Effects of intense and prolonged exercise on insulin sensitivity and glycogen metabolism in hypertensive subjects

BACKGROUND

The information that insulin sensitivity and glycogen synthesis are reduced in hypertension arises primarily from studies using insulin infusions. Whether glycogen metabolism is actually altered in a physiological condition, such as during and after prolonged exercise, is currently unknown.

METHODS AND RESULTS

To examine this issue, 9 hypertensive and 11 normotensive subjects were evaluated on a rest day and after intense and prolonged exercise on a separate day. Insulin sensitivity and hemodynamic variables were measured on both days. On the exercise day, whole-body substrate utilization was assessed and muscle biopsies were taken in the leg at baseline, immediately after exercise, and 2.5 and 4 hours after exercise. Insulin sensitivity at rest was lower in hypertensive than normotensive subjects (P<0.05) and increased after exercise in normotensive (P<0.01) but not in hypertensive (P=NS) subjects. Leg blood flow increased after exercise in both groups but to a lesser extent in hypertensive than normotensive subjects. Baseline glycogen content and maximal glycogen synthase activity were higher in hypertensive than normotensive subjects (P<0.001). Glycogen concentration decreased relatively less (-35 versus -66%) and returned to baseline levels faster in hypertensive subjects after exercise. Hypertensive subjects used approximately 40% less carbohydrates during exercise (P<0.001) at the expense of greater free fatty acid oxidation.

CONCLUSIONS

It is concluded that increased intramuscular glycogen storage and resynthesis in hypertension are independent of blood flow and may represent compensatory mechanisms for the reduced insulin sensitivity and carbohydrate metabolism in this condition.

Skeletal muscle enzymes as predictors of 24-h energy metabolism in reduced-obese persons

BACKGROUND

Little is known about the effects of weight loss on the relation between skeletal muscle enzymes and energy metabolism.

OBJECTIVE

This study was performed retrospectively to investigate the relation between skeletal muscle enzymes and 24-h energy metabolism in obese persons before and after weight loss.

DESIGN

Ten women and 9 men [with body mass indexes (in kg/m(2)) > 30] underwent a 15-wk weight-loss program (-700 kcal/d). Body weight and composition, 24-h energy metabolism (whole-body indirect calorimetry), and maximal activities of phosphofructokinase (EC 2.7.1.11), creatine kinase (CK; EC 2.7.3.2), citrate synthase (CS; EC 4.1.3.7), 3-hydroxyacyl-CoA dehydrogenase (HADH; EC 1.1.1.35), and cytochrome-c oxidase (COX; EC 1.9.3.1) were determined from biopsy samples of the vastus lateralis taken before and after weight loss.

RESULTS

Before weight loss, fat-free mass (FFM) was the only predictor of 24-h energy expenditure (R(2) = 0.70, P < 0.001), whereas the cumulative variance in sleeping metabolic rate explained by FFM and fat mass (FM) was 83% (P < 0.001). After weight loss, CS (r = 0.45, P = 0.05) and COX (r = 0.65, P < 0.01) were significantly associated with 24-h energy expenditure, whereas CK (r = 0.53, P < 0.05), CS (r = 0.45, P < 0.05), COX (r = 0.64, P < 0.01), and HADH (r = 0.45, P = 0.05) were all significant correlates of sleeping metabolic rate. After weight loss, FFM, FM, and COX explained 84% (P < 0.01) of the variance in 24-h energy expenditure, whereas FFM, FM, and CK all contributed to the cumulative variance in sleeping metabolic rate explained by this model (R(2) = 0.82, P < 0.05).

CONCLUSION

Maximal activities of key skeletal muscle enzymes contribute to the variability in 24-h energy metabolism in reduced-obese persons.

Glycogen depletion of human skeletal muscle fibers in response to high-frequency electrical stimulation

The purpose of the present study was to evaluate the pattern of change in muscular glycogen content in response to high-frequency electrical stimulation (HFES). Muscle biopsies were taken from the vastus lateralis muscle of 7 healthy young men before, 15 min after, and 30 min after electrical stimulation delivered at a 50-Hz frequency (15 s on, 45 s off) at an intensity of 100 mA. The glycogen content of type I, IIA, and IIB muscle fibres was evaluated using microphotometry of periodic acid Schiff (PAS) stained fibres. After 15 min of electrical stimulation, the glycogen content in type I, IIA, and IIB muscle fibres significantly decreased from 113 +/- 10 (mean +/- SE) to 103 +/- 10 (p < or = 0.05), 129 +/- 9 to 102 +/- 12 (p < or = 0.01), and 118 +/- 8 to 90 +/- 13 (p < or = 0.01) arbitrary relative units, respectively. No further decrement in glycogen content was observed in all three fibre types following an additional 15 min of HFES. In addition, isometric force decreased by approximately 50%, from 125.9 +/- 20.0 N to 64.2 +/- 7.7 N (p < or = 0.01), during the first 15 contractions. No further decrease in isometric force was observed following an additional 15 contractions of HFES. These results reveal that significant reductions in isometric force of knee extensor muscles and glycogen content of all human skeletal muscle fibre types in vastus lateralis muscle are observable after 15 min of neuromuscular high-frequency transcutaneous electrical stimulation.

Weight loss-induced rise in plasma pollutant is associated with reduced skeletal muscle oxidative capacity

In this study, we examined whether weight loss-induced changes in plasma organochlorine compounds (OC) were associated with those in skeletal muscle markers of glycolytic and oxidative metabolism. Vastus lateralis skeletal muscle enzyme activities and plasma OC (Aroclor 1260, polychlorinated biphenyl 153, p,p'-DDE, beta-hexachlorocyclohexane, and hexachlorobenzene) were measured before and after a weight loss program in 17 men and 20 women. Both sexes showed a similar reduction in body weight (approximately 11 kg) in response to treatment, although men lost significantly more fat mass than women (P < 0.05). Enzymatic markers of glycolysis, phosphofructokinase (PFK) activity, and oxidative metabolism, beta-hydroxyacyl-CoA dehydrogenase (HADH), citrate synthase (CS), and cytochrome c oxidase (COX) activities, remained unchanged after weight loss. A significant increase in plasma OC levels was observed in response to weight loss, an effect that was more pronounced in men. No relationship was observed between changes in OC and those in PFK activity in either sex [-0.31 < r < 0.12, not significant (NS)]. However, the greater the increase in plasma OC levels, the greater the reduction in oxidative enzyme (HADH, CS, COX) activities was in response to weight loss in men (-0.75 < r < -0.50, P < 0.05) but not in women (-0.33 < r < 0.33, NS). These results suggest that the weight loss-induced increase in plasma pollutant levels is likely to be associated with reduced skeletal muscle oxidative metabolism in men but not in women.

Fat content in individual muscle fibers of lean and obese subjects

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+/-3.3 kg/m(2); mean+/-s.d.) and 14 obese (BMI 33.7+/-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+/-135 vs 129+/-75; obese vs lean, P<0.05), IIA (132+/-67 vs 79+/-29; P<0.05), and IIB (103+/-63 vs 51+/-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+/-5.7 vs 19.7+/-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.

Elevated intramyocellular lipid concentration in obese subjects is not reduced after diet and exercise training

To determine the effects of weight loss on intramyocellular energy substrates, vastus lateralis muscle biopsies were taken from six obese subjects (body mass index 34 +/- 5 kg/m(2)) before, after 15 wk of energy restriction (ER; -700 kcal/day), and after a further average 20.7 +/- 1.6 wk of endurance training plus low-fat diet (ET-LFD). Body weight fell from 100 +/- 6 to 89 +/- 6 kg during ER and to 84 +/- 4 kg after ET-LFD. Lipids and glycogen were histochemically measured in type I, IIA, and IIB fibers. Total muscle glycogen content (MGC; per 100 fibers) decreased after ER [from 72 +/- 13 to 55 +/- 8 arbitrary units (AU)]. A similar but not significant decrease was seen in total muscle lipid content (MLC; 14 +/- 5 to 9 +/- 1 AU). After ET-LFD, MGC returned to initial values (74 +/- 8 AU), and MLC approached near-initial values (12 +/- 3 AU). Individual fiber lipid concentration did not change throughout the protocol in all fiber types, whereas glycogen concentration increased after ET-LFD. The training effects of ET-LFD were measured as increasing activities of key mitochondrial enzymes. Although total muscle energy reserves can be reduced after weight loss, their concentration within individual myofibers remains elevated. Weight loss does not appear sufficient to correct the potential detrimental effects of high intracellular lipid concentrations.

The stimulation-induced increase in skeletal muscle glycogen synthase content is impaired in carriers of the glycogen synthase XbaI gene polymorphism

Associations between glycogen synthase gene (GYS1) polymorphism and states of insulin resistance and type 2 diabetes have been reported. The purpose of this study was to establish if the GYS1 genotype impacts on the content of glycogen synthase (GS) protein in muscle measured under basal and stimulated conditions. To examine this, GYS1 XbaI and Met416Val polymorphisms and thigh muscle GYS1 protein content were determined at rest, both before and after several weeks of neuromuscular electrical stimulation in carriers and noncarriers of the mutations. The allelic frequency was 0.086 for the XbaI mutation (A2) and 0.006 for the Met416Val in our cohort of French-Canadian subjects. When measured at rest, the GS protein content in muscle was similar among carriers and noncarriers of the XbaI variant. However, the stimulation-induced increase (23%) in the amount of GS muscle protein normally seen in wildtype individuals was impaired in those carrying the XbaI mutation. These data demonstrate that some individuals, because of their genetic background, are unable to stimulate the process of GS protein accumulation in skeletal muscle. These results could explain why some individuals appear to be genetically predisposed to developing skeletal muscle insulin resistance when exposed to unfavorable metabolic environments.

No association between the angiotensin-converting enzyme ID polymorphism and elite endurance athlete status

Several studies have reported that the insertion (I) allele of the angiotensin-converting enzyme (ACE) I/deletion (D) polymorphism is associated with enhanced responsiveness to endurance training and is more common in endurance athletes than in sedentary controls. We tested the latter hypothesis in a cohort of 192 male endurance athletes with maximal oxygen uptake >/=75 ml. kg(-1). min(-1) and 189 sedentary male controls. The ACE ID polymorphism in intron 16 was typed with the three-primer polymerase chain reaction method. Both the genotype (P = 0.214) and allele (P = 0.095) frequencies were similar in the athletes and the controls. Further analyses in the athletes revealed no excess of the I allele among the athletes within the highest quartile (> 80 ml. kg(-1). min(-1)) or decile (>83 ml. kg(-1). min(-1)) of maximal oxygen uptake. These data from the GENATHLETE cohort do not support the hypothesis that the ACE ID polymorphism is associated with a higher cardiorespiratory endurance performance level.

Muscle fiber area distribution of musculus uvulae in obstructive sleep apnea and non-apneic snorers

OBJECTIVE

To determine whether differences exist in the morphology of upper airway muscles between apneic and non-apneic snorers

DESIGN

Muscle characteristic analysis in patients undergoing upper airway surgery in a tertiary sleep center.

SUBJECTS

10 non-apneic snorers and 10 sleep apnea hypopnea syndrome (SAHS).

MEASUREMENTS

Frequency distribution of musculus uvulae (MU) muscle fiber area determined from 475+/-207 (mean s.d.) and 697 +/- 165 type IIA fibers and 92 +/- 32 and 68 45 type I fibers in snorers and SAHS, respectively.

RESULTS

Histochemical analyses of MU revealed as expected that type IIA fibers occupied a significantly larger area within that muscle in SAHS compared to Snorers (89.4 +/- 5.8% and 76.1 +/- 15.1% respectively, P= 0.01). No difference was found in the frequency distribution of type I and type IIA fiber areas between the two groups comparing the between-groups and within-group variance of individual area distributions found in snorers and SAHS.

CONCLUSION

There is no difference in muscle fiber area frequency distribution between non-apneic snorers and SAHS patients. This suggests that musculus uvulae of these groups of patients does not have a specific prevalence of atrophic or hypertrophic muscle fibers.

Skeletal muscle fatty acid metabolism in association with insulin resistance, obesity, and weight loss

The current study was undertaken to investigate fatty acid metabolism by skeletal muscle to examine potential mechanisms that could lead to increased muscle triglyceride in obesity. Sixteen lean and 40 obese research volunteers had leg balance measurement of glucose and free fatty acid (FFA) uptake (fractional extraction of [9,10 (3)H]oleate) and indirect calorimetry across the leg to determine substrate oxidation during fasting and insulin-stimulated conditions. Muscle obtained by percutaneous biopsy had lower carnitine palmitoyl transferase (CPT) activity and oxidative enzyme activity in obesity (P < 0.05). During fasting conditions, obese subjects had an elevated leg respiratory quotient (RQ, 0.83 +/- 0.02 vs. 0.90 +/- 0.01; P < 0.01) and reduced fat oxidation but similar FFA uptake across the leg. During insulin infusions, fat oxidation by leg tissues was suppressed in lean but not obese subjects; rates of FFA uptake were similar. Fasting values for leg RQ correlated with insulin sensitivity (r = -0.57, P < 0.001). Thirty-two of the obese subjects were restudied after weight loss (WL, -14.0 +/- 0.9 kg); insulin sensitivity and insulin suppression of fat oxidation improved (P < 0.01), but fasting leg RQ (0.90 +/- 0.02 vs. 0.90 +/- 0.02, pre-WL vs. post-WL) and muscle CPT activity did not change. The findings suggest that triglyceride accumulation in skeletal muscle in obesity derives from reduced capacity for fat oxidation and that inflexibility in regulating fat oxidation, more than fatty acid uptake, is related to insulin resistance.

Markers of capacity to utilize fatty acids in human skeletal muscle: relation to insulin resistance and obesity and effects of weight loss

A number of biochemical defects have been identified in glucose metabolism within skeletal muscle in obesity, and positive effects of weight loss on insulin resistance are also well established. Less is known about the capacity of skeletal muscle for the metabolism of fatty acids in obesity-related insulin resistance and of the effects of weight loss, though it is evident that muscle contains increased triglyceride. The current study was therefore undertaken to profile markers of human skeletal muscle for fatty acid metabolism in relation to obesity, in relation to the phenotype of insulin-resistant glucose metabolism, and to examine the effects of weight loss. Fifty-five men and women, lean and obese, with normal glucose tolerance underwent percutaneous biopsy of vastus lateralis skeletal muscle for determination of HADH, CPT, heparin-releasable (Hr) and tissue-extractable (Ext) LPL, CS, COX, PFK, and GAPDH enzyme activities, and content of cytosolic and plasma membrane FABP. Insulin sensitivity was measured using the euglycemic clamp method. DEXA was used to measure FM and FFM. In skeletal muscle of obese individuals, CPT, CS, and COX activities were lower while, conversely, they had a higher or similar content of FABP(C) and FABP(PM) than in lean individuals. Hr and Ext LPL activities were similar in both groups. In multivariate and simple regression analyses, there were significant correlations between insulin resistance and several markers of FA metabolism, notably, CPT and FABP(PM). These data suggest that in obesity-related insulin resistance, the metabolic capacity of skeletal muscle appears to be organized toward fat esterification rather than oxidation and that dietary-induced weight loss does not correct this disposition.

Obesity and increased contractile activity influence the protein content of UCP2 in human skeletal muscle

The newly discovered uncoupling protein-2 (UCP2) has been proposed to play a critical role in thermoregulatory and substrate oxidation processes. Skeletal muscle mRNA expression and, more recently, the protein content of UCP2 were investigated in humans. These studies have shown that the content of this protein varies quite substantially and that several factors could be responsible for its variation in human skeletal muscle. The aim of this review is to determine whether obesity and low-intensity increased contractile activity contribute to variation in muscle UCP2 content. A recent study from our laboratories revealed that, in obesity, UCP2 content in skeletal muscle is over-expressed by about 1.5-fold compared to lean. Body weight loss in obese subjects did not cause any change in skeletal muscle UCP2 content. On the other hand, when increased muscular contractile activity of knee extensor muscles is induced by several weeks of low-frequency electrical stimulation, UCP2 content increased by about 15%. Obesity and increased contractile activity do not appear sufficient, however, to explain the magnitude of the human skeletal muscle variation in UCP2 content. Since intensive efforts are being devoted to this area of research, it is expected that our understanding of the causes contributing to its variation in humans will soon be substantially improved.

Linkage between a muscle-specific CK gene marker and VO2max in the HERITAGE Family Study

PURPOSE

We have reported a significant association between VO2max in the sedentary state and its response (delta VO2max) to an endurance training program with a muscle-specific creatine kinase (CKMM) gene polymorphism. The purpose of this study was to test the hypothesis of genetic linkage between the same CKMM marker and VO2max in the sedentary state as well as delta VO2max.

METHODS

Sib-pair linkage analysis was performed on 277 full sib-pairs from 98 Caucasian nuclear families of the HERITAGE Family Study. VO2max was measured during cycle ergometry tests before and after 20 wk of endurance training. The CKMM polymorphism was detected by the polymerase chain reaction and digestion with the Ncol restriction enzyme.

RESULTS

Frequencies for the rare (1170 base pairs) and common (985 + 185 base pairs) alleles were 0.32 and 0.68, respectively. No significant linkage (t = -0.02, P = 0.49) was detected between the CKMM marker and the age and sex adjusted VO2max (mL x kg(-1) x min(-1)) in the sedentary state. However, after adjustment of delta VO2max for the effects of age, sex, initial VO2max, and body mass, evidence for linkage between the CKMM locus and delta VO2max was suggestive (P = 0.04).

CONCLUSION

The present results provide further support for the notion that the CKMM gene, or some gene in close linkage disequilibrium with it, may contribute to individual differences in the VO2max response to endurance training.

Overexpression of muscle uncoupling protein 2 content in human obesity associates with reduced skeletal muscle lipid utilization

Uncoupling proteins (UCP) may influence thermogenesis. Since skeletal muscle plays an important role in energy homeostasis and substrate oxidation, this study was undertaken to test the hypotheses that skeletal muscle UCP2 content is altered in obesity and could be linked to basal energy expenditure, insulin sensitivity, or substrate oxidation within skeletal muscle under postabsorptive (fasting) conditions. To examine these possibilities, limb basal energy expenditure and respiratory quotient (bRQ) were measured in 18 obese nondiabetic (Ob) and lean individuals (L). Total body fat (%) ranged from 11% to 46%. In addition, insulin-stimulated rates of glucose disposal (Rd) were measured under euglycemic hyperinsulinemic conditions. Biopsy of vastus lateralis muscle was used to measure cytochrome c oxidase (COX) enzyme activity and UCP2 content. Whereas low muscle COX activity was found in the Ob compared to L (6.9+/-1.6 vs. 9.6+/-1.2 U/g; P<0.001), skeletal muscle UCP2 content in Ob was significantly higher than in L (48+/-9 vs. 33+/-12 arbitrary units/g; P<0.05). Moreover, UCP2 content was positively correlated with percent of total body fat (r=0.57; P<0. 05) and bRQ (r=0.59; P<0.01), but not with visceral fat (r=0.17; P=0. 49), basal energy expenditure (r=0.07; P=0.79) or Rd (r=-0.23; P=0. 34). In summary, these results indicate that if development of obesity in humans is mediated by defective expression of UCP2 within skeletal muscle, then this effect is not observed in people with established obesity. The present study also suggests that skeletal muscle UCP2 content is not related to basal energy expenditure or insulin sensitivity in humans. However, the increased content of UCP2 within skeletal muscle in obesity appears to coincide with a reduced postabsorptive lipid utilization by muscle.

Three mitochondrial DNA restriction polymorphisms in elite endurance athletes and sedentary controls

This study examined the associations between elite endurance athlete (EEA) status and three mitochondrial DNA (mtDNA) restriction fragment length polymorphisms (RFLPs) in the subunit 5 of the NADH dehydrogenase (MTND5) locus and one in the D-loop region. A group of 125 Caucasian male EEA well endowed with the phenotypic expression of VO2max (78.9 +/- 3.8 mL x kg(-1) x min(-1), mean +/- SD) and 65 sedentary controls (SCON: VO2max = 39.8 +/- 8.2 mL x kg(-1) x min(-1)) participated in the study. VO2max was determined during an incremental exercise test on a cycle ergometer or a motor-driven treadmill. mtDNA was extracted from white blood cells or lymphoblastoid cell lines and specific regions were amplified by the polymerase chain reaction. The Pearson Chi-square statistic test and Fisher exact test revealed no significant association (P > 0.05) between any of the three mtDNA RFLPs and EEA status. The MTND5-BamHI RFLP at bp 13,470 (morph 3) was found in 12.8% of the EEA and 12.3% of the SCON (chi2 = 0.009, P = 0.92). The prevalence of the MTND5-Ncil RFLP at bp 13,364 (morph 2) was 12.9% and 14% for the EEA and SCON, respectively (chi2 = 0.043, P = 0.83). The D-loop-KpnI RFLP at bp 16,133 (morph 1) was found in 5.8% of the EEA and in 1.6% of the SCON (Fisher exact test = 1.80, P = 0.18). The MTND5-HincII RFLP at bp 12,406 (morph 1) was not present in this study sample. These results indicate no evidence for a difference in the frequency of two polymorphic restriction sites in the subunit 5 of the NADH dehydrogenase gene of mtDNA and one in the D-loop region between elite endurance athletes and sedentary controls.

Muscle-specific creatine kinase gene polymorphisms in elite endurance athletes and sedentary controls

The purpose of this study was to investigate the association between elite endurance athlete (EEA) status and two restriction fragment length polymorphisms (RFLPs) at the muscle-specific creatine kinase (CKMM) gene locus. Genomic DNA was extracted from white blood cells or lymphoblastoid cell lines of 124 unrelated Caucasian male EEA (VO2max > 73 mL.kg-1.min-1) and 115 unrelated Caucasian sedentary male controls (SCON). The genetic polymorphism at the CKMM locus was detected by the polymerase chain reaction and DNA digestion with the NcoI and TaqI restriction endonucleases. The allelic frequencies for the NcoI and TaqI RFLPs were not different (P > 0.05) between EEA and SCON subjects. The three expected genotypes for CKMM-NcoI (1170/1170 bp, 1170/985 + 185 bp, and 985 + 185/985 + 185 bp) and CKMM-TaqI (1170/1170 bp, 1170/1020 + 150 bp, and 1020 + 50/1020 + 150 bp) were observed in the EEA and SCON groups. These genotype frequencies were in Hardy-Weinberg equilibrium, but they were not significantly (P > 0.05) different between the EEA and SCON. A strong (P < 0.001) linkage disequilibrium was detected among the NcoI and TaqI RFLPs in both EEA and SCON. These findings indicate that the skeletal muscle CK-NcoI and CK-TaqI gene polymorphisms are not associated with the elite endurance athlete status.

Subcutaneous abdominal fat and thigh muscle composition predict insulin sensitivity independently of visceral fat

Whether visceral adipose tissue has a uniquely powerful association with insulin resistance or whether subcutaneous abdominal fat shares this link has generated controversy in the area of body composition and insulin sensitivity. An additional issue is the potential role of fat deposition within skeletal muscle and the relationship with insulin resistance. To address these matters, the current study was undertaken to measure body composition, aerobic fitness, and insulin sensitivity within a cohort of sedentary healthy men (n = 26) and women (n = 28). The subjects, who ranged from lean to obese (BMI 19.6-41.0 kg/m2), underwent dual energy X-ray absorptiometry (DEXA) to measure fat-free mass (FFM) and fat mass (FM), computed tomography to measure cross-sectional abdominal subcutaneous and visceral adipose tissue, and computed tomography (CT) of mid-thigh to measure muscle cross-sectional area, muscle attenuation, and subcutaneous fat. Insulin sensitivity was measured using the glucose clamp technique (40 mU.m-2.min-1), in conjunction with [3-3H]glucose isotope dilution. Maximal aerobic power (VO2max) was determined using an incremental cycling test. Insulin-stimulated glucose disposal (Rd) ranged from 3.03 to 16.83 mg.min-1.kg-1 FFM. Rd was negatively correlated with FM (r = -0.58), visceral fat (r = -0.52), subcutaneous abdominal fat (r = -0.61), and thigh fat (r = -0.38) and positively correlated with muscle attenuation (r = 0.48) and VO2max (r = 0.26, P < 0.05). In addition to manifesting the strongest simple correlation with insulin sensitivity, in stepwise multiple regression, subcutaneous abdominal fat retained significance after adjusting for visceral fat, while the converse was not found. Muscle attenuation contributed independent significance to multiple regression models of body composition and insulin sensitivity, and in analysis of obese subjects, muscle attenuation was the strongest single correlate of insulin resistance. In summary, as a component of central adiposity, subcutaneous abdominal fat has as strong an association with insulin resistance as visceral fat, and altered muscle composition, suggestive of increased fat content, is an important independent marker of insulin resistance in obesity.

Muscle-specific creatine kinase gene polymorphism and VO2max in the HERITAGE Family Study

This study examined the association between a DNA polymorphism in the muscle-specific creatine kinase (CKMM) gene and VO2max in the sedentary state, as well as its response (deltaVO2max) to a standardized 20-wk endurance training program. The subjects were 160 biologically unrelated Caucasian parents (80 women, 80 men) and 80 biologically unrelated adult offspring of the HERITAGE Family Study. The CKMM polymorphism was detected by PCR and digestion with the NcoI restriction enzyme. VO2max was measured during maximal cycle ergometer tests. VO2max was 2119 +/- 45 mL x min(-1) (mean +/- SE) or 26 +/- 0.4 mL x kg(-1) x min(-1). Both sexes had a significant (P < 0.05) increase in the deltaVO2max (women = 283 +/- 20 mL x min[-1] and men = 363 +/- 25 mL x min[-1]). Allele and genotype frequencies were not significantly different (P > 0.05) between sexes. Age and sex adjusted VO2max was significantly (P = 0.007) associated with the CKMM genotype in the parents, whereas no association (P > 0.05) was observed in the offspring. DeltaVO2max values adjusted for age, sex, VO2max, and body mass were characterized by genotype differences in both parents (P = 0.0004) and offspring (P = 0.0025). A significantly (P < 0.05) lower deltaVO2max to endurance training was detected in both parents and offspring homozygotes for the rare allele. The genotype accounted for at least 9% of the variance in deltaVO2max. These results indicate that the NcoI polymorphism in the 3' untranslated region of the muscle-specific creatine kinase gene is associated with the deltaVO2max to endurance training.

Time course of training-induced changes in maximal exercise of short duration in men and women

The purpose of the present study was to test the hypothesis that gender differences are present in the extent and time course of exercise training-induced changes in maximal 10- and 90-s performance test. Thirty-six sedentary subjects (19 women and 17 men) were submitted to 15 weeks of training involving both continuous and interval ergocycle exercise sessions, while 13 other subjects (5 women and 8 men) served as a control group. Maximal power output after 10 s (P10) and 90s (P90) of cycling exercise was measured before and at each 5-week interval of the 15-week training period in both groups. Significant (p < 0.01) training-induced increases in performance were noted after 5 weeks, 10 weeks and 15 weeks of training for P10, and after 5 and 10 weeks for P90 in both genders. P10 and P90 were significantly increased in both genders (about 25% in men and 35% in women) following the 15-week training program and overall absolute increases were not statistically different between men and women. Slight increases (about 5%) in performance tests were observed in control subjects, but only during the first 5-week interval. P10 and P90 of women expressed as a percentage of that of men remained the same throughout the 15-week program. No significant relationship between pre-training values of P10 and their responses to training was found in men and women. In conclusion, results of the present study indicate that women have the same capacity to increase maximal short-term performance in response to training in comparison to men.

Altered glycolytic and oxidative capacities of skeletal muscle contribute to insulin resistance in NIDDM

The insulin resistance of skeletal muscle in glucose-tolerant obese individuals is associated with reduced activity of oxidative enzymes and a disproportionate increase in activity of glycolytic enzymes. Because non-insulin-dependent diabetes mellitus (NIDDM) is a disorder characterized by even more severe insulin resistance of skeletal muscle and because many individuals with NIDDM are obese, the present study was undertaken to examine whether decreased oxidative and increased glycolytic enzyme activities are also present in NIDDM. Percutaneous biopsy of vatus lateralis muscle was obtained in eight lean (L) and eight obese (O) nondiabetic subjects and in eight obese NIDDM subjects and was assayed for marker enzymes of the glycolytic [phosphofructokinase, glyceraldehyde phosphate dehydrogenase, hexokinase (HK)] and oxidative pathways [citrate synthase (CS), cytochrome-c oxidase], as well as for a glycogenolytic enzyme (glycogen phosphorylase) and a marker of anaerobic ATP resynthesis (creatine kinase). Insulin sensitivity was measured by using the euglycemic clamp technique. Activity for glycolytic enzymes (phosphofructokinase, glyceraldehye phosphate dehydrogenase, HK) was highest in subjects with subjects with NIDDM, following the order of NIDDM > O > L, whereas maximum velocity for oxidative enzymes (CS, cytochrome-c oxidase) was lowest in subjects with NIDDM. The ratio between glycolytic and oxidative enzyme activities within skeletal muscle correlated negatively with insulin sensitivity. The HK/CS ratio had the strongest correlation (r = -0.60, P < 0.01) with insulin sensitivity. In summary, an imbalance between glycolytic and oxidative enzyme capacities is present in NIDDM subjects and is more severe than in obese or lean glucose-tolerant subjects. The altered ratio between glycolytic and oxidative enzyme activities found in skeletal muscle of individuals with NIDDM suggests that a dysregulation between mitochondrial oxidative capacity and capacity for glycolysis is an important component of the expression of insulin resistance.

Monitoring high-intensity endurance exercise with heart rate and thresholds

Ventilatory and lactate thresholds have been proposed as tools to establish the highest steady-state intensity sustainable during prolonged physical exercise. The purposes of this study were to clarify whether the intensity at the ventilatory threshold could be sustained during prolonged high-intensity exercise and if the corresponding work rate, pulmonary ventilation, and blood lactate concentration could also be maintained. Fifteen young and healthy male subjects were submitted to a VO2max test on ergocycle and a 90-min high-intensity ergocycle endurance exercise test. During the 90-min exercise test, subjects were able to maintain an intensity corresponding to a heart rate 5 beats.min-1 lower than that predetermined from the ventilatory threshold. Heart rate, FeO2, and FeCO2 were stable during the period from 20 to 80 min, VO2 was constant from 30 to 80 min, while work output, pulmonary ventilation, blood lactate, and VCO2 decreased significantly over the 90-min performance. These results show that physiological parameters near the ventilatory threshold are not interchangeable and that some cannot be used to monitor high-intensity long term exercise. Moreover, they clearly demonstrate that the blood lactate concentration fluctuates substantially during a 90-min endurance performance and cannot predict the highest work intensity that can be sustained during prolonged exercise without fatigue. However, heart rate and VO2 at the ventilatory threshold seem to be more suitable markers for that purpose.

Electrical stimulation-induced changes in performance and fiber type proportion of human knee extensor muscles

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.

The effect of non-insulin-dependent diabetes mellitus and obesity on glucose transport and phosphorylation in skeletal muscle

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 18F-2-deoxy-glucose uptake into midthigh muscle was performed using positron emission tomography during basal and insulin-stimulated conditions (40 mU/m2 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[18F]fluoro-D-glucose (FDG) clearance into muscle and significant increases in the rate constants for inward transport and phosphorylation of FDG. In obese individuals, the rate constant for inward transport of glucose was not increased by insulin infusion and did not differ from values in NIDDM. Insulin stimulation of the rate constant for glucose phosphorylation was similar in obese and lean subjects but reduced in NIDDM. Insulin increased by nearly twofold the number and area of sites labeling for Glut 4 at the sarcolemma in lean volunteers, but in obese and NIDDM subjects translocation of Glut 4 was attenuated. Activities of skeletal muscle HK I and II were similar in lean, obese and NIDDM subjects. These in vivo and ex vivo assessments indicate that impaired glucose transport plays a key role in insulin resistance of NIDDM and obesity and that an additional impairment of glucose phosphorylation is evident in the insulin resistance of NIDDM.

Upper airway collapsibility, and contractile and metabolic characteristics of musculus uvulae

Physiologic, metabolic, and histochemical characteristics of one upper airway (UA) dilator muscle (musculus uvulae; MU) differ between sleep apnea hypopnea syndrome (SAHS) and nonapneic snorers. We hypothesized that these differences in MU characteristics could result from the cumulative effects of the diurnal and nocturnal intermittent contractions of UA muscles in order to compensate for a permanent increase in UA collapsibility. The aim of this study was to determine the influence of UA collapsibility on MU characteristics. Seventeen SAHS and three nonapneic snorers, who underwent an uvulo-palato-pharyngoplasty as a treatment for snoring or SAHS, participated in the study. Awake and sleeping UA critical pressure (Pcrit) was measured during continuous positive or negative airway pressure trials by analysis of the relationship between maximal inspiratory flow and the upstream pressure of flow-limited breathing cycles. Maximum isometric twitch (Pt) and tetanic tension (Po), fatigability measurements, activities of marker enzymes for anaerobic and aerobic-oxidative profile, and fiber type proportions and areas of MU were determined. There was a significant positive relationship between Pt, Po, and Pcrit measured during wakefulness and sleep. The fatigability index was negatively correlated with awake Pcrit values (r = -0.79). Activity level of the anaerobic enzymes as well as the percentage of surface occupied by type I and type IIA muscle fibers as correlated witb awake Pcrit. We conclude that the differences in awake UA collapsibility help to determine the contractile properties and metabolic and histochemical characteristics of MU.

Cardiorespiratory and metabolic adaptations to hyperoxic training

This study examined the effects of hyperoxic training on specific cardiorespiratory and metabolic responses. A group of 19 male subjects trained for 5 weeks on a cycle ergometer at 70 percent of hyperoxic or normoxic maximal heart rate, the hyperoxic group (HG) breathing 70 percent O2, the normoxic group (NG) breathing 21 percent O2. The subjects were tested pre- and post-training under both hyperoxia and normoxia. Measurements included cardiac output (Q(c)), stroke volume (SV), heart rate (HR), pulmonary ventilation (V(E)), oxygen consumption (VO(2)), partial pressure of oxygen (PO(2)), partial pressure of inspired carbon dioxide (PCO(2)), blood lactate concentration [La], and fiber type composition. The V(E) was significantly lower at submaximal work rates (P <0.05) and maximal V(E) increased after training in both groups for both test conditions; hyperoxic V(E) was lower than normoxic V(E) (P <0.05). The maximal V0(2) increased significantly (P <0.05) in both groups for both tests and was 11 percent - 12 percent higher during hyperoxia. Post-training maximal heart rate (HR(max)) was significantly decreased (P <0.05) at the same absolute work rate regardless of the training group or test type. The SV was increased at each work rate and Q c was unchanged. The maximal Q(c) increased significantly (P <0.05) for both groups and types of test: for normoxia: NG 27.3-30.41*min(-1) and HG 30.3-32.31*min(-1) and for hyperoxia: NG 24.7-25.6 and HG 27.9-31.21*min(-1). Although working at the same intensity relative to HR(max), HG showed significantly lower [La] following a single training session, yet maximal values were unchanged after training. Both groups showed a significant increase in the percentage of type IIA fibers post-training but HG retained a larger percentage of IIB fibers. Mitochondrial enzymes; citrate kinase, 3-hydroxyacyl CoA dehydrogenase, and cytochrome c-oxidase were increased in the normoxic trained subjects (P <0.05). In summary, training induced adaptive responses in maximal aerobic power, HR, SV, Q(c), [La], and muscle fiber type composition, independent of inspired PO(2). Intramuscular data suggested there may be some differences between hyperoxic and normoxic training and these were substantiated by mitochondrial enzyme and lactate findings. Our data would suggest that transport mechanisms may limit the ability to increase aerobic power.

Genetic determinism of fiber type proportion in human skeletal muscle

Skeletal muscle fiber type distribution is quite heterogeneous, with about 25% of North American Caucasian men and women having either less than 35% or more than 65% of type I fiber in their vastus lateralis muscle. To what extent human skeletal muscle fiber type proportion is under the control of genetic factors is examined in this paper. The results summarized here suggest that about 15% of the total variance in the proportion of type I muscle fibers in human is explained by the error component related to muscle sampling and technical variance, that about 40% of the phenotype variance is influenced by environmental factors, and the remaining variance (about 45%) is associated with inherited factors. These estimates suggest that a difference of about 30% in type I fibers among individuals could be explained exclusively by differences in the local environment and level of muscular contractile activity. However, unidentified genetic factors would have to be invoked to account for the observation that the skeletal muscle of about 25% of the North American Caucasian population have either less than 35% or more than 65% of type I fibers.

Skeletal muscle utilization of free fatty acids in women with visceral obesity

Visceral obesity is strongly associated with insulin resistance. One potential cause is increased availability of FFA. Alternatively, it has been proposed that there is impaired oxidation of lipid in individuals at risk for obesity. The extent to which either concept involves skeletal muscle is uncertain. To examine these opposing hypotheses, 17 healthy lean and obese premenopausal women, among whom cross-sectional area of visceral fat ranged from 18 to 180 cm2, participated in leg balance studies for measurement of FFA and glucose utilization during basal and insulin-stimulated conditions. A metabolic profile of skeletal muscle, based on enzyme activity, was determined in vastus lateralis muscle obtained by percutaneous biopsy. Visceral fat content was negatively correlated with insulin sensitivity (rates of leg glucose uptake and storage), but insulin resistance was not caused by glucose-FFA competition. During hyperinsulinemia, neither leg FFA uptake nor oxidation was increased in women with visceral obesity. During fasting conditions, however, rates of FFA uptake across the leg were negatively correlated with visceral adiposity as were activities of muscle carnitine palmitoyl transferase and citrate synthase. In summary, visceral adiposity is clearly associated with skeletal muscle insulin resistance but this is not due to glucose-FFA substrate competition. Instead, women with visceral obesity have reduced postabsorptive FFA utilization by muscle.

Changes in plasma electrolytes and muscle substrates during short-term maximal exercise in humans

This study compared the effects of 3 short-term maximal exercise tests lasting 10, 30, and 90 sec upon blood volume, plasma electrolytes, glucose, glycerol, lactate and skeletal muscle ATP, PC, glycogen, and lactate concentrations. Seven sedentary male subjects were recruited and 5 of them were randomly assigned to each of the 3 protocols. The tests were performed on a modified ergocycle at workloads of 0.9, 0.075, and 0.05 kp.kg-1 body mass, respectively. Muscle biopsies were taken from the vastus lateralis before and immediately after each exercise. Venous blood samples were collected before, immediately after, and during the recovery (5, 20, 60, and 120 min). Plasma volume decreased during the 30- and 90-sec tests and was increased in all tests after 60 min of recovery. Plasma K+ increased during all tests and returned to normal values 5 min postexercise, except after the 90-sec test where it fell below resting values. Plasma Na+ and Cl- were unaffected. Blood lactate increased in all tests, glucose increased after the 90-sec test, and glycerol increased after the 30- and 90-sec tests. All 3 tests brought relatively similar changes in muscle ATP, PC, and glycogen while muscle lactate changes were related to exercise duration. These results suggest that a normal hydration status is important for a subject undergoing short training bouts, and that high-intensity tests of short duration do not require a special nutritional regimen to enhance glycogen reserves.

Skeletal muscle glycolytic and oxidative enzyme capacities are determinants of insulin sensitivity and muscle composition in obese women

Regional fat distribution is an important determinant of insulin resistance in obesity. In the current study, the relationship between skeletal muscle insulin sensitivity, mid-thigh muscle composition, and the metabolic profile of muscle was investigated. Muscle composition was assessed by computed tomography of the mid-thigh, and by activities of marker enzymes of aerobic-oxidative and glycolytic pathways and muscle fiber typing using biopsies of the vastus lateralis muscle. Muscle with reduced Hounsfield attenuation on computed tomography scans was increased in proportion to obesity, and was strongly related to insulin resistance, reduced muscle oxidative capacity, and increased anaerobic and glycolytic capacities by muscle. These findings suggest that as part of its expression of insulin resistance, skeletal muscle of obese individuals is also poorly equipped for substrate oxidation and manifests increased storage of fat.

Physiologic, metabolic, and muscle fiber type characteristics of musculus uvulae in sleep apnea hypopnea syndrome and in snorers

Upper airway dilator muscles play an important role in the pathophysiology of sleep apnea hypopnea syndrome (SAHS). The mechanical and structural characteristics of these muscles remain unknown. The aim of this study was to compare the physiologic, metabolic, and fiber type characteristics of one upper airway dilator muscle (musculus uvulae, MU) in 11 SAHS and in seven nonapneic snorers. The different analyses were done on MU obtained during uvulo-palato-pharyngoplasty. Snorers and SAHS differed only in their apnea + hypopnea indices (11.5 +/- 5.9 and 34.2 +/- 14.6/h, respectively, mean +/- SD). Absolute twitch and tetanic tension production of MU was significantly greater in SAHS than in snorers while the fatigability index was similar in the two groups. Protein content and anaerobic enzyme activities of MU were significantly greater in SAHS than in snorers; no difference was observed for aerobic enzyme activities. The total muscle fiber cross-sectional area of MU was significantly higher in SAHS (2.2 +/- 0.9 mm2) than in snorers (1.1 +/- 0.7 mm2). The surface occupied by type IIA muscle fibers of MU was larger in SAHS (2.00 +/- 0.96) than in snorers (0.84 +/- 0.63 mm2). We conclude that the capacity for tension production and the anaerobic metabolic activity of MU are greater in SAHS than in snorers.

Skeletal muscle metabolism and body fat content in men and women

The purpose of the present study was to verify the relationships between indicators of body fat content and specific characteristics of skeletal muscle in a large sample of men and women. Six skinfold thicknesses (sigma 6S) and maximal oxygen uptake (VO2 max) were measured in 348 Caucasian subjects (149 women and 199 men). Fiber type proportions (type I, type IIA, and type IIB) and activity levels of marker enzymes for the Krebs cycle (malate dehydrogenase, MDH) and for the fatty acid oxidation (3-hydroxyacyl CoA dehydrogenase, HADH) pathways were determined in vastus lateralis muscle samples. No significant correlation was found between fiber type proportions and sigma 6S. Significant and negative correlations were, however, obtained in both genders between the sigma 6S and MDH enzyme activity (r = -0.23; p < 0.01), but not between the sigma 6S and HADH enzyme activity. When individuals with low and high amount of subcutaneous fat but paired for VO2max were compared, vastus lateralis of fat men exhibited the same proportion of type I fiber (38.6 +/- 10.3 vs 38.5 +/- 13.4%) and HADH activity level (3.43 +/- 1.05 vs. 3.34 +/- 0.81 U/g), but had about 20% less MDH enzyme activity than vastus lateralis of leaner men (158 +/- 35 vs. 198 +/- 43 U/g;p < 0.05). No difference was found in any of these muscle phenotypes when comparisons were made between women with low and high amount of subcutaneous fat but also paired for VO2max. Moreover, no relations were observed between skeletal muscle fiber type proportion or metabolic markers with relative subcutaneous fat distribution.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired free fatty acid utilization by skeletal muscle in non-insulin-dependent diabetes mellitus

This study was undertaken to assess utilization of FFA by skeletal muscle in patients with non-insulin-dependent diabetes mellitus (NIDDM). 11 NIDDM and 9 nondiabetic subjects were studied using leg balance methods to measure the fractional extraction of [3H]oleate. Limb indirect calorimetry was used to estimate RQ. Percutaneous muscle biopsy samples of vastus lateralis were analyzed for muscle fiber type distribution, capillary density, and metabolic potential as reflected by measurements of the activity of seven muscle enzyme markers of glycolytic and aerobic-oxidative pathways. During postabsorptive conditions, fractional extraction of oleate across the leg was lower in NIDDM subjects (0.31 +/- 0.08 vs. 0.43 +/- 0.10, P < 0.01), and there was reduced oleate uptake across the leg (66 +/- 8 vs. 82 +/- 13 nmol/min, P < 0.01). Postabsorptive leg RQ was increased in NIDDM (0.85 +/- 0.03 vs. 0.77 +/- 0.02, P < 0.01), and rates of lipid oxidation by skeletal muscle were lower while glucose oxidation was increased (P < 0.05). In subjects with NIDDM, proportions of type I, IIa, and IIb fibers were 37 +/- 2, 37 +/- 6, and 26 +/- 5%, respectively, which did not differ from nondiabetics; and capillary density, glycolytic, and aerobic-oxidative potentials were similar. During 6 h after ingestion of a mixed meal, arterial FFA remained greater in NIDDM subjects. Therefore, despite persistent reduced fractional extraction of oleate across the leg in NIDDM (0.34 +/- 0.04 vs. 0.38 +/- 0.03, P < 0.05), rates of oleate uptake across the leg were greater in NIDDM (54 +/- 7 vs. 45 +/- 8 nmol/min, P < 0.01). In summary, during postabsorptive conditions there is reduced utilization of FFA by muscle, while during postprandial conditions there is impaired suppression of FFA uptake across the leg in NIDDM. During both fasting and postprandial conditions, NIDDM subjects have reduced rates of lipid oxidation by muscle.

Human skeletal muscle adaptation in response to chronic low-frequency electrical stimulation

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.

Impact of exercise intensity on body fatness and skeletal muscle metabolism

The impact of two different modes of training on body fatness and skeletal muscle metabolism was investigated in young adults who were subjected to either a 20-week endurance-training (ET) program (eight men and nine women) or a 15-week high-intensity intermittent-training (HIIT) program (five men and five women). The mean estimated total energy cost of the ET program was 120.4 MJ, whereas the corresponding value for the HIIT program was 57.9 MJ. Despite its lower energy cost, the HIIT program induced a more pronounced reduction in subcutaneous adiposity compared with the ET program. When corrected for the energy cost of training, the decrease in the sum of six subcutaneous skinfolds induced by the HIIT program was ninefold greater than by the ET program. Muscle biopsies obtained in the vastus lateralis before and after training showed that both training programs increased similarly the level of the citric acid cycle enzymatic marker. On the other hand, the activity of muscle glycolytic enzymes was increased by the HIIT program, whereas a decrease was observed following the ET program. The enhancing effect of training on muscle 3-hydroxyacyl coenzyme A dehydrogenase (HADH) enzyme activity, a marker of the activity of beta-oxidation, was significantly greater after the HIIT program. In conclusion, these results reinforce the notion that for a given level of energy expenditure, vigorous exercise favors negative energy and lipid balance to a greater extent than exercise of low to moderate intensity. Moreover, the metabolic adaptations taking place in the skeletal muscle in response to the HIIT program appear to favor the process of lipid oxidation.

Muscle fiber types of women after resistance training--quantitative ultrastructure and enzyme activity

Muscle biopsies of the vastus lateralis muscle taken before and after 18 weeks of resistance training were compared by preparing frozen cross sections for electron microscopy and using adjacent sections for fiber typing by myosin ATPase activity. Quantitative ultrastructural changes were observed in histochemically-identified muscle fiber types of twelve young women who underwent the training. The percentage of type IIB fibers decreased and IIA fibers increased. The cross-sectional area of all major fiber types increased with training. The absolute volume of myofibrils, intermyofibrillar space, and mitochondria increased with training for most major fiber types (type I, IIA and IIAB), but the relative volume percentages were not significantly changed because of corresponding fiber hypertrophy. Mean mitochondrial size for types I and IIA and myofibril size for types IIC and IIB increased significantly with training. The capillary number per fiber and density did not change with training. Activity levels were measured for selected glycolytic and oxidative enzymes. Cytochrome oxidase and hexokinase increased significantly with training, while creatine kinase, citrate synthase, phosphofructokinase, glyceraldehyde phosphate dehydrogenase and hydroxyacyl CoA dehydrogenase enzymes were not significantly altered. The results suggest that this type of high-repetition resistance training causes the intracellular components of all fiber types to increase proportionally with an increase in fiber size. In addition, the enzyme analysis indicates the muscle as a whole may increase its oxidative phosphorylation capacity in conjunction with the decreased percentage of type IIB fibers.

Interaction between glucose and free fatty acid metabolism in human skeletal muscle

The mechanism by which FFA metabolism inhibits intracellular insulin-mediated muscle glucose metabolism in normal humans is unknown. We used the leg balance technique with muscle biopsies to determine how experimental maintenance of FFA during hyperinsulinemia alters muscle glucose uptake, oxidation, glycolysis, storage, pyruvate dehydrogenase (PDH), or glycogen synthase (GS). 10 healthy volunteers had two euglycemic insulin clamp experiments. On one occasion, FFA were maintained by lipid emulsion infusion; on the other, FFA were allowed to fall. Leg FFA uptake was monitored with [9,10-3H]-palmitate. Maintenance of FFA during hyperinsulinemia decreased muscle glucose uptake (1.57 +/- 0.31 vs 2.44 +/- 0.39 mumol/min per 100 ml tissue, P < 0.01), leg respiratory quotient (0.86 +/- 0.02 vs 0.93 +/- 0.02, P < 0.05), contribution of glucose to leg oxygen consumption (53 +/- 6 vs 76 +/- 8%, P < 0.05), and PDH activity (0.328 +/- 0.053 vs 0.662 +/- 0.176 nmol/min per mg, P < 0.05). Leg lactate balance was increased. The greatest effect of FFA replacement was reduced muscle glucose storage (0.36 +/- 0.20 vs 1.24 +/- 0.25 mumol/min per 100 ml, P < 0.01), accompanied by decreased GS fractional velocity (0.129 +/- 0.26 vs 0.169 +/- 0.033, P < 0.01). These results confirm in human skeletal muscle the existence of competition between glucose and FFA as oxidative fuels, mediated by suppression of PDH. Maintenance of FFA levels during hyperinsulinemia most strikingly inhibited leg muscle glucose storage, accompanied by decreased GS activity.

Asynchronous increases in oxidative capacity and resistance to fatigue of electrostimulated muscles of rat and rabbit

1. The present study investigates to what extent increases in resistance to fatigue and aerobic oxidative capacity of energy metabolism are correlated in fast-twitch tibialis anterior muscles of rat and rabbit subjected to chronic low-frequency stimulation. 2. Changes in the aerobic oxidative capacity of the stimulated muscles were judged from increases in citrate synthase activity, representing the constant-proportion enzyme group of the citric acid cycle. 3. Resistance to fatigue reached maximal values in both rat and rabbit tibialis anterior muscles after stimulation periods of 14 days, whereas citrate synthase activity continued to increase with longer stimulation periods. 4. Different time courses of the changes in resistance to fatigue and citrate synthase activity were observed not only with prolonged stimulation periods but also during the first week, when pronounced increases in resistance to fatigue were accompanied by only moderate elevations in citrate synthase activity. 5. The dissociation between the changes of the two parameters studied suggests that factors other than elevated aerobic oxidative capacity contribute to enhanced resistance to fatigue.

Electrical stimulation-induced changes in skeletal muscle enzymes of men and women

The purpose of the present study was to determine the effects of low-frequency electrical stimulation (LFES) on the skeletal muscle metabolic profile of men and women. The knee extensor muscles of sedentary men (N = 16) and women (N = 10) were submitted to 3 h.d-1 of 8-Hz neuromuscular electrical stimulation with the use of a portable stimulator (Respond II, Medtronic), 6 d.wk-1 for 6 wk. Enzyme activity levels of creatine kinase (CK), hexokinase (HK), glyceraldehydephosphate dehydrogenase (GAPDH), 3-hydroxyacyl CoA dehydrogenase (HADH), citrate synthase (CS), phosphofructokinase (PFK), and cytochrome c oxidase (COX) were determined in vastus lateralis muscle samples taken before and after the LFES protocol. The analyses of variance revealed no change in CK and in GAPDH. However, a small decrease in PFK activity, the rate-limiting enzyme of glycolysis, was observed in female (8%) and in male subjects (10%), but it reached significance in males only (P < 0.05). The activity level of HK, a regulatory enzyme of the skeletal muscle glucose phosphorylation (HK), increased significantly in female subjects only (36%; P < 0.01) in response to the stimulation protocol. Activity level of marker enzymes of the Krebs cycle (CS) and of the electron-transfert chain (COX) significantly increased in males (18% and 16%; P < 0.05) as well as in females (31% and 19%; P < 0.05). Increment in the marker enzyme activity of the fatty acid oxidation (HADH) was significant in female subjects (30%; P < 0.01) and, although significant, rather modest in male subjects (12%; P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of thyroid status on the expression of metabolic enzymes during chronic stimulation

The effect of thyroid status on the expression of cytochrome c oxidase (CYTOX) and the activities of citrate synthase (CS) and phosphofructokinase (PFK) were examined in chronically stimulated (10 Hz; 35 days) and contralateral, nonstimulated rat tibialis anterior muscle of hypothyroid, hyperthyroid, and euthyroid animals. Stimulation increased CYTOX activity by 2.7-, 3.2-, and 4.9-fold in hyperthyroid, euthyroid, and hypothyroid animals, respectively, to similar absolute values. CS displayed similar increases. Stimulation reduced PFK activity in hypothyroid and euthyroid animals to 45% and 60% of control values. This effect was abolished with hyperthyroidism. Thus stimulation and thyroid hormone act antagonistically on PFK activity. Stimulation increased CYTOX subunit III (mitochondrially encoded) mRNA by 2.5- and 2.9-fold in hyperthyroid and euthyroid animals. Similar increases were observed in the nuclear-encoded mRNAs of CYTOX subunit VIc in euthyroid muscle. In hyperthyroid and euthyroid conditions, the mRNA changes paralleled the increases in enzyme activity. In hypothyroid muscle, the increase in mRNA was less for subunit VIc than III, suggesting that hypothyroidism upsets the coordinate expression of nuclear and mitochondrial genes. Further, the increases in CYTOX activity exceeded that of both subunit mRNAs in hypothyroid muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Aerobic and anaerobic energy contribution during maximal work output in 90 s determined with various ergocycle workloads

The aim of this experiment was to evaluate the effect of different workloads on the relative contribution of the various energy delivery systems during a 90-s ergocycle test. Nine male subjects, 22 +/- 1 (mean +/- SD) years of age and weighing 71.4 +/- 6.8 kg, were submitted to a VO2max test, as 10-s test (0.1 kp/kg) and three 90-s tests at different loads (LO: 0.05, ME: 0.075 and HI: 0.1 kp/kg) on an ergocycle. No difference was found between peak power output during the 10-s and HI tests. No differences were observed in the total work output performed during 90 s at different workloads (between 481 and 495 J/kg) as well as in the contribution of aerobic and anaerobic pathways to total energy production. However, VO2max was reached earlier during the ME and HI tests than during the LO test. These results indicate that variation in workload did not influence the total work output and the total contribution of aerobic and anaerobic systems during maximal 90-s ergocycle performances. However, variation in workload had an impact on the relative aerobic and anaerobic contribution at various time points. It is concluded that a 90-s ergocycle test with a resistance of at least 0.1 kp/kg is required to appropriately assess maximal anaerobic power while anaerobic capacity might be measured with workloads as low as 0.05 kp/kg.

Maximal anaerobic performance of the knee extensor muscles during growth

The extent of the growth changes in maximal work output during 10 s (MWO10), 30 s (MWO30), and 90 s (MWO90) of maximal repetitive knee flexions and extensions assessed on a modified Hydra-Gym machine was investigated in 84 boys and 83 girls, 9-19 yr of age. Body weight, fat mass and fat free mass by underwater weighing, and thigh volume and cross-sectional area were also determined. No difference was observed in the absolute MWO10, MWO30, and MWO90 between girls and boys at 9 and 11 yr of age. However, significant differences appeared between genders from 13 yr of age onward, anaerobic performances of the knee extensor muscles of girls representing about 75% or even less of those of boys. The analysis of variance revealed that maximal work ouput during the three knee extension tests was significantly greater in males as well as in females from 9 to 18 yr, regardless how performance was related to morphological characteristics. Performance in absolute values or expressed per unit of body weight, fat free mass, and thigh cross-sectional area for the MWO10, MWO30, and MWO90 tests were almost always significantly lower in both genders when performances of the 9-yr-old group were compared with those of the 13-yr-old group or older groups. Improvement in maximal work output during the 10-s, 30-s, or 90-s knee extension tests with age occurred mainly between 9 and 15 yr in both genders. The results of the present study show that there are gender differences in predominantly anaerobic performances during growth and reveal that increase in muscle mass does not appear to be the only factor responsible for the age-related increment in the anaerobic working capacity of the knee extensor muscles.

Enzyme activities of fatty acid oxidation and the respiratory chain in chronically stimulated fast-twitch muscle of the rabbit

Fast-twitch tibialis anterior muscle of the rabbit was subjected to chronic low-frequency (10 Hz, 10 h/day) stimulation for different time periods up to 28 days. Total cellular activities of carnitine:palmitoyl-CoA transferase, crotonase, 3-hydroxyacyl-CoA dehydrogenase, 3-keto-acyl-CoA thiolase, citrate synthase, NADH:cytochrome c oxidoreductase, succinate: cytochrome c oxidoreductase, and cytochrome c oxidase were measured in contralateral and stimulated muscles at various times. With the exception of crotonase, which increased only 1.6-fold after 28 days of stimulation, the other enzymes increased in parallel displaying 3-fold elevated absolute activities. These results, by supporting and extending our previous findings, indicate that the expression of the enzymes of the main metabolic systems of aerobic substrate oxidation, i.e. the citric acid cycle, the fatty acid oxidation and the respiratory chain, is regulated in a coordinate manner.

Anaerobic performances in black and white subjects

The anaerobic performance characteristics of 15 Black males of African ancestry (25 +/- 2 yr; mean +/- SD) and 17 White males of French Canadian ancestry (22 +/- 2 yr) were compared. All subjects were sedentary. Morphological characteristics and body composition were similar in both groups. They were tested for maximal force during voluntary isometric contraction of the knee extensors and for total work output during 10 s, 30 s, and 90 s of maximal and repetitive knee extensions. Results indicated no significant differences between Blacks and Whites in maximal force of the knee extensors (736 +/- 78 N vs 722 +/- 11 N, respectively) and in total work output during the 10-s (1134 +/- 246 J vs 1124 +/- 207 J) and 30-s (2735 +/- 519 J vs 2779 +/- 647 J) tests. There was a difference of about 400 J between Blacks and Whites in the total work output during the 90-s test, but this difference was not statistically significant. However, significant differences were found between Blacks and Whites in the peak power output decrement during the last 60 s of the 90-s anaerobic test. These differences in peak power output between the two groups ranged between 7 and 10 W during the last 30 s of the 90-s test. The results indicate that knee extensor muscles of sedentary Black and White males have similar anaerobic performance power and capacities. However, the results reveal that sedentary Black subjects experience a greater degree of fatigue than sedentary Whites during an anaerobic exercise lasting longer than 30 s.

Effect of chronic hypoxia on muscle enzyme activities

Biopsies from the vastus lateralis muscle of seven participants in the Swiss expedition to Mt. Everest and Lhotse in 1986 were taken before departure to and after return from high altitude, and used for measurements of maximal activities of 12 reference enzymes of anaerobic and aerobic-oxidative metabolic pathways. The results indicated that strenuous exercise at high altitude induced increases in enzyme activities of glycolysis and decreases in enzyme activities of terminal substrate oxidation (the citric acid cycle, fatty acid oxidation, ketone body utilization, respiratory chain). The decreases in enzyme activities of aerobic-oxidative metabolism were related to similar decrements in mitochondrial volume density, which suggests that the enzymic changes resulted from a loss of mitochondrial structure rather than from qualitative changes of the mitochondrial population. These changes indicated that strenuous exercise may intensify the stress of high-altitude exposure and, thus, induce an aerobic to anaerobic shift of muscle energy metabolism.

Human variation in skeletal muscle fiber-type proportion and enzyme activities

The purpose of the present study was to describe the extent of the variation in some of the common characteristics of human skeletal muscle. A total of 418 biopsies was obtained from the vastus lateralis muscle of 270 healthy sedentary and 148 physically active individuals of both sexes. The lowest and highest proportion of type I muscle fiber observed were 15 and 85%, respectively. Coefficients of variation (CV) reached approximately 30% for the proportion of types I and IIA fibers and were two times higher for the proportion of type IIB fiber. The smallest and largest mean muscle fiber cross-sectional areas (CSA) were approximately 1,100 microns 2 and 9,500 microns 2, respectively. Mean CSA of the various fiber types exhibited CV of approximately 23%. CV reached 30% for the activity of creatine kinase, ranged between 28 and 41% for the glycolytic enzyme markers, and between 34 and 44% for the aerobic-oxidative enzyme markers. The mean proportion of type I fiber was lower in male than in female muscles, whereas the mean CSA of all fiber types was smaller in female than in male muscles. Levels of glycolytic enzyme markers were higher in male than in female skeletal muscles. However, activities of aerobic-oxidative enzyme markers were similar in males and females. These results reveal the existence of large interindividual variability and gender differences in the most common characteristics of the human skeletal muscle.

Relations between chronic stimulation-induced changes in contractile properties and the Ca2+-sequestering system of rat and rabbit fast-twitch muscles

This study compares changes in contractile properties, parvalbumin content, and Ca2+-uptake by the sarcoplasmic reticulum (SR) of low-frequency stimulated rat and rabbit tibialis anterior (TA) muscles. Time to peak tension increased 1.8-fold in 35-day stimulated rabbit TA, while no change occurred in rat TA. Isometric twitch tension increased 2-fold in rabbit TA, but was unaltered in rat TA. Parvalbumin (PA) content was more than 90% reduced in rabbit TA, but only 60% in rat TA after 35 days. At this time, PA content of the stimulated rat TA was still higher than that of normal rabbit TA. Taking into account the suggested role of PA as a cytosolic Ca2+ buffer, its decrease could lead to an impaired free Ca2+-decay with a prolonged active state and a higher tension output during a single twitch. This would explain why chronic stimulation led to an increase in isometric twitch tension in rabbit TA, but not in rat TA. The 1.6-fold rise in half-relaxation time of 35-day stimulated rat and rabbit TA most likely resulted from a 50% reduced Ca2+-uptake by the SR, due to a still unknown modification of the Ca2+-transport ATPase.

Rapid isolation of total RNA from small mammal and human skeletal muscle

The present study was designed to assess the efficacy and reliability of a method (Anal. Biochem. 162: 156-59, 1987) of total RNA isolation from small muscle samples. The RNA content of skeletal muscles in mouse, rabbit, guinea pig, rat, and human was also investigated. Enough RNA was extractable from muscle samples as small as 25 mg in weight to provide material for the analysis of specific mRNA sequences using RNA blot hybridization with a cDNA probe for a mitochondrially encoded subunit of cytochrome-c oxidase. The RNA was free of detectable levels of protein or DNA. Gel electrophoresis and cDNA probe hybridization indicated that the RNA was relatively undegraded. Significant differences (P less than 0.05) in RNA content were found among fiber types (type I fibers greater than type II fibers) and among the soleus muscles of various species (guinea pig greater than rat = mouse greater than rabbit). Human muscle possessed the lowest RNA content (350 ng/mg) among the species investigated. The results indicate that intact RNA can be rapidly and reliably isolated from small skeletal muscle, including samples typically obtained from human muscle biopsies. This technique should permit the analysis of gene expression in muscle subject to a variety of physiological treatments.

Species-specific responses of muscle lactate dehydrogenase isozymes to increased contractile activity

Analyses of lactate dehydrogenase (LDH) isozymes were performed in normal and chronically stimulated (10 Hz, 10 h/d) tibialis anterior (TA) muscles of the mouse, rat, guinea pig, and rabbit. The electrophoretic mobilities of the individual LDH isozymes of normal muscles revealed species-specific patterns. Stimulation up to 28 days evoked increases in the percentage of the H-subunit of LDH. The absolute increase was highest in the rabbit, intermediate in the guinea pig and rat, and lowest in the mouse. The extent of the M- to H-subunit exchange, thus, appeared to be inversely correlated with the basal aerobic-oxidative capacities of the investigated muscles. In addition, a relationship between the expression of the H-subunit of LDH and the increase in aerobic-oxidative capacity became evident from these stimulation-induced processes of metabolic adaptation.

Electrostimulation-induced increases in fatty acid-binding protein and myoglobin in rat fast-twitch muscle and comparison with tissue levels in heart

Chronic stimulation of rat fast-twitch muscle increased the content of both fatty acid-binding protein (FABP) and myoglobin. The increases in FABP, which reached values close to that of cardiac muscle, exceeded those in myoglobin and those in citrate synthase and 3-hydroxyacyl-CoA dehydrogenase activities.