Luminal 5‐HT stimulates colonic bicarbonate secretion in rats

Abstract

BACKGROUND AND PURPOSEThe bioactive monoamine 5-HT, implicated in the pathogenesis of functional gastrointestinal disorders, is abundantly synthesized and stored in rat proximal colonic mucosa and released to the gut lumen and subepithelial space. Despite much data regarding its expression and function, the effects of luminal 5-HT on colonic anion secretion have not been fully investigated. EXPERIMENTAL APPROACHWe measured short-circuit current (Isc) as an indicator of ion transport in mucosa-submucosa or mucosa-only preparations of rat proximal colon. Total CO2 output was measured in vitro and in vivo. Immunohistochemistry was performed to investigate the localization of 5-HT4, NOS1 and NOS2. KEY RESULTSLuminal 5-HT gradually increased the amplitude and sustained the elevation of Isc. Luminal 5-HT-evoked ΔIsc was acetazolamide sensitive and HCO3− dependent, consistent with cytosolic carbonic anhydrase-dependent electrogenic HCO3 − secretion, while not affected by tetrodotoxin (TTX), atropine or indomethacin. Pretreatment with the selective 5-HT4 antagonist GR113808, but not antagonists for 5-HT3, 5-HT6 or 5-HT7, inhibited luminal 5-HT-evoked ΔIsc. Furthermore, luminal cisapride and tegaserod increased Isc to the same extent as did 5-HT in the presence of indomethacin and TTX. Removal of the submucosa or pretreatment with NOS inhibitors enhanced luminal 5-HT-evoked ΔIsc, suggesting that NO synthesized in the submucosa suppresses mucosal anion secretion. NOS1 and NOS2 were immunostained in the submucosal neurons and glial cells respectively. Luminal 5-HT-evoked HCO3 − secretion was confirmed in vivo, inhibited by co-perfusion of GR113808, but not by ondansetron. CONCLUSIONS AND IMPLICATIONSA novel apical 5-HT4-mediated HCO3− secretory pathway and an NO-dependent inhibitory mechanism are present in the proximal colon. Luminal 5-HT-evoked HCO3 − secretion may be important for the maintenance of mucosal integrity by regulating luminal pH. Abbreviations

Reduction of epithelial secretion in male rat distal colonic mucosa by bile acid receptor TGR5 agonist, INT‐777: role of submucosal neurons

“…Colon segments were pinned on a Sylgard‐plated Petri dish containing ice‐cold Krebs‐Ringer solution (pH 7.4, 4 °C), and the seromuscular layer was peeled off using microdissection forceps. In some experiments, the submucosa was also removed to obtain neuron‐free samples as previously done …”

“…INT‐777 (100 μ M, 10 μ L) was added on the serosal side of (i) seromuscular stripped distal colon preparations pretreated 10 min before with either TTX (1 μ M, 5 μ L) or lidocaine HCl (1 μ M, 5 μ L), or (ii) neuron‐free (seromuscular and submucosa stripping) distal colon preparation …”

“…INT‐777 (100 μ M, 10 μ L) was added on seromuscular stripped distal colon preparations pretreated 10 min before with atropine sulfate (10 μ M, 5 μ L), hexamethonium bromide (100 μ M, 5 μ L), RS‐39604 (10 μ M, 5 μ L), indomethacin (10 μ M, 5 μ L serosal/mucosal), or L‐NAME (200 μ M, 10 μ L)…”

“…To assess the inhibitory potency of TGR5 on stimulated secretion, seromuscular stripped distal colon preparations were pretreated with a) either DMSO (4 μ L), or INT‐777 (100 μ M, 4 μ L) with cisapride (100 μ M, 4 μ L), carbachol (10 μ M, 4 μ L) or nicotine (100 μ M, 4 μ L) added 15 min after on the serosal side; b) indomethacin (10 μ M, 4 μ L serosal/mucosal), followed 15 min after by DMSO (4 μ L) or INT‐777 (100 μ M, 4 μ L) followed 15 min after by PGE2 (10 μ M, 4 μ L, serosal) and then nicotine (100 μ M, 4 μ L, serosal).…”

Gastrointestinal defense mechanisms

“…GPR 41 (FFAR3) was recently identified as an important nutrient chemosensor transducing GLP-2 release from duodenal L-cells [32], associated with an increase in the rate of duodenal mucosal bicarbonate secretion (DBS). GPR 43 (FFAR2) activation also increased the rate of DBS, believed to be independent of GLP-2 release and occurring through serotonin-dependent pathways [33]. Although GPR43 activation increases DBS independently of GLP-2 pathways, Psichas et al co-localized GPR43 with GLP-1 and PYY on L-cells, reporting that activation of GPR43 substantially increases the release of these satiety-inducing products [34].…”