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

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Abstract
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.