Mitochondrial coupling and capacity of oxidative phosphorylation in skeletal muscle of Inuit and Caucasians in the arctic winter

During evolution, mitochondrial DNA haplogroups of arctic populations may have been selected for lower coupling of mitochondrial respiration to ATP production in favor of higher heat production. We show that mitochondrial coupling in skeletal muscle of traditional and westernized Inuit habituating northern Greenland is identical to Danes of western Europe haplogroups. Biochemical coupling efficiency was preserved across variations in diet, muscle fiber type, and uncoupling protein-3 content. Mitochondrial phenotype displayed plasticity in relation to lifestyle and environment. Untrained Inuit and Danes had identical capacities to oxidize fat substrate in arm muscle, which increased in Danes during the 42 days of acclimation to exercise, approaching the higher level of the Inuit hunters. A common pattern emerges of mitochondrial acclimatization and evolutionary adaptation in humans at high latitude and high altitude where economy of locomotion may be optimized by preservation of biochemical coupling efficiency at modest mitochondrial density, when submaximum performance is uncoupled from VO2max and maximum capacities of oxidative phosphorylation.

Low-intensity training increases peak arm VO2 by enhancing both convective and diffusive O2 delivery

AIM

It is an ongoing discussion the extent to which oxygen delivery and oxygen extraction contribute to an increased muscle oxygen uptake during dynamic exercise. It has been proposed that local muscle factors including the capillary bed and mitochondrial oxidative capacity play a large role in prolonged low-intensity training of a small muscle group when the cardiac output capacity is not directly limiting. The purpose of this study was to investigate the relative roles of circulatory and muscle metabolic mechanisms by which prolonged low-intensity exercise training alters regional muscle VO2 .

METHODS

In nine healthy volunteers (seven males, two females), haemodynamic and metabolic responses to incremental arm cycling were measured by the Fick method and biopsy of the deltoid and triceps muscles before and after 42 days of skiing for 6 h day(-1) at 60% max heart rate.

RESULTS

Peak pulmonary VO2 during arm crank was unchanged after training (2.38 ± 0.19 vs. 2.18 ± 0.2 L min(-1) pre-training) yet arm VO2 (1.04 ± 0.08 vs. 0.83 ± 0.1 L min(1) , P < 0.05) and power output (137 ± 9 vs. 114 ± 10 Watts) were increased along with a higher arm blood flow (7.9 ± 0.5 vs. 6.8 ± 0.6 L min(-1) , P < 0.05) and expanded muscle capillary volume (76 ± 7 vs. 62 ± 4 mL, P < 0.05). Muscle O2 diffusion capacity (16.2 ± 1 vs. 12.5 ± 0.9 mL min(-1) mHg(-1) , P < 0.05) and O2 extraction (68 ± 1 vs. 62 ± 1%, P < 0.05) were enhanced at a similar mean capillary transit time (569 ± 43 vs. 564 ± 31 ms) and P50 (35.8 ± 0.7 vs. 35 ± 0.8), whereas mitochondrial O2 flux capacity was unchanged (147 ± 6 mL kg min(-1) vs. 146 ± 8 mL kg min(-1) ).

CONCLUSION

The mechanisms underlying the increase in peak arm VO2 with prolonged low-intensity training in previously untrained subjects are an increased convective O2 delivery specifically to the muscles of the arm combined with a larger capillary-muscle surface area that enhance diffusional O2 conductance, with no apparent role of mitochondrial respiratory capacity.

Maximal heart rate does not limit cardiovascular capacity in healthy humans: insight from right atrial pacing during maximal exercise

In humans, maximal aerobic power (VO2 max ) is associated with a plateau in cardiac output (Q), but the mechanisms regulating the interplay between maximal heart rate (HRmax) and stroke volume (SV) are unclear. To evaluate the effect of tachycardia and elevations in HRmax on cardiovascular function and capacity during maximal exercise in healthy humans, 12 young male cyclists performed incremental cycling and one-legged knee-extensor exercise (KEE) to exhaustion with and without right atrial pacing to increase HR. During control cycling, Q and leg blood flow increased up to 85% of maximal workload (WLmax) and remained unchanged until exhaustion. SV initially increased, plateaued and then decreased before exhaustion (P < 0.05) despite an increase in right atrial pressure (RAP) and a tendency (P = 0.056) for a reduction in left ventricular transmural filling pressure (LVFP). Atrial pacing increased HRmax from 184 ± 2 to 206 ± 3 beats min(-1) (P < 0.05), but Q remained similar to the control condition at all intensities because of a lower SV and LVFP (P < 0.05). No differences in arterial pressure, peripheral haemodynamics, catecholamines or VO2 were observed, but pacing increased the rate pressure product and RAP (P < 0.05). Atrial pacing had a similar effect on haemodynamics during KEE, except that pacing decreased RAP. In conclusion, the human heart can be paced to a higher HR than observed during maximal exercise, suggesting that HRmax and myocardial work capacity do not limit VO2 max in healthy individuals. A limited left ventricular filling and possibly altered contractility reduce SV during atrial pacing, whereas a plateau in LVFP appears to restrict Q close to VO2 max .

Blood profiles in elite cross-country skiers: a 6-year follow-up

Following the doping scandals at the World Championships in cross-country skiing in 2001, the International Ski Federation decided to generate individual blood profiles. From 2001 to 2007, 7081 blood samples from 1074 male and female elite cross-country skiers were collected and analyzed for hemoglobin concentration [Hb] and % reticulocytes (%rets). Data were applied to blood algorithms wherefrom blood model scores were calculated. From 1997-1999 to 2001-2002, the mean [Hb] was reduced by 0.9 g/dL to 15.3 g/dL in male skiers and by 0.4 g/dL to 13.8 in female skiers. From 2002-2003 to 2006-2007, the combination of increases in [Hb] and decreases in %rets led to pronounced increases in mean OFF-model scores. [Hb] was 0.2 g/dL higher at Olympic Games/World Championships (WOCs) than at World Cups competitions <4 weeks before and after WOCs. [Hb] and %rets increased with altitude in both genders. Since the introduction of an enlarged blood testing program, the mean [Hb] values were lowered to close to normal levels, but over the last 2-3 years there has been a small elevation and an increase in OFF-model scores, which may indicate a change in the manipulations used to elevate the [Hb].

Low-intensity training dissociates metabolic from aerobic fitness

This study investigated the effect of prolonged whole-body low-intensity exercise on blood lipids, skeletal muscle adaptations and aerobic fitness. Seven male subjects completed a 32-day crossing of the Greenland icecap on cross-country skies and before and after this arm or leg cranking was performed on two separate days and biopsies were obtained from arm and leg muscle, and venous blood was sampled. During the crossing, subjects skied for 342+/-42 min/day and body mass was decreased by 7.1+/-0.7 kg. Peak leg oxygen uptake (4.6+/-0.2 L/min) was decreased (P<0.05) by 7% whereas peak arm oxygen uptake (3.0+/-0.2 L/min) remained unchanged. Total and low-density lipoprotein cholesterol (5.0+/-0.2 and 3.20.2 mmol/L) were decreased by 8% and 20%, respectively. Muscle beta-hydroxy-acyl-CoA dehydrogenase activity was increased with 22% in arm (P=0.08) and remained unchanged in leg muscle. Hormone sensitive lipase activity was similar in arm and leg muscle prior to the expedition and was not significantly affected by the crossing. In conclusion, an improved blood lipid profile and thus metabolic fitness was present after prolonged low-intensity training and this occurred in spite of a decreased aerobic fitness and an unchanged arm and leg muscle hormone-sensitive lipase activity.

Exercise performance in hypoxia after novel erythropoiesis stimulating protein treatment

Maximal aerobic power at high altitude (<4000 m) does not increase with altitude acclimatization. In order to investigate the isolated effects of increased arterial oxygen content (CaO2) on maximal oxygen uptake (VO2max) in hypoxia, we studied 10 subjects during exercise in acute exposure to 12.6% O2 before and after novel erythropoiesis stimulating protein (NESP) induced increases in CaO2. Over a period of 1 month, weekly NESP treatment increased resting hemoglobin (Hb) from 13.8+/-0.9 to 16.2+/-0.5 g/dL, hematocrit from 42.1+/-0.6% to 49.0+/-1.5%, and CaO2 from 189.7+/-3.0 to 218.6+/-5.7 mL/L. At maximal exercise CaO2 was increased from 172.3+/-3.7 to 191.5+/-3.8 mL/L with NESP treatment, and although maximal heart rate was similar in both conditions (178.4+/-2.6 and 180.9+/-2.5 b.p.m.) VO2max remained unaltered, the values being 3.12+/-2.0 and 3.12+/-2.0, before and after NESP treatment, respectively. NESP-injections in human subjects causes Hb and accordingly CaO2 to increase both in normoxia and hypoxia. Despite increases in CaO2 at maximal exercise in hypoxia VO2max is not increased.

Skeletal muscle mitochondrial DNA content in exercising humans

Several weeks of intense endurance training enhances mitochondrial biogenesis in humans. Whether a single bout of exercise alters skeletal muscle mitochondrial DNA (mtDNA) content remains unexplored. Double-stranded mtDNA, estimated by slot-blot hybridization and real time PCR and expressed as mtDNA-to-nuclear DNA ratio (mtDNA/nDNA) was obtained from the vastus lateralis muscle of healthy human subjects to investigate whether skeletal muscle mtDNA changes during fatiguing and nonfatiguing prolonged moderate intensity [2.0–2.5 h; ∼60% maximal oxygen consumption (V̇o2 max)] and short repeated high-intensity exercise (5–8 min; ∼110% V̇o2 max). In control resting and light exercise (2 h; ∼25% V̇o2 max) studies, mtDNA/nDNA did not change. Conversely, mtDNA/nDNA declined after prolonged fatiguing exercise (0.863 ± 0.061 vs. 1.101 ± 0.067 at baseline; n = 14; P = 0.005), remained lower after 24 h of recovery, and was restored after 1 wk. After nonfatiguing prolonged exercise, mtDNA/nDNA tended to decline ( n = 10; P = 0.083) but was reduced after three repeated high-intensity exercise bouts (0.900 ± 0.049 vs. 1.067 ± 0.071 at baseline; n = 7; P = 0.013). Our findings indicate that prolonged and short repeated intense exercise can lead to significant reductions in human skeletal muscle mtDNA content, which might function as a signal stimulating mitochondrial biogenesis with exercise training.

The influence of intermittent altitude exposure to 4100 m on exercise capacity and blood variables

This study was performed to investigate the effects of intermittent hypoxic exposure on blood and exercise parameters. Eight sea level residents were exposed to 2 h daily stimulus to 4100 m altitude in a hypobaric chamber for a total of 14 days. Exercise performance was evaluated at sea level before and after the hypoxic stimulation. Blood samples were obtained before, during, and at time points up to 14 days after the hypoxic exposure. No changes were observed in haemoglobin, haematocrit, reticulocytes, serum transferrin receptors, or EPO levels in the blood. Submaximal cycle (150 W) ergometer exercise corresponded to a oxygen uptake of 1.9+/-0.1 and 1.9+/-0.1 L min(-1) before and after the intermittent altitude exposure, respectively. At maximal exercise the workloads attained were 343+/-17 and 354+/-27 W before and after the exposure, with corresponding oxygen uptakes of 4.0+/-0.2 and 4.2+/-0.2 L min(-1). It is concluded that intermittent hypoxic exposure to 4100 m altitude for 2 h daily and a total of 14 days does not affect exercise capacity.

Do blood cells mimic gene expression profile alterations known to occur in muscular adaptation to endurance training ?

Exercise is known to upregulate mRNA synthesis for carnitine palmitoyl transferase1 (CPT1) and possibly also other mitochondrial carnitine acyltransferases in muscle tissue. The aim of this study was to test whether such an adaptation of oxidative metabolism in skeletal muscle is a systemic process and consequently, also affects other cells. Messenger RNA levels of five genes [carnitine palmitoyl transferases 1 and 2 (CPT1 and CPT2), carnitine acetyltransferase (CRAT), carnitine palmitoyltransferase 2 (CPT2), microsomal carnitine palmitoyltransferase (GRP58) and organic cation transporter (OCTN2)] were determined with quantitative real time polymerase chain reaction (PCR) in blood cells and in muscle biopsy samples from six cross country skiers before and 6 months after a high volume/low intensity exercise training, when training had elicited a significantly slower rate of lactate accumulation. Quantitative real time PCR showed that levels of mRNA in blood cells correlated significantly (CPT1B: P< 0.001) with those in muscle tissue from the same donors. After 6-months training, there was a 15-fold upregulation of CPT1B mRNA, a six to ninefold increase of CRAT mRNA, of CPT2 mRNA, GRP58 mRNA, and of OCTN2 mRNA. The observation of a concordant stimulation of CPT1, CPT2, CRAT, GRP58 and OCTN2 transcription in blood cells and muscle tissue after 6-month-endurance training leads the hypothesis of a common stimulation mechanism other than direct mechanical stress or local chemical environment, but rather humoral factors.

Anaerobic power and muscle strength characteristics of 11 years old elite and non-elite boys and girls from gymnastics, team handball, tennis and swimming

The aim of the present investigation was to study the possible effects of specificity of training on muscle strength and anaerobic power in children from different sports and at different performance levels in relation to growth and maturation status. Hundred and eighty-four children of both gender participating either in swimming, tennis, team handball or gymnastics were recruited from the best clubs in Denmark. Within each sport, the coach had divided the children into an elite (E) and non-elite (NE) group according to performance level and talent. Tanner stage assessment and body weight and height measurements were performed by a physician. The anaerobic performances were assessed by Wingate tests and jumping performance in squat jump (SJ), countermovement jump (CMJ) and drop jump (DJ) from two heights. Most of the differences between groups in Wingate performance disappeared when the data were normalised to body mass. The gymnasts were the best jumpers and their superiority were increased in the more complex motor coordination tasks like DJ. The results may indicate some influence of training specificity, especially on the more complex motor tasks as DJ and there may be an effect of training before puberty. The performance in the less complex motor tasks like cycling and SJ and CMJ may also be influenced by specific training, but not to the same extent, and heritance may be an important factor for performance in these anaerobic tasks.

Body proportions, body composition and pubertal development of children in competitive sports

The aim of this study was primarily to investigate anthropometric variables, body composition and pubertal development in children aged 9-13 participating in competitive sports. Secondly, the influence of age, sport, training hours and pubertal development/maternal menarcheal age on body composition and pubertal development was explored. A total of 183 (96 girls, 87 boys) children performing swimming (Sw), tennis (Te), European team handball (TH), and gymnastics (Gy) took part in the study. Anthropometric measurements and pubertal development were determined. The participants completed a questionnaire regarding hours of training per week and maternal menarcheal age. Significant differences in stature (z-scores) were found in both boys (Sw=0.06; Te=0.04; TH=0.05; Gy=-0.66, P<0.004) and girls (Sw=0.12; Te=0.19; TH=0.25; Gy=-0.96, P<0.004). In girls, sum of skinfolds in millimetres (Sw=33.4; Te=33.3; TH=41.0; Gy= 27.2, P<0.02) and body mass index z-scores (SW=0.00; Te=-0.27; TH=0.35; Gy=-0.25, P<0.001) were different between the sports. A regression analysis revealed that in girls, age and maternal menarcheal age were significantly associated with pubertal development (P<0.005 and P<0.01), respectively, and sport was associated with the sum of skinfolds (P<0.05). In boys, only age was significantly associated with pubertal development (P<0.005). In conclusion, anthropometric and body composition differences exist in athletes of both sexes from different sports but are more evident in females. Most importantly, we did not find any effect of training on body composition or pubertal development, confirming previous data that children in competitive sports are selected due to constitutional factors.

Is prepubertal growth adversely affected by sport?

PURPOSE

To study the effect of genetic factors, birth weight, early childhood growth, sport, hours of training, and pubertal status on the stature and body mass index (BMI) of children aged 9-13 participating in sports at a competitive level.

METHODS

A total of 184 children (96 girls, 88 boys), competing in swimming, tennis, team handball, and gymnastics, were investigated, assessing their height, weight, pubertal development, and BMI. Of these, 137 (76 girls, 61 boys) returned a questionnaire, which enabled us to determine height and BMI at age 2-4, birth weight, and parental heights.

RESULTS

Significant differences in standard deviation scores (SDS) for actual height and for height at age 2-4 were found in both sexes between the four sports. In girls, BMI SDS was significantly different between the four sports, whereas no difference was found in boys. Each sport investigated separately showed no change in height SDS and BMI SDS between ages 2-4 and 9-13. A regression analysis showed that target height, height at age 2-4, and pubertal status had a significant impact on actual height. Interestingly, the type of sport and hours of training per week had no effect on height SDS. In boys, BMI at age 2-4 and pubertal status had a significant effect on actual BMI, whereas in girls, only BMI at age 2-4 was significant.

CONCLUSIONS

The results suggest that prepubertal growth is not adversely affected by sport at a competitive level and that constitutional factors are of importance for choice of sport in children.