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The role of ruminal epithelium in the absorption of sodium chloride is well established. Previous studies suggest that apical uptake utilizes coupling of Na+/H+ and Cl-/HCO3- exchange (1,2). Basolaterally, Na is actively extruded via the Na/K-ATPase. In this study, we attempted to identify the route for the basolateral excretion of chloride.
Methods: The whole cell configuration of the patch-clamp technique was used to measure conductances of cultured, isolated ruminal epithelial cells. Cells were stimulated with voltages ranging from –120 mV to 100 mV. Resulting currents were measured after stabilization and before the next change in solution, and divided by the cell capacitance to obtain current densities (IC). Reversal potentials designate the potential at which transmembrane current becomes negligible and were corrected for liquid junction potential. Data were tested for significance using student’s t-test.
Results: Cells were filled with a high potassium, low chloride solution and superfused with an NaCl buffer solution. Depolarization of the cells to 100 mV resulted in a current density of 37 ± 10 pA/pF (n = 7), referred to as “outward IC” in the following, and corresponding to positive ions flowing out of the cell, or negative ions flowing inwards. In the same cells, a pipette potential of –120 mV induced a current density of –16 ± 5 pA/pF (“inward IC”; see Table 1). After replacement of chloride by gluconate in the external solution, outward current dropped immediately to a significantly lower level (Table 1), indicating a decrease in the influx of chloride and leading to a significant increase in reversal potential.
In a second series of experiments, we compared cells filled with CsCl solution in NaCl bath solution with Cs-methanesulfonate filled cells (also in NaCl). Reversal potential was significantly lower in the cells in which chloride had been replaced by methanesulfonate (see Table 2), reflecting a reduction in the efflux of chloride.