Kinetics of activation of the sodium conductance in the squid giant axon.

The time course of the rise in sodium conductance during positive voltage-clamp pulses was measured in squid giant axons perfused with CsF and immersed in low-sodium solutions. The initial transients were eliminated by subtraction of records made after blocking the sodium channels with tetrodotoxin. The value of tau m as defined by Hodgkin & Huxley (1952) passed through a well defined maximum at a membrane potential of about -35 mV. On fitting the initial inflexion in the rise of INa to the expression mXh instead of m3h, the value of X was found to vary from axon to axon between 2.9 and 4.4, with an average of 3.5. For any given axon, X did not vary significantly with pulse potential. Measurements of tau m were made on approaching each value of the membrane potential both from the negative and from the positive side. The cube law kinetics of the Hodgkin-Huxley equations were closely obeyed. Application of a negative prepulse to -180 mV delayed the rise of conductance by 20 musec at 7 degrees C without obviously changing tau m. Comparisons of the voltage dependence of tau m with that of the time constant tau 1 of the fast relaxation of the asymmetry current measured in the same axon, showed that tau 1 was smaller than tau m except at positive potentials, was less steeply voltage-dependent, and reached its maximum at a more positive potential.