Modulation of electroneutral Na transport in sheep rumen epithelium by luminal ammonia

Ammonia is an abundant fermentation product in the forestomachs of ruminants and the intestine of other species. Uptake as N[H.sub.3] or N[H.sup.+.sub.4] should modulate cytosolic pH and sodium-proton exchange via [Na.sup.+]/[H.sup.+] exchanger (NHE). Transport rates of [Na.sup.+], N[H.sup.+.sub.4], and N[H.sub.3] across the isolated rumen epithelium were studied at various luminal ammonia concentrations and pH values using the Ussing chamber method. The patch-clamp technique was used to identify an uptake route for N[H.sup.+.sub.4]. The data show that luminal ammonia inhibits electroneutral Na transport at pH 7.4 and abolishes it at 30 mM (P < 0.05). In contrast, at pH 6.4, ammonia stimulates Na transport (P < 0.05). Flux data reveal that at pH 6.4, ~70% of ammonia is absorbed in the form of N[H.sup.+.sub.4], whereas at pH 7.4, uptake of N[H.sub.3] exceeds that of NH[sup.+.sub.4] by a factor of approximately four. The patch-clamp data show a quinidine-sensitive permeability for NH[sup.+.sub.4] and [K.sup.+] but not [Na.sup.+]. Conductance was 135 [+ or -] 12 pS in symmetrical N[H.sub.4]Cl solution (130 mM). Permeability was modulated by the concentration of permeant ions, with [P.sub.K] > [P.sub.NH4] at high and [P.sub.NH4] > [P.sub.K] at lower external concentrations. Joint application of both ions led to anomalous mole fraction effects. In conclusion, the luminal pH determines the predominant form of ammonia absorption from the rumen and the effect of ammonia on electroneutral Na transport. Protons that enter the cytosol through potassium channels in the form of NH[sup.+.sub.4] stimulate and nonionic diffusion of N[H.sub.3] blocks NHE, thus contributing to sodium transport and regulation of pH. rumen; ammonia; sodium transport; sodium-hydrogen exchanger; potassium channel