Endocrine Abstracts

Aging process is accompanied by structural and functional changes in different organs including bone and muscle tissues. Between these tissues there is a mutual interaction mediated by endocrine factors such as the peptide hormone osteocalcin, produced by osteoblasts. Since the skeletal muscles is composed of two basic fiber types, slow-twitch (type I) and fast-twitch (type II), the aim of this study was to examine the age-related changes in rat fast-twitch skeletal muscle fibers using morphometrical, immunohistochemical and biochemical analysis. Experimental animals included three age group of male Wistar rats: young adult (3-months old), middle-aged (16-months old) and old (24-months old). For immunohistochemical characterization of fast myosin heavy chains (MHC-fast) in skeletal muscle fibers, anti-fast myosin skeletal heavy chain served as primary antibody. Morphometric analysis of muscle fibers was performed using Olympus microscope (BX-51), equipped with a microcator, a motorised stage and a CCD video camera, and controlled by the newCAST stereological software package. Blood serum was used for determination of serum osteocalcin (OC) and testosterone (T) concentrations. Morphometric analysis of fast-twitch muscle fibers, in gastrocnemius muscle cross-sections, showed a significant reduction in mean MHC-fast fiber perimeter (MHC-fast FP) by 15% (P<0.01) and 10% (P<0.05) in old rats compared to the young adult and middle-aged rats, respectively. The value of MHC-fast fiber cross-sectional area (MHC-fast FCSA) was markedly reduced by 27% (P<0.01) and 19% (P<0.05) in the old rats, related to young adult and middle-aged groups, respectively. The MHC-fast fiber minimal Feret’s diameter (MHC-fast FMFD) in old rats decreased by 17% (P<0.01) when compared to young adult. Evaluation of the biochemical parameters showed a significant reduction in serum OC concentration by 61% (P<0.0001) and 63% (P<0.0001), when the middle-aged and old rats were compared to the younger animals. Serum T was significantly lower by 26% (P<0.05) and 56% (P<0.0001) in the middle-aged and old groups respectively, compared to young adults. The reduction of serum T measured 40% (P<0.01) when old rats were compared to middle-aged animals. These findings indicate age-related changes in the morphometric parameters of fast-twitch muscle fibers, as a consequence of their size reduction in the rat gastrocnemius. This is supported by the decline in serum levels of osteocalcin and testosterone, which may contribute to the reduction of their muscle anabolic effects in old age.