Batrachotoxin decreases the sensitivity of sodium channels to the blocking action of phenobarbital.

In earlier experiments on frog myelinated nerves it was found that the steroidal alkaloid batrachotoxin (BTX) considerably decreases the affinity of Na channel binding sites to various amine blockers (local anesthetics, antiarrythmics, strych-nine, yohimbine) (for review see Khodorov 1978; 1981) as well to oenanthotoxin (Dubois and Khodorov 1982) however, the blocking action of neutral benzocaine (Zaborovskaya and Khodorov 1982) and butanol (Dubois and Khodorov 1982) was not appreciably affected. The aim of the present work was to examine the effect of BTX on Na channel blockage by phenobarbital (PB): the latter drug lacks amine groups and presents both neutral and anionic forms when dissolved. Experiments were carried out on frog (Rana ridibunda) myelinated nerves under voltage clamp conditions. Fig. IA illustrates a typical experiment with the administration of 2.5 and 5 mmol/1 PB to the node of Ranvier pretreated with 10 5 mol/1 BTX. The first "hump" of the current-voltage curve reflects the voltage dependence of stationary Na currents, I Na , in BTX-modified channels. The second "hump" corresponds to peak J Na in Na channels escaping modification by BTX ("normal" channels). Net conductances, g N a, were calculated from these curves and plotted against membrane potential, E (Fig. 1B,C). It can be seen that BTX protected Na channels from the blocking action of PB: it decreased the maximum g Na (G Nil) in population of "normal" channels, leaving G Na of modified channels unaffected. By contrast, another effect of PB, the concentration-dependent shift of g Nd-E curve to more positive E values, was principally similar in both the "normal" and modified channels. A similar shift of the g Na —E relation under the action of PB could be observed in normal (nontreated) fibres (Bolotina et al. 1982). It is tempting to suppose that a decrease in gf Na would result from an interaction of PB (apparently in neutral form) with the Na channel binding site, while the positive shift in g Na curve is due to an effect of the anionic form of PB, present in the axoplasm, on the inner surface potential of the nodal membrane. Note that neither amine local anestetics nor benzocaine, butanol or oenanthotoxin induce such a shift in the g Nd —E relation. The similarity in the protective action of BTX on Na channels treated with