Cellular Mechanisms Underlying the Low Cardiotoxicity of Istaroxime

Background Istaroxime is an inhibitor of Na⁺/K⁺ ATPase with proven efficacy to increase cardiac contractility and to accelerate relaxation attributable to a relief in phospholamban-dependent inhibition of the sarcoplasmic reticulum Ca²⁺ ATPase. We have previously shown that pharmacologic Na⁺/K⁺ ATPase inhibition promotes calcium/calmodulin-dependent kinase II activation, which mediates both cardiomyocyte death and arrhythmias. Here, we aim to compare the cardiotoxic effects promoted by classic pharmacologic Na⁺/K⁺ ATPase inhibition versus istaroxime. Methods and Results Ventricular cardiomyocytes were treated with ouabain or istaroxime at previously tested equi-inotropic concentrations to compare their impact on cell viability, apoptosis, and calcium/calmodulin-dependent kinase II activation. In contrast to ouabain, istaroxime neither promoted calcium/calmodulin-dependent kinase II activation nor cardiomyocyte death. In addition, we explored the differential behavior promoted by ouabain and istaroxime on spontaneous diastolic Ca²⁺ release. In rat cardiomyocytes, istaroxime did not significantly increase Ca²⁺ spark and wave frequency but increased the proportion of aborted Ca²⁺ waves. Further insight was provided by studying cardiomyocytes from mice that do not express phospholamban. In this model, the lower Ca²⁺ wave incidence observed with istaroxime remains present, suggesting that istaroxime-dependent relief on phospholamban-dependent sarcoplasmic reticulum Ca²⁺ ATPase 2A inhibition is not the unique mechanism underlying the low arrhythmogenic profile of this drug. Conclusions Our results indicate that, different from ouabain, istaroxime can reach a significant inotropic effect without leading to calcium/calmodulin-dependent kinase II-dependent cardiomyocyte death. Additionally, we provide novel insights regarding the low arrhythmogenic impact of istaroxime on cardiac Ca²⁺ handling.