APICAL K+ CONDUCTANCE IN MATURING RABBIT PRINCIPAL CELL

Net K+ secretion is not detected in cortical collecting ducts (CCDs) i solated from newborn rabbits and perfused in vitro. To establish wheth er a low apical K+ permeability of the neonatal principal cell limits K+ secretion early in life, we used the patch-clamp technique in split -open CCDs isolated from maturing rabbits to study the properties and density of conducting K+ channels in principal cells. With KCl in the pipette and a NaCl solution warmed to 37 degrees C in the bath, inward currents with a conductance of similar to 42 pS were observed in 0% ( 0 out of 13 or 0/13), 10% (2/21), 18% (5/28), 29% (4/14), and 56% (10/ 18) of cell-attached patches obtained in 1-, 2-, 3-, 4-, and 5-wk-old animals, respectively. The conductance and reversal potential of this channel led us to suspect that it represented the low-conductance K+ c hannel previously described in the rat CCD by L. G. Palmer, L. Antonia n, and G. Frindt (J. Gen. Physiol. 104: 693-710, 1994). The mean numbe r of open channels per patch (NPo) increased progressively (P < 0.05) after birth, from 0 at 1 wk, to 0.06 +/- 0.04 at 2 wk, to 0.40 +/- 0.1 8 at 3 wk, to 0.74 +/- 0.41 at 4 wk, and to 1.06 +/- 0.28 at 5 wk. The increase in NPo appeared to be due primarily to a developmental incre ase in N, which is the number of channels; open probability, P-o, rema ined constant at similar to 0.5 for all channels identified after the 2nd wk of life. The increase in number of conducting K+ channels durin g postnatal life is likely to contribute to the maturational increase in net K+ secretion in the CCDs.