Specific alpha-adrenoceptor blocking effect of droperidol on isolated smooth muscles

The present study was conducted so as to obtain more insight into the controversies concerning the alpha-adrenoceptor blocking properties of droperidol, a short-acting neuroleptic agent used in neuroleptanalgesia. The effect of droperidol on the vasoconstriction induced by norepinephrine, sympathetic nerve stimulation, histamine and potassium ions was studied on isolated, perfused ear arteries; its effect on norepinephrine-induced contraction was studied on isolated aorta, spleen and vas deferens. In addition, the onset and duration of action of droperidol was studied. Low doses of droperidol inhibit the vasoconstriction induced by norepinephrine and sympathetic nerve stimulation in the ear artery of the rabbit (3.3 X 10(-9) M and 1.3 X 10(-8) M respectively). At similar low doses, droperidol inhibits norepinephrine-induced contractions in the other tissues studied and has a potency comparable to that of phentolamine; its action is rapid in onset and of short duration. High doses of droperidol (10(-6) M) also inhibit the vasoconstriction of the ear artery induced by histamine and by potassium ions. These findings indicate that a low doses, droperidol has specific and competitive alpha-adrenoceptor blocking effects.