Initial conditions and the kinetics of the sodium conductance in Myxicola giant axons. I. effects on the time-course of the sodium conductance.

The effects of conditioning polarizations, ranging from--150 to 0 mV and of durations from 50 mus to 30 ms, on the time-course of GNa during test steps in potential were studied in Myxicola giant axons. Beyond the effects of conditioning polarizations on the amplitude of GNa, the only effect was to produce a translation of GNa(t) along the time axis without a change in shape. For depolarizing conditioning potentials, Hodgkin-Huxley kinetics predict time shifts about threefold greater than found experimentally, whereas the predictions of the coupled model of Goldman (1975. Biophys. J. 15:119--136) were in approximate agreement with our experiments. The time shifts developed over an exponential time-course as the conditioning pulse duration was increased. The time constant of development of the time shift was considerably faster than, and showed the opposite dependency on potential from, the values predicted by both models. It had a mean Q10 of 1/2.50. This fast activation process cannot account for the observed rise time behavior of GNa, suggesting that there is an additional activation process. All results are consistent with the idea that the gating structure displays more than three states, with state intermediate between rest and conducting.