A role for C-protein in the regulation of contraction and intracellular Ca2+ in intact rat ventricular myocytes

1. C-protein is a major component of muscle thick filaments whose function is unknown. We have examined for the first time the role of the regulatory binding domain of C-protein in modulating contraction and intracellular Ca2 + concentration ([Ca2+](i)) in intact cardiac myocytes. 2. Rat ventricular myocytes were reversibly permeabilised with the pore-for ming toxin streptolysin O. Myosin S2 (which binds to the regulatory domain of C-protein) was introduced into cells during permeabilisation to compete with the endogenous C-protein-thick filament interaction. 3. Introduction of S2 into myocytes increased contractility by similar to 3 0%, significantly lengthened the time to peak of the contraction and the ti me to half-relaxation,but had no effect on [Ca2+](i) transient amplitude. 4. Our data are consistent with increased myofilament Ca2+ sensitivity when there is reduced binding of C-protein to myosin near the head-tail junctio n. 5. We propose that the effects of introducing S2 into intact cardiac cells can be equated with the consequences of selectively phosphorylating C-prote in in vivo, and that the regulation of contraction by C-protein is mediated by the effects of crossbridge cycling on the Ca2+ affinity of troponin C.