Kinetics of grayanotoxin evoked modification of sodium channels in squid giant axons.

Kinetics of modification of the sodium channel by alpha-dihydrograyanotoxin II (GTX) were studied with voltage-clamped squid giant axons. GTX modified the channel to generate sustained inward current, only when the membrane was kept depolarized to levels more positive than -80mV, in a voltage-dependent manner, increasing the depolarization. Repetitive depolarizing pulses suppressed rather than increased the degree of GTX-evoked modification. GTX-evoked modification proceeded with a dual exponential time course, regardless of the presence or absence of the inactivation gate, but the elimination by pronase of the inactivation gate accelerated GTX-evoked modification. GTX unbound from the sodium channel with a time constant of 30 s at -150 mV in a manner independent of the concentration. The effective concentration that produced a half-maximal sustained sodium current (EC50), which represents GTX-modified channel activity, was estimated to be about 10 microM with one-to-one stoichiometry. The activation/voltage relationship for the sustained sodium current was shifted in the hyperpolarizing direction by as much as 63-94 mV compared with that of peak sodium current. At a GTX concentration of 100 microM and at +20mV, 64% of the sodium channel population was modified. A kinetics model is proposed to account for the behavior of GTX -modified sodium channels.