Transport of sodium across the isolated bovine rumen epithelium: interaction with short-chain fatty acids, chloride and bicarbonate.

Unidirectional transport rates of sodium (22Na+) and chloride (36Cl-) across bovine rumen epithelium were measured in vitro by the Ussing chamber technique. The active and short-chain fatty acid (SCFA)-stimulated sodium transport was shown to fit Michaelis-Menten kinetics, and was rate limited mainly by one transport system, characterized by a Km of 43 mmol l-1 Na+ and a Jmax (maximal transport rate) of 6.2 mumol cm-2 h-1 Na+. It was confirmed that the basolateral Na+,K(+)-ATPase was essential for active sodium transport, and that an apical amiloride-sensitive sodium transport system (Na(+)-H+ exchange) was involved in a minimum of 60-70% of the active sodium transport in the presence of SCFAs (butyrate). The main part of both the mucosal-serosal (MS) and serosal-mucosal (SM) sodium flux was sensitive to an applied electrical potential difference (PD). It is noteworthy that an applied PD, equal to the in vivo PD (+30 mV, lumen as reference), abolished net transport of sodium. The stimulating effect of a mixture of acetate, propionate and butyrate on active sodium transport was confirmed, and it was further shown that the stimulating effect of each of the three SCFAs was nearly equal. Analogues of naturally occurring SCFAs (isobutyrate and 2-ethyl-butyrate) did not stimulate active sodium transport, but inhibited the stimulating effect of butyrate. The stimulating effect of butyrate was clearly concentration dependent and showed a maximum at approximately 20 mmol l-1 butyrate. Above this limit active sodium transport was decreased with increasing butyrate concentration. This suggests that there was a limit to the amount of butyrate that could be handled by the epithelium. The active sodium transport was clearly correlated with the chloride concentration, and was significantly reduced, but not abolished, by replacement of chloride with gluconate. Active transport of chloride was stimulated by butyrate and reduced by the Na(+)-H+ exchange inhibitor amiloride (3 mmol l-1). There was no effect of the Cl(-)-HCO3- exchange inhibitor DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid; 0.5 mmol l-1) on sodium transport. HCO3- (13 mmol l-1) and CO2 (5%) themselves had only a small and non-significant stimulating effect on sodium fluxes, however, in the presence, but not the absence of HCO3- and CO2 in the experimental solutions acetazolamide (1 mmol l-1) significantly reduced active sodium transport. It is concluded that SCFAs could stimulate the active sodium and chloride transport as a result of their metabolism. The CO2 produced could stimulate apical Na(+)-H+ and Cl(-)-HCO3- exchangers running in parallel via increased H+ and HCO3- gradients.