Histochemical, enzymatic, and contractile properties of skeletal muscle fibers in the lizard Dipsosaurus dorsalis

Lizard skeletal muscle fiber types were investigated in the iliofibularis (IF) muscle of the desert iguana (Dipsosaurus dorsalis). Three fiber types were identified based on histochemical staining for myosin ATPase (mATPase), succinic dehydrogenase (SDH), and alphaglycerophosphate dehydrogenase (alphaGPDH) activity. The pale region of the IF contains exclusively fast-twitch-glycolytic (FG) fibers, which stain dark for mATPase and alphaGPDH, light SDH. The red region of the IF contains fast-twitch-oxidative-glycolytic (FOG) fibers, which stain dark for all three enzymes, and tonic fibers, which stain light for mATPase, dark for SDH, and moderate for alphaGPDH. Enzymatic activities of myofibrillar ATPase, citrate synthase, and alphaGPDH confirm these histochemical interpretations. Lizard FG and FOG fibers possess twitch contraction times and resistance to fatigue comparable to analogous fibers in mammals, but are one-half as oxidative and several times as glycolytic as analogous fibers in rats. Lizard tonic fibers demonstrate the acetylcholine sensitivity common to other vertebrate tonic fibers.     

Comparative histochemical study of prosimian primate hindlimb muscles. I. Muscle fiber types

The profiles of fiber types in hindlimb muscles from the tree shrew (Tupaia glis), lesser bushbaby (Galago senegalensis), and the slow loris (Nycticebus coucang) were determined using histochemical techniques. Fibers were classified as fast-twitch oxidative-glycolytic (FOG), fast-twitch glycolytic (FG), slow-twitch oxidative (SO), or fast-twitch oxidative (FO), according to reactions for alkaline-stable ATPase, acid-stable ATPase, alpha-glucan phosphorylase, reduced nicotinamide adenine dinucleotide diaphorase, succinate dehydrogenase, mitochondrial alpha-glycerophosphate dehydrogenase (MaGPDH), and beta-hydroxybutyric dehydrogenase, as well as glycogen staining by the periodic acid-Schiff technique. Prolonged dissection of numerous muscles was carried out on hindlimbs submersed in cold Tyrode's solution; such treatment had no qualitative effect on enzyme staining reactions, but it is not a suitable procedure if one wishes to stain for glycogen. Fast-twitch oxidative (FO) fibers are alkaline-stable ATPase-positive and possess low MalphaGPDH enzyme activity. These fibers have not been reported previously in any hindlimb muscles. No muscles of any species studies were homogeneous with respect to fiber type. Slow loris muscles lacked FG fibers. The majority of the muscles of the slow loris contained numerous SO fibers. The relationship between enzyme activities and locomotor pattern is discussed.     

Function and position determine relative proportions of different fiber types in limb muscles of the lizard Tropidurus psammonastes

Skeletal muscles can be classified as flexors or extensors according to their function, and as dorsal or ventral according to their position. The latter classification evokes their embryological origin from muscle masses initially divided during limb development, and muscles sharing a given position do not necessarily perform the same function. Here, we compare the relative proportions of different fiber types among six limb muscles in the lizard Tropidurus psammonastes. Individual fibers were classified as slow oxidative (SO), fast glycolytic (FG) or fast oxidative-glycolytic (FOG) based on mitochondrial content; muscles were classified according to position and function. Mixed linear models considering one or both effects were compared using likelihood ratio tests. Variation in the proportion of FG and FOG fibers is mainly explained by function (flexor muscles have on average lower proportions of FG and higher proportions of FOG fibers), while variation in SO fibers is better explained by position (they are less abundant in ventral muscles than in those developed from a dorsal muscle mass). Our results clarify the roles of position and function in determining the relative proportions of the various muscle fibers and provide evidence that these factors may differentially affect distinct fiber types.     

Catalase-positive microperoxisomes in rat soleus and extensor digitorum longus muscle fiber types

The size, distribution, and content of catalase-reactive microperoxisomes were studied cytochemically in slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG), and fast-twitch glycolytic (FG) fibers of soleus and extensor digitorum longus (EDL) rat muscles. Fiber types were classified on the basis of mitochondrial content and distribution, Z-band widths, and myofibril size and shape. Microperoxisomes were generally located between myofibrils at the I-bands. The absence of crystalloid inclusions prevented positive identification of microperoxisomes in nonreacted and aminotriazole-inhibited muscles. EDL and soleus SO fibers possessed the largest microperoxisomes, whereas FOG and FG fibers of the EDL contained small- to medium-sized microperoxisomes. Comparing either microperoxisome number per muscle fiber area or microperoxisome area per fiber area revealed significant differences between fiber types with this ranking: soleus SO greater than EDL SO greater than EDL FOG greater than EDL FG. The present observations demonstrate that the content of catalase-positive microperoxisomes is greatest in the oxidative muscle fiber types. These cytochemical findings account for the higher catalase activity in homogenates of soleus muscles as compared to that of EDL muscles, because the soleus contains more oxidative fibers than EDL.     

Comparative analysis of fiber-type composition in the iliofibularis muscle of phrynosomatid lizards (Squamata).

The lizard family Phrynosomatidae comprises three subclades: the closely related sand and horned lizards, and their relatives the Sceloporus group. This family exhibits great variation in ecology, behavior, and general body plan. Previous studies also show that this family exhibits great diversity in locomotor performance abilities; as measured on a high-speed treadmill, sand lizards are exceptionally fast sprinters, members of the Sceloporus group are intermediate, and horned lizards are slowest. These differences are paralleled by differences in relative hindlimb span. To determine if muscle fiber-type composition also varies among the three subclades, we examined the iliofibularis (IF), a hindlimb muscle used in lizard locomotion, in 11 species of phrynosomatid lizards. Using histochemical assays for myosin ATPase, an indicator of fast-twitch capacity, and succinic dehydrogenase, denoting oxidative capacity, we classified fiber types into three categories based on existing nomenclature: fast-twitch glycolytic (FG), fast-twitch oxidative-glycolytic (FOG), and slow-twitch oxidative (SO). Sand lizards have a high proportion of FG fibers (64-70%) and a low proportion of FOG fibers (25-33%), horned lizards are the converse (FG fibers 25-31%, FOG fibers 56-66%), and members of the Sceloporus group are intermediate for both FG (41-48%) and FOG (42-45%) content. Hence, across all 11 species %FOG and %FG are strongly negatively correlated. Analysis with phylogenetically independent contrasts indicate that this negative relationship is entirely attributable to the divergence between sand and horned lizards. The %SO also varies among the three subclades. Results from conventional nested ANCOVA (with log body mass as a covariate) indicate that the log mean cross-sectional area of individual muscle fibers differs among species and is positively correlated with body mass across species, but does not differ significantly among subclades. The log cross-sectional area of the IF varies among species, but does not vary among subclades. Conversely, the total thigh muscle cross-sectional area does not vary among species, but does vary among subclades; horned lizards have slimmer thighs. Muscle fiber-type composition appears to form part of a coadapted suite of traits, along with relative limb and muscle sizes, that affect the locomotor abilities of phrynosomatid lizards.     

Histochemical determination of the fiber composition of locomotory muscles in a lizard, Dipsosaurus dorsalis

A histochemical survey was done on the fiber composition of 12 different locomotory muscles in the lizard Dipsosaurus dorsalis. Three types of fibers were found in all muscles: (1) fast-twitch-glycolytic (FG); (2) fast-twitch-oxidative-glycolytic (FOG); and (3) tonic fibers. Virtually all locomotory muscles contain some tonic fibers. Most muscles have bulk white regions (containing mostly FG fibers) and distinct red, oxidative regions (with FOG and tonic fibers). These red regions are predominantly located around the joints in the hind limb muscles, and probably serve a postural and joint-stabilizing function. The predominance of FG fibers in the bulk white regions is well-correlated with the rapid, anaerobically supported predator escape behavior of D. dosalis.     

Enzyme profiles of single muscle fibers never exposed to normal neuromuscular activity.

Do muscle fiber properties commonly associated with fiber types in adult animals and the population distribution of these properties require normal activation patterns to develop? To address this issue, the activity of an oxidative [succinic dehydrogenase (SDH)] and a glycolytic [alpha-glycerophosphate dehydrogenase (GPD)] marker enzyme, the characteristics of myosin adenosinetriphosphatase (myosin ATPase, alkaline preincubation), and the cross-sectional area of single fibers were studied. The soleus and medial gastrocnemius of normal adult cats were compared with cats that 6 mo earlier had been spinally transected at T12-T13 at 2 wk of age. In control cats, SDH activity was higher in dark than light ATPase fibers in the soleus and higher in light than dark ATPase fibers in the medial gastrocnemius. After transection, SDH activity was similar to control in both muscles. GPD activity appeared to be elevated in some fibers in each fiber type in both muscles after transection. The cross-sectional areas most affected by spinal transection were light ATPase fibers of the soleus and dark ATPase fibers of the medial gastrocnemius, the predominant fiber type in each muscle. These data demonstrate that although the muscle fibers of cats spinalized at 2 wk of age presumably were never exposed to normal levels of activation, the activity of an oxidative marker enzyme was maintained or elevated 6 mo after spinal transection. Furthermore, although the absolute enzyme activities in some fibers were elevated by transection, three functional protein systems commonly associated with fiber types, i.e., hydrolysis of ATP by myosin ATPase and glycolytic (GPD) and oxidative (SHD) metabolism, developed in a coordinated manner typical of normal adult muscles.     

Ultrastructural localization of lectin receptors on the bone-marrow sinusoidal endothelium of the rat

The purpose of this study was to estimate the absolute and relative masses of the three types of skeletal muscle fibers in the total hindlimb of the male Sprague-Dawley rat (Rattus norvegicus). For six rats, total body mass was recorded and the following weights taken from dissection of one hindlimb: 32 individual major muscles or muscle parts, remaining skeletal musculature (small hip muscles and intrinsic foot muscles), bone, inguinal fat pad, and skin. The fibers from the 32 muscles or muscle parts (which constituted 98% of the hindlimb skeletal muscle mass) were classified from histochemistry as fast-twitch oxidative glycolytic (FOG), fast-twitch glycolytic (FG), or slow-twitch oxidative (SO), and their populations were determined. Fiber cross-sectional areas from the same muscles were measured with a digitizer. Mass of each of the fiber types within muscles and in the total hindlimb was then calculated from fiber-type population, fiber-type area, and muscle-mass data. Skeletal muscle made up 71% of the total hindlimb mass. Of this, 76% was occupied by FG fibers, 19% by FOG fibers, and 5% by SO fibers. Thus, the FG fiber type is clearly the predominant fiber type in the rat hindlimb in terms of muscle mass. Fiber-type mass data are compared with physiological (blood flow) and biochemical (succinate dehydrogenase activities) data for the muscles taken from previous studies, and it is demonstrated that these functional properties are closely related to the proportions of muscle mass composed of the various fiber types.     

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.     

Histochemical and biochemical properties of flight muscle fibers in the little brown bat,Myotis lucifugus

Summary The purpose of this investigation was (1) to determine the fiber composition of pectoralis muscle of the little brown bat,Myotis lucifugus; (2) to compare the fiber composition of this muscle with two of the animal's accessory flight muscles; and (3) to study the effect of hibernation on pectoralis muscle fiber composition. Bat skeletal muscle fibers were also compared with those of white laboratory rats (Rattus norvegicus). Bat pectoralis muscles possessed exceptionally high oxidative capacities as indicated by their succinate dehydrogenase activities, but relatively low glycolytic potentials (phosphofructokinase activities). Muscle histochemistry demonstrated that fiber composition of bat pectorlis muscle was homogeneous; all fibers possessed high aerobic and low glycolytic potentials, and high myofibrillar ATPase activities indicating fast contractile properties. In contrast, accessory flight muscles possessed three distinguishable fiber types. During hibernation there was a significant decline in oxidative potential, no change in glycolytic potential, and no alteration in basic fiber composition of bat pectoralis muscle. The findings of this study suggest that pectoralis muscles ofM. lucifugus may approach the ultimate adaptation of a mammalian locomotory muscle for aerobic generation of muscular power.Abbreviations FG fast-twich glycolytic - FOG fast-twitch-oxydative-glycolytic - -GPDH -glycerophosphate dehydrogenase - LDH lactate dehydrogenase - NADH-D reduced nicotinamide adenine dinucleotide diaphorase - PFK phosphofructokinase - SDH succinate dehydrogenase - SO slowtwich-oxidative     

Fiber‐type composition in the perivertebral musculature of lizards: Implications for the evolution of the diapsid trunk muscles

The perivertebral musculature of lizards is critical for the stabilization and the mobilization of the trunk during locomotion. Some trunk muscles are also involved in ventilation. This dual function of trunk muscles in locomotion and ventilation leads to a biomechanical conflict in many lizards and constrains their ability to breathe while running (“axial constraint”) which likely is reflected by their high anaerobic scope. Furthermore, different foraging and predator‐escape strategies were shown to correlate with the metabolic profile of locomotor muscles in lizards. Because knowledge of muscle's fiber‐type composition may help to reveal a muscle's functional properties, we investigated the distribution pattern of muscle fiber types in the perivertebral musculature in two small lizard species with a generalized body shape and subjected to the axial constraint (Dipsosaurus dorsalis, Acanthodactylus maculatus) and one species that circumvents the axial constraint by means of gular pumping (Varanus exanthematicus). Additionally, these species differ in their predator‐escape and foraging behaviors. Using refined enzyme‐histochemical protocols, muscle fiber types were differentiated in serial cross‐sections through the trunk, maintaining the anatomical relationships between the skeleton and the musculature. The fiber composition in Dipsosaurus and Acanthodactylus showed a highly glycolytic profile, consistent with their intermittent locomotor style and reliance on anaerobic metabolism during activity. Because early representatives of diapsids resemble these two species in several postcranial characters, we suggest that this glycolytic profile represents the plesiomorphic condition for diapsids. In Varanus, we found a high proportion of oxidative fibers in all muscles, which is in accordance with its high aerobic scope and capability of sustained locomotion. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.     

Motor units of the primary ankle extensor muscles of the opossum (Didelphis virginiana): functional properties and fiber types

Motor units of the medial gastrocnemius (MG) and the single lateral gastrocnemius/soleus (LG/S) muscles of the opossum (Didelphis virginiana) were found to have uniformly slow contraction times relative to homologous muscles of the cat. Though a broad range of peak tetanic tensions was found among motor units from both muscles, most of the motor units were quite large relative to tension of the whole muscle. Comparison of the relative sizes of motor units showed that those of LG/S are significantly larger and slower than the units of MG. This suggests that the motor units of the two muscles may be differentially recruited during different behaviors. All of the MG and LG/S motor units were highly or moderately resistant to fatigue. Histochemical staining for NADH-diaphorase activity indicated consistently high levels of the enzyme in all of the fibers of both muscles. Apparently, all of the fast motor units consist of fast oxidative/glycolytic (FOG)-type muscle fibers. Our data provide functional evidence that the types of myofibrillar ATPase demonstrated by Brooke and Kaiser ('70), are not necessarily correlated to physiological classification of fiber types as slow oxidative (SO), fast oxidative/glycolytic (FOG), and fast glycolytic (FG) (Peter et al., '72). Perhaps compartmentalization of muscle fiber types may be a first step in the separation of muscles into multiple heads during the evolution of specialization to diverse locomotor habits among the mammals.     

Enzyme‐ and immunohistochemical aspects of skeletal muscle fibers in brown bear (Ursus arctos)

To further elucidate the pattern of MHC isoform expression in skeletal muscles of large mammals, in this study the skeletal muscles of brown bear, one of the largest mammalian predators with an extraordinary locomotor capacity, were analyzed. Fiber types in longissimus dorsi, triceps brachii caput longum, and rectus femoris muscles were determined according to the myofibrillar ATPase (mATPase) histochemistry and MHC isoform expression, revealed by a set of antibodies specific to MHC isoforms. The oxidative (SDH) and glycolytic enzyme (α‐GPDH) capacity of fibers was demonstrated as well. By mATPase histochemistry five fiber types, i.e., I, IIC, IIA, IIAX, IIX were distinguished. Analyzing the MHC isoform expression, we assume that MHC‐I, ‐IIa, and ‐IIx are expressed in the muscles of adolescent bears. MHC‐I isoform was expressed in Type‐I fibers and coexpressed with presumably ‐IIa isoform, in Type‐IIC fibers. Surprisingly, two antibodies specific to rat MHC‐IIa stained those fast fibers, that were histochemically and immunohistochemically classified as Type IIX. This assumption was additionally confirmed by complete absence of fiber staining with antibody specific to rat MHC‐IIb and all fast fiber staining with antibody that according to our experience recognizes MHC‐IIa and ‐IIx of rat. Furthermore, quite high‐oxidative capacity of all fast fiber types and their weak glycolytic capacity also imply for MHC‐IIa and ‐IIx isoform expression in fast fibers of bear. However, in adult, full‐grown animal, only MHC‐I and MHC‐IIa isoforms were expressed. The expression of only two fast isoforms in bear, like in many other large mammals (humans, cat, dog, goat, cattle, and horse) obviously meets the weight‐bearing and locomotor demands of these mammals. J. Morphol., 2009. © 2008 Wiley‐Liss, Inc.     

Changes in the metabolic profile of the equine gluteus medius as a function of sampling depth

1. Cross sections from the middle of the gluteus medius were removed from 10 adult horses and used to evaluate changes in histochemically determined muscle fiber type and biochemically determined metabolic enzyme activities as a function of sample depth. 2. Muscle fiber types determined using histochemical methods for myosin ATPase (pH 9.4) and succinic dehydrogenase (SDH) activity indicated percent fast-twitch glycolytic (FG) muscle fibers decreased and slow-twitch oxidative (SO) fibers increased as a function of increasing sampling depth. 3. Percent histochemically determined fast-twitch oxidative glycolytic (FOG) fibers decreased slightly only in the deepest region of the gluteus medius. 4. Citrate synthase (CS) enzymatic activity, used as a marker for mitochondrial oxidative potential, increased 2.5-fold in activity per g of muscle protein from 1 to 8 cm sampling depth. 5. 3-hydroxyacyl-CoA dehydrogenase (HAD) enzymatic activity, used as a marker for lipid oxidation potential, increased 3-fold in activity per g of muscle protein when the depth increased from 1 to 8 cm. 6. Phosphorylase (PS) enzymatic activity, used as a marker for potential glycogen utilization, decreased 50% in activity per g of muscle protein when going from 1 to 8 cm. 7. Lactate dehydrogenase (LDH) enzymatic activity, used as a marker for anaerobic glycolytic potential, decreased about 50% in activity as the sampling depth increased from 1 to 8 cm. 8. In summary, the superficial portion of the equine gluteus medius was found to be more glycolytic and less aerobic in its metabolic profile than deeper regions. The muscle became progressively more aerobic and less glycolytic with increasing sampling depth.     

Loss of thick filaments from fast-twitch glucolytic muscle fibers of the pigeon pectoralis after chronic administration of dantrolene sodium.

Adult pigeons received dantrolene sodium, a skeletal muscle relaxant which blocks the release of calcium during excitation-contraction coupling, for 12 to 16 weeks. The pectoralis muscles of these birds were analyzed for changes occurring in the various fiber types of the muscle. Both histochemistry (ATPase and SDH activity) and electron microscopy (mitochondrial and lipid volume percentages) differentiated two fiber types. The two fiber-types consisted of fast-twitch glycolytic fibers (FG) and fast-twitch oxidative-glycolytic (FOG) fibers. After dantrolene treatment some FG fibers showed little or no ATPase activity. Dantrolene treatment also produced a disappearance of thick filaments in some FG fibers. We infer that the fibers without thick filaments are the ones lacking ATPase activity. The FOG fibers were nearly normal. Since drug-fed birds lose weight, a few birds were starved to determine whether the filament loss was related solely to the bird's loss in weight. No fibers in starved birds showed reduced ATPase activity or loss of thick filaments. In fibers that showed thick filament disappearance, the I-bands remained organized and intact, suggesting that the I-band maintains its integrity without interaction with the thick filaments. Changes in activity patterns may cause loss of thick filaments by inhibiting either their synthesis or assembly.     

Fiber subtype distribution of pharyngeal dilator muscles and diaphragm in the cat

In previous studies differences were frequently found between the pharyngeal dilator muscles and the thoracic respiratory muscles in their patterns of electrical and mechanical activity during the respiratory cycle, with both resting and stimulated breathing. However, little is known about the intrinsic properties of the pharyngeal muscles and how they relate to the intrinsic properties of the diaphragm. In the present study, the fiber subtype distributions of two pharyngeal dilator muscles, the geniohyoid and the sternohyoid, were ascertained histochemically in the cat. The geniohyoid and the sternohyoid muscles had a preponderance of fast glycolytic (FG) fibers (mean 48 and 55%, respectively), a smaller number of fast oxidative-glycolytic (FOG) fibers (mean 36 and 31%, respectively), and few slow oxidative (SO) fibers (mean 16 and 14%, respectively). The percentages of SO fibers of both hyoid muscles were significantly (P less than 0.01) lower than that of the costal diaphragm, and the percentages of FOG and FG fibers were significantly higher than that of the diaphragm. In conclusion, the geniohyoid and sternohyoid muscles have histochemical characteristics usually associated with fast contraction and intermediate endurance properties.     

Adaptations in metabolic capacity of rat soleus after paralysis.

To determine whether long-term reductions in neuromuscular activity result in alterations in metabolic capacity, the activities of oxidative, i.e., succinate dehydrogenase (SDH) and citrate synthase (CS), and glycolytic, i.e., alpha-glycerophosphate dehydrogenase (GPD), enzyme markers were quantified in rat soleus muscles 1, 3, and 6 mo after a complete spinal cord transection (ST). In addition, the proportional content of lactate dehydrogenase (LDH) isozymes was used as a marker for oxidative and glycolytic capacities. The myosin heavy chain (MHC) isoform content of a fiber served as a marker of phenotype. In general, MHC isoforms shifted from MHC1 toward MHC2, particularly MHC2x, after ST. Mean SDH and CS activities were higher in ST than control at all time points. The elevated SDH and CS activities were indicative of an enhanced oxidative capacity. GPD activities were higher in ST than control rats at all time points. The increase in activity of SDH was larger than GPD. Thus the GPD-to-SDH (glycolytic-to-oxidative) ratio was decreased after ST. Compared with controls, total LDH activity increased transiently, and the LDH isozyme profile shifted from LDH-1 toward LDH-5, indicative of an enhanced glycolytic capacity. Combined, these results indicate that 1) the metabolic capacities of soleus fibers were not compromised, but the interrelationships among oxidative and glycolytic capacity and MHC content were apparently dissociated after ST; 2) enhancements in oxidative and glycolytic enzyme activities are not mutually exclusive; and 3) chronic reductions in skeletal muscle activity do not necessarily result in a reduced oxidative capacity.     

Effect of growth on muscle capillarity and fiber type composition in rat diaphragm

The effect of growth on the capillarity and fiber type composition of the diaphragm, soleus and extensor digitorum longus (EDL) muscles of rats weighing between 55 and 330 g have been studied. Muscle samples obtained from the anesthetized rat were rapidly frozen and sliced transversely in a cryostat. The sections were stained histochemically by the SDH method and the myosin ATPase method after preincubation at pH 4.3 to typify fibers (FG, FOG and SO fibers). To visualize capillaries, the myosin ATPase method after preincubation at pH 4.0 was used. The percentage of FOG fibers decreased in all muscles with growth. While the FG and SO fibers increased in the diaphragm, SO fibers increased in the soleus, and FG fibers increased in the EDL. The capillary density showed a hyperbolic decrease with growth in all muscles, while the number of capillaries around each fiber increased in all muscles with growth. It is concluded that growth causes the changing properties of the motoneurons and the new capillary formation in the diaphragm muscle, as well as the soleus and EDL muscles.     

Fiber composition and capillarity in growing guinea pigs acclimated to cold and cold plus hypoxia

The central portion of the medial head of the gastrocnemius of control (normoxic and normothermic), hypoxia-, cold-, and cold plus hypoxia-acclimated guinea pigs was analyzed for capillary supply and fiber composition to elucidate changes in capillarity induced by environmental stresses. The muscle was cut at midbelly, frozen, sectioned, and stained for myosin ATPase. Fiber cross-sectional areas; percentages of slow-twitch oxidative (SO), fast-twitch oxidative-glycolytic (FOG), and fast-twitch glycolytic (FG) fibers; and numbers of capillaries around each fiber type were measured. Growth rates of all four guinea pig groups were similar. Capillarity was not affected by acclimation to hypoxia. Cold and cold plus hypoxia acclimation led to increased numbers of capillaries around the fiber in all three fiber types. In addition, significant increases in the percentage of FOG fibers and concomitant decreases in the percentage of FG fibers compared to controls were found in cold and in cold plus hypoxia indicating that a transformation of fiber type from FG to FOG had occurred. The increase in FOGs at the expense of the FGs did not occur in the guinea pigs grown in a hypoxic environment. The increased total capillarity in those muscles studied was the result of more capillaries around all fiber types and was not due to simple transformation of fibers.