Fumonisin, a toxin from the fungus Fusarium moniliforme sheld, blocks both the calcium current and the mechanical activity in frog atrial muscle.

The effects of fumonisin, a toxin isolated from the fungus Fusarium moniliforme sheld were studied on transmembrane potentials and currents of the frog heart muscle using the double sucrose gap technique. Fumonisin (280 microM) shortened the plateau duration of the action potential. Under voltage clamp conditions, fumonisin inhibited the Ca current. The block occurred without alteration of either the kinetic parameters or the apparent reversal potential of the current suggesting that the toxin blocked the maximal Ca conductance. Fumonisin reduced mainly the phasic component of the peak tension. The time to peak tension was unchanged whereas its relaxation phase was accelerated at positive membrane potentials suggesting that Na-Ca exchange was accelerated by fumonisin. Dose-response curves for the Ca current and the peak tension indicated half inhibition at about 100 microM fumonisin and a stoichiometric parameter of 1.7, suggesting that more than one toxin molecule interacted with the Ca channel. The data suggested that the effect of fumonisin on the Ca current and the tension might account for cardiac failure reported in animal intoxication by Fusarium moniliforme.