Journal of applied physiology
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Phenylephrine is generally regarded as a "pure" alpha(1)-agonist. However, after treatment of the forearm with the alpha-adrenergic-blocking drug phentolamine, brachial artery infusion of phenylephrine can cause transient forearm vasodilation. To determine whether this response was beta-receptor mediated, phenylephrine, phentolamine, and propranolol were infused into the brachial arteries of six healthy volunteers. ⋯ Propranolol had no effect on baseline flow, and subsequent phenylephrine infusion after alpha- and beta-blockade caused a small, but significant, sustained decrease in FVC from 5.1 +/- 1.0 to 3.6 +/- 0.8 units. There were no systemic effects from the infusions, and saline infusion at the same rate (1-2 ml/min) had no forearm vasoconstrictor or dilator effects. These data indicate that in humans phenylephrine can exert transient beta(2)-vasodilator activity when its predominant alpha-constrictor effects are blocked.