The novel cardiac myosin activator omecamtiv mecarbil increases the calcium sensitivity of force production in isolated cardiomyocytes and skeletal muscle fibres of the rat.

<bold>Background and Purpose: </bold>Omecamtiv mecarbil (OM) is a novel cardiac myosin activator drug for inotropic support in systolic heart failure. Here we have assessed the concentration-dependent mechanical effects of OM in permeabilized cardiomyocyte-sized preparations and single skeletal muscle fibres of Wistar-Kyoto rats under isometric conditions.<bold>Experimental Approaches: </bold>Ca2+ -dependent active force production (Factive ), its Ca2+ sensitivity (pCa50 ), the kinetic characteristics of Ca2+ -regulated activation and relaxation, and Ca2+ -independent passive force (Fpassive ) were monitored in Triton X-100-skinned preparations with and without OM (3nM-10 μM).<bold>Key Results: </bold>In permeabilized cardiomyocytes, OM increased the Ca2+ sensitivity of force production (ΔpCa50 : 0.11 or 0.34 at 0.1 or 1 μM respectively). The concentration-response relationship of the Ca2+ sensitization was bell-shaped, with maximal effects at 0.3-1 μM OM (EC50 : 0.08 ± 0.01 μM). The kinetics of force development and relaxation slowed progressively with increasing OM concentration. Moreover, OM increased Fpassive in the cardiomyocytes with an apparent EC50 value of 0.26 ± 0.11 μM. OM-evoked effects in the diaphragm muscle fibres with intrinsically slow kinetics were largely similar to those in cardiomyocytes, while they were less apparent in muscle fibres with fast kinetics.<bold>Conclusions and Implications: </bold>OM acted as a Ca2+ -sensitizing agent with a downstream mechanism of action in both cardiomyocytes and diaphragm muscle fibres. The mechanism of action of OM is connected to slowed activation-relaxation kinetics and at higher OM concentrations increased Fpassive production. [ABSTRACT FROM AUTHOR]