Relationship of transepithelial electrical potential to membrane potentials and conductance ratios in frog skin

Previous studies in anuran epithelia have shown that, after clamping the transepithelial voltage in symmetrical sequences for 4-6 min there is near-constancy of the rate of active Na transport and the associated oxidative metabolism, with a near-linear potential dependence of both. Here we have investigated in frog skin the cellular electrophysiolgical events associated with voltage clamping (Vt = inside-outside potential). Increase and decrease of Vt produced converse effects, related directly to the magnitude of Vt. Hyperpolarization resulted in prompt decrease in inward transepithelial current It and increase in fractional outer membrane resistance fRo (as evaluated from small transient voltage perturbations) and in outer membrane potential Vo. Overshoot of Vo was followed by relaxation to a quasi-steady state in minutes. Changes in fRo were progressive, with half times of some 1-5 sec. Changes in transepithelial slope conductance gt were more variable, usually preventing precise evaluation of the outer and inner cell membrane conductances go and gi. Nevertheless, it was shown that go is related inversely to Vt and Vo. Presuming insensitivity of gi to Vt, the dependence of Go on Vo in the steady state much exceeds that predicted by the constant field equation. Apparent inconsistencies with earlier results of others may be attributable to differences in protocol and the complex dependence of go on Vo and/or cellular-current. In contrast to previous findings in tight epithelia at open circuit, differences in Vt were associated with substantial differences in fRo and inner membrane potential Vi. Hyperpolarization of Vt over ranges commonly employed in studies of active transport ad metabolism appears to increase significantly the electrochemical work per Na ion transported.