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From many previous studies it has been known that ammonia influences the intracellular pH (pHi). The diffusion of NH3 into the cell leads to an increase in intracellular pH due to its protonation (NH3 + H+ -> NH4+), whereas the influx of NH4+ ions via (potassium) channel on the other hand, leads to a decrease in intracellular pH due to its dissociation (NHS ---> NH3 + W). This change in intracellular pH - elevation or reduction - should have consequences on cellular functions, most probably on pH -dependent transport processes.Ammonia in the ruminal fluid is a normal physiological fermentation product of the nitrogenous component of the diet. Its intraruminal concentration shows a considerable fluctuation, and its absorption occurs mainly from the rumen. Ammonia absorption occurs probably predominantly in its uncharged lipid soluble NH3 form (Bödeker et al., 1990; Remond et al., 1993a). However, NH4+ ion can also takes part in this absorption (Bödeker and Kemkowski, 1996). The entry of NH3 and / or NH4+ into epithelial cell could therefore, leads to an alteration in intracellular pH, and consequently could influence pH -dependent transport mechanisms; primarily the electroneutral transport of Na+ and Cl- by the coupled exchange systems (Na+/H+ - and Cl-/HC03- -exchanger).It was therefore, the intention of this study to test the effect of increasing mucosal ammonia concentration on Na+ and Cl- transport across isolated rumen epithelium of sheep, using conventional Ussing-chamber technique, in vitro. The following results were obtained :1. Increasing mucosal ammonia concentration resulted in a dose -dependent reduction of Na+ transport, when an isolated rumen epithelium from hay-fed sheep were incubated at mucosal pH of 7.4. This effect could be explained according to the diffusion of NH3 into the cell and its protonation, with the consequent elevation of the intracellular pH. Under these conditions, the activity of Na+/H+ -exchanger is reduced.2. Cl- transport across the rumen epithelium was stimulated under the same above mentioned experimental conditions. This result support the acceptance of intracellular pH elevation with an increase in intracellular availability of HC03- and the consequent stimulation of Cl-/HC03- -exchanger.3. These results support the previous reports on the only indirectly coupled electroneutral Na+ and Cl- transport.4. Stimulation of Na+ transport across the rumen epithelium by increasing mucosal ammonia concentration were observed, when: - isolated rumen epithelium from concentrate-fed sheep were used, - the mucosal pH were decreased to 6.4.5. The stimulatory effect of ammonia on Na+ ransport, could be due to the predominant NH4+ uptake via apical K+ channel, its intracellular dissociation offering protons to the Na+/H+ -exchanger, and the consequent simulation of Na+ transport.6. Concentrate-fed sheep obviously developed an adaptation process, in form of an increased metabolism of ammonia in the rumen mucosa, and / or a relatively increased uptake of the charged ammonium ion (NH4+) by the rumen epithelium. Consequently, NH4+ ion dissociates intracellulary increasing the availability of protons, and inturn stimulating Na+/H+ exchange.7. These results, support the previous reports on the dietary functional adaptation of sheep rumen epithelium.According to these results it could be concluded that, effect of ammonia on Na+ transportacross the rumen epithelium of sheep (inhibition or stimulation) is dependent on the relativeflux rate of NH3/NH4+, and their consequent effect on intracellular pH, and hence on theactivity of the Na+/H+ -exchanger. Mucosal conditions favouring a high relative NH3/NH4flux inhibit, while those favouring a low relative NH3/NH4 flux stimulate Na+ transport across the umen epithelium of sheep.