Vascular responses after alpha adrenergic receptor blockade. 1. Responses of capacitance and resistance vessels to norepinephrine in

hypothesis that alpha receptor blockers antagonize more effectively venous than arterial responses to norepinephrine in man. Systemic arterial blood pressure, venous pres-sure in the forearm, blood flow through the fore-arm, and the volume of the forearm at a venous pressure of 30 mm Hg were measured using pres-sure transducers and a mercury strain-gauge plethysmograph. Infusions of norepinephrine into the brachial artery reduced forearm blood flow and venous distensibility without changing arterial pressure. After intraarterial infusion of phentola-mine the decrease in venous distensibility during administration of norepinephrine was blocked al-most completely whereas the decrease in blood flow through the forearm was not altered. The results indicate that alpha adrenergic re-ceptor blockade can antagonize constriction of capacitance vessels more effectively than constric-tion of resistance vessels