Effect of batrachotoxin on the electroplax of electric eel: evidence for voltage dependent interaction with sodium channels

Batrachotoxin under certain conditions has a strong depolarizing effect on the innervated membrane of the monocellular electroplax preparation from the electric eel, Electrophorus electricus. No effect is observed when the toxin (50-200 nM) is applied to the resting membrane for periods up to 1 hr. However, if the membrane is exposed to batrachotoxin and the cell is subjected to stimulation at a stimulus voltage slightly above the threshold for action potential firing, a progressive prolongation of the action potential and concomitant progressive depolarization of the innervated membrane is observed. When the membrane is depolarized by 15-20 mV, a further abrupt all or none depolarization occurs, and the potential attains a steady state value between 0 and -10 mV. Brief stimulation of a cell in the presence of batrachotoxin is sufficient to define a batrachotoxin treated cell, even though negligible depolarization occurs. If depolarizing agents such as carbamoylcholine or potassium chloride are introduced to such a cell in concentrations that normally produce a 20-30 mV depolarization, the abrupt all or none depolarization immediately occurs. All or none depolarizations arising from either electrical stimulation or depolarizing agents are unaffected by d tubocurarine but are completely reversed by tetrodotoxin. Batrachotoxin thus appears to activate only the action potential sodium channels. In the batrachotoxin treated membrane, these channels can attain stable steady states in either a closed configuration at the normal resting potential or in an open configuration after complete depolarization. A striking hysteresis cycle thus can be generated, which is strongly indicative of a voltage dependent interaction of the toxin with the action potential sodium channels.