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1. Voltage clamp experiments were carried out on squid giant axons internally perfused with 300 mM-NaF + sucrose. K-free artificial sea-water, -0·3 to 3·5° C, was used externally.

2. Membrane currents were corrected for capacitative and leakage components, and the resulting Na current was converted to Na conductance, g_Na. An attempt was made to fit changes in g_Na according to the Hodgkin—Huxley model, namely [Formula: see text].

According to the model _Na is a constant, m and h are steady-state values which depend only on voltage, _m^-1 and _h^-1 are rate constants which also are functions only of voltage.

3. Stepwise depolarizations from the holding potential (-67 to -83 mV) to a potential which varied from -10 to +63 mV resulted in an exponential decline of h from its initial level to a final, non-zero level. If the test depolarization was preceded by a positive prepulse (duration, 19-105 msec; voltage, -6 to 94 mV) the rate constant for h, _h^-1, was increased roughly threefold with practically no change in the final level.

4. The steady-state level of h was studied by using prepulses of varying amplitude followed by a test depolarization. In one such experiment a value of 0·34 was obtained for a 105 msec prepulse to -49 mV. The same value for the steady level of h was obtained from analysing a record taken at +52 mV. If the potential was switched from -49 to +52 mV there was a transient increase in g_Na although h had the same value at these two potentials.

5. Recovery from depolarization was studied by repolarizing the fibre for varying lengths of time, then applying a test depolarization. If the first depolarization was strongly positive (for example, 70 mV), so that the steady level of h was large (0·39), the currents associated with the test pulse could not be fitted on the basis of an exponential increase in h during the recovery period. Rather, the results suggested that on repolarization h rapidly decreased initially, then slowly increased.

6. These results can be explained by assuming that h is given by the sum of two components, h₁ and h₂. Changes are represented kinetically by h₁ x h₂, where x signifies the inactive state. The distribution is shifted to the left at negative potentials and to the right for positive ones. The resulting Na conductance is comprised of two types: the first type, _Nam³h₁, is similar to the Hodgkin—Huxley system and underlines the usual transient increase in g_Na associated with depolarization; the second type, _Nam³h₂, is maintained with depolarization and gives rise to a steady level of g_Na.

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