Objective—To determine the growth-related changes in metabolic and anatomic properties in equine muscle fiber type, including hybrid fibers identified with immunohistochemical analysis.
Animals—24 2-, 6-, 12-, and 24-month-old female Thoroughbreds.
Procedure—Samples were obtained from the gluteus medius muscle of all horses. Expression of myosin heavy chain (MHC) isoforms MHC-I, -IIa, -IIb, and -IIx in each muscle fiber was detected by use of 4 primary monoclonal antibodies: BA-D5, SC-71, BFF3, and BF-35, respectively. Five muscle fiber types (types I, I/IIA, IIA, IIA/IIX, and IIX) were immunohistochemically identified. The area and activity of succinic dehydrogenase (SDH) in each fiber type were determined by use of quantitative histochemical staining and image analysis.
Results—Although the proportion of type I and IIX fibers did not change with age, the proportion of type IIA and IIA/IIX fibers significantly increased and decreased, respectively, from 2 months to 24 months of age. The increase in proportion of type IIA fibers with growth may have been attributable to muscle fiber-type transition from type IIA/IIX fibers but not from type IIX fibers. Values for SDH activity and fiber area in hybrid fiber types were intermediate to those for their respective pure phenotypes.
Conclusions and Clinical Relevance—Hybrid fibers have an important role for determining the proportion of muscle fiber type in horses < 24 months old, and the metabolic and anatomic properties of the hybrid fibers are well coordinated, as in mature horses. (Am J Vet Res 2005;66:401–405)