β-Myosin Heavy Chain Is Induced by Pressure Overload in a Minor Subpopulation of Smaller Mouse Cardiac Myocytes
Rationale: Induction of the fetal hypertrophic marker gene β-myosin heavy chain (β-MyHC) is a signature feature of pressure overload hypertrophy in rodents. β-MyHC is assumed present in all or most enlarged myocytes.
Objective: To quantify the number and size of myocytes expressing endogenous β-MyHC by a flow cytometry approach.
Methods and Results: Myocytes were isolated from the left ventricle of male C57Bl/6J mice after transverse aortic constriction (TAC), and the fraction of cells expressing endogenous β-MyHC was quantified by flow cytometry on 10 000 to 20 000 myocytes with use of a validated β-MyHC antibody. Side scatter by flow cytometry in the same cells was validated as an index of myocyte size. β-MyHC+ myocytes constituted 3±1% of myocytes in control hearts (n=12), increasing to 25±10% at 3 days to 6 weeks after TAC (n=24, P<0.01). β-MyHC+ myocytes did not enlarge with TAC and were smaller at all times than myocytes without β-MyHC (≈70% as large, P<0.001). β-MyHC+ myocytes arose by addition of β-MyHC to α-MyHC and had more total MyHC after TAC than did the hypertrophied myocytes that had α-MyHC only. Myocytes positive for β-MyHC were found in discrete regions of the left ventricle in 3 patterns: perivascular, in areas with fibrosis, and in apparently normal myocardium.
Conclusions: β-MyHC protein is induced by pressure overload in a minor subpopulation of smaller cardiac myocytes. The hypertrophied myocytes after TAC have α-MyHC only. These data challenge the current paradigm of the fetal hypertrophic gene program and identify a new subpopulation of smaller working ventricular myocytes with more myosin.
- Received February 21, 2011.
- Revision received July 6, 2011.
- Accepted July 8, 2011.
- © 2011 American Heart Association, Inc.