Basolateral Cl- channels in the larval bullfrog skin epithelium

The addition of 150 U/ml nystatin to the mucosal surface of isolated skin from larval bullfrogs increases apical membrane permeability and allows a voltage clamp to be applied to the basolateral membrane. With identical Ringer's solutions bathing either side of the tissue the short-circuit current (I-SC) averaged 7.60 +/- 0.78 muA/cm(2), and this current could be increased or decreased by imposing a Cl- concentration gradient. Fluctuation analysis of the I-SC gave power spectra that could be fit with low- and high-frequency Lorentzian functions having corner frequencies of 1.48 +/- 0.06 Hz and 48.5 +/- 11.4 Hz, respectively. The Lorentzian plateau was minimal at the lowest I-SC and increased as the I-SC became greater in the positive or negative direction. Current-voltage plots with identical Ringer's on either side of the tissue showed a pattern of outward rectification. Cell attached patches of cells isolated from the skin with collagenase-trypsin treatment showed spontaneous channel activity with a conductance of 20.9 pS at a pipette potential, -Vp=20 mV. Current-voltage plots of single channels showed a similar pattern of rectification to that of the intact skin, and partial replacement of Cl- by gluconate in the pipette solution shifted the reversal potential from zero to about 40 mV, which is close to the expected shift of the reversal potential of the chloride current through a Cl- selective ion. channel. These results suggest that the basolateral Cl- conductance of the larval skin is mediated by a channel with properties that resemble a volume-sensing outwardrectifier anion channel that has been described in a variety of cell types.