Neural and non-neural activation of electrogenic secretion by 5-hydroxytryptamine in the rat ileum in vitro.

5-hydroxytryptamine (5-HT) stimulates electrogenic Cl−secretion in rat ileum stripped of its outer smooth musculature and myenteric plexus. The myenteric plexus, however, is a site of 5-HT synthesis in the gut, and the plexus mediates electrogenic ion secretion activated by luminal enterotoxin STa and taurocholate. Thus, we investigated the role of the myenteric plexus in 5-HT-induced electrogenic secretion in vitro by measuring short-circuit current (Isc, microamps) with voltage-clamp apparatus as an index of electrogenic Cl−secretion in rat ileum which was either stripped of the myenteric plexus or was left intact. Serosally added 5-HT stimulated electrogenic Cl−secretion in muscle-stripped and intact ileum in a concentration-dependent manner. Pre-treatment of stripped ileum with atropine (1 μm), hexamethonium (100 μm), tetrodotoxin (1.25 μm) and capsaicin (1 μm) for 15 min did not effect the maximum Isc induced by 5-HT which would implicate a direct action on the enterocyte. In intact ilea, however, tetrodotoxin (TTX) and capsaicin reduced significantly the maximum values of Isc stimulated by 5-HT, and the nitric oxide synthase inhibitor Nω-nitro- L-arginine methyl ester ( L-NAME) caused a significant decrease in the maximum response to 5-HT. These results suggest that electrogenic secretion induced by 5-HT in rat ileum in vitro occurs partly by activation of a non-neural pathway probably involving a direct interaction with the enterocyte, and partly via a nitrinergic-myenteric secretory reflex activated by sensory afferent fibres. These data highlight the danger of characterising intestinal secretory activity from in vitro experiments by using muscle-stripped tissue only.