Ouabain worsens diastolic sarcomere length in myocytes from a cardiomyopathy mouse model

Diastolic dysfunction is a major feature of hypertrophic cardiomyopathy (HCM). Data from patient tissue and animal models associate increased Ca²⁺ sensitivity of myofilaments with altered Na⁺ and Ca²⁺ ion homeostasis in cardiomyocytes with diastolic dysfunction. In this study, we tested the acute effects of ouabain on ventricular myocytes of an HCM mouse model. The effects of ouabain on contractility and Ca²⁺ transients were tested in intact adult mouse ventricular myocytes (AMVMs) of Mybpc3-targeted knock-in (KI) and wild-type (WT) mice. Concentration-response assessment of contractile function revealed low sensitivity of AMVMs to ouabain (10 μM) compared to literature data on human cardiomyocytes (100 nM). Three hundred μM ouabain increased contraction amplitude (WT ~1.8-fold; KI ~1.5-fold) and diastolic intracellular Ca²⁺ in both WT and KI (+12-18%), but further decreased diastolic sarcomere length in KI cardiomyocytes (-5%). Western Blot analysis of whole heart protein extracts revealed 50% lower amounts of Na⁺/K⁺ ATPase (NKA) in KI than in WT. Ouabain worsened the diastolic phenotype of KI cardiomyocytes at concentrations which did not impair WT diastolic function. Ouabain led to an elevation of intracellular Ca²⁺, which was poorly tolerated in KI showing already high cytosolic Ca²⁺ at baseline due to increased myofilament Ca²⁺ sensitivity. Lower amounts of NKA in KI could amplify the need to exchange excessive intracellular Na⁺ for Ca²⁺ and thereby explain the general tendency to higher diastolic Ca²⁺ in KI.