Expression of Myosin Isozymes during the Developmental Stage and Their Redistribution Induced by Pressure Overload

Cardiac muscles contain at least two isozymes — referred to as a(HCa) and /3(HC/3) — of the myosin heavy chain. The proportional ratio, of these isozymes varies depending upon the developmental stage and the physiological and/or the hormonal milieu of the cell. Using monoclonal antibodies (MoAb) specific for human cardiac HCa and HC/3, we have examined the expression of these isozymes in fetal through adult cardiac tissues and investigated whether isozymic redistribution occurs in pressure overloaded human ventricles. We found that 1) although HCa was expressed in the. atrium from the early embryonic stage, in embryonic ventricular myofibers, only HC/3 was expressed without expression of HCa, but some myofibers replace HC/3 by HCa after birth, and 2) these HCa containing ventricular myofibers were found to be decreased by pressure overload, which suggested that isozymic redistribution from HCa to HC/3 also occurred in the ventricles, as well as the atrium. In addition, we also found two subtypes of HC/3 (/3l, /32) in the human heart. In the ventricle, both /3l and (32 was present in all myofibers; in contrast, some myofibers contained (31 or (32 or both with or without expression of HCa in the atrium. (31 and (32 were distinctive in their expression during the developmental stage, since (31 was present in the embryonic heart from the early developmental stage, whereas (32 was not present in the early embryonic heart, but began to be expressed in the late embryonic stage. Therefore, there are at least three types of myosin isozymes, HCa, (31 and (32, whose expression is regulated by pressure overload and developmental stage.