The Pool of Free Veratridine Molecules

Since the binding hypothesis assumes the channel modification to be due to de novo drug binding from a pool of free alkaloid molecules (Leibowitz et al. 1986), the location of such a pool is of particular interest. Pertinent points are (a) the reactions to fast application and washout of veratridine, 

The superfused node of Ranvier is particularly suited to fast perfusate changes. As mentioned above, when veratridine (4.5 or 45 pM at pH 8.1) is suddenly applied to the moderately depolarized nodal membrane, an inward current develops exponentially, with a time constant. Ton of 3.8 and 

respectively. Early (after 6 s) washout of these concentrations causes the current to disappear exponentially, but unexpectedly with a time constant, Toff, that depends on the previously applied [VT], 5.6 and 7.5 s, respectively. Longer applications are best monitored by measuring tail currents at the end of 2-s depolarizing pulses elicited every 5 s (see also 

It remains, however, unclear why only the longer time constants behave as those of a reaction. Diffusion through the membrane and intracellular accumulation is also thought to account for the onset of veratridine action on cardiomyocytes, with a time constant of about 1 min (20°C Zong et al. 

In crayfish giant axons most veratridine effects persist, but the maintained current at the holding potential between pulses during a train vanishes on washout (Warashina 1985). Only partial recovery is reported for externally applied alkaloid to squid giant axons (Ohta et al. 1973), but when this preparation is perfused internally, full reversibility is achieved, possibly because washing on either side of the membrane was feasible (Meves 1966). In this connection it is interesting to note that externally applied veratridine (100 M at 5°C) slowly but markedly depolarizes non-perfused squid axons whereas depolarization is much less if at the same time the axons are internally perfused and thus washed with toxin-free solution (Seyama et al. 1988). 

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