Combined Action of Intraaxonal Iodate and External Sea Anemone Toxin ATX II on Sodium Channel Inactivation of Frog Nerve Fibres

Voltage clamp experiments were done on single myelinated nerve fibres of the frog, Rana esculenta, with 10 mM TEA+ in the external solutions to block potassium channels. Iodate (20, 40 or 100 mM KIO3) was applied to the axoplasmic side of the nodal membrane by diffusion from a cut internode. The effect of 20 mM started within a few minutes and reached a stationary value after ca. 20 min which was maintained for another 15 min. The size of the effect was independent of the iodate concentrations tested. Iodate action could not be reversed even after only a 2-min application. When the effect was fully established, iodate increased the faster time constant of inactivation ca. 1.2 times, the slower one ca. 1.7 times. Iodate also induced a persistent (for seconds) INa component that was, at the end of a 15-ms pulse (I15ms), 6% of the early peak INa of the control. Experiments with conditioning prepulses revealed a non-monotonic h infinity -V curve in iodate with finite h infinity values throughout. Increasing [Ca2+]0 from 2 to 10 mM shifted peak INa (V) and I15ms (V) by 10-15 mV to more positive potentials. In contrast, as shown in previous experiments, I15ms induced by sea anemone toxin ATX II was nearly abolished (h infinity approximately equal to 0) for 60 less than V less than 80 mV on increasing [Ca2+]0.(ABSTRACT TRUNCATED AT 250 WORDS)