Histochemical and biochemical analysis of myosin heavy chain expression during cardiogenesis in the rat.

Little is known about the tissue-specific expression of contractile proteins during cardiogenesis in the mammalian heart. Since the myosin heavy chain (HC) isoform expressed in the adult correlates with myocardial functional capacity, we undertook an analysis of myosin HC expression in atrial and ventricular myocardia during fetal cardiogenesis in the rat heart. Cardiac HCs were separated by electrophoresis under denaturing conditions. The expression of the predominant adult isoform HC alpha was localized within developing fetal cardiac chambers by immunohistochemistry with a specific monoclonal antibody (R 37). Results demonstrated that myosin HC isoform expression followed tissue-specific patterns during cardiogenesis in the rat. Atrial myocytes expressed HC alpha throughout development. The ventricles expressed exclusively HC alpha in the adult, but HC beta expression predominated in fetal ventricles. Fetal ventricles also expressed minor amounts of HC alpha, whose amount and distribution varied with developmental stage. HC alpha was initially confined to tracts of cells in the trabeculae, suggestive of future conduction system cells. A more extensive population of HC alpha-expressing cells appeared several days before birth in a pattern which could represent the prenatal initiation of HC alpha expression in working myocardial cells. These results indicate that there is tissue-specific developmental modulation of myosin isoform expression during fetal development. Results also demonstrated that this modulation may include expression of a third electrophoretically distinct myosin HC in fetal hearts.