Heterogeneity of β-type myosin isozymes in the human heart and regulational mechanisms in their expression. Immunohistochemical study using monoclonal antibodies

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.