Myosin Types in Cultured Muscle

Fluorescent antibodies against fast skeletal, slow skeletal, and ventricular myosins were applied to muscle cultures from embryonic pectoralis and ventricular myocardium of the chicken. A number of spindle-shaped mononucleated cells, presumably myoblasts, and all myotubes present in skeletal muscle cultures were labeled by all three antimyosin antisera . In contrast, in cultures from ventricular myocardium all muscle cells were labeled by anti-ventricular myosin, whereas only part of them were stained by anti-slow skeletal myosin and rare cells reacted with anti-fast skeletal myosin . The findings indicate that myosin(s) present in cultured embryonic skeletal muscle cells contains antigenic determinants similar to those present in adult fast skeletal, slow skeletal, and ventricular myosins. The nature of myosin(s) synthesized by striated muscle cells in the course of differentiation is controversial. Myosin in differentiating muscle cells differs from that present in nonmuscle cells (1) but it is not clear whether (a) it is identical to adult fast skeletal myosin (10, 11, 12), (b) it is a distinct embryonic type (4, 5, 16, 17, 18), or (c) it is a mixture of fast skeletal and slow skeletal myosins (3) or even a mixture of skeletal and cardiac myosins (6, 7) . The discrepancy between different results may be attributable to the material examined and the techniques used. However, even when comparable systems were analyzed by the same procedure, conflicting results were reported . For instance, in one immunofluorescence study (6), cultured muscle cells from chick embryo pectoralis were found to react with antibodies against both skeletal and cardiac myosins, whereas, in a more recent study (10), muscle cells were found to react with antifast skeletal but not with anti-slow skeletal myosin . Using antimyosin antibodies specific for fast skeletal, slow skeletal, and ventricular myosins, we have therefore reinvestigated the antigenic properties of myosin(s) present in cultured muscle cells from embryonic chicken pectoralis muscle . Cultures of ventricular myocardium and co-cultures J . CELL BIOLOGY ©The Rockefeller University Press 0021-9525/80/06/0903/07 $1 .00 Volume 85 June 1980 903-909 RAPID COMMUNICATIONS of pectoralis and ventricular myocardium were also examined for comparison. A preliminary report on antimyosin staining ofcardiac muscle cells in culture has been previously presented (14) . MATERIALS AND METHODS