Phosphorylation of Myosin Binding Protein-C Alters the Proximity of Cross-Bridges to Actin and Accelerates Myocardial Twitch Kinetics

Abstract

The strength and kinetics of cardiac contraction vary on a beat-to-beat basis in efforts to match cardiac output in response to changing circulatory demands. In living myocardium, the beta-adrenoreceptor agonist dobutamine initiates protein kinase A (PKA)-mediated phosphorylations of Ca2+ handling proteins and contractile proteins including cardiac myosin binding protein-C (cMyBP-C) and cardiac troponin I (cTnI), which leads to potentiation of twitch force and faster kinetics of force development and relaxation. Our previous studies in skinned myocardium suggest that PKA phosphorylation of cMyBP-C disrupts its interaction with myosin subfragment 2 (S2), which relieves the tether-like constraint of myosin heads imposed by cMyBP-C, and thereby accelerates cross-bridge cycling kinetics. To examine the relative role of cMyBP-C phosphorylation in altering twitch kinetics, we recorded twitch force and low-angle x-ray diffraction patterns in between twitches and near maximum twitch force in intact trabeculae isolated from murine myocardium electrically stimulated at 0.5 Hz in the presence and absence of dobutamine. Our data suggest that phosphorylation of cMyBP-C caused a radial or azimuthal displacement of cross-bridges towards the thin filament in vivo prior to the twitch, which contributes to the accelerated contraction kinetics following twitch stimulation. These results suggest that interactions between cMyBP-C and the S2-domain of myosin heavy chain are dynamically regulated by phosphorylation of cMyBP-C and function to modulate the availability and cooperative binding of cross-bridges to actin during the myocardial twitch.