Effects of aconitine and batrachotoxin on Na currents and gating currents in the frog node of Ranvier.

The effects of aconitine and batrachotoxin (BTX) on Na currents and gating currents were studied on voltage clamped frog nodes of Ranvier. The relation between steady-state values of charge movement and test pulse potential ( Q-E) was compared with the fraction of open channels at a given potential ( F-E). In aconitine-treated nodes the half potential, E, of the F-E curve was −87 mV and the slope factor, k, was 6.6 mV. Compared to normal nodes aconitine shifted the F-E curve by −53 mV along the voltage axis without significantly altering the slope. The parameters of the Q- E curve of aconitine-treated nodes were E=−63 mV and k=18.8 mV compared with E=−30 mV and k=17 mV in a normal node. For the Q- E curve E was −76 mV, fairly close to E of the F-E curve, but the slope remained much smaller than that of the F-E curve. Thus, in aconitine-treated nodes, the normalized Q- E and Q- E curves stay below the F-E curve over almost the whole potential range, a situation different from that in untreated or chloramine-T-treated nodes (Drews 1987). Similar results were obtained with BTX. The F-E curve of BTX-treated fibres had a half potential E=−85.1 mV and a slope factor k=4.7 mV. In contrast to aconitine-treated nodes, the F-E curve showed a secondary rise at E>−50 mV. Compared to the F-E curve the half potential of the Q- E curve was more positive ( E=−49.2 mV) and the slope was smaller ( k=18.7 mV, similar to k=17 mV measured in normal nodes). Even when Q was used instead of Q the slope of the two curves remained very different. By contrast, Dubois et al. (1983) found that both curves coincide after BTX treatment. I could reproduce this result by using pulses of relatively long duration (4 or 15 ms). However, my experiments suggest that such long pulses are likely to produce artefacts in both the on- and off-response. Q-E and F-E curves similar to those observed on aconitine- or BTX-treated nodes can be calculated from the models of Neumcke et al. (1978) and Edmonds (1987 a, b). [ABSTRACT FROM AUTHOR]