Abstract 1578: Myosin Binding Protein C is Essential for Beta-adrenergic Mediated Acceleration of Cardiac Relaxation
Beta-adrenergic enhancement of cardiac function is mediated by an increase in the intracellular calcium [Ca2+]in transient and improved myofilament performance in response to changes in [Ca2+]in. Cardiac myosin binding protein C (cMyBP-C), which is a thick filament accessory protein, regulates crossbridge kinetics via PKA phosphorylation. Previous experiments on skinned myocardium from cMyBP-C knock-out (KO) mice showed that ablation of cMyBP-C constitutively accelerated crossbridge kinetics to rates equivalent to wild-type (WT) myocardium following beta-adrenergic stimulation. KO hearts also demonstrate reduced contractile reserve during dobutamine treatment. Thus, we hypothesized that the absence of cMyBP-C limits myofilament responsiveness to beta-adrenergic stimulation. We tested this possibility with force and [Ca2+]in measurements on intact papillary muscles paced at 3 Hz −/+ 1uM dobutamine. At baseline, KO myocardium shows accelerated kinetics compared to WT, evidenced by a faster decay to 50% of peak twitch force despite a slower decay in [Ca2+]in. KO also exhibits a similar (−dF/dt)Max/(+dF/dt)Max ratio as WT at baseline, indicating equivalent rates of relaxation when scaled to the rate of force generation. Dobutamine increased (−dF/dt)Max/(+dF/dt)Max in WT but not in KO. The absence of effect in KO occurs even though the decay in [Ca2+]in is accelerated. Also, phosphoprotein staining shows that dobutamine increases troponin I (TnI) phosphorylation to similar levels in both KO and WT; therefore, the absence of cMyBP-C prevented acceleration of relaxation by beta-adrenergic stimulation. Together, these results indicate that cMyBP-C is necessary for acceleration of relaxation either through direct phosphorylation of cMyBP-C or by slowing baseline cross-bridge kinetics to unmask possible effects of TnI phosphorylation to accelerate kinetics.