Specificity of different anti-myosin heavy chain antibodies in bovine muscle

Preliminary Results about Lamb Meat Tenderness Based on the Study of Novel Isoforms and Alternative Splicing Regulation Pathways Using Iso-seq, RNA-seq and CTCF ChIP-seq Data

Tenderness is an important indicator of meat quality. Novel isoforms associated with meat tenderness and the role of the CCCTC-binding factor (CTCF) in regulating alternative splicing to produce isoforms in sheep are largely unknown. The current project studied six sheep from two crossbred populations (Dorper × Hu × Hu, DHH and Dorper × Dorper × Hu, DDH) with divergent meat tenderness. Pooled Iso-seq data were used to annotate the sheep genomes. Then, the updated genome annotation and six RNA-seq data were combined to identify differentially expressed isoforms (DEIs) in muscles between DHH and DDH. These data were also combined with peaks detected from CTCF ChIP-seq data to investigate the regulatory role of CTCF for the alternative splicing. As a result, a total of 624 DEIs were identified between DDH and DHH. For example, isoform 7.524.18 transcribed from CAPN3 may be associated with meat tenderness. In addition, a total of 86 genes were overlapped between genes with transcribed DEIs and genes in differential peaks identified by CTCF ChIP-seq. Among these overlapped genes, ANKRD23 produces different isoforms which may be regulated by CTCF via methylation. As preliminary research, our results identified novel isoforms associated with meat tenderness and revealed the possible regulating mechanisms of alternative splicing to produce isoforms.

Identification of novel mRNA isoforms associated with meat tenderness using RNA sequencing data in beef cattle

The Warner-Bratzler shear force (WBSF) and myofibrillar fragmentation index (MFI) are complementary methodologies used to measure beef tenderness. Longissimus thoracis samples from the 20 most extreme bulls (out of 80 bulls set) for WBSF (tender (n = 10) and tough (n = 10)) and MFI (high (n = 10) and low (n = 10)) traits were collected to perform transcriptomic analysis using RNA-Sequencing. All analysis were performed through CLC Genomics Workbench. A total of 39 and 27 transcripts for WBSF and MFI phenotypes were DE, respectively. The possible DE novel mRNA isoforms, for WBSF and MFI traits, are myosin encoders (e.g. MYL1 and MYL6). In addition, we identified potential mRNA isoforms related to genes affecting the speed fibers degradation during the meat aging process. The DE novel transcripts are transcripted by genes with biological functions related to oxidative process, energy production and striated muscle contraction. The results suggest that the identified mRNA isoforms could be used as potential candidate to select animals in order to improve meat tenderness.

Comparison of beef quality influenced by freeze-thawing among different beef cuts having different muscle fiber characteristics

The association of muscle fiber characteristics (MFC) with freeze-thawed meat quality was investigated using four different cuts (loin, M. longissimus thoracis; tenderloin, M. psoas major; top round, M. semimembranosus; eye of round, M. semitendinosus) obtained from beef carcasses (n = 10; Hanwoo steer; 28 months of age). Tenderloin had higher (P < .05) composition of muscle fiber type I than the others. Its quality traits except cooking loss were not affected by freeze-thawing (P > .05). However, top round having lower type I and higher type IIX compositions showed lower water-holding capacity and larger discoloration (P < .01) than other beef cuts after freeze-thawing. Increased tenderness after freeze-thawing was observed only for loin (P < .01) which had relatively larger size of type I fibers. In conclusion, individual muscle has different susceptibilities to freezing due to their different MFC. Considering different MFC by meat type when freezing meat will help to prevent excessive deterioration of meat quality.

Effects of supplementing rumen-protected niacin on fiber composition and metabolism of skeletal muscle in dairy cows during early lactation

Nicotinic acid (NA) has been shown to induce muscle fiber switching toward oxidative type I fibers and a muscle metabolic phenotype that favors fatty acid (FA) utilization in growing rats, pigs, and lambs. The hypothesis of the present study was that supplementation of NA in cows during the periparturient phase also induces muscle fiber switching from type II to type I fibers in skeletal muscle and increases the capacity of the muscle to use free FA, which may help to reduce nonesterified fatty acid (NEFA) flow to the liver, liver triglyceride (TG) accumulation, and ketogenesis. Thirty multiparous Holstein dairy cows were allocated to 2 groups and fed a total mixed ration without (control group) or with ∼55 g of rumen-protected NA per cow per day (NA group) from 21 d before expected calving until 3 wk postpartum (p.p.). Blood samples were collected on d -21, -14, -7, 7, 14, 21, 35, and 63 relative to parturition for analysis of TG, NEFA, and β-hydroxybutyrate. Muscle and liver biopsies were collected on d 7 and 21 for gene expression analysis and to determine muscle fiber composition in the musculus semitendinosus, semimembranosus, and longissimus lumborum by immunohistochemistry, and liver TG concentrations. Supplementation of NA did not affect the proportions of type I (oxidative) or the type II:type I ratio in the 3 muscles considered. A slight shift from glycolytic IIx fibers toward oxidative-glycolytic fast-twitch IIa fibers was found in the semitendinosus, and a tendency in the longissimus lumborum, but not in the semimembranosus. The transcript levels of the genes encoding the muscle fiber type isoforms and involved in FA uptake and oxidation, carnitine transport, tricarboxylic acid cycle, oxidative phosphorylation, and glucose utilization were largely unaffected by NA supplementation in all 3 muscles. Supplementation of NA had no effect on plasma TG and NEFA concentrations, liver TG concentrations, and hepatic expression of genes involved in hepatic FA utilization and lipogenesis. However, it reduced plasma β-hydroxybutyrate concentrations in wk 2 and 3 p.p. by 18 and 26% and reduced hepatic gene expression of fibroblast growth factor 21, a stress hormone involved in the regulation of ketogenesis, by 74 and 56%. In conclusion, a high dosage of rumen-protected NA reduced plasma β-hydroxybutyrate concentrations in cows during early lactation, but failed to cause an alteration in muscle fiber composition and muscle metabolic phenotype.

ATP Sensitive Potassium Channels in the Skeletal Muscle Function: Involvement of the KCNJ11(Kir6.2) Gene in the Determination of Mechanical Warner Bratzer Shear Force

The ATP-sensitive K⁺-channels (KATP) are distributed in the tissues coupling metabolism with K⁺ ions efflux. KATP subunits are encoded by KCNJ8 (Kir6.1), KCNJ11 (Kir6.2), ABCC8 (SUR1), and ABCC9 (SUR2) genes, alternative RNA splicing give rise to SUR variants that confer distinct physiological properties on the channel. An high expression/activity of the sarco-KATP channel is observed in various rat fast-twitch muscles, characterized by elevated muscle strength, while a low expression/activity is observed in the slow-twitch muscles characterized by reduced strength and frailty. Down-regulation of the KATP subunits of fast-twitch fibers is found in conditions characterized by weakness and frailty. KCNJ11 gene knockout mice have reduced glycogen, lean phenotype, lower body fat, and weakness. KATP channel is also a sensor of muscle atrophy. The KCNJ11 gene is located on BTA15, close to a QTL for meat tenderness, it has also a role in glycogen storage, a key mechanism of the postmortem transformation of muscle into meat. The role of KCNJ11 gene in muscle function may underlie an effect of KCNJ11 genotypes on meat tenderness, as recently reported. The fiber phenotype and genotype are important in livestock production science. Quantitative traits including meat production and quality are influenced both by environment and genes. Molecular markers can play an important role in the genetic improvement of animals through breeding strategies. Many factors influence the muscle Warner-Bratzler shear force including breed, age, feeding, the biochemical, and functional parameters. The role of KCNJ11gene and related genes on muscle tenderness will be discussed in the present review.

A simplified immunohistochemical classification of skeletal muscle fibres in mouse

The classification of muscle fibres is of particular interest for the study of the skeletal muscle properties in a wide range of scientific fields, especially animal phenotyping. It is therefore important to define a reliable method for classifying fibre types. The aim of this study was to establish a simplified method for the immunohistochemical classification of fibres in mouse. To carry it out, we first tested a combination of several anti myosin heavy chain (MyHC) antibodies in order to choose a minimum number of antibodies to implement a semi-automatic classification. Then, we compared the classification of fibres to the MyHC electrophoretic pattern on the same samples. Only two anti MyHC antibodies on serial sections with the fluorescent labeling of the Laminin were necessary to classify properly fibre types in Tibialis Anterior and Soleus mouse muscles in normal physiological conditions. This classification was virtually identical to the classification realized by the electrophoretic separation of MyHC. This immuno-histochemical classification can be applied to the total area of Tibialis Anterior and Soleus mouse muscles. Thus, we provide here a useful, simple and time-efficient method for immunohistochemical classification of fibres, applicable for research in mouse.

Development of image analysis tool for the classification of muscle fibre type using immunohistochemical staining

An accurate characterisation of muscle fibres is essential for studying muscle plasticity. During some transient events such as ageing, myogenesis, physical activity or conversion of muscle to meat, the morphological parameters and/or the fibre type distribution may change. Nowadays, this information is generally obtained using immunohistology techniques, but these analyses are acknowledged to be laborious and time-consuming. In fact, each myofibre, from thousands, must be measured individually and its expression profile in response to different anti-myosin antibodies must be established step by step. In this paper, we describe a new histological approach using double-labelling (laminin, myosin) serial sections, fluorescence microscopy visualisation and, finally, semi-automatic image analysis. The goal of the study was to propose a tool allowing faster fibre type characterisation, including the identification of hybrid fibres from pure ones. The steps in the image processing prone to subjectivity have been fully automated. On the other hand, the expert retained control of all image analysis procedures requiring visual diagnosis. The tool that we developed with the Visilog software allowed a rapid and objective fibre typing and morphometric characterisation of two different bovine muscles. The results were in agreement with our previous histological and densitometric assays. The method and the tool proved to be potentially more efficient than other techniques used in our institute or described in the literature. A more global evaluation will be considered in other laboratories as well as on other animal species.

Eastern grey kangaroo (Macropus giganteus) myofibres. 1. A simplified classification method using two commercially available antibodies

Skeletal muscles from eastern grey kangaroos (Macropus giganteus) were assessed for myofibre contractile and metabolic characteristics using immunocytochemical and histological staining of serial sections. Myofibre classification using monoclonal antibodies that typically bind to mammalian slow (clone WB-MHC), fast (clone MY-32) and Types 1, 2X and 2B (clone S5 8H2) myosin heavy chains was validated using acid- and alkali-preincubated myofibrillar ATPase, NADH and α-glycerophosphate dehydrogenase stains. Myofibres were classified as Type 1 (slow oxidative), Type 2A (fast oxidative-glycolytic), Type 2X/2B (fast glycolytic) or intermediate or transitional myofibre Types 2C (Type 1–Type 2A intermediate) and 2AX/B (Type 2A–Type 2X/2B intermediate). The Type 2 (fast) antibody (clone MY-32) used in the present study did not bind to a subset of fast myofibres in any of the eight kangaroo muscles investigated. These myofibres were identified as Type 2A using clone S5 8H2 and on the basis of the histochemical staining profile. Hence, a simplified immunostaining system using only clones WB-MHC (anti-Type 1) and MY-32 (anti-Type 2X/2B) allowed five myofibre types to be identified based on the staining pattern and intensity of staining for the two antibodies. It is concluded that the myofibres of muscles from kangaroos can be quickly classified into five types using two commercially available antibodies. This method is directly applicable for routine investigations into the myofibre properties of commercially important muscles from the kangaroo musculature and, when combined with enzymatic assays for oxidative and glycolytic activity, will allow for a better understanding of factors influencing the quality of meat from kangaroos.

In vivo proteome dynamics during early bovine myogenesis

Myogenesis is a complex process of which the underlying mechanisms are conserved between species, including birds and mammals. Despite a good understanding of the stages of myogenesis, many of the mechanisms involved in the regulation of proliferation of the successive myoblast generations, the cellular transitions cell proliferation/alignment of myoblasts/fusion of myoblasts into myotubes/differentiation of myofibres and the control of total myofibre number still remain unknown. An in vivo proteomic analysis of the semitendinosus muscle from Charolais foetuses, at three specific stages of myogenesis (60, 110 and 180 days postconception), was conducted using 2-DE and MS. Expression profiles of more than 170 proteins were revealed and analysed using two way hierarchical clustering and statistical analysis. Our studies identify, for the first time, distinct proteins of varied biological functions and protein clusters with myogenic processes, such as the control of cell cycle activity and apoptosis, the establishment of cellular metabolism and muscle contractile properties and muscle cell reorganisation. These results are of fundamental interest to the field of myogenesis in general, and more specifically to the control of muscle development in meat producing animals.

A comprehensive characterisation of the fibre composition and properties of a limb (flexor digitorum superficialis, membri thoraci) and a trunk (psoas major) muscle in cattle

BACKGROUND

The fibre type attributes and the relationships among their properties play an important role in the differences in muscle capabilities and features. Comprehensive characterisation of the skeletal muscles should study the degree of association between them and their involvement in muscle functionality. The purposes of the present study were to characterise the fibre type composition of a trunk (Psoas major, PM) and a limb (Flexor digitorum, membri thoraci, FD) muscle in the bovine species and to study the degree of coordination among contractile, metabolic and histological properties of fibre types. Immunohistochemical, histochemical and histological techniques were used.

RESULTS

The fibre type composition was delineated immunohistochemically in calf muscle samples, identifying three pure (I, IIA, and IIX) and two hybrid type fibres (I+IIA, and IIAX). Most of the fibres in FD were types I and IIA, while pure IIX were absent. All fibre types were found in PM, the IIX type being the most frequent. Compared to other species, small populations of hybrid fibres were detected. The five fibre types, previously identified, were ascribed to three different acid and alkaline mATPase activity patterns. Type I fibres had the highest oxidative capacity and the lowest glycolytic capacity. The reverse was true for the IIX fibres, whereas the type IIA fibres showed intermediate properties. Regarding the histological properties, type I fibres tended to be more capillarised than the II types. Correlations among contractile, metabolic and histological features on individual fibres were significantly different from zero (r values varied between -0.31 and 0.78). Hybrid fibre values were positioned between their corresponding pure types, and their positions were different regarding their metabolic and contractile properties.

CONCLUSION

Coordination among the contractile, metabolic and histological properties of fibres has been observed. However, the magnitude of the correlation among them is always below 0.8, suggesting that the properties of muscles are not fully explained by the fibre composition. These results support the concept that, to some extent, muscle plasticity can be explained by the fibre type composition, and by the properties derived from their metabolic and histological profiles.

Effect of ractopamine-hydrochloride and trenbolone acetate on longissimus muscle fiber area, diameter, and satellite cell numbers in cull beef cows1

The objective of this study was to evaluate the effects of coadministration of ractopamine-HCl (RAC) and trenbolone acetate plus estradiol (TBA) on LM fiber cross-sectional area (CSA), diameter, fiber-associated myonuclei, and satellite cell number. Culled crossbred beef cows (n = 98; 11 +/- 1.8 yr old; BCS 4.3 +/- 0.03) from a single ranch in south Florida were fed a concentrate diet for 92 d in a 2 x 2, randomized block design. Cows were blocked by BW on arrival into light (initial BW = 369.75 +/- 2.68 kg and end BW = 501.96 +/- 6.90 kg) and heavy (initial BW = 418.31 +/- 2.75 kg and end BW = 522.15 +/- 7.09 kg) groups before assignment to treatment. Factors included dietary treatment (0 or 15 ppm) and implant status (0 or 80 mg of trenbolone acetate + 16 mg of estradiol). Ractopamine was provided in the diet to 2 pens or half the treatments during the final 35 d of feeding. Cows were slaughtered on d 92. Forty-eight hours postmortem, the 6th-rib portions of the LM were obtained from 10 randomly selected carcasses from each treatment group (n = 40). Cryosections (12 mum) were immunostained for dystrophin and myosin heavy chain I or II for the measurement of fiber CSA and type, respectively. Fiber-associated nuclei and satellite cell numbers were measured in serial cryosections. There was a RAC x TBA interaction (P < 0.05). Type I fiber CSA and diameter were increased (P < 0.05) by TBA and RAC. Type I CSA and diameter were larger (P < 0.05) in TBA + RAC than RAC only. Type II fiber CSA and diameter were not affected by TBA (P = 0.48), RAC (P = 0.15), or TBA + RAC (P = 0.60). Satellite cell numbers and fiber-associated nuclei were not affected (P > 0.05) by implant status or ractopamine supplementation. These results indicate that TBA and RAC preferentially increase the size of type I fibers in cull cows.

Identification of myosin heavy chain isoforms in skeletal muscle of four Southern African wild ruminants

The aim was to separate and characterize the myosin heavy chain (MHC) isoforms of four southern African wild ruminants, namely Blesbuck (Damaliscus dorcas phillipsi), Kudu (Tragelaphus strepsiceros), Black Wildebeest (Connochaetes gnou) and Blue Wildebeest (Connochaetes taurinus). Longissimus dorsi muscle samples were subjected to SDS-PAGE and Western blot analyses using antibodies raised against MHC isoforms. The specificity of these antibodies was assessed using immunohistochemistry combined with ATPase histochemistry, Three MHC isoforms were separated and the bands were identified from fastest to slowest migrating as MHC I, MHC IIx and MHC IIa. The mobility of the MHC isoforms was similar for all four species, including that of bovine, but differed from human muscle. Kudu muscle exhibited the lowest proportion of MHC I and the highest proportion of MHC IIx, whereas Blesbuck muscle had the least MHC IIx. The two Wildebeest species were intermediate in isoform content. In conclusion, when new species are studied, existing electrophoretic protocols may need to be modified to achieve quantifiable separation and isoform migration pattern must be verified in order to reach correct interpretations. Furthermore, antibody specificity may differ between techniques as well as species and needs confirmation.

Expression of eight distinct MHC isoforms in bovine striated muscles: evidence for MHC-2B presence only in extraocular muscles

This study aimed to analyse the expression of myosin heavy chain (MHC) isoforms in bovine muscles, with particular attention to the MHC-2B gene. Diaphragm, longissimus dorsi, masseter, several laryngeal muscles and two extraocular muscles (rectus lateralis and retractor bulbi) were sampled in adult male Bos taurus (age 18-24 months, mass 400-500 kg) and analysed by RT-PCR, gel electrophoresis and immunohistochemistry. Transcripts and proteins corresponding to eight MHC isoforms were identified: MHC-alpha and MHC-beta/slow (or MHC-1), two developmental isoforms (MHC-embryonic and MHC-neonatal), three adult fast isoforms (MHC-2A, MHC-2X and MHC-2B) and the extraocular isoform MHC-Eo. All eight MHC isoforms were found to be co-expressed in extrinsic eye muscles, retractor bulbi and rectus lateralis, four (beta/slow, 2A, 2X, neonatal) in laryngeal muscles, three (beta/slow, 2A and 2X) in trunk and limb muscles and two (beta/slow and alpha) in masseter. The expression of MHC-2B and MHC-Eo was restricted to extraocular muscles. Developmental MHC isoforms (neonatal and embryonic) were only found in specialized muscles in the larynx and in the eye. MHC-alpha was only found in extraocular and masseter muscle. Single fibres dissected from masseter, diaphragm and longissimus were classified into five groups (expressing, respectively, beta/slow, alpha, slow and 2A, 2A and 2X) on the basis of MHC isoform electrophoretical separation, and their contractile properties [maximum shortening velocity (v(0)) and isometric tension (P(0))] were determined. v(0) increased progressively from slow to fast 2A and fast 2X, whereas hybrid 1-2A fibres and fibres containing MHC-alpha were intermediate between slow and fast 2A.

New insights into skeletal muscle fibre types in the dog with particular focus towards hybrid myosin phenotypes

Electrophoresis, immunoblots, immunohistochemistry and image analysis methods were applied to characterise canine trunk and appendicular muscle fibres according to their myosin heavy chain (MyHC) composition and to determine, on a fibre-to-fibre basis, the correlation between contractile [MyHC (s), myofibrillar ATPase (mATPase) and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) isoforms], metabolic [succinate dehydrogenase (SDH) and glycerol-3-phosphate dehydrogenase (GPDH) activities and glycogen and phospholamban (PLB) content] and morphological (cross-sectional area and capillary and nuclear densities) features of individual myofibres. An accurate delineation of MyHC-based fibre types was obtained with the developed immunohistochemical method, which showed high sensitivity and objectivity to delineate hybrid fibres with overwhelming dominance of one MyHC isoform. Phenotypic differences in contractile, metabolic and morphological properties seen between fibre types were related to MyHC content. All canine skeletal muscle fibre types had a relatively high histochemical SDH activity but significant differences existed in the order IIA>I>IIX. Mean GPDH was ranked according to fibre type such that I<IIA<IIX. Type IIA fibres were the smallest, type IIX fibres the largest and type I of intermediate size. Capillary and nuclear density decreased in the order IIA>I>IIX. Hybrid fibres, which represented nearly one third of the whole pool of skeletal muscle fibres analysed, had mean values intermediate between their respective pure phenotypes. Slow fibres expressed the slow SERCA isoform and PLB, whereas type II fibres expressed the fast SERCA isoform. Discrimination of myofibres according to their MyHC content was possible on the basis of their contractile, metabolic and morphological features. These intrafibre interrelationships suggest that myofibres of control dogs exhibit a high degree of co-ordination in their physiological, biochemical and morphological characteristics. This study demonstrates that canine skeletal muscle fibres have been misclassified in numerous previous studies and offers useful baseline data and new prospects for future work on muscle-fibre-typing in canine experimental studies.

Neuromuscular partitioning, architectural design, and myosin fiber types of the M. vastus lateralis of the llama (Lama glama)

The llama (Lama glama) is one of the few mammals of relatively large body size in which three fast myosin heavy chain isoforms (i.e., IIA, IIX, IIB) are extensively expressed in their locomotory muscles. This study was designed to gain insight into the morphological and functional organization of skeletal musculature in this peculiar animal model. The neuromuscular partitioning, architectural design, and myosin fiber types were systematically studied in the M. vastus lateralis of adult llamas (n = 15). Four nonoverlapping neuromuscular partitions or compartments were identified macroscopically (using a modified Sihler's technique for muscle depigmentation), although they did not conform strictly to the definitions of "neuromuscular compartments." Each neuromuscular partition was innervated by primary branches of the femoral nerve and was arranged within the muscle as paired partitions, two in parallel (deep-superficial compartmentalization) and the other two in-series (proximo-distal compartmentalization). These neuromuscular partitions of the muscle varied in their respective architectural designs (studied after partial digestion with diluted nitric acid) and myosin fiber type characteristics (identified immunohistochemically with specific anti-myosin monoclonal antibodies, then examined by quantitative histochemistry and image analysis). The deep partitions of the muscle had longer fibers, with lower angles of pinnation, and higher percentages of fast-glycolytic fibers than the superficial partitions of the muscle. These differences clearly suggest a division of labor in the whole M. vastus lateralis of llamas, with deep partitions exhibiting features well adapted for dynamic activities in the extension of stifle, whereas superficial portions seem to be related to the antigravitational role of the muscle in preserving the extension of the stifle during standing and stance phase of the stride. This peculiar structural and functional organization of the llama M. vastus lateralis does not confirm the generalized idea that deep muscles or the deepest portions within the same muscles somehow develop postural and/or low-intensity isometric functions. Rather, it suggests a primacy of architecture over intramuscular location in determining fiber type composition and hence division of labor within the muscle. A compartmentalization in the distribution of the three fast-subtype fibers (IIA, IIX, and IIB) also occurred, and this could also be relevant functionally, since these fiber types differed significantly in size (IIA < IIX < IIB), oxidative capacity (IIA > IIX > IIB), and capillarization (IIA = IIX > IIB). Furthermore, a typical spatial pattern in fiber type distribution was encountered in llama muscle (i.e., fiber types were consistently ranked in the order I --> IIA --> IIX --> IIB from the center to the periphery of fascicles), suggesting again peculiar and not well understood functional adaptations in these species.

Coordinated expression of myosin heavy chains, metabolic enzymes, and morphological features of porcine skeletal muscle fiber types

Combined methodologies of electrophoresis, immunoblots, immunohistochemistry, histochemistry, and photometric image analysis were applied to characterize porcine skeletal muscle fibers according to their myosin heavy chain (MyHC) composition, and to determine on a fiber-to-fiber basis the correlation between contractile [MyHC (s), myofibrillar ATPase (mATPase), and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) isoforms], metabolic [succinate dehydrogenase (SDH), and glycerol-3-phosphate dehydrogenase (GPDH) activities, glycogen, and phospholamban (PLB) contents], and morphological [cross-sectional area (CSA), capillary, and nuclear densities] features of individual myofibers. An accurate delineation of MyHC-based fiber types was obtained with the immunohistochemical method developed. This protocol showed a high sensitivity and objectivity to delineate hybrid fibers with overwhelming dominance of one MyHC isoform. The phenotypic differences in contractile, metabolic, and morphological properties seen between fiber types were related with MyHC content. Slow fibers had the lowest mATPase activity (related to shortening velocity), the highest SDH activity (oxidative capacity), the lowest GPDH activity (glycolytic metabolism), and glycogen content, the smallest CSA, the greatest capillary, and nuclear densities, and expressed slow SERCA isoform and PLB, but not the fast SERCA isoform. The reverse pattern was true for pure IIB fibers, whereas type IIA and IIX fibers had intermediate properties. Hybrid fibers had mean values intermediate in-between their respective pure phenotypes. Discrimination of myofibers according to their MyHC content was possible on the basis of their contractile and non-contractile profiles. These intrafiber interrelationships suggest that myofibers of control pigs exhibit a high degree of co-ordination in their physiological, biochemical, and anatomical features. This study may well be a useful baseline for future work on the pig meat industry and also offers new prospects for muscle fiber typing in porcine experimental studies.

New insights into muscle fiber types in the pig

The accurate classification of skeletal muscle fiber types according to myosin heavy chain (MyHC) polymorphism remains a difficult task in the pig. Combined myofibrillar ATPase and metabolic enzyme histochemistry, in situ hybridization, and immunocytochemistry were performed on serial transverse sections of pig longissimus (L) and rhomboideus (R) muscles at 100 kg body weight to give a new insight into muscle fiber typing in the pig. Several monoclonal antibodies (MAbs) either specific for a single MyHC (I, IIa, or IIb) or of multiple MyHCs (IIa + IIx or I + IIx + IIb) were used. No monospecific IIx antibody was available for the pig. All three adult Type II isoforms were expressed in the white L muscle, whereas no IIb was observed in the red R muscle, which was confirmed using RNase protection analysis. In most fibers, the distribution of the transcripts closely matched that of the corresponding proteins. When observed, co-expression of MyHCs mostly occured for IIx and IIb in L muscle, and was more common at the protein (11.5%) than at the mRNA (2.2%) level. A minor proportion of myofibers showed a mismatch between MyHC mRNA and protein. According to the type grouping distribution of myofibers encountered in pig muscle, MyHC isoform expression followed the rank order of I-->IIa-->IIx-->IIb from the center to the periphery of the islets, concomitantly with a decrease in oxidative metabolism and an increase in fiber size. The developmental origin and functional significance of the type grouping distribution are discussed.

Limb myosin heavy chain isoproteins and muscle fiber types in the adult goat (Capra hircus)

The primary focus of this study was the accurate classification of limb skeletal muscle fiber types in adult goats (Capra hircus) according to the myosin heavy chain (MHC) isoform they express. Combined methodologies of gel electrophoresis, immunoblotting, immunohistochemistry, myofibrillar ATPase (mATPase), and quantitative metabolic enzyme histochemistry of M. semitendinosus samples were developed. Three MHCs were identified and tentatively designated as types I, IIA, and IIX. Five fiber types were defined immunohistochemically according to their MHC content: I, I+IIA, IIA, IIAX, and IIX. The hybrid fast-twitch fibers (IIAX) totaled 21% of the fiber population analyzed. The three major pure fibers (I, IIA, and IIX) could be objectively separated upon the basis of their mATPase activities after acid and alkaline preincubations. The prominent number of hybrid fibers, however, could not be delineated with these mATPase methods. Metabolic and size properties of muscle fibers varied according to their MHC content, but overlapped the full range of muscle fiber phenotypes. These integrated data demonstrate that type II skeletal muscle fibers of small ruminants have been misclassified in previous studies. The immunohistochemical approach developed in the present study offers new prospects for muscle fiber typing in caprine experimental studies and meat production technologies.

Genetic variability of foetal bovine myoblasts in primary culture

Selected strains of adult bovines and those which either have high muscle growth capacity or are double-muscled present particular characteristics of muscle fibres and collagen at slaughter that favour meat tenderness. For double-muscled bovines, it has been shown that these characteristics originated during foetal life. However, no studies have been done to determine the origin of muscle growth superiority in bovine with high muscle growth capacity compared to those with a low muscle growth capacity. Therefore, the objective of this study was to examine the proliferation and differentiation phases of myoblasts in primary culture taken from high and low muscle growth capacity foetuses at 110 days post-conception. These cultures were analysed on 1, 2, 3, 4, 6, 8, 10 days of culture. The proliferation phase was monitored by appropriate marker antibodies. The differentiation was studied by immunocytochemistry with specific antibodies for foetal, I, II (IIa and IIb), I and IIb, I and IIa myosin heavy chains (MHCs) and connectin respectively, and by immunoblotting with desmin antibody. A higher proliferation, a lower fusion and a delayed differentiation of the 'late markers' namely MHCs fast (IIa and IIb) and connectin were shown in high muscle growth capacity foetuses compared to low capacity ones. The results indicate that the muscle growth superiority of high muscle growth capacity bovines seems, therefore, to have a similar foetal origin to that of double-muscled ones.