Ventricular performance and Na+-K+ ATPase activity are reduced early and late after myocardial infarction in rats

ISSN 0100-879X Volume 42 (10) 870-992 October 2009 BIOMEDICAL SCIENCES AND CLINICAL INVESTIGATION www.bjournal.com.br Braz J Med Biol Res, October 2009, Volume 42(10) 902-911 Ventricular performance and Na+-K+ ATPase activity are reduced early and late after myocardial infarction in rats I. Stefanon, J.R. Cade, A.A. Fernandes, R.F. Ribeiro Junior, G.P. Targueta, J.G. Mill and D.V. Vassallo The Brazilian Journal of Medical and Biological Research is partially financed by Institutional Sponsors Campus Ribeirão Preto Faculdade de Medicina de Ribeirão Preto Brazilian Journal of Medical and Biological Research (2009) 42: 902-911 ISSN 0100-879X Ventricular performance and Na+-K+ ATPase activity are reduced early and late after myocardial infarction in rats I. Stefanon1, J.R. Cade1, A.A. Fernandes1, R.F. Ribeiro Junior1, G.P. Targueta1, J.G. Mill1 and D.V. Vassallo1,2 1Departamento de Ciências Fisiológicas, Universidade Federal do Espírito Santo, Vitória, ES, Brasil 2Departamento de Ciências Fisiológicas, Escola Superior de Ciências da Santa Casa de Misericórdia de Vitória, Vitória, ES, Brasil Abstract Myocardial infarction leads to compensatory ventricular remodeling. Disturbances in myocardial contractility depend on the active transport of Ca2+ and Na+, which are regulated by Na+-K+ ATPase. Inappropriate regulation of Na+-K+ ATPase activity leads to excessive loss of K+ and gain of Na+ by the cell. We determined the participation of Na+-K+ ATPase in ventricular performance early and late after myocardial infarction. Wistar rats (8-10 per group) underwent left coronary artery ligation (infarcted, Inf) or sham-operation (Sham). Ventricular performance was measured at 3 and 30 days after surgery using the Langendorff technique. Left ventricular systolic pressure was obtained under different ventricular diastolic pressures and increased extracellular Ca2+ concentrations (Ca2+e) and after low and high ouabain concentrations. The baseline coronary perfusion pressure increased 3 days after myocardial infarction and normalized by 30 days (Sham 3 = 88 ± 6; Inf 3 = 130 ± 9; Inf 30 = 92 ± 7 mmHg; P < 0.05). The inotropic response to Ca2+e and ouabain was reduced at 3 and 30 days after myocardial infarction (Ca2+ = 1.25 mM; Sham 3 = 70 ± 3; Inf 3 = 45 ± 2; Inf 30 = 29 ± 3 mmHg; P < 0.05), while the Frank-Starling mechanism was preserved. At 3 and 30 days after myocardial infarction, ventricular Na+-K+ ATPase activity and contractility were reduced. This Na+-K+ ATPase hypoactivity may modify the Na+, K+ and Ca2+ transport across the sarcolemma resulting in ventricular dysfunction. Key words: Heart failure; Ouabain; Na+-K+ ATPase; Myocardial infarction; Calcium handling Introduction Myocardial infarction induces a progressive geometric, structural, and functional remodeling of both ventricles (1-4). Several adaptive changes resulting from the myocardium infarction, including myocyte hypertrophy (5), increased deposition of extracellular matrix components (6), and chamber enlargement (7,8), seem to depend either on hemodynamic disturbances determined by the loss of contractile tissue (9) or on the neurohumoral activation triggered by myocardial ischemia (10-12). Studies in intact and isolated preparations have suggested that contractile abnormalities may depend on changes in transient calcium concentrations (13) and on the decreased Ca2+ sensitivity in the contractile machinery of the ventricular myocardium (4,14). The influences of the coronary vasculature and its perfusion on the diastolic properties of the heart and on cardiac contraction have also been well established. Coronary perfusion affects the diastolic ventricular properties through changes in the lengthwise stiffness of the coronary vasculature. Under low oxygen conditions, an increase in perfusion enhances cardiac muscle contractility via the increased oxygen supply (15). However, the best described disturbances in myocardial contractility are intimately dependent on the active transport of ions, namely Ca2+ and Na+, which are regulated by SERCA and Na+-K+ ATPase. Na+-K+-ATPase is a heteromeric protein consisting of α and β subunits. While the α subunit contains the amino acids involved in catalytic function, ion transport and cardiac glycoside binding, the function of the β subunit is not yet fully understood, although it is essential for the normal activity of the enzyme and is involved in the transport of the functional Na+-K+ ATPase in the plasma membrane (16). Ouabain binds to Correspondence: I. Stefanon, Departamento de Ciências Fisiológicas, Universidade Federal do Espírito Santo, Centro de Ciências da Saúde, Av. Marechal Campos, 1468, 29040-093 Vitória, ES, Brasil. Fax: +55-27-3335-7340. E-mail: Research supported by CNPq and FAPES/FUNCITEC. Received November 28, 2008. Accepted June 26, 2009. Available online September 4, 2009. Braz J Med Biol Res 42(10) 2009 www.bjournal.com.br Reduced Na+-K+ ATPase activity after myocardial infarction Na+-K+ ATPase at a site formed in the extracellular part of the catalytic α subunit by the H1-H2, H3-H4, and H5-H6 loops. Reversible interaction of cardiotonic steroids with this site may induce a conformational change in the Na+/K+ ATPase protein. In the active Na+-K+ ATPase, cardiotonic steroids are fixed leading to the enzyme’s inactivation. In cardiac myocytes, ouabain induces an intracellular increase of Na+ concentration, resulting from inhibition of the Na+K+ ATPase, subsequently increasing the intracellular Ca2+ concentration via Na+/Ca2+ exchanger inhibition, causing a positive inotropic effect (17). Thus, inappropriate regulation of Na+-K+ ATPase activity, which leads to excessive loss of K+ from the cell and gain of Na+ into the cell, could result in disturbances in Ca2+ signaling and contractility. The Na+-K+ ATPase has a binding site for endogenous cardiac glycosides (18,19). It has been proposed that an increase in endogenous ouabain levels could be involved in the pathogenesis of congestive heart failure (20). Furthermore, although inhibition of Na+-K+ ATPase by cardiac glycosides is well documented and increases in intracellular Na+ and Ca2+ have been observed in the presence of cardiotonic steroids, a direct connection between a reduction in Na+-K+ ATPase activity and contractility in the heart failure following myocardial infarction (MI) has not been demonstrated. Since levels of endogenous digitalis-like factor increase in the plasma of humans (18,19) after MI and during heart failure, we designed this study to assess the exogenous effect of ouabain on heart contractility performance early and late after myocardial infarction in rats. Since rodents exhibit an ouabain-insensitive α1-isoenzyme (21), we perfused a higher concentration of ouabain in the isolated heart (50 nM) than usually observed in human plasma during heart failure (18). Our hypothesis was that a decrease in Na+-K+ ATPase activity could contribute to the reduced myocardial cont (...truncated)