K-SKIN( TRANSPORT AND CAPACITANCE OF THE BASOLATERAL MEMBRANE OF THE LARVAL FROG)

Skin from larval bullfrogs was mounted in an Ussing-type chamber in wh ich the apical surface was bathed with a Ringer solution containing 11 5 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 possibl e to calculate the resistance and capacitance of the apical and basola teral membranes of the epithelium (R-a and R-b, C-a and C-b, respectiv ely). With SO42- as the anion, R-m decreased rapidly within 5 min foll owing the addition of 150 U/ml nystatin to the apical solution, wherea s I-sc increased from 0.66 to 52.03 mu A/cm(2) over a 60-min period. T hese results indicate that nystatin becomes rapidly incorporated into the apical membrane and that the increase in basolateral K+ permeabili ty 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 s olution. 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 increa sed from 2.03 to 89.51 mu A/cm(2) folloning treatment with 150 U/ml ny statin, whereas with gluconate as the anion I-sc was only increased fr om 0.63 to 11.64 mu A/cm(2). This suggests that the increase in basola teral K+ permeability produced by nystatin treatment, in the presence of more permeable anions, is due to swelling of the epithelial cells o f the tissue rather than the gradient for apical K+ entry. Finally, C- b was not different among skins exposed to Cl-, SO42-, or gluconate, d espite the large differences in I-sc, nor did inhibition of I-sc by tr eatment with hyperosmotic dextrose cause significant changes in C-b. T hese results support the hypothesis that increases in cell volume acti vate K+ channels that are already present in the basolateral membrane of epithelial cells.