Nonhormonal regulation of apical membrane sodium transport in A6 epithelia

Cultured A6 epithelia and native Na⁺ transporting tissues exhibit large transient increases of short-circuit currents (I_sc) during equilibration in Ussing chambers that require between 30-120 minutes to stabilize. In both control and aldosterone pre-treated A6 epithelia, peak currents averaged about twice those measured in inserts prior to mounting the tissues in the chambers, despite constancy of the growth medium bathing the tissues in the chambers and the inserts. To understand the nature of these transients, we used a pulse method of blocker-induced noise analysis to determine the changes of Na⁺ channel densities (N_T), open probabilities (P_o) and single-channel currents (i_Na) during the equilibration periods as the currents returned towards those originally measured in the inserts. Underlying the changes of the I_sc were large changes of i^Na, P_o and N_T, mainly due in decreases in N_T from 1117±527 to 103±32 channels/100 μm² (control tissues, n=12) and from 2761±604 (n=8) to 220±19 (n=20) channels/100 μm² for aldosterone-pretreated tissues within 40 minutes. Both P_o and i_Na increased during this time. i_Na increased from 0.20±0.02 to 0.32±0.02 pA (control) and from 0.12±.0.01 to 0.30±0.01 pA (aldosterone). P_o increased from 0.15±0.02 to 0.36±0.03 (control) and from 0.13±0.02 to 0.30±0.02 (aldosterone).