Na⁺/K⁺-ATPase inhibition by ouabain induces CaMKII-dependent apoptosis in adult rat cardiac myocytes

Abstract 

The positive inotropic effect produced by Na⁺/K⁺-ATPase inhibition has been used for the treatment of heart failure for over 200years. Recently, administration of toxic doses of ouabain has been shown to induce cardiac myocyte apoptosis. However, whether prolonged administration of non-toxic doses of ouabain can also promote cardiac myocyte cell death has never been explored. The aim of this study was to assess whether non-toxic doses of ouabain can induce myocyte apoptosis and if so, to examine the underlying mechanisms. For this purpose, cardiac myocytes from rat and cat, two species with different sensitivity to digitalis, were cultured for 24h in the presence or absence of 2µM (rat) and 25nm–2µM ouabain (cat). Cell viability and apoptosis assays showed that ouabain produced, in the rat, a 43±5% decrease in cell viability due to apoptosis (enhanced caspase-3 activity, increased Bax/Bcl-2 and TUNEL-positive nuclei) and necrosis (LDH release and trypan blue staining). Similar results were obtained with 25nM ouabain in the cat. Ouabain-induced reduction in cell viability was prevented by the NCX inhibitor KB-R7943 and by the CaMKII inhibitors, KN93 and AIP. Furthermore, CaMKII overexpression exacerbated ouabain-induced cell mortality which in contrast was reduced in transgenic mice with chronic CaMKII inhibition. However, KN93 failed to affect ouabain-induced inotropy. In addition, whereas ERK½ inhibition with PD-98059 had no effect on cell mortality, PI3K inhibition with wortmannin, exacerbated myocyte death. We conclude that ouabain triggers an apoptotic cascade that involves NCX and CaMKII as a downstream effector. Ouabain simultaneously activates an antiapoptotic cascade involving PI3K/AKT which is however, insufficient to completely repress apoptosis. The finding that KN93 prevents ouabain-induced apoptosis without affecting inotropy suggests the potential use of CaMKII inhibitors as an adjunct to digitalis treatment for cardiovascular disease.

Keywords: Cardiotonic steroids, Apoptosis, CaMKII, Heart failure, Na⁺/Ca²⁺ exchanger, Na⁺/K⁺ ATPase