Secophalloidin as a novel activator of skinned cardiac muscle

Secophalloidin (SPH) is known to activate skinned cardiac muscle in the absence of Ca²⁺. We hypothesized that SPH-induced changes in cross-bridge properties underlie muscle activation. We found that force responsiveness to orthovanadate was drastically reduced in SPH activated muscles compared to Ca²⁺-activated contraction. Moreover, SPH caused ∼30% increase in Ca²⁺-independent force in muscles where Ca²⁺ sensitivity was totally destroyed by troponin I extraction with 10mM vanadate. Thus, SPH and Ca²⁺ activation differ in both properties of the cross-bridge cycle and protein requirements for thin filament regulation. In addition, we tested the relationship between the activating effects SPH and EMD 57033, a Ca²⁺ sensitizer that increases resting force in cardiac muscle. After maximal activation by either SPH or EMD 57033, the other compound was found to further increase force, indicating that SPH activates muscle via a novel mechanism.