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

To study and define the early time-dependent response (≤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 μA/cm², 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 μA/cm²). 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 ~0.5 to ~0.15 at the highest rates of Na⁺ transport or ~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).