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

Active [Na.sup.+] absorption across rumen epithelium comprises [Na.sup.+]/[H.sup.+] exchange and a nonselective cation conductance (NSCC). Luminal chloride is able to stimulate [Na.sup.+] absorption, which has been attributed to an interaction between [Cl.sup.-]/HC[O.sup.-.sub.3] and [Na.sup.+]/[H.sup.+] exchangers. However, isolated rumen epithelial cells also express a [Cl.sup.-] conductance. We investigated whether [Cl.sup.-] has an additional effect on electrogenic [Na.sup.+] absorption via NSCC. NSCC was estimated from short-circuit current ([I.sub.sc]) across epithelia of goat and sheep rumen in Ussing chambers. Epithelial surface pH (p[H.sub.s]) was measured with 5-N-hexadecanoyl-aminofluorescence. Membrane potentials were measured with microelelectrodes. Luminal, but not serosal, [Cl.sup.-] stimulated the [Ca.sup.2+] and [Mg.sup.2+] sensitive [I.sub.sc]. 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 [+ or -] 0.03 in a low-[Cl.sup.-] solution. It was increased by 0.21 pH units when luminal [Cl.sup.-] was increased from 10 to 68 mM. Increasing mucosal pH from 7.5 to 8.0 also increased the [Ca.sup.2+] and [Mg.sup.2+] sensitive [I.sup.sc] and transepithelial conductance and reduced the fractional resistance of the apical membrane. Luminal [Cl.sup.-] depolarized the apical membrane of rumen epithelium. 5-Nitro-2-(3-phenylpropylamino)-benzoate reduced the divalent cation sensitive [I.sub.sc], but only in low-[Cl.sup.-] solutions. The results show that luminal [Cl.sup.-] can increase the microclimate pH via apical [Cl.sup.-]/HC[O.sup.-.sub.3] or [Cl.sup.-]/O[H.sup.-] exchangers. Electrogenic [Na.sup.+] absorption via NSCC increases with pH, explaining part of the [Cl.sup.-] effects on [Na.sup.+] absorption. The data further show that the [Cl.sup.-] conductance of rumen epithelium must be located at the basolateral membrane. electrolyte transport; sodium absorption; forestomach; ruminants; microclimate doi:10.1152/ajpgi.00419.2005