Developmental aspects of cardiac contractile proteins.

The change in the isomyosin complement of avian and mammalian hearts was examined during the embryonic period in primary cultures of embryonic myocytes and in the cross-section of the adult ventricular wall. The type of myosin was determined by immunofluorescence using Abs specific for heavy chains of V1 and V3 isomyosins and by cytochemical staining for Ca2+ activated myosin ATPase. Our analysis indicates that the first isomyosin to appear in both chambers of avian heart is of the V3 type (HC beta). With advancing development, however, the atria initiate the expression of HC alpha and repress that of HC beta while the ventricle retains HC beta. In cultured myocytes derived from rat embryos cellular heterogeneity was detected in response to thyroid hormone. The cells are not synchronized in their response. Two populations are discernible with the minor one being thyroid hormone insensitive. Heterogeneity of the cellular populations was also seen in the left ventricle of adult rabbits. Myocytes with a similar isomyosin complement appear clustered with a predominance of V1 in the epicardium. Heterogeneous myocytes are, however, also frequently seen connected by an intercalated disc.