Modumodulation by Inotropic Interventions of the Regulatory State of the Cardiac Thick Filament in Diastole

Abstract

X-ray diffraction from electrically paced intact trabeculae of rat heart (frequency 0.5 Hz, temperature 28°C) shows that in diastole almost all the myosin motors lie in three stranded helical tracks on the surface of the thick filament, unavailable for actin attachment and ATP splitting (OFF state) and that they are switched ON during systole in relation to the loading condition (Reconditi et al. PNAS 114:3240, 2017). Inotropic interventions able to potentiate up to twofold the peak of the isometric twitch force in physiological solution with 1 mM Ca2+, such as increase in sarcomere length from 1.95 to 2.22 μm and addition of the β-adrenergic effector isoprenaline (10-7 M) do not affect the OFF state of the myosin motors in diastole, solidifying the idea that in the intact myocyte myosin motors are switched ON only during systole by an energetically well-suited downstream mechanism as thick filament mechano-sensing (Caremani et al., J. Gen. Physiol. 151:53, 2019). Here we show that, in electrically paced intact trabeculae mounted at the beamline ID02 of the European Synchrotron, the putatively inotropic agent Omecamtiv Mecarbil (1 μM), which in vitro prolongs the actin-bond lifetime while preventing the working stroke (Woody et al., Nature Commun. 9:3838, 2018) and in demembranated myocytes increases the Ca2+ sensitivity of the filaments (Nagy et al., Brit. J. Pharmacol. 172:4506, 2015), significantly affects all the signals marking the OFF state of the thick filament in diastole. Specifically, the intensity of the 1st myosin layer line due to the three stranded helical symmetry of the motors in the OFF state is reduced to ∼60% of the control, indicating that ∼20% the myosin motors are switched ON. Supported by Cassa di Risparmio di Firenze (2016-2018), Italy.