title: Effect of growth and training on muscle adaptation in Thoroughbred horses.
abstract: OBJECTIVE
  To determine the effect of growth and training on metabolic properties in muscle fibers of the gluteus medius muscle in adolescent Thoroughbred horses.
  
  
  METHODS
  Twenty 2-year-old Thoroughbreds.
  
  
  METHODS
  Horses were randomly assigned to 2 groups. Horses in the training group were trained for 16 weeks, and control horses were kept on pasture without training. Samples were obtained by use of a needle-biopsy technique from the middle gluteus muscle of each horse before and after the training period. Composition and oxidative enzyme (succinic dehydrogenase [SDHI) activity of each fiber type were determined by use of quantitative histochemical staining procedures. Whole-muscle activity of SDH and glycolytic enzyme (phosphofructokinase) as well as myosin heavy-chain isoforms were analyzed biochemically and electrophoretically, respectively.
  
  
  RESULTS
  The SDH activity of type-I and -IIA fibers increased during growth, whereas whole-muscle activity was unchanged. Percentage of type-IIX/B muscle fibers decreased during training, whereas that of myosin heavy-chain IIa increased. The SDH activity of each fiber type as well as whole-muscle SDH activity increased during training. An especially noticeable increase in SDH activity was found in type-IIX/B fibers.
  
  
  CONCLUSIONS
  Changes in muscle fibers of adolescent Thoroughbreds are caused by training and not by growth. The most noticeable change was for the SDH activity of type-IIX/B fibers. These changes in the gluteus medius muscle of adolescent Thoroughbreds were considered to be appropriate adaptations to running middle distances at high speeds.
authors.affiliation: Department of Biological Sciences, Faculty of Agriculture, Yamaguchi University, Japan.
authors.forename: Seiko
authors.initials: S
authors.lastname: Yamano
authors.forename: Daisuke
authors.initials: D
authors.lastname: Eto
authors.forename: Takao
authors.initials: T
authors.lastname: Sugiura
authors.forename: Makoto
authors.initials: M
authors.lastname: Kai
authors.forename: Atsushi
authors.initials: A
authors.lastname: Hiraga
authors.forename: Mikihiko
authors.initials: M
authors.lastname: Tokuriki
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Miyata
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D006736:Horses / Q000502:physiology*; D007527:Isoenzymes / Q000378:metabolism; D018485:Muscle Fibers, Skeletal / Q000201:enzymology / Q000502:physiology; D018482:Muscle, Skeletal / Q000201:enzymology / Q000254:growth & development* / Q000502:physiology*; D025221:Phosphofructokinases / Q000378:metabolism; D010805:Physical Conditioning, Animal / Q000502:physiology*; D013385:Succinate Dehydrogenase / Q000378:metabolism
chemical_list: D007527:Isoenzymes; D013385:Succinate Dehydrogenase; D025221:Phosphofructokinases