K+ transport and capacitance of the basolateral membrane of the larval frog skin

Skin from larval bullfrogs was mounted in an Ussing-type chamber in which the apical surface was bathed with a Ringer solution containing 115 mM K+ and the basolateral surface was bathed with a Ringer solution containing 115 mM Na+. Ion transport was measured as the short-circuit current (I-sc) with a low-noise voltage clamp, and skin resistance (R-m) was measured by applying a direct current voltage pulse. Membrane impedance was calculated by applying a voltage signal consisting of 53 sine waves to the command stage of the voltage clamp. From the ratio of the Fourier-transformed voltage and current signals, it was possible to calculate the resistance and capacitance of the apical and basolateral membranes of the epithelium (R-a and R-b, C-a and C-b, respectively). With SO42- as the anion, R-m decreased rapidly within 5 min following the addition of 150 U/ml nystatin to the apical solution, whereas I-sc increased from 0.66 to 52.03 mu A/cm(2) over a 60-min period. These results indicate that nystatin becomes rapidly incorporated into the apical membrane and that the increase in basolateral K+ permeability requires a more prolonged time course. Intermediate levels of I-sc were obtained by adding 50, 100, and 150 U/ml nystatin to the apical solution. This produced a progressive decrease in R-a and R-b while C-a, and C-b remained constant. With Cl- as the anion, I-sc values increased from 2.03 to 89.51 mu A/cm(2) folloning treatment with 150 U/ml nystatin, whereas with gluconate as the anion I-sc was only increased from 0.63 to 11.64 mu A/cm(2). This suggests that the increase in basolateral K+ permeability produced by nystatin treatment, in the presence of more permeable anions, is due to swelling of the epithelial cells of the tissue rather than the gradient for apical K+ entry. Finally, C-b was not different among skins exposed to Cl-, SO42-, or gluconate, despite the large differences in I-sc, nor did inhibition of I-sc by treatment with hyperosmotic dextrose cause significant changes in C-b. These results support the hypothesis that increases in cell volume activate K+ channels that are already present in the basolateral membrane of epithelial cells.