Myocardial Na+,K+-ATPase in tachycardia induced cardiomyopathy

Na+,K+-ATPase is a major determinant of myocyte homeostasis and excitation-contraction. Cardiac glycosides such as digitalis and ouabain increase the inotropic state of the heart through the inhibition of Na+,K+-ATPase. While cardiac glycosides are commonly used in the setting of congestive heart failure, optimal therapy would depend upon an intact Na+,K+-ATPase system. Changes in Na+,K+-ATPase activity and glycoside receptor density with the development of cardiomyopathy have not been well defined. Accordingly, left ventricular (LV) function and Na+,K+-ATPase activity and glycoside binding were examined in 7 pigs with dilated cardiomyopathy and in 7 controls. Dilated cardiomyopathy was produced by pacing induced supraventricular tachycardia (SVT) for 3 weeks at 240 bpm. Left ventricular function was examined by simultaneous echocardiography and catheterization. Left ventricular fractional shortening significantly decreased with SVT (34 ± 2 vs. 10 ± 2%, P<0.05) and LV diastolic dimension and pressure significantly increased (3.8 ± 0.3 vs. 5.1 ± 0.4 cm, and 8 ± 2 vs. 27 ± 2 mmHg, respectively, P<0.05) as compared to controls. Na+,K+-ATPase activity was assayed as potassium dependent p-nitrophenol-phosphatase activity. Glycoside receptor density (Bmax) and affinity (KD) was determined using [3H]-ouabain binding assays. Na+,K+-ATPase activity, Bmax, and KD all significantly fell from control values with SVT induced cardiomyopathy (0.64 ± 0.06 vs. 0.45 ± 0.12 μg pNP/mg/h, 5.5 ± 0.4 vs. 1.9 ± 0.4 pmol/mg, and 15 ± 3 vs. 9 ± 3 nm, respectively, P<0.05). The distribution of Na+,K+-ATPase in LV sections taken from control and SVT hearts were examined using immunohistochemical techniques. A patchy distribution of Na+,K+-ATPase along the sarcolemma in SVT sections was observed as opposed to a more uniform distribution in control myocytes. There was no observable change in the relative content and distribution of the Na+,K+-ATPase isoforms α2 and α3 in the SVT sections as compared to controls. In an additional set of experiments, changes in LV as well as isolated myocyte responsiveness to ouabain were examined. Left ventricular fractional shortening and peak were measured following administration of 20–60 μg/Kg of ouabain in control (n = 3) and SVT (n = 3) pigs. In the control group, 40 μg/Kg caused a 25% in LV fractional shortening and a 60% increase in peak from baseline. Cumulative doses of 60 μg/Kg in the control pigs resulted in over a 75% increase in peak from baseline values. Administration of 40 μg/Kg of ouabain produced only a 16% and 20% increase in LV fractional shortening and peak in the SVT group respectively; both significantly lower than controls (P<0.05). In contrast to controls, cumulative doses of 60 μg/Kg in the SVT group were not tolerated and resulted in irreversible ventricular fibrillation. Myocyte contractile responsiveness to cumulative doses of ouabain (0.05–8 μm) was examined in cardiocytes isolated from 3 additional control and SVT hearts. In control myocytes, 2 μm ouabain produced over a 50% increase and 4 μm caused over a 100% increase in velocity of shortening from baseline values. In contrast, there was no significant increase in the velocity of shortening of SVT myocytes at any ouabain concentration. In summary, SVT induced cardiomyopathy caused a reduction in glycoside receptor density and Na+,K+-ATPase activity. These changes were associated with an attenuated LV and myocyte responsiveness to ouabain as well as increased sensitivity to the toxic effects of this cardiac glycoside. These changes in Na+,K+-ATPase activity and glycoside responsiveness may have important implications in the treatment of dilated cardiomyopathies.