Initial conditions and the kinetics of the sodium conductance in Myxicola giant axons. II. Relaxation experiments.

The time-course of the decay of INa on resetting the membrane potential to various levels after test steps in potential was studied. The effects of different initial conditions on these Na tail currents were also studied. For postpulse potentials at or negative to -35 mV, these currents may be attributed nearly entirely to the shutdown of the activation process, inactivation being little involved. Several relaxations may be detected in the tail currents. The slower two are well defined exponentials with time constants of approximately 1 ms and 100 mus in the hyperpolarizing potential range. The fastest relaxation is only poorly resolved. Different initial conditions could alter the relative weighting factors on the various exponential terms, but did not affect any of the individual time constants. The activation of the sodium conductance cannot be attributed to any number of independent and identical two-state subunits with first order transitions. The results of this and the previous paper are discussed in terms of the minimum kinetic scheme consistent with the data. Evidence is also presented suggesting that there may exist a small subpopulation of channels with different kinetics and a faster rate of recovery from TTX block than the rest of the population.