Effect of chloride on pH microclimate and electrogenic Na+ absorption across the rumen epithelium of goat and sheep

Active Na+ absorption across rumen epithelium comprises Na+/H+ exchange and a nonselective cation conductance (NSCC). Luminal chloride is able to stimulate Na+ absorption, which has been attributed to an interaction between Cl−/HCO3− and Na+/H+ exchangers. However, isolated rumen epithelial cells also express a Cl− conductance. We investigated whether Cl− has an additional effect on electrogenic Na+ absorption via NSCC. NSCC was estimated from short-circuit current ( Isc) across epithelia of goat and sheep rumen in Ussing chambers. Epithelial surface pH (pHs) was measured with 5- N-hexadecanoyl-aminofluorescence. Membrane potentials were measured with microelelectrodes. Luminal, but not serosal, Cl− stimulated the Ca2+ and Mg2+ sensitive Isc. This effect was independent of the replacing anion (gluconate or acetate) and of the presence of bicarbonate. The mean pHs of rumen epithelium amounted to 7.47 ± 0.03 in a low-Cl− solution. It was increased by 0.21 pH units when luminal Cl− was increased from 10 to 68 mM. Increasing mucosal pH from 7.5 to 8.0 also increased the Ca2+ and Mg2+ sensitive Isc and transepithelial conductance and reduced the fractional resistance of the apical membrane. Luminal Cl− depolarized the apical membrane of rumen epithelium. 5-Nitro-2-(3-phenylpropylamino)-benzoate reduced the divalent cation sensitive Isc, but only in low-Cl− solutions. The results show that luminal Cl− can increase the microclimate pH via apical Cl−/HCO3− or Cl−/OH− exchangers. Electrogenic Na+ absorption via NSCC increases with pH, explaining part of the Cl− effects on Na+ absorption. The data further show that the Cl− conductance of rumen epithelium must be located at the basolateral membrane.