Muscle Oxygen Supply and Use in Type 1 Diabetes, From Ambient Air to the Mitochondrial Respiratory Chain: Is There a Limiting Step?

OBJECTIVE

Long before clinical complications of type 1 diabetes (T1D) develop, oxygen supply and use can be altered during activities of daily life. We examined in patients with uncomplicated T1D all steps of the oxygen pathway, from the lungs to the mitochondria, using an integrative ex vivo (muscle biopsies) and in vivo (during exercise) approach.

RESEARCH DESIGN AND METHODS

We compared 16 adults with T1D with 16 strictly matched healthy control subjects. We assessed lung diffusion capacity for carbon monoxide and nitric oxide, exercise-induced changes in arterial O₂ content (SaO₂, PaO₂, hemoglobin), muscle blood volume, and O₂ extraction (via near-infrared spectroscopy). We analyzed blood samples for metabolic and hormonal vasoactive moieties and factors that are able to shift the O₂-hemoglobin dissociation curve. Mitochondrial oxidative capacities were assessed in permeabilized vastus lateralis muscle fibers.

RESULTS

Lung diffusion capacity and arterial O₂ transport were normal in patients with T1D. However, those patients displayed blunted exercise-induced increases in muscle blood volume, despite higher serum insulin, and in O₂ extraction, despite higher erythrocyte 2,3-diphosphoglycerate. Although complex I- and complex II-supported mitochondrial respirations were unaltered, complex IV capacity (relative to complex I capacity) was impaired in patients with T1D, and this was even more apparent in those with long-standing diabetes and high HbA_1c. [Formula: see text]O_2max was lower in patients with T1D than in the control subjects.

CONCLUSIONS

Early defects in microvascular delivery of blood to skeletal muscle and in complex IV capacity in the mitochondrial respiratory chain may negatively impact aerobic fitness. These findings are clinically relevant considering the main role of skeletal muscle oxidation in whole-body glucose disposal.

Salbutamol: beware of the paradox!

A 7-year-old known asthmatic presented with an acute severe asthma attack to the Accident and Emergency department. Following a poor response to salbutamol and ipratropium nebulisers, he was given intravenous salbutamol and aminophylline. Over the course of the following 3 h, there was improvement in his bronchospasm with decreasing oxygen requirement, however, his respiratory rate showed an upward trend. Serial blood gas estimations showed a worsening metabolic acidosis unresponsive to two fluid boluses of 20 ml/kg of normal saline. Lactate levels were subsequently measured and found to be high, accounting for the metabolic acidosis. High lactate levels were attributed to intravenous salbutamol. His blood gases and lactate level returned to normal within 3 h after stopping intravenous salbutamol. He was recommended on salbutamol nebulisers while still continuing on intravenous aminophylline. He continued to improve and was discharged home after 4 days.

Steps in the evolution of heteromorphic sex chromosomes

We review some recently published results on sex chromosomes in a diversity of species. We focus on several fish and some plants whose sex chromosomes appear to be 'young', as only parts of the chromosome are nonrecombining, while the rest is pseudoautosomal. However, the age of these systems is not yet very clear. Even without knowing what proportions of their genes are genetically degenerate, these cases are of great interest, as they may offer opportunities to study in detail how sex chromosomes evolve. In particular, we review evidence that recombination suppression occurs progressively in evolutionarily independent cases, suggesting that selection drives loss of recombination over increasingly large regions. We discuss how selection during the period when a chromosome is adapting to its role as a Y chromosome might drive such a process.

Effects of spontaneously chosen crank rate variations on electromyographic responses in sub-maximal arm exercise in inexperienced subjects

The aim of the present study was to compare electromyographic responses during arm exercises with a crank rate chosen spontaneously ( T(S)) or set at 20% below or above ( T(-20), T(+20)) the spontaneously chosen crank rate (SCCR). Ten male physical education students performed arm exercises with intensities ranging from 20% to 80% of maximal power. Muscular activity levels were analysed for the biceps brachii and the triceps brachii muscles using integrated rectified surface electromyography (iEMG). All values were presented as the mean and standard deviation. During T(S), the sum of iEMG for the two muscles studied was significantly ( P<0.05) lower than during T(+20) for each power output. No significant differences were observed in iEMG values between T(S) and T(-20). The hypothesis that SCCR relates to a minimisation of muscle activation during an upper body exercise was not confirmed. Variations superior or inferior to a 20% increase of the iEMG responses do not influence it. Moreover, the selection of crank rates depends on the power output and the SCCR increased significantly ( P<0.05) with increasing power output.

Recombination and base composition: the case of the highly self-fertilizing plant Arabidopsis thaliana

The effects of recombination and self-fertilization on base composition were investigated both theoretically and experimentally in the Arabidopsis genome. Levels of inbreeding modulate the effect of recombination on base composition.

Background

Rates of recombination can vary among genomic regions in eukaryotes, and this is believed to have major effects on their genome organization in terms of base composition, DNA repeat density, intron size, evolutionary rates and gene order. In highly self-fertilizing species such as Arabidopsis thaliana, however, heterozygosity is expected to be strongly reduced and recombination will be much less effective, so that its influence on genome organization should be greatly reduced.

Results

Here we investigated theoretically the joint effects of recombination and self-fertilization on base composition, and tested the predictions with genomic data from the complete A. thaliana genome. We show that, in this species, both codon-usage bias and GC content do not correlate with the local rates of crossing over, in agreement with our theoretical results.

Conclusions

We conclude that levels of inbreeding modulate the effect of recombination on base composition, and possibly other genomic features (for example, transposable element dynamics). We argue that inbreeding should be considered when interpreting patterns of molecular evolution.

Strength and Conditioning (Michael Stone Sub‐editor

This critical review reflects the current state of cadence research during cyclical activities at different intensities. Moreover, this review aims at making suggestions in the areas of evaluation, therapy and sporting performance in the light of all the different results reported in studies. A large number of researchers have tried to determine the 'optimal' cadence from imposed, preferred or spontaneously chosen cadences in order to improve efficiency and performances. Results are sometimes conflicting and difficult to explain or interpret. The authors have studied the variations in cadences using a reduced number of parameters, without links between energetic (oxygen consumption, ventilation), biomechanical (force, electromyography) and/or perceived parameters (rating of perceived exertion). Conclusions point out that the 'optimal' cadence cannot be unique and must be associated with the objectives and individual characteristics of the subject (skills and training level, anthropometric parameters). In the area of training and reconditioning, cadences would have to be set in relation to the nature of cyclical activities and the subjects' condition.

Ventilatory thresholds in arm and leg exercises with spontaneously chosen crank and pedal rates

The present study assessed whether the first and the second ventilatory thresholds (VT1 and VT2) were dependent on the muscle groups solicited when spontaneously chosen crank and pedal rates are used. 20 physical education male students (22 +/- 2.2 yr.) performed two maximal incremental tests randomly assigned using an increment of 15 and 30 W every minute for arm and leg exercises, respectively. These tests were used to measure the maximal oxygen uptake (VO2 max) and to identify VT1 and VT2. The absolute oxygen uptake (VO2) values measured at VT1, VT2, and at maximal workload were significantly (p < .05) lower during arm and leg exercises. However, VT1 and VT2 expressed in percent of VO2 max were not significantly different between arm and leg exercises (54.1 +/- 8.2 vs 57.2 +/- 11.4%; and 82.5 +/- 6.4 vs 84.6 +/- 5.1% at VT1 and VT2, respectively). In addition, at the two thresholds, none of the variables measured during arm and leg exercises were significantly correlated with the exception of spontaneously chosen crank and pedal rates (p < .01; r = .75 and r = .69 for VT1 and VT2, respectively). Probably due to the different training status and skill level, no extrapolation can be made to specify the arm thresholds from the leg. These results underline the need to specify the ventilatory thresholds from specific arm ergometer measures obtained from tests performed with spontaneously chosen crank and pedal rates and, thus, close to sport and recreational activities, when they are used for training and rehabilitation programs.

Cardiorespiratory and efficiency responses during arm and leg exercises with spontaneously chosen crank and pedal rates

The aim of this study was to compare the cardiorespiratory and efficiency responses between upper (T(UBE)) and lower (T(LBE)) body exercises at the same relative power outputs and with spontaneously chosen crank (SCCR) or pedal (SCPR) rates. Twelve participants performed exercise bouts set at 20, 40, 60 and 80% of maximal power (MP) separated by passive recovery periods. Oxygen uptake, ventilation, gross and work efficiencies during T(LBE) were significantly (P < 0.05) higher than during T(UBE). These results suggest that these responses were not directly related to the relative intensities. However, no significant difference was found for delta efficiency and heart rate values. During T(UBE) and T(LBE), gross efficiency increased significantly (P < of MP for T(UBE) and T(LBE) and the same SCCR and SCPR could explain these results. The present results confirm that the cardiorespiratory and efficiency responses between arm and leg exercises are not always similar, although the power output are normalized in relation to MP and add to the understanding of differences between upper and lower body.

Plasma lactate recovery from maximal exercise with correction for variations in plasma volume

BACKGROUND

To compare plasma lactate concentrations and plasma lactate kinetics during recovery, for measured and corrected values for changes in plasma volume, after a maximal aerobic exercise.

METHODS

Sixteen male subjects performed an incremental and maximal exercise in order to reach maximal aerobic power. Prior to the exercise, at the end and during recovery (2, 5, 12 and 30 min), blood samples were collected through an antecubital catheter. Samples were analysed for lactate, hematocrit and hemoglobin in order to calculate changes in plasma volume. Plasma lactate concentrations ([La]p) were corrected for changes in plasma volume. Plasma lactate kinetics was estimated through the ratio between [La]p after 5 min recovery minus [La]p after 30 min to time (25 min) and expressed in percentage per minute.

RESULTS

Maximal changes in plasma volume (-19.7 +/- 3.8%) were correlated to maximal measured [La]p (r=0.66, p<0.01). Maximal measured [La]p values (14.9 +/- 2.6 mmol x l-1) were 17.3% higher (p<0.001) than corrected values (12.7 +/-2.0 mmol x l-1). The kinetics of [La]p decrease was significantly higher (p<0.001) for measured values (2.38 +/- 0.29 % x min-1) than for corrected values (2.22 +/- 0.33 % x min-1).

CONCLUSIONS

These results suggested that changes in plasma volume must be taken into account when peak postexercise plasma lactate concentration or lactate recovery curves are analysed.

Spontaneously chosen crank rate variations in submaximal arm exercise with inexperienced subjects. Effects on cardiorespiratory and efficiency parameters

The aim of the present study was to compare the physiological responses during arm exercises when the crank rate was chosen spontaneously (TS) or set at +/- 20 % (T-20, T+ 20) of the spontaneously chosen crank rate (SCCR). Eight physical education male students, aged 22 +/- 3.2 years, performed an upper body exercise in which intensities ranged from unload to 80 % of maximal power. No significant difference was observed in oxygen uptake, ventilation, gross and net efficiency values between TS and T+ 20 or T-20. Nevertheless, oxygen uptake and ventilation were significantly (p < 0.05) lower and gross and net efficiencies higher (p < 0.05) during T-20 than T+ 20. No significant difference was noticed for heart rate, delta and work efficiency between TS, T-20 and T+ 20. The hypothesis that SCCR is the most economical one according to the efficiency parameters was not quite verified. However, crank rates lower than SCCR could be interesting because they increase gross efficiency compared to higher crank rates. Moreover, the selection of crank rates depends on power output. Indeed, SCCR increased significantly (p < 0.05) with power output. In the physical reconditioning of injured or handicapped subjects, the latter are very sensitive to the power output, and the crank rate could be another constraint.

Does recombination improve selection on codon usage? Lessons from nematode and fly complete genomes

Understanding the factors responsible for variations in mutation patterns and selection efficacy along chromosomes is a prerequisite for deciphering genome sequences. Population genetics models predict a positive correlation between the efficacy of selection at a given locus and the local rate of recombination because of Hill-Robertson effects. Codon usage is considered one of the most striking examples that support this prediction at the molecular level. In a wide range of species including Caenorhabditis elegans and Drosophila melanogaster, codon usage is essentially shaped by selection acting for translational efficiency. Codon usage bias correlates positively with recombination rate in Drosophila, apparently supporting the hypothesis that selection on codon usage is improved by recombination. Here we present an exhaustive analysis of codon usage in C. elegans and D. melanogaster complete genomes. We show that in both genomes there is a positive correlation between recombination rate and the frequency of optimal codons. However, we demonstrate that in both species, this effect is due to a mutational bias toward G and C bases in regions of high recombination rate, possibly as a direct consequence of the recombination process. The correlation between codon usage bias and recombination rate in these species appears to be essentially determined by recombination-dependent mutational patterns, rather than selective effects. This result highlights that it is necessary to take into account the mutagenic effect of recombination to understand the evolutionary role and impact of recombination.

Synonymous codon usage, accuracy of translation, and gene length in Caenorhabditis elegans

In many unicellular organisms, invertebrates, and plants, synonymous codon usage biases result from a coadaptation between codon usage and tRNAs abundance to optimize the efficiency of protein synthesis. However, it remains unclear whether natural selection acts at the level of the speed or the accuracy of mRNAs translation. Here we show that codon usage can improve the fidelity of protein synthesis in multicellular species. As predicted by the model of selection for translational accuracy, we find that the frequency of codons optimal for translation is significantly higher at codons encoding for conserved amino acids than at codons encoding for nonconserved amino acids in 548 genes compared between Caenorhabditis elegans and Homo sapiens. Although this model predicts that codon bias correlates positively with gene length, a negative correlation between codon bias and gene length has been observed in eukaryotes. This suggests that selection for fidelity of protein synthesis is not the main factor responsible for codon biases. The relationship between codon bias and gene length remains unexplained. Exploring the differences in gene expression process in eukaryotes and prokaryotes should provide new insights to understand this key question of codon usage.

RPE responses during arm and leg exercises: effect of variations in spontaneously chosen crank rate

The aim of this study was two-fold. First, the rating of perceived exertion (RPE) was compared between two different upper and lower body exercises. Subjects (n = 12) performed with spontaneously chosen crank or pedal rates: (i) incremental maximum power tests (Test 1), with an initial work rate of 50% of maximal power followed by increases of 10% at each 120-sec. work stage and (ii) tests (Test 2) with exercise bouts set at 20, 40, 60, and 80% of maximal power separated by passive recovery periods. Second, the effects of variations in spontaneously chosen crank rate on RPE was analysed using the second test performed only with upper body. Subjects performed Test 2 three times with crank rates spontaneously chosen by the subjects, set at plus or minus 20% of spontaneously chosen crank rate. During both Tests 1 and 2 for upper or lower body, RPE increased linearly (p<.01) with power output. No significant difference was noticed between upper and lower body tests; however, RPE was significantly different (p<.05) between Test 1 results for upper and lower body at 70, 80, 90, and 100% of maximal power. The greater RPE at high power output could be linked to the important effect of fatigue during upper body exercise. Among the three crank-rate conditions, no significant difference in RPE was noticed. The choice of crank rate does not seem to influence the perception of exertion in upper body cycling exercise.

Transposons but not retrotransposons are located preferentially in regions of high recombination rate in Caenorhabditis elegans

We analyzed the distribution of transposable elements (TEs: transposons, LTR retrotransposons, and non-LTR retrotransposons) in the chromosomes of the nematode Caenorhabditis elegans. The density of transposons (DNA-based elements) along the chromosomes was found to be positively correlated with recombination rate, but this relationship was not observed for LTR or non-LTR retrotransposons (RNA-based elements). Gene (coding region) density is higher in regions of low recombination rate. However, the lower TE density in these regions is not due to the counterselection of TE insertions within exons since the same positive correlation between TE density and recombination rate was found in noncoding regions (both in introns and intergenic DNA). These data are not compatible with a global model of selection acting against TE insertions, for which an accumulation of elements in regions of reduced recombination is expected. We also found no evidence for a stronger selection against TE insertions on the X chromosome compared to the autosomes. The difference in distribution of the DNA and RNA-based elements along the chromosomes in relation to recombination rate can be explained by differences in the transposition processes.

Physiological effects of variations in spontaneously chosen crank rate during sub-maximal and supra-maximal upper body exercises

The aim of the present study was to compare the physiological responses when the crank rate was chosen spontaneously (Ts) or set at +/- 10% (T-10%, T+10%) of the freely chosen rate, during two upper body exercises: i) a sub-maximal test (T(SUB)) in which intensities ranged from 50 to 80% (118.4 +/- 10.2 to 189.5 +/- 16.3 watts) of maximal power (MP) and ii) a supramaximal test (T(SUPRA)) in which power output was set at 110 and 120% (260.5 +/- 22.4 and 284.2 +/- 24.4 watts) of MP. Eight nationally and internationally ranked kayakers, aged 20 +/- 2 years, performed these tests in which power outputs were normalised in relation to the maximal power output determined during T(MP). In T(SUB+10%), oxygen uptake and ventilation were significantly (P< 0.05) higher than during T(SUBxS). In T(SUB+10%) and T(SUB-10%), energy expenditure was significantly (P<0.05) higher and gross and net efficiencies lower than during T(SUBxS). During T(SUPRA-10%) when the power output was set at 110% of MP, time to exhaustion was significantly higher (P<0.05) than during T(SUPRAxS). The findings of the present study suggest that upper body exercise performed on an ergocycle should be conducted using the freely and spontaneously chosen crank rate.

Relationship in humans between spontaneously chosen crank rate and power output during upper body exercise at different levels of intensity

The aim of this study was to assess the relationship between spontaneously chosen crank rate (SCCR) and power output during two upper body exercise tests: firstly, an incremental maximal aerobic power test (T1), with an initial intensity of 50 W followed by 15-W increases at each subsequent 90-s stage and secondly, a test (T2) with consecutive exercise periods set at 50%, 60%, 70%, 80%, 110% and 120% of maximal power (Pmax) separated by passive recovery periods. Eight nationally and internationally ranked kayakers, aged 20 (SD 2) years, performed the tests. During both T1 and T2, mean SCCR values were correlated (r = 1) and increased significantly (P < 0.05) in association with the increases in power output. The finding that the subjects consistently increased their crank rate as the power output increased in different tests, i.e. at submaximal, maximal and supramaximal intensities, strongly suggests that SCCR depended on power output and not on the type of exercise (incremental or rectangular exercise). Moreover, the equation relating crank rate and power output determined from T1 suggests that it may be used to predict the crank rate which will be chosen in upper body exercise, whatever the intensity. Finally, the results of testing at 110% and 120% of Pmax would suggest that a high crank rate (>90 rpm) should be used during the test procedure using supramaximal exercises where accumulated oxygen deficit is calculated, and more particularly when exercise is performed using the upper body.

Physiological effects of variations in spontaneously chosen crank rate during incremental upper-body exercise

The aims of the present study were: first, to assess the interindividual variations of a spontaneously chosen crank rate (SCCR) in relation to the power developed during an incremental upper body exercise on an arm ergometer set at a constant power regime, and second, to compare heart rate (HR) responses, expired minute ventilation (V[E]) and oxygen consumption (VO2) when the pedal rates were chosen spontaneously (T[SCCR]) or set at +/- 10% of the freely chosen rates (T[+10%] and T[-10%], respectively). The mean pedal rate values were linearly related (P < 0.01) with the power developed during arm cranking (r = 0.96), although large variations of pedalling rate strategies were observed between subjects. Maximal power (MP) and time to exhaustion values were significantly higher (P < 0.05) during T(SCCR) than during T(+10%) and T(-10%). Peak VO2 values were significantly higher (P < 0.05) in T(+10%) than in T(SCCR) and T(-10%). The increase in HR, V(E), and VO2 mean values, in relation to the increase in the power developed, was significantly higher (P < 0.05) when the pedal rate was set at plus 10% of the SCCR (T[+/-10%]) than in the two other conditions. The findings of the present study suggest that the use of an electromagnetically braked ergometer, which automatically adjusts the resistance component to maintain a constant work rate, should be used in order to achieve the highest MP values during an incremental upper body exercise. A 10% increase of the SCCR should be used in order to provide the highest peak VO2 value.