Changes in cardiac contractility related to calcium-mediated changes in phosphorylation of myosin-binding protein C

Ca ions can influence the contraction of cardiac muscle by activating kinas es that specifically phosphorylate the myofibrillar proteins myosin-binding protein C (MyBP-C) and the regulatory light chain of myosin (RLC). To inve stigate the possible role of Ca-regulated phosphorylation of MyBP-C on cont raction, isolated quiescent and rhythmically contracting cardiac trabeculae were exposed to different concentrations of extracellular Ca and then chem ically skinned to clamp the contractile system. Maximum Ca-activated force (F-max) was measured in quiescent cells soaking in 1) 2.5 mM Ca for 120 min , 2) 1.25 mM for 120 min, or 3) 1.25 mM for 120 min followed by 10 min in 7 .5 mM, and 4) cells rhythmically contracting in 2.5 mM for 20 min. F-max wa s, respectively, 21.5, 10.5, 24.7, and 32.6 mN/mm(2). Changes in F-max. wer e closely associated with changes in the degree of phosphorylation of MyBP- C and occurred at intracellular concentrations of Ca below levels associate d with phosphorylation of RLC. Monophosphorylation of MyBP-C by a Ca-regula ted kinase is necessary before beta -adrenergic stimulation can produce add itional phosphorylation. These results suggest that Ca-dependent phosphoryl ation of MyBP-C modulates contractility by changing thick filament structur e.