Potential dependence of unidirectional chloride fluxes across isolated frog skin.

Isolated frog skins were voltage clamped at transepithelial potentials (Vt) ranging from -60 mV to 60 mV to measure transepithelial 36Cl- fluxes from the apical to the basolateral bathing solution (J13) and in the opposite direction (J31). The potential dependence of fluxes obtained in Na+-free choline Ringer's indicates the presence of conductive and nonconductive components that probably correspond to fluxes through paracellular and cellular pathways, respectively. Rectification of fluxes with reversal of the potential reflects a structural asymmetry, presumably in surface charge density. The data are consistent with a charge density of one negative charge per 280 A2 on the apical side. A new model for passive Cl- transport was developed that includes surface charge asymmetry and specifically accounts for the observed variation of conductance with potential. In normal frog Ringer's, J13 was larger than J31 at zero potential (active Cl- transport), J13 rose exponentially with increasing positive potential to reach a maximum at 40 mV (approximately open-circuit), and the predicted partial Cl- conductance exceeded the measured conductance leading to the conclusion that when J13 is largely driven by Na+ transport, much of the coupling occurs via nonconductive pathways. Theophylline stimulates Cl- transport that also occurs via nonconductive pathways as Vt becomes more positive.