The energy-saving effect of a new myosin activator, omecamtiv mecarbil, on LV mechanoenergetics in rat hearts with blood-perfused isovolumic contraction model

A novel myosin activator, omecamtiv mecarbil (OM), is a cardiac inotropic agent with a unique new mechanism of action, which is thought to arise from an increase in the transition rate of myosin into the actin-bound force-generating state without increasing calcium (Ca2+) transient. There remains, however, considerable controversy about the effects of OM on cardiac contractility and energy expenditure. In the present study, we investigated the effects of OM on left ventricular (LV) mechanical work and energetics, i.e., mechanoenergetics in rat normal hearts (CTL) and failing hearts induced by chronic administration of isoproterenol (1.2 mg/kg/day) for 4 weeks (ISO-HF). We analyzed the LV end-systolic pressure-volume relation (ESPVR) and the linear relation between the myocardial oxygen consumption per beat (VO2) and systolic pressure-volume area (PVA; a total mechanical energy per beat) in isovolumically contracting rat hearts at 240- or 300-bpm pacing in the absence or presence of OM. OM did not change the ESPVR in CTL and ISO-HF. OM, however, significantly decreased the slope of VO2–PVA relationship in both CTL and ISO-HF, and significantly increased the mean VO2 intercept without changes in basal metabolism in ISO-HF. These results suggested that OM improved the oxygen cost of PVA (contractile efficiency) with the unchanged LV contractility in both CTL and ISO-HF but increased VO2 for Ca2+ handling in excitation–contraction (E–C) coupling in ISO-HF. We concluded that OM improves contractile efficiency in normal and failing hearts but increases O2 consumption of Ca2+ handling in failing hearts in isovolumically contracting rat model.