Ca2+ Channel Activating Function of Maitotoxin, the Most Potent Marine Toxin Known, in Clonal Rat Pheochromocytoma Cells

Actions of maitotoxin, the most potent marine toxin known obtained from toxic dinoflagellate, Gambier-discus toxicus, were studied using clonal rat pheochromocytoma cells (PC12), rat liver mitochondria and liposomes. Maitotoxin induced a profound release of norepinephrine and dopamine from PC12 cells and the molar ratio of norepinephrine to dopamine was almost the same as that stored in the cells. This releasing action was apparent at a concentration of 5 X 10(-10) g/ml or more, the releasing rate increased with an increase in the concentration of applied maitotoxin and attained maximum at about 10(-6) g/ml. The [3H]norepinephrine release induced by maitotoxin was abolished in the absence of external Ca2+ and increased with increasing concentration of external Ca2+ up to 10 mM. The release gradually decreased as the external Na+ concentrations were reduced from 130 to 20 mM, but maitotoxin is still able to induce a profound release in the absence of external Na+. The releasing action of maitotoxin was markedly suppressed by various Ca2+ channel blockers, such as Mn2+, verapamil, and nicardipine, and by a local anesthetic, tetracaine. The inhibitory actions of Ca2+ channel blockers were antagonized by external Ca2+ and became less obvious in the higher Ca2+ concentration range. Maitotoxin did not exhibit any ionophoretic activities on rat mitochondrial and liposomal membranes. These results suggest that maitotoxin has the ability to activate voltage-dependent Ca2+ channels of PC12 cells.