Myosin isoforms and muscle fiber characteristics in equine gluteus medius muscle

Background To date, four different myosin heavy chain (MyHC) isoforms have beenidentified in adult skeletal muscle of a number of species: types I, IIa, IIx or IId, and IIb. The aimof this study was to investigate the distribution of various MyHC isoforms in the equine gluteusmedius and gluteus profundus muscles in relation with several morphometric variables of musclefibers. Methods Samples from different depths of the gluteus medius muscle (2, 4, 6, and 8cm) and gluteus profundus muscle of five sedentary horses were examined by MyHC gelelectrophoresis, monoclonal antibodies staining against fast, slow and neonatal MyHC isoforms,myosin adenosine triphosphatase (m‐ATPase) activity, nicotinamide adenine dinucleotidetetrazolium reductase, α‐glycerophosphate dehydrogenase, and α‐amylase‐PAS. Dataabout relative frequencies, sizes, and capillaries of the various histochemical fiber types werecollected by morphometry. Results Three MyHC isoforms were present in the gluteus medius muscle. Two ofthem comigrated with type I and IIa MyHC isoforms of rat diaphragm (used as a control). Thethird isoform showed an electrophoretic mobility closer to type IIx than to the IIb MyHC isoformof rat diaphragm. Only two MyHC isoforms (type I and IIa) were detected in the gluteusprofundus muscle. In both muscles, type I fibers (high m‐ATPase activity at pH 4.5) only reactedwith the anti slow‐MyHC antibody and both type IIA and IIB fibers (low and moderatem‐ATPase activity at pH 4.5, respectively) only reacted with the anti fast‐MyHC antibody. Nocross‐reactivity of fibers positive for both antibodies was found except for the scarce type IICfibers. Fiber types and capillaries were heterogeneously distributed across the gluteus mediusmuscle. The deeper regions of this muscle were found to contain a higher percentage of type Ifibers, a large number of capillaries and a lower proportion of type IIB fibers compared to thesuperficial regions of the muscle. The gluteus profundus muscle had more abundant and largertype I fibers than the deepest sampling site of the gluteus medius muscle. Conclusions These results show the existence of three different MyHC isoforms inthe equine gluteus medius muscle and that fiber types and MyHC isoforms are heterogeneouslydistributed within this muscle. The distribution of slow‐twitch and fast‐twitch MyHCs among thefibers determined by immunohistochemistry was in agreement with histochemically identified typeI and type II fibers, respectively. © 1996 Wiley‐Liss, Inc.