OBJECTIVE: To examine skeletal muscle intracellular triglyceride concentration in different fiber types in relation to obesity. DESIGN: Skeletal muscle fiber type distribution and intracellular lipid content were measured in vastus lateralis samples obtained by needle biopsy from lean and obese individuals. SUBJECTS: Seven lean controls (body mass index (BMI) 23.0 AE 3.3 kg=m 2 ; mean AE s.d.) and 14 obese (BMI 33.7 AE 2.7 kg=m 2 ) individuals; both groups included comparable proportions of men and women. MEASUREMENTS: Samples were histochemically stained for the identification of muscle fiber types (myosin ATPase) and intracellular lipid aggregates (oil red O dye). The number and size of fat aggregates as well as their concentration within type I, IIA and IIB muscle fiber types were measured. The cellular distribution of the lipid aggregates was also examined. RESULTS: The size of fat aggregates was not affected by obesity but the number of lipid droplets within muscle fibers was twice as abundant in obese compared to lean individuals. This was seen in type I (298 AE 135 vs 129 AE 75; obese vs lean, P < 0.05), IIA (132 AE 67 vs 79 AE 29; P < 0.05), and IIB (103 AE 63 vs 51 AE 13; P < 0.05) muscle fibers. A more central distribution of lipid droplets was observed in muscle fibers of obese compared to lean subjects (27.2 AE 5.7 vs 19.7 AE 6.4%; P < 0.05). CONCLUSION: The higher number of lipid aggregates and the disposition to a greater central distribution in all fiber types in obesity indicate important changes in lipid metabolism and=or storage that are fiber type-independent.
Defects of glucose transport and phosphorylation may underlie insulin resistance in obesity and non-insulin-dependent diabetes mellitus (NIDDM). To test this hypothesis, dynamic imaging of 18 F-2-deoxy-glucose uptake into midthigh muscle was performed using positron emission tomography during basal and insulin-stimulated conditions (40 mU/m 2 per min), in eight lean nondiabetic, eight obese nondiabetic, and eight obese subjects with NIDDM. In additional studies, vastus lateralis muscle was obtained by percutaneous biopsy during basal and insulin-stimulated conditions for assay of hexokinase and citrate synthase, and for immunohistochemical labeling of Glut 4. Quantitative confocal laser scanning microscopy was used to ascertain Glut 4 at the sarcolemma as an index of insulin-regulated translocation. In lean individuals, insulin stimulated a 10-fold increase of 2-deoxy-2[
The purpose of the study was to verify the influence of several weeks of chronic low-frequency electrical stimulation (LFES) on the metabolic profile and functional capacity of human skeletal muscle. Knee extensor muscles (KEM) of eight subjects were electrically stimulated at 8 Hz for 8 h/day and 6 days/wk. Vastus lateralis muscle samples were taken before, after 4 wk, and after 8 wk of LFES, and activities of anaerobic (creatine kinase, phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase) and aerobic-oxidative (citrate synthase, 3-hydroxyacyl-CoA dehydrogenase, cytochrome-c oxidase) enzyme markers were determined. KEM dynamic performance was also assessed before, after 4 wk, and after 8 wk of LFES. Activity levels of anaerobic enzymes were not altered, whereas the activity levels of citrate synthase (29%),3-hydroxyacyl-CoA dehydrogenase (22%), and cytochrome-c oxidase (25%) were significantly increased after 4 wk of LFES but were not further increased after 4 additional wk of LFES. KEM performance was also improved (P < 0.05) but leveled off after 4 wk of LFES. Although significant changes were observed, the results of the present study suggest that the muscle characteristics investigated in the current study have a limited capacity of adaptation in response to this form of chronic LFES.
The purpose of the present study was to look at the changes in the performance of human knee extensor muscles (KEM) induced by 6 weeks of low-frequency (8 Hz) electrical stimulation (LFES). KEM performance of 20 sedentary (before and after stimulation), ten active, and five endurance-trained subjects was evaluated during 25 consecutive 10-s isometric contractions, each separated by a rest period of 5 s. The mean force maintained during six consecutive 10-s contractions was expressed as a relative percentage of that of the first contraction. The mean performance of the first series of six contractions was not altered in response to stimulation, whereas that of the other four series was significantly increased. No significant difference was noted among the three groups in terms of KEM performance during the first series of six contractions. However, after the first series of six contractions, KEM performance of endurance-trained subjects was better in comparison to the other groups. Citrate synthase (CS) activity, capillary number per type IIA and IIB fibers, and the percentage of type IIA muscle fibers determined from vastus lateralis samples were significantly increased in response to the stimulation protocol. No significant change was observed in the proportion or capillary number of type I fibers, or in the areas of type I, IIA, and IIB fibers. The present study provides evidence that resistance to fatigue of human skeletal muscle can be significantly altered in response to 6 weeks of transcutaneous low-frequency electrical stimulation. The improvement in KEM resistance to fatigue of the sedentary subjects was such that, at the end of the stimulation protocol, resistance to fatigue was similar to that of active subjects. However, the ability of endurance-trained subjects to withstand fatigue was still superior compared to that of the other untrained or active subjects.
Insulin stimulation of glucose transport in skeletal muscle is considered to involve translocation of the skeletal muscle/adipose tissue glucose transporter isoform, Glut 4, from cytosolic vesicles to the cell surface. The current study was undertaken to investigate Glut 4 translocation in skeletal muscle of healthy volunteers during euglycaemic insulin infusion. Previous quantitative studies of glucose transport have depended on differential centrifugation methods, which demand large biopsy samples. In this study we have developed and applied a quantitative method using confocal laser microscopy, well suited to the small needle biopsies that are typically available clinically. Percutaneous biopsy of vastus lateralis skeletal muscle was performed during basal and euglycaemic insulin-stimulated conditions, and Glut 4 translocation was assessed using immunohistochemical labelling and confocal laser microscopy imaging in 14 healthy lean subjects. At physiological hyperinsulinaemia (536 +/- 16 pM), mean systemic glucose utilization was 9.27 +/- 0.78 mg/kg-min, indicative of normal insulin sensitivity. The presence of Glut 4 at the sarcolemma increased significantly (p < 0.01), with a ratio of insulin-stimulated to basal sarcolemmal Glut 4 of 1.85 +/- 0.33, indicative of insulin-stimulated Glut 4 translocation. The area of Glut 4-labelled sites also increased significantly (p < 0.01) in response to insulin infusion; this ratio was 1.56 +/- 0.13. Thus, at physiological hyperinsulinaemia, the amount of Glut 4 at the cell surface of skeletal muscle in healthy, lean individuals increases approximately twofold over basal conditions, and this process can be measured using immunohistochemical labelling imaged by confocal laser scanning microscopy.
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