Tetrodotoxin and manganese ion: effects on action potential of the frog heart

Tetrodotoxin, which effectively suppresses the permeability of various tissues to sodium ions, has no effect on the calcium action potential of the fiber of barnacle muscle, which potential is produced by increase in permeability to calcium ions. Manganese ions, however, suppress the action potential. When applied to fiber of frog cardiac ventricle, tetrodotoxin suppresses the rate of rise of the action potential without affecting the overshoot; the suppressive effect of manganese ion is mainly on the overshoot of the action potential. This suggests that, in the action potential of the car-diac ventricle of the frog, the platealu phase is related primarily to the increase in permeability of the membrane to calcium ions.