BACKGROUND AND PURPOSECalcium handling is known to be deranged in heart failure. Interventions aimed at improving cell Ca 2+ cycling may represent a promising approach to heart failure therapy. Istaroxime is a new luso-inotropic compound that stimulates cardiac contractility and relaxation in healthy and failing animal models and in patients with acute heart failure (AHF) syndrome. Istaroxime is a Na-K ATPase inhibitor with the unique property of increasing sarcoplasmic reticulum (SR) SERCA2a activity as shown in heart microsomes from humans and guinea pigs. The present study addressed the molecular mechanism by which istaroxime increases SERCA2a activity. EXPERIMENTAL APPROACHTo study the effect of istaroxime on SERCA2a-phospholamban (PLB) complex, we applied different methodologies in native dog healthy and failing heart preparations and heterologous canine SERCA2a/PLB co-expressed in Spodoptera frugiperda (Sf21) insect cells. KEY RESULTSWe showed that istaroxime enhances SERCA2a activity, Ca 2+ uptake and the Ca 2+ -dependent charge movements into dog healthy and failing cardiac SR vesicles. Although not directly demonstrated, the most probable explanation of these activities is the displacement of PLB from SERCA2a.E2 conformation, independently from cAMP/PKA. We propose that this displacement may favour the SERCA2a conformational transition from E2 to E1, thus resulting in the acceleration of Ca 2+ cycling. CONCLUSIONS AND IMPLICATIONSIstaroxime represents the first example of a small molecule that exerts a luso-inotropic effect in the failing human heart through the stimulation of SERCA2a ATPase activity and the enhancement of Ca 2+ uptake into the SR by relieving the PLB inhibitory effect on SERCA2a in a cAMP/PKA independent way. LINKED ARTICLE AbbreviationsAHF, acute heart failure; CPA, cyclopiazonic acid; +dP/dt, rate of LV pressure rise; −dP/dt, rate of LV pressure decay; LV, left ventricle; PLB, phospholamban; SERCA, sarcoplasmic reticulum Ca 2+

The seven patients have recessive nemaline myopathy caused by absence of alpha-skeletal muscle actin. The level of retention of alpha-cardiac actin, the skeletal muscle fetal actin isoform, may determine alpha-skeletal muscle actin disease severity. This has implications for possible future therapy.

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