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 alpha(HC alpha) and beta(HC beta)--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 HC alpha and HC beta, 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 although HC alpha was expressed in the atrium from the early embryonic stage, in embryonic ventricular myofibers, only HC beta was expressed without expression of HC alpha, but some myofibers replace HC beta by HC alpha after birth, and these HC alpha containing ventricular myofibers were found to be decreased by pressure overload, which suggested that isozymic redistribution from HC alpha to HC beta also occurred in the ventricles, as well as the atrium. In addition, we also found two subtypes of HC beta (beta 1, beta 2) in the human heart. In the ventricle, both beta 1 and beta 2 was present in all myofibers; in contrast, some myofibers contained beta 1 or beta 2 or both with or without expression of HC alpha in the atrium. beta 1 and beta 2 were distinctive in their expression during the developmental stage, since beta 1 was present in the embryonic heart from the early developmental stage, whereas beta 2 was not present in the early embryonic heart, but began to be expressed in the late embryonic stage.(ABSTRACT TRUNCATED AT 250 WORDS)