Abstract
The synthetic ciguatoxin, CTX3C enabled us to explore the physiological effects of ciguatoxin, scarcely obtained as a natural product, on Na+ channels. We have previously reported, 1) the toxin speeds up time to peak, 2) activates Na+ channels at hyperpolarized potentials (shifts the activation curve) and 3) slows the recovery from “slow inactivation”. Site-2 toxins, such as grayanotoxin or batrachotoxin are known to similarly activate Na+ channels. In the present study, therefore, as a step toward identifying CTX binding sites, we tested whether the residues in Nav1.4, providing binding sites for Site-2 toxins, also influence CTX3C affinity. In addition to Site-2 residues, we explored some residues in D4S5, which had been previously recognized as a binding region by a photo affinity labeling technique. The CTX3C sensitivity of mutants for Site2-toxins (I433A, F1579A, F1586A) or those at D4S5, (F1483A or F1492A) were similar to that of wild type channels in all respects. However, one mutant of Site2-resides located at D1S6 (N434A) was resistant to the effect of CTX3C on channel activation: CTX3C (3 µM) did not shift the activation curve in the hyperpolarizing direction, while preserving other effects, such as effects on “slow inactivation”. These results indicate the residue N434 is involved in the mechanism utilized by CTX3C to enhance activation kinetics of Na channels. [Jpn J Physiol 55 Suppl:S127 (2005)]
