ABSORPTION OF SHORT-CHAIN FATTY-ACIDS ACROSS RUMINAL EPITHELIUM OF SHEEP

Investigations on the absorption of short-chain fatty acids across rum inal epithelium of sheep were performed both in vitro (Ussing chamber technique, using propionic acid representatively for short-chain fatty acids) and in vivo (washed, isolated reticulorumen). A pH-induced, ne arly tenfold increase in the concentration of undissociated propionate led to an only twofold increase in mucosal-to-serosal flux of propion ate (in vitro). Neither amiloride (1 mmol . l(-1), in vitro) nor theop hylline (10 mmol . l(-1), in vivo), inhibitors of the ruminal Na+/H+ e xchanger, exerted any significant influence on propionate fluxes or sh ort-chain fatty acids absorption, respectively. Total replacement of l uminal Na+ (by choline) did not alter short-chain fatty acids absorpti on (in vivo). Mucosal 4,4'-diisothiocyanatostilbene-2,2'-disulfonic ac id (0.1 mmol . l(-1)) or mucosal nitrate (40 mmol . l(-1)) markedly re duced propionate net flux (in vitro). Increasing mucosal Cl- concentra tion brought about a significant drop in mucosal-to-serosal flux of pr opionate (in vitro) and in short-chain fatty acids net absorption (in vivo), respectively. The results obtained suggest that short-chain fat ty acids are absorbed both as anions and as undissociated acids across ruminal epithelium of sheep. It is concluded that short-chain fatty a cids anions either compete with Cl- for binding sites at a common anio n-exchange mechanism or that they are absorbed by an short-chain fatty acids anion/HCO3- exchanger indirectly coupled to a Cl-/HCO3- exchang er via intracellular bicarbonate.