Tenotomy of the avian anterior latissimus dorsi muscle. II. Can regeneration from the stump occur in the pigeon?

Because the chick's anterior latissimus dorsi muscle (ALD) regenerates a fast-twitch muscular connection after tenotomy, the pigeon's ALD was tenotomized, either at the origin or through the muscle 0.5 cm from the origin, to determine whether this muscle behaves similarly to the chick muscle. These procedures were compared in pigeons operated upon at 7 weeks, versus 5 to 9 months of age. The pigeon's ALD did not regenerate a new connection, and other differences were observed between the pigeon and chick ALD. The pigeon ALD has only a single slow muscle-fiber type, has fewer fast fibers, and transforms to a fast-twitch muscle more readily than the chick ALD after tenotomy. The transformation of muscle fiber types occurred more readily in the older pigeons than those tenotomized at 7 weeks of age. Tenotomy induced morphological alterations of the muscle fiber structure in all of the pigeons, which is in contrast to the absence of changes in the tenotomized chick ALD. Therefore the pigeon and chick ALD respond completely differently to tenotomy.     

Skeletal muscle satellite cell diversity: satellite cells form fibers of different types in cell culture

Following skeletal muscle injury, new fibers form from resident satellite cells which reestablish the fiber composition of the original muscle. We have used a cell culture system to analyze satellite cells isolated from adult chicken and quail pectoralis major (PM; a fast muscle) and anterior latissimus dorsi (ALD; a slow muscle) to determine if satellite cells isolated from fast or slow muscles produce one or several types of fibers when they form new fibers in vitro in the absence of innervation or a specific extracellular milieu. The types of fibers formed in satellite cell cultures were determined using immunoblotting and immunocytochemistry with monoclonal antibodies specific for avian fast and slow myosin heavy chain (MHC) isoforms. We found that satellite cells were of different types and that fast and slow muscles differed in the percentage of each type they contained. Primary satellite cells isolated from the PM formed only fast fibers, while up to 25% of those isolated from ALD formed fibers that were both fast and slow (fast/slow fibers), the remainder being fast only. Fast/slow fibers formed from chicken satellite cells expressed slow MHC1, while slow MHC2 predominated in fast/slow fibers formed from quail satellite cells. Prolonged primary culture did not alter the relative proportions of fast to fast/slow fibers in high density cultures of either chicken or quail satellite cells. No change in commitment was observed in fibers formed from chicken satellite cell progeny repeatedly subcultured at high density, while fibers formed from subcultured quail satellite cell progeny demonstrated increasing commitment to fast/slow fiber type formation. Quail satellite cells cloned from high density cultures formed colonies that demonstrated a similar change in commitment from fast to fast/slow, as did serially subcloned individual satellite cell progeny, indicating that the observed change from fast to fast/slow differentiation resulted from intrinsic changes within a satellite cell. Thus satellite cells freshly isolated from adult chicken and quail are committed to form fibers of at least two types, satellite cells of these two types are found in different proportions in fast and slow muscles, and repeated cell proliferation of quail satellite cell progeny may alter satellite cell progeny to increasingly form fibers of a single type.     

Curare-induced transformation of myosin pattern in developing skeletal muscle fibers

The effects of neuromuscular block on the pattern of distribution of myosin isozymes in developing skeletal muscle fibers was examined by immunocytochemistry. The homogeneous population of fibers in the anterior latissimus dorsi (ALD) of the 18-day chick embryo was converted by curare to a mosaic of at least two categories of fibers. Normally all fibers in this slow muscle reacted with antibodies against slow myosin (anti-ALD). They also reacted with an antibody specific for the alkali 1 light chain (anti-delta 1) but not the alkali 2 light chain (anti-delta 2) of fast myosin. After treatment with curare, which inhibits neuronal cell death and increases the number of axonal endings, ALD muscle fibers continued to react with anti-delta 1, but many now reacted with anti-delta 2 as well. The same fibers failed to react with anti-ALD. From this it can be concluded that the myosin in this population was converted to a type not normally present. The changes, therefore, are not merely a result of the preferential loss of a slow type of fiber, nor are they a result of delayed maturation. In contrast, curare had no apparent effect on the fast posterior latissimus dorsi (PLD). As in the normal muscle at 18 days, all fibers reacted strongly with anti-delta 1 and to variable degrees with anti-delta 2, and very few fibers reacted with anti-ALD. Our observations suggest that the dual response to antibodies against fast and slow myosin during development is not a necessary consequence of multiple axon terminals. We present evidence that curare induces the expression of a different myosin in the embryonic ALD, and we suggest that the selective transformation of the fiber population may be a manifestation of a change in composition of the motoneuron pool.     

Regional injury and the terminal differentiation of satellite cells in stretched avian slow tonic muscle.

In the avian stretch model, the application of a weight overload to the humerus induces enlargement of the anterior latissimus dorsi (ALD) muscle and an increase in muscle fiber number which is accompanied by satellite cell activation. Myofiber injury may be an important stimulus to muscle fiber hyperplasia; therefore, light and electron microscopic evaluation was undertaken to determine if myofiber injury occurs in the stretch-enlarged ALD muscle of the adult quail. Autoradiographic studies were used to determine the terminal differentiation of labeled myogenic cells. A weight equal to 10% of body mass was attached to one wing of 27 adult quail and 3 birds were euthanized at 9 intervals of stretch, from 1 to 30 days. Birds were injected with tritiated thymidine at intervals ranging from 1 hr to 3 days prior to euthanization. Labeled nuclei were detected by light microscopic examination and identified by electron microscopy of a serial section. Three regions of the muscle were examined for disorganization of contractile elements, presence of cytoplasmic vacuoles, and/or phagocytic cell infiltration. The percentage of fibers exhibiting one or more of these criterion was significantly greater in the stretched ALD by Days 5 and 7 and declined at Day 10, reaching near control values by Day 14. Myofiber necrosis and phagocytic cell infiltration were only observed in the middle and distal regions of the stretched ALD muscle. Traditional signs of regeneration and repair were observed, including clusters of labeled myoblast-like cells and myotube formation within an existing basal lamina. New myotube formation with labeled central nuclei was also noted in the interstitial space, outside of basal lamina of persisting fibers. Labeled myonuclei were observed in the stretched fibers. These results demonstrate that chronic stretch produces regional injury and fiber degeneration and resultant regeneration in the ALD muscle of the adult quail. This may be an important stimulus for new fiber formation in this model.     

Muscle and Muscle Fiber Type Transformation In Clawed Crustaceans

SYNOPSIS. The first pair of thoracic limbs in many crustaceansis elaborated into claws in which the principal muscle is thecloser. Changes in the fiber composition of the closer muscleduring claw development, regeneration and reversal are reviewedhere and the hypothesis is advanced that such changes are nerve-dependent.In adult lobsters, Homarus amencanus, the paired claws and closermuscles are bilaterally asymmetric, consisting of a minor orcutter claw with predominantly fast fibers and a small ventralband of slow and a major or crusher claw with 100% slow fibers.Yet in the larval and early juvenile stages the paired clawsand closer muscles are symmetric consisting of a central bandof fast fibers sandwiched by slow. Differentiation into a cutteror crusher muscle during subsequent juvenile development isby appropriate fiber type transformation. Experimental manipulationof the claws or the environment in early juvenile stages whenthe claws are equipotent revealed that the determination ofclaw and closer muscle asymmetry is dependent on the convergenceof neural input from the paired claws: the point of convergencemost likely being the CNS. Bilaterally symmetrical input resultsin the development of paired cutter claws while bilaterallyasymmetric input gives rise to dimorphic, cutter and crusherclaws. In the northern crayfish, Orconectes rusticus, wherethe paired claws are bilaterally similar, the closer muscletransforms its central band of fast fibers to slow, both duringprimary development and regeneration. Whether these fiber typetransformations are nerve-dependent is unknown. In adult snappingshrimps, Alpheus sp., the paired claws and closer muscles areasymmetric: the minor or pincer claw has a central band of fastfibers flanked by slow while the major or snapper claw has 100%slow fibers. Claw reversal occurs with removal of the snapperresulting in the transformation of the existing pincer to asnapper and the regeneration of a new pincer at the old snappersite. Transformation of the closer muscle from pincer to snappertype is by degeneration of the fast fiber band and hypertrophyof the slow fibers. Claw transformation can be either preventedif the pincer nerve is sectioned at the time of snapper removalor promoted if the snapper nerve is sectioned: both resultsimplicating a neural basis for muscle transformation.     

Changes in fiber composition of soleus muscle during rat hindlimb suspension

Chronic reduction of gravitational load in the rear limbs of rats to simulate the influence of near-zero gravity in skeletal muscles has been shown previously to elicit atrophy in the soleus muscle. Use of this model by the present investigation indicates that soleus atrophy was characterized by a decline in the number of fibers in groups that contained the slow isoenzyme of myosin and which were classified as type I from intensity of staining to myofibrillar actomyosin adenosinetriphosphatase (ATPase) and to NADH tetrazolium reductase. Furthermore total fiber number was not changed, whereas fibers containing the intermediate isoenzyme and those classified as type IIa increased. There results could be explained by either a change in the composition within existing fibers or a simultaneous loss of slow fibers and de novo synthesis of intermediate and fast fibers. Evidence for transformation included an absence of embryonic or neonatal myosin in muscles from suspended rats and the constant fiber number that was unchanged by 4 wk of suspension. Furthermore although fiber areas of both groups of type I and IIa fibers declined during suspension, variability of the fiber areas within each group did not increase.     

Composition of muscle fibers in the slow loris,using the m. biceps brachii as an example

Histological examination of the skeletal muscle of the slow loris, which displays slow movement and locomotion among the prosimians, revealed a muscle fiber composition which differed from the general condition in mammals. Three types of muscle fiber cells were therefore analyzed quantitatively in order to elucidate their specificity. The skeletal muscle of the limbs of the slow loris was predominantly composed of red muscle fibers (type I) showing persistent tonic contraction.     

Morphology and histochemistry of the ambiens muscle of the red-eared turtle (Pseudemys scripta)

Six fiber types have been described in the ambiens muscle of red-eared turtles. These include one slow oxidative type, two fast oxidative types, two fast oxidative and glycolytic types, and one fast glycolytic type. Fiber types are non-randomly distributed throughout cross sections of the muscle. There is a decreasing gradient of oxidative staining and an increasing gradient of glycolytic staining along an axis from the superficial to deep regions of the muscle. The slow oxidative fibers are predominantly located within one or two fascicles of the superficial surface of the muscle. The fast glycolytic fibers are predominant in deep fascicles. In contrast to previous reports of histochemically monotypic intrafusal fibers in turtle muscle, ambiens muscle spindles have been observed containing one to eleven intrafusal fibers, including two fiber types. Fiber diameter and area are consistently smaller than observed in most extrafusal fibers. Spindles are predominantly located in superficial and cranial fascicles of the ambiens muscle and are located in regions characterized by extrafusal fibers with high oxidative activity.     

Fiber type homogeneity of the flight musculature in small birds

Studies of medium- and large-bodied avian species have suggested that variation in flight muscle composition is related to differences in flight behavior. For example, slow-twitch or tonic fibers are generally found only in the flight muscles of non-volant or soaring/gliding birds. However, we know comparatively little about fiber composition of the muscles of the smallest birds. Here we describe the fiber composition of muscles from the wings, shoulders, and legs of two small avian species, which also display very high wingbeat frequencies: Anna's hummingbirds (Calypte anna) and zebra finches (Taeniopygia guttata). All flight muscles examined in both species contained exclusively fast oxidative glycolytic (FOG) fibers. These unique results suggest that fast oxidative fibers are both necessary and sufficient for the full range of flight behaviors in these small-bodied birds. Like all other studied birds, the zebra finch gastrocnemius, a tarsometatarsal extensor, contained a mixture of FOG (27.1%), slow oxidative (SO, 12.7%), and fast glycolytic (FG, 60.2%) fibers. By contrast, the hummingbird gastrocnemius lacked FG fibers (85.5% FOG, 14.5% SO), which may reflect the reduced role of the hindlimb during take-off. We further hypothesize that thermogenic requirements constrain fiber type heterogeneity in these small endothermic vertebrates.     

Myosin light chain patterns in histochemically typed single fibers of the rat skeletal muscle.

1. This study was conducted to investigate the relationship between histochemical fiber types and myosin light chain patterns in rat single muscle fibers. 2. The hybrid of fast and slow light chains was observed in type I and II fibers of the soleus and type II fibers of the red portion of lateral head of the gastrocnemius muscles. 3. We also observed 7 types of light chain composition. Of the 7 types, 5 types were explainable by assuming the coexistence of isomyosins with either fast or slow light chains. However, the other 2 types could not be accounted for without hypothesizing the presence of isomyosins with promiscuous light chain distribution.     

Distribution of myosin isoenzymes among skeletal muscle fiber types.

Using an immunocytochemical approach, we have demonstrated a preferential distribution of myosin isoenzymes with respect to the pattern of fiber types in skeletal muscles of the rat. In an earlier study, we had shown that fluorescein-labeled antibody against "white" myosin from the chicken pectoralis stained all the white, intermediate and about half the red fibers of the rat diaphragm, a fast-twitch muscle (Gauthier and Lowey, 1977). We have now extended this study to include antibodies prepared against the "head" (S1) and "rod" portions of myosin, as well as the alkali- and 5,5'dithiobis (2-nitrobenzoic acid) (DTNB)-light chains. Antibodies capable of distinguishing between alkali 1 and alkali 2 type myosin were also used to localize these isoenzymes in the same fast muscle. We observed, by both direct and indirect immunofluorescence, that the same fibers which had reacted previously with antibodies against white myosin reacted with antibodies to the proteolytic subfragments and to the low molecular-weight subunits of myosin. These results confirm our earlier conclusion that the myosins of the reactive fibers in rat skeletal muscle are sufficiently similar to share antigenic determinants. The homology, furthermore, is not confined to a limited region of the myosin molecule, but includes the head and rod portions and all classes of light chains. Despite the similarities, some differences exist in the protein compositions of these fibers: antibodies to S1 did not stain the reactive (fast) red fiber as strongly as they did the white and intermediate fibers. Non-uniform staining was also observed with antibodies specific for A2 myosin; the fast red fiber again showed weaker fluorescence than did the other reactive fibers. These results could indicate a variable distribution of myosin isoenzymes according to their alkali-light chain composition among fiber types. Alternatively, there may exist yet another myosin isoenzyme which is localized in the fast red fiber. Those red fibers which did not react with any of the antibodies to pectoralis myosin, did react strongly with an antibody against myosin isolated from the anterior latissimus dorsi (ALD), a slow red muscle of the chicken. The myosin in these fibers (slow red fibers) is, therefore, distinct from the other myosin isoenzymes. In the rat soleus, a slow-twitch muscle, the majority of the fibers reacted only with antibody against ALD myosin. A minority, however, reacted with antiboddies to pectoralis as well as ALD myosin, which indicates that both fast and slow myosin can coexist within the same fiber of a normal adult muscle. These immunocytochemical studies have emphasized that a wide range of isoenzymes may contribute to the characteristic physiological properties of individual fiber types in a mixed muscle.     

A freeze-fracture study of postembryonic differentiation of latissimus dorsi muscles of the chicken

The differentiation of fiber type characteristics in the anterior (ALD) and posterior (PLD) latissimus dorsi muscles is examined by the freeze-fracture technique in 1-, 7- and 30-day-old chicks. Several characteristics of plasma membrane (caveolae, rectilinear arrays, intramembranous particles) and sarcoplasmic reticulum which show fiber type differences in the adult ALD and PLD muscles are compared in the developmental stages. The caveolar density in the ALD fibers is about 20/microns2 at 1 day increasing to about 37/microns2 at 30 days, whereas in the PLD fibers it remains at about 20/microns2 during this period. The distribution of the caveolae in the two muscles is different from the beginning; in the ALD fibers the caveolae are distributed throughout the plasma membrane and in PLD fibers they are patterned into clusters overlying the I band regions. The density of intramembranous particles of 1-day ALD and PLD plasma membranes appears similar, but by 7 days the particle counts in the sarcolemma of the ALD muscle are about twice as numerous as those in the PLD muscle. The rectilinear arrays are virtually absent in the ALD muscle, whereas in the PLD muscle their density is about 10/microns2 at 1 day and about 20/microns2 at 7 days. Already at 1 day posthatching the SR in ALD and PLD fibers has the adult configuration, i.e., an open irregular network in ALD fibers and periodically arranged tubules with triadic expansions in the PLD fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Myosin heavy chain composition of muscle spindles in human biceps brachii.

Data on the myosin heavy chain (MyHC) composition of human muscle spindles are scarce in spite of the well-known correlation between MyHC composition and functional properties of skeletal muscle fibers. The MyHC composition of intrafusal fibers from 36 spindles of human biceps brachii muscle was studied in detail by immunocytochemistry with a large battery of antibodies. The MyHC content of isolated muscle spindles was assessed with SDS-PAGE and immunoblots. Four major MyHC isoforms (MyHCI, IIa, embryonic, and intrafusal) were detected with SDS-PAGE. Immunocytochemistry revealed very complex staining patterns for each intrafusal fiber type. The bag(1) fibers contained slow tonic MyHC along their entire fiber length and MyHCI, alpha-cardiac, embryonic, and fetal isoforms along a variable part of their length. The bag(2) fibers contained MyHC slow tonic, I, alpha-cardiac, embryonic, and fetal isoforms with regional variations. Chain fibers contained MyHCIIa, embryonic, and fetal isoforms throughout the fiber, and MyHCIIx at least in the juxtaequatorial region. Virtually each muscle spindle had a different allotment of numbers of bag(1), bag(2) and chain fibers. Taken together, the complexity in intrafusal fiber content and MyHC composition observed indicate that each muscle spindle in the human biceps has a unique identity.     

Gamma irradiation prevents compensatory hypertrophy of overloaded mouse extensor digitorum longus muscle.

Mouse extensor digitorum longus (EDL) muscle was subjected to a dose of gamma irradiation that causes reproductive death of satellite cells and/or to chronic compensatory overload, achieved by removal of the distal portion of the tibialis anterior muscle. Four weeks later the mass, fiber type percentage, and fiber size of the EDL muscle were measured. Both the irradiated + overloaded and the irradiated only EDL muscles were significantly lighter and contained significantly smaller fibers than untreated muscle or muscle subjected to chronic overload only. Overload muscle, whether irradiated or not, had a larger percentage of type IIx fibers and a smaller percentage of type IIb fibers than muscle that had not been overloaded. The results confirm that satellite cell proliferation is a prerequisite for muscle hypertrophy induced by synergist incapacitation, but it appears not to be required for the maintenance of, or change in, normal muscle fiber myosin heavy chain phenotype expression.     

Coexistence of fast and slow types of myosin light chains in a single fiber of rat soleus muscle

Single muscle fibers were isolated from soleus and extensor digitorum longus muscle of adult rats. The muscle fiber type of single fibers was determined physiologically by the skinned fiber method according to the sensitivity to strontium (Sr) ions. The fiber type of single fibers was contrasted to the pattern of myosin light chains analyzed by one and two dimensional gel-electrophoreses. All the type 2 fibers isolated from soleus muscle contained both fast and slow types of myosin light chains.     

Denervation and reinnervation of fast and slow muscles. A histochemical study in rats.

A histochemical study, using myosin-adenosine triphosphatase activity at pH 9.4, was conducted in soleus and plantaris muscles of adult rats, after bilateral crushing of the sciatic nerve at the sciatic notch. The changes in fiber diameter and per cent composition of type I and type II fibers plus muscle weights were evaluated along the course of denervation-reinnervation curve at 1, 2, 3, 4 and 6 weeks postnerve crush. The study revealed that in the early denervation phase (up to 2 weeks postcrush) both the slow and fast muscles, soleus and plantaris, resepctively, atrophied similarly in muscle mass. Soleus increased in the number of type II fibers, which may be attributed to "disuse" effect. During the same period, the type I fibers of soleus atrophied as much or slightly more than the type II fibers; whereas the type II fibers of plantaris atrophied significantly more than the type I fibers, reflecting that the process of denervation, in its early stages, may affect the two fiber types differentially in the slow and fast muscles. It was deduced that the type I fibers of plantaris may be essentially different in the slow (soleus) and fast (plantaris) muscles under study. The onset of reinnervation, as determined by the increase in muscle weight and fiber diameter of the major fiber type, occurred in soleus and plantaris at 2 and 3 weeks postcrush, respectively, which confirms the earlier hypotheses that the slow muscles are reinnervated sooner than the fast muscles. It is suggested that the reinnervation of muscle after crush injury may be specific to the muscle type or its predominant fiber type.     

Innervation of regenerated spindles in muscle grafts of the rat

Features of the nerve supply and the encapsulated fibers of muscle spindles were assessed in grafted and normal extensor digitorum longus (EDL) muscles of rats by analysis of serial 10-microns frozen transverse sections stained for enzymes which delineated motor and sensory endings, oxidative capacity and muscle fiber type. The number of fibers was significantly more variable, and branched fibers were more frequently observed in regenerated spindles than in control spindles. Forty-eight percent of regenerated spindles received sensory innervation. Spindles reinnervated by afferents had a larger periaxial space than did spindles which were not reinnervated by afferents. Regenerated fibers innervated by afferents had small cross-sectional areas, equatorial regions with myofibrils restricted to the periphery of fibers, unpredictable patterns of nonuniform and nonreversible staining along the length of the fiber for 'myofibrillar' adenosine triphosphatase (mATPase) after acid and alkaline preincubation. In contrast, regenerated fibers devoid of sensory innervation resembled extrafusal fibers in that they usually exhibited myofibrils throughout the length of the fiber, no central aggregations of myonuclei, uniform staining for mATPase and a reversal of staining for mATPase after preincubation in an acid or alkaline medium. Approximately thirty percent of encapsulated fibers devoid of sensory innervation stained analogous to a type I extrafusal fiber, a pattern of staining never observed in intrafusal fibers of normal spindles. Groups of encapsulated fibers all exhibiting this pattern of staining reflect that either these fibers may have been innervated by collaterals of skeletomotor axons that originally innervated type I extrafusal fibers or that fibers innervated by only fusimotor neurons express patterns of staining for mATPase similar to extrafusal fibers in the absence of sensory innervation. Sensory innervation may also influence the reestablishment of multiple sites of motor endings on regenerated intrafusal fibers. Those regenerated fibers innervated by afferents had more motor endings than did regenerated fibers devoid of sensory innervation. Differences in size, morphology, and patterns of staining for mATPase and numbers of motor endings between fibers innervated by afferents and fibers devoid of sensory innervation reflect that afferents can influence the differentiation of muscle cells and the reestablishment of motor innervation other than during the late prenatal/early postnatal period when muscle spindles form and differentiate in rats.     

Expression of myosin isoforms during notexin-induced regeneration of rat soleus muscles

Myosin isozymes and their fiber distribution were studied during regeneration of the soleus muscle of young adult (4-6 week old) rats. Muscle degeneration and regeneration were induced by a single subcutaneous injection of a snake toxin, notexin. If reinnervation of the regenerating muscle was allowed to occur (functional innervation nearly complete by 7 days), then fiber diameters continued to increase and by 28 days after toxin treatment they attained the same values as fibers in the contralateral soleus. If the muscles were denervated at the time of toxin injection, the early phases of regeneration still took place but the fibers failed to continue to increase in size. Electrophoresis of native myosin showed multiple bands between 3 and 21 days of regeneration which could be interpreted as indicating the presence of embryonic, neonatal, fast and slow myosins in the innervated muscles. Adult slow myosin became the exclusive from in innervated regenerates. In contrast, adult fast myosin became the predominant form in denervated regenerating muscles. Immunocytochemical localization of myosin isozymes demonstrated that in innervated muscles the slow form began to appear in a heterogeneous fashion at about 7 days, and became the major form in all fibers by 21-28 days. Thus, the regenerated muscle was almost entirely composed of slow fibers, in clear contrast to the contralateral muscle which was still substantially mixed. In denervated regenerating muscles, slow myosin was not detected biochemically or immunocytochemically whereas fast myosin was detected in all denervated fibers by 21-28 days. The regenerating soleus muscle therefore is clearly different from the developing soleus muscle in that the former is composed of a uniform fiber population with respect to myosin transitions. Moreover the satellite cells which account for the regeneration process in the soleus muscle do not appear to be predetermined with respect to myosin heavy chain expression, since the fibers they form can express either slow or fast isoforms. The induction of the slow myosin phenotype is entirely dependent on a positive, extrinsic influence of the nerve.     

Correlation between myofibrillar ATPase activity and myosin heavy chain composition in single human muscle fibers.

Single human muscle fibers were analysed using a combination of histochemical and biochemical techniques. Routine myofibrillar adenosine triphosphatase (mATPase) histochemistry revealed a continuum of staining intensities between the fast fiber types IIA and IIB (type IIAB fibers) after preincubation at pH 4.6. Electrophoretic analysis of single, histochemically-identified fibers demonstrated a correlation between the staining intensity and the myosin heavy chain (MHC) composition. All fibers classified as type I contained exclusively MHCI and all type IIA fibers contained only MHCIIa. Type IIAB fibers displayed variable amounts of both MHCIIa and MHCIIb; the greater the staining intensity of these fibers after preincubation at pH 4.6, the greater the percentage of MHCIIb. Those fibers histochemically classified as type IIB contained either entirely MHCIIb or, in addition to MHCIIb, a small amount of MHCIIa. These data establish a correlation between the mATPase activity and MHC content in single human muscle fibers.