Longer term effects of ouabain on the contractility of rat isolated cardiomyocytes and on the expression of Ca and Na regulating proteins

Abstract

Cardiac glycosides like ouabain are used in the therapy of heart failure and atrial fibrillation. They exert a positive inotropic effect on cardiomyocytes by inhibiting the plasma membrane sodium pump (Na,K-ATPase), decreasing the Ca-extrusion by the sarcolemmal cardiac sodium/calcium exchanger (NCX) and increasing the intracellular Ca-concentration and Ca-release during subsequent contraction cycles.The longer term effects of ouabain treatment on the expression of proteins important for Ca- and Na-homeostasis are not well known and were investigated in this study. Isolated adult rat cardiomyocytes were cultured in the presence or absence of ouabain (30 microM). In these cells, the expression of the Na,K-ATPase, Na,Ca-exchanger (NCX), the sarcoplasmic reticulum Ca-ATPase (SERCA 2a) and phospholamban (PLB) were studied by Western blot. In addition, the contractile function of these cells was studied after electrical stimulation. After 2 days of ouabain treatment immunoreactivity of the NCX was increased significantly relative to control which was set 1 (1.78 +/- 0.16 vs. 1 +/- 0.13; n = 8; P = 0.003) and at day 4 (1.96 +/- 0.35 vs. 1 +/- 0.20; n = 6; P = 0.02). All other proteins (SERCA 2a, PLB and Na,K-ATPase a1 and b1) remained unchanged (n>/= 4). Ouabain treatment increased the fractional shortening of isolated cardiomyocytes at day 0 (1 Hz: 9.64 +/- 0.73 %, n = 24, vs. 7.18 +/- 0.60 %; n = 21; P = 0.01), whereas at day 2 the contractility was unchanged (1 Hz: 7.23 +/- 1.08 %, n = 9 vs. 7.70 +/- 0.63 %; n = 10, P = 0.71). The inhibition of SERCA 2a (10 microM cyclopiazonic acid (CPA)) decreased contractility in both the ouabain treated group and in controls, at day 0 and at day 2. These results show that chronic ouabain treatment increases the protein expression of the NCX. The positive inotropic effect of ouabain can no longer be observed after a chronic treatment for 2 days. Thus, both protein expression and contractile function of the cells are specifically altered by longer term cardiac glycoside exposure. Whether such regulation can be found in human cardiomyocytes and the resulting consequences in the clinical setting remain to be determined.

Ouabain induces cardiac remodeling in rats independent of blood pressure

“…However, there are data suggesting that long-term administration of ouabain has no effect on myocardial contractility in rats. According to Muller-Ehmsen et al [27] , chronic ouabain treatment increases the protein expression of the NCX, and the positive inotropic effect can no longer be observed after chronic treatment for 2 d. Further more, NCX overexpression impairs ouabain-dependent cell shortening in adult rat cardiomyocytes [28] . In this study, chronic ouabain treatment for 4 or 6 weeks might induce NCX overexpression, and myocardial contractility might be impaired subsequently; this may be one of the ways through which ouabain affects systolic performance.…”

Inhibition of Na/K-ATPase promotes myocardial tumor necrosis factor-alpha protein expression and cardiac dysfunction via calcium/mTOR signaling in endotoxemia

“…In cardiomyocytes, Na/K-ATPase activity is inhibited by phospholemman [22] and glutathionylation induced by NADPH oxidase [13]. In the heart, Na/K-ATPase modulates cardiomyocyte apoptosis [47], contractility [3,39] and hypertrophy [25] via Ca 2? -dependent mechanisms.…”

Ouabain-induced hypertension enhances left ventricular contractility in rats

“…Studies that investigated cardiac changes on long-term ouabain treatment were performed only during 2 or 4 days in isolated adult cardiomyocytes (Müller-Ehmsen et al, 2003) and papillary muscles (ElArmouche et al, 2004). Moreover, the increment in blood pressure induced by ouabain treatment does not develop within this time.…”