Amiloride-sensitive cation channel in apical membrane of inner medullary collecting duct

Ion channels in the apical membrane of rat inner medullary collecting duct (IMCD) cells in primary culture were studied with the patch-clamp technique. A 27.5 +/- 1.2 pS non-selective cation channel was characterized in inside-out patches. The channel did not discriminate between Na+ and K+ (n = 8) and had a Na permeability-to-Cl permeability (PNa:PCl) ratio of 13:1 (n = 3). Amiloride (5 x 10(-7) M, n = 5) on the extracellular side of the membrane inhibited channel activity 10-fold at negative membrane voltages (voltage refers to cell interior with respect to patch pipette). This diuretic decreased the mean open time and increased the mean closed time without altering single-channel conductance. Voltage-dependent inhibition of this channel by amiloride distinguishes its behavior from other known nonselective cation channels. Neither voltage, Ba2+ (1 mM), tetraethylammonium (5 mM) nor changes in the bath pH (6.1 to 8.0) altered channel activity. Although the channels were active in 41% of the patches in the inside-out configuration, with a percent open time lying between 35 and 60 (-70 to +60 mV), channels were only active in 9% of the cell-attached patches. In preliminary microelectrode studies we have observed an amiloride-inhibited conductance in the apical membrane of isolated and perfused rat IMCD. Therefore, this novel nonselective cation channel identified in IMCD cells in culture may represent the amiloride-sensitive conductance observed in isolated and perfused IMCD and may mediate electrogenic Na+ absorption.