Tail currents in the myelinated axon of Xenopus laevis suggest a two-open-state Na channel.
Na tail currents in the myelinated axon of Xenopus laevis were measured at -70 mV after steps to -10 mV. The tail currents were biexponential, comprising a fast and a slow component. The time constant of the slow tail component, analyzed in the time window 0.35-0.50 ms, was independent of step duration, and had a value of 0.23 ms. The amplitude, extrapolated back to time 0, varied, however, with step duration. It reached a peak after 0.7 ms and inactivated relatively slowly (at 2.1 ms the absolute value was reduced by approximately 30%). The amplitude of the fast component, estimated by subtracting the amplitude of the slow component from the calculated total tail current amplitude, reached a peak (three times larger than that of the slow component) after 0.5 ms and inactivated relatively fast (at 2.1 ms it was reduced by approximately 65%). The results were explained by a novel Na channel model, comprising two open states bifurcating from a common closed state and with separate inactivating pathways. A voltage-regulated use of the two pathways explains a number of findings reported in the literature.
Elinder
F
F
Nobel Institute for Neurophysiology, Karolinska Institutet, Stockholm, Sweden.
Arhem
P
P
D000818:Animals; D001369:Axons / Q000502:physiology*; D008564:Membrane Potentials; D008954:Models, Biological; D009413:Nerve Fibers, Myelinated / Q000502:physiology*; D012584:Sciatic Nerve / Q000502:physiology*; D015222:Sodium Channels / Q000502:physiology*; D013696:Temperature; D013997:Time Factors; D014982:Xenopus laevis
D015222:Sodium Channels