Myosin heavy chain isoforms in histochemically defined fiber types of rat muscle

Combined histochemical and biochemical analyses were performed on rat skeletal muscles in order to determine the myosin heavy chain patterns in specific fiber types. Four myosin heavy chain isoforms were separated by gradient polyacrylamide gel electrophoresis of extracts from single fibers and whole muscle homogenates. Their electrophoretic mobility increased in the order HCIIa, HCIIb, and HCI. HCIIa, HCIIb and HCI were present as unique isoforms in histochemically defined fiber types IIA, IIB and I, respectively. The isoforms HCI and HCIIa coexisted at variable ratios in type IC and IIC fibers. An additional fast myosin heavy chain isoform with an electrophoretic mobility between HCIIa and HCIIb was designated as HCIId because of its abundance in fast fibers of large diameter in the diaphragm. With the exception of slight differences in mATPase staining intensity after acid preincubation, these fibers were almost indistinguishable from type IIB fibers. In view of their specific myosin heavy chain composition (HCIId), these fibers were named type IID. In the extensor digitorum longus muscle, type IID fibers were of smaller size than type IIB and differed from the latter by higher NADH tetrazolium reductase activities. Circumstantial evidence suggests that type IID fibers are identical with the 2X fibers, previously described by Schiaffino et al. (1986).