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

Abstract

Ammonia is an abundant fermentation product in the forestomachs of ruminants and the intestine of other species. Uptake as NH3 or NH4+ should modulate cytosolic pH and sodium-proton exchange via Na+/H+ exchanger (NHE). Transport rates of Na+, NH4+, and NH3 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 NH4+. 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, approximately 70% of ammonia is absorbed in the form of NH4+, whereas at pH 7.4, uptake of NH3 exceeds that of NH4+ by a factor of approximately four. The patch-clamp data show a quinidine-sensitive permeability for NH4+ and K+ but not Na+. Conductance was 135 +/- 12 pS in symmetrical NH(4)Cl solution (130 mM). Permeability was modulated by the concentration of permeant ions, with P(K) > P(NH4) at high and P(NH4) > P(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 NH4+ stimulate and nonionic diffusion of NH3 blocks NHE, thus contributing to sodium transport and regulation of pH.

Evidence for the functional involvement of members of the TRP channel family in the uptake of Na+ and NH4 + by the ruminal epithelium

“…Past studies have shown that at the pH found physiologically in the rumen (<6.8), channel-mediated uptake of NH 4 + greatly exceeds efflux in the form of NH 3 [1] and efflux of protons via this pathway may contribute to ruminal pH homeostasis [4]. The purpose of the current study was to identify candidate genes for this conductance.…”

Short-chain fatty acids and acidic pH upregulate UT-B, GPR41, and GPR4 in rumen epithelial cells of goats

“…The mechanism responsible for this pH-dependent effect of ammonia cannot be determined from the experiments performed. However, it is very plausible that a large amount of ammonia is absorbed as NH 4 ϩ under low pH conditions (6), and the NH 4 ϩ dissociates into H ϩ and NH 3 in the cytosol (2). It is also possible that H ϩ and/or NH 3 exert effects on UT-B expression (3).…”

Modulation of urea transport across sheep rumen epithelium in vitro by SCFA and CO2

“…The incubation of rumen epithelium has been described in detail by Abdoun et al (2). Briefly, sheep were killed in a local slaughterhouse, and the reticulorumen was removed from the abdominal cavity within 2-3 min.…”

“…Chloride and mannitol fluxes were measured essentially as described previously (2,73,74). All measurements were performed under classical Ussing chamber conditions without chemical gradients and with transepithelial potential clamped to 0 mV in the short-circuit mode.…”

“…The presence of the protein involved in sodium-proton exchange (NHE) in ruminal epithelium is well documented (2,22,27,31,74), and SCFA are known to stimulate this pH-regulating transport protein in the rumen (25,76). If urea transport is mediated by changes in cytosolic pH, inhibition of NHE should lead to measurable effects.…”