The effect of physiological and pharmacological inotropic interventionson the regulatory state of the thick filament in intact cardiac trabeculae.

Abstract

In the cardiac muscle in diastole myosin motors are packed on the surface of the thick filament in three-stranded helical tracks, folded toward the centre of the sarcomere and unable to bind actin and hydrolyse ATP (OFF state; Woodhead et al. 2005, Nature 436:1195). In the X-ray diffraction pattern of intact trabeculae from rat heart this OFF state is marked by the intensity of the corresponding myosin-based layer lines and axial reflections (Reconditi et al. 2017, PNAS 114:3240). In trabeculae electrically paced at 0.5 Hz (temperature 27 °C), during diastole none of the X-ray signals related to the OFF state of the myosin motors are affected by inotropic interventions which increase by twofold the systolic peak force, like increase in sarcomere length (1.95-2.25 μm) or addition of 10−7 M isoprenaline, indicating that these inotropic interventions acts downstream with respect to Ca2+-activation (Caremani et al. 2019, J. Gen. Physiol. 151:53). Instead, the addition of the cardiac activator omecamtiv mecarbil (OM, 1 μM), a small molecule that binds to the catalytic domain of cardiac myosin and has been found to increase the systolic ejection fraction in animal models (Malik et al. 2011, Science 331:1439) and the Ca2+ sensitivity of force in demembranated cardiac myocytes (Nagy et al. 2015, Brit. J. Pharmacol. 172:4506), induces a partial switching ON of motors in diastole, mainly at the expense of the filament regions outside the central 1/3 containing the Myosin Binding Protein C. The specific effect that different inotropic interventions exert on the regulatory state of the thick filament in diastole reflects on the different mechanisms by which these interventions potentiate the systolic performance. Supported by ECR, University of Florence and MUR (Italy) and ESRF (France).