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

Abstract

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 electrophysiological events associated with voltage clamping (V _t =inside-outside potential). Increase and decrease ofV _t produced converse effects, related directly to the magnitude ofV _t . Hyperpolarization resulted in prompt decrease in inward transepithelial currentI_t and increase in fractional outer membrane resistancefR₀ (as evaluated from small transient voltage perturbations) and in outer membrane potentialV₀. Overshoot ofV₀ was followed by relaxation to a quasi-steady state in minutes. Changes infR₀ were progressive, with half times of some 1–5 sec. Changes in transepithelial slope conductanceg _t were more variable, usually preventing precise evaluation of the outer and inner cell membrane conductancesg₀ andg _i . Nevertheless, it was shown thatg₀ is related inversely toV _t andV₀. Presuming insensitivity ofV _i toV _t , the dependence ofg₀ onV₀ 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 ofg₀ onV₀ and/or cellular current. In contrast to previous findings in tight epithelia at open circuit, differences inV _t were associated with substantial differences infR₀ and inner membrane potentialV _i . Hyperpolarization ofV _t over ranges commonly employed in studies of active transport and metabolism appears to increase significantly the electrochemical work per Na ion transported.