Coordinated expression of myosin heavy chains, metabolic enzymes, and morphological features of porcine skeletal muscle fiber types

Abstract

Combined methodologies of electrophoresis, immunoblots, immunohistochemistry, histochemistry, and photometric image analysis were applied to characterize porcine skeletal muscle fibers according to their myosin heavy chain (MyHC) composition, and to determine on a fiber-to-fiber basis the correlation between contractile [MyHC (s), myofibrillar ATPase (mATPase), and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) isoforms], metabolic [succinate dehydrogenase (SDH), and glycerol-3-phosphate dehydrogenase (GPDH) activities, glycogen, and phospholamban (PLB) contents], and morphological [cross-sectional area (CSA), capillary, and nuclear densities] features of individual myofibers. An accurate delineation of MyHC-based fiber types was obtained with the immunohistochemical method developed. This protocol showed a high sensitivity and objectivity to delineate hybrid fibers with overwhelming dominance of one MyHC isoform. The phenotypic differences in contractile, metabolic, and morphological properties seen between fiber types were related with MyHC content. Slow fibers had the lowest mATPase activity (related to shortening velocity), the highest SDH activity (oxidative capacity), the lowest GPDH activity (glycolytic metabolism), and glycogen content, the smallest CSA, the greatest capillary, and nuclear densities, and expressed slow SERCA isoform and PLB, but not the fast SERCA isoform. The reverse pattern was true for pure IIB fibers, whereas type IIA and IIX fibers had intermediate properties. Hybrid fibers had mean values intermediate in-between their respective pure phenotypes. Discrimination of myofibers according to their MyHC content was possible on the basis of their contractile and non-contractile profiles. These intrafiber interrelationships suggest that myofibers of control pigs exhibit a high degree of co-ordination in their physiological, biochemical, and anatomical features. This study may well be a useful baseline for future work on the pig meat industry and also offers new prospects for muscle fiber typing in porcine experimental studies.