Dethiophalloidin increases Ca2+ responsiveness of skinned cardiac muscle

Phalloidin, an F-actin stabilizing peptide, is known to enhance Ca2+ responsiveness in skinned cardiac muscle. Here we studied the effects of dethiophalloidin (DTPH), a phalloidin derivative which, binding much more weakly to F-actin, on skinned bovine left ventricle muscle. When added to activated skinned muscle, DTPH (15-80 micron), similarly to phalloidin, caused a rapid (within several minutes) enhancement of active force; the relative force enhancement by DTPH became greater as Ca2+ concentration was decreased. Unlike phalloidin, DTPH effects were reversible. Using a value of the force enhancement at 15 micron DTPH (76% of maximum), an apparent equilibrium constant for DTPH binding to myofilaments was estimated at about 5 micron. Force-pCa plots showed that DTPH (80 micron) brought about a 10% increase in the maximal Ca(2+)-activated force and a 0.34 pCa units increase in the Ca2+ sensitivity. Both changes are stronger than those caused by phalloidin in similar conditions (6% and 0.2 pCa units, respectively). As with phalloidin, DTPH did not change the value of the Hill coefficient in the fit tothe force-pCa curve. DTPH and phalloidin interacted as follows: (1) pre-treatment with phalloidin entirely prevented the response to DTPH, indicating the absence of any non-specific DTPH action; and (2) when added after DTPH, phalloidin decreased the force enhancement due to DTPH, reflecting a stronger effect of DTPH to increase force. In conclusion, the stabilization of F-actin structure is not a major factor in the mechanism by which phalloidin modifies contraction.