Binding of tetrodotoxin and saxitoxin to Na+ channels at different holding potentials: fluctuation measurements in frog myelinated nerve.

The number of available Na+ channels in nodes of frog nerve fibres was determined from nonstationary Na+ current fluctuations recorded during a train of depolarizing test pulses. Mean numbers in Ringers's solution were 90,000 at a hyperpolarizing holding potential VH = -40 mV, 50,000 at the resting potential (VH = 0 mV) and 30,000 at a depolarizing holding potential VH = 30 mV. Addition of the cationic channel blockers tetrodotoxin (TTX) or saxitoxin (STX) to Ringer reduced the channel number by a factor which was independent of the holding potential. The reduction factor was 4 for 9.3 nM TTX and 3 for 3.5 nM STX. Thus, in the state of repetitive stimulation, TTX or STX blockage of Na+ channels is hardly affected by the membrane potential. Taking into account use-dependent TTX and STX effects [1], it is concluded that binding of both toxins exhibits a weak voltage dependence with toxin affinities decreasing at more negative holding potentials. The results suggest that binding of TTX and STX occurs at an external superficial receptor near the Na+ channel and that the toxin affinity of the receptor may be modulated by fast Na+ activation and slow inactivation gating processes.