Synergistic regulation of vertebrate muscle development by Dach2, Eya2, and Six1, homologs of genes required for Drosophila eye formation

We have identified a novel vertebrate homolog of the Drosophila gene dachshund, Dachshund2 (Dach2). Dach2 is expressed in the developing somite prior to any myogenic genes with an expression profile similar to Pax3, a gene previously shown to induce muscle differentiation. Pax3 and Dach2 participate in a positive regulatory feedback loop, analogous to a feedback loop that exists in Drosophila between the Pax gene eyeless (a Pax6 homolog) and the Drosophila dachshund gene. Although Dach2 alone is unable to induce myogenesis, Dach2 can synergize with Eya2 (a vertebrate homolog of the Drosophila gene eyes absent) to regulate myogenic differentiation. Moreover, Eya2 can also synergize with Six1 (a vertebrate homolog of the Drosophila gene sine oculis) to regulate myogenesis. This synergistic regulation of muscle development by Dach2 with Eya2 and Eya2 with Six1 parallels the synergistic regulation of Drosophila eye formation by dachshund with eyes absent and eyes absent with sine oculis. This synergistic regulation is explained by direct physical interactions between Dach2 and Eya2, and Eya2 and Six1 proteins, analogous to interactions observed between the Drosophila proteins. This study reveals a new layer of regulation in the process of myogenic specification in the somites. Moreover, we show that the Pax, Dach, Eya, and Six genetic network has been conserved across species. However, this genetic network has been used in a novel developmental context, myogenesis rather than eye development, and has been expanded to include gene family members that are not directly homologous, for example Pax3 instead of Pax6.     

Expression of slow and fast myosin heavy chains in overloaded muscles of the developing rat

Summary The present study examines the developmental accumulation of slow myosin heavy chain in the extensor digitorum longus, soleus and plantaris muscles of rats after early post-natal imposition of mechanical overload by removal of synergistic muscles. The proportions of slow and fast myosin heavy chain were measured in each muscle by ELISA. Fibres expressing slow myosin were examined immunocytochemically using a monoclonal antibody specific for slow MHC. Between 30 and 60 days of age, MHC increases by 15% (p<0.001) in the soleus and by 27% (p<0.001) in the plantaris of normally developing, unoperated animals. The effect of overload on the soleus and plantaris is to accelerate the rate of increase in slow MHC accumulation so that levels are respectively 16 and 39% higher than controls by 30 days of age (p<0.001). By 60 days, the control soleus and plantaris attain levels of slow MHC roughly equivalent to their overloaded counterparts. In overloaded plantaris, the increase in levels of slow myosin does not occur at the expense of fast myosin expression. In the EDL there is a normal developmentally regulated decrease in slow MHC accumulation, reflected by a 40% decrease in levels of slow MHC (p<0.0001) and a 50% decrease in the number of slow fibres (p<0.001), between 30 days and 20 weeks of age. This elimination of slow myosin accumulation in the EDL is unimpeded by chronic overload. Thus, muscles react to mechanical overload in a tissue specific manner. The pattern of response is conservative and potentiates normal, long term maturational shifts in myosin heavy chain expression characteristic of each muscle.     

Expression of monocarboxylate transporter (MCT) 1 and MCT4 in overloaded mice plantaris muscle

A number of studies have shown that changes in muscle contractile activity regulate the expression of monocarboxylate transporters (MCTs) in the skeletal muscle. The aim of this study was to investigate the effect of functional overload on MCT1 and MCT4 protein expression. Plantaris muscles were functionally overloaded for 15 days by ablation of the synergistic muscles. MCT1 and MCT4 mRNA abundance increased by 160–161% (p < 0.01) and 265–325% (p < 0.05), respectively, after 1–3 days of functional overload. MCT1 and MCT4 protein expression increased by 92 and 61%, respectively, after 12 days of functional overload (p < 0.05). AMP-activated protein kinase (AMPK) phosphorylation status [phospho-AMPK (Thr172)/total AMPK] was significantly elevated after 3–9 days of functional overload. Plasma testosterone concentration was elevated after 12 days of functional overload, while blood lactate concentration was not altered. Thus, the current study demonstrated that heavy mechanical loading induces increase in MCT1 and MCT4 protein expression in the muscles with increase in AMPK phosphorylation status and plasma testosterone concentration.     

Muscle endurance and mitochondrial function after chronic normobaric hypoxia: contrast of respiratory and limb muscles

Skeletal muscle adaptation to chronic hypoxia includes loss of oxidative capacity and decrease in fiber size. However, the diaphragm may adapt differently since its activity increases in response to hypoxia. Thus, we hypothesized that chronic hypoxia would not affect endurance, mitochondrial function, or fiber size in the mouse diaphragm. Adult male mice were kept in normoxia (control) or hypoxia (hypoxia, FIO₂ = 10%) for 4 weeks. After that time, muscles were collected for histological, biochemical, and functional analyses. Hypoxia soleus muscles fatigued faster (fatigue index higher in control, 21.5 ± 2.6% vs. 13.4 ± 2.4%, p < 0.05), but there was no difference between control and hypoxia diaphragm bundles. Mean fiber cross-sectional area was unchanged in hypoxia limb muscles, but it was 25% smaller in diaphragm (p < 0.001). Ratio of capillary length contact to fiber perimeter was significantly higher in hypoxia diaphragm (28.6 ± 1.2 vs. 49.3 ± 1.4, control and hypoxia, p < 0.001). Mitochondrial respiration rates in hypoxia limb muscles were lower: state 2 decreased 19%, state 3 31%, and state 4 18% vs. control, p < 0.05 for all comparisons. There were similar changes in hypoxia diaphragm: state 3 decreased 29% and state 4 17%, p < 0.05. After 4 weeks of hypoxia, limb muscle mitochondria had lower content of complex IV (cytochrome c oxidase), while diaphragm mitochondria had higher content of complexes IV and V (F ₁/F ₀ ATP synthase) and less uncoupling protein 3 (UCP-3). These data demonstrate that diaphragm retains its endurance during chronic hypoxia, apparently due to a combination of morphometric changes and optimization of mitochondrial energy production.     

Skeletal muscle buffering capacity is higher in the superficial vastus than in the soleus of spontaneously running rats

Skeletal muscle buffering capacity (βm_titr) was determined in soleus (type I) and superficial vastus (type II) muscles of 16 Long–Evans rats with differing levels of spontaneous activity and in 11 sedentary control rats. βm_titr was 24% higher (P<0.001) in superficial vastus muscle than in soleus muscle (268±50 vs. 216±30 μmol H⁺ g muscle dry wt^-1 pH unit^-1) (mean±SD). There was no relationship between βm_titr and mean weekly running distance amongst spontaneously running rats, nor was βm_titr any greater in these rats than in a group of sedentary control rats. Protein to wet wt ratio was 31% higher (P<0.0001) in the superficial vastus muscle when compared with soleus muscle (22.04±3.74 vs. 16.77±3.00 mg protein, 100 mg wet wt muscle^-1), but there was no relationship between protein to wet wt ratio and running distance. Initial muscle homogenate pH (pH_i) was lower in superficial vastus muscle compared with soleus muscle (6.36±0.25 vs. 6.63±0.16). Running rats had a significantly lower pH_i in both soleus and superficial vastus than sedentary controls. There was an exponential relationship between weekly running distance and pH_i in both the superficial vastus muscle (r=-0.86, P<0.001) and the soleus muscle (r=-0.73, P<0.01). Citrate synthase activity correlated with weekly running distance in superficial vastus muscle (r=0.66, P<0.01) but not in soleus muscle. The results confirm a higher βm_titr in the type II superficial vastus muscle when compared with the predominantly type I soleus muscle. We suggest that this may be partly the result of a higher protein concentration in type II muscle. Future studies measuring βm_titr in mixed muscle (e.g. human vastus lateralis) should report fibre type composition.     

A rehabilitation exercise program to remediate skeletal muscle atrophy in an estrogen-deficient organism may be ineffective

To determine rehabilitation exercise program effects under hormone deficient (ovariectomy or OVX) and hormone supplemented [OVX + 17-beta estradiol (E2)] conditions. Mature female rats (n = 123) were assigned to OVX or OVX + E2-supplemented groups. OVX and OVX + E2 groups were allocated to one of four conditions: (1) control, (2) hindlimb unweighted (HLU) for 4 weeks to induce muscle atrophy, (3) cage Recovery for 2 weeks after HLU, and (4) Recovery with 2 weeks of rehabilitation exercise program after 4 weeks of HLU. Atrophy following HLU was comparable for OVX and OVX + E2-supplemented rats and was significant in all muscles examined (soleus, tibialis anterior, plantaris, gastrocnemius, quadriceps). Also significant with HLU was the decline in muscle force (P < 0.05) in soleus, plantaris, gastrocnemius and tibialis anterior (quadriceps not tested). There were trends toward return of muscle mass in Recovery OVX and Recovery OVX + E2 groups but only the E2 supplemented OVX rats had return of muscle mass (4/5 muscles studied) with exercise. Peak tetanic tension (Po) returned to control values in the E2 supplemented Exercise rats but not in the unsupplemented Exercise group. For example, gastrocnemius Po for OVX HLU, OVX Recovery and OVX-Exercise groups was 82%*, 82%* and 76%* of control. Gastrocnemius Po for E2 supplemented HLU, Recovery and Exercise groups was 72%*, 95% and 106% of control (*P < 0.05 compared to control). H&E cross-sections from OVX-Exercise rats showed central nuclei. In conclusion, a rehabilitation exercise program to remediate acute atrophy in females appears more effective if E2 is present.     

Additivity of adrenaline and contractions on hormone‐sensitive lipase,but not on glycogen phosphorylase,in rat muscle

Aim: Hormone‐sensitive lipase (HSL) has been proposed to regulate triacylglycerol (TG) breakdown in skeletal muscle. In muscles with different fibre type compositions the influence on HSL of two major stimuli causing TG mobilization was studied. Methods: Incubated soleus and extensor digitorum longus (EDL) muscles from 70 g rats were stimulated by adrenaline (5.5 μm , 6 min) or contractions (200 ms tetani, 1 Hz, 1 min) in maximally effective doses or by both adrenaline and contractions. Results: Hormone‐sensitive lipase activity was increased significantly by adrenaline as well as contractions, and the highest activity (P < 0.05) was seen with combined stimulation [Soleus: 0.40 ± 0.03 (SE) m‐unit mg protein^?1 (basal), 0.65 ± 0.02 (adrenaline), 0.65 ± 0.03 (contractions), 0.78 ± 0.03 (adrenaline and contractions); EDL: 0.18 ± 0.01, 0.30 ± 0.02, 0.26 ± 0.02, 0.32 ± 0.01]. Glycogen phosphorylase activity was always increased more by adrenaline compared with contractions [Soleus: 60 ± 4 (a/a + b)% vs. 46 ± 3 (P < 0.05); EDL: 60 ± 5 vs. 39 ± 6 (P < 0.05)]. After combined stimulation glycogen phosphorylase activity in soleus [59 ± 3 (a/a + b)%] was identical to and in EDL [45 ± 4 (a/a + b)%] smaller (P < 0.05) than the activity after adrenaline only. Conclusions: In slow‐twitch oxidative as well as in fast‐twitch glycolytic muscle HSL is activated by both adrenaline and contractions. These stimuli are partially additive indicating at least partly different mechanisms of action. Contractions may impair the enhancing effect of adrenaline on glycogen phosphorylase activity in muscle.     

Differential gene expression of muscle-specific ubiquitin ligase MAFbx/Atrogin-1 and MuRF1 in response to immobilization-induced atrophy of slow-twitch and fast-twitch muscles

We examined muscle-specific ubiquitin ligases MAFbx/Atrogin-1 and MuRF1 gene expression resulting from immobilization-induced skeletal muscle atrophy of slow-twitch soleus and fast-twitch plantaris muscles. Male C57BL/6 mice were subjected to hindlimb immobilization, which induced similar percentage decreases in muscle mass in the soleus and plantaris muscles. Expression of MAFbx/Atrogin-1 and MuRF1 was significantly greater in the plantaris muscle than in the soleus muscle during the early stage of atrophy. After a 3-day period of atrophy, total FOXO3a protein level had increased in both muscles, while phosphorylated FOXO3a protein had decreased in the plantaris muscle, but not in the soleus muscle. PGC-1α protein expression did not change following immobilization in both muscles, but basal PGC-1α protein in the soleus was markedly higher than that in plantaris muscles. These data suggest that although soleus and plantaris muscles atrophied to a similar extent and that muscle-specific ubiquitin protein ligases (E3) may contribute more to the atrophy of fast-twitch muscle than to that of slow-twitch muscle during immobilization.     

Effect of diffusion distance on measurement of rat skeletal muscle glucose transport in vitro

The relationships between muscle size, diffusion distance, and glucose uptake were studied using the Type II b epitrochlearis (13 ± 1 mg intact), Type I soleus (25± 1 mg), and mixed Type II a/II b extensor digitorum longus (25 ± 1 mg) from 60–70 g rats. Using intact muscles, the relative rates of 3-O-methyl-glucose uptake in response to 2 mUml^-1 insulin were soleus = epitrochlearis > extensor digitorum longus, a finding inconsistent with the fibre-type compositions and the relative GLUT-4 protein levels (soleus > extensor digitorum longus > epitrochlearis). To test whether these results were influenced by substrate diffusion limitations in the tubular muscles, soleus and extensor digitorum longus were split longitudinally from tendon to tendon into strips of comparable size (13 ± 1 mg) to the epitrochlearis. Insulin-stimulated rates of 3-O-methyl-glucose uptake were significantly enhanced in the split soleus (+120%) and split extensor digitorum longus (+200%), but not in the epitrochlearis, with the relative rates being soleus > extensor digitorum longus > epitrochlearis. Diffusion distances of the split soleus and extensor digitorum longus, as reflected by [¹⁴C]mannitol space equilibration time, were markedly enhanced (by at least 50%) relative to the intact muscles, and were comparable to that of the epitrochlearis. These results indicate that when muscles of different size and/or shape are used for in vitro measurement of glucose transport, the muscle preparations used must have similar diffusion distances for physiologically meaningful comparisons to be made.     

Functional maximal strength training induces neural transfer to single-joint tasks

The purpose of this study was to investigate whether neural adaptations following functional multiple-joint leg press training can induce neural adaptations to the plantar flexor muscles in a single-joint contraction task. Subjects were randomised to a maximal strength training (MST) (n = 10) or a control group (n = 9). MST consisted of 24 sessions (8 weeks) of 4 × 4 repetitions of horizontal leg press using maximal intended velocity in the concentric phase with the movement ending in a plantar flexion. Neural adaptations in the soleus and gastrocnemius medialis (GM) were assessed by surface electromyographic activity and V-waves during maximum voluntary isometric contraction (MVIC), and also by H-reflexes in the soleus during rest and 20% MVIC. One repetition maximum leg press increased by 44 ± 14% (mean ± SD; P < 0.01). Plantar flexion MVIC increased by 20 ± 14% (P < 0.01), accompanied by 13 ± 19% (P < 0.05) increase in soleus, but not GM surface electromyography. Soleus V/M_SUP increased by 53 ± 66% and in GM by 59 ± 64% (P < 0.05). Normalised soleus H-reflexes remained unchanged by training. No changes occurred in the control group. These results suggest that leg press MST can induce neural adaptations in a single-joint plantar flexion MVIC task.     

Carbohydrate supplementation delays DNA damage in elite runners during intensive microcycle training

The aim of this study was to evaluate the effect of carbohydrate supplementation on free plasma DNA and conventional markers of training and tissue damage in long-distance runners undergoing an overload training program. Twenty-four male runners were randomly assigned to two groups (CHO group and control group). The participants were submitted to an overload training program (days 1–8), followed by a high-intensity intermittent running protocol (10 × 800 m) on day 9. The runners received maltodextrin solution (CHO group) or zero energy placebo solution as the control equivalent before, during, and after this protocol. After 8 days of intensive training, baseline LDH levels remained constant in the CHO group (before: 449.1 ± 18.2, after: 474.3 ± 22.8 U/L) and increased in the control group (from 413.5 ± 23.0 to 501.8 ± 24.1 U/L, p < 0.05). On day 9, LDH concentrations were lower in the CHO group (509.2 ± 23.1 U/L) than in the control group (643.3 ± 32.9 U/L, p < 0.01) post-intermittent running. Carbohydrate ingestion attenuated the increase of free plasma DNA post-intermittent running (48,240.3 ± 5,431.8 alleles/mL) when compared to the control group (73,751.8 ± 11,546.6 alleles/mL, p < 0.01). Leukocyte counts were lower in the CHO group than in the control group post-intermittent running (9.1 ± 0.1 vs. 12.2 ± 0.7 cells/μL; p < 0.01) and at 80 min of recovery (10.6 ± 0.1 vs. 13.9 ± 1.1 cells/μL; p < 0.01). Cortisol levels were positively correlated with free plasma DNA, leukocytes, and LDH (all r > 0.4 and p < 0.001). The results showed that ingestion of a carbohydrate beverage resulted in less DNA damage and attenuated the acute post-exercise inflammation response, providing better recovery during intense training.     

Neural and morphological changes in response to a 20-day intense eccentric training protocol

The purpose of this study was to examine the time course of adaptation through 20 days of eccentric training and 5 days of detraining. A total of 22 untrained subjects trained one arm every 2nd day for 20 days. Subjects performed maximal isokinetic eccentric biceps brachii training at 90°/s (six sets of eight reps). Muscle thickness (reported in cm) via ultrasound, strength (reported in Nm) and muscle activation (electromyography) were measured before, during, and after training (nine time points). Strength in the trained arm decreased after 8 days of training (65.6 ± 4.1 to 57.5 ± 3.5; p < 0.05) and remained decreased throughout the study. Agonist muscle activation amplitude of the trained arm increased after 14 days of training (p < 0.05) and remained elevated throughout the study. Antagonist muscle activation decreased after 20 days of training (p < 0.05). Muscle thickness increased after 8 days of training (3.66 ± 0.11 to 3.90 ± 0.12; p < 0.05) and remained above baseline until the end of training (3.97 ± 0.12). After 5 days of detraining, muscle thickness decreased (3.97 ± 0.12 vs. 3.85 ± 0.11; p < 0.05), but remained higher than baseline (p < 0.05). Muscle thickness did not change significantly in the untrained arm at any time point. In conclusion, the early increase in biceps brachii muscle thickness coupled with a significant decrease in strength is an indicator of muscle damage leading to swelling and impaired muscle function. The persistent decrease in strength, despite an increase in muscle activation, suggests that the recovery interval was inadequate to allow complete repair of muscle damage. Intense eccentric training performed every 2nd day leads to a prolonged impairment of muscle strength in previously untrained individuals.     

Inter-individual variability in the adaptation of human muscle specific tension to progressive resistance training

Considerable variation exists between people in the muscle response to resistance training, but there are numerous ways muscle might adapt to overload that might explain this variable response. Therefore, the aim of this study was to quantify the range of responses concerning the training-induced change in maximum voluntary contraction (MVC) knee joint torque, quadriceps femoris (QF) maximum muscle force (F), physiological cross-sectional area (PCSA) and specific tension (F/PCSA). It was hypothesized that the variable change in QF specific tension between individuals would be less than that of MVC. Fifty-three untrained young men performed progressive leg-extension training three times a week for 9 weeks. F was determined from MVC torque, voluntary muscle activation level, antagonist muscle co-activation and patellar tendon moment arm. QF specific tension was established by dividing F by QF PCSA, which was calculated from the ratio of QF muscle volume to muscle fascicle length. MVC torque increased by 26 ± 11% (P < 0.0001; range −1 to 52%), while F increased by 22 ± 11% (P < 0.0001; range −1 to 44%). PCSA increased by 6 ± 4% (P < 0.001; range −3 to 18%) and specific tension increased by 17 ± 11% (P < 0.0001; range −5 to 39%). In conclusion, training-induced changes in F and PCSA varied substantially between individuals, giving rise to greater inter-individual variability in the specific tension response compared to that of MVC. Furthermore, it appears that the change in specific tension is responsible for the variable change in MVC.     

Reduced soleus muscle injury at long muscle length during contraction in the rat

Muscle injury was studied to test the hypotheses that maintaining the soleus muscle at a long muscle length during contraction prevents muscle injuries and that the prevention of initial muscle injuries reduces subsequent muscle damage. The rat sciatic nerve was stimulated for 30 min with plantar or dorsal flexion of the foot, and the time course of contraction-induced injuries was examined. The soleus muscle injuries were first classified into one of five types, and the percentages of aberrant sarcomere areas observed in the soleus muscle were then separately quantified by electron microscopy at 0, 1, 6, 12, and 24 h (n = 3) post-stimulation. At a short muscle length (plantar flexion) during contraction, the soleus muscle showed sarcomere hypercontraction (9.8 ± 2.5%, mean ± standard error) and Z-band disarrangement (31.0 ± 4.5%) at 0 h, sarcomere hypercontraction (6.7 ± 1.9%), Z-band disarrangement (28.0 ± 4.9%), and sarcomere hyperstretching (1.3 ± 1.3%) at 1 h, the absence of sarcomere hypercontraction, but Z-band disarrangement (6.7 ± 1.9%) and sarcomere hyperstretching (5.0 ± 1.8%) at 6 h, and myofilament disorganization at 12 and 24 h (5.2 ± 1.5 and 2.5 ± 1.0%, respectively). In contrast, the soleus muscles at a long muscle length (dorsal flexion) during contraction using a self-made brace showed alterations in 1.2–2.4% of sarcomeres at 0 h and afterwards. Desmin disappeared, and α-actinin immunostaining was weaker in areas of sarcomere hypercontraction, whereas dystrophin was always detected along the sarcoplasmic membrane, suggesting that the integrity of the sarcolemma was intact. These results indicate that initial and subsequent muscle injuries were significantly reduced at long muscle length during contraction, probably through the prevention of sarcomere hypercontraction, and that initial muscle injuries rapidly progress to other injuries or normal structure.     

Increased nitric oxide synthase activity and Hsp90 association in skeletal muscle following chronic exercise

Exercise training results in dynamic changes in skeletal muscle blood flow and metabolism. Nitric oxide (NO) influences blood flow, oxidative stress, and glucose metabolism. Hsp90 interacts directly with nitric oxide synthases (NOS), increasing NOS activity and altering the balance of superoxide versus NO production. In addition, Hsp90 expression increases in various tissues following exercise. Therefore, we tested the hypothesis that exercise training increases Hsp90 expression as well as Hsp90/NOS association and NOS activity in skeletal muscle. Male, Sprague–Dawley rats were assigned to either a sedentary or exercise trained group (n = 10/group). Exercise training consisted of running on a motorized treadmill for 10 weeks at 30 m/min, 5% grade for 1 h. Western blotting revealed that exercise training resulted in a 1.9 ± 0.1-fold increase in Hsp90 expression in the soleus muscle but no increase in neuronal nitric oxide synthase (nNOS), inducible nitric oxide synthase, or endothelial nitric oxide synthase (eNOS). Exercise training also resulted in a 3.4 ± 1.0-fold increase in Hsp90 association with nNOS, a 2.3 ± 0.4-fold increase association with eNOS measured by immunoprecipitation as well as a 1.5 ± 0.3-fold increase in eNOS phosphorylation at Ser-1179. Total NOS activity measured by the rate of conversion of L-[¹⁴C]arginine to L-[¹⁴C]citrulline was increased by 1.42 ± 0.9 fold in soleus muscle following exercise training compared to controls. In summary, a 10-week treadmill training program in rats results in a significant increase in total NOS activity in the soleus which may be due, in part, to increased NOS interaction with Hsp90 and phosphorylation. This interaction may play a role in altering muscle blood flow and skeletal muscle redox status.     

The relationship between monocarboxylate transporters 1 and 4 expression in skeletal muscle and endurance performance in athletes

The purpose of this study was to examine the relationship between skeletal muscle monocarboxylate transporters 1 and 4 (MCT1 and MCT4) expression, skeletal muscle oxidative capacity and endurance performance in trained cyclists. Ten well-trained cyclists (mean ± SD; age 24.4 ± 2.8 years, body mass 73.2 ± 8.3 kg, VO_2max 58 ± 7 ml kg⁻¹ min⁻¹) completed three endurance performance tasks [incremental exercise test to exhaustion, 2 and 10 min time trial (TT)]. In addition, a muscle biopsy sample from the vastus lateralis muscle was analysed for MCT1 and MCT4 expression levels together with the activity of citrate synthase (CS) and 3-hydroxyacyl-CoA dehydrogenase (HAD). There was a tendency for VO_2max and peak power output obtained in the incremental exercise test to be correlated with MCT1 (r = −0.71 to −0.74; P < 0.06), but not MCT4. The average power output (P _average) in the 2 min TT was significantly correlated with MCT4 (r = −0.74; P < 0.05) and HAD (r = −0.92; P < 0.01). The P _average in the 10 min TT was only correlated with CS activity (r = 0.68; P < 0.05). These results indicate the relationship between MCT1 and MCT4 as well as cycle TT performance may be influenced by the length and intensity of the task.     

REM sleep and muscle atonia in brainstem stroke: A quantitative polysomnographic and lesion analysis study

Important brainstem regions are involved in the regulation of rapid eye movement sleep. We hypothesized that brainstem stroke is associated with dysregulated rapid eye movement sleep and related muscle activity. We compared quantitative/qualitative polysomnography features of rapid eye movement sleep and muscle activity (any, phasic, tonic) between 15 patients with brainstem stroke (N = 46 rapid eye movement periods), 16 patients with lacunar/non-brainstem stroke (N = 40 rapid eye movement periods), 15 healthy controls (N = 62 rapid eye movement periods), and patients with Parkinson's disease and polysomnography-confirmed rapid eye movement sleep behaviour disorder. Further, in the brainstem group, we performed a magnetic resonance imaging-based lesion overlap analysis. The mean ratio of muscle activity to rapid eye movement sleep epoch in the brainstem group (“any” muscle activity 0.09 ± 0.15; phasic muscle activity 0.08 ± 0.14) was significantly lower than in the lacunar group (“any” muscle activity 0.17 ± 0.2, p < 0.05; phasic muscle activity 0.16 ± 0.19, p < 0.05), and also lower than in the control group (“any” muscle activity 0.15 ± 0.17, p < 0.05). Magnetic resonance imaging-based lesion analysis indicated an area of maximum overlap in the medioventral pontine region for patients with reduced phasic muscle activity index. For all groups, mean values of muscle activity were significantly lower than in the patients with Parkinson's disease and polysomnography-confirmed REM sleep behaviour disorder group (“any” activity 0.51 ± 0.26, p < 0.0001 for all groups; phasic muscle activity 0.42 ± 0.21, p < 0.0001 for all groups). For the tonic muscle activity in the mentalis muscle, no significant differences were found between the groups. In the brainstem group, contrary to the lacunar and the control groups, “any” muscle activity index during rapid eye movement sleep was significantly reduced after the third rapid eye movement sleep phase. This study reports on the impact of brainstem stroke on rapid eye movement atonia features in a human cohort. Our findings highlight the important role of the human brainstem, in particular the medioventral pontine regions, in the regulation of phasic muscle activity during rapid eye movement sleep and the ultradian distribution of rapid eye movement-related muscle activity.     

Respiratory muscle training reduces the work of breathing at depth

Resistance respiratory muscle training (RRMT) increases respiratory muscle and swimming performance at depths down to 17 msw. It is unknown if RRMT improves swimming performance at greater depths and if the improvements are associated with a reduced work of breathing (WOB), altered respiratory mechanics and/or improved respiratory muscle performance. Eight male subjects (30.3 ± 6.0 years) were tested swimming underwater in a hyperbaric chamber at 37 m of depth against a pre-determined load (70% [(V)\dot]_{\textO 2} \dot{V}_{{{\text{O}}_{ 2} }} ) until exhausted. End expiratory lung volume (EELV) was determined by subtracting inspiratory capacity from total lung capacity throughout the swims. The mechanical WOB on the lung was calculated as the integrated product of the transpulmonary pressure and ventilatory flow. Maximal expiratory (P _EMAX) and inspiratory pressures (P _IMAX) were measured pre- and post-RRMT. RRMT was performed every 30 s against spring loaded inspiratory and expiratory valves 30 min/day, 5 days/week, for 4 weeks. RRMT increased P _IMAX and P _EMAX by 40% (110 ± 11 cmH₂O (SD) vs. 155 ± 22, p < 0.001) and 30% (148 ± 33 cmH₂O vs. 192 ± 49, p < 0.001), respectively, respiratory endurance by 75% (19.7 ± 15.4 min vs. 34.4 ± 27.3, p = 0.010), and swimming endurance by 87% (26.4 ± 9.7 min vs. 49.4 ± 21.6, p = 0.004). The longer swimming time was associated with reduced [(V)\dot]_\textE \dot{V}_{\text{E}} and [(V)\dot]_\textA \dot{V}_{\text{A}} (p < 0.001), f _b (p < 0.001), [(V)\dot]_\textCO2 \dot{V}_{{{\text{CO}}_{2} }} (p < 0.001) and WOB (p < 0.001). There were no changes in EELV post-RRMT. These results suggest the improved exercise performance post-RRMT was associated with stronger respiratory muscles, a decreased f _b, and a reduced WOB.