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Targeted overexpression of the sarcoplasmic reticulum Ca2+-ATPase increases cardiac contractility in transgenic mouse
D L Baker1, K Hashimoto, I L Grupp
1Division of Cardiology, University of Cincinnati College of Medicine, Cincinnati, Ohio, USA.
Abstract:
Cardiac hypertrophy and heart failure are known to be associated with a reduction in Ca2+-ATPase pump levels of the sarcoplasmic reticulum (SR). To determine whether, and to what extent, alterations in Ca2+ pump numbers can affect contraction and relaxation parameters of the heart, we have overexpressed the cardiac SR Ca2+-ATPase specifically in the mouse heart using the alpha-myosin heavy chain promoter. Analysis of 2 independent transgenic lines demonstrated that sarco(endo)plasmic reticulum Ca2+-ATPase isoform (SERCA2a) mRNA levels were increased 3.88+/-0. 4-fold and 7.90+/-0.2-fold over those of the control mice. SERCA2a protein levels were increased by 1.31+/-0.05-fold and 1.54+/-0. 05-fold in these lines despite high levels of mRNA, suggesting that complex regulatory mechanisms may determine the SERCA2a pump levels. The maximum velocity of Ca2+ uptake (Vmax) was increased by 37%, demonstrating that increased pump levels result in increased SR Ca2+ uptake function. However, the apparent affinity of the SR Ca2+-ATPase for Ca2+ remains unchanged in transgenic hearts. To evaluate the effects of overexpression of the SR Ca2+ pump on cardiac contractility, we used the isolated perfused work-performing heart model. The transgenic hearts showed significantly higher myocardial contractile function, as indicated by increased maximal rates of pressure development for contraction (+dP/dt) and relaxation (-dP/dt), together with shortening of the normalized time to peak pressure and time to half relaxation. Measurements of intracellular free calcium concentration and contractile force in trabeculae revealed a doubling of Ca2+ transient amplitude, with a concomitant boost in contractility. The present study demonstrates that increases in SERCA2a pump levels can directly enhance contractile function of the heart by increasing SR Ca2+ transport.
Insights
Overexpressing the sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) in mouse hearts improved cardiac function. Increased SERCA2a levels enhanced calcium transport, boosting heart contraction and relaxation rates.
Area of Science:
- Cardiovascular Physiology
- Molecular Cardiology
- Biochemistry
Background:
- Cardiac hypertrophy and heart failure are linked to reduced sarcoplasmic reticulum (SR) Ca2+-ATPase pump levels.
- Understanding the impact of SR Ca2+-ATPase levels on cardiac function is crucial for treating heart conditions.
Purpose of the Study:
- To investigate the direct effects of increased SR Ca2+-ATPase pump numbers on cardiac contraction and relaxation.
- To determine the extent to which altering Ca2+ pump levels influences heart performance.
Main Methods:
- Genetically engineered mice to overexpress cardiac SR Ca2+-ATPase (SERCA2a) using the alpha-myosin heavy chain promoter.
- Analyzed SERCA2a mRNA and protein levels in two independent transgenic lines.
- Utilized the isolated perfused work-performing heart model to assess cardiac contractility.
- Measured intracellular free calcium concentration and contractile force in cardiac trabeculae.
Main Results:
- Transgenic mice showed significantly elevated SERCA2a mRNA and protein levels.
- Maximum velocity of Ca2+ uptake (Vmax) increased by 37% in transgenic hearts.
- Overexpression led to enhanced myocardial contractile function, including increased rates of pressure development (+dP/dt) and relaxation (-dP/dt).
- Ca2+ transient amplitude doubled, correlating with a significant boost in contractility.
Conclusions:
- Increased SERCA2a pump levels directly enhance cardiac contractile function.
- Elevated SR Ca2+ transport is a key mechanism for improving heart contractility.
- This study provides evidence for SERCA2a as a therapeutic target for heart failure.