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Regulation of in vivo cardiac contractility by phospholemman: role of Na+/Ca2+ exchange

By JuFang Wang, Erhe Gao, Joseph Rabinowitz, Jianliang Song, Xue-Qian Zhang, Walter J. Koch, Amy L. Tucker, Tung O. Chan, Arthur M. Feldman and Joseph Y. Cheung

Abstract

Phospholemman (PLM), when phosphorylated at serine 68, relieves its inhibition on Na+-K+-ATPase but inhibits Na+/Ca2+ exchanger 1 (NCX1) in cardiac myocytes. Under stress when catecholamine levels are high, enhanced Na+-K+-ATPase activity by phosphorylated PLM attenuates intracellular Na+ concentration ([Na+]i) overload. To evaluate the effects of PLM on NCX1 on in vivo cardiac contractility, we injected recombinant adeno-associated virus (serotype 9) expressing either the phosphomimetic PLM S68E mutant or green fluorescent protein (GFP) directly into left ventricles (LVs) of PLM-knockout (KO) mice. Five weeks after virus injection, ∼40% of isolated LV myocytes exhibited GFP fluorescence. Expression of S68E mutant was confirmed with PLM antibody. There were no differences in protein levels of α1- and α2-subunits of Na+-K+-ATPase, NCX1, and sarco(endo)plasmic reticulum Ca2+-ATPase between KO-GFP and KO-S68E LV homogenates. Compared with KO-GFP myocytes, Na+/Ca2+ exchange current was suppressed, but resting [Na+]i, Na+-K+-ATPase current, and action potential amplitudes were similar in KO-S68E myocytes. Resting membrane potential was slightly lower and action potential duration at 90% repolarization (APD90) was shortened in KO-S68E myocytes. Isoproterenol (Iso; 1 μM) increased APD90 in both groups of myocytes. After Iso, [Na+]i increased monotonically in paced (2 Hz) KO-GFP but reached a plateau in KO-S68E myocytes. Both systolic and diastolic [Ca2+]i were higher in Iso-stimulated KO-S68E myocytes paced at 2 Hz. Echocardiography demonstrated similar resting heart rate, ejection fraction, and LV mass between KO-GFP and KO-S68E mice. In vivo closed-chest catheterization demonstrated enhanced contractility in KO-S68E compared with KO-GFP hearts stimulated with Iso. We conclude that under catecholamine stress when [Na+]i is high, PLM minimizes [Na+]i overload by relieving its inhibition of Na+-K+-ATPase and preserves inotropy by simultaneously inhibiting Na+/Ca2+ exchanger

Topics: Muscle Mechanics and Ventricular Function
Publisher: American Physiological Society
OAI identifier: oai:pubmedcentral.nih.gov:3064295
Provided by: PubMed Central
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