Effect of nifedipine and verapamil on alpha-receptor-activation in patients with essential hypertension.

The question of whether the hypotensive effect of calcium entry blockers involves an interaction with alpha-adrenergic receptors was examined. The effect of nifedipine subl. (20 mg, n = 9) and of verapamil p.o. (160 mg, n = 9) on the pressor effect of the unselective alpha-adrenergic agonist noradrenaline, as well as on 3H-yohimbine binding to platelet alpha 2-adrenoceptors was studied in patients with essential hypertension. In addition, the effect of nifedipine on reactivity to the selective alpha 1-adrenergic agonist phenylephrine was investigated (n = 9). Nifedipine caused a significant reduction of reactivity to noradrenaline (P less than 0.01), along with a significant decrease in binding sites (P less than 0.01). Affinity to the alpha 2-receptors was unchanged. Verapamil, although equally effective in lowering blood pressure, had no effect on the pressor response or binding sites. The pressor effect of the alpha 1-agonist phenylephrine was reduced (P less than 0.01) by nifedipine. Nifedipine may therefore affect both alpha 1- and alpha 2-adrenoceptors in patients with essential hypertension. Since verapamil did not affect the pressor response to noradrenaline or yohimbine-binding, the interaction with alpha 2-adrenoceptors does not appear to be a general prerequisite for the hypotensive action of calcium entry blockers.