To investigate the existance of heterogeneity of β-type myosin isozymes (HCβ) in human hearts, immunohistochemical studies using monoclonal antibodies (MoAbs) raised against human ventricular myosin heavy chains were performed. Two types of MoAbs recognized some muscle fibers in the atrium, whereas both reacted with all ventricular muscle fibers. Since atrial muscle fibers reactive with each MoAb were found to be clearly different, the existence of two immunologically distinct HCβ (β1, and β2) was suggested in the atrium. By using affinity chromatography, two molecular variants of HCβ were isolated from the bovine atrium, and differences in the primary structure of β1 and β2 were confirmed by analysis of peptides produced by chymotriptic digestion. In pressure-overloaded human atria, myofibers containing β1 and/or β2 increased in accordance with decrement of myofibers containing α-type myosin isozyme (P < 0.01). But they differed in expression during the developmental stage, since β2 did not exist in the early embryonic bovine heart, but β1 did. Thus, there are two distinct HCβ whose expression is regulated by at least two factors: pressure overlaod and developmental stage.
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