Forearm vascular α₁-adrenergic blockade by verapamil

With use of direct brachial artery infusion and measurement of forearm blood flow and vascular resistance by strain gauge plethysmography, the effect of verapamil on phenylephrine-induced vasoconstriction was determined. Seven healthy men (age range, 19 to 47 years; weight range, 68 to 108 kg; mean blood pressure, 74 to 100 mm Hg; five nonsmokers) were systemically β-blocked with intravenous administrations of 10 mg propranolol. Each subject then received ascending doses of phenylephrine (0.191 to 7.6 μg/min) alone and with concurrent verapamil (19.1 μg/min) by brachial arterial infusion. Dose-ratio during verapamil infusion compared with control was 8.1 (p < 0.05). No change in slope of the phenylephrine dose-response curve was noted; however, consistent with the dose ratio, verapamil shifted the curve to the right with a decrease in the y intercept determined by linear regression (60.0 versus 40.3 mm Hg ml/100 ml forearm volume/min; p < 0.05). Verapamil-induced attenuation of forearm vasoconstriction elicited by phenylephrine indicates that, in humans, in vivo verapamil forearm vasodilating effects are, in part, a result of α₁-adrenergic blockade.