Identification of a novel type 2 fiber population in mammalian skeletal muscle by combined use of histochemical myosin ATPase and anti-myosin monoclonal antibodies.

A novel type of myosin heavy chain (MHC), called 2X, has been recently identified in type 2 fibers of rat skeletal muscles using an immunochemical approach. In the present study, the same panel of anti-MHC monoclonal antibodies was used in immunohistochemistry combined with enzyme histochemistry to identify and compare type 2X fibers in hindlimb skeletal muscles of rat, mouse, and guinea pig. Immunohistochemistry shows that 2X MHC is localized in a large subset of type 2 fibers and is co-expressed with 2A or 2B MHC in a small number of fibers. Enzyme histochemistry shows that type 2X fibers display low myosin ATPase activity after pre-incubation at pH 4.3 and high activity after paraformaldehyde pre-incubation at pH 10.4. After pre-incubation at pH 4.6, myosin ATPase shows intermediate and high activity in rat and mouse 2X fibers, respectively, whereas it is low in guinea pig 2X fibers. Succinate dehydrogenase displays moderate to high activity in 2X fibers of all species. Taken together, these staining patterns allow this novel fiber population to be distinguished from the other type 2 fibers using only enzyme histochemistry. Nevertheless, the combined use of immuno- and enzyme histochemistry prevents incorrect fiber typing due to the interspecies variability of myosin ATPase activity among the correspondent fiber types, and completely modifies the presently used classification of mouse type 2 fibers.