Basolateral FFA2 of enterochromaffin cells contributes to 5‐HT release in rat and mouse duodenum

Abstract

Enteroendocrine cells (EEC) express nutrient receptors that when activated release hormones such as serotonin (5‐HT). Although nutrient receptors are thought to be localized on the apical membrane of EEC, where they are well placed to sense luminal nutrients, immunohistochemistry mostly localizes the receptors to the cytosol or to either membrane pole. Since villous cells transport luminal nutrients via nutrient transporters or by passive diffusion to the submucosal space, it is possible that absorbed nutrients activate basolaterally‐expressed nutrient receptors rather than detect luminal nutrients. One of the short‐chain fatty acid receptors, FFA2, is expressed on enterochromaffin (EC) cells of rat duodenum; its activation increases HCO3− secretion in vivo, accompanied by 5‐HT release into the portal vein and mediated by 5‐HT4 receptor activation. We thus used muscle‐stripped mucosa‐submucosa preparations of rat and mouse duodenum mounted in Ussing chambers to pharmacologically localize FFA2 to the apical or basolateral membranes of EC cells.The selective and allosteric FFA2 agonist phenylacetamide‐1 (PA1) was applied to the mucosal or serosal bath of the Ussing chamber, where short‐circuit current (Isc) was measured in the presence of serosal indomethacin (10 μM).PA1 (1 – 100 μM) in the mucosal bath had no effect or slightly decreased Isc in both rat and mouse duodenum. In contrast, PA1 in the serosal bath dose‐dependently increased Isc that was greater in mouse than in rat duodenum; in both, the PA1‐induce Isc increase was reduced by the 5‐HT3 receptor antagonist granisetron (Gran, 10 μM) and abolished by the 5‐HT4 receptor antagonist GR113801 (GR; 10 μM). The effect of serosal PA1 was inhibited by the selective FFA2 antagonist GLPG0974 (0.1 mM). Furthermore, the native FFA2 agonists sodium acetate and sodium propionate (pH adjusted to pH 7.4) dose‐dependently increased Isc inhibited by GR, Gran, the selective Nav1.8 antagonist A803467, and the selective neurokinin receptor‐1 (NK‐1) antagonist CP99994, but not by TTX or atropine. Serosal PA1 increased serosal 5‐HT concentrations, which were not affected by TTX+A803467. FFA2 immunoreactivity was limited to EC cells, suggesting that EC cells rather than myenteric neurons are the primary source of 5‐HT.These results suggest that FFA2 is localized at least to the basolateral membrane of EC cells and that its activation increases electrogenic anion secretion. The basolateral location of FFA2 is important for informing the design of experimental and clinical studies aimed at altering 5‐HT release from EC cells, which is related to mucosal defenses and dyspeptic symptoms.Support or Funding InformationVA Merit Review and NIDDK R01 DK54221This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.