Ruminal transport and metabolism of short-chain fatty acids (SCFA) in vitro: effect of SCFA chain length and pH

The unidirectional transport and metabolism of C-14-labeled acetate, propio nate and butyrate across the isolated bovine rumen epithelium was measured in vitro by the Ussing chamber technique. There was a significant, but rela tively small, net secretion of acetate and propionate, and a large and sign ificant net absorption of butyrate. The results demonstrate that the mucosa l-serosal (MS) pathway for short-chain fatty acids (SCFA) is different from the serosal-mucosal (SM) pathway, and that butyrate is treated differently from acetate and propionate by the epithelium. The results support that th e main route for epithelial SCFA transport is transcellular. The correlatio n between SCFA lipophility and the flux rate was positive but weak at both pH 7.3 and 6.0. Decreasing pH increased all SCFA fluxes significantly, but not proportionally to the increase of protonized SCFA in the bathing soluti on. There was a significant and apparently non-competitive interaction betw een the transport of acetate, propionate and butyrate. It seems that mediat ed transport mechanisms must be involved in epithelial SCFA transport in th e bovine rumen, but the data do not exclude that passive diffusion could ac count for a significant part of the Aux. The metabolism of SCFA in the Ussi ng chamber system was considerable, and there was a clear preference for ex cretion of CO2, from this metabolism to the mucosal side, while side prefer ence for non-CO2, metabolite excretion was not studied. Of the propionate a nd butyrate transported in the MS direction, 78 and 95% was metabolised, wh ile only 37 and 38% was metabolised in the SM direction (acetate metabolism could not be measured). There was, however, no simple relation between the degree of metabolism and the transport rate or the transport asymmetry of the SCFA. (C) 1999 Elsevier Science Inc. All rights reserved.