Correlation of myosin isoforms with anatomical divisions in equine musculus biceps brachii

Overview of horse body composition and muscle architecture: implications for performance

Locomotion requires skeletal muscle to sustain and generate force. A muscle's force potential is proportional to its weight. Since the larger the muscle the larger its potential power output, a better understanding of the proportion of skeletal muscle a horse possesses may lead to a better understanding of horse performance. Several techniques exist to assess body composition, which include dual energy X-ray absorption, underwater (hydrostatic) weighing, derivation from total body water, bio-electric impedance, air displacement, body condition scoring, cadaver dissection and ultrasound. The relevance of each method to the equine industry will be discussed as will the practical information that the existing horse body composition studies have provided. Attention will be given to the data regarding the implications of body composition on the performance horse. The limited number of studies discussing different varieties of muscle architectures and the functional importance of these muscles will also be addressed. These body composition data may provide a better understanding of important issues in horse care that can lead to more optimal horse care techniques and a healthier and safer environment for horses.

Muscle fibre types and their distribution in the biceps and triceps brachii of the rat and rabbit

Muscle fibre type composition and distribution in the biceps brachii (long head) and triceps brachii (long head) of the rat and rabbit were investigated using the following histochemical techniques: myosin ATPase, with preincubation at pH 10.4 and 4.35; succinate dehydrogenase (SDH) and glycogen phosphorylase. The muscle fibres were classified into slow-twitch (SO), fast-twitch glycolytic (FG), fast-twitch oxidative glycolytic (FOG and FOg) and fast-twitch oxidative fibres (FO). Significant differences in the regional distribution of muscle fibre types have been observed between the rat and the rabbit. In the rat, SO fibres were restricted to the deep regions of both biceps and triceps brachii, whereas FG fibres were located in the intermediate and superficial regions (the superficial regions contained the highest percentages of FG fibres). In the rabbit, SO and FG fibres were spread over the entire muscle, although SO and FG fibres were most abundant in the deep and superficial regions respectively. These findings indicate that the biceps and triceps brachii are more regionalised in the rat than in the rabbit.

A sensitive electrophoretic method for the quantification of myosin heavy chain isoforms in horse skeletal muscle: histochemical and immunocytochemical verifications

In adult horses, three myosin heavy chain (MyHC) isoforms can be identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunohistochemistry using specific anti-MyHC monoclonal antibodies. This report studies the suitability of a consistent SDS-PAGE technique for quantifying MyHC profiles in homogenized cryostate sections of equine gluteus medius muscle biopsies (n = 18). The method used (previously described by R. J. Talmadge and R. R. Roy; J. Appl. Physiol. 1993, 75, 2337-2340) resolved MyHCs in three bands: I, IIB or IIX, and IIA from the fastest to the slowest migration band. The success rate of the protocol for yielding three well-differentiated MyHC bands was 100% and a subsequent quantification by densitometry for each MyHC isoform was obtained in all 18 muscle biopsies. The results obtained with this electrophoretic method were compared with routine myofibrillar adenosine triphosphatase histochemistry and immunohistochemistry using specific anti-MyHC monoclonal antibodies. The percent composition of the three electrophoretically separated MyHC isoforms (I, IIA and IIB or IIX) showed strong positive correlation with percentages of the area occupied in the biopsies by the three major fiber types (I, IIA, and IIB) identified histochemically (r = 0.96, P < 0.001) and immunohistochemically (r = 0.94, P < 0.01). It can be concluded that the electrophoretic method used here for measuring MyHC content is a valid alternative for muscle fiber typing in horses. As it is less costly and time-consuming than both qualitative histochemistry and immunohistochemistry, the method offers new prospects for application in equine experimental studies and veterinary medicine.

Myosin heavy chain isoforms in adult equine skeletal muscle: an immunohistochemical and electrophoretic study

BACKGROUND

The aim of this study was to characterize the myosin heavy chain (MyHC) isoforms present in equine skeletal muscle.

METHODS

Muscle biopsies were removed from the superficial region of the gluteus medius muscle of five mature horses and analyzed by immunohistochemistry (using a battery of monoclonal antibodies specific for rat MyHC isoforms) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

RESULTS

Immunohistochemistry allowed subdivision of three different muscle fiber populations containing a single MyHC, one slow and two fast, and two hybrid populations, one containing slow and fast MyHCs and another with both fast-MyHC isoforms. Electrophoresis of MyHC confirmed the existence of three resolvable bands, with an electrophoretic mobility parallel to type I, IIa, and IIx rat MyHCs. The identities of two of these MyHCs were easily comparable with slow type I and fast type IIa MyHCs from rat skeletal muscle. However, a precise identification of the second fast MyHC was not made.

CONCLUSIONS

These results show the presence of three different MyHC isoforms in mature equine skeletal muscle, whose differential distribution defines three fiber types containing a single MyHC and two hybrid fiber populations containing either both slow and fast type IIa MyHCs or both fast MyHC isoforms.

Myosin isoforms and muscle fiber characteristics in equine gluteus medius muscle

BACKGROUND

To date, four different myosin heavy chain (MyHC) isoforms have been identified in adult skeletal muscle of a number of species: types I, IIa, IIx or IId, and IIb. The aim of this study was to investigate the distribution of various MyHC isoforms in the equine gluteus medius and gluteus profundus muscles in relation with several morphometric variables of muscle fibers.

METHODS

Samples from different depths of the gluteus medius muscle (2, 4, 6, and 8 cm) and gluteus profundus muscle of five sedentary horses were examined by MyHC gel electrophoresis, monoclonal antibodies staining against fast, slow and neonatal MyHC isoforms, myosin adenosine triphosphatase (m-ATPase) activity, nicotinamide adenine dinucleotide tetrazolium reductase, alpha-glycerophosphate dehydrogenase, and alpha-amylase-PAS. Data about relative frequencies, sizes, and capillaries of the various histochemical fiber types were collected by morphometry.

RESULTS

Three MyHC isoforms were present in the gluteus medius muscle. Two of them comigrated with type I and IIa MyHC isoforms of rat diaphragm (used as a control). The third isoform showed an electrophoretic mobility closer to type IIx than to the IIb MyHC isoform of rat diaphragm. Only two MyHC isoforms (type I and IIa) were detected in the gluteus profundus muscle. In both muscles, type I fibers (high m-ATPase activity at pH 4.5) only reacted with the anti slow-MyHC antibody and both type IIA and IIB fibers (low and moderate m-ATPase activity at pH 4.5, respectively) only reacted with the anti fast-MyHC antibody. No cross-reactivity of fibers positive for both antibodies was found except for the scarce type IIC fibers. Fiber types and capillaries were heterogeneously distributed across the gluteus medius muscle. The deeper regions of this muscle were found to contain a higher percentage of type I fibers, a large number of capillaries and a lower proportion of type IIB fibers compared to the superficial regions of the muscle. The gluteus profundus muscle had more abundant and larger type I fibers than the deepest sampling site of the gluteus medius muscle.

CONCLUSIONS

These results show the existence of three different MyHC isoforms in the equine gluteus medius muscle and that fiber types and MyHC isoforms are heterogeneously distributed within this muscle. The distribution of slow-twitch and fast-twitch MyHCs among the fibers determined by immunohistochemistry was in agreement with histochemically identified type I and type II fibers, respectively.

Neonatal development of the diaphragm of the horse, Equus caballus

The diaphragm of neonatal horses is significantly different from the diaphragm of adult horses in terms of histochemical fiber type composition, myosin heavy chain isoform, and native myosin isoform composition. There is a significant increase in the percentage of type I fibers present in the diaphragm with increasing age from birth through about seven months postnatal age. A possible lack of postural tone in the hiatal region of the neonatal diaphragm is suggested to account for increased incidence of vomiting or aspiration pneumonia in younger horses. The isoform data lead to rejection of the hypothesis that the diaphragm of the horse should, as an ungulate, be relatively precocial in its rate of maturation relative to other non-ungulate mammals that have been studied.

Morphological, histochemical, and myosin isoform analysis of the diaphragm of adult horses, Equus caballus

The horse provides an interesting model for study of the structure and function of the mammalian diaphragm. Multiple regions of diaphragm from seven adult horses were prepared for histochemistry, immunocytochemistry, myosin heavy chain electrophoresis, and native myosin electrophoresis. Two additional adults were dissected to demonstrate myofiber and central tendon morphology and stained for acetylcholinesterase to demonstrate motor endplates. All regions of the adult diaphragm were histochemically characterized by a preponderance of type I fibers with some type IIa fibers. Type IIb fibers were absent in all adult specimens. Myosin heavy chain electrophoresis supported the histochemical study: two isoform bands were present on SDS gels that comigrated at the same rate as rat type I and IIa myosin heavy chain isoforms. No isoform was determined to comigrate with rat type IIb heavy chain isoforms. Native myosin isoform analysis revealed two isoforms that comigrated with rat FM-4 and FM-3 (FM = fast myosin) and two isoforms that comigrated with rat SM-1 and SM-2 (SM = slow myosin) isoforms. In some samples, a third slow native myosin isoform was observed that comigrated at the same rate as the SM-3 of the equine biceps brachii muscle. This doublet (or "triplet") of slow isoforms is unique to some horse muscles compared with other adult animals studied. It is not known if these multiple slow native myosin isoforms confer some functional advantage to the equine muscles. The adult equine diaphragm also differs in its morphology by having a large central tendon compared to that in other mammals, and is predominantly slow in fiber type and myosin isoform composition.

Myosin polymorphism and differential expression in adult human skeletal muscle

1. Myosin heavy chain (HC) and light chain (LC) isoforms are expressed in a tissue-specific and developmentally-regulated manner in human skeletal muscle. 2. At least seven myosin HC isoforms are expressed in skeletal muscle of the adult. 3. Histochemically-delineated fibre types (based on the stability of myofibrillar actomyosin adenosine triphosphatase activity) in limb muscles correlate with the myosin HC content. 4. Alterations in the phenotypic expression of myosin provides a mechanism of adaptation to stresses placed upon the muscle (e.g. increased and decreased usage).