Effects of avian pancreatic peptide on smooth muscle relaxations by vasoactive intestinal peptide and inhibitory nerve stimulation

Abstract

Experiments were carried out to investigate whether avian pancreatic peptide (APP) antagonized relaxations of vascular and gastrointestinal smooth muscles induced by vasoactive intestinal peptide (VIP) and non-adrenergic, non-cholinergic inhibitory nerve stimulation using the cat submandibular gland, cat colon and dog stomach in vivo and several isolated smooth muscles in vitro. APP caused a dose-dependent inhibition of an atropine-resistant vasodilatation induced by chorda-lingual and pelvic nerve stimulation and by VIP. APP did not reduce VIP output induced by pelvic nerve stimulation. APP shifted the dose-response curve for VIP and the frequency-response curve for pelvic nerve stimulation to the right. The vasodilatation induced by isoproterenol or bradykinin was inhibited by somewhat higher doses of APP. APP failed to inhibit the vago-vagal reflex relaxation of the dog stomach. In guinea pig and rat fundic strips and in isolated chick rectum, APP did not exert any significant effects on relaxation caused by VIP or transmural stimulation in the presence of atropine and guanethidine. The result indicates that APP inhibits VIP-mediated relaxation of the vascular smooth muscle, but not that of the gastrointestinal smooth muscle. The inhibitory effect of APP on VIP-mediated vasodilatation is suggested to be due to the physiological competition.