Muscle interstitial potassium kinetics during intense exhaustive exercise: effect of previous arm exercise

Unaccustomed eccentric contractions impair plasma K+ regulation in the absence of changes in muscle Na+,K+-ATPase content

The Na⁺,K⁺-ATPase (NKA) plays a fundamental role in the regulation of skeletal muscle membrane Na⁺ and K⁺ gradients, excitability and fatigue during repeated intense contractions. Many studies have investigated the effects of acute concentric exercise on K⁺ regulation and skeletal muscle NKA, but almost nothing is known about the effects of repeated eccentric contractions. We therefore investigated the effects of unaccustomed maximal eccentric knee extensor contractions on K⁺ regulation during exercise, peak knee extensor muscle torque, and vastus lateralis muscle NKA content and 3-O-MFPase activity. Torque measurements, muscle biopsies, and venous blood samples were taken before, during and up to 7 days following the contractions in six healthy adults. Eccentric contractions reduced peak isometric muscle torque immediately post-exercise by 26±11% and serum creatine kinase concentration peaked 24 h post-exercise at 339±90 IU/L. During eccentric contractions, plasma [K⁺] rose during Set 1 and remained elevated at ∼4.9 mM during sets 4–10; this was despite a decline in work output by Set 4, which fell by 18.9% at set 10. The rise in plasma [K⁺].work⁻¹ ratio was elevated over Set 2 from Set 4– Set 10. Eccentric contractions had no effect on muscle NKA content or maximal in-vitro 3-O-MFPase activity immediately post- or up to 7 d post-exercise. The sustained elevation in plasma [K⁺] despite a decrease in work performed by the knee extensor muscles suggests an impairment in K⁺ regulation during maximal eccentric contractions, possibly due to increased plasma membrane permeability or to excitation-contraction uncoupling.

An integrated analysis of match-related fatigue in professional rugby league

This study examined the changes in external outputs, including metabolic power variables, and internal response whilst considering contextual factors on physical performance variables during rugby league match play. Physical performance (total distance, high-speed running and high-power distances, average metabolic power), heart-rate (percentage heart-rate peak and training impulse), collisions (attacking and defensive) and contextual (time in attack, time in defence, time out of play) data were collected from 18 rugby league players during 38 games throughout two National Rugby League seasons. Physical variables were highest in the first 10-min period of each half (P < 0.001). Heart-rate indices peaked in the second 10-min period and were lower during second half periods (P < 0.001). Few differences existed in collisions and contextual factors across 10-min periods. Physical variables were highest during the first 5-min period compared to the final (P < 0.001). There was no difference in heart-rate response, attacking collisions or contextual factors between these periods. Following the peak 5-min period in the match, there were reductions in physical, heart-rate, defensive collisions and contextual factors (P < 0.001). The data show temporal changes in physical performance, heart-rate response and collisions during rugby league match play, although these are affected by contextual factors.

Knee extension fatigue attenuates repeated force production of the elbow flexors

Non-local muscle fatigue has been demonstrated with unilateral activities, where fatiguing one limb alters opposite limb forces. Fewer studies have examined if non-local fatigue occurs with unrelated muscles. The purpose of this study was to investigate if knee extensors fatigue alters elbow flexors force and electromyography (EMG) activity. Eighteen males completed a control and fatiguing session (randomised). Blood lactate was initially sampled followed by three maximal voluntary contractions (MVC) with the elbow flexors and two with the knee extensors. Thereafter, subjects either sat (control) or performed five sets of bilateral dynamic knee extensions to exhaustion using a load equal to the dominant limb MVC (1-min rest between sets). Immediately afterwards, subjects were assessed for blood lactate and unilateral knee extensors MVC, and after 1 min performed a single unilateral elbow flexor MVC. Two minutes later, subjects performed 12 unilateral elbow flexor MVCs (5 s contraction/10 s rest) followed by a third blood lactate test. Compared to control, knee extensor force dropped by 35% (p < 0.001; ES = 1.6) and blood lactate increased by 18% (p < 0.001; ES = 2.8). Elbow flexor forces were lower after the fatiguing protocol only during the last five MVCs (p < 0.05; ES = ∼ 0.58; ∼ 5%). No changes occurred between conditions in EMG. Elbow flexor forces significantly decreased after knee extensors fatigue. The effect was revealed during the later stages of the repeated MVCs protocol, demonstrating that non-local fatigue may have a stronger effect on repeated rather than on single attempts of maximal force production.

Effects of 12 weeks high-intensity & reduced-volume training in elite athletes

It was investigated if high-intensity interval training (HIT) at the expense of total training volume improves performance, maximal oxygen uptake and swimming economy. 41 elite swimmers were randomly allocated to a control (CON) or HIT group. For 12 weeks both groups trained ∼12 h per week. HIT comprised ∼5 h vs. 1 h and total distance was ∼17 km vs. 35 km per week for HIT and CON, respectively. HIT was performed as 6-10×10-30 s maximal effort interspersed by 2–4 minutes of rest. Performance of 100 m all-out freestyle and 200 m freestyle was similar before and after the intervention in both HIT (60.4±4.0 vs. 60.3±4.0 s; n = 13 and 133.2±6.4 vs. 132.6±7.7 s; n = 14) and CON (60.2±3.7 vs. 60.6±3.8 s; n = 15 and 133.5±7.0 vs. 133.3±7.6 s; n = 15). Maximal oxygen uptake during swimming was similar before and after the intervention in both the HIT (4.0±0.9 vs. 3.8±1.0 l O₂×min⁻¹; n = 14) and CON (3.8±0.7 vs. 3.8±0.7 l O₂×min⁻¹; n = 11) group. Oxygen uptake determined at fixed submaximal speed was not significantly affected in either group by the intervention. Body fat % tended to increase (P = 0.09) in the HIT group (15.4±1.6% vs. 16.3±1.6%; P = 0.09; n = 16) and increased (P<0.05) in the CON group (13.9±1.5% vs. 14.9±1.5%; n = 17). A distance reduction of 50% and a more than doubled HIT amount for 12 weeks did neither improve nor compromise performance or physiological capacity in elite swimmers.

Nanomechanics of vascular endothelium

The mechanical characteristics of endothelial cells reveal four distinct compartments, namely glycocalyx, cell cortex, cytoplasm and nucleus. There is accumulating evidence that endothelial nanomechanics of these individual compartments control vascular physiology. Depending on protein composition, filament formation and interaction with cross-linker proteins, these four compartments determine endothelial stiffness. Structural organization and mechanical properties directly influence physiological processes such as endothelial barrier function, nitric oxide release and gene expression. This review will focus on endothelial nanomechanics and its impact on vascular function.

Physiology and pathophysiology of carnosine

Carnosine (β-alanyl-l-histidine) was discovered in 1900 as an abundant non-protein nitrogen-containing compound of meat. The dipeptide is not only found in skeletal muscle, but also in other excitable tissues. Most animals, except humans, also possess a methylated variant of carnosine, either anserine or ophidine/balenine, collectively called the histidine-containing dipeptides. This review aims to decipher the physiological roles of carnosine, based on its biochemical properties. The latter include pH-buffering, metal-ion chelation, and antioxidant capacity as well as the capacity to protect against formation of advanced glycation and lipoxidation end-products. For these reasons, the therapeutic potential of carnosine supplementation has been tested in numerous diseases in which ischemic or oxidative stress are involved. For several pathologies, such as diabetes and its complications, ocular disease, aging, and neurological disorders, promising preclinical and clinical results have been obtained. Also the pathophysiological relevance of serum carnosinase, the enzyme actively degrading carnosine into l-histidine and β-alanine, is discussed. The carnosine system has evolved as a pluripotent solution to a number of homeostatic challenges. l-Histidine, and more specifically its imidazole moiety, appears to be the prime bioactive component, whereas β-alanine is mainly regulating the synthesis of the dipeptide. This paper summarizes a century of scientific exploration on the (patho)physiological role of carnosine and related compounds. However, far more experiments in the fields of physiology and related disciplines (biology, pharmacology, genetics, molecular biology, etc.) are required to gain a full understanding of the function and applications of this intriguing molecule.

Angiogenic response to passive movement and active exercise in individuals with peripheral arterial disease

Peripheral arterial disease (PAD) is caused by atherosclerosis and is associated with microcirculatory impairments in skeletal muscle. The present study evaluated the angiogenic response to exercise and passive movement in skeletal muscle of PAD patients compared with healthy control subjects. Twenty-one PAD patients and 17 aged control subjects were randomly assigned to either a passive movement or an active exercise study. Interstitial fluid microdialysate and tissue samples were obtained from the thigh skeletal muscle. Muscle dialysate vascular endothelial growth factor (VEGF) levels were modestly increased in response to either passive movement or active exercise in both subject groups. The basal muscle dialysate level of the angiostatic factor thrombospondin-1 protein was markedly higher (P < 0.05) in PAD patients compared with the control subjects, whereas soluble VEGF receptor-1 dialysate levels were similar in the two groups. The basal VEGF protein content in the muscle tissue samples was ∼27% lower (P < 0.05) in the PAD patients compared with the control subjects. Analysis of mRNA expression for a range of angiogenic and angiostatic factors revealed a modest change with active exercise and passive movement in both groups, except for an increase (P < 0.05) in the ratio of angiopoietin-2 to angiopoietin-1 mRNA in the PAD group with both interventions. PAD patients and aged individuals showed a similar limited angiogenic response to active exercise and passive movement. The limited increase in muscle extracellular VEGF combined with an elevated basal level of thrombospondin-1 in muscle extracellular fluid of PAD patients may restrict capillary growth in these patients.

Effect of intensified training on muscle ion kinetics, fatigue development, and repeated short-term performance in endurance-trained cyclists

The effects of intensified training in combination with a reduced training volume on muscle ion kinetics, transporters, and work capacity were examined. Eight well-trained cyclists replaced their regular training with speed-endurance training (12 × 30 s sprints) 2–3 times per week and aerobic high-intensity training (4–5 × 3–4 min at 90–100% of maximal heart rate) 1–2 times per week for 7 wk and reduced training volume by 70% (intervention period; IP). The duration of an intense exhaustive cycling bout (EX2; 368 ± 6 W), performed 2.5 min after a 2-min intense cycle bout (EX1), was longer ( P < 0.05) after than before IP (4:16 ± 0:34 vs. 3:37 ± 0:28 min:s), and mean and peak power during a repeated sprint test improved ( P < 0.05) by 4% and 3%, respectively. Femoral venous K+ concentration in recovery from EX1 and EX2 was lowered ( P < 0.05) after compared with before IP, whereas muscle interstitial K+ concentration and net muscle K+ release during exercise was unaltered. No changes in muscle lactate and H+ release during and after EX1 and EX2 were observed, but the in vivo buffer capacity was higher ( P < 0.05) after IP. Expression of the ATP-sensitive K+ (KATP) channel (Kir6.2) decreased by IP, with no change in the strong inward rectifying K+ channel (Kir2.1), muscle Na+-K+ pump subunits, monocarboxylate transporters 1 and 4 (MCT1 and MCT4), and Na+/H+ exchanger 1 (NHE1). In conclusion, 7 wk of intensified training with a reduced training volume improved performance during repeated intense exercise, which was associated with a greater muscle reuptake of K+ and muscle buffer capacity but not with the amount of muscle ion transporters.

Resveratrol blunts the positive effects of exercise training on cardiovascular health in aged men

Ageing is thought to be associated with decreased vascular function partly due to oxidative stress. Resveratrol is a polyphenol, which in animal studies has been shown to decrease atherosclerosis, and improve cardiovascular health and physical capacity, in part through its effects on Sirtuin 1 signalling and through an improved antioxidant capacity. We tested the hypothesis that resveratrol supplementation enhances training-induced improvements in cardiovascular health parameters in aged men. Twenty-seven healthy physically inactive aged men (age: 65 ± 1 years; body mass index: 25.4 ± 0.7 kg m(-2); mean arterial pressure (MAP): 95.8 ± 2.2 mmHg; maximal oxygen uptake: 2488 ± 72 ml O2 min(-1)) were randomized into 8 weeks of either daily intake of either 250 mg trans-resveratrol (n = 14) or of placebo (n = 13) concomitant with high-intensity exercise training. Exercise training led to a 45% greater (P < 0.05) increase in maximal oxygen uptake in the placebo group than in the resveratrol group and to a decrease in MAP in the placebo group only (-4.8 ± 1.7 mmHg; P < 0.05). The interstitial level of vasodilator prostacyclin was lower in the resveratrol than in the placebo group after training (980 ± 90 vs. 1174 ± 121 pg ml(-1); P < 0.02) and muscle thromboxane synthase was higher in the resveratrol group after training (P < 0.05). Resveratrol administration also abolished the positive effects of exercise on low-density lipoprotein, total cholesterol/high-density lipoprotein ratio and triglyceride concentrations in blood (P < 0.05). Resveratrol did not alter the effect of exercise training on the atherosclerosis marker vascular cell adhesion molecule 1 (VCAM-1). Sirtuin 1 protein levels were not affected by resveratrol supplementation. These findings indicate that, whereas exercise training effectively improves several cardiovascular health parameters in aged men, concomitant resveratrol supplementation can blunt these effects.

Effects of high-frequency stimulation and doublets on dynamic contractions in rat soleus muscle exposed to normal and high extracellular [K(+)]

The development of maximal velocity and power in muscle depends on the ability to transmit action potentials (AP) at very high frequencies up to about 400 Hz. However, for every AP there is a small loss of K⁺ to the interstitium, which during intense exercise, may build up to a point where excitability is reduced, thus limiting the intensity of further exercise. It is still unknown how the muscle responds to high-frequency stimulation when exposed to high [K⁺]. Contractile parameters of the muscles (force [F], velocity [V], power [P], rate of force development [RFD], and work) were examined during dynamic contractions, performed in vitro using rat soleus muscles incubated in buffers containing 4 or 8 mmol/L K⁺ and stimulated with constant trains of tetanic or supratetanic frequency or with trains initiated by a high-frequency doublet, followed by tetanic or subtetanic trains. At 4 mmol/L K⁺, an increase in frequency increased P_max when using constant train stimulation. When stimulating with trains containing high-frequency doublets an increase in 120-msec work was seen, however, no increase in P_max was observed. At 8 mmol/L K⁺, no differences were seen for either P_max or 120-msec work when increasing frequency or introducing doublets. In all experiments where the frequency was increased or doublets applied, an increase in RFD was seen in both normal and high [K⁺]. The results indicate that stimulation with supratetanic frequencies can improve dynamic muscle contractility, but improvements are attenuated when muscles are exposed to high extracellular [K⁺].

Effect of extraluminal ATP application on vascular tone and blood flow in skeletal muscle: implications for exercise hyperemia

During skeletal muscle contractions, the concentration of ATP increases in muscle interstitial fluid as measured by microdialysis probes. This increase is associated with the magnitude of blood flow, suggesting that interstitial ATP may be important for contraction-induced vasodilation. However, interstitial ATP has solely been described to induce vasoconstriction in skeletal muscle. To examine whether interstitial ATP induces vasodilation in skeletal muscle and to what extent this vasoactive effect is mediated by formation of nitric oxide (NO) and prostanoids, three different experimental models were studied. The rat gluteus maximus skeletal muscle model was used to study changes in local skeletal muscle hemodynamics. Superfused ATP at concentrations found during muscle contractions (1-10 μM) increased blood flow by up to 400%. In this model, the underlying mechanism was also examined by inhibition of NO and prostanoid formation. Inhibition of these systems abolished the vasodilator effect of ATP. Cell-culture experiments verified ATP-induced formation of NO and prostacyclin in rat skeletal muscle microvascular endothelial cells, and ATP-induced formation of NO in rat skeletal muscle cells. To confirm these findings in humans, ATP was infused into skeletal muscle interstitium of healthy subjects via microdialysis probes and found to increase muscle interstitial concentrations of NO and prostacyclin by ~60% and ~40%, respectively. Collectively, these data suggest that a physiologically relevant elevation in interstitial ATP concentrations increases muscle blood flow, indicating that the contraction-induced increase in skeletal muscle interstitial [ATP] is important for exercise hyperemia. The vasodilator effect of ATP application is mediated by NO and prostanoid formation.

Dietary nitrate supplementation improves team sport-specific intense intermittent exercise performance

Recent studies have suggested that dietary inorganic nitrate (NO₃(-)) supplementation may improve muscle efficiency and endurance exercise tolerance but possible effects during team sport-specific intense intermittent exercise have not been examined. We hypothesized that NO₃(-) supplementation would enhance high-intensity intermittent exercise performance. Fourteen male recreational team-sport players were assigned in a double-blind, randomized, crossover design to consume 490 mL of concentrated, nitrate-rich beetroot juice (BR) and nitrate-depleted placebo juice (PL) over ~30 h preceding the completion of a Yo-Yo intermittent recovery level 1 test (Yo-Yo IR1). Resting plasma nitrite concentration ([NO₂(-)]) was ~400% greater in BR compared to PL. Plasma [NO₂(-)] declined by 20% in PL (P < 0.05) and by 54 % in BR (P < 0.05) from pre-exercise to end-exercise. Performance in the Yo-Yo IR1 was 4.2% greater (P < 0.05) with BR (1,704 ± 304 m) compared to PL (1,636 ± 288 m). Blood [lactate] was not different between BR and PL, but the mean blood [glucose] was lower (3.8 ± 0.8 vs. 4.2 ± 1.1 mM, P < 0.05) and the rise in plasma [K(+)] tended to be reduced in BR compared to PL (P = 0.08). These findings suggest that NO₃(-) supplementation may promote NO production via the nitrate-nitrite-NO pathway and enhance Yo-Yo IR1 test performance, perhaps by facilitating greater muscle glucose uptake or by better maintaining muscle excitability. Dietary NO₃(-) supplementation improves performance during intense intermittent exercise and may be a useful ergogenic aid for team sports players.

Unchanged [3H]ouabain binding site content but reduced Na+-K+ pump α2-protein abundance in skeletal muscle in older adults

Aging is associated with reduced muscle mass, weakness, and increased fatigability. In skeletal muscle, the Na+-K+ pump (NKA) is important in regulating Na+-K+ gradients, membrane excitability, and thus contractility, but the effects of aging on muscle NKA are unclear. We investigated whether aging is linked with reduced muscle NKA by contrasting muscle NKA isoform gene expression and protein abundance, and NKA total content in 17 Elderly (66.8 ± 6.4 yr, mean ± SD) and 16 Young adults (23.9 ± 2.2 yr). Participants underwent peak oxygen consumption assessment and a vastus lateralis muscle biopsy, which was analyzed for NKA α1-, α2-, α3-, β1-, β2-, and β3-isoform gene expression (real-time RT-PCR), protein abundance (immunoblotting), and NKA total content ([3H]ouabain binding sites). The Elderly had lower peak oxygen consumption (−36.7%, P = 0.000), strength (−36.3%, P = 0.001), NKA α2- (−24.4%, 11.9 ± 4.4 vs. 9.0 ± 2.7 arbitrary units, P = 0.049), and NKA β3-protein abundance (−23.0%, P = 0.041) than Young. The β3-mRNA was higher in Elderly compared with Young ( P = 0.011). No differences were observed between groups for other NKA isoform mRNA or protein abundance, or for [3H]ouabain binding site content. Thus skeletal muscle in elderly individuals was characterized by decreased NKA α2- and β3-protein abundance, but unchanged α1 abundance and [3H]ouabain binding. The latter was likely caused by reduced α2 abundance with aging, preventing an otherwise higher [3H]ouabain binding that might occur with a greater membrane density in smaller muscle fibers. Further study is required to verify reduced muscle NKA α2 with aging and possible contributions to impaired exercise capability and daily living activities.

Metabolic limitations of performance and fatigue in football

Football participation requires considerable utilization of both aerobic and anaerobic energy systems to match the high energetic demands of the sport. The consequent stresses imposed on the physiological and metabolic systems carries players to the threshold of exhaustion during match-play, from which they are required to recover in preparation for the subsequent game. A high number of players experience fatigue during the high-intensity bouts and a consequent decline in their performance towards the end of the game is a likely outcome during match-play. The current review aims to establish the current understanding that relates to metabolic limitations of performance and the associated mechanisms for the onset of fatigue that may be instrumental in further development of evidence-based nutritional and training interventions in this event.

Lifelong physical activity prevents an age-related reduction in arterial and skeletal muscle nitric oxide bioavailability in humans

Ageing has been proposed to be associated with increased levels of reactive oxygen species (ROS) that scavenge nitric oxide (NO). In eight young sedentary (23 ± 1 years; Y), eight older lifelong sedentary (66 ± 2 years; OS) and eight older lifelong physically active subjects (62 ± 2 years; OA), we studied the effect of ROS on systemic and skeletal muscle NO bioavailability and leg blood flow by infusion of the antioxidant N-acetylcysteine (NAC). Infusion of NAC increased the bioavailability of NO in OS, as evidenced by an increased concentration of stable metabolites of NO (NOx) in the arterial and venous circulation and in the muscle interstitium. In OA, infusion of NAC only increased NOx concentrations in venous plasma whereas in Y, infusion of NAC did not affect NOx concentrations. Skeletal muscle protein levels of endothelial and neuronal NO synthase were 32% and 24% higher, respectively, in OA than in OS. Exercise at 12 W elicited a lower leg blood flow response that was associated with a lower leg oxygen uptake in OS than in Y. The improved bioavailability of NO in OS did not increase blood flow during exercise. These data demonstrate that NO bioavailability is compromised in the systemic circulation and in the musculature of sedentary ageing humans due to increased oxidative stress. Lifelong physical activity opposes this effect within the trained musculature and in the arterial circulation. The lower blood flow response to leg exercise in ageing humans is not associated with a reduced NO bioavailability.

Cortical actin nanodynamics determines nitric oxide release in vascular endothelium

The release of the main vasodilator nitric oxide (NO) by the endothelial NO synthase (eNOS) is a hallmark of endothelial function. We aim at elucidating the underlying mechanism how eNOS activity depends on cortical stiffness (К(cortex)) of living endothelial cells. It is hypothesized that cortical actin dynamics determines К(cortex) and directly influences eNOS activity. By combined atomic force microscopy and fluorescence imaging we generated mechanical and optical sections of single living cells. This approach allows the discrimination between К(cortex) and bulk cell stiffness (К(bulk)) and, additionally, the simultaneous analysis of submembranous actin web dynamics. We show that К(cortex) softens when cortical F-actin depolymerizes and that this shift from a gel-like stiff cortex to a soft G-actin rich layer, triggers the stiffness-sensitive eNOS activity. The results implicate that stiffness changes in the ∼100 nm phase of the submembranous actin web, without affecting К(bulk), regulate NO release and thus determines endothelial function.

Pro- and anti-angiogenic factors in human skeletal muscle in response to acute exercise and training

This study examined the effect of acute exercise and 4 weeks of aerobic training on skeletal muscle gene and protein expression of pro- and anti-angiogenic factors in 14 young male subjects. Training consisted of 60 min of cycling (∼60% of ), 3 times/week. Biopsies were obtained from vastus lateralis muscle before and after training. Muscle interstitial fluid was collected during cycling at weeks 0 and 4. Training increased (P < 0.05) the capillary: fibre ratio and capillary density by 23% and 12%, respectively. The concentration of interstitial vascular endothelial growth factor (VEGF) in response to acute exercise increased similarly (>6-fold; P < 0.05) before and after training. Resting protein levels of soluble VEGF receptor-1 in interstitial fluid, and of VEGF, thrombospondin-1 (TSP-1) and tissue inhibitor of matrix metalloproteinase-1 (TIMP1) in muscle were unaffected by training, whereas endothelial nitric oxide synthase protein levels in muscle increased by 50% (P < 0.05). Before and after training, acute exercise induced a similar increase (P < 0.05) in the mRNA level of angiopoietin 2, matrix metalloproteinase 9 and TSP-1. After training, TIMP1 mRNA content increased with exercise (P < 0.05). In conclusion, acute exercise induced a similar increase in the gene-expression of both pro- and anti-angiogenic factors in untrained and trained muscle. We propose that the increase in anti-angiogenic factors with exercise is important for modulation of angiogenesis. The lack of effect of training on basal muscle VEGF protein levels and VEGF secretion during exercise suggests that increased VEGF levels are not a prerequisite for exercise-induced capillary growth in healthy muscle.

Exercise training alters the balance between vasoactive compounds in skeletal muscle of individuals with essential hypertension

The effects of physical training on the formation of vasodilating and vasoconstricting compounds, as well as on related proteins important for vascular function, were examined in skeletal muscle of individuals with essential hypertension (n=10). Muscle microdialysis samples were obtained from subjects with hypertension before and after 16 weeks of physical training. Muscle dialysates were analyzed for thromboxane A(2), prostacyclin, nucleotides, and nitrite/nitrate. Protein levels of thromboxane synthase, prostacyclin synthase, cyclooxygenase 1 and 2, endothelial nitric oxide synthase (eNOS), cystathionine-γ-lyase, cytochrome P450 4A and 2C9, and the purinergic receptors P2X1 and P2Y2 were determined in skeletal muscle. The protein levels were compared with those of normotensive control subjects (n=12). Resting muscle dialysate thromboxane A(2) and prostacyclin concentrations were lower (P<0.05) after training compared with before training. Before training, dialysate thromboxane A(2) decreased with acute exercise, whereas after training, no changes were found. Before training, dialysate prostacyclin levels did not increase with acute exercise, whereas after training there was an 82% (P<0.05) increase from rest to exercise. The exercise-induced increase in ATP and ADP was markedly reduced after training (P<0.05). The amount of eNOS protein in the hypertensive subjects was 40% lower (P<0.05) than in the normotensive control subjects, whereas cystathionine-γ-lyase levels were 25% higher (P<0.05), potentially compensating for the lower eNOS level. We conclude that exercise training alters the balance between vasodilating and vasoconstricting compounds as evidenced by a decrease in the level of thromboxane, reduction in the exercise-induced increase in ATP and a greater exercise-induced increase in prostacyclin.

Caffeine intake improves intense intermittent exercise performance and reduces muscle interstitial potassium accumulation

The effect of oral caffeine ingestion on intense intermittent exercise performance and muscle interstitial ion concentrations was examined. The study consists of two studies (S1 and S2). In S1, 12 subjects completed the Yo-Yo intermittent recovery level 2 (Yo-Yo IR2) test with prior caffeine (6 mg/kg body wt; CAF) or placebo (PLA) intake. In S2, 6 subjects performed one low-intensity (20 W) and three intense (50 W) 3-min (separated by 5 min) one-legged knee-extension exercise bouts with (CAF) and without (CON) prior caffeine supplementation for determination of muscle interstitial K(+) and Na(+) with microdialysis. In S1 Yo-Yo IR2 performance was 16% better (P < 0.05) in CAF compared with PLA. In CAF, plasma K(+) at the end of the Yo-Yo IR2 test was 5.2 ± 0.1 mmol/l with no difference between the trials. Plasma free fatty acids (FFA) were higher (P < 0.05) in CAF than PLA at rest and remained higher (P < 0.05) during exercise. Peak blood glucose (8.0 ± 0.6 vs. 6.2 ± 0.4 mmol/l) and plasma NH(3) (137.2 ± 10.8 vs. 113.4 ± 13.3 μmol/l) were also higher (P < 0.05) in CAF compared with PLA. In S2 interstitial K(+) was 5.5 ± 0.3, 5.7 ± 0.3, 5.8 ± 0.5, and 5.5 ± 0.3 mmol/l at the end of the 20-W and three 50-W periods, respectively, in CAF, which were lower (P < 0.001) than in CON (7.0 ± 0.6, 7.5 ± 0.7, 7.5 ± 0.4, and 7.0 ± 0.6 mmol/l, respectively). No differences in interstitial Na(+) were observed between CAF and CON. In conclusion, caffeine intake enhances fatigue resistance and reduces muscle interstitial K(+) during intense intermittent exercise.

Relationship between performance at different exercise intensities and skeletal muscle characteristics

The hypothesis investigated whether exercise performance over a broad range of intensities is determined by specific skeletal muscle characteristics. Seven subjects performed 8–10 exhaustive cycle trials at different workloads, ranging from 150 to 700 W (150 min to 20 s). No relationships between the performance times at high and low workloads were observed. A relationship ( P < 0.05) was noticed between the percentage of fast-twitch x fibers and the exercise time at 579 ± 21 W (∼30 s; r2 = 0.88). Capillary-to-fiber-ratio ( r2: 0.58–0.85) was related ( P < 0.05) to exercise time at work intensities ranging from 395 to 270 W (2.5–21 min). Capillary density was correlated ( r2 = 0.68; P < 0.05) with the net rate of plasma K+ accumulation during an ∼3-min bout and was estimated to explain 50–80% ( P < 0.05) of the total variance observed in exercise performances lasting ∼30 s to 3 min. The Na+-K+ pump β1-subunit expression was found to account for 13–34% ( P < 0.05) during exhaustive exercise of ∼1–4 min. In conclusion, exercise performance at different intensities is related to specific physiological variables. A large distribution of fast-twitch x fibers may play a role during very intense efforts, i.e., ∼30 s. Muscle capillaries and the Na+-K+ pump β1-subunit seem to be important determinants for performance during exhaustive high-intensity exercises lasting between 30 s and 4 min.

The effect of passive movement training on angiogenic factors and capillary growth in human skeletal muscle

The effect of a period of passive movement training on angiogenic factors and capillarization in skeletal muscle was examined. Seven young males were subjected to passive training for 90 min, four times per week in a motor-driven knee extensor device that extended one knee passively at 80 cycles min₋₁. The other leg was used as control. Muscle biopsies were obtained from m. v. lateralis of both legs before as well as after 2 and 4 weeks of training. After the training period, passive movement and active exercise were performed with both legs, and muscle interstitial fluid was sampled from microdialysis probes in the thigh. After 2 weeks of training there was a 2-fold higher level of Ki-67 positive cells, co-localized with endothelial cells, in the passively trained leg which was paralleled by an increase in the number of capillaries around a fibre (P <0.05). Capillary density was higher than pre-training at 4 weeks of training (P <0.05). The training induced an increase in the mRNA level of endothelial nitric oxide synthase (eNOS), the angiopoietin receptor Tie-2 and matrix metalloproteinase (MMP)-9 in the passively trained leg and MMP-2 and tissue inhibitor of MMP (TIMP)-1 mRNA were elevated in both legs. Acute passive movement increased (P <0.05) muscle interstitial vascular endothelial growth factor (VEGF) levels 4- to 6-fold above rest and the proliferative effect, determined in vitro, of the muscle interstitial fluid ~16-fold compared to perfusate. The magnitude of increase was similar for active exercise. The results demonstrate that a period of passive movement promotes endothelial cell proliferation and angiogenic factors and initiates capillarization in skeletal muscle.

Interstitial and plasma adenosine stimulate nitric oxide and prostacyclin formation in human skeletal muscle

One major unresolved issue in muscle blood flow regulation is that of the role of circulating versus interstitial vasodilatory compounds. The present study determined adenosine-induced formation of NO and prostacyclin in the human muscle interstitium versus in femoral venous plasma to elucidate the interaction and importance of these vasodilators in the 2 compartments. To this end, we performed experiments on humans using microdialysis technique in skeletal muscle tissue, as well as the femoral vein, combined with experiments on cultures of microvascular endothelial versus skeletal muscle cells. In young healthy humans, microdialysate was collected at rest, during arterial infusion of adenosine, and during interstitial infusion of adenosine through microdialysis probes inserted into musculus vastus lateralis. Muscle interstitial NO and prostacyclin increased with arterial and interstitial infusion of adenosine. The addition of adenosine to skeletal muscle cells increased NO formation (fluorochrome 4-amino-5-methylamino-2',7-difluorescein fluorescence), whereas prostacyclin levels remained unchanged. The addition of adenosine to microvascular endothelial cells induced an increase in NO and prostacyclin levels. These findings provide novel insight into the role of adenosine in skeletal muscle blood flow regulation and vascular function by revealing that both interstitial and plasma adenosine have a stimulatory effect on NO and prostacyclin formation. In addition, both skeletal muscle and microvascular endothelial cells are potential mediators of adenosine-induced formation of NO in vivo, whereas only endothelial cells appear to play a role in adenosine-induced formation of prostacyclin.

Contraction-induced secretion of VEGF from skeletal muscle cells is mediated by adenosine

The role of adenosine and contraction for secretion of vascular endothelial growth factor (VEGF) in skeletal muscle was investigated in human subjects and rat primary skeletal muscle cells. Microdialysis probes were inserted in the thigh muscle of seven male subjects, and dialysate was collected at rest, during infusion of adenosine, and during knee extensor exercise. The dialysate was analyzed for content of VEGF protein and adenosine. The mechanism of VEGF secretion from muscle cells in culture was examined in resting and electrostimulated cells and in response to the adenosine analog NECA and the adenosine A2A receptor specific analog CGS-21680. Adenosine receptors A1, A2A, and A2B were blocked with DPCPX, ZM-241385, and enprofylline, respectively. cAMP-dependent protein kinase A (PKA) and mitogen-activated protein kinase (MAPK) were inhibited by H-89 and PD-98509, respectively. The human experiment showed that adenosine infusion enhanced ( P < 0.05) the interstitial concentration of VEGF protein approximately fourfold above baseline. Exercise increased ( P < 0.05) the interstitial VEGF concentration approximately sixfold above rest in parallel with an approximately threefold increase in adenosine concentration. In accordance, in cultured muscle cells, NECA and contraction caused secretion of VEGF ( P < 0.05). The contraction-induced secretion of VEGF was abolished by the A2B antagonist enprofylline and by inhibition of PKA or MAPK. The results demonstrate that adenosine causes secretion of VEGF from human skeletal muscle cells and that the contraction-induced secretion of VEGF protein is partially mediated via adenosine acting on A2B adenosine receptors. Moreover, the contraction-induced secretion of VEGF protein from muscle is dependent on both PKA and MAPK activation, but only the MAPK pathway appears to be adenosine dependent, revealing involvement of additional pathways in VEGF secretion.

Human muscle net K+ release during exercise is unaffected by elevated anaerobic metabolism, but reduced after prolonged acclimatization to 4,100 m

It was investigated whether skeletal muscle K+ release is linked to the degree of anaerobic energy production. Six subjects performed an incremental bicycle exercise test in normoxic and hypoxic conditions prior to and after 2 and 8 wk of acclimatization to 4,100 m. The highest workload completed by all subjects in all trials was 260 W. With acute hypoxic exposure prior to acclimatization, venous plasma [K+] was lower ( P < 0.05) in normoxia (4.9 ± 0.1 mM) than hypoxia (5.2 ± 0.2 mM) at 260 W, but similar at exhaustion, which occurred at 400 ± 9 W and 307 ± 7 W ( P < 0.05), respectively. At the same absolute exercise intensity, leg net K+ release was unaffected by hypoxic exposure independent of acclimatization. After 8 wk of acclimatization, no difference existed in venous plasma [K+] between the normoxic and hypoxic trial, either at submaximal intensities or at exhaustion (360 ± 14 W vs. 313 ± 8 W; P < 0.05). At the same absolute exercise intensity, leg net K+ release was less ( P < 0.001) than prior to acclimatization and reached negative values in both hypoxic and normoxic conditions after acclimatization. Moreover, the reduction in plasma volume during exercise relative to rest was less ( P < 0.01) in normoxic than hypoxic conditions, irrespective of the degree of acclimatization (at 260 W prior to acclimatization: −4.9 ± 0.8% in normoxia and −10.0 ± 0.4% in hypoxia). It is concluded that leg net K+ release is unrelated to anaerobic energy production and that acclimatization reduces leg net K+ release during exercise.

Faster oxygen uptake kinetics during recovery is related to better repeated sprinting ability

This study was designed to test the hypothesis that subjects having faster oxygen uptake (VO(2)) kinetics during off-transients to exercises of severe intensity would obtain the smallest decrement score during a repeated sprint test. Twelve male soccer players completed a graded test, two severe-intensity exercises, followed by 6 min of passive recovery, and a repeated sprint test, consisting of seven 30-m sprints alternating with 20 s of active recovery. The relative decrease in score during the repeated sprint test was positively correlated with time constants of the primary phase for the VO(2) off-kinetics (r = 0.85; p < 0.001) and negatively correlated with the VO(2) peak (r = -0.83; p < 0.001). These results strengthen the link found between VO(2) kinetics and the ability to maintain sprint performance during repeated sprints.

Muscle carnosine metabolism and beta-alanine supplementation in relation to exercise and training

Carnosine is a dipeptide with a high concentration in mammalian skeletal muscle. It is synthesized by carnosine synthase from the amino acids L-histidine and beta-alanine, of which the latter is the rate-limiting precursor, and degraded by carnosinase. Recent studies have shown that the chronic oral ingestion of beta-alanine can substantially elevate (up to 80%) the carnosine content of human skeletal muscle. Interestingly, muscle carnosine loading leads to improved performance in high-intensity exercise in both untrained and trained individuals. Although carnosine is not involved in the classic adenosine triphosphate-generating metabolic pathways, this suggests an important role of the dipeptide in the homeostasis of contracting muscle cells, especially during high rates of anaerobic energy delivery. Carnosine may attenuate acidosis by acting as a pH buffer, but improved contractile performance may also be obtained by improved excitation-contraction coupling and defence against reactive oxygen species. High carnosine concentrations are found in individuals with a high proportion of fast-twitch fibres, because these fibres are enriched with the dipeptide. Muscle carnosine content is lower in women, declines with age and is probably lower in vegetarians, whose diets are deprived of beta-alanine. Sprint-trained athletes display markedly high muscular carnosine, but the acute effect of several weeks of training on muscle carnosine is limited. High carnosine levels in elite sprinters are therefore either an important genetically determined talent selection criterion or a result of slow adaptation to years of training. beta-Alanine is rapidly developing as a popular ergogenic nutritional supplement for athletes worldwide, and the currently available scientific literature suggests that its use is evidence based. However, many aspects of the supplement, such as the potential side effects and the mechanism of action, require additional and thorough investigation by the sports science community.

Potassium-transporting proteins in skeletal muscle: cellular location and fibre-type differences

Abstract Potassium (K(+)) displacement in skeletal muscle may be an important factor in the development of muscle fatigue during intense exercise. It has been shown in vitro that an increase in the extracellular K(+) concentration ([K(+)](e)) to values higher than approx. 10 mm significantly reduce force development in unfatigued skeletal muscle. Several in vivo studies have shown that [K(+)](e) increases progressively with increasing work intensity, reaching values higher than 10 mm. This increase in [K(+)](e) is expected to be even higher in the transverse (T)-tubules than the concentration reached in the interstitium. Besides the voltage-sensitive K(+) (K(v)) channels that generate the action potential (AP) it is suggested that the big-conductance Ca(2+)-dependent K(+) (K(Ca)1.1) channel contributes significantly to the K(+) release into the T-tubules. Also the ATP-dependent K(+) (K(ATP)) channel participates, but is suggested primarily to participate in K(+) release to the interstitium. Because there is restricted diffusion of K(+) to the interstitium, K(+) released to the T-tubules during AP propagation will be removed primarily by reuptake mediated by transport proteins located in the T-tubule membrane. The most important protein that mediates K(+) reuptake in the T-tubules is the Na(+),K(+)-ATPase alpha(2) dimers, but a significant contribution of the strong inward rectifier K(+) (Kir2.1) channel is also suggested. The Na(+), K(+), 2Cl(-) 1 (NKCC1) cotransporter also participates in K(+) reuptake but probably mainly from the interstitium. The relative content of the different K(+)-transporting proteins differs in oxidative and glycolytic muscles, and might explain the different [K(+)](e) tolerance observed.

Effects of membrane depolarization and changes in extracellular [K(+)] on the Ca (2+) transients of fast skeletal muscle fibers. Implications for muscle fatigue

Repetitive activation of skeletal muscle fibers leads to a reduced transmembrane K⁺ gradient. The resulting membrane depolarization has been proposed to play a major role in the onset of muscle fatigue. Nevertheless, raising the extracellular K⁺ (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document}) concentration (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document}) to 10 mM potentiates twitch force of rested amphibian and mammalian fibers. We used a double Vaseline gap method to simultaneously record action potentials (AP) and Ca²⁺ transients from rested frog fibers activated by single and tetanic stimulation (10 pulses, 100 Hz) at various \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} and membrane potentials. Depolarization resulting from current injection or raised \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} produced an increase in the resting [Ca²⁺]. Ca²⁺ transients elicited by single stimulation were potentiated by depolarization from −80 to −60 mV but markedly depressed by further depolarization. Potentiation was inversely correlated with a reduction in the amplitude, overshoot and duration of APs. Similar effects were found for the Ca²⁺ transients elicited by the first pulse of 100 Hz trains. Depression or block of Ca²⁺ transient in response to the 2nd to 10th pulses of 100 Hz trains was observed at smaller depolarizations as compared to that seen when using single stimulation. Changes in Ca²⁺ transients along the trains were associated with impaired or abortive APs. Raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} to 10 mM potentiated Ca²⁺ transients elicited by single and tetanic stimulation, while raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} to 15 mM markedly depressed both responses. The effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} on Ca²⁺ transients, but not those of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document}, could be fully reversed by hyperpolarization. The results suggests that the force potentiating effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} might be mediated by depolarization dependent changes in resting [Ca²⁺] and Ca²⁺ release, and that additional mechanisms might be involved in the effects of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} on force generation.

Effect of extracellular potassium accumulation on muscle fiber conduction velocity: a simulation study

A progressive reduction in muscle fiber conduction velocity is typically observed during fatiguing muscle contraction. Although the exact causes of the conduction velocity decrease have not yet been fully established, increasing evidence suggests that changes in extracellular potassium concentration may be largely responsible. In this study, a mathematical model was developed to examine the effect of extracellular potassium concentration on the muscle fiber action potential and conduction velocity. The model was used to simulate changes in extracellular potassium concentration at a range of temperatures and extracellular potassium accumulation during repetitive stimulation of the muscle fiber at 37 degrees C. The action potential broadened, and its amplitude and conduction velocity decreased as extracellular potassium concentration increased. The potassium-induced changes in action potential shape and conduction velocity were eliminated when the inward rectifier channels were removed from the model. The results support the hypothesis that accumulation of extracellular potassium ions may be a major contributor to the reduction in muscle fiber conduction velocity and loss of membrane excitability during fatiguing contractions. They additionally suggest that inward rectifier currents play a critical role in potassium-induced membrane depolarization, leading to increased sodium inactivation and resulting in the observed reduction in conduction velocity and membrane excitability.

Potassium softens vascular endothelium and increases nitric oxide release

In the presence of aldosterone, plasma sodium in the high physiological range stiffens endothelial cells and reduces the release of nitric oxide. We now demonstrate effects of extracellular potassium on stiffness of individual cultured bovine aortic endothelial cells by using the tip of an atomic force microscope as a mechanical nanosensor. An acute increase of potassium in the physiological range swells and softens the endothelial cell and increases the release of nitric oxide. A high physiological sodium concentration, in the presence of aldosterone, prevents these changes. We propose that the potassium effects are caused by submembranous cortical fluidization because cortical actin depolymerization induced by cytochalasin D mimics the effect of high potassium. In contrast, a low dose of trypsin, known to activate sodium influx through epithelial sodium channels, stiffens the submembranous cell cortex. Obviously, the cortical actin cytoskeleton switches from gelation to solation depending on the ambient sodium and potassium concentrations, whereas the center of the cell is not involved. Such a mechanism would control endothelial deformability and nitric oxide release, and thus influence systemic blood pressure.

Effects of calcitonin gene-related peptide on rat soleus muscle excitability: mechanisms and physiological significance

Intense exercise causes a large loss of K+ from contracting muscles. The ensuing elevation of extracellular K+ ([K+]o) has been suggested to cause fatigue by depressing muscle fiber excitability. In isolated muscles, however, repeated contractions confer some protection against this effect of elevated K+. We hypothesize that this excitation-induced force-recovery is related to the release of the neuropeptide calcitonin gene-related peptide (CGRP), which stimulates the muscular Na+-K+ pumps. Using the specific CGRP antagonist CGRP-(8-37), we evaluated the role of CGRP in the excitation-induced force recovery and examined possible mechanisms. Intact rat soleus muscles were stimulated to evoke short tetani at regular intervals. Increasing extracellular K+ ([K+]o) from 4 to 11 mM decreased force to ∼20% of initial force ( P < 0.001). Addition of exogenous CGRP (10−9 M), release of endogenous CGRP with capsaicin, or repeated electrical stimulation recovered force to 50–70% of initial force ( P < 0.001). In all cases, force recovery could be almost completely suppressed by CGRP-(8-37). At 11 mM [K+]o, CGRP (10−8 M) did not alter resting membrane potential or conductance but significantly improved action potentials ( P < 0.001) and increased the proportion of excitable fibers from 32 to 70% ( P < 0.001). CGRP was shown to induce substantial force recovery with only modest Na+-K+ pump stimulation. We conclude that the excitation-induced force recovery is caused by a recovery of excitability, induced by local release of CGRP. The data suggest that the recovery of excitability partly was induced by Na+-K+ pump stimulation and partly by altering Na+ channel function.

Passive leg movement enhances interstitial VEGF protein, endothelial cell proliferation, and eNOS mRNA content in human skeletal muscle

The present study used passive limb movement as an experimental model to study the effect of increased blood flow and passive stretch, without enhanced metabolic demand, in young healthy male subjects. The model used was 90 min of passive movement of the leg leading to a 2.8-fold increase ( P < 0.05) in blood flow without a significant enhancement in oxygen uptake. Muscle interstitial fluid was sampled with microdialysis technique and analyzed for vascular endothelial growth factor (VEGF) protein and for the effect on endothelial cell proliferation. Biopsies obtained from the musculus vastus lateralis were analyzed for mRNA content of VEGF, endothelial nitric oxide synthase (eNOS), and matrix metalloproteinase-2 (MMP-2). The passive leg movement caused an increase ( P < 0.05) in interstitial VEGF protein concentration above rest (73 ± 21 vs. 344 ± 83 pg/ml). Addition of muscle dialysate to cultured endothelial cells revealed that dialysate obtained during leg movement induced a 3.2-fold higher proliferation rate ( P < 0.05) than dialysate obtained at rest. Passive movement also enhanced ( P < 0.05) the eNOS mRNA level fourfold above resting levels. VEGF mRNA and MMP-2 mRNA levels were unaffected. The results show that a session of passive leg movement, elevating blood flow and causing passive stretch, augments the interstitial concentrations of VEGF, the proliferative effect of interstitial fluid, and eNOS mRNA content in muscle tissue. We propose that enhanced blood flow and passive stretch are positive physiological stimulators of factors associated with capillary growth in human muscle.

Effect of dexamethasone on skeletal muscle Na+,K+ pump subunit specific expression and K+ homeostasis during exercise in humans

The effect of dexamethasone on Na(+),K(+) pump subunit expression and muscle exchange of K(+) during exercise in humans was investigated. Nine healthy male subjects completed a randomized double blind placebo controlled protocol, with ingestion of dexamethasone (Dex: 2 x 2 mg per day) or placebo (Pla) for 5 days. Na(+),K(+) pump catalytic alpha1 and alpha2 subunit expression was approximately 17% higher (P < 0.05) and the structural beta1 and beta2 subunit expression was approximately 6-8% higher (P < 0.05) after Dex compared with Pla. During one-legged knee-extension for 10 min at low intensity (LI; 18.6 +/- 1.0 W), two moderate intensity (51.7 +/- 2.4 W) exercise bouts (MI(1): 5 min; 2 min recovery; MI(2): exhaustive) and two high-intensity (71.7 +/- 2.5 W) exercise bouts (HI(1): 1 min 40 s; 2 min recovery; HI(2): exhaustive), femoral venous K(+) was lower (P < 0.05) in Dex compared with Pla. Thigh K(+) release was lower (P < 0.05) in Dex compared with Pla in LI and MI, but not in HI. Time to exhaustion in MI(2) tended to improve (393 +/- 50 s versus 294 +/- 41 s; P = 0.07) in Dex compared with Pla, whereas no difference was detected in HI(2) (106 +/- 10 s versus 108 +/- 9 s). The results indicate that an increased Na(+),K(+) pump expression per se is of importance for thigh K(+) reuptake at the onset of low and moderate intensity exercise, but less important during high intensity exercise.

Muscle K+, Na+, and Cl− disturbances and Na+-K+ pump inactivation: implications for fatigue

Membrane excitability is a critical regulatory step in skeletal muscle contraction and is modulated by local ionic concentrations, conductances, ion transporter activities, temperature, and humoral factors. Intense fatiguing contractions induce cellular K+ efflux and Na+ and Cl− influx, causing pronounced perturbations in extracellular (interstitial) and intracellular K+ and Na+ concentrations. Muscle interstitial K+ concentration may increase 1- to 2-fold to 11–13 mM and intracellular K+ concentration fall by 1.3- to 1.7-fold; interstitial Na+ concentration may decline by 10 mM and intracellular Na+ concentration rise by 1.5- to 2.0-fold. Muscle Cl− concentration changes reported with muscle contractions are less consistent, with reports of both unchanged and increased intracellular Cl− concentrations, depending on contraction type and the muscles studied. When considered together, these ionic changes depolarize sarcolemmal and t-tubular membranes to depress tetanic force and are thus likely to contribute to fatigue. Interestingly, less severe local ionic changes can also augment subtetanic force, suggesting that they may potentiate muscle contractility early in exercise. Increased Na+-K+-ATPase activity during exercise stabilizes Na+ and K+ concentration gradients and membrane excitability and thus protects against fatigue. However, during intense contraction some Na+-K+ pumps are inactivated and together with further ionic disturbances, likely precipitate muscle fatigue.

Potassium, Na+,K+-pumps and fatigue in rat muscle

During contractile activity, skeletal muscles undergo a net loss of cytoplasmic K(+) to the interstitial space. During intense exercise, plasma K(+) in human arterial blood may reach 8 mm, and interstitial K(+) 10-12 mm. This leads to depolarization, loss of excitability and contractile force. However, little is known about the effects of these physiological increases in extracellular K(+) ([K(+)](o)) on contractile endurance. Soleus muscles from 4-week-old rats were mounted on transducers for isometric contractions in Krebs-Ringer bicarbonate buffer containing 4-10 mm K(+), and endurance assessed by recording the rate of force decline during continuous stimulation at 60 Hz. Increasing [K(+)](o) from 4 to 8 or 10 mm and equilibrating the muscles for 40 or 20 min augmented the rate of force decline 2.4-fold and 7.2-fold, respectively (P < 0.001). The marked loss of endurance elicited by exposure to 8 or 10 mm K(+) was alleviated or significantly reduced by stimulating the Na(+),K(+)-pumps by intracellular Na(+) loading, the beta(2)-agonist salbutamol, adrenaline, calcitonin gene related peptide, insulin or repeated excitation. In conclusion, excitation-induced increase in [K(+)](o) is an important cause of high-frequency fatigue, and the Na(+),K(+)-pumps are essential for the maintenance of contractile force in the physiological range of [K(+)](o). Recordings of contractile force during continuous stimulation at 8-10 mm K(+) may be used to analyse the effects of agents or conditions influencing the excitability of working isolated muscles.

Metabolic Response and Fatigue in Soccer

The physical demands in soccer have been studied intensively, and the aim of the present review is to provide an overview of metabolic changes during a game and their relation to the development of fatigue. Heart-rate and body-temperature measurements suggest that for elite soccer players the average oxygen uptake during a match is around 70% of maximum oxygen uptake (VO2 max). A top-class player has 150 to 250 brief intense actions during a game, indicating that the rates of creatine-phosphate (CP) utilization and glycolysis are frequently high during a game, which is supported by findings of reduced muscle CP levels and several-fold increases in blood and muscle lactate concentrations. Likewise, muscle pH is lowered and muscle inosine monophosphate (IMP) elevated during a soccer game. Fatigue appears to occur temporarily during a game, but it is not likely to be caused by elevated muscle lactate, lowered muscle pH, or change in muscle-energy status. It is unclear what causes the transient reduced ability of players to perform maximally. Muscle glycogen is reduced by 40% to 90% during a game and is probably the most important substrate for energy production, and fatigue toward the end of a game might be related to depletion of glycogen in some muscle fibers. Blood glucose and catecholamines are elevated and insulin lowered during a game. The blood free-fatty-acid levels increase progressively during a game, probably reflecting an increasing fat oxidation compensating for the lowering of muscle glycogen. Thus, elite soccer players have high aerobic requirements throughout a game and extensive anaerobic demands during periods of a match leading to major metabolic changes, which might contribute to the observed development of fatigue during and toward the end of a game.

Additive protective effects of the addition of lactic acid and adrenaline on excitability and force in isolated rat skeletal muscle depressed by elevated extracellular K+

During strenuous exercise, extracellular K(+) ([K(+)](o)) is increased, which potentially can reduce muscle excitability and force production. In addition, exercise leads to accumulation of lactate and H(+) and increased levels of circulating catecholamines. Individually, reduced pH and increased catecholamines have been shown to counteract the depressing effect of elevated K(+). This study examines (i) whether the effects of addition of lactic acid and adrenaline on the excitability of isolated muscles are caused by separate mechanisms and are additive and (ii) whether the effect of adding lactic acid or increasing CO(2) is related to a reduction of intra- or extracellular pH. Rat soleus muscles were incubated at a [K(+)](o) of 15 mM, which reduced tetanic force by 85%. Subsequent addition of 20 mM lactic acid or 10(-5) M adrenaline led to a small recovery of force, but when added together induced an almost complete force recovery. Compound action potentials showed that the force recovery was associated with recovery of muscle excitability. The improved excitability after addition of adrenaline was associated with increased Na(+)-K(+) pump activity resulting in hyperpolarization and an increase in the chemical Na(+) gradient. In contrast, addition of lactic acid had no effect on the membrane potential or the Na(+)-K(+) pump activity, but most likely increased excitability via a reduction in intracellular pH. It is concluded that the protective effects of acidosis and adrenaline on muscle excitability and force took place via different mechanisms and were additive. The results suggest that circulating catecholamines and development of acidosis during exercise may improve the tolerance of muscles to elevated [K(+)](o).

Effects of extracellular HCO3(-) on fatigue, pHi, and K+ efflux in rat skeletal muscles

Elevated plasma HCO(3)(-) can improve exercise endurance in humans. This effect has been related to attenuation of the work-induced reduction in muscle pH, which is suggested to improve performance via at least two mechanisms: 1) less inhibition of muscle enzymes and 2) reduced opening of muscle K(ATP) channels with less ensuing reduction in excitability. Aiming at determining whether the ergogenic effect of HCO(3)(-) is related to effects on muscles, we examined the effect of elevating extracellular HCO(3)(-) from 25 to 40 mM (pH from 7.4 to 7.6) on fatigue, intracellular pH (pH(i)), and K(+) efflux in isolated rat skeletal muscles contracting isometrically. Fatigue induced by 30-Hz stimulation at 30 and 37 degrees C was similar between soleus muscles incubated in high and normal HCO(3)(-) concentrations. In extensor digitorum longus muscles stimulated at 60 Hz, elevated HCO(3)(-) did not affect fatigue at 30 degrees C. In soleus muscles, 30-Hz stimulation induced a approximately 0.2 unit reduction in pH(i), as determined by using the pH-sensitive probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. This reduction in pH(i) was not affected by elevated HCO(3)(-). Estimation of K(+) efflux using (86)Rb(+) showed that elevated HCO(3)(-) did not affect K(+) efflux at rest or during contractions. Similarly, other modifications of the intra- and extracellular pH had little effect on K(+) efflux during contraction. In conclusion, elevated extracellular HCO(3)(-) had no significant effect on muscle fatigue, pH(i), and K(+) efflux. These findings indicate that alternative mechanisms must be considered for the ergogenic effect of HCO(3)(-) observed in integral exercise studies.

Energy conservation attenuates the loss of skeletal muscle excitability during intense contractions

High-frequency stimulation of skeletal muscle has long been associated with ionic perturbations, resulting in the loss of membrane excitability, which may prevent action potential propagation and result in skeletal muscle fatigue. Associated with intense skeletal muscle contractions are large changes in muscle metabolites. However, the role of metabolites in the loss of muscle excitability is not clear. The metabolic state of isolated rat extensor digitorum longus muscles at 30 degrees C was manipulated by decreasing energy expenditure and thereby allowed investigation of the effects of energy conservation on skeletal muscle excitability. Muscle ATP utilization was reduced using a combination of the cross-bridge cycling blocker N-benzyl-p-toluene sulfonamide (BTS) and the SR Ca2+ release channel blocker Na-dantrolene, which reduce activity of the myosin ATPase and SR Ca2+-ATPase. Compared with control muscles, the resting metabolites ATP, phosphocreatine, creatine, and lactate, as well as the resting muscle excitability as measured by M-waves, were unaffected by treatment with BTS plus dantrolene. Following 20 or 30 s of continuous 60-Hz stimulation, BTS-plus-dantrolene-treated muscles showed a 25% lower ATP utilization compared with control muscles. Furthermore, the ability of muscles to maintain excitability during high-frequency stimulation was significantly improved in BTS-plus-dantrolene-treated muscles, indicating a strong link between metabolites, energetic state, and the excitability of the muscle.

Effect of two different intense training regimens on skeletal muscle ion transport proteins and fatigue development

This study examined the effect of two different intense exercise training regimens on skeletal muscle ion transport systems, performance, and metabolic response to exercise. Thirteen subjects performed either sprint training [ST; 6-s sprints (n = 6)], or speed endurance training [SET; 30-s runs approximately 130% Vo(2 max), n = 7]. Training in the SET group provoked higher (P < 0.05) plasma K(+) levels and muscle lactate/H(+) accumulation. Only in the SET group was the amount of the Na(+)/H(+) exchanger isoform 1 (31%) and Na(+)-K(+)-ATPase isoform alpha(2) (68%) elevated (P < 0.05) after training. Both groups had higher (P < 0.05) levels of Na(+)-K(+)-ATPase beta(1)-isoform and monocarboxylate transporter 1 (MCT1), but no change in MCT4 and Na(+)-K(+)-ATPase alpha(1)-isoform. Both groups had greater (P < 0.05) accumulation of lactate during exhaustive exercise and higher (P < 0.05) rates of muscle lactate decrease after exercise. The ST group improved (P < 0.05) sprint performance, whereas the SET group elevated (P < 0.05) performance during exhaustive continuous treadmill running. Improvement in the Yo-Yo intermittent recovery test was larger (P < 0.05) in the SET than ST group (29% vs. 10%). Only the SET group had a decrease (P < 0.05) in fatigue index during a repeated sprint test. In conclusion, turnover of lactate/H(+) and K(+) in muscle during exercise does affect the adaptations of some but not all related muscle ion transport proteins with training. Adaptations with training do have an effect on the metabolic response to exercise and specific improvement in work capacity.

Beta3-adrenoceptor agonist stimulation of the Na+, K+ -pump in rat skeletal muscle is mediated by beta2- rather than beta3-adrenoceptors

BACKGROUND AND PURPOSE

In cardiac muscle, BRL 37344, a selective beta3-adrenoceptor agonist, activates the Na+, K+ -pump via NO signalling. This study investigated whether BRL 37344 also activates the Na+, K+ -pump via beta3-adrenoceptors in skeletal muscle.

EXPERIMENTAL APPROACH

Isolated rat soleus muscles were incubated between 1 and 60 min in buffer. Intracellular Na+, K+ content and Na+, K+ -pump activity were measured using flame photometry and ouabain-suppressible 86Rb+ uptake, respectively. Additional muscles were mounted on force transducers and stimulated (60 Hz for 2 s) every 10 min.

KEY RESULTS

BRL 37344 (10(-8) -10(-5) M) induced a concentration- and time-dependent reduction in intracellular Na+, and increased ouabain-suppressible 86Rb+ uptake by up to 112%. BRL 37344-induced reductions in intracellular Na+ were blocked by the beta1/beta2-adrenoceptor antagonist, nadolol (10(-7) M), and the beta2-adrenoceptor antagonist, ICI 118,551 (10(-7) -10(-5) M), but not by beta3- or beta1-adrenoceptor antagonists, SR 59230A (10(-7) M) and CGP 20712A (10(-7) -10(-5) M), respectively. Another beta3-adrenoceptor agonist, CL 316,243, did not alter intracellular Na+. BRL 37344-induced reductions in intracellular Na+ were not blocked by L-NAME, an NOS inhibitor, or ODQ, a guanylyl cyclase inhibitor. The NO donors, SNP and SNAP, did not alter intracellular Na+. BRL 37344 rapidly recovered force in muscles depressed by high [K+]o, an effect that was blocked by nadolol, but not L-NAME.

CONCLUSIONS AND IMPLICATIONS

In rat soleus muscle, the beta3-adrenoceptor agonist BRL 37344 stimulated the Na+, K+ -pump via beta2-adrenoceptors. A more selective beta3-adrenoceptor agonist did not affect Na+, K+ homeostasis in skeletal muscle. NO did not seem to mediate Na+, K+ -pump stimulation in skeletal muscle.

Interspersed normoxia during live high, train low interventions reverses an early reduction in muscle Na+, K +ATPase activity in well-trained athletes

Hypoxia and exercise each modulate muscle Na(+), K(+)ATPase activity. We investigated the effects on muscle Na(+), K(+)ATPase activity of only 5 nights of live high, train low hypoxia (LHTL), 20 nights consecutive (LHTLc) versus intermittent LHTL (LHTLi), and acute sprint exercise. Thirty-three athletes were assigned to control (CON, n = 11), 20-nights LHTLc (n = 12) or 20-nights LHTLi (4 x 5-nights LHTL interspersed with 2-nights CON, n = 10) groups. LHTLc and LHTLi slept at a simulated altitude of 2,650 m (F(I)O(2) 0.1627) and lived and trained by day under normoxic conditions; CON lived, trained, and slept in normoxia. A quadriceps muscle biopsy was taken at rest and immediately after standardised sprint exercise, before (Pre) and after 5-nights (d5) and 20-nights (Post) LHTL interventions and analysed for Na(+), K(+)ATPase maximal activity (3-O-MFPase) and content ([(3)H]-ouabain binding). After only 5-nights LHTLc, muscle 3-O-MFPase activity declined by 2% (P < 0.05). In LHTLc, 3-O-MFPase activity remained below Pre after 20 nights. In contrast, in LHTLi, this small initial decrease was reversed after 20 nights, with restoration of 3-O-MFPase activity to Pre-intervention levels. Plasma [K(+)] was unaltered by any LHTL. After acute sprint exercise 3-O-MFPase activity was reduced (12.9 +/- 4.0%, P < 0.05), but [(3)H]-ouabain binding was unchanged. In conclusion, maximal Na(+), K(+)ATPase activity declined after only 5-nights LHTL, but the inclusion of additional interspersed normoxic nights reversed this effect, despite athletes receiving the same amount of hypoxic exposure. There were no effects of consecutive or intermittent nightly LHTL on the acute decrease in Na(+), K(+)ATPase activity with sprint exercise effects or on plasma [K(+)] during exercise.

N-acetylcysteine attenuates the decline in muscle Na+,K+-pump activity and delays fatigue during prolonged exercise in humans

Reactive oxygen species (ROS) have been linked with both depressed Na(+),K(+)-pump activity and skeletal muscle fatigue. This study investigated N-acetylcysteine (NAC) effects on muscle Na(+),K(+)-pump activity and potassium (K(+)) regulation during prolonged, submaximal endurance exercise. Eight well-trained subjects participated in a double-blind, randomised, crossover design, receiving either NAC or saline (CON) intravenous infusion at 125 mg kg(-1) h(-1) for 15 min, then 25 mg kg(-1) h(-1) for 20 min prior to and throughout exercise. Subjects cycled for 45 min at 71% , then continued at 92% until fatigue. Vastus lateralis muscle biopsies were taken before exercise, at 45 min and fatigue and analysed for maximal in vitro Na(+),K(+)-pump activity (K(+)-stimulated 3-O-methyfluorescein phosphatase; 3-O-MFPase). Arterialized venous blood was sampled throughout exercise and analysed for plasma K(+) and other electrolytes. Time to fatigue at 92% was reproducible in preliminary trials (c.v. 5.6 +/- 0.6%) and was prolonged with NAC by 23.8 +/- 8.3% (NAC 6.3 +/- 0.5 versus CON 5.2 +/- 0.6 min, P < 0.05). Maximal 3-O-MFPase activity decreased from rest by 21.6 +/- 2.8% at 45 min and by 23.9 +/- 2.3% at fatigue (P < 0.05). NAC attenuated the percentage decline in maximal 3-O-MFPase activity (%Deltaactivity) at 45 min (P < 0.05) but not at fatigue. When expressed relative to work done, the %Deltaactivity-to-work ratio was attenuated by NAC at 45 min and fatigue (P < 0.005). The rise in plasma [K(+)] during exercise and the Delta[K(+)]-to-work ratio at fatigue were attenuated by NAC (P < 0.05). These results confirm that the antioxidant NAC attenuates muscle fatigue, in part via improved K(+) regulation, and point to a role for ROS in muscle fatigue.

Aging augments interstitial K+concentrations in active muscle of rats

Skeletal muscle performance declines with advancing age, and the underlying mechanism is not completely understood. A large body of convincing evidence has demonstrated a crucial role for interstitial K+concentration ([K+]o) in modulating contractile function of skeletal muscle. The present study tested the hypothesis that during muscle contraction there is a greater accumulation of [K+]oin aged compared with adult skeletal muscle. Twitch muscle contraction was induced by electrical stimulation of the sciatic nerves of 8- and 32-mo-old Fischer 344 × Brown Norway rats. Levels of [K+]owere measured continuously by a microdialysis technique with the probes inserted into the gastrocnemius muscle. Stimulation at 1, 3, and 5 Hz elevated muscle [K+]oby 52, 64, and 88% in adult rats, and by 78, 98, and 104% in aged rats, respectively, and the increase was significantly higher in aged than in adult rats. Recovery for [K+]o, as measured by the time for [K+]oto recover by 20 and 50% from peak response after stimulation, was slower in aged rats. Ouabain (5 mM), a specific inhibitor of the Na+-K+pump, was added in the perfusate to inhibit the reuptake of K+into the cells to assess the role of the pump in the overall K+balance. Ouabain elevated muscle [K+]oat rest, and the effect was significantly attenuated in aged animals. The present data demonstrated an augmented [K+]oin aged skeletal muscle compared with adult skeletal muscle, and the data suggested that an alteration in the function of the Na+-K+pump may contribute, in part, to the deficiency in K+balance in skeletal muscle of aged rats.