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Receptors of the sympathetic system may be divided into alpha (α), beta (β), and dopamine (D) receptors. Drugs that bind to these receptors and modulate or mimic the function of the sympathetic nervous system may be divided into those which augment the system (sympathomimetics) and those which antagonize the system (sympatholytics). The sympathomimetics constitute a very important group of agonists used for cardiovascular, respiratory, and other conditions. They are readily divided into subgroups on the basis of their spectrum of affinity for α, β, or D receptors. Alternatively, sympathomimetics may be divided into subgroups based on whether their mode of action is direct or indirect. Sympatholytics are an important group of antagonists used in cardiovascular and other conditions. These drugs are divided into primary subgroups on the basis of their receptor (α and β) selectivity.


Mode of Action


Sympathomimetics (also called adrenomimetics) may directly activate their adrenoceptors, or they may act indirectly to increase the concentration of catecholamine transmitter in the synapse (Figure 6–1). Amphetamine derivatives and tyramine cause the release of stored catecholamines; these sympathomimetics are, therefore, mainly indirect in their mode of action. Another form of indirect action is seen with cocaine and the tricyclic antidepressants; these drugs inhibit reuptake of catecholamines by presynaptic nerve terminals that release them (Figure 4–3), and thus increase the synaptic activity of released transmitter.

Figure 6–1.
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Sympathomimeticdrugs are readily divided into subgroups on the basis of which receptors they activate: alpha, beta, or dopamine (not shown). Alternatively, these drugs are divided into subgroups on the basis of whether their mode of action is direct (at postsynaptic receptors) or indirect (other than at postsynaptic receptors).


Blockade of metabolism (i.e., block of catechol-O-methyltransferase [COMT] and monoamine oxidase [MAO]) has little direct effect on autonomic activity, but MAO inhibition increases the stores of catecholamines in adrenergic synaptic vesicles and thus may potentiate the action of other indirect-acting sympathomimetics subsequently discussed.


Spectrum of Action


Both α and β receptors are further subdivided into subgroups. The distribution of these receptors is set forth in Table 4–3. Epinephrine may be considered a single prototype with effects at all receptor types (α1, α2, β1, β2, and β3). In addition, separate prototypes—phenylephrine for α receptors and isoproterenol for β receptors—have been characterized. Dopamine receptors constitute a third class of adrenoceptors. The just-mentioned drugs have relatively little effect on dopamine receptors, but dopamine itself is a potent dopamine receptor agonist and when given as a drug can also activate β receptors (intermediate doses) and α receptors (large doses). The relative affinities of these representative drugs are presented in Table 6–1.

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Table 6–1. Relative Selectivity ...

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