Apical Na+ conductance in maturing rabbit principal cell

Net Na+ absorption in microperfused rabbit cortical collecting ducts (CCDs) is low during the 1st wk of postnatal life, increasing substantially thereafter [L. M. Satlin. Am. J. Physiol. 266 (Renal Fluid Electrolyte Physiol. 35): F57-F65, 1994]. To establish whether the low rate of Na+ absorption observed immediately after birth is due to a low apical Na+ permeability of the neonatal principal cell, we used the patch-clamp technique in split-open CCDs isolated from maturing rabbits to estimate conductance, number (N), and open probability (P-o) of apical Na+ channels in principal cells. With LiCl in the pipette and a NaCl or potassium gluconate solution, warmed to 37 degrees C, in the bath, inward currents with a conductance of similar to 11 pS (n = 23) were observed in 17% of cell-attached patches at 1 wk, 41% of patches at 2 wk, and 43% of patches at 5 wk. The mean N per patch in the 1st wk (0.22 +/- 0.09; n = 36) was significantly less than that observed in the 2nd (1.38 +/- 0.39; n = 34) and 5th (1.24 +/- 0.37; n = 21) wk of life. P-o, studied at positive pipette voltages, was significantly lower in the 1st wk (0.085 +/- 0.035; n = 5) than in the 2nd wk (0.345 +/- 0.063; n = 9) and 5th wk (0.291 +/- 0.058; n = 4). To confirm that the 11-pS channel represented the amiloride-sensitive apical Na+ channel, cell-attached patches in CCDs isolated from 2-wk-old rabbits were studied with 0.5 mu M amiloride added to the LiCl pipette solution. Amiloride led to >90% reductions in mean open and closed times of the 11-pS conductance, consistent with blockade of the channel. These data indicate that N and P-o of apical amiloride-sensitive Na+ channels in principal cells increase significantly after birth.