Omecamtiv mecarbil augments cardiomyocyte contractile activity both at resting and systolic Ca2+ levels.

AIMS: Heart failure with reduced ejection fraction (HFrEF) is a disease with high mortality and morbidity. Recent positive inotropic drug developments focused on cardiac myofilaments, that is, direct activators of the myosin molecule and Ca2+ sensitizers for patients with advanced HFrEF. Omecamtiv mecarbil (OM) is the first direct myosin activator with promising results in clinical studies. Here, we aimed to elucidate the cellular mechanisms of the positive inotropic effect of OM in a comparative in vitro investigation where Ca2+ -sensitizing positive inotropic agents with distinct mechanisms of action [EMD 53998 (EMD), which also docks on the myosin molecule, and levosimendan (Levo), which binds to troponin C] were included. METHODS: Enzymatically isolated canine cardiomyocytes with intact cell membranes were loaded with Fura-2AM, a Ca2+ -sensitive, ratiometric, fluorescent dye. Changes in sarcomere length (SL) and intracellular Ca2+ concentration were recorded in parallel at room temperature, whereas cardiomyocyte contractions were evoked by field stimulation at 0.1 Hz in the presence of different OM, EMD, or Levo concentrations. RESULTS: SL was reduced by about 23% or 9% in the presence of 1 µM OM or 1 µM EMD in the absence of electrical stimulation, whereas 1 µM Levo had no effect on resting SL. Fractional sarcomere shortening was increased by 1 µM EMD or 1 µM Levo to about 152%, but only to about 128% in the presence of 0.03 µM OM. At higher OM concentrations, no significant increase in fractional sarcomere shortening could be recorded. Contraction durations largely increased, whereas the kinetics of contractions and relaxations decreased with increasing OM concentrations. One-micromole EMD or 1 µM Levo had no effects on contraction durations. One-micromole Levo, but not 1 µM EMD, accelerated the kinetics of cardiomyocyte contractions and relaxations. Ca2+ transient amplitudes were unaffected by all treatments. CONCLUSIONS: Our data revealed major distinctions between the cellular effects of myofilament targeted agents (OM, EMD, or Levo) depending on their target proteins and binding sites, although they were compatible with the involvement of Ca2+ -sensitizing mechanisms for all three drugs. Significant part of the cardiotonic effect of OM relates to the prolongation of systolic contraction in combination with its Ca2+ -sensitizing effect.