Istaroxime improves diabetic diastolic dysfunction through SERCA stimulation

Abstract

Calcium handling is generally impaired in heart failure (HF). Mechanisms that can restore cardiac relaxation improving cellular Ca2 + cycling, represent a promising therapeutic approach for HF. Istaroxime is a Na-K ATPase (NKA) inhibitor with the property of accelerating Ca2 + re-uptake into sarcoplasmic reticulum (SR) through the SR Ca2 + pump (SERCA) stimulation by displacing the interaction between SERCA and its inhibitor, phospholamban (PLB). To characterize Istaroxime effects in a model of mild diabetes (type 1) with diastolic dysfunction and preserved global systolic function. Istaroxime was tested at a concentration mostly unaffecting NKA to isolate its effects dependent on SERCA only. Streptozotocin (STZ)-treated rats were evaluated at 9 weeks after STZ injection in comparison to control (CTR) ones. STZ-induced changes were evaluated in vivo (echocardiography), in isolated left ventricular myocytes and in SERCA2a-enriched microsomes. SERCA and PLB protein levels were measured by western blot and SERCA activity as 32P hydrolysis. Action potential rate-dependency and intracellular Ca2 + handling were evaluated in patch clamped or field-stimulated (2 Hz) cells. STZ-induced cardiomyopathy was characterized by impaired diastolic relaxation which was associated to reduced SERCA protein level and activity at the cellular level. Moreover, the monomeric PLB/SERCA ratio was increased. In STZ group, action potentials (AP) were significantly prolonged at each cycle length; Ca2 + transients were characterized by slower decay, delayed onset and increased diastolic Ca2 + . Istaroxime at a concentration of 100 nM significantly stimulated SERCA activity and SR Ca re-uptake after caffeine depletion in STZ group only. Moreover, Istaroxime reduced STZ-induced diastolic Ca2 + enhancement but not affected AP prolongation. SERCA stimulation can be considered a promising therapeutic approach for diastolic dysfunction treatment.