WHOLE-CELL AND UNITARY AMILORIDE-SENSITIVE SODIUM CURRENTS IN M-1 MOUSE CORTICAL COLLECTING DUCT CELLS

Amiloride-sensitive whole cell currents have been reported in M-1 mous e cortical collecting duct cells (Korbmacher et al., J. Gen. Physiol. 102: 761-793, 1993). We have confirmed that amiloride inhibits the who le cell currents but not necessarily the measured whole cell conductan ce. This discrepancy reflects the action of the transepithelial potent ial in modifying measured whole cell currents. Anomalous responses wer e eliminated by removing external Na+ and/or introducing paraepithelia l shunts. The amiloride-sensitive whole cell currents displayed Goldma n rectification. The ionic selectivity sequence of the amiloride-sensi tive conductance was Li+ > Na+ much greater than K+. Growth of M-1 cel ls on permeable supports increased the amiloride-sensitive whole cell permeability, compared with cells grown on plastic. Single amiloride-s ensitive channels were observed, which conformed to the highly selecti ve low-conductance amiloride-sensitive class [Na(5)] of epithelial Na channels. Hypotonic pretreatment markedly slowed run-down of channel activity. The gating of the M-1 Na+ channel in excised patches was com plex. Open- and closed-state dwell-time distributions from patches tha t display one operative channel were best described with two or more e xponential terms each. We conclude that 1) study of M-1 whole cell Na currents is facilitated by reducing the transepithelial potential to zero, 2) these M-1 currents reflect the operation of Na(5) channels, a nd 3) the Na+ channels display complex kinetics, involving greater tha n or equal to 2 open and greater than or equal to 2 closed states.