THE EFFECT OF VASOACTIVE INTESTINAL POLYPEPTIDE (VIP) ON RABBIT CAVERNOSAL SMOOTH-MUSCLE CONTRACTILITY

Vasoactive intestinal polypeptide (VIP) has emerged as a possible cand idate for a nonadrenergic, noncholinergic inhibitory neurotransmitter in penile erection. In this study, the effect of VIP and its relations hip to the adrenergic and cholinergic mechanisms were examined using i solated corpus cavernosal strips from the rabbit penis. The mechanism of action of VIP on corporal relaxation was investigated with respect to the activation of cyclic GMP and the mobilization of calcium and po tassium ions. VIP caused a dose-dependent relaxation of the cavernosal strip. Pretreatment with VIP had no effect on the contraction induced by norepinephrine, phenylephrine, and clonidine. VIP had no synergist ic effect on the relaxation produced by acetylcholine or isoproterenol . Neither atropine nor propranolol had any blocking effect on the VIP- induced relaxation. Methylene blue decreased the VIP-induced relaxatio n of the cavernosal strip. VIP had no effect on the contraction induce d by KCl at either 20 or 80 mM. In calcium-free high-potassium physiol ogic salt solution, VIP inhibited the calcium-induced contraction. The se results suggests that the mechanism of action of VIP is not mediate d through classical adrenergic and cholinergic neurotransmission on ra bbit cavernosal strips, but that VIP may exert its action by the activ ation of cyclic GMP, which may be associated, in part, with the inhibi tion of calcium influx.