Short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, are the major anions in the large intestinal lumen. They are produced from dietary fiber by bacterial fermentation and are known to have a variety of physiological and pathophysiological effects on the intestine. In the present study, we investigated the expression of the SCFA receptor, GPR43, in the rat distal ileum and colon. Expression of GPR43 was detected by reverse transcriptase/polymerase chain reaction (RT-PCR), Western blotting, and immunohistochemistry. mRNA for GPR43 was detected, by RT-PCR, in extracts of the whole wall and separated mucosa from the ileum and colon and from muscle plus submucosa from the ileum, but not from muscle plus submucosa preparations from the colon. We raised a rabbit antiserum against a synthesized fragment of rat GPR43; this was specific for rat GPR43. GPR43 protein was detected by Western blot analysis in extracts of whole wall and separated mucosa, but not in muscle plus submucosa extracts. By immunohistochemistry, GPR43 immunoreactivity was localized to enteroendocrine cells expressing peptide YY (PYY), whereas 5-hydroxytryptamine (5-HT)-immunoreactive (IR) enteroendocrine cells were not immunoreactive for GPR43. Mast cells of the lamina propria expressing 5-HT were also GPR43-IR. The results of the present study suggest that the PYY-containing enteroendocrine cells and 5-HT-containing mucosal mast cells sense SCFAs via the GPR43 receptor. This is consistent with physiological data showing that SCFAs stimulate the release of PYY and 5-HT from the ileum and colon.

Short-chain fatty acids (SCFAs), including propionate, butyrate and acetate, are fermentation products of carbohydrates in the colon. We investigated the contractile effects of SCFAs on the rat distal colon. Mechanical activity of the circular muscle in strip preparations was recorded in vitro. Propionate and butyrate concentration-dependently (10 micromol L(-1)-10 mmol L(-1)) induced rapid, large amplitude phasic contractions (the first phase) followed by tonic contractions (the second phase). Acetate itself had no effect on muscle activity, although preincubation with acetate attenuated both phases of the propionate-induced response. The propionate-induced phasic contraction was attenuated by atropine, tetrodotoxin and the 5-HT4 receptor antagonist SB-204070. The propionate-induced tonic contraction was attenuated by the cyclo-oxygenase inhibitor piroxicam. Antagonists of 5-HT1A, 5-HT2A and 5-HT3 receptors had no effect on the responses. Propionate-induced responses were not observed in mucosa-free preparations. These results suggest that propionate acts on receptors in the mucosa causing the release of 5-HT from enterochromaffin cells. 5-HT acts through 5-HT4 receptors on the endings of intrinsic primary afferent neurones that in turn activate cholinergic motor neurones that contract the circular muscle. Propionate also causes tonic contraction, via prostaglandin release, in the rat distal colon.

BACKGROUND AND PURPOSEIn suburothelial venules of rat bladder, pericytes (perivascular cells) develop spontaneous Ca 2+ transients, which may drive the smooth muscle wall to generate spontaneous venular constrictions. We aimed to further explore the morphological and functional characteristics of pericytes in the mouse bladder. EXPERIMENTAL APPROACHThe morphological features of pericytes were investigated by electron microscopy and fluorescence immunohistochemistry. Changes in diameters of suburothelial venules were measured using video microscopy, while intracellular Ca 2+ dynamics were visualized using Fluo-4 fluorescence Ca 2+ imaging. KEY RESULTSA network of a-smooth muscle actin immunoreactive pericytes surrounded venules in the mouse bladder suburothelium. Scanning electron microscopy revealed that this network of stellate-shaped pericytes covered the venules, while transmission electron microscopy demonstrated that the venular wall consisted of endothelium and adjacent pericytes, lacking an intermediate smooth muscle layer. Pericytes exhibited spontaneous Ca 2+ transients, which were accompanied by phasic venular constrictions. Nicardipine (1 mM) disrupted the synchrony of spontaneous Ca 2+ transients in pericytes and reduced their associated constrictions. Residual asynchronous Ca 2+ transients were suppressed by cyclopiazonic acid (10 mM), 2-aminoethoxydiphenyl borate (10 mM), U-73122 (1 mM), oligomycin (1 mM) and SKF96365 (10 mM), but unaffected by ryanodine (100 mM) or YM-244769 (1 mM), suggesting that pericyte Ca 2+ transients rely on Ca 2+ release from the endoplasmic reticulum via the InsP3 receptor and also require Ca 2+ influx through store-operated Ca 2+ channels. CONCLUSIONS AND IMPLICATIONSThe pericytes in mouse bladder can generate spontaneous Ca 2+ transients and contractions, and thus have a fundamental role in promoting spontaneous constrictions of suburothelial venules. Abbreviations2-APB, 2-aminoethoxydiphenyl borate; CPA, cyclopiazonic acid; CCCP, carbonyl cyanide 3-chlorophenylhydrazone; CRAC, Ca 2+ release-activated Ca 2+ channels; InsP3, inositol 1,4,5-trisphosphate; LNA, L-nitro-arginine; NCX, Na + -Ca 2+ exchanger; PDGFR, platelet-derived growth factor receptor; SERCA, sarco/endoplasmic reticulum Ca 2+ -ATPase; a-SMA, a-smooth muscle actin; YM-244769, N-(3-aminobenzyl)-6-{4-[(3-fluorobenzyl)oxy] phenoxy}nicotinamide BJP British Journal of Pharmacology