Distension of rat rectal colon causes electrogenic Cl- secretion via the plexus submucosus Meissner. This study aimed to identify the neurotransmitter(s) of this reflex pathway. Distension was applied to partially stripped rat rectal colon in Ussing chambers. Baseline short-circuit current (Isc) increased and then slowly declined again within 30 min. The increase in Isc 10 min after distension (delta Isc10) was 1.8 +/- 0.3 mumol.h-1.cm-2. Atropine (1 microM) did not alter delta Isc10. Thus cholinergic neurons with muscarinic synapses were not involved. Tissues were then desensitized to vasoactive intestinal peptide (VIP) or substance P. This required continuous infusion of VIP or substance P into the chamber; otherwise, desensitization was only temporary due to rapid degradation of VIP or substance P. During substance P desensitization, distension still induced a secretory response (delta Isc10 not significant vs. control), whereas during VIP desensitization distension no longer had an effect. Furthermore, a polyclonal anti-VIP antiserum blocked 81% and the VIP antagonist [p-Cl-D-Phe6,Leu17]VIP blocked 89% of the distension-induced delta Isc10, supporting the results of the desensitization experiments. To localize the site of VIP action, tetrodotoxin (TTX) was used. The TTX effect on Isc during VIP stimulation was not different from its effect on baseline Isc. This is in accord with the concept that the VIP receptors are mainly located on the enterocytes. We conclude that VIP, but not substance P or acetylcholine (via muscarinic receptors), acts as a neurotransmitter in the distension-induced reflex pathway, causing Cl- secretion in rat rectal colon.
