We have determined that Ca2+ does not alter the affinity of bovine cardiac myosin subfragment-1 (S-1) for actin regulated by cardiac troponin-tropomyosin. Under conditions where the addition of Ca-2+ mediated a 27-fold stimulation of the actin-activated MgATPase rate, Ca-2+ effected negligible change in the binding between the two proteins producing this ATP hydrolysis, actin and myosin S-1. This suggests that the predominant mechanism producing cardiac contraction is not Ca-2+ facilitated binding of myosin cross-bridges to the actin filament. Rather, regulation of cardiac contraction primarily depends upon control of other parts of the cross-bridge cycle. We have also investigated the MgATPase of cardiac myosin S-1 and regulated actin at intermediate Ca-2+ concentrations. The MgATPase rate increases very cooperatively with increasing Ca-2+ concentration. This cooperatively appears to reside solely in interactions between adjacent troponin-tropomyosin complexes along the actin filament.
