HISTOCHEMICAL, BIOCHEMICAL, AND ULTRASTRUCTURAL ANALYSES OF SINGLE HUMAN MUSCLE-FIBERS, WITH SPECIAL REFERENCE TO THE C-FIBER POPULATION

A muscle biopsy from the vastus lateralis muscle of a strength-trained woman was found to contain an unusual fiber type composition and was analyzed by histochemical, biochemical, and ultrastructural techniques . Special attention was given to the C-fibers, which comprised over 15 % of the total fiber number in the biopsy. The mATPase activity of the C-fibers remained stable to varying degrees over the pH range normall y used for routine mATPase histochemistry. Although a continuum existe d, the C-fibers were histochemically subdivided into three main fiber types: IC, IIC, and IIAC. The IC fibers were histochemically more simi lar to the Type I, the IIAC were more similar to the Type IIA, and the IIC were darkly stained throughout the pH range. Biochemical analysis revealed that all C-fibers coexpressed myosin heavy chains (MHC) I an d IIa in variable ratios. The histochemical staining intensity correla ted with the myosin heavy chain composition such that the Type IC fibe rs contained a greater ratio of MHCI/MHCIIa, the IIAC contained a grea ter ratio of MHCIIa/MHCI, and the Type IIC contained equal amounts of these two heavy chains. Ultrastructural data of the C-fiber population revealed an oxidative capacity between fiber Types I and IIA and sugg ested a range of mitochondrial volume percent from highest to lowest s uch that I > IC > IIC > IIAC > IIA. Under physiological conditions, it appears that the IC fibers represent Type I fibers that additionally express some fast characteristics, whereas the Type IIAC are Type IIA fibers that additionally express some slow characteristics. Fibers exp ressing a 50:50 mixture of MHCI and MHCIIa (IIC fibers) were rarely fo und. It is not known whether C-fibers represent a distinct population between the fast- and slow-twitch fibers that is specifically adapted to a particular usage or whether they are transforming fibers in the p rocess of going from fast to slow or slow to fast.