is adrenaline.
† In the United Kingdom the drug name is isoprenaline.
‡ In the United Kingdom the drug name is noradrenaline.
§ In the United Kingdom and Japan the drug name is salbutamol.
Schaak S, Mialet-Perez J, Flordellis C, Paris H. Genetic variation of human adrenergic receptors: From molecular and functional properties to clinical and pharmacogenetic implications. Curr Top Med Chem. 2007;7:217-231.
Shin J, Johnson JA. Pharmacogenetics of beta-blockers. Pharmacotherapy. 2007;27:874-887.
1. Metoprolol would be most effective in blocking the ability of Epi to:
A. Reduce secretion of insulin from the pancreas.
B. Increase release of renin from juxtaglomerular apparatus.
C. Increase secretion of glucagon from the pancreas.
D. Produce mydriasis (dilatation of pupil).
E. Increase secretion of saliva.
2. Which of the following drugs is most likely to produce orthostatic hypotension?
3. Which of the following drugs would be most likely to increase airway resistance in a patient with pulmonary obstructive disease?
4. Systemic administration of which of the following drugs would most likely cause bradycardia?
5. Terbutaline would be expected to cause all of the following effects except:
B. Reduced pulmonary airway resistance.
E. Increased blood flow in skeletal muscle.
6. The cardiovascular effects of Epi in a person treated with phentolamine will most closely resemble the responses after the administration of: