Effects of exogenous adrenaline and noradrenaline on vascular postsynaptic alpha 1- and alpha 2-adrenoceptors in man.

The effects of adrenaline and noradrenaline on vascular postsynaptic alpha 1- and alpha 2-adrenoceptors were investigated in six healthy volunteers. The catecholamines were infused intra-arterially, in three cumulative doses, together with a continuous infusion of saline, doxazosin (alpha 1-selective antagonist), yohimbine (alpha 2-selective antagonist) or the combination of the two antagonists, and changes in forearm blood flow were measured by plethysmography. beta-adrenoceptor mediated effects of the catecholamines were prevented by concomitant intra-arterial infusion of propranolol. Adrenaline and noradrenaline reduced forearm blood flow dose-dependently and to the same extent. The vasoconstrictive effect of adrenaline and of noradrenaline was significantly reduced by doxazosin and by yohimbine, and, to a greater extent, by the combination of doxazosin and yohimbine. The magnitude of these reductions were approximately the same for adrenaline and noradrenaline. No changes in heart rate or blood pressure were observed during the infusions. It is concluded that exogenous adrenaline and noradrenaline produce vasoconstriction in the vasculature of the human forearm by stimulation of both postsynaptic alpha 1- and alpha 2-adrenoceptors.