REGULATION OF LOW-AMILORIDE-AFFINITY SODIUM-CHANNELS IN ALVEOLAR TYPE-II CELLS

We determined the mechanisms by which beta-agonists increase sodium (Na+) currents across rat alveolar type II (ATII) cells grown in primary culture. When ATII cells were patched in the cell-attached mode using symmetrical Na+ solutions (150 mM Na+-glutamate), single-channel currents were observed for holding potentials between -80 and 30 mV (referenced to the pipette solution) with a single-channel conductance of 27 +/- 3 pS, a mean open time (tau(1)) of 3.3 +/- 0.15 ms and an open probability (P-o) of 0.36 +/- 0.06 (n = 7). Addition of 10 mu M terbutaline into the bath increased tau(1) to 6.43 +/- 0.5 ms and P-o to 0.62 +/- 0.06 (n = 7) without affecting channel conductance. Single-channel currents with a conductance of 25 +/- 2 pS were also recorded across ATII cells patched in the inside-out mode. Addition of 250 U/ml of protein kinase A (PKA), 1 mM ATP, and 5 mM MgCl2 in the bath solution (150 mM Na+-glutamate) increased the single channel tau(1) from 3.26 +/- 0.15 to 7.38 +/- 0.38 and P-o from 0.41 +/- 0.06 to 0.72 +/- 0.07 (n = 6) without altering conductance. Addition of 1 mu M amiloride or ethylisopropylamiloride (EIPA) in the pipette solution (150 mM Na+- glutamate) blocked single-channel activity almost completely. Ionic substitution experiments showed the relative permeability of Na+ to K+ and Na+ to Cl- to be 7:1 and 8:1, respectively. These results indicate that the apical membranes of ATII cells contain highly selective, low-amiloride affinity Na+ channels, the activity of which can be upregulated by an increase of the level of intracellular adenosine 3',5'-cyclic monophosphate.