Histochemical demonstration of skeletal muscle fibre types and capillaries on the same transverse section

Summary. Serial transverse sections of m. vastus lateralis biopsies from six healthy men were reacted:

• 1 for myofibrillar adenosine triphosphatase (mATPase) to identify fibre types;
• 2 with α-amylase, periodic acid-Schiff (αPAS) to visualize capillaries. Sections were also processed with a new histochemical method for identification of fibre types and capillaries on the same skeletal muscle slice (mATPase/αPAS). Fibre type composition using either method was 41% type I, 37% type IIA and 22% type IIB. Types I, HA and IIB least diameter areas (mean±SE, μm²) were similar in sections processed for mATPase/αPAS or mATPase (3976±338, 5187±373 and 4389±514 vs. 4092±345, 5100±360 and 4289±474, respectively). The number of capillaries per mm² and per fibre did not differ in sections processed using the αPAS (315±29 and 1·48 ±0·12) or mATPase/αPAS (317±25 and 1·43±0·10) method. The number of capillaries was greater (P<0·05) around types I or IIA than type IIB fibres whether a section was processed for mATPase/αPAS (4·5±0·2 or 4·6±0·2 vs. 3·4±0·3) or when fibre profiles of serial sections reacted for mATPase or αPAS were ‘matched’ (4·5±0·2 or 4·4±0·2 vs. 3·4±0·3). The results indicate that histochemical and morphometric measures can be made on the same transverse section using the new method with substantial savings of time, cost and energy.

     

Capillary supply and muscle fibre types in patients with intermittent claudication: relationships between morphology and metabolism

Abstract There have been previous reports on an increased oxidative capacity in muscle tissue from the diseased legs of patients with intermittent claudication. The present study was designed to correlate metabolic and morphological data and to investigate whether the metabolic adaptive changes in muscle tissue of claudicating legs were also reflected in morphological variables such as capillary supply, fibre type distribution, and fibre area. The activity of cytochro-me-c-oxidase in gastrocnemius muscle was determined and the insulin and glucose uptakes were measured across the leg in the basal state and 10 min following intravenous administration of 25 g glucose. The finding of a reduced relative number of Type II B fibres and a reduced ratio Type II B/II A fibre area, as well as an increased capillary supply to Type II A, indicated that the most extensive morphologic changes in muscle tissue of claudicating legs had occurred in Type II fibres. The increased number of capillaries in contact with Type IIA fibres in muscle tissue from claudicating legs, compared with muscle tissue from control legs, suggested that the most apparent metabolic changes occurred in this fibre type in the adaptation process of these patients. The more pronounced morphologic and metabolic changes in Type II fibres suggest that these fibres are more intensely activated than Type I fibres during physical activity in claudicating legs. The insulin uptake correlated positively with the number of capillaries per fibre, suggesting that the endothelial surface area is one of the determining factors for insulin uptake. The percentage of Type II B fibres reflected to a certain extent the metabolic adaptation in muscle tissue.     

Capillary supply and muscle fibre types in patients with intermittent claudication: relationships between morphology and metabolism

Abstract. There have been previous reports on an increased oxidative capacity in muscle tissue from the diseased legs of patients with intermittent claudication. The present study was designed to correlate metabolic and morphological data and to investigate whether the metabolic adaptive changes in muscle tissue of claudicating legs were also reflected in morphological variables such as capillary supply, fibre type distribution, and fibre area. The activity of cytochro-me-c-oxidase in gastrocnemius muscle was determined and the insulin and glucose uptakes were measured across the leg in the basal state and 10 min following intravenous administration of 25 g glucose. The finding of a reduced relative number of Type II B fibres and a reduced ratio Type II B/II A fibre area, as well as an increased capillary supply to Type II A, indicated that the most extensive morphologic changes in muscle tissue of claudicating legs had occurred in Type II fibres. The increased number of capillaries in contact with Type IIA fibres in muscle tissue from claudicating legs, compared with muscle tissue from control legs, suggested that the most apparent metabolic changes occurred in this fibre type in the adaptation process of these patients. The more pronounced morphologic and metabolic changes in Type II fibres suggest that these fibres are more intensely activated than Type I fibres during physical activity in claudicating legs. The insulin uptake correlated positively with the number of capillaries per fibre, suggesting that the endothelial surface area is one of the determining factors for insulin uptake. The percentage of Type II B fibres reflected to a certain extent the metabolic adaptation in muscle tissue.     

Effects of strength and endurance training on muscle fibrecharacteristics in elderly women

The effects of 18weeks’ intensive strength and endurance training on fibre characteristics of the vastuslateralis muscle were studied in 76- to 78-year-old women. Type I and type IIa fibresconstituted over 90% of the cell population and were almost equally represented. Nochanges were observed in the proportions of the different fibre types. When comparing thebaseline and the 18-week measurements within the groups, the strength group showed a meanincrease of 34% (P=0·028) in mean type I fibre area. Thefrequency histograms showed an increased proportion of larger type I fibres after strengthtraining and a decreased proportion of smaller type IIa fibres after endurance training. In thecontrol subjects, the proportion of smaller type I and type IIa fibres increased during theexperimental period. The results indicate that intensive strength training induces type I fibrehypertrophy, whereas the effects of endurance training are less evident. The considerablevariation found in the change in muscle fibre cross-sectional areas is also noteworthy.     

Effect of immobilization of short duration on the muscle fibre size

Summary. In 10 patients with unilateral knee joint lesions, the injured leg was immobilized for 72 h by a plaster cast reaching from the hip to the ankle. At removal of the casts, open muscle biopsies were taken from the vastus medialis muscle on both sides. The muscle specimens were stained for myofibrillar ATPase activity and muscle fibre areas were measured on transverse sections. The differences in fibre size between the two sides were registered and statistically evaluated. The reduction in fibre size on the immobilized side was 14.4% for the type 1 fibres and 17.3% for the type 2 fibres, respectively. However, these differences were not significant because of considerable variations from case to case, with an increase in fibre size on the immobilized side in three cases. It is concluded that short time post-traumatic immobilization may have variable effects on the muscle fibre size and that one single biopsy might be insufficient to disclose early disuse atrophy whereas the method may be useful in cases of longer duration.     

ATP and IMP in single human muscle fibres after high intensity exercise

Summary. Within a muscle there is large variation in the activity of various enzymes among single fibres. It is reasonable, therefore, to expect a corresponding variation in their metabolic response to exercise. This was examined by obtaining muscle biopsies from five men at rest and after intense short-term exercise consisting of three bouts of 30 knee extensions each and intervened by 60 s rest. Freeze-dried dissected single fibres identified as type 1 or type 2 were analysed for ATP and IMP contents by liquid chromatography. Rest. Average ATP tended to be higher in type 2 than in type 1 fibres. The ATP range was 14–30 and 14–32 mmol × kg^-1 dry muscle (dm) in type 1 and 2 fibres, respectively. IMP was less than 1 mmol × kg^-1 dm in most fibres and similar in types 1 and 2. Exercise. Muscle force decreased 70% during exercise. The average decrease in ATP content was significant for both fibre types with somewhat larger response for type 2 (20%) than type 1 (10%) fibres. The ATP range was 10–28 and 10–30 mmol × kg^-1 dm in type 1 and 2 fibres, respectively. Average IMP content increased substantially in both fibre types, more so for type 2, and the range for individual fibres was 0–13 (type 1) and 0–21 (type 2) mmol × kg^-1 dm. Conclusion. After exhaustive short-term exercise, a large variation in ATP and IMP contents was evident among single type 1 and type 2 fibres. None of the fibres, however, showed an ATP content lower than 10 mmol × kg^-1 dm. It is suggested that the muscle force loss, demonstrated in the present study, is not attributed to a lowered ATP content per se.     

High blood pressure and muscle morphology/metabolism – causal relationship or only associated factors?

As muscle tissue constitutes a main target organ for glucose metabolism and is responsible for the development of insulin resistance, it seems plausible to elucidate the relationship between blood pressure and muscle morphology and metabolism. The association between blood pressure and capillarization/morphology of the vastus lateralis muscle and metabolic variables was evaluated in 24 perimenopausal obese women [body mass index (BMI) 34·9 ± 1·1; waist–hip ratio (WHR) 0·90 ± 0·02]. The muscle enzyme activity of lipoprotein lipase (LPL), citrate synthase and glycogen synthase was determined. There was a significant negative correlation between the percentage of type I fibres and relative fibre area of type I on the one hand and systolic and diastolic blood pressure on the other. There was a negative correlation between the capillary density (i.e. number of capillaries/muscle fibre) and a positive correlation between the diffusion distance (fibre area supplied by one capillary) and diastolic blood pressure. The activities of LPL and citrate synthase were positively correlated with the percentage of type I and negatively correlated with the percentage of type II muscle fibres. The activity of LPL was also negatively correlated with plasma glucose and the insulin/C-peptide ratio. The insulin/C-peptide ratio was positively correlated with the percentage of type II muscle fibres. In stepwise multiple regression analyses, 20–30% of the variation in systolic and diastolic blood pressure could be explained by the variables of muscle fibre distribution. Excluding muscle morphological variables from the regression model, the insulin/C-peptide ratio accounted for 13% of the variation in systolic and diastolic blood pressure. The results of the study show the close association between muscle morphology and blood pressure. It remains to be elucidated whether this association indicates a causal relationship.     

Muscle Fibre Types,Ubiquinone Content and Exercise Capacity in Hypertension and Effort Angina

The composition of skeletal muscle fibre expressed as a percentage of slow twitch (ST), type I or “red” and fast twitch (FT), type II or “white” were determined in patients with hypertension (HT) or with severe ischaemic heart disease (IHD) and compared to age matched controls. Similarly, exercise capacity expressed as the cycle intensity eliciting a blood lactate concentration corresponding to 2.0 mmol × 1–¹ were compared with healthy controls. Both patient groups had a higher percentage of FT fibres with relatively lower exercise capacities than their controls. The exercise capacities were reduced even when the relationship of decreased capacity with the percentage of increased FT was considered. There was an increase IHD but not in HT in patients with fibre subgroup FTc, which most probably reflected fibre trauma. Both patient groups were low in the skeletal muscle mitochondrial electron carrier and unspecific antioxidant ubiquinone, coenzyme Q₁₀ or CoQ₁₀. Patients with IHD but not HT showed, however, a faster fall in the ratio CoQ₁₀ over ST% the higher the percentage value of ST. The ratio reflects the antioxidant activity related to CoQ₁₀ in the fibre hosting most of the oxidative metabolism. A low ratio indicates a risk of metabolic lesion and cell trauma. This could explain fibre plasticity and offer an alternative cause to heredity in elucidating in deviating muscle fibre composition in patients with HT and IHD.     

Human skeletal muscle metabolism and morphology after temporary incomplete ischaemia

Abstract. Incomplete ischaemia was caused by clamping the aorta during reconstructive vascular surgery. After restoration of the blood flow extensive metabolic and morphological changes in the muscle tissue were observed. The adenylate (ATP + ADP + AMP) and the creatine (PCr + Cr) pools declined 30–40% and the energy charge of the adenine nucleotides dropped significantly. The metabolic pool changes were closely related to the changes in the lactate/pyruvate ratios. Morphological signs of membrane disturbances, such as fibre oedema and swelling of mitochondria, were seen in many muscle fibres 30 min after declamping. 5 days postoperatively, a number of mitochondrial abnormalities were observed and fibre regeneration was seen in places. The relative number of Type 2 fibres and the width of the capillary lumen were both related to the decrease of the metabolic parameters. Thus, the metabolic state and the fine structure of the incompletely ischaemic muscle were closely related to each other. The amount of Type 2 fibres seemed to be of special importance for the patho-physiological events in intermittently ischaemic muscle fibres.     

Higher proportion of fast-twitch (type II) muscle fibres in idiopathic inflammatory myopathies - evident in chronic but not in untreated newly diagnosed patients

Objective: Polymyositis and dermatomyositis are idiopathic, inflammatory myopathies characterized by proximal muscle fatigue. Conventional immunosuppressive treatment gives a variable response. Biopsies from chronic patients display a low proportion type I and a high proportion of type II muscle fibres. This raised a suspicion that the low proportion of type I fibres might play a role in the muscle fatigue. Aim: To investigate whether the muscle fibre attributes evident in chronic myositis are characteristic for the polymyositis and dermatomyosistis diseases themselves. Methods: Muscle biopsies were obtained from thigh muscle from untreated patients (n = 18), treated responders (n = 14) and non‐responders (n = 6) and from healthy controls (n = 11), respectively. For clinical evaluations, creatine kinase, functional index of myositis and cumulative dose of cortisone were established. Results: Chronic patients had a lower proportion of type I fibres and a higher proportion of type II fibres compared to untreated myositis patients and healthy controls. Fibre cross‐sectional area (CSA) did not differ between patients and healthy individuals but all women had a 20% smaller type II fibre CSA compared to men. Conclusions: Untreated polymyositis and dermatomyositis patients and healthy controls have a different fibre type composition than chronic polymyositis and dermatomyositis patients. Fibre CSA did not differ between healthy controls or any of the patient groups. A low proportion of oxidative muscle fibres can therefore be excluded as a contributing factor causing muscle fatigue at disease onset and the gender difference should be taken into consideration when evaluating fibre CSA in myositis.     

Glycogen Phosphorylase Activity in Biopsy Samples and Single Muscle Fibres of Musculus Quadriceps Femoris of Man at Rest

Harris, R. C, Essén, B. & Hultman, E. Glycogen Phosphorylase Activity in Biopsy Samples and Single Muscle Fibres of Musculus Quadriceps Femoris of Man at Rest. Scand. J. clin. Lab. Invest. 36, 521–526, 1976.

Glycogen phosphorylase activity in biopsy samples of the quadriceps muscle of man taken at rest was measured in the direction of glycogen breakdown. The conditions of assay used were pH 6.8 and 35 d`C. Mean phosphorylase activity per kg fresh muscle in 8 male volunteers was: phosphorylase a - 259 U, and total phosphorylase - 627 U, where 1U is the production of 1 μmol glucose-1-phosphate/s under the conditions of assay. The mean value of total activity, transposed to the in vivo condition, is sufficient to support a maximum rate of glycogen degradation calculated as glucosyl units utilized per kg fresh muscle, of 627 μmol/s. This is approximately the rate at which glycolysis occurs during a maximum voluntary isometric contraction. The mean ratio of phosphorylase a activity to total activity at rest was 0.40. Estimates of the mean total phosphorylase activity in type I (‘slow’) muscle fibres isolated from the quadriceps of 9 volunteers ranged from 210–385 U/kg fresh muscle, and in type II (‘fast’) muscle fibres from 493–934 U/kg fresh muscle. The average ratio of activity in type II fibres compared with that in type I fibres was 2.5.     

Effects of ischaemic training on force development and fibre-type composition in human skeletal muscle

Summary. Force (peak torque) of m. quadriceps femoris was measured during 60 repeated, voluntary dynamic knee extensions in 10 men before and after a 4-week training regimen of one-legged cycle exercise. Biopsies for histochemical analysis were obtained from the lateral vastus muscle after the training period. One leg was trained with the blood flow to the leg muscles reduced by local supra-atmospheric external pressure of 50 mmHg (‘Ischaemic leg, I-leg’). Employing the same work-load profile the other leg was trained at normal atmospheric pressure (‘Non-restricted-flow leg, N-leg’). In response to I-training, Maximum Peak Torqued (MPT; the highest torque produced in any contraction) and Initial Peak Torque (IPT; the average peak torque of the initial 12 contractions) decreased by 8% (P<0.01) and 9% (P<0.001), respectively. Final Peak Torque (FPT; the average peak torque of the final 12 contractions) increased by 13% (P<0.05) after I-training. No changes in MPT, IPT or FPT occurred following N-training. After training the proportion of slow-twitch fibres was higher (P<0.05) and the mean slow-twitch fibre area was larger (P<0.05) in the I-than in the N-trained leg. The results indicate that blood flow-restricted training, in contrast to non-restricted-flow training, decreases maximum voluntary dynamic force, possibly by inducing an increase in the share of the muscle cross-sectional area consisting of slow-twitch fibres. That flow-restricted training improves maintenance of force during short-term local exercise may reflect ischaemically induced changes in the metabolic characteristics of skeletal muscle.     

Muscle relaxation rate, fibre-type composition and energy turnover in hyper- and hypo-thyroid patients

1. Quadriceps strength, relaxation rate, fibre-type composition and energy-turnover rate during a submaximal contraction have been measured in hypo- and hyper-thyroid patients and compared with findings in normal subjects. 2. Six out of eight hypothyroid patients had normal strength whereas four out of five hyperthyroid patients were weak. 3. Relaxation rate was decreased in all the hypothyroid patients but increased in only three out of five hyperthyroid patients. 4. In hypothyroidism there was a marked reduction in the percentage contributed by type II fibres to muscle cross-section, partly due to type II atrophy but also due to a decrease in the relative frequency of type II fibres. In hyperthyroidism both fibre types tended to atrophy. 5. The rate of ATP turnover during submaximal contraction held to fatigue was reduced in hypothyroidism. This was probably due to decreased ATP utilization rather than an impaired supply of energy-supplying substrates. In hyperthyroidism the rate of ATP turnover was increased. 6. Altered relaxation rate and ATP-turnover rate may be explained on the basis of changes in myosin ATPase activity with thyroid status. Changes in muscle-fibre-type composition, as determined histochemically, could not per se account for the functional abnormalities.     

Serum creatine kinase activity and its changes after a muscle biopsy

Summary. The significance of the absolute elevations of serum creatine kinase (CK) levels after intense exercise and injuries was studied by measuring CK activities from seven healthy active males during a 2-week period, with a muscle biopsy taken between the first and second week. Most of the subjects (three lifters and two runners) carried on their normal exercise activities, while two lifters stopped training during the 2 weeks. The weight of the biopsy, number of fibres, percentage of fibre types, and cross-sectional areas of the muscle fibres were measured. The CK levels of the nonactive subjects and runners remained consistently low during the control week, whereas those of the lifters were usually 500% greater than those of the other two groups, and fluctuated with the intensity of their workouts. A muscle biopsy, having a mean weight of 71.3 mg and containing 1800 fibres, increased the CK values by approximately 100 units litre^-1 (U1^-1) in most of the subjects. One runner injured his right hamstring muscles 2 days prior to the biopsy, and his CK values rose from 50 to 4400 U I^-1. The increases in CK after the biopsy were not related to fibre type, activity, weight of the biopsy, or number or size of fibres removed. These results indicate that:

• 1 CK values are consistently lower in normal subjects and runners than in lifters.
• 2 Weight training results in chronic elevations of CK.
• 3 Compared to a muscle biopsy, muscular injury dramatically increases CK levels.
• 4 Elevation of serum CK is observed as early as 1 h after an intense weight-lifting session.
• 5 The elevation of serum CK by 100 U 1^-1 is associated with damage to approximately 2000 fibres.

     

Impact of blood flow‐restricted bodyweight exercise on skeletal muscle adaptations

This study ascertains the ability of bodyweight blood flow‐restricted (BFR) exercise training to promote skeletal muscle adaptations of significance for muscle accretion and metabolism. Six healthy young individuals (three males and three females) performed six weeks of bodyweight BFR training. Each session consisted of five sets of sit‐to‐stand BFR exercise to volitional failure with 30‐second inter‐set recovery. Prior to, and at least 72 h after training, muscle biopsies were taken from m. vastus lateralis to assess changes in fibre type‐specific cross‐sectional area (CSA), satellite cell (SC) and myonuclei content and capillarization, as well as mitochondrial protein expression. Furthermore, magnetic resonance imaging was used to assess changes in whole thigh muscle CSA. Finally, isometric knee extensor muscle strength was evaluated. An increase in knee extensor whole muscle CSA was observed at middle and distal localizations after training (3·2% and 3·5%, respectively) (P<0·05), and a trend was observed towards an increase in type I fibre CSA, whereas muscle strength did not increase. Additionally, the number of SCs and myonuclei associated with type I fibres increased by 65·7% and 20%, respectively (P<0·05). No significant changes were observed in measures of muscle capillarization and mitochondrial proteins. In conclusion, six weeks of bodyweight‐based BFR exercise promoted myocellular adaptations related to muscle accretion, but not metabolic properties. Moreover, the study revealed that an appropriate total training volume needs further investigation before recommending bodyweight BFR to patient populations.     

Calf muscle adaptation in intermittent claudication. Side-differences in muscle metabolic characteristics in patients with unilateral arterial disease

Summary. The adaptation of enzyme activities, notably in the oxidative metabolism, and of prerequisites for tissue transport of oxygen in the claudication leg was evaluated by comparing muscle biopsies from the gastrocnemius muscle of the claudication and the symptom-free leg of seven patients with unilateral claudication. The claudication leg had higher activities of a marker enzyme for mitochondrial oxidative capacity, citrate synthase (CS), as well as of the MB and the mitochondrial isoenzyme of creatine kinase (CK), which are considered to be involved in the transfer of high energy phosphate from the mitochondria to the resynthesis of ATP in the cytoplasm. The difference between claudication and healthy leg in activities of these CK isoenzymes were well correlated with the corresponding side difference in CS activity. No significant differences between claudication and healthy leg were found in distribution of muscle fibre types or fibre dimension, capillary density or myoglobin content, nor was there any side difference in phosphofructokinase or lactate dehydrogenase. Side differences tended to be greater in those patients with the most advanced obstructive arterial disease as estimated from non-invasive pressure measurements. It is concluded that in reasonably physically-active patients, the mode of ischaemia to which the claudication leg is subjected leads to a metabolic adaptation characterized by increased activities of enzymes involved in the oxidative metabolism, but no significant adaptation of either the conditions for local oxygen transport, as estimated by myoglobin content, and capillary density, or capacity for anaerobic metabolism.     

Muscle morphology,enzyme activity and muscle strength in elderly men and women

Summary. The quadriceps muscle were investigated with respect to isometric and isokinetic muscle strength, morphology and as regards some enzymes and metabolites of importance for the contractile processes. The material comprises 52 men (66–76 years) and 13 women (61–71 years) without clinical symptoms. Some participants used antihypertensive and/or heart-regulating medicine but were symptom-free. None of the subjects participated in any systematic training. All of them except for one physically inactive woman carried out moderate physical activities for at least 4 hweek. The body cell mass was higher in the men than in the women, and was highly correlated to muscle strength in both sexes. The fibre distribution was similar in both sexes with an average of 48 % Type I fibres in the men and 54 % Type I fibres in the women. The women had fewer Type II B fibres (4%) than the men (18 %). The average Type I fibre area did not differ between the sexes. The average Type II fibre area and the mean fibre area were smaller in women than in men, so that the ratio between the mean Type II and Type I fibre area was smaller in the women (0–76) than in the men (0–95). Signs of denervation/reinnervation, such as type grouping and an increase number of enclosed fibres, were observed very rarely and no great number of atrophic fibres indicating neuropathy was found. In both sexes, there was a reduction in the Type II fibre size, specially in relation to Type I fibre size, compared with young people. There was no difference between the enzymatic (Mg²⁺ATPase, MK and LDH) and the phosphagen (ATP and CP) content values between the sexes and no correlation between these variables and muscle strength. The capillarization was similar in the men and the women.     

Determinants of insulin-stimulated skeletal muscle glycogen metabolism in man

Abstract. To examine factors which influence skeletal muscle glycogen synthesis in man, we related insulin sensitivity measured by euglycaemic insulin clamp in 43 healthy males to muscle glycogen synthase (GS) activity, GS protein content (Western blot), glycogen concentrations and fibre composition. Insulin increased muscle glycogen content (P<0.05) and the change in glycogen content correlated with the GS protein content (r=0.90, P=0.01). GS protein concentration correlated inversely with age (r=-0.69, P=0.04). Non-oxidative glucose disposal was inversely related to per cent type 2B fibres (r=-0.52, P< 005). The influence of age on these relationships was separately studied in young (n=12, age=26 ± 2 years) and elderly (n=15, age = 56 ± 2 years) males. Insulin increased GS activity significantly only in young subjects (from 17.8 ± 30 to 25.3 ± 3.2 nmol mg protein ' min^-1; P=0.015). GS activity and non-oxidative glucose disposal correlated in young (r=0.69, P=001) but not in the elderly (r=0.064, P = 0.82) males, and this relationship was not influenced by the degree of obesity. In conclusion, muscle fibre type and GS activity are both determinants of muscle glycogen metabolism in healthy, normoglycae-mic males. The close relationship between non-oxidative glucose metabolism and GS activity in young males is altered in ageing.     

Effect of hyperthyroidism on fibre-type composition,fibre area,glycogen content and enzyme activity in human skeletal muscle

Summary. Seven hyperthyroid patients were studied by repeated muscle biopsies (vastus lateralis) before and after a period of medical treatment which averaged 10 months. The biopsies were analysed with regard to fibre-type composition, fibre area, capillary density, glycogen content and enzyme activities representing the glycolytic capacity (hexokinase, 6-phosphofructokinase), oxidative capacity (oxoglutarate dehydrogenase, citrate synthase) and Ca²⁺- and Mg²⁺-stimulated ATPase in muscle. In the pretreatment biopsy (hyperthyroid state), there was a significantly lower proportion of type I fibres (30% vs. 41%), a higher capillary density (23%), lower glycogen content (33%), and higher hexokinase activity (32%) compared with the post-treatment biopsy. No significant changes in the activity of the remaining enzymes were observed. The present study indicates that hyperthyroidism induces a transformation from type I to type II fibres in human skeletal muscle. The increase in hexokinase activity probably reflects a higher glucose utilization by skeletal muscle in order to compensate partially for the reduced glycogen content.     

Intramyocellular lipids and their dynamics assessed by 1H magnetic resonance spectroscopy

This report provides an overview on the present knowledge on intramyocellular lipids (IMCL) and their dynamics in the course of interventions with physical activity of variable type and intensity in different population groups, as accessible by examinations using non‐invasive volume‐selective ¹H magnetic resonance spectroscopy (¹H MRS). IMCL serve as energy source in skeletal muscle for fat oxidation in the mitochondria and became intensively studied after discovery of their relation with insulin sensitivity. While baseline levels of IMCL concentration have been shown to be mainly dependent on the metabolic status (insulin sensitivity), on the level of training and on fibre composition in the muscles, studies applying different physical activity protocols revealed the dynamic of their depletion and replenishment. From the findings in human studies, it can be concluded that IMCL levels are potentially useful markers for monitoring metabolic adaptation of skeletal muscle to sportive activities and training.