Regulation of Na+ channels by luminal Na+ in rat cortical collecting tubule

1. The idea that luminal Na+ can regulate epithelial Na+ channels was tested in the cortical collecting tubule of the rat using whole-cell and single-channel recordings. Here we report results consistent with the idea of Na+ self-inhibition. 2. Macroscopic amiloride-sensitive currents (I-Na) were measured by conventional whole-cell clamp. I-Na was a saturable function of external Na+ concentration ([Na+](0)) with an apparent K-m of 9 mM. Single channel currents (i(Na)) were measured in cell-attached patches, i(Na) increased with pipette Na+ concentration with an apparent K-m of 48 mM. Since I-Na = (i(Na))NP0, the different K-m values imply that the channel density (N) and/or open probability (P-0) increase as [Na+](0) decreases. Reduction of [Na+](0) after increasing intracellular Na+ concentration also increased the outward amiloride-sensitive conductance, consistent with activation of the Na+ channels. 3. The underlying mechanism was studied by changing pipette Na+ concentration while recording from cell-attached patches. No increase in NP0 was observed, suggesting that the effect is not a direct interaction between [Na+](0) and the channel. 4. [Na+](0) was varied outside the patch-clamp pipette while recording from cell-attached patches. When amiloride was in the bath to prevent Na+ entry, no change in NP0 was observed. 5. Activation of the channels by hyperpolarization was observed with 140 mM Na-0(+) but not with 14 mM Na-0(+). 6. The results are consistent with the concept of self-inhibition of Na+ channels by luminal Na+. Activation or the channels by lowering [Na+](0) is not additive with that achieved by hyperpolarization.