Whole-cell currents in rat cortical collecting tubule: low-Na diet increases amiloride-sensitive conductance.

Individual principal cells within the rat cortical collecting tubule were studied under voltage-clamp conditions using the whole-cell variation of the patch-clamp technique. Isolated tubules were split to expose the apical membrane surface and bathed in NaCl medium at 23 degrees C. When carboxyfluorescein was included in the patch pipette, the dye diffused rapidly into the cell being clamped but did not spread to neighboring cells, indicating a lack of cell-to-cell coupling. Average cell capacitance under whole-cell clamp conditions with KCl in the pipette was 18 +/- 2 pF (n = 10 cells) in rats maintained on a normal diet, consistent with that expected from morphometric measurements of cell surface area. The capacitance increased to 36 +/- 7 pF (n = 8 cells) for rats kept on a low-Na diet, indicating that cell membrane area was increased under these conditions. The amiloride-sensitive whole-cell conductance (GNa), assumed to equal the conductance through apical Na channels, was determined as the slope of the current-voltage relation near zero holding potential. GNa was 6.0 +/- 1.7 nS/cell (n = 12) for rats maintained on a low-Na diet compared with 0.06 +/- 0.08 nS/cell (n = 13) for rats kept on a normal diet. The amiloride-insensitive whole-cell conductance averaged 9.1 +/- 2.0 nS/cell, with no significant difference between low-Na and normal groups. Sodium channel density (N) was estimated from GNa, the mean open probability of the channel, and the single-channel conductance. N equals 3,000 channels/cells in rats on a low-Na diet compared with N less than 100 channels/cell for rats on a normal diet.