Species-Dependent Limitations of Murine Myocardium for Investigating the Combined Inotropic Effects of SERCA2a Stimulation and Myofilament Ca2+ Sensitization

Impaired sarcoplasmic reticulum Ca2+ handling and myofilament function both contribute to contractile dysfunction in heart failure. A previous study in failing human myocardium showed that combining the SERCA2a activator istaroxime (ISTA) with the Ca2+ sensitizer levosimendan (LEVO) produces greater positive inotropy than either drug alone. However, human multicellular preparations did not allow comprehensive investigation of the underlying cellular mechanisms. We therefore assessed contractile performance and Ca2+ dynamics in murine multicellular myocardial preparations and ventricular cardiomyocytes treated with ISTA (20 μM), LEVO (0.5 μM), or their combination. ISTA improved developed force, contraction–relaxation kinetics, and mechanical restitution. These effects were associated with increased Ca2+ transient amplitude, accelerated Ca2+ rise and decay, and enhanced sarcoplasmic reticulum reloading. Adding LEVO produced no further mechanical benefit and only minor changes in Ca2+ handling. LEVO alone elicited heterogeneous contractile responses and increased myofilament Ca2+ sensitivity in only a minority of cardiomyocytes, while consistently accelerating Ca2+ transient decay. This effect was reproduced by the phosphodiesterase-3 inhibitor cilostamide, after which LEVO caused no significant additional change, consistent with a contribution of downstream cAMP-PKA signaling to the acceleration of Ca2+ removal. Thus, limited murine responsiveness to LEVO-mediated myofilament sensitization prevented mechanistic reconstruction of the additive inotropic effect observed in human myocardium, highlighting a species-dependent limitation of the mouse model for studying this combined therapy.