Heterogeneity of spectrin distribution among avian muscle fiber types

Muscle spectrin has been examined in avian fast, slow, and mixed muscles using the techniques of immunofluorescence microscopy and immunoautoradiography. By immunofluorescence, fibers of the fasttwitch pectoralis major (PM) are seen to contain α‐spectrin antigen primarily at the sarcolemma, while in the slow‐tonic anterior latissimus dorsi (ALD), α‐spectrin antigen is found in high concentrations throughout the sarcoplasm as well as being present in association with the sarcolemma. In mixed (fast‐ and slow‐twitch) muscles of the leg, two populations of fibers can be distinguished: those which resemble the fibers of the PM and another group which displays interior staining similar to the fibers of the ALD. Histochemical staining for actomyosin ATPase reveals that the fibers of mixed muscles which contain the most spectrin antigen correspond to the slow‐twitch fibers. Supportive data were obtained using sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE) followed by immunoautoradiography. In slow muscle, an approximate threefold increase in α‐spectrin concentration relative to other proteins is evident. These results suggest that the distribution of α‐spectrin may be modified by the physiological state of the myofiber.