IDENTIFICATION, DISTRIBUTION, AND MYOSIN SUBUNIT COMPOSITION OF TYPE-IIX FIBERS IN MOUSE MUSCLES

The purpose of the present investigation was to study the distribution and subunit composition of type IIX fibers in mouse muscles. The exis tence of a population of type IIX fibers in fast-twitch muscles of the mouse was shown by mean of immunohistochemistry and gel electrophores is. In the hindlimb muscles, tibialis anterior (TA) and extensor digit orum longus (EDL), type IIX fibers account for approximately one third of the total fiber number, with the superficial portion of the TA (TA S) being composed exclusively of type IIB and IIX fibers. A similar pr oportion of IIX fibers was found in diaphragm (DIA) while in tongue mu scles approximately 40% of the fibers were IIX. Single fiber gel elect rophoresis revealed a significant number of fibers in TAS that contain both IIB and IIX myosin heavy chain (MyHC). This was confirmed with i mmunohistochemistry, which revealed the presence of fibers with variou s degrees of staining intensity. This suggests that there may exist a degree of plasticity which results in the conversion of IIX fibers to IIB fibers and vice versa. Analysis of myosin light chain (MyLC) compo sition of type IIX fibers revealed that the ratio of MyLC3f to MyLC1f was significantly lower than in type IIB fibers. (C) 1994 John Wiley a nd Sons, Inc.