Abstract 9592: The Super Relaxed State of Myosin Contributes to the Pathogenesis of Right Ventricular Dysfunction in Pulmonary Hypertension

Abstract

Introduction: Patients with pulmonary hypertension (PH) often succumb to right ventricular (RV) failure yet little is known regarding the underlying pathological mechanisms. Recently, the transition of myosin from the disordered relaxed (DRX) to the super relaxed (SRX) state during diastole has been identified as a pathological feature contributing to myocardial dysfunction in heart failure. Using synchrotron radiation (SR) x-ray diffraction (SAXS), the proximity of myosin heads relative to actin in the in situ beating rat heart can be evaluated in different layers of the myocardial wall. We performed SR SAXS at different timepoints post-PH induction with simultaneous PV-loop acquisition to assess if SRX transition contributes to progressive global RV dysfunction in a rat model of PH. Methods: PH was induced in rats using the Sugen5416/3-week 10% hypoxia method, with rats being returned to normoxia for 3- (SuHx3wk) or 6-weeks (SuHx6wk) after the hypoxic period. SR SAXS was performed at BL40XU, SPring-8 Synchrotron, Japan. Global and site-specific RV myofilament phosphorylation status was also examined. Results: Compared to control rats (n=7), SuHx3wk (n=7) and SuHx6wk (n=7) rats exhibited progressive increases in RV size, RV end systolic pressure (ESP), RV dP/dt max and dP/dt min (all P<0.05 vs. Control). In the deeper subendocardial layer of the RV free wall, there were virtually no myosin heads within the proximity of actin at end diastole in SuHx6wk rats, whereas in the control and SuHx3wk rats, around 30-50% of myosin heads were in proximity to actin (P<0.05, SuHx6wk vs. Control and SuHx3wk). No significant differences in diastolic sarcomere length were found, suggesting a transition of myosin from DRX to SRX in the SuHx6wk rats. Importantly, indices for global RV function (ESP, dP/dt max, dP/dt min ) significantly correlated with diastolic myosin mass transfer values. Analysis of RV myofilament phosphorylation revealed a progressive and significant reduction in MLC-2v phosphorylation over the time course of PH in rats. Conclusion: PH is associated with progressive changes in global RV function, which may be attributed to a transition from DRX to SRX state of myosin in the RV. Reduced MLC-2v phosphorylation may underlie the DRX to SRX transition in PH.