Time-dependent stimulation by aldosterone of blocker-sensitive ENaCs in A6 epithelia

To study and define the early time dependent response (less than or equal to 6 h) of blocker-sensitive epithelial Na+ channels (ENaCs) to stimulation of Na+ transport by aldosterone, we used a new modified method of blocker-induced noise analysis to determine the changes of single-channel current (i(Na)) channel open probability (P-o), and channel density (N-T) under transient conditions of transport as measured by macroscopic short-circuit currents (I-sc). In three groups of experiments in which spontaneous baseline rates of transport averaged 1.06, 5.40, and 15.14 mu A/cm(2), stimulation of transport occurred due to increase of blocker-sensitive channels. N-T varied linearly over a 70-fold range of transport (0.5-35 mu A/cm(2)). Relatively small and slow time-dependent but aldosterone-independent decreases of P-o occurred during control (10-20% over 2 h) and aldosterone experimental periods (10-30% over 6 h). When the P-o of control and aldosterone-treated tissues was examined over the 70-fold extended range of Na+ transport, P-o was observed to vary inversely with I-sc, falling from similar to 0.5 to similar to 0.15 at the highest rates of Na+ transport or similar to 25% per 3-fold increase of transport. Because decreases of P-o from any source cannot explain stimulation of transport by aldosterone, it is concluded that the early time-dependent stimulation of Na+ transport in A6 epithelia is due exclusively to increase of apical membrane N-T.