Abstract
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. The MTX-induced inotropic effect was almost abolished by Co2+ or verapamil, but was little affected by propranolol, reserpine or tetrodotoxin. 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. 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. 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.