VASOACTIVE-INTESTINAL-PEPTIDE AND IMPOTENCE IN EXPERIMENTAL DIABETES-MELLITUS

Objective To determine whether a defect in vasoactive intestinal pepti de (VIP)-mediated vasodilatation underlies diabetic impotence. Materia ls and methods Rats treated with streptozotocin for 8 weeks developed diabetes, as shown by hyperglycaemia and glycosuria, and had significa nt impairment of sexual function, as determined by tests of sexual beh aviour. The VIP content of the penis and major pelvic ganglion, the VI P release by the penis in vitro and the responsiveness of the vasculat ure of the penis in vivo to intracavernous VIP injection were determin ed. Results In diabetic rats, the VIP content of the major pelvic gang lion and penis was markedly increased, while the acetylcholine content of the penis was normal. The amount of VIP released in vitro by high potassium concentration or veratridine was similar for penile tissue s lices of normal and diabetic rats. Intracavernous injection of VIP ind uced erection in the control rats but not in diabetic rats, whereas in tracavernous injection of the adenylate-cyclase activator forskolin pr oduced erection in both control and diabetic rats. Conclusion Because VIP induces vasodilatation by activating adenylate cyclase, and forsko lin produced erection in the diabetic rats, the failure of VIP to prod uce erection in these rats is unlikely to be due to a defect in the se cond-messenger mechanism or in the properties of vascular smooth muscl e. Thus, a defect at the level of the VIP receptor or of the associate d G-protein possibly explains the failure of intracavernous VIP to pro duce erection in the diabetic rats. Hence, an abnormality in VIP is a component of sexual dysfunction in the diabetic rat and the defect is at the level of the VIP receptor or associated G-protein.