Verapamil interacts in vitro with alpha 2-adrenoceptors but does not modify alpha 2-receptivity in vivo.

The kind of interaction of two structurally different calcium channel blockers (verapamil and nicardipine) with both alpha 2-adrenergic agonist and antagonist binding on human platelets was investigated. Only verapamil, but not nicardipine, interacted in vitro with platelet alpha 2-adrenoceptors on [3H]-yohimbine or [3H]-UK 14,304 binding. Verapamil behaves as a weak antagonist competitor for alpha 2-adrenoceptors. In patients with mild essential arterial hypertension, the number of platelet alpha 2-adrenoceptors as well as velocity of aggregatory response to adrenaline, are significantly decreased: -21 and -25%, respectively (p less than 0.05). Verapamil (120 mg t.i.d. orally during 1 month) failed to modify the platelet alpha 2-adrenoceptor number or the adrenaline-induced platelet aggregation in hypertensive patients. These results show that, although interacting in vitro, verapamil does not modify the alpha 2-adrenergic receptivity after 1 month treatment in humans.