Na transport in sheep rumen is modulated by voltage-dependent cation conductance in apical membrane

The effects of clamping the transepithelial potential difference (PDt; mucosa reference) have been studied in sheep rumen epithelium. Pieces of ruminal epithelium were examined in Ussing chambers, in a part of the experiments combined with conventional intracellular recordings. After equilibration, the tissue conductance (Gt) was 2.50 +/- 0.09 mS/cm(2), the potential difference of the apical membrane (PD(a)) was -47 +/- 2 mV, and the fractional resistance of the apical membrane (fRa) was 68 +/- 2% under short-circuit conditions. Hyperpolarization of the tissue (bloodside positive) depolarized PDa, decreased fRa, and increased Gt significantly. Clamping PDt at negative values caused converse effects on PDa and fRa. All changes were completely reversible. The determination of individual conductances revealed that the conductance of the apical membrane increased almost linearly with depolarization of PDa. The PD-dependent changes were significantly reduced by total replacement of Na. These observations support the assumption of a PD-dependent conductance in the apical membrane that permits enhanced apical uptake of Na even at depolarized PDa. This mechanism appears to be important for the regulation of osmotic pressure in forestomach fluid.