Demonstration of 'cardiac-specific' myosin heavy chain in masticatory muscles of human and rabbit

Differential expression of equine myosin heavy-chain mRNA and protein isoforms in a limb muscle

The horse is one of the few animals kept and bred for its athletic performance and is therefore an interesting model for human sports performance. The regulation of the development of equine locomotion in the first year of life, and the influence of early training on later performance, are largely unknown. The major structural protein in skeletal muscle, myosin heavy-chain (MyHC), is believed to be primarily transcriptionally controlled. To investigate the expression of the MyHC genes at the transcriptional level, we isolated cDNAs encoding the equine MyHC isoforms type 1 (slow), type 2a (fast oxidative), and type 2d/x (fast glycolytic). cDNAs encoding the 2b gene were not identified. The mRNA expression was compared to the protein expression on a fiber-to-fiber basis using in situ hybridization (non-radioactive) and immunohistochemistry. Marked differences were detected between the expression of MyHC transcripts and MyHC protein isoforms in adult equine gluteus medius muscle. Mismatches were primarily due to the presence of hybrid fibers expressing two fast (2ad) MyHC protein isoforms, but only one fast (mainly 2a) MyHC RNA isoform. This discrepancy was most likely not due to differential mRNA expression of myonuclei.

Different effects on human skeletal myosin heavy chain isoform expression: strength vs. combination training

Myosin heavy chain (MHC) isoform expression changes with physical training. This may be one of the mechanisms for muscular adaptation to exercise. We aimed to investigate the effects of different strength-training protocols on MHC isoform expression, bearing in mind that alpha- MHC(slow) (newly identified MHC isoform) mRNA may be upregulated in response to training. Twelve volunteers performed a 6-wk strength training with maximum contractions (Max group), and another 12 of similar age performed combination training of maximum contractions and ballistic and stretch-shortening movements (Combi group). Muscle samples were taken from triceps brachii before and after training. MHC isoform composition was determined by SDS-PAGE silver staining, and mRNA levels of MHC isoforms were determined by RT-PCR. In Max group, there was an increase in MHC(2A) (49.4 to 66.7%, P < 0.01) and a decrease in MHC(2X) (33.4 to 19.5%, P < 0.01) after training, although there was no significant change in MHC(slow). In Combi group, there was also an increase in MHC(2A) (47.7 to 62.7%, P < 0.05) and a decrease in MHC(slow) (18.2 to 9.2%, P < 0.05) but no significant change in MHC(2X). An upregulation of alpha-MHC(slow) mRNA was, therefore, found in both groups as a result of training. The strength training with maximum contractions led to a shift in MHC isoform composition from 2X to 2A, whereas the combined strength training produced an MHC isoform composition shift from slow to 2A.

Myosin heavy-chain isoform composition of human single jaw-muscle fibers

Diversity in muscle contractile properties is based on the variability of contractile properties of single muscle fibers which in turn is related to the presence of different myosin heavy-chain (MyHC) isoforms. Human jaw muscles are featured by many hybrid fibers expressing more than one MyHC isoform. The purpose of this study was to determine the proportion of each isoform within these fibers for evaluation of the fiber's capacity of producing a large diversity in contractile properties. Electrophoretic separation of MyHC isoforms was performed on 218 single fibers of the temporalis and digastric muscles. Of these fibers, 100 were classified as hybrid fibers. Most hybrid fibers co-expressed MyHC-IIA and -IIX (n = 62); a smaller number co-expressed MyHC-I and -IIA (n = 14), MyHC-I and -IIX (n = 12), and MyHC-I, -IIA, and -IIX (n = 12). The proportions of the individual MyHC isoforms in the hybrid fibers varied highly, suggesting a large range of contractile properties among these fibers.

Influence of food consistency on the rabbit masseter muscle fibres

The plasticity of the masseter muscle was studied by comparing two groups of rabbits that were fed soft- and hard-diet for 87 d. Incisors of the soft-diet group were cut back to minimize the bite forces. Muscle fibres were immunohistochemically defined as fast- or slow-contracting fibres and their cross-sectional area was measured. The muscles of animals fed with the hard-diet were composed of fibres with larger cross-sectional areas than the soft-diet group. The relative difference was larger in slow-contracting fibres than in fast-contracting fibres. The results were similar for the different regions of the muscle. No changes in fibre composition were found. In conclusion, the difference in food consistency, as induced in this study, caused changes in the muscle fibre cross-sectional area that can be recognized from the altered necessary occlusal forces, which result from the modified forces developed by the masseter muscle.

Myosin heavy chain composition in human masticatory muscles by immunohistochemistry and gel electrophoresis

In this study we compared the immunohistochemically quantified fiber type area with the myosin heavy chain (MyHC) contents of a bundle of fibers from a human masticatory muscle. The total cross-sectional areas were determined immunohistochemically for the three major fiber types (I, IIA, and IIX) in bundles of fibers (n=42) taken from the anterior and posterior belly of the human digastric muscle (n=7). The relative MyHC contents of the same fiber bundles were determined electrophoretically (MyHC-I, -IIA, and -IIX; anterior, 32%, 35%, and 33%; posterior, 39%, 42%, and 19%) and compared with the immunohistochemical data (MyHC-I, -IIA, and -IIX; anterior, 32%, 31%, and 37%; posterior, 39%, 45%, and 15%). No significant differences were seen in the mean fiber type distribution between the two techniques; the correlation coefficient ranged from 0.71 to 0.96. The correlation coefficient was higher for MyHC type I and MyHC type IIX than for MyHC type IIA. The MyHC contents of single fibers taken from the posterior belly indicated that many fibers in this belly co-express MyHC-IIA and MyHC-IIX. Despite the presence of these hybrid fibers, the correspondence between both methods was relatively large.

Molecular and cellular mechanisms involved in the generation of fiber diversity during myogenesis

Skeletal muscles have a characteristic proportion and distribution of fiber types, a pattern which is set up early in development. It is becoming clear that different mechanisms produce this pattern during early and late stages of myogenesis. In addition, there are significant differences between the formation of muscles in head and those found in rest of the body. Early fiber type differentiation is dependent upon an interplay between patterning systems which include the Wnt and Hox gene families and different myoblast populations. During later stages, innervation, hormones, and functional demand increasingly act to determine fiber type, but individual muscles still retain an intrinsic commitment to form particular fiber types. Head muscle is the only muscle not derived from the somites and follows a different development pathway which leads to the formation of particular fiber types not found elsewhere. This review discusses the formation of fiber types in both head and other muscles using results from both chick and mammalian systems.

Dynamic nature of fibre-type specific expression of myosin heavy chain transcripts in 14 different human skeletal muscles

The main goal of this study was to find out, whether the appearance of fibres without evident myosin heavy chain (MyHC) transcript expression (negative fibres) implies the existence of additional MyHC transcripts in human muscle fibres. Fourteen different skeletal muscles were analysed also to verify how MyHC transcript expression matches histochemical phenotypes of fibres. For this purpose, the expression of beta-slow, 2a and 2x MyHC transcripts, demonstrated by in situ hybridisation technique, was analysed within type I, IIC, IIA, IIAX and IIX fibres, determined according to the activity of myofibrillar ATPase. Additionally, MyHC isoform expression was immunohistochemically demonstrated and metabolic profiles of negative fibres were estimated. From a total of 4444 muscle fibres analysed, only 0.8% of fibres were negative, among them type I prevailed, the remainder were type IIA and IIX fibres. The majority of fibres expressed only beta, 2a and 2x MyHC transcripts and they mostly matched type I, IIA and IIX fibres respectively, but two minor hybrid fibre groups (beta/2a and 2ax) exhibited variable histochemical phenotype. The infrequency, the prevailing oxidative-glycolytic metabolic profile of negative type I fibres and frequent co-appearance with transitional type IIC fibres imply that the negative fibres rather result from fibre type transition than express an additional slow or even 2b MyHC transcripts. The appearance of hybrid and mismatched fibres additionally indicates that fibre type transition occurs also in presumably normal skeletal muscles, what enables the muscles to tune even with minimal changes in mechanical demands.

Muscle-derived CD45-SCA-1+c-kit- progenitor cells give rise to skeletal muscle myotubes in vitro

A stem cell population isolated from murine skeletal muscle has recently been shown to differentiate into hematopoietic cells after transplantation in vivo. In the present study, we tested the hypothesis that this cell population would also, under appropriate culture conditions, differentiate into skeletal muscle cells in vitro. Lower-extremity skeletal muscle tissue isolated from 3- to 4-wk-old mice was dissected free from bone and vessels, enzymatically digested, and flow cytometrically sorted to yield CD45(-)Sca-1(+)c-Kit(-) (S+) cells. These cells were further sorted into CD34(+) and CD34(-) fractions and examined for skeletal, cardiac, and hematopoietic lineage-specific messenger RNA (mRNA) transcripts immediately after isolation and after a 10- to 14-d culture period. Freshly isolated S(+)CD34(+) cells lacked expression of skeletal-, cardiac-, or hematopoietic-specific mRNA transcripts, whereas S(+)CD34(-) cells expressed c-met, a marker for skeletal muscle satellite cells. During 10-14 d in culture, both S(+)CD34(+) and S(+)CD34(-) cell populations underwent a period of attachment followed by elongation and, ultimately, fusion to create large multinucleated contractile myotubes expressing skeletal muscle lineage mRNA transcripts but not hematopoietic or cardiac lineage transcripts. We conclude that murine skeletal muscle possesses two populations of progenitor cells that can be directly isolated. One population expressing the phenotype S(+)CD34(-) may contain satellite cells, whereas the S(+)CD34(+) population is devoid of satellite cell markers. Both populations possess the ability to differentiate into skeletal muscle cells in vitro.

`Superfast' or masticatory myosin and the evolution of jaw-closing muscles of vertebrates

There are four fibre types in mammalian limb muscles, each expressing a different myosin isoform that finely tunes fibre mechanics and energetics for locomotion. Functional demands on jaw-closer muscles are complex and varied,and jaw muscles show considerable phylogenetic plasticity, with a repertoire for myosin expression that includes limb, developmental, α-cardiac and masticatory myosins. Masticatory myosin is a phylogenetically ancient motor with distinct light chains and heavy chains. It confers high maximal muscle force and power. It is highly jaw-specific in expression and is found in several orders of eutherian and marsupial mammals including carnivores,chiropterans, primates, dasyurids and diprotodonts. In exceptional species among these orders, masticatory myosin is replaced by some other isoform. Masticatory myosin is also found in reptiles and fish. It is postulated that masticatory myosin diverged early during gnathostome evolution and is expressed in primitive mammals. During mammalian evolution, mastication of food became important, and in some taxa jaw closers replaced masticatory myosin with α-cardiac, developmental, slow or fast limb myosins to adapt to the variety of diets and eating habits. This occurred early in some taxa(rodents, ungulates) and later in others (macropods, lesser panda, humans). The cellular basis for the uniqueness of jaw-closing muscles lies in their developmental origin.

Post-translational phosphorylation of the slow/beta myosin heavy chain isoform in adult rabbit masseter muscle

Four different phenotypes of slow muscle fibers, characterized by differential epitope expression in the slow/beta myosin heavy chain (MyHC) isoform, have been identified in adult rabbit masseter muscle. We investigated the role of post-translational phosphorylation in the expression of these four phenotypes. Serial cryostat sections were treated either with alkaline phosphatase to dephosphorylate proteins in the tissue, or with a brain kinase solution and ATP to phosphorylate them, and then stained, using four antibodies that bind specifically to the slow/beta MyHC isoform. In sections pre-treated with phosphatase, immunoreactivity to antibody A4.840 was abolished, but it could be restored by subsequent kinase/ATP treatment or ATP alone, indicating that the expression of its epitope requires phosphorylation. Phosphatase treatment resulted in an exposure of the epitope for antibody A4.951 in cells that normally bind this antibody only weakly or not at all, but since heat treatment alone produced similar effects, the role of phosphorylation in this enhancement is less certain. Immunoreactivity to antibodies S58 and BA-D5 were not influenced by phosphatase pre-treatment. Kinase/ATP treatment was only effective in changing antibody binding when tissues already had been phosphatase treated. We interpret these results to mean that sites of potential phosphorylation may already be occupied by O-linked glycosylation.

Influence of early postnatal cold exposure on myofiber maturation in pig skeletal muscle

Early after birth, piglets rely almost exclusively on muscular shivering thermogenesis to produce heat in the cold and this can possibly modulate skeletal muscle development. An experiment involving 10 individually housed piglets was conducted to determine the influence of cold (24-15 degrees C, D5C group) vs. thermoneutrality (34-30 degrees C, D5TN group) between birth and 5 days on myosin heavy chain (MyHC) polymorphism and metabolic characteristics of longissimus lumborum (LL) and rhomboideus (RH) muscles. Five additional piglets were sacrificed at birth. Piglets exposed to cold received 43% more artificial milk on a liveweight basis in order to achieve similar growth rates. D5C piglets produced 93% more heat and exhibited intense shivering during the whole experiment. Contractile and metabolic characteristics of muscles were determined by immunocytochemistry, electrophoresis and enzyme activities. At least eight MyHC isoforms were detected, including atypical expressions of the alpha-cardiac and extraocular isoforms. Dramatic changes in MyHC composition, myofiber cross-sectional area (CSA) and energy metabolism occurred between birth and 5 days. Cold exposure did not affect either the total number of fibers or the CSA, but it did influence muscle maturation. In particular, it increased the expression of alpha-cardiac and type I MyHC, and decreased that of fetal MyHC, confirming an acceleration in the rate of postnatal maturation. An increase in oxidative enzyme activities was observed in both muscles in the cold, whereas the activity of a glycolytic enzyme, lactate dehydrogenase, remained unchanged. Cold exposure also induced an increase in T3 plasma levels. The extent to which these changes are the result of sustained shivering or are due to the action of hormonal factors, such as thyroid hormones, are discussed.

Abnormal palatopharyngeal muscle morphology in sleep-disordered breathing

The aim of the present study was to investigate whether histopathological changes can be detected in two soft palate muscles, the palatopharyngeus and the uvula, in 11 patients with long duration of sleep-disordered breathing (SDB). Muscle samples were collected from patients undergoing uvulo-palatopharyngoplasty (UPPP). Reference samples from the corresponding areas were obtained at autopsy from five previously healthy subjects. Muscle morphology, fibre type and myosin heavy chain (MyHC) compositions were analysed with enzyme-histochemical, immunohistochemical and biochemical techniques. The muscle samples from the patients, and especially those from the palatopharyngeus, showed several morphological abnormalities. The most striking findings were (i) increased amount of connective tissue, (ii) abnormal variability in fibre size, (iii) increased proportion of small-sized fibres, (iv) alterations in fibre type and MyHC compositions, (v) increased frequency of fibres containing developmental MyHC isoforms. Our findings point towards a pathological process of denervation and degeneration in the patient samples. Conclusively, the morphological abnormalities suggest a neuromuscular disorder of the soft palate in SDB patients.

Changes in fibre type composition of gluteus medius and semitendinosus muscles of Dutch Warmblood foals and the effect of exercise during the first year postpartum

In order to obtain broader insights into the equine musculoskeletal system, we studied the fibre type composition of 2 locomotory muscles in biopsies from Dutch Warmblood foals taken at 3 different ages in the first year postpartum. The muscle fibre types were determined histochemically as well as immunohistochemically. ATPase-characterised IIB fibres appear to express either IId or type lIa plus IId myosin heavy chain (MHC). A high percentage of fibres classified as IIA with ATPase expressed both fast types of MHC. The type I classification by the 2 methods matched almost completely. There was an increase with age of fibres expressing I and IIa MHC in the gluteus medius. At the same time, there was a decrease of fibres expressing IId MHC and fibres co-expressing MHC IIa and IId. MHC expression of the semitendinosus muscle did not change over time at first, but from age 22-48 weeks there was a decrease in the percentage of type IId fibres. In general, the gluteus medius contained more type I fibres but fewer type IId fibres compared to the semitendinosus. At most ages the fibre type compositions of both muscles correlated with one another. To examine the effect of exercise, one-third of the foals were given box rest, one-third received training and one-third kept at pasture during the first 22 weeks of life. The 3 exercise groups differed in their fibre type composition; however, these differences could not be attributed to the effect of exercise.

Spatial distribution of myosin heavy-chain isoforms in mouse masseter

There is a paucity of information regarding the anatomy and muscle fiber phenotype of the masseter. The objective of this study was to characterize the distribution of each myosin heavy-chain (MyHC) isoform within different anatomical regions of male and female mouse masseters. Masseters from male and female CD-1 mice (2-4 months old) were examined for description of the anatomical partitioning of muscle fibers and endplate distribution. The spatial distribution of MyHC isoforms--embryonic, neonatal, slow, alpha-cardiac, IIa, and IIb--was determined within the defined masseter partitions by means of Western blot analysis and immunofluorescent localization. Types IIa, IIx, and IIb were the predominant MyHC isoforms observed. Distinct differences in the spatial distribution of these MyHC isoforms were found between muscle regions and varied between sexes. The regionalization of muscle fiber types in the mouse masseter is consistent with the functional compartmentalization of the masseter observed in other species.

Abundant expression of myosin heavy-chain IIB RNA in a subset of human masseter muscle fibres

Type IIB fast fibres are typically demonstrated in human skeletal muscle by histochemical staining for the ATPase activity of myosin heavy-chain (MyHC) isoforms. However, the monoclonal antibody specific for the mammalian IIB isoform does not detect MyHC IIB protein in man and MyHC IIX RNA is found in histochemically identified IIB fibres, suggesting that the IIB protein isoform may not be present in man; if this is not so, jaw-closing muscles, which express a diversity of isoforms, are likely candidates for their presence. ATPase histochemistry, immunohistochemistry polyacrylamide gel electrophoresis and in situ hybridization, which included a MyHC IIB-specific mRNA riboprobe, were used to compare the composition and RNA expression of MyHC isoforms in a human jaw-closing muscle, the masseter, an upper limb muscle, the triceps, an abdominal muscle, the external oblique, and a lower limb muscle, the gastrocnemius. The external oblique contained a mixture of histochemically defined type I, IIA and IIB fibres distributed in a mosaic pattern, while the triceps and gastrocnemius contained only type I and IIA fibres. Typical of limb muscle fibres, the MyHC I-specific mRNA probes hybridized with histochemically defined type I fibres, the IIA-specific probes with type IIA fibres and the IIX-specific probes with type IIB fibres. The MyHC IIB mRNA probe hybridized only with a few histochemically defined type I fibres in the sample from the external oblique; in addition to this IIB message, these fibres also expressed RNAs for MyHC I, IIA and IIX. MyHC IIB RNA was abundantly expressed in histochemical and immunohistochemical type IIA fibres of the masseter, together with transcripts for IIA and in some cases IIX. No MyHC IIB protein was detected in fibres and extracts of either the external oblique or masseter by immunohistochemistry, immunoblotting and electrophoresis. Thus, IIB RNA, but not protein, was found in the fibres of two different human skeletal muscles. It is believed this is the first report of the substantial expression of IIB mRNA in man as demonstrated in a subset of masseter fibres, but rarely in limb muscle, and in only a few fibres of the external oblique. These findings provide further evidence for the complexity of myosin gene expression, especially in jaw-closing muscles.

Maximum shortening velocity and myosin heavy-chain isoform expression in human masseter muscle fibers

While human masseter muscle is known to have unusual co-expression of myosin heavy-chain proteins, cellular kinetics of individual fibers has not yet been tested. Here we examine if myosin heavy-chain protein content is closely correlated to fiber-shortening speed, as previously reported in other human muscles, or if these proteins do not correlate well to shortening speeds, as has been demonstrated previously in rat muscle. Slack-test recordings of single, skinned human masseter fibers at 15 degrees C revealed maximum shortening velocities generally slower and much more variable than those recorded in human limb muscle. The slowest fiber recorded had a maximum shortening velocity (V0) value of 0.027 muscle lengths x s(-1), several times slower than the slowest type I fibers previously measured in humans. By contrast, human limb muscle controls produced V0 measurements comparable with previously published results. Analysis by gel electrophoresis found 63% of masseter fibers to contain pure type I MyHC and the remainder to co-express mostly type I in various combinations with IIA and IIX isoforms. V0 in masseter fibers forms a continuum in which no clear relationship to MyHC isoform content is apparent.

Differences in myosin heavy-chain composition between human jaw-closing muscles and supra- and infrahyoid muscles

Jaw-closing muscles have architectural features suited to force production; supra- and infrahyoid muscles are better adapted to produce velocity and displacement. It was hypothesized that this difference in function would be reflected in myosin heavy-chain (MyHC) composition (equivalent to contraction velocity) and fibre-type cross-sectional area (equivalent to force). MyHC composition was determined in muscles obtained from eight human cadavers, using monoclonal antibodies against MyHC isoforms. Jaw closers contained 4.2 times fewer type IIA fibres and 5.2 times more hybrid fibres than suprahyoid muscles, and 3.9 times fewer type IIA fibres and 3.2 times more hybrid fibres than the infrahyoid muscles. In the jaw closers, MyHC-I was expressed in approx. 70% of all fibres (pure+hybrid), in the suprahyoid muscles in approx. 40%, and in the infrahyoid muscles in approx. 46%. In the jaw closers, type I fibres were 40% larger in diameter than in the supra- and infrahyoid muscles. It can be concluded that the jaw closers have characteristics of slow muscles, and that the supra-/infrahyoid muscles have characteristics of fast muscles.

Muscle fibre types in the suprahyoid muscles of the rat

Five muscle fibre types (I, IIc, IIa, IIx and IIb) were found in the suprahyoid muscles (mylohyoid, geniohyoid, and the anterior and posterior bellies of the digastric) of the rat using immuno and enzyme histochemical techniques. More than 90% of fibres in the muscles examined were fast contracting fibres (types IIa, IIx and IIb). The geniohyoid and the anterior belly of the digastric had the greatest number of IIb fibres, whilst the mylohyoid was almost exclusively formed by aerobic fibres. The posterior belly of the digastric contained a greater percentage of aerobic fibres (83.4%) than the anterior belly (67.8%). With the exception of the geniohyoid, the percentage of type I and IIc fibres, which have slow myosin heavy chain (MHCbeta), was relatively high and greater than has been previously reported in the jaw-closing muscles of the rat, such as the superficial masseter. The geniohyoid and mylohyoid exhibited a mosaic fibre type distribution, without any apparent regionalisation, although in the later MHCbeta-containing fibres (types I and IIc) were primarily located in the rostral 2/3 region. In contrast, the anterior and posterior bellies of the digastric revealed a clear regionalisation. In the anterior belly of the digastric 2 regions were observed: both a central region, which was almost exclusively formed by aerobic fibres and where all of the type I and IIc fibres were located, and a peripheral region, where type IIb fibres predominated. The posterior belly of the digastric showed a deep aerobic region which was greater in size and where type I and IIc fibres were confined, and a superficial region, where primarily type IIx and IIb fibres were observed.

Coordinate expression of Ca2+-ATPase slow-twitch isoform and of beta calmodulin-dependent protein kinase in phospholamban-deficient sarcoplasmic reticulum of rabbit masseter muscle

Modulation of sarcoplasmic reticulum (SR) Ca(2+) transport by endogenous calmodulin-dependent protein kinase II (CaM K II) involves covalent changes of regulatory protein phospholamban (PLB), as a common, but not the only mechanism, in limb slow-twitch muscles of certain mammalian species, such as the rabbit. Here, using immunofluorescent techniques in situ, and biochemical and immunological methods on the isolated SR, we have demonstrated that rabbit masseter, a muscle with a distinct embryological origin, lacks PLB. Accommodating embryological heterogeneity in the paradigm of neural-dependent expression of specific isogenes in skeletal muscle fibers, our results provide novel evidence for the differential expression in the SR of 72 kDa beta components of CaM K II, together with the expression of a slow-twitch sarcoendoplasmic reticulum Ca(2+)-ATPase isoform, both in limb muscle and in the masseter.

Intermuscular and intramuscular differences in myosin heavy chain composition of the human masticatory muscles

Among and within the human masticatory muscles a large number of anatomical differences exists indicating that different muscles and muscle portions are specialized for certain functions. In the present study we investigated whether such a specialization is also reflected by intermuscular and intramuscular differences in fibre type composition and fibre cross-sectional area. Fibre type compositions and fibre cross-sectional areas of masticatory muscles were determined in eight cadavers using monoclonal antibodies against myosin heavy chain (MyHC). The temporalis, masseter and pterygoid muscles could be characterized by a relatively large number of fibres containing more than one MyHC isoform (hybrid fibres). In these muscles a large number of fibres expressed MyHC-I, MyHC-fetal and MyHC-cardiac alpha. Furthermore, in these muscles type I fibres had larger cross-sectional areas than type II fibres. In contrast, the mylohyoid, geniohyoid and digastric muscle were characterized by less hybrid fibres, and by less fibres expressing MyHC-I, MyHC-fetal, and MyHC-cardiac alpha, and by more fibres expressing MyHC-IIA; the cross-sectional areas of type I and type II fibres in these muscles did not differ significantly. Compared to the masseter and pterygoid muscles, the temporalis had significantly larger fibres and a notably different fibre type composition. The mylohyoid, geniohyoid, and digastric muscles did not differ significantly in their MyHC composition and fibre cross-sectional areas. Also intramuscular differences in fibre type composition were present, i.e., a regionally higher proportion of MyHC type I fibres was found in the anterior temporalis, the deep masseter, and the anterior medial pterygoid muscle portions; furthermore, significant differences were found between the bellies of the digastric.

Investigation of sexual dimorphism in the rabbit masseter muscle showing different effects of androgen deprivation in adult and young adult animals

To evaluate the role played by androgens in the development and maintenance of sex differences in the proportion of muscle fibres of different phenotypes, the effects of castration in adult (>6 months old) and in young adult (2-3 months old) male rabbits was examined. Immunohistochemical methods were used to evaluate the proportion of muscle fibres containing different myosin heavy-chain isoforms in 10 different neuromuscular compartments of the masseter. In young adult animals of both sexes, the proportion of fibres of different phenotypes in different compartments was not significantly different from that of normal adult females. In animals castrated as young adults, the development of adult male phenotype proportions was completely blocked in most compartments. In animals castrated as adults, proportions were not significantly different from those of the intact males. For most masseter compartments, androgens produced permanent changes in muscle fibre phenotype during a critical period of postnatal development. However, in the posterior deep compartment, androgen deprivation in young adults had no effect on phenotype proportions, but castration of adults resulted in a striking increase in the proportion of fibres containing the IIa myosin heavy-chain isoform.

Characterisation of human soft palate muscles with respect to fibre types, myosins and capillary supply

Four human soft palate muscles, and palatopharyngeus, the uvula, the levator and tensor veli palatini were examined using enzyme-histochemical, immunohistochemical and biochemical methods and compared with human limb and facial muscles. Our results showed that each palate muscle had a distinct morphological identity and that they generally shared more similarities with facial than limb muscles. The palatopharyngeus and uvula muscles contained 2 of the highest proportions of type II fibres ever reported for human muscles. In contrast, the levator and tensor veli palatini muscles contained predominantly type I fibres. A fetal myosin heavy chain isoform (MyHC), not usually found in normal adult limb muscles, was present in a small number of fibres in all palate muscles. The mean muscle fibre diameter was smaller than in limb muscles and the individual and intramuscular variability in diameter and shape was considerable. All palate muscles had a high capillary density and an unusually high mitochondrial enzyme activity in the type II fibres, in comparison with limb muscles. No ordinary muscle spindles were observed. The fibre type and MyHC composition indicate that the palatopharyngeus and uvula muscles are functionally involved in quick movements whereas the levator and tensor veli palatini muscles perform slower and more continuous contractions. The high aerobic capacity and the rich capillarisation suggest that the palate muscles are relatively fatigue resistant. Absence of ordinary muscle spindles indicates a special proprioceptive control system. The special morphology of the palate muscles may be partly related to the unique anatomy with only one skeletal insertion, a feature consistent with muscle work at low load and tension and which may influence the cytoarchitecture of these muscles. Other important factors determining the special morphological characteristics might be specific functional requirements, distinct embryological origin and phylogenetic factors.

Myosin isoform composition of the human medial and lateral pterygoid muscles

The medial and lateral pterygoid muscles are different in structure as well as in function. The medial pterygoid muscle is concentrically active during jaw closing, and the superior head of the lateral pterygoid muscle is eccentrically active during jaw closing, while its inferior head is concentrically active during jaw opening. Architecturally, the medial pterygoid can deliver higher forces than the lateral pterygoid. We investigated whether these differences are reflected in the myosin heavy-chain (MyHC) composition and the fiber cross-sectional area (f-csa) of these muscles. The pterygoid muscles from eight cadavers were investigated by means of monoclonal antibodies against different isoforms of MyHC. The proportions of pure MyHC type I fibers did not differ significantly among the muscles (32% in medial pterygoid, 34% in superior head, and 36% in the inferior head of the lateral pterygoid), nor did the total proportions of pure MyHC type IIA and IIX fibers (16% in medial pterygoid, 26% in the superior head, and 19% in the inferior head of the lateral pterygoid). The mean f-csa of type I fibers was 1315 microm2, which did not differ significantly among the muscles, and was significantly larger than the f-csa of type IIA fibers. The relative proportions of hybrid fibers, which expressed more than one MyHC isoform, were 52% in the medial pterygoid, 40% in the superior head, and 45% in the inferior head of the lateral pterygoid and did not differ significantly among the muscles. The most abundant hybrid fiber types found were fibers expressing MyHCs-cardiac alpha+IIA and MyHCs-cardiac alpha+I+IIA. Significant regional differences were found in the proportions of MyHC type I fibers in the medial pterygoid and in the inferior head of the lateral pterygoid. Although the form and function of the muscles are different, we conclude that this is not reflected in their myosin isoform composition.

Myosin heavy chain composition of the human lateral pterygoid and digastric muscles in young adults and elderly

The myosin heavy chain (MyHC) content in different parts of, two jaw opening muscle, the human lateral pterygoid and the digastric muscles of five young adult and five elderly subjects (mean age 22 and 73 years, respectively) was determined, using gel electrophoresis and immunohistochemical methods. The lateral pterygoid of both young and elderly contained predominantly slow MyHC, and fast A MyHC was the major fast isoform. In contrast, the digastric was composed of slow, fast A and fast X MyHCs in about equal proportions in both age groups. About half of the lateral pterygoid fibres contained mixtures of slow and fast MyHCs, often together with alpha-cardiac MyHC. In the digastric, co-existence of slow and fast MyHCs was rare, and alpha-cardiac MyHC was lacking. On the other hand, co-expression of fast A and fast X MyHCs was found more often in the digastric than in the lateral pterygoid. In both age groups about half of the digastric IIB fibres contained solely fast X MyHC. In the lateral pterygoid, type IIB fibres with pure fast X MyHC was found in only one subject. The lateral pterygoid in elderly showed a significant amount of fibres with solely fast A MyHC, which were occasionally found in young adults. In the digastric, no significant differences were found between young and elderly, although the muscles of elderly contained lower mean value of slow MyHC, as compared to that of young muscles. It is concluded that the lateral pterygoid and the digastric muscles differ not only in the MyHC composition but also in modifications of the MyHC phenotypes during aging, suggesting that they have separate roles in jaw opening function.

Stapedius muscle fibre composition in the rat

The stapedius muscle (SM) is supposed to prevent cochlear damage by noise. Consequently functional demands are the ability of fast contraction with long endurance. This implies the presence of a large fraction of myosin type II fibres with an appreciable oxidative capacity. We determined the myosin composition of SM fibres using consecutive complete SM cross-sections (6 week old rats) which were processed by enzyme histochemistry (EHC) to determine acid/alkali lability of myofibrillar adenosine triphosphatase (mATPase) or by immunohistochemistry (IHC) using myosin heavy chain (MyHC) antibodies. Method accuracy was determined in co-processed extensor digitorum longus (EDL). Four hundred SM and 200 EDL fibres were assigned to mATPase type I, IIA, IIB, IIX or 'miscellaneous' ('Misc') categories. Per mATPase category the fibres were attributed to groups with specific MyHC composition. In the EDL, mATPase type I and IIB fibres expressed only MyHC I and IIB respectively, whereas about 10% of the type IIA and 40% of the type IIX fibres expressed more than one MyHC. Thus IHC detects amounts of myosin isoforms which are not detected by EHC. The mATPase IIX category criterion leaves the possibility that this category contains fibres with myosin type IIA and/or IIB in larger amounts. The criteria of the mATPase categories type I, IIA or IIB preclude assignment to these categories of fibres which also contain other myosin isoforms in larger amounts. Such fibres were classified in one of the mATPase 'Misc' categories. Thus in the EDL the capability of the EHC criteria to select 'pure' fibres in terms of myosin differs per mATPase category. None of the SM fibres were assigned to the mATPase type I or IIB categories, about 25% to the type IIA, 60% to type IIX and 15% (including most fibres which expressed MyHC I) to a 'Misc' category. All SM fibres expressed two or more MyHC isoforms, MyHC IIB occurring in all fibres and substantial amounts of MyHC IIA and/or IIX in most. These findings confirm the hypothesis that such fibres have the capacity to contract fast and have the better fatigue resistance.

Skeletal muscle function and fibre types: the relationship between occlusal function and the phenotype of jaw-closing muscles in human

Mammalian skeletal muscle cells are composed of repeated sarcomeric units containing thick and thin filaments of myosin and actin, respectively. Excitation of the myosin ATPase enzyme is possible only with presence of Mg-ATP and Ca(2+). Skeletal muscle fibres may be classified into several types according to the isoform of myosin they contain. Nine isoforms of myosin heavy chain are known to exist in mammalian skeletal muscle including type I, IIA, IIB, IIX, IIM, alpha, neonatal, embryonic, and extra-ocular. Healthy adult human limb skeletal muscle contains type I, IIA, IIB, and IIX myosin heavy chains. The jaw-closing muscles of most carnivores and primates have tissue-specific expression of the type IIM or 'type II masticatory' myosin heavy chain. Adult human jaw-closing muscles, however, do not contain IIM myosin. Rather, they express type I, IIA, IIX (as in human limb muscle), and myosins typically expressed in developing or cardiac muscle. The morphology of human jaw-closing muscle fibres is also unusual in that the type II fibres are of smaller diameter that type I fibres, except in cases of increased function and hypertrophy. This paper describes the relationship of fibre types and motor unit function to changes in human occlusion and masticatory activity. Refereed Scientific Paper

Postnatal muscle fibre composition of the gluteus medius muscle of Dutch Warmblood foals; maturation and the influence of exercise

The fibre type composition of the deep gluteus muscle was studied in biopsies of Dutch Warmblood foals from birth until age 48 weeks. Half the foals were given box-rest, the other half received exercise consisting of an increasing number of gallop sprints. The muscle fibre types were determined using monoclonal antibodies discriminating against the following myosin heavy chain (MHC) isoforms: types I, IIa, IId, Cardiac-alpha and Developmental. During the first 48 weeks there was a consistent increase of fibres expressing types IIa MHC, replacing fibres expressing IId MHC. This change was reflected in the presence of a quite large population of fibres co-expressing MHC IIa and IId. The difference between the exercised (training) and nonexercised (box-rest) groups was small but suggested that the increase of type IIa fibres was larger in the training group. It appeared that after birth a significant number of fibres coexpress either Developmental and type IIa-MHC or Cardiac-alpha and type I-MHC. The Developmental isoform disappears during the first 10 weeks after birth and almost all the alpha isoform expression during the first 22 weeks. It is concluded that a fast turnover of fibre types takes place in the deep gluteus medius in the first months postpartum. Potentially, exercise could have an effect on the rate of change of these fibre types.

Neuromuscular fatigue during repeated exhaustive submaximal static contractions of knee extensor muscles in endurance-trained, power-trained and untrained men

The neural and muscular changes during fatigue produced in repeated submaximal static contractions of knee extensors were measured. Three groups of differently adapted male subjects (power-trained, endurance-trained and untrained, 15 in each) performed the exercise that consisted of 10 trials of submaximal static contractions at the level of 40% of maximal voluntary contraction (MVC) force till exhaustion with the inter-trial rest intervals of 1 min. MVC force, reaction time and patellar reflex time components before and after the fatiguing exercise and following 5, 10 and 15 min of recovery were recorded. Endurance-trained athletes had a significantly longer holding times for all the 10 trials compared with power-trained athletes and untrained subjects. However, no significant differences in static endurance between power-trained athletes and untrained subjects were noted. The fatigue test significantly prolonged the time between onset of electrical and mechanical activity (electromechanical delay) in voluntary and reflex contractions. The electromechanical delay in voluntary contraction condition for power-trained and untrained subjects and in reflex condition for endurance-trained subjects had not recovered 15 min after cessation of exercise. No significant changes in the central component of visual reaction time (premotor time of MVC) and latency of patellar reflex were noted after fatiguing static exercise. It is concluded, that in this type of exercise the fatigue development may be largely owing to muscle contractile failure.

Opposite changes in myosin heavy chain composition of human masseter and biceps brachii muscles during aging

The myosin heavy chain (MyHC) content in functionally different parts of the human masseter muscle of six elderly and five young adult subjects (mean age 74 and 22 years, respectively) was determined, using gel electrophoresis. The MyHC composition of the old masseter was also studied by enzyme- and immunohistochemical methods and compared with previous data for young adults. For comparison, the biceps brachii muscle of the same subjects was also analysed. The old masseter contained smaller amounts of slow and larger amounts of fast and fetal MyHCs. These differences were region-dependent and were more pronounced in the superficial portion. There was also a larger proportion of "hybrid" fibres, containing two to four MyHC isoforms (42%), compared with the young adult masseter (23%). No such differences were observed between old and young biceps. In contrast to the masseter, the old biceps contained more slow MyHC and less fast MyHC. This investigation demonstrates that the aging process in human skeletal muscle is accompanied by a modification in the muscle phenotype which is both muscle and region specific; a transformation towards a fast and fetal phenotype concomitant with an increased number of fibres with a mixture of different MyHC isoforms in the masseter; and an opposite shift towards a slower phenotype in the biceps brachii. The results might reflect differences between jaw and limb muscles in genetic programs and adaptive responses to changed functional demands following aging.

Regional differences in fibre type composition in the human temporalis muscle

Anatomical and electromyographic studies point to regional differences in function in the human temporalis muscle. During chewing and biting the anterior portions of the muscle are in general more intensively activated and they are capable of producing larger forces than the posterior portions. It was hypothetised that this heterogeneity in function is reflected in the fibre type composition of the muscle. The composition and surface area of different fibre types in various anteroposterior portions of the temporalis muscle were investigated in 7 cadavers employing immunohistochemistry with a panel of monoclonal antibodies against different isoforms of myosin heavy chain. Pure slow muscle fibres, type I, differed strongly in number across the muscle. In the most posterior portion of the muscle there were 24% type I fibres, in the intermediate portion 57%, and in the most anterior portion 46%. The mean fibre cross-sectional area (m-fcsa) of type I fibres was 1849 microm2, which did not differ significantly across the muscle. The proportion of pure fast muscle fibres, type IIA and IIX, remained more or less constant throughout the muscle at 13% and 11% respectively; their m-fcsa was 1309 microm2 and 1206 microm2, respectively, which did not differ significantly throughout the muscle. Pure type IIB fibres were not found. The relative proportion of hybrid fibres was 31% and did not differ significantly among the muscle portions. Fibre types I + IIA and cardiac alpha + I + IIA were the most abundant hybrid fibre types. In addition, 5% of the type I fibres had an additional myosin isoform which has only recently been described by means of electrophoresis and was named Ia. In the present study they were denoted as hybrid type I + Ia muscle fibres. It is concluded that intramuscular differences in type I fibre distribution are in accordance with regional differences in muscle function.

Transient expression of myosin heavy chain MHCI alpha in rabbit muscle during fast-to-slow transition

The expression of an alpha-cardiac-like myosin heavy chain, MHCI alpha, was investigated at both the mRNA and protein levels in rabbit tibialis anterior muscle undergoing fast-to-slow transition by continuous chronic low-frequency stimulation (CLFS). According to sequence analyses of the PCR product, the MHCI alpha isoform was found to be identical to the alpha-cardiac MHC expressed in rabbit atrium. In muscles at different degrees of transformation, the upregulation of MHCI alpha mRNA preceded that of the MHCI beta mRNA. At more advanced stages of the transformation, MHCI alpha mRNA decayed while MHCI beta mRNA persisted at high levels. The expression of MHCI alpha, therefore, was transitory. Studies at the protein level were based on immunoblotting using a monoclonal antibody (F88 12F8,1), characterized to be specific to MHCI alpha in rabbit muscle. These studies revealed a similar relationship between initial increase and successive decline of the MHCI alpha protein as seen at the mRNA level. Immunohistochemistry of 30-day stimulated muscle revealed that up to 65% of the fibres expressed the MHCI alpha isoform in combination with other adult MHC isoforms. The most frequent patterns of coexistence were MHCIIa + MHCI alpha + MHCI beta (28%), MHCI alpha + MHCI beta (18%), and MHCIIa + MHCI alpha (11%). According to these combinations, the upregulation of MHCI alpha may be assigned as an intermediate step in the transformation of existing fibres during the MHCIIa-->MHCI beta transition. A small fraction of fibres contained, in addition to the MHCI alpha + MHCI beta and MHCIIa + MHCI alpha combinations, developmental myosin, suggesting that MHCI alpha was also expressed in regenerating fibres originating from satellite cell-derived myotubes.

Skeletal fiber types and spindle distribution in limb and jaw muscles of the adult and neonatal opossum, Monodelphis domestica

The South American opossum, Monodelphis domestica, is very immature at birth, and we wished to assess its potential for studies of jaw muscle development. Given the lack of prior information about any Monodelphis fiber types or spindles, our study aimed to identify for the first time fiber types in both adult and neonatal muscles and the location of spindles in the jaw muscles. Fiber types were identified in frozen sections of adult and 6-day-old jaw and limb muscles by using myosin ATPase and metabolic enzyme histochemistry and by immunostaining for myosin isoforms. The distribution of fiber types and muscle spindles throughout the jaw-closer muscles was identified by immunostaining of sections of methacarnoy-fixed, wax-embedded heads. Most muscles contained one slow (type I) and two fast fiber types (equivalent to types IIA and IIX), which were similar to those in eutherian muscle, and an additional (non-IIB) fast type. In jaw-closer muscles, the main extrafusal fiber type was IIM (characteristic of these muscles in some eutherians), and almost all spindles were concentrated in four restricted areas: one in masseter and three in temporalis. Six-day neonatal muscles were very immature, but future spindle-rich areas were revealed by immunostaining and corresponded in position to the adult areas. Extrafusal and spindle fiber types in Monodelphis share many similarities with eutherian mammalian muscle. This finding, along with the immaturity of myosin isoform expression observed 6 days postnatally, indicates that Monodelphis could provide a valuable model for studying early developmental events in the jaw-closer muscles and their spindles.

Different phenotypes among slow/beta myosin heavy chain-containing fibres of rabbit masseter muscle: a novel type of diversity in adult muscle

Difference in the phenotype of different mammalian muscle fibres are usually attributed to differences in the expression of the product of different myosin heavy chain (MyHC) genes, which are known as isoforms. We studied differences in phenotype among fibres containing a single MyHC isoform (slow/beta) of the masseter muscle of adult rabbits. Four different monoclonal antibodies to slow/beta MyHC were used to stain serial sections from muscles in males and females. All antibodies recognize a single band on immunoblots and stain the same set of fibres in rabbit postcranial muscles. However, differential staining was observed in the masseter muscles. Antibody BA-D5 reacts with the most fibres, antibody A4.951 reacts with a subset of these fibres, and antibody A4.840 reacts with a subset of these fibres, and antibody A4.840 reacts with a subset of A4.951-positive fibres. Antibody S58 reacts only with an even smaller subset of fibres. Even though differential staining using four antibodies might allow for the expression of as many as 15 different staining patterns, or patterns, or phenotypes, only four were observed on > 99% of over 30 000 fibres studied. In females, nearly 40% of the fibres stain exclusively with antibody BA-D5, while in males, fewer than 8% of the fibres express this phenotype. The proportions of fibres of the other phenotypes do not differ so strikingly with gender. We conclude that an epitope diversity exists among muscle fibres in the adult rabbit masseter and that it is not necessarily a consequence of differences in gene expression. We feel that it is a regulated process and that, at least for some phenotypes, this regulation may be hormonally influenced.

Cardiac and extracardiac expression of Csx/Nkx2.5 homeodomain protein

Csx/Nkx2.5 is an evolutionary conserved homeobox gene related to the Drosophila tinman gene, which is essential for the dorsal mesoderm formation. Expression of Csx/Nkx2.5 mRNA is the earliest marker for heart precursor cells in all vertebrates so far examined. Previous studies have demonstrated that Csx/Nkx2.5 mRNA is highly expressed in the heart and at lower levels in the spleen, tongue, stomach, and thyroid in the murine embryo. Since some developmental genes are regulated by posttranscriptional mechanisms, we analyzed the developmental pattern of Csx protein expression at the single-cell level using Csx-specific antibodies. Immunohistochemical analysis of murine embryos at 7.8 days post coitum revealed that Csx protein is strongly expressed in the nucleus of endodermal and mesodermal cells in the cardiogenic plate. Subsequently, in the heart, Csx protein was detected only in the nucleus of myocytes of the atrium and the ventricle through the adult stage. During the fetal period, Csx protein expression in the nucleus was also noted in the spleen, stomach, liver, tongue, and anterior larynx. Unexpectedly, confocal microscopy revealed that Csx immunoreactivity was detected only in the cytoplasm of a subset of cranial skeletal muscles. Csx protein was not detected in the thyroid glands. The expression of Csx protein in all organs was markedly downregulated after birth except in the heart. These results raise the possibility that Csx/Nkx2.5 may play a role in the early developmental process of multiple tissues in addition to its role in early heart development.

Alpha-cardiac-like myosin heavy chain as an intermediate between MHCIIa and MHCI beta in transforming rabbit muscle

To elucidate the sequence of myosin heavy chain (MHC) transitions in fast-to-slow transforming rabbit muscle, direct reverse transcriptase-polymerase chain reaction was applied for detecting mRNAs specific to five MHC isoforms in single fibers from control and low-frequency-stimulated tibialis anterior muscles. The detection of MHCIIb, MHCIId(x), MHCI alpha, and MHCI beta mRNAs was based on previously published methods. The RT-PCR assay for MHCIIa mRNA was based on the identification of a cDNA sequence in the 3'-region from which specific primers were derived. Comparisons between rat, rabbit, and human MHCIIa sequences revealed high degrees of sequence identities. MHC mRNA isoform patterns in single fibers from stimulated muscles showed hybrid fibers expressing the following combinations: MHCIId(x) + MHCIIa, MHCIId(x) + MHCIIa + MHCI alpha, MHCIId(x) + MHCIIa + MHCI alpha + MHCI beta, MHCIIa + MHCI alpha, MHCIIa + MHCI alpha + MHCI beta, and MHCI alpha + MHCI beta. The combination MHCIIa + MHCI beta without MHCI alpha was never seen. These coexpression patterns suggest that the fast-to-slow fiber transition results from sequential isoform expressions in the order MHCIId(x)--> MHCIIa-->MHCI alpha-->MHCI beta. The allocation of MHCI alpha between MHCIIa and MHCI beta seems to be in line with graded differences in sequence identity of the 3'-regions of these mRNA isoforms.

Adverse changes in fibre type composition of the human masseter versus biceps brachii muscle during aging

The fibre composition of functionally different regions, the superficial and the deep portions, of the human masseter and the biceps brachii muscles of six elderly subjects (mean age 74 years) was studied by morphological and enzyme-histochemical methods. When compared to previous data for young adults, the masseter muscle from elderly subjects showed a significant decrease in the proportion of type I fibres and an increase of type IM and II fibre types, concomitant with muscle fibre atrophy. In the old biceps, there were no changes in the type I fibre proportion and fibre diameter, but a significant decrease in the type IIB fibre content. The present results of changes in fibre composition, in addition to previous findings of functional deterioration of the old masseter, probably reflect a combination of age related degenerative and adaptive processes within the trigeminal neuromuscular system. When compared with previous findings in young adult masseter, it can be concluded that the human masseter muscle differs from limb muscles both in fibre composition and in structural response of the muscle during aging. These differences between the masseter and the biceps brachii muscle suggest muscle specific alterations during aging probably due to differences in genetical control, functional performance and nerve and hormonal influences.

Transitory expression of alpha cardiac myosin heavy chain in a subpopulation of secondary generation muscle fibers in the pig

Unlike the random distribution of fiber types seen in skeletal muscles of most mammals, pig muscle exhibits a rosette pattern consisting of islets of slow fibers surrounded by concentric circles of type IIA and IIB fibers. Within each islet of slow fibers, one of the central fibers is a primary myofiber, whereas all others are secondary fibers. The present study demonstrates that a subpopulation of the slow secondary fibers transiently expresses alpha-myosin heavy chain (MHC). Two cDNA libraries were made from longissimus dorsi skeletal muscle of 14-day-old piglet and adult pig atrium; the latter muscle is mainly composed of alpha-MHC. Screening of the libraries with a human anti-alpha-MHC mAb (F8812F8) demonstrated the presence of positive MHC clones in both libraries; the nucleotide sequence of the 3'-untranslated region (3'-UTR) was identical in both libraries. As this MHC 3'-UTR had 75% homology with the human alpha-MHC, it was identified as pig alpha-MHC. Using specific cRNA probes and mAbs against pig alpha-cardiac and beta/slow/type I MHC, we studied the expression of these MHCs in developing pig semitendinosus muscle by combining in situ hybridization and immunocytochemistry on serial sections at 90 days of gestation, and at 1, 6, 35 days and 6 months of age. The results showed that a subpopulation of secondary fibers that directly abut primary fibers, transiently produced alpha-MHC, both at the levels of the protein and its transcript. Subsequently, these fibres expressed beta-MHC. At 1 day, immunocytochemistry showed that 16% of the secondary fibers expressed alpha-MHC, among which 20% did not yet express beta-MHC. At 6 days, alpha- and beta-MHCs were mostly present in the same fibers, i.e., 23% of the secondary fibers. Thereafter, the proportion of secondary fibers reacting with alpha-MHC mAb decreased to 10% at 5 weeks and 0% at 6 months, whereas beta-MHC was still accumulating in about 38% of the secondary fibers. During the period studied, the distribution of alpha- and beta-MHC transcripts closely matched that of the corresponding proteins. Expression of alpha-MHC was not detected in primary type I muscle fibers and slow type I secondary fibers at the periphery of the rosettes of slow fibers. This study is the first unequivocal demonstration of a transitory expression of alpha-MHC in a subpopulation of secondary fibers in a limb skeletal muscle during mammalian development.

Expression of an alpha-cardiac like myosin heavy chain in diaphragm, chronically stimulated, and denervated fast-twitch muscles of rabbit

An additional slow fibre type, type I alpha, is detected in diaphragm and appears in fast-twitch hindlimb muscles of rabbit under the influence of altered neuromuscular activity. Type I alpha fibres were delineated from fibres expressing myosin heavy chain I beta (type I beta) by immunohistochemistry with a monoclonal antibody raised against the alpha-cardiac MHCI alpha. When stained for mATPase after acid and alkaline preincubations, some type I alpha fibres resembled type I beta and type IIA fibres, respectively. Some type I alpha fibres displayed dissimilar mATPase staining, indicating heterogeneity of this fibre population. The appearance of numerous type I alpha fibres in stimulated muscles, which in addition contain type IIA and type I beta fibres, suggested that they may be interspaced between types IIA and I beta. Electrophoresis under nondenaturing conditions disclosed an additional isomyosin both in normal diaphragm and stimulated muscles. This band displayed the same mobility as the slowest isomyosin in rabbit masseter muscle. It was recognized by the same monoclonal (anti-alpha-cardiac MHC) antibody used for immunohistochemistry. Therefore, this isomyosin appeared to be very similar, but perhaps not identical to the alpha-cardiac MHC-based isomyosin, probably resulting from discrete differences in the MHC complement. This assumption agrees with additional findings suggesting an even greater heterogeneity of the MHCs than generally assumed. In support of this, we show in atrium and masseter muscles the existence of an additional, electrophoretically distinct MHC isoform which migrates in close vicinity to MHCI alpha.

Progress in myocardial damage detection: new biochemical markers for clinicians

New clinical requirements for triaging chest pain patients challenge the abilities of the current cardiac markers. Serial measurements of myoglobin, creatine kinase (CK) isoenzyme MB (CKMB) mass, or CK isoforms in emergency rooms help to rapidly rule out acute myocardial infarction (AMI). However, within the first 3 to 4 h from chest pain onset, their sensitivities are too low to contribute significantly to AMI diagnosis during this period. CKMB and lactate dehydrogenase (LDH) isoenzyme 1 are not heart-specific, which hampers reliable diagnosis in patients with concomitant skeletal muscle damage. By contrast, the regulatory proteins troponin I and troponin T are expressed in three different isoforms: one for slow-twitch skeletal muscle fibers, one for fast-twitch skeletal muscle fibers, and one for cardiac muscle (cTnI, cTnT); cardiac-specific cTnI and cTnT assays are already available for routine use. cTnT and cTnI are the most promising markers for risk stratification in patients with unstable angina pectoris. Recent reports on increased cTnT in patients with renal failure or myopathy without evidence of myocardial injury and undetectable cTnI suggest that cTnT could be reexpressed similar to CKMB and LDH-1 in chronically damaged human skeletal muscle. Therefore, cTnI is probably the most heart-specific marker. Among the recently proposed new markers for early AMI diagnosis: glycogen phosphorylase isoenzyme BB (GPBB), fatty acid binding protein, phosphoglyceric acid mutase isoenzyme MB, enolase isoenzyme alpha beta, S100a0, and annexin V, GPBB is the most promising because it increases as early as 1 to 4 h from chest pain onset and its early release appears to be essentially dependent on ischemic myocardial injury.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Unloaded shortening velocities of rabbit masseter muscle fibres expressing skeletal or alpha-cardiac myosin heavy chains

1. Some rabbit masseter fibres express the alpha-cardiac myosin heavy chain (MHC). To compare the biochemical and physiological properties of these fibres with other skeletal fibre types, we examined the histochemical and immunohistochemical staining characteristics, maximum velocity of shortening (V(zero)) and MHC isoform content of fibres from rabbit masseter and soleus muscles. 2. The fibre-type composition of muscle sections was determined with MHC antibodies and myofibrillar ATPase histochemistry. Fibres we designated 'type alpha-cardiac' were different from type I and type II fibres in that they stained positively with the alpha-cardiac MHC antibody and they maintained. ATPase reactivity after acid and alkali pre-incubations. Samples of superficial masseter contained a few type I fibres, with the majority of fibres classified as either type IIA or type alpha-cardiac. Soleus samples contained type I, IIA and IIC fibres. 3. The V(zero) of chemically skinned fibres was determined by the slack-test method. Each fibre was subsequently characterized as type I, IIA, IIC or alpha-cardiac from MHC identification using gel electrophoresis (SDS-PAGE). In masseter fibres the V(zero) values were (in muscle lengths s-1): type I, 0.54 +/- 0.05 (mean +/- S.D., n = 3); type IIA, 1.23 +/- 0.34 (n = 27); type alpha-cardiac, 0.78 +/- 0.08 (n = 9). In soleus fibres V(zero) values were: type I, 0.55 +/- 0.06 (n = 14); type IIA, 0.89 +/- 0.04 (n = 8); type IIC, 0.73 (n = 2). 4. We conclude that the rabbit masseter muscle contains an 'alpha-cardiac' fibre type that is distinct from other skeletal fibres. This fibre type expresses only the alpha-cardiac MHC, has unusual myofibrillar ATPase reactivity and has a V(zero) intermediate between type I and type II fibres.

Effect of maturation on 31P magnetic resonance spectroscopy of the rabbit masseter muscle

This work studies the dynamic metabolic changes of the rabbit masseter muscle during post-natal development. The composition and proportion of oxidative and glycolytic muscle fibers alter during maturation. The masseter muscle, as most muscles of the craniofacial region, exhibits unusual development in composition of isoforms of myosin. The effect of this unusual composition on the dynamic metabolic properties of the masseter muscle have not been assessed. The metabolism of the rabbit masseter muscle was studied by means of 31P-nuclear magnetic resonance (NMR) spectroscopy. Contraction was elicited by electrical stimulation of the muscle in the anesthetized animal. Five animals were studied at 8 weeks and 24 weeks so that both the juvenile and adult stages could be evaluated. The dynamic biochemical changes in the masseter muscle were studied by the analysis of NMR spectra. A single-turn surface coil (copper) was used, and the original signal was treated with Fourier transforms to obtain 31P spectra. The low signal-to-noise ratio required averaging 16 acquisitions (acquisition time = 400 msec, repetition rate = 1.8 sec) in 30 sec and then obtaining continuous spectra for 27 min. Each averaged spectrum demonstrated five peaks: inorganic phosphate (Pi), creatine phosphate (PCr), and three peaks related to adenosine triphosphate (ATP). The protocol involved recording an initial three-minute rest period, stimulating the muscle at 5 Hz for 3 min twice, separated by three-minute rest periods, and stimulating the muscle at 50 Hz twice for 3 min separated by rest periods. The Pi/PCr ratio increased significantly in the adult masseter during both 5-Hz stimulations, evoking twitching, and the first 50-Hz stimulation, evoking tetany (repeated ANOVA, P < 0.05). The resting pH (6.96 +/- 0.13) was significantly lowered during both twitching (6.85 +/- 0.10; P < 0.0038) and tetany (6.55 +/- 0.13; P < 0.0001), but only in the adult masseter muscle. These finding suggest that the adult masseter muscle possesses more glycolytic fibers as it modifies its metabolism during postnatal development.

Plasticity of craniomandibular muscle function: 31P magnetic resonance spectroscopy of the rabbit masseter muscle

The masseter muscle was studied during postnatal development of the rabbit from the juvenile to adult stage in which the oral function was altered during maturation by modifying the diet to soft food. The muscle was assessed using phosphate magnetic resonance (31P NMR) spectroscopy with a single-turn copper surface coil to study potential changes in phosphate metabolism. The 31P NMR spectra consisted of five peaks related to unbound forms of inorganic phosphate (Pi), creatine phosphate (PCr), and three peaks related to the adenosine triphosphate (ATP). The masseter was assessed in one group of five rabbits at 8 weeks postnatally (juvenile) and after 4 months of this experimental group masticating on soft food. They were compared with a control group of five rabbits raised on a normal hard diet. The Pi/PCr ratio increased in the adult masseter much higher during twitching, tetany, and periodic contraction than in the juvenile regardless as to whether the adult animal had been raised from the juvenile period on soft or hard diet. There were relatively few differences between the experimental adult animals raised on a soft diet and the normal adult animals despite the soft diet animals demonstrating a significantly lower weight and smaller muscle mass. These findings suggest that chronic underuse of the masseter muscle by decreasing the masticatory loads has a minimal effect on the phosphate metabolism of the maturing masseter.

Reverse transcriptase-polymerase chain reaction detects induction of cardiac-like alpha myosin heavy chain mRNA in low frequency stimulated rabbit fast-twitch muscle

Using reverse transcriptase-polymerase chain reaction we quantified in rabbit skeletal muscles expression levels of the highly homologous cardiac alpha and beta myosin heavy chain (alpha MHC, beta MHC) mRNA isoforms. Masseter muscle displayed highest levels of a cardiac-like alpha MHC mRNA. This isoform was present at 20-fold lower amounts in slow soleus and at 200-fold lower levels in several fast-twitch muscles. Low-frequency stimulation periods exceeding 20 days drastically induced the alpha MHC mRNA in fast tibialis anterior. The alpha MHC mRNA was 140-fold elevated after 60 days when beta MHC mRNA had increased 50-fold. Our results demonstrate the wide distribution of a cardiac-like alpha MHC mRNA in skeletal muscle and its marked induction during fast-to-slow transition as induced by low-frequency stimulation.

Myosin expression in the jaw-closing muscles of the domestic cat and American opossum

Sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-PAGE), glycerol SDS-PAGE, two-dimensional electrophoresis, and protein immunoblotting techniques were used to identify myosin heavy chain (MHC) and light chain (MLC) isoforms in limb and masticatory muscles of the cat and American opossum. The fibre types in which these isoforms are expressed were identified by histochemistry and immunohistochemistry. Antibodies specific for the type IIM MHC isoform characteristic of cat jaw-closing muscles and the type I MHC isoform were produced and characterized. The IIM antibody stained the majority of fibres found in the jaw-closing muscles of both species. These IIM-containing fibres characteristically had a histochemical ATPase that remained active after both acid and alkali pre-incubations. A minority of type I fibres was also present in cat jaw-closing muscles, and these reacted positively with antibody specific for type I MHC. It was confirmed that the vast majority of fibres in the cat jaw-closing muscles contained only the characteristic masticatory MHC (IIM) and masticatory MLCs (LC1m and LC2m). These muscles did not contain either the type II fibre isoforms of limb muscles or the atrial cardiac (alpha-cardiac) MHC. The type IIM MHC could also be identified in jaw-closing muscles of the opossum. Two-dimensional gel electrophoresis was used to identify the MLC composition of single, histochemically defined, type I fibres in the cat soleus and deep masseter. The type I fibres of limb muscle contained the usual slow MLCs, but type I fibres from the jaw-closing muscles contained only the masticatory light chains.

Pattern of muscle fiber type formation in the pig

The aim of this study was to analyze the temporal sequence of expression of the myosin isoforms in the populations of muscle fibers in the pig and to bring more information on the origin of the strikingly different pattern of fiber composition and distribution between the deep medial red (oxido-glycolytic) and superficial white (glycolytic) portions of semitendinosus (ST) muscle. Muscle samples were taken from 49-, 55-, 75-, 90-, 103-, and 113- (birth) day-old fetuses, from 6-, 11-, 21-, 35-, 50-, and 80-day-old piglets, and from a 3-year-old pig. Our results confirm the sequential formation of primary and secondary generation fibers. The use of immunohistochemistry and heterologous monoclonal antibodies (mAb) directed against specific myosin heavy chain (MHC) isoforms revealed a different pattern of gene expression between the two portions of the ST muscle for both generations of fibers. By 75 days of gestation (dg), primary myotubes from the deep medial portion stained positively for the anti-slow MHC mAb and negatively for the adult anti-fast MHC, whereas the opposite was observed in the superficial portion. Secondary fibers never expressed slow MHC until late gestation. Instead, they expressed an adult fast MHC isoform as soon as they formed in the deep medial portion and later on in the superficial portion. From late gestation to the first 3 postnatal weeks, slow MHC began to be expressed in a subpopulation of secondary fibers. These fibers were in the direct vicinity of primary myotubes in the deep medial portion, whereas their location could not be established in the superficial portion. The remaining secondary fibers matured to type IIA in the direct vicinity of these type I fibers and to type IIB at the periphery of the islets. In both portions of the muscle, a subpopulation of secondary fibers, the first ones to express slow MHC, also transitorily expressed a MHC that was identical or closely related to the alpha-cardiac MHC during the early postnatal period. A third generation of small diameter fibers was observed shortly after birth and reacted with the anti-fetal MHC mAb; their destiny remains to be established.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of myosin heavy chain isoforms and myogenesis of intrafusal fibres in rat muscle spindles

This review concerns the pattern of expression and regulation of myosin heavy chain (MHC) isoforms in intrafusal fibres of rat muscle spindles detected by immunocytochemistry. The three types of intrafusal fibres--nuclear bag1, nuclear bag2, and nuclear chain fibres--are unique in co-expressing several MHCs including special isoforms such as slow tonic and alpha cardiac-like MHC and isoforms typical of muscle development, such as embryonic and neonatal MHC. The distinct intrafusal fibre types appear sequentially during rat hind limb development, the nuclear bag2 precursors being first identifiable at 17-18 days in utero as the only primary myotubes expressing slow tonic MHC. Sensory innervation is required for the expression of "spindle-specific" MHC isoforms. Motor innervation contributes to the diversity in distribution of the different MHCs along the length of the nuclear bag fibres. It is suggested that unique populations of myoblasts are destined to become intrafusal fibres during development in the rat hind limb muscles and that the regional heterogeneity in MHC expression is related both to sensory and motor innervation and to the properties of the myoblast lineages. These distinct features make intrafusal fibres an attractive in situ model for investigating myogenesis, myofibrillogenesis, and the mechanisms regulating MHC expression.

Cardiac myosin heavy chain expression during heart development in Xenopus laevis

Muscle-specific gene expression in the heart during Xenopus development was investigated using reverse transcription-polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization to detect transcripts of the gene for the cardiac myosin heavy chain (CMHC). RT-PCR analysis determined that CMHC transcripts are present in the cardiac mesoderm at state 13, demonstrating that muscle-specific gene expression in the primitive myocardium has begun by the early neurula stage, approximately 30 h before the heart beat begins. Xenopus, therefore, is similar to amniotes and mammals in that cardiac precursor cells begin to express muscle-specific gene transcripts soon after commitment to the cardiac myocyte lineage. The earliest CMHC gene transcripts can be detected in the heart using whole-mount in situ hybridization is early tailbud stage 28, which coincides with the onset of heart tube morphogenesis. CMHC gene expression was also detected in skeletal muscle: RT-PCR analysis determined that CMHC transcripts are transiently expressed in the somite during the initial phases of skeletal muscle differentiation. Furthermore, CMHC mRNAs are expressed in a subset of head muscles of the feeding tadpole. CMHC gene expression is induced in ectodermal cells of the animal cap in blastula-stage embryos injected with synthetic MyoD or Myf5 RNA, suggesting that the CMHC gene contains regulatory elements that are responsive to the activity of those skeletal-muscle-specific transcription factors.

Fibre type classification and myosin isoforms in the human masseter muscle

Human masseter muscle is highly unusual since it contains relatively large numbers of fibres with variable myofibrillar ATPase staining as well as fibres that express neonatal and alpha-cardiac myosin heavy chain (MHC). These findings however, have not been organised together into a fibre type classification scheme. Biopsies from the anterior superficial area of masseter were collected from a large sample of healthy young adults. Biopsies were sectioned and stained for myofibrillar ATPase reactivity and the presence of MHC isoforms as detected by a series of antibodies. The MHC composition of the same biopsies was also analysed using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). A series of rectus abdominis muscle biopsies were analysed similarly to serve as a control for type I, IIA and IIB fibres and isoforms. From the histochemical, immunohistochemical and biochemical experiments we found the masseter to contain type I, IM, IIC, IIA and IIB fibres as previously classified, but in addition there were type neonatal, alpha-cardiac, and 'other' (three or more myosins including neonatal and alpha-cardiac). The percentage of each fibre type was highly variable in masseter biopsies, but generally type I fibres were most common, and the proportion of IIB, neonatal, alpha-cardiac and 'other' fibres was low. Even in biopsies that contained relatively large amounts of these last three fibre types, the amount of neonatal and/or alpha-cardiac MHC detected on SDS-PAGE was limited, suggesting that these MHCs are a minor component in the fibres in which they are expressed.