"Fast" isomyosins and fiber types in mammalian skeletal muscle.

Immunohistochemical procedures have been used to correlate antigenic differences among fast isomyosins with their specific localization in particular types of rat muscle fibers. Antisera were produced in rabbits against myosins isolated from guinea pig fast-white (tensor fasciae latae) and fast-red (masseter) muscles. After sequential cross-adsorptions on insolubilized heterologous myosins, antibodies mainly directed against heavy chain determinants were obtained. In indirect immunofluorescence assays, these antibodies selectively stained fast-white (type IIB) and fast-red (type IIA) fibers, respectively. Slow-twitch (type I) fibers, which were reactive with antibodies against soleus muscle myosin, were unreactive with anti-fast-myocin antibodies. In addition to these major types of fibers, with unique immunoreactivity, two groups of fibers with double reactivity were identified: 1) fibers reactive with anti-slow-twitch and anti-fast-red antibodies (corresponding to IIC fibers with enzyme histochemistry) and 2) fibers reactive with anti-fast-red and anti-fast-white antibodies. We propose to identify these fibers with the symbols I in equilibrium with IIA and IIA in equilibrium with IIB, respectively, as they may be considered as intermediate and/or transitional stages between the major fiber types. No fibers were reactive with both anti-slow-twitch and anti-fast-white antibodies. Therefore, muscle fibers may change in a sequential manner, from I leads to IIA leads to IIB or in the reverse direction, suggesting an obligatory sequence of gene activation.