Enhanced Myocardial Contractility and Increased Ca²⁺ Transport Function in Transgenic Hearts Expressing the Fast-Twitch Skeletal Muscle Sarcoplasmic Reticulum Ca²⁺-ATPase

From the Division of Cardiology (E.L., Y.J., D.L.K., D.L.B., T.L., R.A.W., M.P.), Departments of Pharmacology and Cell Biophysics (I.L.G.) and Molecular and Cellular Physiology (G.G.), University of Cincinnati College of Medicine, Cincinnati, Ohio, and Department of Medical Biochemistry, University of Calgary (J.L.), Alberta, Canada.

Correspondence to Muthu Periasamy, PhD, Director of Molecular Cardiology, University of Cincinnati College of Medicine, 231 Bethesda Ave, ML0542, Cincinnati, OH 45267-0542. E-mail muthu.periasamy{at}uc.edu

Abstract—In this study, we investigated whether the fast-twitch skeletal muscle sarco(endo)plasmic reticulum Ca²⁺ transport pump (SERCA1a) can functionally substitute the cardiac SERCA2a isoform and how its overexpression affects cardiac contractility. For this purpose, we generated transgenic (TG) mice that specifically overexpress SERCA1a in the heart, using the cardiac-specific -myosin heavy chain promoter. Ectopic expression of SERCA1a resulted in a 2.5-fold increase in the amount of total SERCA protein. At the same time, the level of the endogenous SERCA2a protein was decreased by 50%, whereas the level of other muscle proteins, including calsequestrin, phospholamban, actin, and tropomyosin, remained unchanged. The steady-state level of SERCA phosphoenzyme intermediate was increased 2.5-fold, and the maximal velocity of Ca²⁺ uptake was increased 1.7-fold in TG hearts, demonstrating that the overexpressed protein is functional. Although the basal cytosolic calcium signal was decreased by 38% in TG cardiomyocytes, the amplitude of cytosolic calcium signal was increased by 71.8%. The rate of calcium resequestration was also increased in TG myocytes, which was reflected by a 51.6% decrease in the normalized time to 80% decay of calcium signal. This resulted in considerably increased peak rates of myocyte shortening and relengthening (50.0% and 66.6%, respectively). Cardiac functional analysis using isolated work-performing heart preparations revealed significantly faster rates of contraction and relaxation in TG hearts (41.9% and 39.5%, respectively). The time to peak pressure and the time to half-relaxation were shorter (29.1% and 32.7%, respectively). In conclusion, our study demonstrates that the SERCA1a pump can functionally substitute endogenous SERCA2a, and its overexpression significantly enhances Ca²⁺ transport and contractile function of the myocardium. These results also demonstrate that the SERCA pump level is a critical determinant of cardiac contractility.

Key Words: SERCA1a • overexpression • transgenic mouse • cardiac contractility

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