The Hyperpolarizing Region of the Current-voltage Curve in Frog Skin

Excitability (action potential and refractory period) has been described by A. Finkelstein in the depolarizing region of the current-voltage (I-V) curve of the isolated frog skin. Recently Fishman and Macey interpreted this phenomenon as a consequence of a region with negative resistance that confers to the I-V curve an N shape. We have studied the I-V relation of the isolated frog skin in the hyperpolarizing region with a current-ramp system. It was found that in Na(2)SO(4) Ringer's, the resistance continuously increases in the hyperpolarizing direction. When hyperpolarization reaches 300 mv an electrical breakdown occurs, occasionally followed by a region of negative resistance. In NaCl Ringer's the breakdown was also found although the I-V relation was reasonably linear. Unidirectional Na(+) outflux was measured at different levels of voltage clamping across the skin and with different Na(+) concentrations in the solutions. The Na(+) outflux was found to be relatively independent of these parameters. Based on these results a Na(+) rectifying structure is postulated. An electrical model for active Na(+) transport including a diode and an oscillator is proposed. The effects of CO(2), nitrogen, amiloride, and ouabain on the I-V relation are described.