title: Serotonin inhibits Na+/H+ exchange activity via 5-HT4 receptors and activation of PKC alpha in human intestinal epithelial cells.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  Caco-2 cells were treated with 5-HT (.1 micromol/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.
  
  
  RESULTS
  NHE activity was inhibited significantly (approximately 50%-75%, P < .05) by .1 micromol/L 5-HT via inhibition of maximal velocity (Vmax) without any changes in apparent affinity (Km) 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 gamma 1 (PLC gamma 1) via activation of src-kinases. Signaling pathways downstream of PLC gamma 1 involved increase of intracellular Ca 2+ levels and subsequent activation of protein kinase C alpha (PKC alpha). The effects of 5-HT on NHE activity were not cell-line specific because T84 cells also showed NHE inhibition.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Section of Digestive Diseases and Nutrition, University of Illinois at Chicago, Jesse Brown VA Medical Center, IL 60612, USA.
authors.forename: Ravinder K
authors.initials: RK
authors.lastname: Gill
authors.forename: Seema
authors.initials: S
authors.lastname: Saksena
authors.forename: Sangeeta
authors.initials: S
authors.lastname: Tyagi
authors.forename: Waddah A
authors.initials: WA
authors.lastname: Alrefai
authors.forename: Jaleh
authors.initials: J
authors.lastname: Malakooti
authors.forename: Zaheer
authors.initials: Z
authors.lastname: Sarwar
authors.forename: Jerrold R
authors.initials: JR
authors.lastname: Turner
authors.forename: Krishnamurthy
authors.initials: K
authors.lastname: Ramaswamy
authors.forename: Pradeep K
authors.initials: PK
authors.lastname: Dudeja
mesh_terms: D018938:Caco-2 Cells; D020013:Calcium Signaling / Q000502:physiology; D002462:Cell Membrane / Q000378:metabolism; D004789:Enzyme Activation; D006801:Humans; D007413:Intestinal Mucosa / Q000201:enzymology / Q000378:metabolism*; D051966:Phospholipase C gamma; D011493:Protein Kinase C / Q000378:metabolism*; D051571:Protein Kinase C-alpha; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D044422:Receptors, Serotonin, 5-HT4 / Q000378:metabolism*; D012701:Serotonin / Q000494:pharmacology*; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors*; D010738:Type C Phospholipases / Q000378:metabolism; D019061:src-Family Kinases / Q000378:metabolism
chemical_list: C081517:SLC9A2 protein, human; C000613385:SLC9A3 protein, human; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D044422:Receptors, Serotonin, 5-HT4; D012701:Serotonin; D011505:Protein-Tyrosine Kinases; D019061:src-Family Kinases; C496628:PRKCA protein, human; D011493:Protein Kinase C; D051571:Protein Kinase C-alpha; D010738:Type C Phospholipases; D051966:Phospholipase C gamma