Studies of hamster cardiac myofibrillogenesis in vivo with antibodies to spectrin, desmin, and alpha-actinin.

The spectrins are a family of cytoskeletal-membrane proteins that have generated much interest in the past decade. In the present study, we utilized immunohistochemical, morphological, and electrophoretic techniques to assess the possible function(s) of spectrin in mammalian cardiac tissue during development. Antibodies generated against alpha-actinin and desmin were also employed to identify myofibrils and intermediate filaments in relation to changes in the distribution of spectrin. Spectrin is localized along the sarcolemma of pre-myofibrillar hamster cardiac myocytes (day 8, postcoitum) and remains associated with the cell membrane throughout development. The staining pattern is somewhat diffuse at first, but eventually the cell margin becomes clearly defined by day 13 postcoitum. A second, more profound change in the distribution of spectrin occurs during the newborn stage, when spectrin begins to appear in the sarcoplasm. It appears as regularly spaced invaginations that are diffuse at first, eventually attaining a position around the Z-bands of adult muscle. The change in the distribution of spectrin coincides temporally with the appearance of T-tubules, which are sarcolemmal invaginations that reside at the Z-bands of adult heart. Thus, spectrin may act as a guidance mechanism for the proper positioning of T-tubules around the Z-discs of mammalian cardiac tissue. Although spectrin does not appear to interact directly with early myofibrils it may assist in the proper alignment of T-tubules and, in doing so, act to stabilize the entire contractile apparatus by enveloping it and attaching it to the sarcolemma.