Pacific ciguatoxin-1b effect over Na + and K + currents, inositol 1,4,5-triphosphate content and intracellular Ca 2+ signals in cultured rat myotubes

Abstract

1. The action of the main ciguatoxin involved in ciguatera fish poisoning in the Pacific region (P-CTX-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 mV) 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 (IP 3) mass levels, which was blocked by TTX. 6. In the presence of P-CTX-1b (10 nM) and in the absence of external Ca 2+, the intracellular Ca 2+ 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 IP 3 over internal stores activated by P-CTX-1b-induced membrane depolarization.