[Effects of vasoactive drugs on isolated rabbit corpus cavernosum penis]

Recently, intracavernous injection of some vasoactive drugs has been performed for the diagnosis and treatment of impotence. Despite extensive studies the mechanism of erection is still obscure. Therefore, the author studied the effects of some vasoactive drugs on isolated rabbit corpus cavernosum penis. Norepinephrine, phenylephrine or clonidine caused contraction of isolated rabbit corpus cavernosum strips in a concentration-dependent manner. This contractile effect was more potent with norepinephrine and phenylephrine than with clonidine. Prazosin was more effective than yohimbine in inhibiting norepinephrine-induced contractions. Papaverine and verapamil strongly relaxed the strips contracted by norepinephrine. Prostaglandin E1 also showed a relaxant effect. Low concentrations of isoproterenol caused relaxation, but in high concentrations it caused contraction. Acetylcholine relaxed norepinephrine-contracted strips in a concentration-dependent manner. Although the relaxant effect of acetylcholine was weaker than that of papaverine at high concentrations, acetylcholine and papaverine were almost equally effective at low concentrations. Vasoactive intestinal polypeptide relaxed the strips, and it was significantly more potent than papaverine. These findings suggest that both postsynaptic alpha 1-adrenoceptors and alpha 2-adrenoceptors are present in rabbit corpus cavernosum penis, and that alpha 1-adrenoceptors are predominant. The flaccid state of the rabbit penis seems to be maintained mainly by alpha 1-adrenoceptors. Verapamil seems to be useful for the diagnosis and treatment of impotence as papaverine. Acetylcholine and vasoactive intestinal polypeptide may be neurotransmitters involved in erection.