Mechanism of Action

All of the p-blockers exert equilibrium-competitive antagonism of the actions of catecholamines and other adrenomimetics at p-receptors. Probably the best-recognized action of these compounds that is not mediated by a p-receptor is depression of cellular membrane excitability. This effect has been described as a membrane-stabilizing action, a quinidinelike effect, or a local anesthetic effect. This action is not too surprising in view of the structural similarities between p-blockers and local anesthetics. However, with the usual therapeu tic doses, the actions of the ß-receptor blocking agents appear to be almost entirely accounted for by their ß-re-ceptor antagonism.

Because the ß-receptors of the heart are primarily of the ß1 type and those in the pulmonary and vascular smooth muscle are ß2 receptors, ß1-selective antagonists are frequently referred to as cardioselective blockers. The intrinsic activity, cardioselectivity, and membrane-stabilizing actions of a number of ß-blockers are summarized in Table 11.2

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