-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 -myosin heavy chain (-MyHC) is a signature feature of pressure overload hypertrophy in rodents. -MyHC is assumed present in all or most enlarged myocytes. Objective: To quantify the number and size of myocytes expressing endogenous -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 -MyHC was quantified by flow cytometry on 10 000 to 20 000 myocytes with use of a validated -MyHC antibody. Side scatter by flow cytometry in the same cells was validated as an index of myocyte size.-MyHC-positive myocytes constituted 31 % of myocytes in control hearts (n12), increasing to 2510 % at 3 days to 6 weeks after TAC (n24, P<0.01). -MyHC-positive myocytes did not enlarge with TAC and were smaller at all times than myocytes without -MyHC (70 % as large, P<0.001). -MyHC-positive myocytes arose by addition of -MyHC to -MyHC and had more total MyHC after TAC than did the hypertrophied myocytes that had -MyHC only. Myocytes positive for -MyHC were found in discrete regions of the left ventricle in 3 patterns: perivascular, in areas with fibrosis, and in apparently normal myocardium. Conclusions: -MyHC protein is induced by pressure overload in a minor subpopulation of smaller cardiac myocytes. The hypertrophied myocytes after TAC have -MyHC only. These data challenge the current paradigm of the fetal hypertrophic gene program and identify a new subpopulation of smaller workin