GATING CURRENT KINETICS IN MYXICOLA GIANT-AXONS - ORDER OF THE BACK TRANSITION RATE CONSTANTS

Gating current, l(g), was recorded in Myxicola axons with series resistance compensation and higher time resolution than in previous studies. l(g) 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. l(g) at OFF also displays two exponentials, neither reflecting recovery from charge immobilization. Deactivation of the two l(ON) components may proceed with well-separated exponentials at - 100 mV. l(Na) 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. At - 100 mV, means for l(Na) tail component time constants (four axons) are 76-mu-s (range: 53-89-mu-s) and 344-mu-s (range: 312-387-mu-s), and for l(OFF) (six axons) 62-mu-s (range: 34-87-mu-s) and 291-mu-s (range: 204-456-mu-s) in reasonable agreement. I(Na) 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 half-arrow-pointing-both-left-and-right closed1 half-arrow-pointing-both-left-and-right open activation model indicated Na tail currents are consistent with a closed1 half-arrow-point-both-left-and-right open rate constant greater than the closed2 half-arrow-pointing-both-left-and-right closed1.