Novel method for detection of reactive oxygen species in vivo in human skeletal muscle.

Excessive production of reactive oxygen species (ROS) are implicated in the pathogenesis of numerous disease states. However, direct measurement of in vivo ROS in humans has remained elusive due to limited access to appropriate tissue beds and the inherently short half-lives and high reactivity of ROS. Herein, we describe a novel technique by which to measure in vivo ROS in human skeletal muscle. Microdialysis probes were inserted into the vastus lateralis of eight healthy volunteers. Amplex Ultrared, a highly specific fluorogenic substrate for hydrogen peroxide (H(2)O(2)), and horseradish peroxidase (HRP), were perfused through microdialysis probes, and outflowing dialysate was collected and fluorescence was measured. Extracellular H(2)O(2) that crossed the microdialysis membrane was measured via fluorescence of the dialysate. Superoxide dismutase (SOD) was then added to the inflowing perfusion media to convert any superoxide crossing the microdialysis membrane to H(2)O(2) within the microdialysis probe. Fluorescence significantly increased (P=0.005) upon SOD addition. These data demonstrate the feasibility of measuring both in vivo H(2)O(2) and superoxide in the extracellular environment of human skeletal muscle, providing a technique with a potential application to a wide range of circulatory and metabolic studies of oxidative stress.

Muscle blood flow response to mental stress and adrenaline infusion in man: microdialysis ethanol technique compared to (133)Xe clearance and venous occlusion plethysmography.

BACKGROUND AND AIM: Adrenaline, administered locally by microdialysis in skeletal muscle, causes vasoconstriction around the microdialysis catheter. This is contrary to the vasodilation that normally occurs when adrenaline is infused intravenously or intra-arterially. The hypothesis was tested that vasoconstriction, measured by microdialysis, would not occur with two interventions causing increased plasma levels of adrenaline, mental stress and intravenous adrenaline infusion (0.1 nmol kg(-1) min(-1)). METHODS: Twenty-four men (27 +/- 1.6 years) underwent these interventions. Blood flow was determined by the microdialysis ethanol technique and (133)Xe clearance (gastrocnemius muscle, medial head) and by venous occlusion plethysmography (calf). RESULTS: The ethanol outflow/inflow ratio, which is inversely related to blood flow, decreased to 92.0 +/- 3.4% of basal, P = 0.014 (mean +/- SEM, n = 16) during the mental stress test, but increased to 108.3 +/- 2.2% of basal, P = 0.001 (n = 16) during the adrenaline infusion. The latter increase was abolished when adrenaline was infused during alpha-receptor blockade by phentolamine. On the contrary, by (133)Xe clearance and venous occlusion plethysmography, blood flow increased during both interventions; 2.0-1.7-fold (mental stress) and 1.3-1.4-fold (adrenaline infusion), respectively, P<0.05. CONCLUSION: Adrenaline causes vasoconstriction in skeletal muscle when blood flow is measured with the microdialysis ethanol technique, irrespective of the mode of administration. The discrepant blood flow result obtained with the microdialysis ethanol technique might, at least partly, be explained by differential diffusion properties of ethanol and (133)Xe. An additional or alternative explanation might be that an inserted microdialysis catheter shifts the balance of vasoconstrictor and vasodilator effects of adrenaline in skeletal muscle.

Long-term exercise training in overweight adolescents improves plasma peptide YY and resistin.

The objective of this study was to investigate the effect of long-term exercise training on concentrations of five hormones related to appetite and insulin resistance in overweight adolescents. In addition, we were interested in the relationships of these hormones with each other and with anthropometric and/or cardiovascular disease marker changes. Participants were >or=the 85th percentile for BMI for age and sex and participated in an 8-month supervised aerobic training program. Anthropometrics, cardiovascular fitness assessment, and fasting blood samples were taken pre- and post-training. Glucose, insulin, total cholesterol (TC), high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, triglycerides, leptin, active ghrelin, total peptide YY (PYY), adiponectin, and resistin concentrations were measured. The participants increased their time to exhaustion on an incremental treadmill test and decreased both percent body fat and blood triglyceride concentrations. Total PYY concentration increased and resistin concentration decreased after long-term exercise training, which are favorable outcomes. Leptin concentrations were related to weight, percent body fat, waist circumference, and triglyceride concentrations pre- and post-training. The changes in resistin concentrations were related to the changes in triglyceride concentrations. We conclude that long-term exercise training has beneficial effects for overweight adolescents with respect to PYY and resistin, hormones related to appetite and insulin sensitivity.

Acute resistance exercise increases skeletal muscle angiogenic growth factor expression.

AIMS: Both aerobic and resistance exercise training promote skeletal muscle angiogenesis. Acute aerobic exercise increases several pro-angiogenic pathways, the best characterized being increases in vascular endothelial growth factor (VEGF). We hypothesized that acute resistance exercise also increases skeletal muscle angiogenic growth factor [VEGF and angiopoietin (Ang)] expression. METHODS: Seven young, sedentary individuals had vastus lateralis muscle biopsies and blood drawn prior to and at 0, 2 and 4 h post-resistance exercise for the measurement of VEGF; VEGF receptor [KDR, Flt-1 and neuropilin 1 (Nrp1)]; Ang1 and Ang2; and the angiopoietin receptor--Tie2 expression. Resistance exercise consisted of progressive knee extensor (KE) exercise to determine one repetition maximum (1-RM) followed by three sets of 10 repetitions (3 x 10) of KE exercise at 60-80% of 1-RM. RESULTS: Resistance exercise significantly increased skeletal muscle VEGF mRNA and protein and plasma VEGF protein at 2 and 4 h. Resistance exercise increased KDR mRNA and Tie2 mRNA at 4 h and Nrp1 mRNA at 2 and 4 h. Skeletal muscle Flt-1, Ang1, Ang2 and Ang2/Ang1 ratio mRNA were not altered by resistance exercise. CONCLUSIONS: These findings suggest that acute resistance exercise increases skeletal muscle VEGF, VEGF receptor and angiopoietin receptor expression. The increases in muscle angiogenic growth factor expression in response to acute resistance exercise are similar in timing and magnitude with responses to acute aerobic exercise and are consistent with resistance exercise promoting muscle angiogenesis.

NO-mediated alterations in skeletal muscle nutritive blood flow and lactate metabolism in fibromyalgia.

UNLABELLED: The purpose of these investigations was to determine if differences exist in skeletal muscle nutritive blood flow and lactate metabolism in women with fibromyalgia (FM) compared to healthy women (HC); furthermore, to determine if differences in nitric oxide-mediated systems account for any detected alterations in blood flow and lactate metabolism and contribute to exertional fatigue in FM. FM (n = 8) and HC (n = 8) underwent a cycle ergometry test of aerobic capacity, a muscle biopsy for determination of nitric oxide synthase (eNOS, nNOS, iNOS) content, and microdialysis for investigation of muscle nutritive blood flow and lactate metabolism. During prolonged (3h) resting conditions, the ethanol outflow/inflow ratio (inversely related to blood flow) increased in FM over time compared to HC (P < 0.05). FM also exhibited a reduced nutritive blood flow response to aerobic exercise (P < 0.05). There was an increase in dialysate lactate in response to acetylcholine in FM, and to sodium nitroprusside in both groups, with a greater rise in dialysate lactate in FM (P < 0.05). The iNOS protein content was higher in FM and was negatively correlated with total exercise time (r(2) = 0.462, P < 0.05). IN CONCLUSION: (1) There is reduced nutritive flow response to aerobic exercise and reduced maximal exercise time in FM that might relate to higher iNOS protein content and contribute to exertional fatigue in FM; (2) The increased dialysate lactate in FM in response to stimulation of NOS or a nitric oxide donor suggest that FM may be more sensitive than HC to the suppressive effect of nitric oxide on oxidative phosphorylation.

Relationship between body composition and skeletal muscle eNOS.

OBJECTIVE: Owing to the relationship between nitric oxide related endothelial dysfunction, insulin resistance and cardiovascular disease in overweight individuals, we investigated if skeletal muscle endothelial nitric oxide synthase (eNOS) protein content and activity are lower in overweight than lean women. DESIGN: A total of 19 women (age 26.0+/-1.7 years) underwent a resting muscle biopsy, body composition analysis by hydrostatic weighing and peak aerobic capacity determination using indirect calorimetry (Study 1). An additional separate set of six lean (< or = 25% fat) and six overweight (>25% fat) women were subsequently studied for the determination of eNOS activity, and to better control for absolute peak aerobic capacity between lean and overweight women (Study 2). RESULTS: Skeletal muscle eNOS content was inversely related to percent body fat (r2 = 0.58, P < 0.01), and body mass index (r2 = 0.35, P < 0.05). Total eNOS activity was lower in overweight than lean women (2.09 +/- 0.22 vs 1.44 +/- 0.17 U, P < 0.05; n = 12), and was inversely related to percent body fat (r2 = 0.32, P = 0.05), and BMI (r 2 = 0.41, P < 0.05). Absolute and relative aerobic capacity were not independent predictors of skeletal muscle eNOS content (r2 = 0.11 and 0.26, respectively). CONCLUSION: There is an inverse relationship between eNOS and percent body fat that may have implications for the previously reported reduced endothelial function and insulin sensitivity in overweight women.

Responses of lipolysis and salivary cortisol to food intake and physical activity in lean and obese children.

This investigation was conducted to determine whether there were differences in lipolytic responses to feeding and physical activity between lean (LN) and obese (OB) children, and if these responses were related to cortisol. Fourteen LN and 11 OB children participated in this study of abdominal lipolysis and salivary cortisol response to breakfast and lunch with an intervening exercise session. Calculated fasting glycerol release was lower in OB than LN (0.645 +/- 0.06 vs. 0.942 +/- 0.11 micromol/ml; P < 0.05). Fasting adipose tissue nutritive flow was lower in OB than in LN subjects, but responses to feeding and exercise were not different. Breakfast elicited a decrease in interstitial glycerol concentration in LN (-33%; P < 0.05), but not in OB (-5%), children, although decreases in glycerol concentration in response to lunch were similar (LN, -41%; OB, -36%). An interaction was evident in the salivary cortisol response to breakfast (LN, no change; OB, increase) and exercise (LN, no change; OB, decrease), but there were no group differences in response to lunch. Alterations in salivary cortisol and lipolysis were not related. These data suggest that salivary cortisol and lipolytic responses are not necessarily linked, but are altered in obesity. Furthermore, prior exercise may improve the antilipolytic response to a meal in OB children.

Fatty acid oxidation in African-American and Caucasian women during physical activity.

The goal of this study was to determine whether differences in physical activity-related fat oxidation exist between lean and obese African-American (LAA and OAA) and lean and obese Caucasian (LC and OC) premenopausal women. Lean AA (28.4 +/- 2.8 yr, n = 7), LC (24.7 +/- 1.8 yr, n = 9), OAA (30.9 +/- 2.2 yr, n = 11), and OC (34.1 +/- 2.5 yr, n = 9) women underwent preliminary assessment of peak aerobic capacity (VO2 peak). On a subsequent testing day, participants exercised after an 8-h fast on a cycle ergometer at 15 W (approximately 40% VO2 peak) for 10 min and then for 10 min at approximately 65% VO2 peak). Fatty acid oxidation was determined using the average respiratory exchange ratio and O2 consumption during minutes 5-9 of the exercise session. Percent body fat and fat-free mass were lower (P < 0.05) in LAA (25.8 +/- 2.8% and 48.3 kg) and LC (26.4 +/- 2.0% and 45.8 +/- 1.7 kg) than in OAA (41.2 +/- 1.3% and 58.8 +/- 3.3 kg) and OC (39.3 +/- 2.7% and 58.6 kg) women. Fat oxidation among the groups was analyzed statistically using analysis of covariance with fat-free mass and VO2 peak) as covariates. During exercise at 15 W, fat oxidation was as low in LAA (0.134 +/- 0.024 g/min) as in OAA (0.144 +/- 0.026 g/min) and OC (0.156 +/- 0.020 g/min) women: all these rates of fat oxidation were lower than in LC women (0.200 +/- 0.021 g/min, P < 0.05, LC vs. all other groups). Fatty acid oxidation during higher-intensity exercise (65% VO2 peak)) was higher in LC than in OC women but was not statistically different between African-American and Caucasian groups. Fatty acid oxidation was therefore lower during low-intensity physical activity in OAA, LAA, and OC than in LC women.

Relationship between fat-to-fat-free mass ratio and decrements in leg strength after downhill running.

The purpose of this study was to determine whether greater body fat mass (FM) relative to lean mass would result in more severe muscle damage and greater decrements in leg strength after downhill running. The relationship between the FM-to-fat-free mass ratio (FM/FFM) and the strength decline resulting from downhill running (-11% grade) was investigated in 24 male runners [age 23.4 +/- 0.7 (SE) yr]. The runners were divided into two groups on the basis of FM/FFM: low fat (FM/FFM = 0.100 +/- 0.008, body mass = 68.4 +/- 1.3 kg) and normal fat (FM/FFM = 0.233 +/- 0.020, body mass = 76.5 +/- 3.3 kg, P < 0.05). Leg strength was reduced less in the low-fat (-0.7 +/- 1.3%) than in the normal-fat individuals (-10.3 +/- 1.5%) 48 h after, compared with before, downhill running (P < 0.01). Multiple linear regression analysis revealed that the decline in strength could be predicted best by FM/FFM (r2 = 0.44, P < 0.05) and FM-to-thigh lean tissue cross-sectional area ratio (r2 = 0.53, P < 0.05), with no additional variables enhancing the prediction equation. There were no differences in muscle glycogen, creatine phosphate, ATP, or total creatine 48 h after, compared with before, downhill running; however, the change in muscle glycogen after downhill running was associated with a higher FM/FFM (r = -0.56, P < 0.05). These data suggest that FM/FFM is a major determinant of losses in muscle strength after downhill running.

Effect of intense training on plasma leptin in male and female swimmers.

PURPOSE: The purpose of this study was to determine whether fasting plasma leptin concentration was altered with an increase in training volume in competitive male and female athletes. METHODS: Intercollegiate male (N = 9) and female (N = 12) swimmers were examined during the preseason and at two times during the mid-season (mid-season 1 and mid-season 2) when training volume was relatively high (33,000 m.wk(-1)). Body composition (hydrostatic weighing), energy intake and expenditure, and fasting plasma leptin concentration were measured. RESULTS: In the women, there was a significant (P < 0.05) decline in fat mass (2 kg) with the increase in training volume, which was not accompanied by a reduction in fasting leptin (12.8 +/- 1.5 vs 11.0 +/- 1.2 vs 11.0 +/- 1.5 ng.mL(-1) for preseason, mid-season 1, and mid-season 2, respectively). In the men, there were no significant changes in body composition, body mass, or fasting leptin (4.4 +/- 0.8 vs 4.3 +/- 0.8 vs 4.6 +/- 0.8 ng.mL(-1), respectively). CONCLUSION: These findings suggest 1) plasma leptin is not sensitive to an increase in training volume and 2) leptin may not be indicative of changes in fat mass with an increase in training volume in female athletes. These data suggest that leptin may not be useful in monitoring relative training stress in athletes.

Diabetic groups as defined by ADA and NDDG criteria have a similar aerobic capacity, blood pressure and body composition. American Diabetes Association and National Diabetes Data Group.

AIMS/HYPOTHESIS: In 1997 the American Diabetes Association (ADA) published new categories for diabetes based on fasting plasma glucose that classified diabetes as a plasma glucose of 7.0 mmol/l, or more, rather than one of 7.8 mmol/l or more, as published previously by the National Diabetes Data Group (NDDG) in 1979. We compared the cardiovascular disease risk factors of subjects classified as having Type II (non-insulin-dependent) diabetes mellitus under the NDDG and ADA criteria. METHODS: We examined a database of approximately 3,700 men (40.4 +/- 11.5 years old) and distributed them into four categories: normal fasting plasma glucose (NFG) of less than 6.1 mmol/l, impaired (IFG) 6.1 to 7.0 mmol/l, ADA diabetic 7.0 to 7.8 mmol/l and NDDG diabetic of 7.8 mmol/l or more. RESULTS: Fasting glucose was 5.2 +/- 0.5, 6.4 +/- 0.2, 7.3 +/- 0.2 and 11.2 +/- 2.9 mmol/l for the subjects of the NFG, IFG, ADA and NDDG groups, respectively. Estimated treadmill VO2max was 41.4 +/- 8.0, 36.0 +/- 7.8, 32.2 +/- 7.6, 30.6 +/- 7.0 ml x kg(-1) x min(-1) in the NFG, IFG, ADA, and NDDG groups, respectively (NFG and IFG > ADA and NDDG: p < 0.05). The ADA and NDDG groups were also similar for resting and exercise blood pressure and body composition. Triglycerides and total: HDL cholesterol ratios were higher and LDL cholesterol concentration was lower, in the NDDG group than in all other groups (p < 0.05). Total and LDL cholesterol in the ADA and NDDG groups were similar. CONCLUSION/INTERPRETATION: The similarities in the aerobic capacities, blood pressure and body composition of the ADA and NDDG groups indicate that the decision to lower the cut-off from 7.8 mmol/l to 7.0 mmol/l blood glucose for the clinical classification of diabetes was appropriate. The ADA and NDDG groups, however, might not have identical risks for cardiovascular disease because of differences between total:HDL cholesterol ratios, circulating HDL cholesterol and triglyceride concentrations.

Lipid oxidation is reduced in obese human skeletal muscle.

The purpose of this study was to discern cellular mechanisms that contribute to the suppression of lipid oxidation in the skeletal muscle of obese individuals. Muscle was obtained from obese [body mass index (BMI), 38.3 +/- 3.1 kg/m(2)] and lean (BMI, 23.8 +/- 0.9 kg/m(2)) women, and fatty acid oxidation was studied by measuring (14)CO(2) production from (14)C-labeled fatty acids. Palmitate oxidation, which is at least partially dependent on carnitine palmitoyltransferase-1 (CPT-1) activity, was depressed (P < 0.05) by approximately 50% with obesity (6.8 +/- 2.2 vs. 13.7 +/- 1.4 nmole CO(2).g(-1).h(-1)). The CPT-1-independent event of palmitoyl carnitine oxidation was also depressed (P < 0.01) by approximately 45%. There were significant negative relationships (P < 0.05) for adiposity with palmitate (r = -0.76) and palmitoyl carnitine (r = -0.82) oxidation. Muscle CPT-1 and citrate synthase activity, an index of mitochondrial content, were also significantly (P < 0.05) reduced ( approximately 35%) with obesity. CPT-1 (r = -0.48) and citrate synthase (r = -0.65) activities were significantly (P < 0.05) related to adiposity. These data suggest that lesions at CPT-1 and post-CPT-1 events, such as mitochondrial content, contribute to the reduced reliance on fat oxidation evident in human skeletal muscle with obesity.

Applications of microdialysis in studies of exercise.

Microdialysis provides a means of determining nutrient blood flow as well as continuous simultaneous determination of the interstitial concentration of nutrients and metabolites. Microdialysis also allows for local delivery of pharmacological agents to tissue without resultant systemic effects. The potential of microdialysis for exercise studies is clear, yet relatively untapped.

Association between muscle fiber composition and blood pressure levels during exercise in men.

Normotensive individuals with a magnified blood pressure (BP) level during exercise have an increased risk for developing hypertension. The purpose of this study was to determine if skeletal muscle fiber type is related to the BP level during exercise. Peak BP was determined in 35 normotensive, middle-aged (mean +/- SE, 46.0 +/- 1.8 years) men during maximal treadmill exercise. Fiber distribution (I, IIa, IIb) was measured in muscle samples (percutaneous needle biopsy) from the vastus lateralis and lateral gastrocnemius. The systolic BP during exercise was significantly (P < .05) related to the percentage of type IIb fibers in both the vastus lateralis (r = 0.37) and gastrocnemius (r = 0.38). Mean arterial pressure BP was also related to the percentage of type IIb fibers in the gastrocnemius (r = 0.39, P < .05), with a similar trend evident in the vastus lateralis (r = 0.31, P = 0.08). The percentage of type IIb muscle fibers in both muscle groups was associated with (P < .05) body fat (vastus lateralis, r = 0.44; gastrocnemius, r = 0.43). There were no relationships between the relative percentage of type I or IIa fibers with any BP parameters. Maximal oxygen consumption was negatively related to BP, but only when expressed relative to body weight (mL x kg(-1) x min(-1)). These data suggest that muscle morphology is related to the blood pressure level during exercise and provides insight into factors that may predispose individuals toward the development of hypertension and cardiovascular disease.

Effects of 10 days of endurance exercise training on the suppression of whole body and regional lipolysis by insulin.

The aim of this study was to evaluate in premenopausal women (10 sedentary obese women) the effects of 10 days of exercise on the suppression of whole body and regional lipolysis by insulin. Lipolysis was determined using 2H5-glycerol infusion and microdialysis of sc adipose tissue during a two-stage hyperinsulinemic-euglycemic clamp [10 (LO) and 20 (MO) mU/m x min]. Microdialysis probes were positioned in abdominal and femoral sc adipose tissue to monitor interstitial glycerol and blood flow. Basal plasma glycerol was 86.7 +/-17.0 and 100.3 +/- 19.8 micromol/L before and after training, respectively (P < 0.05). Plasma glycerol was suppressed to a greater extent after [to 47 +/- 5% (LO) and 42 +/- 5% (MO) of basal] than before [to 62 +/- 8% (LO) and 55 +/- 8% (MO) of basal] training. The rate of appearance of glycerol was suppressed to 49 +/- 7% and 40 +/- 5% of basal during LO and to 38 +/- 5% and 30 +/- 4% of basal during MO (P < 0.05) before and after training, respectively. There were no differences in the suppression of lipolysis in abdominal as well as femoral sc adipose tissue as evidenced by similar reductions in dialysate glycerol levels before and after training in each of these tissues. The results indicate that the antilipolytic response to insulin can be improved through endurance exercise training. Intraabdominal adipose tissue or skeletal muscle may be the site of improved antilipolytic response to insulin after training, as improvement was not evident in abdominal or femoral sc adipose tissue.

Suppression of whole body and regional lipolysis by insulin: effects of obesity and exercise.

The aim of this study was to evaluate in premenopausal women (six endurance-trained nonobese, six sedentary nonobese, and five sedentary obese) the suppression of whole body and regional lipolysis by insulin. Lipolysis was determined using 2H5-glycerol infusion and microdialysis of sc adipose tissue (AT) during a two-stage [6-10 (low; LO) and 12-20 (moderate; MOD) mU/m x min] hyperinsulinemic-euglycemic clamp. Microdialysis probes were positioned in abdominal and femoral sc AT to monitor interstitial glycerol and nutritive blood flow. Basal plasma glycerol was 102 +/- 9, 52 +/- 6, and 143 +/- 30 micromol/L in endurance-trained nonobese, sedentary nonobese, and sedentary obese, respectively (P < 0.05, sedentary nonobese < endurance-trained nonobese, sedentary obese). The plasma glycerol concentration was decreased (P < 0.05) to a greater extent in endurance-trained nonobese and sedentary nonobese [both to approximately 50% (LO) and approximately 45% (MOD) of basal] than in sedentary obese [to 72% (LO) and 63% (MOD) of basal]. The rate of appearance of glycerol was suppressed to 36 +/- 7%, 44 +/- 10%, and 62 +/- 7% of basal during LO in endurance-trained nonobese, sedentary nonobese, and sedentary obese, respectively (P < 0.05, endurance-trained nonobese < sedentary obese), and to 34 +/- 3%, 36 +/- 5%, and 53 +/- 8% of basal during MOD, respectively (P < 0.05, endurance-trained nonobese < sedentary obese). There were no between-group differences in the suppression of lipolysis in abdominal sc AT, as evidenced by similar reductions in dialysate glycerol levels [all to approximately 65% (LO) and approximately 55% (MOD) of basal]. Femoral dialysate glycerol was reduced (P < 0.05) more in sedentary nonobese and endurance-trained nonobese (to approximately 75% of basal) than in sedentary obese (to 90% of basal) during LO, but to a similar extent (to approximately 60% of basal) in all groups during MOD. The results indicate that the sedentary obese women had whole body resistance to the suppression of lipolysis by insulin. Intraabdominal AT may be the site of resistance, as resistance was not evident in abdominal or femoral sc AT.

Leptin response to insulin in humans is related to the lipolytic state of abdominal subcutaneous fat.

Insulin-induced leptinemia in humans appears to be blunted by insulin resistance. We therefore examined the relationship between insulin action and plasma leptin by monitoring regional and whole body lipolysis and plasma leptin levels in 15 premenopausal women (body fat range, 14-59%) during a two-stage euglycemic clamp (insulin was infused 90 min each at 6-10 and 12-20 mU/m2 x min). Microdialysis probes were placed in abdominal and femoral sc adipose tissue. Subjects were given a primed, constant infusion of a stable isotope tracer (2H5-glycerol), and plasma glycerol isotope enrichments were analyzed by mass spectrometry to determine glycerol kinetics. Although there was no mean change in plasma leptin during the clamp (baseline, 16.6 +/- 4.5 ng/mL; final, 16.3 +/- 4.3 ng/mL), there was large interindividual variability in the changes in plasma leptin (range, -18% to +19%). Changes in plasma leptin during the clamp stages were correlated with abdominal dialysate glycerol concentrations (r = -0.44; P < 0.05), but not femoral dialysate glycerol concentrations (r = -0.15), the rate of appearance of glycerol in plasma (r = 0.005), or plasma insulin levels (r = 0.16). The results suggest that insulin-induced changes in plasma leptin are more related to the lipolytic state (i.e. low leptin response when lipolysis is high) of abdominal sc adipose tissue than that of other fat depots.

Effects of endurance exercise training on muscle glycogen accumulation in humans.

The purpose of this investigation was to determine whether endurance exercise training increases the ability of human skeletal muscle to accumulate glycogen after exercise. Subjects (4 women and 2 men, 31 +/- 8 yr old) performed high-intensity stationary cycling 3 days/wk and continuous running 3 days/wk for 10 wk. Muscle glycogen concentration was measured after a glycogen-depleting exercise bout before and after endurance training. Muscle glycogen accumulation rate from 15 min to 6 h after exercise was twofold higher (P < 0.05) in the trained than in the untrained state: 10.5 +/- 0.2 and 4.5 +/- 1.3 mmol. kg wet wt(-1). h(-1), respectively. Muscle glycogen concentration was higher (P < 0.05) in the trained than in the untrained state at 15 min, 6 h, and 48 h after exercise. Muscle GLUT-4 content after exercise was twofold higher (P < 0.05) in the trained than in the untrained state (10.7 +/- 1.2 and 4.7 +/- 0.7 optical density units, respectively) and was correlated with muscle glycogen concentration 6 h after exercise (r = 0.64, P < 0.05). Total glycogen synthase activity and the percentage of glycogen synthase I were not significantly different before and after training at 15 min, 6 h, and 48 h after exercise. We conclude that endurance exercise training enhances the capacity of human skeletal muscle to accumulate glycogen after glycogen-depleting exercise.

Norepinephrine response to exercise at the same relative intensity before and after endurance exercise training.

It is well documented that endurance exercise training results in a blunted norepinephrine (NE) response to exercise of a given absolute exercise intensity. However, it is not clear what effect training has on the catecholamine response to exercise of the same relative intensity because previous studies have provided conflicting results. The purpose of the present study was, therefore, to determine the catecholamine response to exercise of the same relative exercise intensity before and after endurance exercise training. Six women and three men [age 28 +/- 8 (SD) yr] performed 10 wk of training. Maximal O2 uptake (VO2 max) was determined during treadmill exercise. Fifteen-minute treadmill exercise bouts were performed at 60, 65, 70, 75, 80, and 85% of VO2 max before and after training. VO2 max was increased by 20% (from 39.2 +/- 7.7 to 46.9 +/- 8.1 ml. kg-1. min-1; P < 0.05) in response to training. Plasma NE concentrations were higher (P < 0.05) during exercise at the same relative intensity after, compared with before, training at 65-85% of VO2 max. Differences between heart rates and plasma epinephrine concentrations after, compared with before, training were not statistically significant. These results provide evidence that the NE response to exercise is dependent on the absolute as well as the relative intensity of the exercise.

Training-induced alterations in fat and carbohydrate metabolism during exercise in elderly subjects.

Compared with young adults, fat oxidation is lower in elderly persons during endurance exercise performed at either the same absolute or relative intensity. We evaluated the effect of 16 wk of endurance training on fat and glucose metabolism during 60 min of moderate intensity exercise [50% of pretraining peak oxygen consumption (VO2peak)] in six elderly men and women (74 +/- 2 yr). Training caused a 21% increase in mean VO2peak. The average rate of fat oxidation during exercise was greater after (221 +/- 28 mumol/min) than before (166 +/- 17 mumol/min) training (P = 0.002), and the average rate of carbohydrate oxidation during exercise was lower after (3,180 +/- 461 mumol/min) than before (3,937 +/- 483 mumol/min) training (P = 0.003). Training did not cause a significant change in glycerol rate of appearance (Ra), free fatty acid (FFA) Ra, and FFA rate of disappearance during exercise. However, glucose Ra during exercise was lower after (1,027 +/- 95 mumol/min) than before (1,157 +/- 69 mumol/min) training (P = 0.01). These results demonstrate that a 16-wk period of endurance training increases fat oxidation without a significant change in lipolysis (glycerol Ra) or FFA availability (FFA Ra) during exercise in elderly subjects. Therefore, the training-induced increase in fat oxidation during exercise is likely related to alterations in skeletal muscle fatty acid metabolism.