Noise analysis of inward and outward Na+ currents across the apical border of ouabain-treated frog skin.

The passive Na+ transport across the apical membrane of frog skin (Rana catesbeiana) was studied under the following circumstances: (1) control conditions (sulfate Ringer's, K+ depolarised serosal membranes); (2) after blocking the active transport step with ouabain; (3) with an outward oriented Na+ current. The amiloride-induced Na+ current fluctuations were analysed to calculate the density of amiloride blockable channels and the current through one single channel. Despite the large reduction of the macroscopic current by ouabain, the single channel current remained unchanged, while the number of amiloride blockable Na+ channels was reduced by a factor of eight. It is concluded from these observations that the earlier described reduction of the permeability of the apical membrane is caused by a decrease of the number of electrically conductive Na+ channels. The outward oriented single channel currents were less than 50% of the currents in the opposite direction. After ouabain, the number of Na+ channels was independent from the current direction.