Pacific ciguatoxin-1b effect over Na+ and K+ currents, inositol 1,4,5-triphosphate content and intracellular Ca2+ signals in cultured rat myotubes

1 The action of the main ciguatoxin involved in ciguatera fish poisoning in the Pacific region (PCTX-1b) was studied in myotubes originated from rat skeletal muscle cells kept in primary culture. 2 The effect of P-CTX-1b on sodium currents at short times of exposure (up to 1 min) showed a moderate increase in peak Na+ current. During prolonged exposures, P-CTX-1b decreased the peak Na+ current. This action was always accompanied by an increase of leakage currents, tail currents and outward Na+ currents, resulting in an intracellular Na+ accumulation. This effect is blocked by prior exposure to tetrodotoxin (TTX) and becomes evident only after washout of TTX. 3 Low to moderate concentrations of P-CTX-1b (2-5 nm) partially blocked potassium currents in a manner that was dependent on the membrane potential. 4 P-CTX-1b (2 - 12 nm) caused a small membrane depolarization (3 - 5 nV) and an increase in the frequency of spontaneous action potential discharges that reached in general low frequencies (0.1 - 0.5 Hz). 5 P-CTX-1b (10 nm) caused a transient increase of intracellular inositol 1,4,5-trisphosphate (IP3) mass levels, which was blocked by TTX. 6 In the presence of P-CTX-1b (10 nm) and in the absence of external Ca2+, the intracellular Ca2+ levels show a transient increase in the cytoplasm as well as in the nuclei. The time course of this effect may reflect the action of IP3 over internal stores activated by P-CTX-1b-induced membrane depolarization.