The fibre type composition of the striated muscle of the oesophagus in ruminants and carnivores

The fibre type composition of the striated muscle layer of the oesophagus of the cow, sheep, donkey, dog and cat was examined with standard histochemical methods and immunohistochemical staining using type-specific antimyosin sera. The heavy chain and light chain composition of oesophageal myosin was also examined using electrophoretic peptide mapping and 2-dimensional gel electrophoresis respectively. In the ruminants and donkey the oesophagus was composed of fibre types I, IIA and IIC with immunohistochemical characteristics identical to those of the same fibre types found in control skeletal muscle. In the ruminants there was a gradient in the proportion of type I fibres from 1% (at the cervical end) to about 30% (at the caudal end). In the carnivores the oesophageal muscle was composed of a very small percentage of type I and IIC fibres, but the predominant type was very different histochemically and immunohistochemically from all the fibre types (I, IIA, IIB, IIC) present in the control muscles. This oesophageal fibre type ( IIoes ) had an acid- and alkaline-stable m-ATPase activity, a moderate histochemical Ca-Mg actomyosin ATPase activity, and reacted weakly with anti-IIA and anti-IIB myosin sera. Although the light chains of the IIoes myosin were the same as the light chains of a mixture of IIA and IIB myosins, their respective heavy chains gave different peptide maps. Greater differences were obtained between the heavy chains of IIoes and other striated muscle myosins. These observations lead us to conclude that this predominant fibre type of the carnivore oesophageal striated muscle is of the 'fast' type, and contains a distinct isoform of myosin similar but not identical to the other fast type myosins.     

Degree of correspondence between contractile and oxidative capacities in horse muscle fibres: a histochemical study

Samples taken from the middle gluteal muscle of 95 untrained adult horses of different ages and sex were subjected to histochemical analysis using the myosin adenosine triphosphatase (m-ATPase) and nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR) staining techniques. Fibres were classified into types I, IIA and IIB according to m-ATPase activity after preincubation at pH 4.4. The percentage of FT (Fast-Twitch Glycolytic) fibres and the proportion of IIB fibres with "high" and "low" oxidative capacity were determined in serial sections stained for NADH-TR. Statistical analysis revealed a significantly higher proportion of IIB fibres than FT fibres (P less than 0.001), though both percentages were correlated. Thus, 72.2 +/- 17.6% of type IIB fibres showed low oxidative capacity, but the remaining 27.8 +/- 17.6% showed high aerobic potential, and thus did not correspond to FT fibres. These results confirm that the contractile capacity of a muscle fibre does not determine its oxidative profile. The different types of muscle fibre should thus be classified solely according to m-ATPase activity, since this characteristic is related to the molecular structure of contractile proteins. Oxidative capacity should be assessed separately, and not be used as a criterion for fibre classification in horses.     

The fibre type composition of the striated muscle of the oesophagus in ruminants and carnivores

Summary The fibre type composition of the striated muscle layer of the oesophagus of the cow, sheep, donkey, dog and cat was examined with standard histochemical methods and immunohistochemical staining using type-specific antimyosin sera. The heavy chain and light chain composition of oesophageal myosin was also examined using electrophoretic peptide mapping and 2-dimensional gel electrophoresis respectively. In the ruminants and donkey the oesophagus was composed of fibre types I, IIA and IIC with immunohistochemical characteristics identical to those of the same fibre types found in control skeletal muscle. In the ruminants there was a gradient in the proportion of type I fibres from 1% (at the cervical end) to about 30% (at the caudal end).In the carnivores the oesophageal muscle was composed of a very small percentage of type I and IIC fibres, but the predominant type was very different hisotchemically and immunohistochemically from all the fibre types (I, IIA, IIB, IIC) present in the control muscles. This oesophageal fibre type (IIoes) had an acid- and alkaline-stable m-ATP-ase activity, a moderate histochemical Ca-Mg actomyosin ATPase activity, and reacted weakly with anti-IIA and antiIIB myosin sera. Although the light chains of the IIoes myosin were the same as the light chains of a mixture of IIA and IIB myosins, their respective heavy chains gave different peptide maps. Greater differences were obtained between the heavy chains of IIoes and other striated muscle myosins.These observations lead us to conclude that this predominant fibre type of the carnivore oesophageal striated muscle is of the fast type, and contains a distinct isoform of myosin similr but not identical to the other fast type myosins.     

Analysis of myosin light and heavy chain types in single human skeletal muscle fibers

In this study, myosin types in human skeletal muscle fibers were investigated with electrophoretic techniques. Single fibers were dissected out of lyophilized surgical biopsies and typed by staining for myofibrillar ATPase after preincubation in acid or alkaline buffers. After 14C-labelling of the fiber proteins in vitro by reductive methylation, the myosin light chain pattern was analysed on two-dimensional gels and the myosin heavy chains were investigated by one-dimensional peptide mapping. Surprisingly, human type I fibers, which contained only the slow heavy chain, were found to contain variable amounts of fast myosin light chains in addition to the two slow light chains LC1s and LC2s. The majority of the type I fibers in normal human muscle showed the pattern LC1s, LC2s and LC1f. Further evidence for the existence in human muscle of a hybrid myosin composed of a slow heavy chain with fast and slow light chains comes from the analysis of purified human myosin in the native state by pyrophosphate gel electrophoresis. With this method, a single band corresponding to slow myosin was obtained; this slow myosin had the light chain composition LC1s, LC2s and LC1f. Type IIA and IIB fibers, on the other hand, revealed identical light chain patterns consisting of only the fast light chains LC1f, LC2f and LC3f but were found to have different myosin havy chains. On the basis of the results presented, we suggest that the histochemical ATPase normally used for fibre typing is determined by the myosin heavy chain type (and not by the light chains). Thus, in normal human muscle a number of 'hybrid' myosins were found to occur, namely two extreme forms of fast myosins which have the same light chains but different heavy chains (IIA and IIB) and a continuum of slow forms consisting of the same heavy chain and slow light chains with a variable fast light chain composition. This is consistent with the different physiological roles these fibers are thought to have in muscle contraction.     

The multiplicity of combinations of myosin light chains and heavy chains in histochemically typed single fibres. Rabbit tibialis anterior muscle.

1. Combined histochemical and biochemical single-fibre analyses [Staron & Pette (1987) Biochem. J. 243, 687-693], were used to investigate the rabbit tibialis-anterior fibre population. 2. This muscle is composed of four histochemically defined fibre types (I, IIC, IIA and IIB). 3. Type I fibres contain slow myosin light chains LC1s and LC2 and the slow myosin heavy chain HCI, and types IIA and IIB contain the fast myosin light chains LC1f, LC2f and LC3f and the fast heavy chains HCIIa and HCIIb respectively. 4. A small fraction of fibres (IIAB), histochemically intermediate between types IIA and IIB, contain the fast light myosin chains but display a coexistence of HCIIa and HCIIb. 5. Similarly to the soleus muscle, C fibres in the tibialis anterior muscle contain both fast and slow myosin light chains and heavy chains. The IIC fibres show a predominance of the fast forms and the IC fibres (histochemically intermediate between types I and IIC) a predominance of the slow forms. 6. A total of 60 theoretical isomyosins can be derived from these findings on the distribution of fast and slow myosin light and heavy chains in the fibres of rabbit tibialis anterior muscle.     

Heterogeneity and distribution of fast myosin heavy chains in some adult vertebrate skeletal muscles.

Three monoclonal antibodies, LM5, F2 and F39 raised to chicken fast skeletal muscle myosin, specific for myosin heavy chain (MHC) subunit, were used to study the composition and distribution of this protein in some vertebrate skeletal muscles. These antibodies in immunohistochemical investigations did not react with the majority of the type I fibres in most muscles. Antibodies LM5 and F39 stained all the type II fibres in all the adult chicken skeletal muscles studied. Antibody F2 also stained all the type II fibres in most chicken skeletal muscles tested except in gastrocnemius in which a proportion of both the type IIA and IIB fibres either did not stain or stained only weakly. Antibody F2 unlike LM5 and F39 stained most of the type IIIB fibres in anterior latissimus dorsi (ALD) and IB fibres in red strip of chicken Pectoralis muscle. Antibodies LM5 and F2 in the rat diaphragm reacted with all the type IIA and IIB fibres, while antibody F39 stained only the type IIB fibres darkly with most IIA fibres being either not stained or only weakly stained. In the rat extensor digitorum longus (EDL) and tibialis anterior (TA) muscles, antibody LM5 stained all the IIA and IIB fibres. Antibody F2 in these muscles stained all the type IIA fibres but only a proportion of the IIB fibres. The remaining IIB fibres were either unstained or only weakly positive. Antibody F39 in rat EDL and TA muscles did not only distinguish subgroups of IIB fibres (dark, intermediate and negative or very weak) but also of the IIA fibres. These three antibodies used together therefore detected a great deal of heterogeneity in the myosin heavy chain composition and muscle fibre types of several skeletal muscles.     

Heterogeneity and distribution of fast myosin heavy chains in some adult vertebrate skeletal muscles

Summary Three monoclonal antibodies, LM5, F2 and F39 raised to chicken fast skeletal muscle myosin, specific for myosin heavy chain (MHC) subunit, were used to study the composition and distribution of this protein in some vertebrate skeletal muscles. These antibodies in immunohistochemical investigations did not react with the majority of the type I fibres in most muscles. Antibodies LM5 and F39 stained all the type II fibres in all the adult chicken skeletal muscles studied. Antibody F2 also stained all the type II fibres in most chicken skeletal muscles tested except in gastrocnemius in which a proportion of both the type IIA and IIB fibres either did not stain or stained only weakly. Antibody F2 unlike LM5 and F39 stained most of the type IIIB fibres in anterior latissimus dorsi (ALD) and IB fibres in red strip of chicken Pectoralis muscle. Antibodies LM5 and F2 in the rat diaphragm reacted with all the type IIA and IIB fibres, while antibody F39 stained only the type IIB fibres darkly with most IIA fibres being either not stained or only weakly stained. In the rat extensor digitorum longus (EDL) and tibialis anterior (TA) muscles, antibody LM5 stained all the IIA and IIB fibres. Antibody F2 in these muscles stained all the type IIA fibres but only a proportion of the IIB fibres. The remaining IIB fibres were either unstained or only weakly positive. Antibody F39 in rat EDL and TA muscles did not only distinguish subgroups of IIB fibres (dark, intermediate and negative or very weak) but also of the IIA fibres. These three antibodies used together therefore detected a great deal of heterogeneity in the myosin heavy chain composition and muscle fibre types of several skeletal muscles.     

Polymorphism of myofibrillar proteins of rabbit skeletal-muscle fibres. An electrophoretic study of single fibres.

Rabbit predominantly fast-twitch-fibre and predominantly slow-twitch-fibre skeletal muscles of the hind limbs, the psoas, the diaphragm and the masseter muscles were fibre-typed by one-dimensional polyacrylamide-gel electrophoresis of the myofibrillar proteins of chemically skinned single fibres. Investigation of the distribution of fast-twitch-fibre and slow-twitch-fibre isoforms of myosin light chains and the type of myosin heavy chains, based on peptide ''maps'' published in Cleveland. Fischer, Kirschner & Laemmli [(1977) J. Biol. Chem. 252, 1102-1106], allowed a classification of muscle fibres into four classes, corresponding to histochemical types I, IIA, IIB and IIC. Type I fibres with a pure slow-twitch-type of myosin were found to be characterized by a unique set of isoforms of troponins I, C and T, in agreement with the immunological data of Dhoot & Perry [(1979) Nature (London) 278, 714-718], by predominance of the beta-tropomyosin subunit and by the presence of a small amount of an additional tropomyosin subunit, apparently dissimilar from fast-twitch-fibre alpha-tropomyosin subunit. The myofibrillar composition of type IIB fast-twitch white fibres was the mirror image of that found for slow-twitch fibres in that the fast-twitch-fibre isoforms only of the troponin subunits were present and the alpha-tropomyosin subunit predominated. Type IIA fast-twitch red fibres showed a troponin subunit composition identical with that of type IIB fast-twitch white fibres. On the other hand, a unique type of myosin heavy chains was found to be associated with type IIA fibres. Furthermore, the myosin light-chain composition of these fibres was invariably characterized by a small amount of LC3F light chain and by a pattern that was either a pure fast-twitch-fibre light-chain pattern or a hybrid LC1F/LC2F/LC3F/LC1Sb light-chain pattern. By these criteria type IIA fibres could be distinguished from type IIC intermediate fibres, which showed coexistence of fast-twitch-fibre and slow-twitch-fibre forms of myosin light chains and of troponin subunits.     

Purification and characterization of myosins from human and rabbit skeletal muscles by using specific monoclonal antibodies

By using immunoaffinity column chromatography slow (I) and fast (IIA, IIB) myosins were isolated from human (vastus lateralis) and rabbit (tibialis anterior, psoas and conoidal bundle) skeletal muscles. The peptide pattern revealed that slow (I) and fast (IIA, IIB) myosin heavy chains are quite distinct, as are those from pure slow (conoidal bundle) and fast (psoas) rabbit skeletal muscles. Unlike Billeter et al. (1981) the authors observed that fast human myosins were always associated with a small amount of slow myosin light chains. The fast myosins (IIA, IIB) from rabbit tibialis anterior muscle did not appear very distinct and contained only fast myosin light chains. These myosins were different from the IIB myosin from the psoas muscle. Ten per cent of the fibres revealed histochemically as fast IIA also reacted with an anti-slow myosin antibody. The classical histochemical techniques appear inadequate to demonstrate the existing differences among fibre types, but the monoclonal antibodies hold promise.     

Lactate dehydrogenase isozymes in type I,IIA and IIB fibres of rabbit skeletal muscles

Summary Lactate dehydrogenase (LDH) isozyme patterns were analysed by polyacrylamide (PAA) slab gel electrophoresis in extracts prepared from various rabbit skeletal muscles of defined fibre composition and by PAA microelectrophoresis of microdissected, histochemically typed single muscle fibres. The results obtained by electrophoresis of whole muscle extracts generally agreed with the data obtained from single fibre electrophoresis, i.e. the LDH isozyme pattern corresponded to that of the predominant fibre type. Type I Fibres from soleus and semitendinosus muscles were characterized by a unique pattern of all 5 LDH isozymes with a predominance of LDH-1, 2 and 3. The major fraction (80%) of the type II fibres from extensor digitorum longus and tibialis anterior muscles contained only LDH-5 (M₄). About 20% of the type II fibres contained in addition to LDH-5 small amounts of LDH-4 and LDH-3. The fraction of fibres containing LDH-5, LDH-4, and LDH-3 was similar (ca. 20%) in the histochemically defined IIA and IIB subpopulations In view of the fact that the major fractions of rabbit IIB fibres display low and of IIA fibres high aerobic oxidative capacities (Reichmann and Pette 1982), these data indicate that the expression of the H-subunit of LDH is not correlated with the aerobic-oxidative capacity of the fibre. It also appears not to be correlated with the presence of different myosin isoforms in IIA and IIB fibres.     

Type IIC fibres in certain muscles of the adult rat (sedentary and exercised)

Samples were taken, at fixed levels, of the vastus lateralis, the caput lateralis of the gastrocnemius muscle and the longissimus lumbaris of 72 Wistar rats which were either sedentary or subjected to various exercise schedules. The samples were analyzed using the histochemical technique of myosin ATPase (m-ATPase) after preincubation at pH 4.2, and the fibre-types I, II (IIA and IIB) and IIC were identified, calculating the percentage of type IIC fibres as well as their minimum diameter. The percentages of these IIC fibres found in the red and mixed parts of the gastrocnemius (caput lateralis) and the longissimus lumbaris were between 0.7% and 2.6%. However, their presence was not detected in the vastus lateralis or in the white part of the gastrocnemius (caput lateralis). The lack of differences in this fibre type between the males and females of the population was shown statistically. Likewise, no significant modification of the IIC fibres between sedentary and exercised animals was seen. With regard to fibrillar size, females showed a smaller minimum diameter than males, the results showing a small increase in the size of these fibres in both sexes after exercise, although in most cases this was not statistically significant.     

Effects of training on fibre composition in rat gastrocnemius muscle

Forty-eight adult Wistar rats were divided into four groups for experimentation. The group I was used as a control and groups II, III and IV underwent progressive treadmill training. Samples of the red and mixed portions of m. gastrocnemius (lateral head) were stained with the histochemical technique of m-ATPase to determine the percentage of type I, IIA and IIB fibres, and with NADH-TR, in order to quantify variations in "low-oxidative" fibre percentages. The results showed that progressive training did not lead to statistical variations in the percentage of type I fibres. However, the proportion of type IIA fibres rose, while that of IIB fibres fell, in both cases significantly. Variations were more marked in mixed than in red Gastrocnemius muscle. A clear decrease was noted in "low-oxidative" fibres, which were virtually absent from red portion. This decrease was more marked, and occurred more rapidly, than in type IIB fibres.     

Influence of age on fibre type characteristics in the middle gluteal muscle of Andalusian foals.

34 Andalusian foals of both sexes were divided into three age-groups (A = mean age 1 month, B = 7 months, C = 14 months). Samples of the right m. gluteus medius were stained for m-ATPase and NADH-TR in order to determine fibre type composition and size as well as the relative area occupied by each type. Results revealed no statistically significant variation in the proportion of type I fibre among the three age-groups. Significant differences were recorded, however, for type II fibres; an increase in the proportion of IIA fibres was accompanied by a decrease in IIB ones, the difference being most marked between groups A and B. IIB high-oxidative fibres also decreased, while IIB low-oxidative ones showed no significant variation. All fibre types increased significantly in size; types I and IIA recorded a threefold increase, whereas type IIB showed least growth. The relative area occupied by each type increased significantly between groups A and B, but only IIA fibres recorded a significant increase in relative area between groups B and C.     

EXERCISE MYOPATHY: CHANGES IN MYOFIBRILS OF FAST-TWITCH MUSCLE FIBRES

The purpose of the present study was to determine the relationships between the changes of myofibrils in fast-twitch oxidative-glycolytic (type IIA) fibres and fast-twitch glycolytic (type IIB) muscle fibres, protein synthesis and degradation rate in exercise-induced myopathic skeletal muscle. Exhaustive exercise was used to induce myopathy in Wistar rats. Intensity of glycogenolysis in muscle fibres during exercise, protein synthesis rate, degradation rate and structural changes of myofibrils were measured using morphological and biochemical methods. Myofibril cross sectional area (CSA) in type IIA fibres decreased 33% and type IIB fibres 44%. Protein degradation rate increased in both type IIA and IIB fibres, 63% and 69% respectively in comparison with the control group. According to the intensity of glycogenolysis, fast oxidative-glycolytic fibres are recruited more frequently during overtraining. Myofibrils in both types of fast-twitch myopathic muscle fibres are significantly thinner as the result of more intensive protein degradation. Regeneration capacity according to the presence of satellite cells is higher in type IIA fibres than in type IIB fibres in myopathic muscle.     

Variations in fibre composition of the gastrocnemius muscle in rats subjected to speed training

Thirty-six adult Wistar rats were divided into three groups. One group was used as a control, and the other two underwent different training programmes in which greater relevance was attached to the intensity of exercise than to its duration. Samples of the red and mixed portions of m. gastrocnemius (caput lateralis) were stained with m-ATPase to determine the percentage of type I, IIA and IIB fibres, and with NADH-TR in order to quantify variations in the percentage of low staining intensity (FG) fibres. The most notable results obtained were: a) the ratio of type I type II fibres remained unchanged; b) the proportion of IIA fibres increased, while that of IIB fibres decreased correspondingly; c) FG fibres, which were virtually absent from the red portion, recorded a clear decrease which was more marked, and occurred more rapidly, than in IIB fibres. These differences were all statistically significant in the mixed portion of the muscle. Adaptive changes in fibre composition in the red portion were less marked.     

Interrelationships of myofibrillar ATPase activity and metabolic properties of myosin heavy chain-based fibre types in rat skeletal muscle

 Myofibrillar ATPase (mATPase), succinate dehydrogenase (SDH) and α-glycerophosphate dehydrogenase (GPD) activities and cross-sectional area (CSA) were measured in fibres of rat medial gastrocnemius muscle using quantitative histochemistry. The same fibres were typed immunohistochemically using monoclonal antibodies specific to selected myosin heavy chain (MHC) isoforms. The values of mATPase, SDH, GPD and CSA formed a continuum, but significant differences in mean values were observed among fibre types of presumed homogeneous MHC content. Type I fibres had the lowest mATPase activity, followed in rank order by type IIA<type IID/X<type IIB. Type IIA fibres had the highest SDH activity, followed in rank order by type IID/X>type I>type IIB. The mean GPD activity was consistently ranked according to fibre type such that type IIB>type IID/X >type IIA>type I. Type IIA fibres were the smallest, type IIB fibres were the largest and types I and IID/X were of intermediate size. Significant interrelationships between mATPase, SDH, GPD and CSA values were found on a fibre-to-fibre basis. Consequently, discrimination of fibres according to their MHC content was possible on the basis of their mATPase, SDH, GPD and CSA profiles. These intrafibre interrelationships suggest that the MHC isoform is associated with phenotypic differences in contractile, metabolic and size properties of muscle fibre types. Accepted: 30 November 1998     

The effect of glucocorticoids on the myosin heavy chain isoforms' turnover in skeletal muscle

The purpose of this study was to find the effect of dexamethasone on the myosin heavy chain (MyHC) isoforms' composition in different skeletal muscles and glycolytic (G) fibres in relation with their synthesis rate and degradation of MyHC isoforms by alkaline proteinases. Eighteen-week-old male rats of the Wistar strain were treated with dexamethasone (100 microg/100 g bwt) during 10 days. The forelimb strength decreased from 9.52 to 6.19 N (P<0.001) and hindlimb strength from 15.54 to 8.55 N (P<0.001). Daily motor activity decreased (total activity from 933 to 559 and ambulatory activity from 482 to 226 movements/h, P<0.001). The degradation rate of muscle contractile proteins increased from 2.0 to 5.9% per day (P<0.001), as well as the myosin heavy chain IIB isoform degradation with alkaline proteinase in fast-twitch (F-T) muscles (12 +/- 0.9%; P<0.05) and glycolytic muscle fibres (15 +/- 1.1%; P<0.001). The synthesis rate of MyHC type II isoforms decreased in Pla muscles (P<0.05) and MyHC IIA (P<0.05) and IIB in EDL muscle and G fibres (P<0.001). The relative content of MyHC IIB isoform decreased in F-T muscles (P<0.001) and in G fibres (P<0.01), and the relative content of IIA and IID isoforms increased simultaneously. Dexamethasone decreased the MyHC IIB isoform synthesis rate and increased the sensibility of MyHC IIB isoform to alkaline proteinase, which in its turn led to the decrease of MyHC IIB isoform relative content in F-T muscles with low oxidative potential and G muscle fibres.     

Immunohistochemical analysis of the human psoas major muscle with regards to the body side and aging

The aim of our study was to explore the age related changes of the fibre type composition of the human psoas major muscle. Moreover, we wanted to compare the fibre type composition of the left and right muscle. Muscle samples were collected from 15 young and 15 old males. Type I, IIA and IIX muscle fibres were typed using myosin heavy chain identification. The serial transverse sections were analysed using a light microscope. Results of our study showed that the age-related atrophy affected all three fibre types. Type IIA fibres were affected most profoundly while type I fibres were affected most weakly. The percentage of the different fibre types did not change during aging. There were no differences in the fibre type composition between the left and right muscle. Human psoas major muscle undergoes normal aging changes with the atrophy of all three fibre types, whereas atrophy most profoundly affects type IIA fibres. No differences in the fibre type composition between the left and right muscle point to the equal engagement of both legs in normal everyday activities of human.     

Multivariate classification of histochemically stained human skeletal muscle fibres by the SIMCA method

Summary The SIMCA (soft independent modelling of class analogy) method of pattern recognition has been used to classify four muscle fibre types: I, IIA, IIB and IIC. The samples were histochemically stained human skeletal sections from biopsy material. Disjoint (separate) class modelling gave information about variables, i.e., the combinations of alkaline, acidic and Ca²⁺-containing preincubation procedures with appropriate discrimination power, and showed satisfactory separation of the classes (fibre types). Two serial stained muscle sections represent a minimum for a proper classification of the four fibre groups. A comparison of biopsy samples from two different persons showed significant variation in the data structure between similar fibre types, probably caused by intermuscle variations. It is suggested that the introduction of computer-assisted classification by the application of such multivariate analytical techniques both facilitates the classification of muscle fibres and improves the precision and reliability of fibre typing.     

The histochemical profiles of fibre types in porcine skeletal muscle

Using a variety of histochemical methods -mATPase staining after alkaline and acid preincubations, NADH-TR and alpha-MGPDH- we have investigated the fibre types in porcine skeletal muscle. The results reveal that four major fibre types -I, IIA, IIB and II*- can be separated histochemically in Longissimus lumborum muscle of Landrace pigs. The histochemical properties of the muscle fibre type II* are very similar to that of type IIX described in other mammals. The existence of IIX fibres in pig muscle has been recently demonstrated by molecular biology techniques and our results validate the use of histochemistry (mATPase) as an easy methodology to differentiate the three fast myosins (type II fibres) in pig muscle.