The mechanism of action of maitotoxin in relation to Ca2+ movements in guinea‐pig and rat cardiac muscles

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Abstract
1 Maitotoxin (MTX), the most potent marine toxin known, produced a dose-dependent positive inotropic effect on guinea-pig isolated left atria and rat ventricle strips at concentrations of 0.1 ng to 4 ng ml−1. MTX (4 ng ml−1) also exhibited a positive chronotropic effect on guinea-pig right atria. 2 The MTX-induced inotropic effect was almost abolished by Co2+ or verapamil, but was little affected by propranolol, reserpine or tetrodotoxin. 3 The tissue Ca content of guinea-pig left atria was increased by MTX (2–30 ng ml−1) in a dose-dependent manner, and this increase was markedly inhibited by Co2+ or verapamil. 4 Furthermore, on the rat isolated cardiac myocytes MTX (0.01–10 ng ml−1) caused an arrhythmogenic effect which was followed by their transformation into irreversibly rounded cells. The effects of MTX on the isolated cells were inhibited by verapamil or Ca2+-free solution. 5 These results suggest that the excitatory effects of MTX on heart muscle are caused by a direct action on the cardiac muscle membrane mainly due to an increase in Ca2+ permeability.

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