The N-shaped current-potential characteristic in frog skin. I. Time development during step voltage clamp.

A fast (10 musec) voltage-clamp system similar to that used on nerve axons was applied across the frog skin. An electrical analog is used to obtain the electrical parameters and to estimate the time (300 musec) required to voltage clamp the excitable membrane layer in the skin. The speed of the clamp allows observation of the early development in time of an N-shaped current-potential (I-V) relation. The isochronal I-V curves constructed from step clamp data show the beginning of a negative slope in about 250 musec after successively applied step changes in skin potential (> 200 mv). Subsequently, the negative slope reaches a quasi-steady state interval (0.4-1.5 msec) and then decays and disappears in the next 20 msec. The negative slope I-V characteristic is only found in skins which exhibit spike generation under current clamp.