Verapamil antagonizes forearm vasoconstriction mediated by selective alpha 1- and alpha 2-agonists in hypertensive...

Calcium channel blocking agents preferentially antagonize alpha 2-mediated pressor responses in various animal species. Whether the same happens in man is not clear. For this reason we studied the interference exerted by verapamil, a calcium entry blocker, on forearm vasoconstriction mediated by selective alpha 1- (methoxamine) and alpha 2- (B-HT 933) adrenergic agonists in untreated mild-to-moderately hypertensive patients (n = 22). Each patient underwent a single study. Forearm blood flow was recorded by strain gauge venous plethysmography; all drugs were infused into the brachial artery at systemically ineffective rates. Cumulative dose-response curves to intra-arterial methoxamine or B-HT 933 were obtained during saline or two different rates of verapamil infusion (0.9 and 3.1 micrograms/100 ml forearm tissue per min) which increased forearm blood flow dose-dependently without changing systemic blood pressure or heart rate. Either methoxamine or B-HT 933 decreased forearm blood flow during saline infusion, but their effect was blunted in a dose-dependent manner during verapamil. No evidence of preferential alpha 2-antagonism was present. At variance with animal data, calcium entry blockade by verapamil antagonizes either alpha 1- or alpha 2-mediated vasoconstriction in human forearm vessels.