Effect of serotonin on water and electrolyte transport in the in vivo rabbit small intestine.

The influence of intravenously administered serotonin on water and electrolyte fluxes in the in vivo rabbit jejunum and ileum was examined. Animals were divided into four groups: (1) those receiving saline intravenously while a glucose-free isotonic saline solution perfused the jejunum and ileum; (2) serotonin given intravenously while glucose-free intestinal perfusate was used as in group 1; (3) intravenous saline given while a 10 mM glucose-isotonic saline solution perfused the jejunum and ileum; and (4) intravenous serotonin given while the intestinal perfusate was as in group 3. Serotonin administration resulted in highly significant net secretion of H2O and sodium in both jejunum and ileum in the groups with a glucose-free perfusate. In jejunum, serotonin evoked net water and sodium secretion, whereas controls absorbed water and sodium. In ileum, serotonin significantly enhanced secretion. The addition of glucose to the perfusate completely abolished the serotonin effect. Unidirectional 22Na flux analysis revealed a marked diminution in both mucosal to serosal and serosal to mucosal fluxes in serotonin-treated animals. The decrease in mucosal to serosal flux was greater than the decrease in serosal to mucosal flux, thus explaining the enhanced net secretion observed with serotonin in the groups receiving glucose-free perfusate. In spite of its pronounced effect on water and electrolyte transport, serotonin failed to produce any detectable histological alterations in small bowel mucosa, either by light or electron microscopy. We postulate that serotonin may be an important mediator of the diarrhea so frequently noted in the carcinoid syndrome by virtue of its effects on small intestinal H2O and electrolyte transport.