MEDizzy - Acebutolol and pindolol are classified as β blockers with

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Pharmacology Of The Peripheral Nervous Systems 1

Acebutolol and pindolol are classified as β blockers with intrinsic sympathomimetic activity (ISA). In a practical sense, what does this mean?

ExplanationThe β blockers with intrinsic sympathomimetic activity (ISA) are partial agonists—they act simultaneously as both β agonists and competitive β blockers. How can that be? At usual doses, and in the presence of low (e.g., resting) sympathetic tone, they act as weak agonists for β-adrenergic receptors. Under these conditions, then, they may actually but slightly increase such β-mediated responses as heart rate. However, while these drugs are occupying the β-adrenergic receptors, they simultaneously block (antagonize) the effects of more efficacious (“stronger”) β agonists, for example, epinephrine and norepinephrine, or such exogenous agents (drugs) as isoproterenol. Thus, although they weakly increase resting heart rate, when catecholamine levels are high (as with stress, or exercise), such β-mediated responses as acceleration of heart rate and contractility are less intense than they would be had these drugs with ISA not been present. See Question 22 (in the General Principles chapter) for more information, because it was based on the effects of a β blocker with ISA/partial agonist activity.