Published online before print March 13, 2003, doi: 10.1161/01.RES.0000066661.49920.59
© 2003 American Heart Association, Inc.
Cellular Biology |
Chronic SR Ca2+-ATPase Inhibition Causes Adaptive Changes in Cellular Ca2+ Transport
From the Department of Pharmacology and Cell Biophysics (A.G.B., G.C., A.G.S., E.G.K.), College of Medicine, University of Cincinnati, Cincinnati, Ohio; Department of Physiology (K.S.G., D.M.B.), Loyola University Chicago, Stritch School of Medicine, Maywood, Ill; Department of Cell Biology/Molecular Medicine (A.Y.), UMDNJ RWJ, Newark, NJ; Department of Medicine (B.M.W.), University of Illinois at Chicago, Chicago, Ill; and Banting and Best Department of Medical Research (M.A., D.H.M.), University of Toronto, Toronto, Ontario, Canada.
Correspondence to Evangelia G. Kranias, PhD, Department of Pharmacology and Cell Biophysics University of Cincinnati College of Medicine, 231 Albert Sabin Way, PO Box 670575, Cincinnati, OH 45267-0575. E-mail Litsa.Kranias{at}uc.edu
Phospholamban, the critical regulator of the cardiac SERCA2a Ca2+ affinity, is phosphorylated at Ser16 and Thr17 during ß-adrenergic stimulation (eg, isoproterenol). To assess the impact of nonphosphorylatable phospholamban, a S16A, T17A double-mutant (DM) was introduced into phospholamban knockout mouse hearts. Transgenic lines expressing DM phospholamban at levels similar to wild types (WT) were identified. In vitro phosphorylation confirmed that DM phospholamban could not be phosphorylated, but produced the same shift in EC50 of SERCA2a for Ca2+ as unphosphorylated WT phospholamban. Rates of basal twitch [Ca2+]i decline were not different in DM versus WT cardiomyocytes. Isoproterenol increased the rates of twitch [Ca2+]i decline in WT, but not DM myocytes, confirming the prominent role of phospholamban phosphorylation in this response. Increased L-type Ca2+ current (ICa) density, with unaltered characteristics, was the major compensation in DM myocytes. Consequently, the normal ß-adrenergic–induced increase in ICa caused larger dynamic changes in absolute ICa density. Isoproterenol increased Ca2+ transients to a comparable amplitude in DM and WT. There were no changes in myofilament Ca2+ sensitivity, or the expression levels and Ca2+ removal activities of other Ca2+-handling proteins. Nor was there evidence of cardiac remodeling up to 10 months of age. Thus, chronic inhibition of SERCA2a by ablation of phospholamban phosphorylation (abolishing its adrenergic regulation) results in a unique cellular adaptation involving greater dynamic ICa modulation. This ICa modulation may partly compensate for the loss in SERCA2a responsiveness and thereby partially normalize ß-adrenergic inotropy in DM phospholamban mice.
Key Words: sarcoplasmic reticulum • phospholamban • SR Ca2+-ATPase • cardiomyocytes • transgenic mice
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