5-HT induces duodenal mucosal bicarbonate secretion via cAMP- and Ca2+-dependent signaling pathways and 5-HT4 receptors in mice

In previous studies, we have found that 5-hydroxytryptamine (5-HT) is a potent stimulant of duodenal mucosal bicarbonate secretion (DMBS) in mice. The aim of the present study was to determine the intracellular signaling pathways and 5-HT receptor subtypes involved in 5-HT-induced DMBS. Bicarbonate secretion by murine duodenal mucosa was examined in vitro in Ussing chambers. 5-HT receptor involvement in DMBS was inferred from pharmacological studies by using selective 5-HT receptor antagonists and agonists. The expression of 5-HT4 receptor mRNA in duodenal mucosa and epithelial cells was analyzed by RT-PCR. cAMP-dependent signaling pathway inhibitors MDL-12330A, Rp-cAMP, and H-89 and Ca2+-dependent signaling pathway inhibitors verapamil and W-13 markedly reduced 5-HT-stimulated duodenal bicarbonate secretion and short-circuit current (I-sc), whereas cGMP-dependent signaling pathway inhibitors NS-2028 and KT-5823 failed to alter these responses. Both SB-204070 and high-dose ICS-205930 (selective 5-HT4 receptor antagonists) markedly inhibited 5-HT-stimulated bicarbonate secretion and I-sc, whereas methiothepine (5-HT1 receptor antagonist), ketanserin (5-HT2 receptor antagonist), and a low concentration of ICS-205930 (5-HT3 receptor antagonist) had no effect. RS-67506 (partial 5-HT4 receptor agonist) concentration-dependently increased bicarbonate secretion and I-sc, whereas 5-carboxamidotryptamine (5-HT1 receptor agonist), alpha-methyl-5-HT (5-HT2 receptor agonist), and phenylbiguanide (5-HT3 receptor agonist) did not significantly increase bicarbonate secretion or I-sc. RT-PCR analysis confirmed the expression of 5-HT4 receptor mRNA in murine duodenal mucosa and epithelial cells. These results demonstrate that 5-HT regulates DMBS via both cAMP- and Ca2+-dependent signaling pathways and 5-HT4 receptors located in the duodenal mucosa and/or epithelial cells.