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

Abdoun, Khalid, Wolf, Katarina, Arndt, Gisela, Martens, Holger
British journal of nutrition 2003 v.90 no.4 pp. 751-758
ammonia, ammonium compounds, cytoplasm, diet, dissociation, metabolism, nutrient uptake, osmoregulation, osmotic pressure, pH, permeability, protons, rumen epithelium, sheep
The cellular uptake of ammonia affects the intracellular pH (pHi) of polar and non-polar cells. A predominant uptake of NH3 and its intra-cellular 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 intra-cellular 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.