Alteration of cardiac myosin isozymes after whole-body irradiation

Abstract

Three myosin isozymes (Vl, V2, V3) have been shown to exist in the heart. V1 isozyme, containing two α heavy chains, has relatively higher actin- and calcium-stimulated myosin adenosine triphophatase (ATPase) activities than does V3, which is composed of two β heavy chains (Hoh et al, 1978; Martin et al, 1982; Litten et al, 1983). V2 isozyme appears to be a heterodimer composed of one α and one β heavy chain (Hoh et al, 1979; Everett et al, 1983a). The proportions of myosin isozymes are altered under various conditions of stress. During pressure-overload-induced hypertrophy, there is a shift toward the V3 isozyme (Mercadier et al, 1981; Litten et al, 1982). The amount of V3 isozyme is also increased with hypothyroidism and age and is decreased by hyperthyroidism and exercise (Litten et al, 1982; Everett et al, 1983b). Changes in the relative amounts of V1 and V3 isozymes are believed to be responsible, in part, for altered cardiac performance. A high proportion of V3 isozyme is associated with relatively slow, economic tension development within the heart, whereas a high proportion of V1 isozyme is associated with a faster, but less economical force development (Schwartz etal, 1981; Alpert et al, 1983). In this study, measurements of cardiac myosin isozymes were carried out in rats subjected to acute sublethal wholebody irradiation. The studies showed, for the first time, that there is a shift in the pattern of myosin isozymes 9 days after irradiation. This change may result in functional alterations of the heart following whole-body irradiation.