Distribution of fast myosin heavy chain‐based muscle fibres in the gluteus medius of untrained horses: mismatch between antigenic and ATPase determinants

Abstract

The distribution of muscle fibres classified on the basis of their content of different myosin heavy chain (MHC) isoforms was analysed in muscle biopsies from the gluteus medius of adult untrained horses by correlating immunohistochemistry with specific anti-MHC monoclonal antibodies and standard myofibrillar ATPase (mATPase) histochemistry. Percutaneous needle biopsies were taken at 3 depths (20, 40 and 60 mm) from 4 4-y-old Andalusian stallions. The percentage of ' pure ' I MHC fibres increased whereas that for pure IIX MHC fibres decreased from the most superficial to the deepest sampling site. Within the fast fibres, types IIA and IIAX MHC-classified fibres were proportionately more abundant in the deepest sampling site than in the superficial region of the muscle. The immunohistochemical and histochemical characterisation of a large number of single fibres (n l 1375) was compared and correlated on a fibre-to-fibre basis. The results showed that 40 % of the fibres analysed were pure type I (expressing only MHC-I) ; they showed correct matching between their antigenic and mATPase determinants. In contrast, within the fast fibres, a considerable proportion of fibres were found showing a mismatch between their immunohistochemical and mATPase profiles. The most common mismatched fibre phenotypes comprised fibres displaying coexpression of both fast MHCs when analysed by immunocytochemistry, but showing an mATPase profile similar to typical IIX fibres (moderate mATPase reaction after preincubation at pH 4.4). Considered altogether, the total mismatched fibres represented only 4.2 % of the whole fast fibre population in the superficial region of the muscle, but their proportion increased to 15.6 % and 38.4 % in the middle and deep regions, respectively, of gluteus medius. It is concluded that a considerable number of hybrid fast MHC IIAX fibres are present in the gluteus medius of untrained horses, suggesting that equine type II fibres have probably been misclassified in numerous previous publications based on the use of histochemistry alone. This has important implications in attempts to study the physiological properties of fast fibre types adequately in horses.

Differential Expression of Equine Myosin Heavy-chain mRNA and Protein Isoforms in a Limb Muscle

“…Hybrid fibers are usually interpreted as transitory fibers (Pette and Staron 1997). However, because of the regular finding of substantial numbers of especially MyHC type 2ad protein hybrid fibers, these fibers are now more regarded as a "true" type, possibly as a functional intermediate between type 2a and type 2d/x fibers Andersen and Schiaffino 1997;Lefaucheur et al 1998;Andersen et al 1999;Linnane et al 1999;Rivero et al 1999;Stephenson 2001;Dingboom et al 2002). Because at the transcriptional level hardly any hybrid fibers are detected, no functional mRNA intermediate is apparently necessary between the type 2a and type 2d/x fibers.…”

Muscle Fiber Population and Biochemical Properties of Whole Body Muscles in Thoroughbred Horses

“…It is well known that muscles of the Thoroughbred horse are composed of Type I, Type IIa, and Type IIx fibers (Billeter et al, 1988;Sosnicki et al, 1989;Rivero et al, 1996;Serrano et al, 1996). Furthermore, the development of immunohistochemical techniques has permitted identification of hybrid muscle fibers expressing two types of myosin heavy chain (MHC) isoforms (Linnane et al, 1999;Rivero et al, 1999;Quiroz-Rothe and Rivero, 2001). Although some previous studies, including our study, demonstrated that hybrid fibers (I/ IIa and IIa/x) were detected in the gluteus medius muscle in Thoroughbred horses, the percentages of these fibers decreased to less than 4.9% until they were 24 months old (Yamano et al, 2005).…”

Myosin Heavy Chain Isoforms in Equine Gluteus Medius Muscle: Comparison of mRNA and Protein Expression Profiles

“…Hence, regulation of hybrid fibers is transcriptionally controlled in the equine gluteus medius muscle. Although hybrid MyHC 2ad protein fibers are regarded as a 'true' type because of their regular appearance, they can also be classified as transitory fibers as suggested by Pette and Staron (1997) and demonstrated in this paper (Andersen et al 1999b;Linnane et al 1999;Rivero et al 1999;Stephenson 2001;Dingboom et al 2002). The appearance of hybrid and mismatched fibers indicates that fiber-type transition occurs in presumably normal skeletal muscles not subjected to any training protocol.…”

“…They were taken as deep as possible (until resistance from the iliac wing was reached). Tissue samples were either open (horse A and horse D, 500 mg) or needle (horse B and horse C, 100 mg) biopsies, taken from the same location in the muscle to ensure comparable fiber-type composition (Serrano et al 1996;Linnane et al 1999). Samples were divided into small portions (maximum 50 mg) and rolled in talcum powder, mounted on cork blocks with the use of OCT embedding medium, and oriented so that the fibers could be sectioned either transversely or longitudinally and frozen in liquid nitrogen.…”

“…Depending on the type of muscle, a substantial number of fibers express two MyHC isoforms. Specifically, MyHC type 2a and type 2d protein hybrid fibers (2ad) are seen regularly and are regarded as a functional intermediate between the type 2a and type 2d/x fibers (Andersen et al 1999b;Linnane et al 1999;Rivero et al 1999;Stephenson 2001;Dingboom et al 2002). Regulation of the double MyHC expression is unclear.…”