Effect of ammonia on Na+ transport across isolated rumen epithelium of sheep is diet dependent

The cellular uptake of ammonia affects the intracellular pH (pHi) of polar and non-polar cells. A predominant uptake of NH3 and its intracellular protonation tend to alkalinise the cytoplasm, whereas a predominant uptake of NH4+ acidifies the cytoplasm by reversing this reaction. Hence, the well-known absorption of ammonia across the rumen epithelium probably causes a change in the pHi. The magnitude and direction of this change in pHi (acid or alkaline) depends on the relative transport rates of NH3 and NH4+. Consequently, the intracellular availability of protons will influence the activity of the Na+H+ exchanger, which could affect transepithelial Na+ transport. The aim of the present study has been to test this possible interaction between ruminal ammonia concentrations and Na+ transport. The term ammonia is used to designate the sum of the protonated (NH4+) and unprotonated (NH3) forms. Isolated ruminal epithelium of sheep was investigated by using the Ussing-chamber technique in vitro. The present results indicate that ammonia inhibits Na+ transport across the rumen epithelium of hay-fed sheep, probably by binding intracellular protons and thus inhibiting Na+-H+ exchange. By contrast, ammonia stimulates Na+ transport in concentrate-fed and urea-fed sheep, which develop an adaptation mechanism in the form of an increased metabolism of ammonia in the rumen mucosa and/or an increased permeability of rumen epithelium to the charged ammonium ion (NH4+). Intracellular dissociation of NH4+ increases the availability of protons, which stimulate Na+H+ exchange. This positive effect of ruminal ammonia on Na+ absorption may significantly contribute to the regulation of osmotic pressure of the ruminal fluid, because intraruminal ammonia concentrations up to 40 mmol/l have been reported.