Rate constants associated with changes in sodium conductance in axons perfused with sodium fluoride

Membrane currents during step depolarizations were measured in axons which were perfused with 300 mM-NaF and placed in K-free artificial sea-water, -0. 3-4 degrees C. The Na conductance was fitted by the modified Hodgkin-Huxley model, g (Na) = g (Na) m (3) (h (1) + h (2) ). Changes in h (1) and h (2) were assumed to follow Formula: see text where x represents the inactive state. 2. The rate constants and steady-state values for m were in agreement with the Hodgkin-Huxley equations except that the experimental relationship of m (infinity) (3) against V was shifted 10-15 mV in the negative direction. This discrepancy, which was not found in an experiment with choline sea-water, can be explained on the basis of a resistance in series with the membrane between the voltage measuring electrodes. 3. At 0 degrees C the rate constants (in msec (-1) ) associated with changes in h (1) and h (2) were fitted using the following equations: beta (h1) = 0. 5/exp - (V + 32) /10 + D (1) exp (- V/V (1) ), alpha (h2) = pexp (V/V (2) ), beta (h2) = pexp (V/V (2) - V/23. 5) + pD (2), with the condition that at 0 mV, (alpha (h2) + beta (h2) ) = p (D (2) + 2) = 0. 55 msec (-1). The experiments gave average values D (1) = 3. 6, V (1) = 240 mV, p = 0. 08 msec (-1) and V (2) = 70 mV. The average value of g (Na) was 66 mmho/cm (2). 4. At negative voltages where m (infinity) (3) against V is steep, the points for beta (h1) and alpha (h2) /beta (h2) from axons in Na sea-water were not fitted well by the above equations whereas data from an axon in choline sea-water were. These discrepancies can be explained on the basis of a series resistance. 5. Measurements made at 16-17 degrees C indicated that g (Na) has a Q (10) of 1. 6, tau (m) (-1) a Q (10) of 2. 8 and beta (h1) a Q (10) of 3. 5. The ratio alpha (h2) /beta (h2) was decreased relative to the value at 0 degrees C and could be fitted by using Q (10) = 0. 6. 6. Measurements made with 250 mM-NaF + 50 mM-KF inside gave rate constants which were very similar to those obtained with 300 mM-NaF. Perfusion with 300 mM-KF appeared to double the value of beta (h1), relative to that obtained with 300 mM-NaF, and to reduce alpha (h2) /beta (h2) by about half. 7. The voltage dependence of alpha (h2) makes it likely that following depolarization recovery from the inactive state x occurs via x --> h (1) rather than x --> h (2) --> h (1).