Abstract 1580: Phosphorylation of Myosin Binding Protein C Contributes to the Force-Frequency Response and Beta-Adrenergic Enhancement of Cardiac Contraction

Abstract

Beta-adrenergic agonists enhance cardiac function by increasing contractility, lusitropy and heart rate. We propose that phosphorylation of myosin binding protein C (cMyBP-C), a thick filament accessory protein, plays a key role in adrenergic modulation of function by regulating crossbridge interaction kinetics. To test this idea, we developed transgenic mice expressing similar levels of either cMyBP-C with Ser to Ala mutations at 3 known protein kinase A (PKA) sites (t3SA mice) or wild type cMyBP-C (tWT mice) on the cMyBP-C knock-out (KO) background. Using echocardiography, t3SA hearts were found to have concentric hypertrophy and exhibited diastolic and systolic dysfunction, but tWT hearts were normal. We recorded twitch force and intracellular Ca 2+ ([Ca 2+ ] in ) in intact papillary muscles to assess in vivo myofilament function. At 1 Hz stimulation, t3SA and tWT exhibited similar twitch forces and [Ca 2+ ] in . Increasing the frequency to 3 Hz produced a smaller force increase in t3SA but similar increases in [Ca 2+ ] in compared to tWT. t3SA myocardium exhibited a smaller gain in function with increased frequency of stimulation, i.e., a smaller increase in twitch force vs. change in [Ca 2+ ] in and smaller increases in (+dF/dt) Max , and (−dF/dt) Max . These results suggest that basal levels of phosphorylation of cMyBP-C in tWT myocardium contribute to the positive force frequency relationship in myocardium. For both t3SA and tWT, 1 μM dobutamine further increased twitch force and [Ca 2+ ] in at 3 Hz. Phosphoprotein staining showed that t3SA and tWT troponin I (TnI) phosphorylation levels were initially similar and increased to similar levels with dobutamine; therefore, dobutamine effects may involve TnI phosphorylation. These results can be explained by a frequency dependent recruitment of myofilament functional groups that involves cMyBP-C phosphorylation and further effects of PKA phosphorylation of cMyBP-C and TnI to accelerate crossbridge turnover kinetics.