Na+ selective channels in the apical membrane of rabbit late proximal tubules (pars recta)

Using the patch-clamp technique, Na+ selective channels were observed in the luminal membrane of rabbit straight proximal tubule segments. In the cell-attached configuration (NaCl-Ringers in pipette and bath) influx of Na+ ions from the pipette into the cell through fluctuating channels was observed. The current-voltage curve of these Na+ channels yielded a zero-current potential of 84.3 +/- 30.9 mV (n = 10), reflecting the electrochemical driving force for Na+ influx under resting conditions. The single channel conductance was 12.0 +/- 2.1 pS (n = 13). In inside-out oriented cell-excised patches the single channel conductance was not significantly different with NaCl-Ringers on both sides. At clamp potentials ranging from +50 mV to -50 mV the single channel current was ohmic and channel kinetics were independent of the voltage. With KCl-Ringers on the bath side (corresponding to cell interior), the zero current potential was 62 +/- 19 mV (n = 4), indicating a high selectivity of Na+ over K+ ions. Addition of 10(-5) mol/l amiloride to the bathing solution decreased the mean channel open time slightly. This effect was more pronounced with 10(-4) mol/l amiloride, whereas the single channel conductance was unaffected by the diuretic. 10(-3) mol/l amiloride caused a complete block of the channel. It is concluded that amiloride sensitive Na+ channels, with similar properties to those observed in tight epithelia, contribute to Na+ reabsorbtion in the straight portion of proximal tubules.