Vascular responses after alpha adrenergic receptor blockade

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Abstract
Experiments were done to test the hypothesis that alpha receptor blockers antagonize more effectively venous than arterial responses to norepinephrine in man. Systemic arterial blood pressure, venous pressure in the forearm, blood flow through the forearm, and the volume of the forearm at a venous pressure of 30 mm Hg were measured using pressure 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 phentolamine the decrease in venous distensibility during administration of norepinephrine was blocked almost completely whereas the decrease in blood flow through the forearm was not altered. The results indicate that alpha adrenergic receptor blockade can antagonize constriction of capacitance vessels more effectively than constriction of resistance vessels.