Increased serotonin levels have been implicated in the pathophysiology of diarrhea associated with celiac and inflammatory diseases. However, the effects of serotonin on Na⁺/H⁺ exchange (NHE) activity in the human intestine have not been investigated fully. The present studies examined the acute effects of 5-hydroxytryptamine (5-HT) on NHE activity using Caco-2 cells as an in vitro model.

Caco-2 cells were treated with 5-HT (.1 μmol/L, 1 h) and NHE activity was measured as ethyl-isopropyl-amiloride (EIPA)-sensitive ²²Na uptake. The effect of 5-HT receptor-specific agonists and antagonists was examined. The role of signaling intermediates in 5-HT-mediated effects on NHE activity was elucidated using pharmacologic inhibitors and immunoblotting.

NHE activity was inhibited significantly (∼50%–75%, P < .05) by .1 μmol/L 5-HT via inhibition of maximal velocity (V_max) without any changes in apparent affinity (K_m) for the substrate Na⁺. NHE inhibition involved a decrease of both NHE2 and NHE3 activities. Studies using specific inhibitors and agonists showed that the effects of 5-HT were mediated by 5-HT4 receptors. 5-HT-mediated inhibition of NHE activity was dependent on phosphorylation of phospholipase C γ 1 (PLCγ1) via activation of src-kinases. Signaling pathways downstream of PLCγ1 involved increase of intracellular Ca²⁺ levels and subsequent activation of protein kinase C α (PKCα). The effects of 5-HT on NHE activity were not cell-line specific because T84 cells also showed NHE inhibition.

A better understanding of the regulation of Na⁺ absorption by 5-HT offers the potential for providing insights into molecular and cellular mechanisms involved in various diarrheal and inflammatory disorders.