NEURAL MEDIATION OF VASOACTIVE INTESTINAL POLYPEPTIDE INHIBITORY EFFECT ON JEJUNAL ALANINE ABSORPTION

It was recently shown that vasoactive intestinal polypeptide (VIP) inh ibits rat jejunal alanine absorption, an effect that was significantly reduced by vagotomy. This study assesses the role of capsaicin-sensit ive primary afferents (CSPA) and the myenteric plexus in the inhibitio n of rat jejunal alanine absorption by VIP. Continuous intravenous inf usion of VIP (11.2 ng.kg(-1).min(-1)) reduced alanine absorption by 60 % in sham control rats and by 20% in rats neonatally treated with caps aicin (P < 0.01). In in vitro experiments, VIP decreased alanine uptak e by jejunal strips isolated from sham control rats in a dose-dependen t manner. In the presence of 40 nM VIP, alanine uptake by full-thickne ss jejunal strips Mras reduced by 54% in sham control rats and by 25% in rats neonatally treated with capsaicin (P < 0.001). On the other ha nd, VIP reduced alanine uptake by mucosal scrapings by 25% in sham rat s compared with 9% reduction in neonatally treated rats. Chemical abla tion of the extrinsic innervation and jejunal myenteric plexuses by pr etreatment with benzalkonium chloride significantly (P < 0.001) reduce d basal alanine absorption and the inhibitory effect of VIP. Moreover, incubation of intestinal strips with tetrodotoxin and atropine reduce d significantly (P < 0.05) the inhibitory effect of VIP on alanine abs orption. These data suggest that VIP exerts its inhibitory effect on a lanine absorption through the CSPA fibers and the myenteric plexus. Th e neuronal circuitry of this inhibitory process may involve cholinergi c muscarinic mechanisms.