Na currents and action potentials in rat myelinated nerve fibres at 20 and 37° C

Abstract

(1) Action potentials and membrane currents were recorded in single myelinated rat nerve fibres at 20 and 37° C. Three experiments were also performed in single cat nerve fibres. (2) K currents were blocked by internal CsCl and external TEA. The steady state and kinetic parameters of Na activation and inactivation were determined at both temperatures. (3) When the temperature was raised from 20 to 37° C, steady state Na activation,m _∞(V), and inactivation,h _∞(V), did not change significantly. (4) The time constant of Na activation, τ_m, was determined within the potential range of −40 to 125 mV at 20° C andV=40–60 mV at 37° C. The temperature coefficient, Q₁₀, of τ_m was 2.2. (5) The decay in the Na current was described by two exponentials at both temperatures. The amplitude of the slow phase was 1–10%. The time constant of the fast phase of Na inactivation, τ_h1, was determined at both temperatures within the potential range of −50 mV to 125 mV. The Q₁₀ of τ_h1 was 2.9 and did not depend on potential. (6) The Na equilibrium potential was 152 mV at 20° C and 144 mV at 37° C. The leakage conductance was 24 nS at 20° C and 43 nS at 37° C. These differences were interpreted as signs of fibre deterioration at higher temperature. (7) The results from the current and voltage clamp experiments performed in the cat nerve were essentially the same as those in the rat nerve fibres. (8) The action potentials computed on the basis of the voltage clamp results at 20° C were similar to the ones actually measured. This was also true for those action potentials predicted for 37° C on the basis of the 20° C data, theg _L andV _Na values measured at 37° C, and the Q₁₀ values of the time constants. (9) Steady state values and kinetic parameters of K permeability were adopted from the literature. As in the experiments the influence ofP _K on the shape of the predicted action potential was almost negligible at both temperatures.