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(Circulation Research. 2009;104:95.)
© 2009 American Heart Association, Inc.

Cellular Biology

Ca²⁺-Independent Alterations in Diastolic Sarcomere Length and Relaxation Kinetics in a Mouse Model of Lipotoxic Diabetic Cardiomyopathy

Thomas P. Flagg, Olivier Cazorla, Maria S. Remedi, Todd E. Haim, Michael A. Tones, Anthony Bahinski, Randal E. Numann, Attila Kovacs, Jean E. Schaffer, Colin G. Nichols, Jeanne M. Nerbonne

From the Departments of Cell Biology and Physiology (T.P.F., M.S.R., C.G.N.), Molecular Biology and Pharmacology (J.M.N.), and Internal Medicine (A.K., J.E.S.), Washington University School of Medicine, St Louis, Mo; Institut National de la Santé et de la Recherche Médicale, U-637 (O.C.), Université Montpellier 1, Unité de Formation et de Recherche de Médecine, Montpellier, France; and Pfizer Global Research & Development (T.E.H., M.A.T., A.B., R.E.N.), Chesterfield, Mo.

Correspondence to Jeanne M. Nerbonne, PhD, Department of Molecular Biology and Pharmacology, Washington University School of Medicine, 660 S Euclid Ave, Box 8103, St Louis, MO 63110. E-mail jnerbonne{at}wustl.edu

Previous studies demonstrated increased fatty acid uptake and metabolism in MHC-FATP transgenic mice that overexpress fatty acid transport protein (FATP)1 in the heart under the control of the -myosin heavy chain (-MHC) promoter. Doppler tissue imaging and hemodynamic measurements revealed diastolic dysfunction, in the absence of changes in systolic function. The experiments here directly test the hypothesis that the diastolic dysfunction in MHC-FATP mice reflects impaired ventricular myocyte contractile function. In vitro imaging of isolated adult MHC-FATP ventricular myocytes revealed that mean diastolic sarcomere length is significantly (P<0.01) shorter than in wild-type (WT) cells (1.79±0.01 versus 1.84±0.01 µm). In addition, the relaxation rate (dL/dt) is significantly (P<0.05) slower in MHC-FATP than WT myocytes (1.58±0.09 versus 1.92±0.13 µm/s), whereas both fractional shortening and contraction rates are not different. Application of 40 mmol/L 2,3-butadionemonoxime (a nonspecific ATPase inhibitor that relaxes actin–myosin interactions) increased diastolic sarcomere length in both WT and MHC-FATP myocytes to the same length, suggesting that MHC-FATP myocytes are partially activated at rest. Direct measurements of intracellular Ca²⁺ revealed that diastolic [Ca²⁺]_i is unchanged in MHC-FATP myocytes and the rate of calcium removal is unexpectedly faster in MHC-FATP than WT myocytes. Moreover, diastolic sarcomere length in MHC-FATP and WT myocytes was unaffected by removal of extracellular Ca²⁺ or by buffering of intracellular Ca²⁺ with the Ca²⁺ chelator BAPTA (100 µmol/L), indicating that elevated intracellular Ca²⁺ does not underlie impaired diastolic function in MHC-FATP ventricular myocytes. Functional assessment of skinned myocytes, however, revealed that myofilament Ca²⁺ sensitivity is markedly increased in MHC-FATP, compared with WT, ventricular cells. In addition, biochemical experiments demonstrated increased expression of the β-MHC isoform in MHC-FATP, compared with WT ventricles, which likely contributes to the slower relaxation rate observed in MHC-FATP myocytes. Collectively, these data demonstrate that derangements in lipid metabolism in MHC-FATP ventricles, which are similar to those observed in the diabetic heart, result in impaired diastolic function that primarily reflects changes in myofilament function, rather than altered Ca²⁺ cycling.

Key Words: metabolism • diabetes • myofilaments • remodeling

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