SEGMENTAL DIFFERENCES IN SHORT-CHAIN FATTY-ACID TRANSPORT IN RABBIT COLON - EFFECT OF PH AND NA

Short-chain fatty acids (SCFAs) are the predominant luminal anion in t he mammalian colon. Although they are rapidly absorbed in vivo, little is known about the mechanisms of transepithelial transport in vitro. Previous studies have suggested that SCFA transport may be linked to N a absorption or an anion exchange mechanism. We compared the transport of propionate under short-circuit conditions in rabbit proximal and d istal colon to determine whether there were segmental differences, how SCFAs may be linked to either Na absorption or anion transport, and w hether SCFAs, as weak electrolytes, may be affected by transepithelial pH gradients. In distal colon, propionate transport was not significa ntly altered by stimulation of electrogenic Na absorption, epinephrine or Cl removal. However, a modest transepithelial pH gradient (luminal 6.8/serosal 7.4) stimulated propionate absorption. In proximal colon, propionate transport was significantly altered by manuevers that eith er stimulated (lowered [Na] in the bathing media) or inhibited (theoph ylline) apical Na-H exchange. Neither Cl removal, nor the anion exchan ge inhibitor DIDS, nor a transepithelial bicarbonate gradient, altered propionate transport. A transepithelial pH gradient inhibited propion ate secretion, but not in a manner entirely consistent with the effect of pH on the distribution of a weak electrolyte. These results sugges t that there is significant segmental heterogeneity in colonic SCFA tr ansport; that transepithelial propionate fluxes are altered by changes in pH or electroneutral Na absorption (Na-H exchange), but not by chl oride removal, bicarbonate gradients or electrogenic Na absorption. Re gulation of SCFA transport may be an important factor in the physiolog y of colonic fluid balance.