AGE AND SEGMENTAL DIFFERENCES IN 5-HYDROXYTRYPTAMINE-INDUCED HYPERSECRETION IN THE PIG SMALL-INTESTINE

5-Hydroxytryptamine is a mediator in cholera toxin-induced hypersecret ion in the small intestine. Our hypothesis is that the hypersecretion induced by 5-hydroxytryptamine in the small intestine decreases with i ncreasing age and in an aboral direction in the small intestine. In vi vo, measuring accumulated fluid in ligated loops, the apparent maximal efficacy of the 5-hydroxytryptamine-induced jejunal secretion in pig neonates was 4.8 +/- 1.1 mg . mg(-1) dry loop 45 min(-1). The apparent maximal efficacy decreased by 23% and 63% in young and adult pigs, re spectively, compared with neonates, In vitro, measuring changes in sho rt-circuit current in Ussing chambers, the apparent maximal efficacy w as 66.7 +/- 4.8 mu A . cm(-2) in neonates and was reduced by 30% and 5 7% in young and adult pigs, respectively. Young pigs were used in the segmental study. The apparent maximal efficacy in vivo was 3.7 +/- 0.5 mg . mg(-1) dry loop and decreased by 22% and 56% in the mid and dist al small intestine, respectively, By contrast, in vitro the apparent m aximal efficacy was elevated by 56% to 72.0 +/- 5.0 mu A . cm(-2) in t he distal compared with the proximal part. In conclusion, the secretor y response to 5-hydroxptryptamine in pig small intestine decreases wit h increasing age and in the aboral direction according to in vivo resu lts. We suggest that the decrease in sensitivity to 5-hydroxytryptamin e can explain a part of the reduced secretory response to cholera toxi n with age and in the aboral direction of the small intestine.