Bicarbonate-dependent transport of acetate and butyrate across the basolateral membrane of sheep rumen epithelium

Aim This study aimed to assess the role of HCO3− in the transport of acetate and butyrate across the basolateral membrane of rumen epithelium and to identify transport proteins involved. Methods The effects of basolateral variation in HCO3− concentrations on acetate and butyrate efflux out of the epithelium and the transepithelial flux of these short‐chain fatty acids were tested in Ussing chamber experiments using 14C‐labelled substrates. HCO3−‐dependent transport mechanisms were characterized by adding specific inhibitors of candidate proteins to the serosal side. Results Effluxes of acetate and butyrate out of the epithelium were higher to the serosal side than to the mucosal side. Acetate and butyrate effluxes to both sides of rumen epithelium consisted of HCO3−‐independent and ‐dependent parts. HCO3−‐dependent transport across the basolateral membrane was confirmed in studies of transepithelial fluxes. Mucosal to serosal fluxes of acetate and butyrate decreased with lowering serosal HCO3− concentrations. In the presence of 25 mm HCO3−, transepithelial flux of acetate was inhibited effectively by p‐hydroxymercuribenzoic acid or α‐cyano‐4‐hydroxycinnamic acid, while butyrate flux was unaffected by the blockers. Fluxes of both acetate and butyrate from the serosal to the mucosal side were diminished largely by the addition of NO3− to the serosal side, with this effect being more pronounced for acetate. Conclusion Our results indicate the existence of a basolateral short‐chain fatty acid/HCO3− exchanger, with monocarboxylate transporter 1 as a primary candidate for acetate transfer.