Effect of phalloidin on the ATPase activity of striated muscle myofibrils

Summary

Phalloidin was shown to increase the ATPase activity and Ca²⁺ sensitivity of both bovine cardiac and rabbit psoas myofibrils when assayed in a solution containing 50 mm KCl, 100 mm MOPS (pH 7.0), 2 mm MgCl₂, 1 mm ATP, 2 mm EGTA, and varying concentrations of Ca²⁺ (temperature 21–22°C). The phalloidin effect in cardiac myofibrils developed over a time course of several minutes in the presence of 50 μm phalloidin. Relative increase of ATPase activity was maximal at pCa 8 and decreased with decrease in pCa. In cardiac myofibrils the increase was about 70% at pCa 8 and 20% at pCa 4 following 20–30 min pre-incubation with 2 μm or 50 μm phalloidin. The effect persisted after excess phalloidin was washed out. The increase in Ca²⁺ sensitivity was approximately 0.15 pCa units. For skeletal myofibrils treated with 2 μm phalloidin all changes were considerably less than those seen with cardiac myofibrils and the changes were even less when the myofibrils were exposed to 50 μm phalloidin. These results show that when specifically bound to actin, phalloidin can change the kinetic parameters of the cross-bridge cycle and may also alter the Ca²⁺ sensitivity of the contractile system. The effects of phalloidin seem to vary with muscle type.