Ca-dependent arrhythmogenic effects of maitotoxin, the most potent marine toxin known, on isolated rat cardiac muscle cells.

Maitotoxin (MTX) (0.1-10 ng/ml) caused an increase in the degree and the rate of contraction and subsequent arrhythmogenic actions in rat myocardial cells. These effects of MTX were inhibited by verapamil and were abolished by Ca2+-free medium. The MTX-induced effects on cardiac myocytes were enhanced by increasing the Ca2+ concentration. These results suggest that MTX elevates the permeability of cardiac muscle membrane to Ca2+ to increase the intracellular Ca2+ concentration, and thus induces excitatory effects.