Evidence for two types of sodium conductance in axons perfused with sodium fluoride solution

Abstract

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 ḡ_Na is a constant, m_∞ and h_∞ are steady-state values which depend only on voltage, τ_m^-1 and τ_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, τ_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, ḡ_Nam³h₁, is similar to the Hodgkin—Huxley system and underlines the usual transient increase in g_Na associated with depolarization; the second type, ḡ_Nam³h₂, is maintained with depolarization and gives rise to a steady level of g_Na.