Distribution of Polymorphic Forms of Troponin Components in Extra- and Intrafusal Fibers of an Avian Slow Muscle

By indirect immunofluorescence microscopy, the reactivities of extra- and intrafusal muscle fibers with antibodies against troponin (TN) components were studied in an avian slow muscle, the anterior latissimus dorsi (ALD) of the chicken. Serial cross sections of the muscle were exposed to antibodies specific to TN components (TN-T, -I, and -C) from adult chicken breast and ventricular muscles. In extrafusal fibers, four distinct categories were identified on the basis of differential reactivity with these antibodies. The predominant population of fibers (greater than 95%) reacted weakly only with antiventricular TN-C. The second type of fibers (less than 5%) was stained with antibodies raised against breast TN components. The third group of fibers (less than 1%) was labeled not only with antibreast TN components, but also with antiventricular TN-T and -C. The last class of fibers (less than 1%) reacted with antibodies directed against ventricular TN-T and -C. These results were correlated with myofibrillar ATPase staining patterns of fibers. In intrafusal muscle fibers of this muscle, the same four types of fibers were observed; in these fibers, however, there appeared to be a longitudinal variation in the reactivity. In conclusion, the slow ALD muscle of the adult chicken contains populations of both extrafusal and intrafusal fibers which are heterogeneous in reactivity with TN component antibodies.