Lysosomes and proteolytic enzyme activities in cultured striated muscle cells.

Primary cell cultures prepared from chick embryonic skeletal muscle and the rat myogenic line L6 were examined morphologically and biochemically during several stages of development. The L6 cells were cultured to provide three morphologically distinct populations: prefusion, postfusion, and a subclone of cells that did not fuse even at high density. Ultrastructural studies revealed the characteristic morphology of healthy myoblasts. Acridine orange staining and cytochemical localization of acid phosphatase suggest the presence of presumptive lysosomal material. Enzymatic studies of lysosomal cathepsins B, D, H, and L revealed unusually high enzyme specific activities in these homogeneous myoblast populations. No activity was detected for the two nonlysosomal enzymes Ca2+-proteinase and serine proteinase. It is suggested that the lysosomal apparatus and its complement of enzymes play a significant role in the differentiation of muscle myotubes.