Effect of phoratoxin B, a toxin isolated from mistletoe, on frog skeletal muscle fibres

The effect of phoratoxin B on transmembrane potentials and currents of frog skeletal muscle was studied using intracellular microelectrode recording and double sucrose gap voltage clamp techniques. Phoratoxin B irreversibly depolarized the membrane. The depolarization was insensitive to tetrodotoxin, tetraethylammonium, 4-amino-pyridine, cesium, cadmium and D-600. In voltage clamp experiments, phoratoxin B induced an inward resting current (Irest) which did not inactivate. Irest was blocked by increasing the external calcium concentration in the Ringer solution; it was not blocked when Sr2+ replaced Ca2+. Analysis of the peak sodium current indicated that while both Ca2+ and Sr2+ screen membrane surface charges, Ca2+ bound and reversed the phoratoxin B induced Irest whereas Sr2+ did not. The inward Irest induced by phoratoxin B developed in the presence of external chloride and remained unchanged in the presence of bicarbonate. The data suggest that the depolarizing action of the toxin might be attributed to an increase in the non-selective leak current and that it might act as a detergent.