Effect of histrionicotoxin on ion channels in synaptic and conducting membranes of electroplax of Electrophorus electricus.

Histrionicotoxin (HTX) at low concentrations of 5-10 microM blocks the postsynaptic potential of the electroplax of Electrophorus electricus. At 100-fold higher concentrations, HTX blocks the directly evoked action potentials of the conducting membrane. The pH dependence of the blockade by HTX at synaptic channels is different from that at the conducting membrane. At the synapse HTX is more potent at acid pH, while at the conducting membrane it is more potent at basic pH. HTX at high concentrations antagonizes the effects of batrachotoxin, indicative of an effect on the batrachotoxin-sensitive sodium channels involved in action potential generation. While the effects of HTX on the synaptic channels are concentration, time, and pH dependent, the effects on the channels of the conducting membrane are, in addition, use dependent, suggesting interactions of HTX with the activated forms of these channels.