Early response of A6 epithelia to aldosterone is mediated by vesicle trafficking of apical Na⁺ channels and not open probability

New non-invasive methods (FASEB J. 9:A64, 1995; J. Gen. Phvsiol. 105:42a, 1995) were used to measure the time-dependent changes of Na' channel density (NT), single channel current (iNa), open probability (P0) and capacitance in response to 2.7 uM aldosterone. During control (2 hrs) and experimental periods (6-8 hrs), A6 epithelia bathed with serum free growth medium were subjected to pulse increases of the weak Na' channel blocker (10-30 uM CDPC) at intervals of 20 minutes. From the changes of macroscopic rates of Na transport and the Lorentzians of the blockerinduced current noise, control iNa, P0 and NT averaged 0.37 ±0.04 pA, 0.44 ± 0.06, and 10.9 ±3.0 x 10' channels/cm2, respectively. After a delay of about 1 -2 hrs, aldosterone caused a gradual increase of the IN> from 1.18 ±0.14 uA/cm2 (N=6 ) to 3.45 ±0.51 uA/cm2 at 6 hrs due solely to increase of the NT ( 43.5 ±9.0 x 10' channels/cm2) with no change of P0. To test for vesicle trafficking of channels in contrast to direct activation of pre-resident channels, the audio frequency dielectric domains of apical membrane capacitance were measured by dielectric spectrsocopy (0.25 Hz - 10.5 kHz) in K⁺-depolarized tissues. Capacitance was increased timewise by 0.3 ± 0.01 μF/cm2 at 8.5 hrs from control (0.53 ±0.01 μF/cm2).