Exercise training in normobaric hypoxia in endurance runners. I. Improvement in aerobic performance capacity.

This study investigates whether a 6-wk intermittent hypoxia training (IHT), designed to avoid reductions in training loads and intensities, improves the endurance performance capacity of competitive distance runners. Eighteen athletes were randomly assigned to train in normoxia [Nor group; n = 9; maximal oxygen uptake (VO2 max) = 61.5 +/- 1.1 ml x kg(-1) x min(-1)] or intermittently in hypoxia (Hyp group; n = 9; VO2 max = 64.2 +/- 1.2 ml x kg(-1) x min(-1)). Into their usual normoxic training schedule, athletes included two weekly high-intensity (second ventilatory threshold) and moderate-duration (24-40 min) training sessions, performed either in normoxia [inspired O2 fraction (FiO2) = 20.9%] or in normobaric hypoxia (FiO2) = 14.5%). Before and after training, all athletes realized 1) a normoxic and hypoxic incremental test to determine VO2 max and ventilatory thresholds (first and second ventilatory threshold), and 2) an all-out test at the pretraining minimal velocity eliciting VO2 max to determine their time to exhaustion (T(lim)) and the parameters of O2 uptake (VO2) kinetics. Only the Hyp group significantly improved VO2 max (+5% at both FiO2, P < 0.05), without changes in blood O2-carrying capacity. Moreover, T(lim) lengthened in the Hyp group only (+35%, P < 0.001), without significant modifications of VO2 kinetics. Despite similar training load, the Nor group displayed no such improvements, with unchanged VO2 max (+1%, nonsignificant), T(lim) (+10%, nonsignificant), and VO2 kinetics. In addition, T(lim) improvements in the Hyp group were not correlated with concomitant modifications of other parameters, including VO2 max or VO2 kinetics. The present IHT model, involving specific high-intensity and moderate-duration hypoxic sessions, may potentialize the metabolic stimuli of training in already trained athletes and elicit peripheral muscle adaptations, resulting in increased endurance performance capacity.     

Exercise endurance and arterial desaturation in normobaric hypoxia with increased chemosensitivity

We studied whether exercise endurance under normobaric hypoxia can be enhanced by increasing hypoxic ventilatory sensitivity with almitrine bismesylate (ALM). On both ALM and placebo (PL) days, resting subjects breathed a hypoxic gas mixture (an inspired O2 fraction of 10.4-13.2%), which lowered resting arterial O2 saturation (SaO2) to 80%. After 15 min of rest there was a 3-min warm-up period of exercise at 50 W (light) on a cycle ergometer, followed by a step increase in load to 60% of the previously determined maximum power output with room-air breathing (moderate), which was maintained until exhaustion. With PL, SaO2 decreased rapidly with the onset of exercise and continued to fall slowly during moderate exercise, averaging 71.0 +/- 1.8% (SE) at exhaustion. With ALM, saturation did not differ from PL during air breathing but significantly exceeded SaO2 with PL, by 3.4% during resting hypoxia, by 4.0% at the start of exercise, and by 5.9% at exhaustion. Ventilation was not affected by ALM during air breathing and was slightly, although not significantly, increased during hypoxic rest and exercise. ALM was associated with an increased heart rate during room air breathing but not during hypoxia. Endurance time was 20.6 +/- 1.6 min with ALM and 21.3 +/- 0.9 min with PL. During hypoxic exercise, the potential benefit of greater saturation with ALM is apparently offset by other unidentified factors.     

Downregulation of Na+-K+-ATPase pumps in skeletal muscle with training in normobaric hypoxia.

To investigate the effects of training in normoxia vs. training in normobaric hypoxia (fraction of inspired O2 = 20.9 vs. 13.5%, respectively) on the regulation of Na+-K+-ATPase pump concentration in skeletal muscle (vastus lateralis), 9 untrained men, ranging in age from 19 to 25 yr, underwent 8 wk of cycle training. The training consisted of both prolonged and intermittent single leg exercise for both normoxia (N) and hypoxia (H) during a single session (a similar work output for each leg) and was performed 3 times/wk. Na+-K+-ATPase concentration was 326 +/- 17 (SE) pmol/g wet wt before training (Control), increased by 14% with N (371 +/- 18 pmol/g wet wt; P < 0.05), and decreased by 14% with H (282 +/- 20 pmol/g wet wt; P < 0.05). The maximal activity of citrate synthase, selected as a measure of mitochondrial potential, showed greater increases (P < 0.05) with H (1.22 +/- 0.10 mmol x h-1 x g wet wt-1; 70%; P < 0.05) than with N (0.99 +/- 0.10 mmol x h-1 x g wet wt-1; 51%; P < 0.05) compared with pretraining (0.658 +/- 0.09 mmol x h-1 x g wet wt-1). These results demonstrate that normobaric hypoxia induced during exercise training represents a potent stimulus for the upregulation in mitochondrial potential while at the same time promoting a downregulation in Na+-K+-ATPase pump expression. In contrast, normoxic training stimulates increases in both mitochondrial potential and Na+-K+-ATPase concentration.     

Effects of endurance training on a mitochondrial reticulum in limb skeletal muscle

High voltage electron microscopy at 1500 kV, was used to examine the effects of endurance training on mitochondrial morphology in rat skeletal muscle. The soleus, deep portions of the vastus lateralis, and superficial portions of the vastus lateralis muscles were examined to represent slow-twitch-oxidative, fast-twitch-oxidative-glycolytic, and fast-twitch-glycolytic skeletal muscle fiber types, respectively. Muscle samples were removed from endurance trained and untrained control female Wistar rats (n = 6, each group). Tissues were fixed using standard electron microscopic techniques and sectioned transversely with respect to muscle fiber orientation to approximately, 0.5 micron thickness. The sections were stained on grids with uranyl acetate and Reynolds' lead citrate. Results confirmed the presence of a mitochondrial reticulum in all three skeletal muscle fiber types of both groups. Stereologic analyses indicated volume densities of intermyofibrillar mitochondria increased significantly (P less than 0.05) with endurance training in the three skeletal muscle fiber types. Surface-to-volume ratio of mitochondria was significantly decreased (P less than 0.05) after training only in the deep portion of the vastus lateralis muscle. It was concluded that the mitochondria in mammalian limb skeletal muscle are a reticulum which adapts to endurance training by proliferating.     

Adaptation of mitochondrial ATP production in human skeletal muscle to endurance training and detraining.

The adaptation of mitochondrial ATP production rate (MAPR) to training and detraining was evaluated in nine healthy men. Muscle samples (approximately 60 mg) were obtained before and after 6 wk of endurance training and after 3 wk of detraining. MAPR was measured in isolated mitochondria by a bioluminometric method. In addition, the activities of mitochondrial and glycolytic enzymes were determined in skeletal muscle. In response to training, MAPR increased by 70%, with a substrate combination of pyruvate + palmitoyl-L-carnitine + alpha-ketoglutarate + malate, by 50% with only pyruvate + malate, and by 92% with palmitoyl-L-carnitine + malate. With detraining MAPR decreased by 12-28% from the posttraining rate (although not significantly for all substrates). No differences were found when MAPR was related to the protein content in the mitochondrial fraction. The largest increase in mitochondrial enzyme activities induced by training was observed for cytochrome-c oxidase (78%), whereas succinate cytochrome c reductase showed only an 18% increase. The activity of citrate synthase increased by 40% and of glutamate dehydrogenase by 45%. Corresponding changes in maximal O2 uptake were a 9.6% increase by training and a 6.0% reversion after detraining. In conclusion, both MAPR and mitochondrial enzyme activities are shown to increase with endurance training and to decrease with detraining.     

Mitochondrial enzymatic adaptation of skeletal muscle to endurance training.

Some mitochondrial enzymatic activities (succinate dehydrogenase, NADH cytochrome reductase, cytochrome oxidase) were studied in the gastrocnemius and soleus muscle of the rat. The modifications of the enzyme activity, induced by endurance training, were found to be functions of 1) daily work load and 2) total training time. The treatment with an effective dose of vasodilating substances (papaverine, nicergoline, dipyridamole, and bamethan) showed that 1) nicergoline, bamethan, and dipyridamole were differently able to shorten the time of appearance of the increase in the enzymatic activities; 2) however, long-term treatments with these drugs did not prove able to modify the plateau level of the enzymatic activity increase, for a given amount of endurance training; 3) the pharmacodynamic effect on enzymatic activities was in no way related to the vasodilating effect of these drugs, since the effect was not observed with papaverine. The transition from a given level of endurance training to a lower one led to a proportional decrease of the mitochondrial enzymatic activities, thus pointing out the relation between amount of training and enzymatic activity. The drugs studied were unable to modify the decrease of enzymatic activity induced by lower work load.     

Adaptive response of hypertrophied skeletal muscle to endurance training

The response of hypertrophied soleus and plantaris muscle of rats to endurance training was studied. Hypertrophy was produced by bilateral extirpation of the gastrocnemius muscle. A 13-wk training program of treadmill running initiated 30 days after removal of the gastrocnemius muscle accentuated (P less than 0.01) the hypertrophy. Succinate dehydrogenase activities of the enlarged muscles of sedentary rats were similar to those of normal animals, as were the increases associated with training. Phosphorylase and hexokinase activities were unaltered as a result of the experimental perturbations. Rates of glycogen depletion during exercise were lower (P less than 0.01) in the liver and soleus and plantaris muscles of endurance-trained animals. No difference existed in the rate of glycogen depletion of normal and hypertrophied muscle within the sedentary or trained groups. These data demonstrate that extensively hypertrophied muscle responds to training and exercise in a manner similar to that of normal muscle.     

Nonuniform effects of endurance exercise training on vasodilation in rat skeletal muscle.

Endurance exercise training (Ex) has been shown to increase maximal skeletal muscle blood flow. The purpose of this study was to test the hypothesis that increased endothelium-dependent vasodilation is associated with the Ex-induced increase in muscle blood flow. Furthermore, we hypothesized that enhanced endothelium-dependent dilation is confined to vessels in high-oxidative muscles that are recruited during Ex. To test these hypotheses, sedentary (Sed) and rats that underwent Ex (30 m/min x 10% grade, 60 min/day, 5 days/wk, 8-12 wk) were studied using three experimental approaches. Training effectiveness was evidenced by increased citrate synthase activity in soleus and vastus lateralis (red section) muscles (P < 0.05). Vasodilatory responses to the endothelium-dependent agent acetylcholine (ACh) in situ tended to be augmented by training in the red section of gastrocnemius muscle (RG; Sed: control, 0.69 +/- 0.12; ACh, 1.25 +/- 0.15; Ex: control, 0.86 +/- 0.17; ACh, 1.76 +/- 0.27 ml x min(-1) x 100 g(-1) x mmHg(-1); 0.05 < P < 0.10 for Ex vs. Sed during ACh). Responses to ACh in situ did not differ between Sed and Ex for either the soleus muscle or white section of gastrocnemius muscle (WG). Dilatory responses of second-order arterioles from the RG in vitro to flow (4-8 microl/min) and sodium nitroprusside (SNP; 10(-7) through 10(-4) M), but not ACh, were augmented in Ex (vs. Sed; P < 0.05). Dilatory responses to ACh, flow, and SNP of arterioles from soleus and WG muscles did not differ between Sed and Ex. Content of the endothelial isoform of nitric oxide synthase (eNOS) was increased in second-order, fourth-order, and fifth-order arterioles from the RG of Ex; eNOS content was similar between Sed and Ex in vessels from the soleus and WG muscles. These findings indicate that Ex induces endothelial adaptations in fast-twitch, oxidative, glycolytic skeletal muscle. These adaptations may contribute to enhanced skeletal muscle blood flow in endurance-trained individuals.     

Do skeletal muscle phenotypic characteristics of Xhosa and Caucasian endurance runners differ when matched for training and racing distances?

Although East African black athletes dominate endurance running events, it is unknown whether black and white endurance runners with similar racing ability, matched for training, may differ in their skeletal muscle biochemical phenotype. Thirteen Xhosa (XR) and 13 Caucasian (CR) endurance runners were recruited and matched for 10-km performance, average preferred racing distance (PRD(A)), and training volume. Submaximal and maximal exercise tests were done, and vastus lateralis muscle biopsies were taken. XR were significantly lighter and shorter than CR athletes but had similar maximum oxygen consumption corrected for body weight and peak treadmill speed (PTS). XR had lower plasma lactate concentrations at 80% PTS (P < 0.05) compared with CR. Also, XR had more type IIA (42.4 +/- 9.2 vs. 31.3 +/- 11.5%, P < 0.05) and less type I fibers (47.8 +/- 10.9 vs. 63.1 +/- 13.2%, P < 0.05), although oxidative enzyme activities did not differ. Furthermore, XR compared with CR had higher lactate dehydrogenase (LDH) activity in homogenate muscle samples (383 +/- 99 vs. 229 +/- 85 mumol.min(-1).g dry weight(-1), P < 0.05) and in both type IIa (P < 0.05) and type I (P = 0.05) single-fiber pools. A marked difference (P < 0.05) in the composition of LDH isoform content was found between the two groups with XR having higher levels of LDH(5-4) isoforms (skeletal muscle isozymes; LDH-M) than CR, which was not accounted for by fiber-type differences alone. These results confirm differences in muscle phenotype and physiological characteristics, particularly associated with high-intensity running.     

Changes in skeletal muscle in males and females following endurance training

Gender differences in substrate selection have been reported during endurance exercise. To date, no studies have looked at muscle enzyme adaptations following endurance exercise training in both genders. We investigated the effect of a 7-week endurance exercise training program on the activity of beta-oxidation, tricarboxylic acid cycle and electron transport chain enzymes, and fiber type distribution in males and females. Training resulted in an increase in VO2peak, for both males and females of 17% and 22%, respectively (P < 0.001). The following muscle enzyme activities increased similarly in both genders: 3-beta-hydroxyacyl CoA dehydrogenase (38%), citrate synthase (41%), succinate-cytochrome c oxidoreductase (41%), and cytochrome c oxidase (COX; 26%). The increase in COX activity was correlated (R2 = 0.52, P < 0.05) with the increase in VO2peak/fat free mass. Fiber area, size, and % area were not affected by training for either gender, however, males had larger Type II fibers (P < 0.05) and females had a greater Type I fiber % area (P < 0.05). Endurance training resulted in similar increases in skeletal muscle oxidative potential for both males and females. Training did not affect fiber type distribution or size in either gender.     

Plasma leptin levels of elite endurance runners after heavy endurance training

A decrease in testosterone levels and an increase in cortisol levels are observed in male athletes with the overtraining syndrome (OTS). Cortisol causes blood leptin levels to rise and testosterone has an inverse relationship with blood leptin levels. Therefore, we hypothesized that the hormonal changes as a result of OTS induce an increase in leptin. To test this hypothesis, we examined the relationship among changes in leptin, testosterone and cortisol in thirteen male collegiate distance runners (aged 20.3+/-1.1 years) before and after an 8-day strenuous training camp. Runners ran 284.1+/-48.2 km during the training camp. Body fat percentages and plasma glucose concentrations decreased significantly after the training. Non-ester fatty acids and total cholesterol concentrations in blood were unchanged. Serum cortisol concentrations showed a significant increase after the training camp (from 11.82+/-2.00 microg/dl to 16.78+/-3.99 microg/dl), and serum testosterone decreased significantly (from 408.0+/-127.6 ng/dl to 265.2+/-97.6 ng/dl). The ratio of testosterone to cortisol (TCR) dropped by 50% after training (from 35.62+/-13.69 to 16.94+/-8.47). These results suggest that the subjects reached a state of the OTS. Contrary to our hypothesis, plasma leptin was not significantly changed (from 1.34+/-0.29 ng/ml to 1.49+/-0.18 ng/ml). Delta Plasma leptin was not significantly correlated with delta serum cortisol, delta TCR or delta fat percentage. However, delta serum testosterone was positively correlated with delta plasma leptin (r=596, p<0.05). Plasma leptin concentrations might modulate the secretion of testosterone in overtraining conditions. In conclusion, the change in blood leptin level is independent of the changes in cortisol, TCR and fat percentage in highly trained male athletes in the state of the OTS.     

Molecular adaptations in human skeletal muscle to endurance training under simulated hypoxic conditions.

This study was performed to explore changes in gene expression as a consequence of exercise training at two levels of intensity under normoxic and normobaric hypoxic conditions (corresponding to an altitude of 3,850 m). Four groups of human subjects trained five times a week for a total of 6 wk on a bicycle ergometer. Muscle biopsies were taken, and performance tests were carried out before and after the training period. Similar increases in maximal O(2) uptake (8.3-13.1%) and maximal power output (11.4-20.8%) were found in all groups. RT-PCR revealed elevated mRNA concentrations of the alpha-subunit of hypoxia-inducible factor 1 (HIF-1) after both high- (+82.4%) and low (+78.4%)-intensity training under hypoxic conditions. The mRNA of HIF-1alpha(736), a splice variant of HIF-1alpha newly detected in human skeletal muscle, was shown to be changed in a similar pattern as HIF-1alpha. Increased mRNA contents of myoglobin (+72.2%) and vascular endothelial growth factor (+52.4%) were evoked only after high-intensity training in hypoxia. Augmented mRNA levels of oxidative enzymes, phosphofructokinase, and heat shock protein 70 were found after high-intensity training under both hypoxic and normoxic conditions. Our findings suggest that HIF-1 is specifically involved in the regulation of muscle adaptations after hypoxia training. Fine-tuning of the training response is recognized at the molecular level, and with less sensitivity also at the structural level, but not at global functional responses like maximal O(2) uptake or maximal power output.     

Lactate removal is not enhanced in nonstimulated perfused skeletal muscle after endurance training.

The effects of endurance training (running 40 m/min grade for 60 min, 5 days/wk for 8 wk) on skeletal muscle lactate removal was studied in rats by utilizing the isolated hindlimb perfusion technique. Hindlimbs were perfused (single-pass) with Krebs-Henseleit bicarbonate buffer, fresh bovine erythrocytes (hematocrit approximately 30%), 10 mM lactate, and [U-14C]lactate (30,000 dpm/ml). Arterial and venous blood samples were collected every 10 min for the duration of the experiment to assess lactate uptake. During perfusions, no significant differences in skeletal muscle lactate uptake were observed between trained (7.31 +/- 0.20 micromol/min) and control hindlimbs (6.98 +/- 0.43 micromol/min). In support, no significant differences were observed for [14C]lactate uptake in trained (22,776 +/- 370 dpm/min) compared with control hindlimbs (21,924 +/- 1,373 dpm/min). Concomitant with these observations, no significant differences were observed between groups for oxygen consumption (4.93 +/- 0.18 vs. 4.92 +/- 0.13 micromol/min), net skeletal muscle glycogen synthesis (7.1 +/- 0.4 vs. 6.5 +/- 0.3 micromol x 40 min(-1) x g(-1)), or 14CO2 production (2,203 +/- 185 vs. 2,098 +/- 155 dpm/min), trained and control, respectively. These findings indicate that endurance training does not affect lactate uptake or alter the metabolic fate of lactate in quiescent skeletal muscle.     

Extreme endurance training: evidence of capillary and mitochondria compartmentalization in human skeletal muscle.

Biopsies from the medial gastrocnemius muscle of three experienced endurance runners who had completed an ultramarathon run (160 km) the previous day were assessed for their oxidative characteristics (fibre types, capillarization and mitochondria content). Also, a regional comparison was made for fibres located centrally (completely surrounded by other fibres) versus fibres located peripherally (next to the interfascicular space) and the capillarization of these peripheral fibres was determined. Subsarcolemmal mitochondria were abundant and 'compartmentalized' close to the capillaries. The number of capillaries around fibres ranged from 5.8 to 8.5 and 5.7 to 8.5, and the number of capillaries.mm-2 ranged from 665 to 810 and 727 to 762, for type I (slow twitch) and type II (fast twitch) fibres, respectively. Central fibres contained a greater number of capillaries and more capillaries.mm-2 than their peripheral counterparts. Peripheral fibres contained more capillaries.micron-1 between fibres than at the interfascicular space. Type I fibres were more distributed (63%-78%) and larger than type II fibres. An abundance of subsarcolemmal mitochondria located close to the capillaries, efficient capillary proliferation between fibres where sharing can occur and greater relative distribution and size of type I fibres are, collectively, efficient characteristics of extreme endurance training.     

Dynamic exercise training in foxhounds. II. Analysis of skeletal muscle

The purpose of this study was to determine whether 8-12 wk of endurance training produces biochemical and histochemical adaptations in skeletal muscle in foxhounds. Analyses were performed on samples removed from gastrocnemius, triceps, and semitendinosus muscles of foxhounds before and after a treadmill running program. Biochemical analysis showed that training did not alter the activities of phosphofructokinase, beta-hydroxyacyl-CoA dehydrogenase, succinate dehydrogenase, or total phosphorylase. Histochemical analysis of myofibrillar actomyosin ATPase demonstrated three distinct classes of type II fibers and one type I fiber in the semitendinosus and triceps muscles and two type II and two type I fibers in the gastrocnemius muscle. Fiber type distribution and oxidative and glycolytic potentials, as indicated by nicotinamide adenine dinucleotide tetrazolium reductase or alpha-glycerophosphate dehydrogenase staining intensity, were unaltered by training. Similarly, capillary density, capillary-to-fiber ratios, and capillary area-to-fiber area ratios did not change with training. Thus, unlike humans and other mammals (i.e., rat), these foxhounds did not manifest biochemical or histochemical adaptations in skeletal muscle as the result of endurance training. This is consistent with the results of the study in which endurance training produced a 27% increase in maximal cardiac output and a 4% increase in maximal arteriovenous O2 extraction in foxhounds.