Gating current kinetics in Myxicola giant axons. Order of the back transition rate constants

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Gating current, Ig, was recorded in Myxicola axons with series resistance compensation and higher time resolution than in previous studies. Ig at ON decays as two exponentials with time constants, tau ON-F and tau ON-S, very similar to squid values. No indication of an additional very fast relaxation was detected, but could be still unresolved. Ig at OFF also displays two exponentials, neither reflecting recovery from charge immobilization. Deactivation of the two I(ON) components may proceed with well-separated exponentials at -100 mV. INa tail currents at OFF also display two exponentials plus a third very slow relaxation of 5-9% of the total tail current. The very slow component is probably deactivation of a very small subpopulation of TTX sensitive channels. A -100 mV, means for INa tail component time constants (four axons) are 76 microseconds (range: 53-89 microseconds) and 344 microseconds (range: 312-387 microseconds), and for IOFF (six axons) 62 microseconds (range: 34-87 microseconds) and 291 microseconds (range: 204-456 microseconds) in reasonable agreement. INa ON activation time constant, tau A, is clearly slower than tau ON-F at all potentials. Except for the interval -30 to -15 mV, tau A is clearly faster than tau ON-S, and has a different dependency on potential. tau ON-S is several fold smaller than tau h. Computations with a closed2----closed1----open activation model indicated Na tail currents are consistent with a closed1----open rate constant greater than the closed2----closed1.