Normal hearts increase contractile force in response to mechanical stretch caused by increasing volume. Although this phenomenon has been extensively studied from myofilaments spacing perspective, possible coupling of mechanical-sensing signaling to modulation of myofilament function remains unknown. Cardiac myosin binding protein-C (MyBPC3) is a component of heart muscle thick filament. Phosphorylation of MyBPC3 releases its inhibition on cross-bridge cycling to increase cardiac contractility. Thus, we postulate that mechanical stretch of the myocardium causes phosphorylation of MyBPC3 to increase contractility. We tested this hypothesis by performing static stretch of 20% from baseline on cultured neonatal rat cardiac myocytes (NRCM) at durations of 2, 5, 15, 30, and 60 minutes. NRCM culture provides the advantage of cells living in an environment free of adrenergic stimulation to avoid catecholamine stimulation mediated phosphorylation of MyBPC3. Site-specific phospho-serine antibodies were used to detect phosphorylation of rat equivalent of S282 and S302 of mouse MyBPC3. We used MyBPC3 antibody made from a different species than site-specific phospho-serine antibodies to account for loading. S282P transiently peaked after 5 minutes of stretch, whereas S302P continued to increase with time through 60 minutes (see figure ). Consequently, our data show that mechanical stretch alone can cause phosphorylation of MyBPC3 as a mechanism that couples different signaling pathways to myofilament function.
