High Succinate Dehydrogenase Activity Research Articles

Oxidative capacity of human muscle fiber types: effects of age and training status

Morphometry and oxidative capacity of slow-twitch (type I) and fast-twitch (type IIa and IIb) muscle fibers obtained from vastus lateralis needle biopsies were compared between younger (21-30 yr) and older (51-62 yr) normal fit (maximal O2 uptake = 47.0 vs. 32.3 ml.kg-1.min-1) and endurance-trained (66.3 vs. 52.7 ml.kg-1.min-1) men (n = 6/group). The older groups had smaller type IIa (31%) and IIb (40%) fiber areas and fewer capillaries surrounding these fibers than did younger groups. The reduced type II fiber areas and capillary contacts associated with aging were also observed in the older trained men. However, the capillary supply per unit type II fiber area was not affected by age but was enhanced by training. Additionally, on the basis of quantitative histochemical analysis, succinate dehydrogenase activities of type IIa fibers in the older trained men [4.07 +/- 0.68 (SD) mmol.min-1.l-1] were similar to those observed in younger trained men (4.00 +/- 0.48 mmol.min-1.l-1) and twofold higher than in older normal fit men (2.01 +/- 0.65 mmol.min-1.l-1; age x fitness interaction, P < 0.05). Type I muscle fibers were unaffected by age but were larger and had more capillary contacts and higher succinate dehydrogenase activities in the trained groups. The findings of this study suggest that aging results in a decrease in type II fiber size and oxidative capacity in healthy men and that this latter effect can be prevented by endurance training. Conclusions regarding the effects of age and training status on muscle capillarization depend largely on how these data are expressed.

Ultrastructural characteristics of zebrafish spinal motoneurons innervating glycolytic white, and oxidative red and intermediate muscle fibers

Spinal motoneurons in the zebrafish were classified using morphological criteria. Dorsomedial white motoneurons which innervate the fast, glycolytic white muscle fiber compartment were distinguished from ventrolateral red and intermediate motoneurons which innervate the slow, oxidative, red and intermediate muscle fiber compartments. Synapses on cell somata and cell organelles were studied in detail. The motoneurons which innervate white muscle fibers (W motoneurons) are considerably larger than those which innervate red and intermediate muscle fibers (RI motoneurons; W > RI). Significant differences were also found in the size of the nucleus (W > RI) and in the ratio size nucleus/size soma (W < RI); small differences were found regarding endoplasmic reticulum (W > RI) and mitochondria (W < RI). There were no differences in synaptic apposition length or percentage of terminals with flat vesicles. Small differences were discerned with regard to covering percentages (W < RI) and percentage of terminals with round vesicles (W > RI). Terminals with dense cored vesicles appeared on W motoneuron somata only. Within the motoneuron population, there was a positive correlation between the coverage of terminals containing flat vesicles and the perimeter of the cell soma. In RI motoneurons, there was a positive correlation between the perimeter of the cell and the amount of endoplasmic reticulum. A negative correlation was found between the RI cell perimeter and mitochondria, which is in line with a high succinate dehydrogenase activity in small cells.

Increased mitochondrial DNA in blood vessels and ragged-red fibers in mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS).

Using in situ hybridization, we studied muscle biopsy specimens from 4 patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS). Three of the 4 patients with MELAS had a mutation at position 3243 of mitochondrial DNA (mtDNA) in the transfer RNALeu(UUR) gene, and the other patient had a mutation at position 3271 in the same transfer RNALeu(UUR) gene. Quantitative analysis using Southern blot hybridization and polymerase chain reaction showed 80 to 90% mutant mtDNA in muscle. In situ hybridization analysis showed that total mtDNAs (both normal and mutant) were extremely increased in blood vessels with high succinate dehydrogenase activity (strongly succinate dehydrogenase-reactive blood vessels) and ragged-red fibers. Cytochrome c oxidase activity in most of these reactive blood vessels and ragged-red fibers was positive. The similar morphological behavior in these vessels and fibers suggests that an increase in mutant mtDNA is responsible for mitochondrial proliferation and dysfunction in both tissues where cytochrome c oxidase is not a primarily defective enzyme. The pattern of expression of genes for mtDNA-encoded ribosomal RNA and the protein-coding region cytochrome c oxidase subunit II were similar in muscle specimens of patients with MELAS, patients with chronic progressive external ophthalmoplegia, and normal control subjects, and also between the two MELAS mutations. These results do not support the hypothesis that impaired transcription termination is a molecular defect in MELAS.

Strongly succinate dehydrogenase–reactive blood vessels in muscles from patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke‐like episodes

Intramuscular blood vessels were examined with succinate dehydrogenase stain in skeletal muscle biopsy specimens from 6 patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS). Almost all arteries had large granular deposits with high succinate dehydrogenase activity in their walls. Electron microscopic examination of serial frozen sections of these biopsies showed that the smooth muscle cells of the strongly succinate dehydrogenase-reactive blood vessels contained markedly proliferated mitochondria, characteristic of patients with MELAS. The presence of strongly succinate dehydrogenase-reactive blood vessels in muscle biopsy specimens provides an important clue toward understanding the underlying pathogenetic mechanism in patients with MELAS as well as another approach to the diagnosis of this disorder.

Structure of Wood and Cambial Variant in the Stem of Dalbergia Paniculata Roxb.

The wood of Dalbergia paniculata is unique as it consists of concentric layers of broad xylem, alternating with bands of narrow phloem. This anomaly results from the periodic formation of successive cambia in the secondary phloem. Some phloem parenchyma cells dedifferentiate to form a discontinuous ring of cambium. Such parenchyma cells have higher succinate dehydrogenase activity than the neighbouring cells of secondary phloem. The newly differentiated cambial layer functions bidirectionally, and its products give rise to xylem internally and phloem externally. The phloem along with cambium present internal to the newly formed xylem becomes included.The wood is diffuse-porous and the intervessel pits are vestured. The phloem has welldifferentiated sieve tube members and companion cells.

Redistribution of the flux-control coefficients in mitochondrial oxidative phosphorylations in the course of brain edema

This work describes the control exerted by dicarboxylate carrier and succinate dehydrogenase activities on the oxidative phosphorylations in rabbit brain mitochondria as an edema develops. Vasogenic edema leads to an uncompetitive inhibition of succinate dehydrogenase activity and to a large decrease of oxidative phosphorylations linked to succinate utilisation. Naftidrofuryl treatment in vivo restores both a high succinate dehydrogenase activity and a normal respiratory rate. In order to quantify the control of oxidative phosphorylations by the succinate dehydrogenase step, we applied the control analysis (Kacser, H. and Burns, J.A. (1973) in Rate Control of Biological Processes (Davies, D.D., ed.), pp. 65–104, Cambridge University Press, London; Heinrich, R. and Rapoport, T.A. (1974) Eur. J. Biochem. 42, 89–95). By using two inhibitors, one (phenylsuccinate) acting only on the dicarboxylate carrier and another (malonate) acting on both the dicarboxylate carrier and the succinate dehydrogenase, a method was developed to calculate the control coefficients of these two steps. The main result is that in mitochondria isolated from normal tissue succinate dehydrogenase exerted no control, but in the course of edema this enzymatic step became a controlling one: a transition from zero to a high control coefficient (0.5) was observed from the onset of intracellular edema for the threshold value of water/dry-weight tissue of 4.6.

Differential distribution of B16F10 melanoma cells in the liver lobule.

The distribution pattern and the number of tumor cells arrested in the liver were studied in mouse livers. Mice were perfused intravascularly with a suspension of B16F10 melanoma cells. The animals were sacrificed at 0, 1, 5, and 20 min after tumor cell perfusion. The pattern of tumor cell distribution was studied by morphological methods, and by a combined method of fluorescent-tumor cell labelling and histochemical succinate dehydrogenase activity on frozen sections, in order to define the localization of tumor cells arrested in the liver lobule. The results show that the tumor cells have an exclusive distribution in the periportal regions of the liver lobule (identified as the high succinate dehydrogenase activity areas), and that the cells are not arrested in the pericentral regions (identified as the low succinate dehydrogenase activity areas). In addition, indomethacin treatment (2 mg/kg/day) induced an increase in the number of melanoma cells arrested in the liver, but a different distribution with respect to controls was not observed. These results show that periportal regions of the liver lobule constitute a particular domain in which the B16F10 melanoma cells present a special retention ability that can be modulated by indomethacin treatment.

Mitochondrial and peroxisomal β-oxidations in pig coronary smooth muscle cells

1. 1. The total subcellular membranes of pig coronary media were fractionated using a sucrose density gradient. 2. 2. A fraction with high succinate dehydrogenase activity and a mean density of 1.165 was separated from a fraction with high catalase activity and a mean density of 1.145. 3. 3. Acyl CoA β-oxidation activity measured in the absence of BSA was present in both fractions with 47% of the total activity in the succinate dehydrogenase fraction and 47% in the catalase fraction. 4. 4. In the succinate dehydrogenase fraction bovine serum albumin stimulated the acyl CoA β-oxidation (maximal sitmulation, 3.2 times at a concentration of 15 mg%) while in the catalase fraction it had no effect. 5. 5. It is concluded that, in pig coronary media, the β-oxidation system has two components, i.e. mitochondrial and peroxisomal β-oxidation.

Slow type muscle cells in the earthworm gizzard with a distinct, Ca2+-regulated myosin isoform

Histochemical staining of the gizzard from the earthworm,Lumbricus terrestris, reveals low ATPase and high succinic dehydrogenase activity for all muscle cells as compared to the main part of the body wall. In accordance with the presence of slow type muscle cells in the gizzard, isolated actomyosin shows an ATPase activity three times lower than the body wall actomyosin. Gizzard myosin represents an isoform, distinct from those of the body wall muscle, by comparison of the light chain pattern in isoelectric focusing. No difference was observed in the Ca2+-regulatory properties between gizzard and body wall actomyosin. Gizzard actomyosin is dual-regulated, and the myosin contains a regulatory light chain which is reversibly dissociated by EDTA. Isolated gizzard binds two molecules of Ca2+ per molecule, in the same range of free Ca2+ concentrations over which actomyosin is activated, suggesting that the myosin-linked regulatory system is mediated by direct binding of Ca2+. The molar ratios of the major contractile proteins of body wall and gizzard actomyosins differ considerably, indicating a structural diversity of fast and slow type muscle cells.

Degeneration-regeneration as a mechanism contributing to the fast to slow conversion of chronically stimulated fast-twitch rabbit muscle.

Extensor digitorum longus muscles of male adult White New Zealand rabbits were indirectly stimulated at 10 Hz for 12 h daily for periods ranging up to 28 days. After four weeks the stimulated muscles showed a nearly uniform profile of high succinate dehydrogenase activity and, when incubated after acid preincubation for myofibrillar adenosine triphosphatase, displayed more dark- and intermediate-staining fibers than their contralateral counterparts. Muscles stimulated from between 6 to 21 days revealed degenerative foci and phagocytosis of degenerated fibers. These fibers were mostly of the fast-twitch, glycolytic type. Small myofibers, which often contained central nuclei, and structures identified as myoblasts or myotubes, reacted with a monoclonal antibody prepared against embryonic myosin heavy chains. The data suggest that under the employed conditions the fast to slow conversion of chronically stimulated fast-twitch rabbit muscle is not exclusively caused by adult fiber transformation, but results in part from the substitution of fast-twitch glycolytic fibers with newly formed fibers that have a high oxidative profile.

Succinate dehydrogenase and peroxidase activities in leaf epidermis and stem of some angiosperm species growing near a fertilizer complex

The enzymes succinate dehydrogenase and peroxidase were localized histochemically in the leaf epidermal cells and in the young stems of:Annona squamosa L.,Coccinia indica W. and A.,Bougainvillea spectabilis Willd.,Azadirechta indica L.,Ficus benghalensis L.,Lantana camara L. var.aculeata, Mangifera indica L.,Manilkara hexandra (Roxb) Dub.,Streblus asper Lour,Syzygium cumini L.,Tamarindus indica L., andZizyphus oenoplia L., from the areas around the Gujarat State Fertilizer Company, near Baroda (22°−30′ N latitude, 73°−60′ E longitude), and nearly-normal area of Vallabh Vidyanagar (22°−50′ N latitude, 73°−10′ E longitude). The soil types, temperature, rainfall, humidity and agricultural patterns of the two localities are identical. Intensities of enzyme activity vary in different cell types of the epidermis and in various tissues of the stem. Intensities of succinate dehydrogenase and peroxidase in epidermal cells is low in majority of the pollution-affected plants. Except inAzadirechta indica andManilkara hexandra, stem of all the other pollution-affected species showed lower succinate dehydrogenase activity than in the normal. Vascular parenchyma in the stems of pollution-affected plants ofAzadirechta indica andManilkara hexandra showed high succinate dehydrogenase activity than that in the normal, whereas in extravascular regions it is almost identical with that of the normal stem. Peroxidase activity in the stem of pollution-affected plants ofManilkara hexandra andStreblus asper is higher than that in normal, and in rest of the species it is lower than or almost the same as in the normal.

Glycerolphosphate oxidase and succinate dehydrogenase activities in IIA and IIB fibres of mouse and rabbit tibialis anterior muscles.

Quantitative microphotometric measurements of two mitochondrial flavoproteins, glycerolphosphate oxidase (GP-OX) and succinate dehydrogenase (SDH), were performed on serial sections of mouse and rabbit tibialis anterior (TA) muscles in order to study the distribution of these two enzymes and their activity ratios in IIA and IIB fibres. The measurements showed a large scatter of the two enzyme activities in these two myosin-based fibre types. In rabbit TA, IIA and IIB fibres have similar GP-OX activities, whereas generally IIA fibres have higher SDH activities than IIB fibres. An inverse distribution of the two enzymes exists in mouse muscle. Generally, IIA fibres of mouse TA display low SDH and IIB fibres high SDH activities. The mean activity of GP-OX is slightly higher in IIA than in IIB fibres of mouse TA. Since measurements of both enzymes were taken in the same fibres, the ratio of their activities in each fibre could be evaluated. The SDH/GP-OX activity ratios vary significantly between the two fibre populations both in rabbit and in mouse. The ratio is high in IIA and low in IIB fibres of rabbit TA, whereas it is low in IIA and high in IIB fibres of mouse TA.

Succinate dehydrogenase activity in fibres classified by myosin ATPase in three hind limb muscles of rat.

1. Succinate dehydrogenase (SDH) activity was assessed in situ in single fibres of cross-sectioned extensor hallucis longus, extensor digitorum longus, and soleus muscles of rat by means of microphotometric recordings of initial maximum reaction rates. 2. Each fibre assessed for SDH activity was subjectively classified into myosin subgroups by its histochemical reaction for myofibrillar actomyosin ATPase (myosin ATPase) following preincubation at pH 4.6 according to Brooke & Kaiser (1970). 3. The majority of fibres classified into myosin types I and IIa were highly reactive for SDH, such that those myosin groups could be interchangeable with the metabolic subgroups of Peter, Barnard, Edgerton, Gillespie & Stempel (1972); myosin I = slow-twitch oxidative, myosin IIa = fast-twitch oxidative glycolytic. 4. The myosin type IIb fibres, however, demonstrated marked variability in activity levels of SDH. Over 40% of those fibres had high SDH activity, and thus could not be equated with the metabolic subgroup fast-twitch glycolytic. 5. The histochemical reaction for myosin ATPase in muscle fibres therefore cannot be used as a reliable means to predict the fibres' metabolic characteristics.

Histochemistry of Heartwood and Injury-induced Discoloured Wood in Samanea saman (Jacq.) Merrill

Summary A comparative histochemical study of the heartwood and discoloured wood formation in Samanea saman (J acq .) M errill showed that both the processes have general similarities. The difference noted was that the heartwood formation was much slower and gradual and was not accompanied by succinate dehydrogenase activity, whereas a high succinate dehydrogenase activity was observed in the cells of the discoloured wood. The starch disappeared and lipid globules enlarged both during the formation of heartwood and discoloured wood. While there was a little increase in the amount of proteins containing aromatic amino acids at the heartwood boundary in normal wood, the discoloured wood showed greater concentration of these proteins. Peroxidase activity was confined to the ray cells in normal wood. Its highest activity was found in the cambial zone and the middle sapwood. In the discoloured wood all the fibers and prenchyma cells show high peroxidase activity. The nuclei of the fibers in the injury-affected outermost sapwood divide amitotically followed by septa formation. Normal wood has aseptate fibers.

A possible mechanism of phenotypic expression of normal and dystrophic genomes on succinic dehydrogenase activity and fiber size within a single myofiber of muscle transplants

Muscle transplantation was used to evaluate the ability of normal and dystrophic chickens to support regeneration of both normal and dystrophic muscle fragments. Pectoralis muscles were grafted into the site of the biceps muscle of host chickens. Identification of dystrophic characteristics of intact and regenerating muscle fibers was made by cytochemical analysis of mitochondrial succinic dehydrogenase (SDH) and by fiber size. In the biceps muscle of dystrophic chicks at 40 days ex ovo, the mean size of muscle fibers with low activity of SDH and fibers with high SDH activity was 29.0 ± 5.9 μm and 42.0 ± 10.4 μm, respectively. The mean size of normal muscle fibers was notably smaller than in dystrophic muscle and was 17.8 + 3.1 μm. The hypertrophy of fibers coupled with elevation of SDH activity tended to increase with age. Transplants were examined at 56 days postoperatively. The results of cross-transplantation between normal and dystrophic genotypes were similar to unoperated muscles in the correlation between SDH activity and fiber size. Donor muscles determined the type of myofibers regenerated in transplants regardless of whether the host was normal or dystrophic. In addition, combined transplantation was attempted to produce a single hybrid myofiber in which normal and dystrophic nuclei coexist. The minced muscle fragments removed from both normal and dystrophic pectoralis muscle were mixed in equal volume. The mixtures were then allowed to regenerate in host chicks. A number of mosaic myofibers appeared in transplants and had regional differences in SDH activity along their length. It was concluded that: (1) The characteristics of high SDH activity and fiber hypertrophy are an expression of dystrophic nuclei, (2) combined transplantation of both normal and dystrophic muscle fragments can produce mosaic myofibers in SDH reaction; and (3) the local control of SDH activity and fiber size within nuclear territories in mosaic myofibers seems likely to be due to phenotypic expression of either normal or dystrophic genomes.

Comparative study of the concentrating mechanism in the kidney of the big gerbil ( Rhombomys opimus L.) and the water vole ( Arvicola terrestris L.)

1. 1. Rodents of the species studied are able to produce a hypoosmotic urine at water diuresis; the urine to plasma osmotic concentration ratio under dehydration in the water vole is 3.8 ± 0.3, in the albino rat—7.0 ± 0.2, and in the big gerbil—12.7 ± 0.9. 2. 2. Microanatomical data agree with the hypothesis of the countercurrent multiplication. 3. 3. The sodium and urea content, under dehydration, increases in the renal papilla of the big gerbil and the water vole as well; the level of potassium, calcium and magnesium remains unchanged. The papilla to cortex gradient of sodium and urea concentrations in the desert rodent is significantly higher than in the vole. 4. 4. Both in the big gerbil and the water vole, the highest succinate dehydrogenase activity is characteristic of tubules of the cortex and the outer medulla but it is not observed in tubular cells of the papilla; a high level of the lactate dehydrogenase activity is also found in the renal papilla structures. The increase of the ability to produce a hyperosmotic urine does not depend on a change of the cellular energetic metabolism type. 5. 5. The adaptive evolution of the renal concentrating ability in water-loving and desert rodents was followed by the change of the Henle's loop length and by the modification of some medullar interstitial structures.

The development of vibrissae representation in subcortical trigeminal centers of the neonatal rat.

In every station of the trigeminal system of the young rat, the segmented activity of the mitochondrial enzyme succinic dehydrogenase (SDH) clearly delineates the representation of the mystacial vibrissae. In the trigeminal complex of the medulla, three parallel representation can be seen, two in the spinal trigeminal nucleus and one in the principal trigeminal nucleus. In the next station, the ventrobasal complex of the thalamus, a single representation occurs. Likewise, layer IV of somatosensory cortex contains one representation of the vibrissae. Further, neonatal damage to the mystacial vibrissae results in anomalies within each representation. The present study delineates both the normal development of subcortical trigeminal stations and the aberrant organization seen after vibrisse removal. The results of a similar study on somatosensory cortex (Killackey and Belford, '79) and the present data allow the comparison of the development of each of the five vibrissae representations in the trigeminal system. In the brainstem, each of the three trigeminal complex representations are present at birth, although the pattern becomes more distinct over the first several days of life. Interestingly, vibrissae removal at birth induces an aberrant pattern that is distinct by postnatal Day 3. Although details are not equally discernible in each representation, the abnormalities appear to be similar. The SDH segmentation in the ventrobasal complex develops during postnatal Days 1 through 4. At Day 1, portions of the matrix of high density SDH activity break up into bands. Clusters can be discerned within these bands on Day 2. By Day 4 the pattern is sharply delineated. Vibrissae removal at birth results in anomalies that are a part of the initial development of segmentation, not a later reorganization. Comparison of the present data with that of our previous studies indicates that there is a sequential development of the central somatosensory structures related to the vibrissae, beginning with the most peripheral station. Further, there are many similarities in the development of each station. There are also differences which are particularly important in comparing the trigeminal nuclei with the later stations. The unique features in the abnormal development of the trigeminal nuclei are likely due to their direct connections with the periphery.

Fiber types in red and white segments of rat sternomastoid muscle

AbstractThe contractile, histochemical and morphological properties of fibers from red and white rat sternomastoid segments have been studied. Previous mechanical experiments suggested that both segments were fast‐twitch and the present experiments were done to confirm this observation and to determine other essential characteristics of the fibers.The ATP'ase and succinic dehydrogenase (SDH) activities of fibers were determined using standard histochemical techniques. Seventy‐six percent of fibers in the red segment had the high ATP'ase and high succinic dehydrogenase activity typical of fast‐twitch‐red fibers. Other fibers in the red segment were typically fast‐twitch‐white (13%) or slow‐twitch‐red (11%). The average cross‐sectional area of red fibers was 2,104 m̈m2. Fibers in the white segment were mostly (84%) fast‐twitch‐white with intermediate ATP'ase activity and low to intermediate SDH activity. The remaining fibers in the white segment were histochemically typical of fast‐twitch‐red fibers. The average cross‐sectional area of white fibers was 5,109 m̈m2. Morphologically, red fibers had three times as many mitochondria as white fibers and a Z‐line thickness 38% greater than that of white fibers.Although fibers from the red segment were histochemically and morphologically different from fibers in the white segment, the isometric tension characteristics, i.e., contraction time, post‐tetanic potentiation and fatigueability, were very similar and were typical of fast‐twitch‐fatigueable preparations. Thus, histochemical and morphological characteristics of fibers in the sternomastoid muscle are not indicative of the mechanical properties of the fibers.

Separation of inner and outer membranes of Rickettsia prowazeki and characterization of their polypeptide compositions.

Rickettsia prowazeki were disrupted in a French pressure cell and fractionated into soluble (cytoplasm) and envelope fractions. The envelope contained 25% of the cell protein, with the cytoplasm containing 75%. Upon density gradient centrifugation, the envelope fraction separated into a heavy band (1.23 g/cm3) and a lighter band (1.19 g/cm3). The heavy band had a high content of 2-keto-3-deoxyoctulosonic acid, a marker for bacterial lipopolysaccharide, but had no succinic dehydrogenase, a marker for cytoplasmic membrane activity, and therefore represented outer membrane. The lighter band exhibited a high succinate dehydrogenase activity, and thus contained inner (cytoplasmic) membrane. Outer membrane purified by this method was less than 5% contaiminated by cytoplasmic membrane; however, inner membrane from the gradient was as much as 30% contaminated by outer membrane. The protein composition of each cellular fraction was characterized by sodium dodecyl sulfate--polyacrylamide gel electrophoresis. The outer membrane contained four major proteins, which were also major proteins of the whole cell. The cytoplasmic membrane and soluble cytoplasm exhibited a more complex pattern on gels.

Comparison of contractile, histo- and biochemical properties of different skeletal muscles of the brown rat (Rattus norvegicus) with those of the domesticated Wistar rat

1. Muscle properties of rat strains showing different locomotor activities were studied. The isometric contraction times (CT) of the slow soleus muscle are shorter in both young and adult domesticated Wistar than in wild brown rats; no difference was found in the fast extensor digitorum longus (EDL) muscles. No significant difference was seen with respect to half relaxation time (HRT) in either fast or slow muscles. CT and HRT of the diaphragm are longer in young brown rats than in domesticated animals, while the diaphragm of the adult wild rat differs only by a longer HRT. 2. Histochemically, the fast EDL muscle and diaphragm show different types of muscle fibres, i.e. fibres with low and high myofibrillar ATPase activity (type I and II), and fibres with low, intermediate and high succinic dehydrogenase (SDH) activity (type A, B and C). The EDL and diaphragm of brown rats have a higher percentage of “fast-red” fibres with high ATPase and SDH activities (type II C) and phospholipid content, than the Wistar rats. The slow soleus muscles of young and adult brown rats are uniform with respect to low ATPase activity and heterogeneous with respect to SDH activity; in the Wistar rat the soleus muscles are heterogeneous with respect to both myosin ATPase and SDH activities, i.e. they contain “slow-intermediate” (I B) and “fast-red” (II C) fibres. The pattern of phospholipid content is similar to the SDH pattern in each case. No difference was found in phosphorylase activity. The glycogen content is higher in both fast and slow muscles of the adult rat than in the muscles of the domesticated rat.