Gill RK, Pant N, Saksena S, Singla A, Nazir TM, Vohwinkel L, Turner JR, Goldstein J, Alrefai WA, Dudeja PK. Function, expression, and characterization of the serotonin transporter in the native human intestine. Am J Physiol Gastrointest Liver Physiol 294: G254-G262, 2008. First published November 8, 2007 doi:10.1152/ajpgi.00354.2007.-The enteric serotonin transporter (SERT) plays a critical role in modulating serotonin availability and thus has been implicated in the pathogenesis of various intestinal disorders. To date, SERT expression and function in the human intestine have not been investigated. Current studies were designed to characterize the function, expression, distribution, and membrane localization of SERT in the native human intestine. Real-time PCR studies showed relatively higher SERT mRNA expression in the human small intestine compared with colon (ileum ϾϾ duodenum ϾϾ jejunum). Northern blot analysis revealed three mRNA hybridizing species encoding SERT (3.0, 4.9, and 6.8 kb) in the human ileum. Consistent with SERT mRNA expression, SERT immunostaining was mainly detected in the epithelial cells of human duodenal and ileal resected tissues. Notably, SERT expression was localized predominantly to the apical and intracellular compartments and was distributed throughout the crypt-villus axis. Immunoblotting studies detected a prominent protein band (ϳ70 kDa) in the ileal apical plasma membrane vesicles (AMVs) isolated from mucosa obtained from organ-donor intestine. Functional studies showed that uptake of [ 3 H]serotonin (150 nM) in human ileal AMVs was 1) significantly increased in the presence of both Na ϩ and Cl Ϫ ; 2) inhibited (ϳ50%) by the neuronal SERT inhibitor, fluoxetine (10 M) and by unlabeled 5-HT; and 3) exhibited saturation kinetics indicating the presence of a carrier-mediated process. Our studies demonstrated differential expression of SERT across various regions of the human intestine and provide evidence for the existence of a functional SERT capable of removing intraluminal serotonin in human ileal epithelial cells. serotonin uptake; serotonin transporter expression SEROTONIN (5-hydroxytryptamine, 5-HT) is a neurotransmitter and hormone that influences diverse physiological functions. About 90% of the whole body content of 5-HT is present in the gastrointestinal (GI) tract, where it plays a critical role in modulation of gut motility and fluid and electrolyte transport (10,12,28,30). The majority of 5-HT in the GI tract is synthesized and stored in secretory granules of mucosal enterochromaffin (EC) cells. EC cells continuously release small amounts of 5-HT in response to a number of chemical and mechanical stimuli (13). Because 5-HT exerts its effects via specific 5-HT receptor subtypes, a mechanism for 5-HT deactivation is necessary to prevent the receptors from desensitization. The enzymes that catabolize 5-HT, including monoamine oxidases and glucuronyl transferases, are localized intracellularly and thus require transport of 5-HT across plasma membranes (5, 31). However, 5-...