Rat and avian myofibers having similar innervation share antigenic determinants

The antigenic relationship between extrafusal myofiber types of avian and rat muscles was investigated. Antibodies specific for myosin heavy-chain isozymes of chicken fast-twitch myofibers (anti-twitch) or chicken slow-tonic myofibers (anti-tonic) were used in the immunohistochemical examination of embryonic and adult rat muscle. Anti-twitch antibodies reacted with chicken fast-twitch myofibers, with all extrafusal myofibers of adult rat and with embryonic rat myofibers. Anti-tonic antibodies reacted with chicken slow-tonic myofibers and with embryonic rat myofibers, but not with any extrafusal myofibers of adult rat. All intrafusal myofibers of adult rat reacted with both antibodies. However, individual intrafusal myofibers reacted most strongly with one or the other of the antibodies and weakly with the remaining antibody, thereby segregating the intrafusal myofibers into two classes. These results indicate shared antigenic determinants between the myosin heavy chains of avian and rat twitch myofibers and between the myosin heavy chains of avian tonic myofibers and embryonic rat myofibers. Therefore, antigenically related myosin heavy-chain isozymes are present in avian and rat myofibers having similar patterns of innervation (twitch or tonic). This tonic vs twitch comparison presents a unique perspective for the immunochemical analysis of myosin isozymes, compared with the traditional division based on speed of contraction, slow or fast.