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

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Abstract
The time-course of the decay of INa [ionic Na] on resetting the membrane potential to various levels after test steps in potential was studied. The effects of different initial conditions of these Na tail currents were also studied. For postpulse potentials at or negative to -35 mV, these currents may be attributed almost entirely to the shutdown of the activation process. Several relaxations may be detected in the tail currents, the slower 2 well-defined exponentials with time constants of .apprx. 1 ms and 100 .apprx. 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 Na conductance cannot be attributed to any number of independent and identical 2-state subunits with 1st-order transitions. There may exist a small subpopulation of channels with different kinetics and a faster rate of recovery from TTX [tetrodotoxin] block than the rest of the population.