Effects of VIP and related peptides and Gila monster venom on genitourinary smooth muscle.

The pharmacological effects of peptide histidine isoleucine (PHI), glucagon and secretin were compared with vasoactive intestinal polypeptide (VIP) on rabbit urethra and anococcygeus muscle. VIP and PHI dose-dependently inhibited induced contractions of both smooth muscle preparations. Cross-tachyphylaxis between VIP and PHI was demonstrated in the urethra preparation, suggesting that their activity is mediated via a common receptor or second messenger. Glucagon and secretin were without effect on either preparation. Radioimmunoassays demonstrated substantial concentrations of VIP and PHI in both urethra and anococcygeus tissue extracts. These observations suggest that PHI is an additional candidate together with VIP to mediate relaxation of rabbit urethra and anococcygeus muscle. When compared with VIP, Gila monster venom was found to inhibit both smooth muscle preparations, producing concentration-response curves parallel to those produced by VIP.