TAIL CURRENTS IN THE MYELINATED AXON OF XENOPUS-LAEVIS SUGGEST A 2-OPEN-STATE NA CHANNEL

Na tail currents in the myelinated axon of Xenopus laevis were measure d at -70 mV after steps to -10 mV. The tail currents were biexponentia l, comprising a fast and a slow component, The time constant of the sl ow tail component, analyzed in the time window 0.35-0.50 ms, was indep endent of step duration, and had a value of 0.23 ms, The amplitude, ex trapolated back to time 0, varied, however, with step duration. It rea ched a peak after 0.7 ms and inactivated relatively slowly (at 2.1 ms the absolute value was reduced by similar to 30%), The amplitude of th e fast component, estimated by subtracting the amplitude of the slow c omponent from the calculated total tail current amplitude, reached a p eak (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 similar t o 65%). The results were explained by a novel Na channel model, compri sing two open states bifurcating from a common closed state and with s eparate inactivating pathways. A voltage-regulated use of the two path ways explains a number of findings reported in the literature.