Myosin binding protein-C modulates loaded sarcomere shortening in rodent permeabilized cardiac myocytes

During the ejection phase of the cardiac cycle, left ventricular (LV) cardiac myocytes undergo loaded shortening and generate power. However, few studies have measured sarcomere shortening during loaded contractions. Here, we simultaneously monitored muscle length (ML) and sarcomere length (SL) during isotonic contractions in rodent permeabilized LV cardiac myocyte preparations. In permeabilized cardiac myocyte preparations from rats, we found that ML and SL traces were closely matched, as SL velocities were within ∼77% of ML velocities during half-maximal Ca²⁺ activations. We next tested whether cardiac myosin binding protein-C (cMyBP-C) regulates loaded shortening and power output by modulating cross-bridge availability. We characterized force–velocity and power–load relationships in wildtype (WT) and cMyBP-C deficient (Mybpc3^−/−) mouse permeabilized cardiac myocyte preparations, at both the ML and SL level, before and after treatment with the small molecule myosin inhibitor, mavacamten. We found that SL traces closely matched ML traces in both WT and Mybpc3^−/− cardiac myocytes. However, Mybpc3^−/− cardiac myocytes exhibited disproportionately high sarcomere shortening velocities at high loads. Interestingly, in Mybpc3^−/− cardiac myocytes, 0.5 µM mavacamten slowed SL-loaded shortening across the force–velocity curve and normalized SL shortening velocity at high loads. Overall, these results suggest that cMyBP-C moderates sarcomere-loaded shortening, especially at high loads, at least in part, by modulating cross-bridge availability.