Use-dependent block with tetrodotoxin and saxitoxin at frog Ranvier nodes. I. Intrinsic channel and toxin parameters

The use-dependent phasic blockage of sodium channels by tetrodotoxin (TTX) and saxitoxin (STX) was examined in frog nodes of Ranvier using trains of depolarizing pulses. The decline of the peak Na+ current from its initial value (I0) before the train to a stationary value (I infinity) after the train was more pronounced at more negative holding potentials. The relationship between I infinity/I0 and holding potential was fitted by a sigmoid function which yielded values for the steepness of the voltage dependencies of around -15 mV for TTX and -8 mV for STX. Similar values were obtained at toxin concentrations of 4 and 8 nM. The higher voltage sensitivity of STX versus TTX is interpreted in terms of the higher charge and the faster binding kinetics of STX. These differences also explain the frequency dependence of the decline of Na+ currents with STX (between 0.5 and 2 Hz) and the frequency independence with TTX. Variation of the pulse amplitude in a train of conditioning pulses revealed that the magnitude of the use-dependent actions of STX parallels the steady-state Na+ inactivation curve h infinity. Inhibition of inactivation, by pre-treatment with chloramine-T, did not, however, abolish the use dependence. Instead, it introduced a change in the time constants of the decline of the Na+ currents and the magnitude became independent of the holding potential.