Apical membrane of native OMCD(i) cells has nonselective cation channels

The purpose of this study was to examine cation channel activity in the apical membrane of the outer medullary collecting duct of the inner stripe (OMCD(i)) using the patch-clamp technique. In freshly isolated and lumen-opened rabbit OMCD(i), we have observed a single channel conductance of 23.3 +/- 0.6 pS (n = 17) in cell-attached (c/a) patches with high KCl in the bath and in the pipette at room temperature. Channel open probability varied among patches from 0.06 +/- 0.01 at -60 mV (n = 5) to 0.31 +/- 0.04 at 60 mV (n = 6) and consistently increased upon membrane depolarization. In inside-out (i/o) patches with symmetrical KCl solutions, the channel conductance (22.8 +/- 0.8 pS; n = 10) was similar as in the c/a configuration. Substitution of the majority of Cl- with gluconate from KCl solution in the pipette and bath did not significantly alter reversal potential (E(rev)) or the channel conductance (19.7 +/- 1.1 pS in asymmetrical potassium gluconate, n = 4; 21.4 +/- 0.5 pS in symmetrical potassium gluconate, n = 3). Experiments with 10-fold lower KCl concentration in bath solution in i/o patches shifted E(rev) to near the E(rev) of K+. The estimated permeability of K+ vs. Cl- was over 10, and the conductance was 13.4 +/- 0.1 pS (n = 3). The channel did not discriminate between K+ and Na+, as evidenced by a lack of a shift in the E(rev) with different K+ and Na+ concentration solutions in i/o patches (n = 3). The current studies demonstrate the presence of cation channels in the apical membrane of native OMCD(i) cells that could participate in K+ secretion or Na+ absorption.