Interaction between ammonia, sodium and chloride transport across the rumen epithelium in sheep

The effect of increasing mucosal ammonia concentration [0 (control), 5, 15 and 30 mmol/l NH4Cl] on the electrophysiology of isolated rumen epithelium and the transport of Na+ and Cl− across it were studied, in vitro. Seventy-three isolated rumen epithelium samples from 14 sheep of different breeds, age and sex and solely fed on hay were used in this study. The presence of ammonia in the incubation solution on the mucosal side evoked a positive short-circuit current (Isc) and increased the transepithelial conductance (Gt). The unidirectional flux rates of Na+ were significantly (p < 0.05) reduced, while the mucosal to serosal and net flux rates for Cl− were significantly (p < 0.05) stimulated. It was determined that an 8 mmol/l intra-ruminal ammonia concentration could cause 50% inhibition of the maximal inhibitory effect of ammonia on Na absorption. It is proposed that ammonia interacts with the electroneutral Na+/H+ and Cl−/HCO3− exchangers of the rumen epithelium, probably by binding intracellular protons, thus elevating intracellular pH (pHi) and consequently inhibiting Na+ transport and stimulating Cl− transport across the isolated rumen epithelium of sheep. Inhibition of Na absorption could increase the osmolality of the rumen fluid, which has been reported to be a predisposing factor of diet-induced left displacement in the abomasum. Further, the increased osmolality of the rumen fluid could adversely affect the microbial growth and fermentation in the rumen. © 2005 Elsevier B.V. All rights reserved.