Distribution of myosin isozymes within single cardiac cells. An immunohistochemical study

Isozymes of myosin have been localized with respect to individual cardiac myocytes in hearts from 3-week-old, adult controls, and adult hypophysectomized rats, and in cultured cardiac cells. For this purpose, affinity-purified antibodies reacting specifically with the heavy chains of each of the two major myosin isozymes of adult rat heart, V1 and V3, were used. The distribution of the two isomyosins was determined by double immuno-labeling of the same cell, V1 myosins being revealed by rhodamine and V3 myosins by fluorescein. A procedure is described which allows optimum immunological visualization of the myosin filaments of rod-shaped isolated myocytes. It was found that the response of the cardiac cells to the two antimyosins varied depending on the state of the animal. In 3-week-old rats, all cells were stained with the anti-V1, and almost none with the anti-V3 myosin. In the hypophysectomized animals, on the contrary, all cells were stained with the anti-V3 and none with the anti-V1. A mixed pattern of reactivity was observed in adult controls since 50% of the cells reacted with the anti-V1, 10% with the anti-V3, and 40% with both antibodies. In the latter case, the distributions of V1 and V3 reactivities were homogeneous throughout the cell, and absolutely superimposable. The same double reactivity and homogeneous repartition were observed in cultured cells. These findings indicate that myocytes from adult rat myocardium are heterogeneous in terms of their isomyosins content and show for the first time that two isomyosins can coexist and be equally distributed in one cardiac cell. These observations are relevant to the regulation of individual heart cell contractility.