Inhibition of nitric oxide synthase by L-NAME speeds phase II pulmonary .VO2 kinetics in the transition to moderate-intensity exercise in man

A Single Dose of Beetroot Juice not Enhance Performance during Intervallic Swimming Efforts

Despite the numerous scientific evidence on the topic, there is no clear and consistent answer that clarifies the true effects of beetroot juice (BJ) supplementation on different types of physical performance. This study examined whether an acute intake of BJ improves swimming performance, physiological variables of anaerobic metabolism, or subjective measures during high-intensity interval exercise with incomplete rest in competitive swimmers. Eighteen competitive swimmers (nine females and nine males) participated in this cross-over randomized, placebo-controlled, double-blind and counterbalanced study. In two trials, swimmers ingested BJ (70 mL, 6.4 mmol/400 mg NO3-) or placebo (PLA) (70 mL, 0.04 mmol/3 mg NO3-) three hours before a 2×6×100 m maximal effort with 40 seconds rest between repetitions and three minutes between blocks. The 100 m times showed no differences between groups (p > 0.05), but there was an interaction between block×repetition×condition (F5 = 3.10; p = 0.046; ηp2 = 0.54), indicating that the BJ group decreased the time of the sixth repetition of block2 compared to block1 (p = 0.01). Lactate concentration showed no differences between conditions (p > 0.05), but there was a main effect of block (ηp2 = 0.60) and a block×repetition interaction (ηp2 = 0.70), indicating higher values in block2 and increasing values between repetitions in block1. The subjective scales, perception of exertion (RPE) and Total Quality Recovery (TQR), showed no effects of condition (p > 0.05), but BJ swimmers had a greater TQR in the last repetitions of each block. In conclusion, a single dose of BJ did not enhance intermittent swimming performance or modified the physiological (lactate and heart rate) or subjective (RPE and TQR) variables; although there was a possible positive effect on the exercise tolerance at the end of effort.

Effects of hydroxyurea on skeletal muscle energetics and force production in a sickle cell disease murine model

Hydroxyurea (HU) is commonly used as a treatment for patients with sickle cell disease (SCD) to enhance fetal hemoglobin production. This increased production is expected to reduce anemia (which depresses oxygen transport) and abnormal Hb content alleviating clinical symptoms such as vaso-occlusive crisis and acute chest syndrome. The effects of HU on skeletal muscle bioenergetics in vivo are still unknown. Due to the beneficial effects of HU upon oxygen delivery, improved skeletal muscle energetics and function in response to a HU treatment have been hypothesized. Muscle energetics and function were analyzed during a standardized rest-exercise-recovery protocol, using ³¹P-magnetic resonance spectroscopy in Townes SCD mice. Measurements were performed in three groups of mice: one group of 2-mo-old mice (SCD_2m, n = 8), another one of 4-mo-old mice (SCD_4m, n = 8), and a last group of 4-mo-old mice that have been treated from 2 mo of age with HU at 50 mg/kg/day (SCD_4m-HU, n = 8). As compared with SCD_2m mice, SCD_4m mice were heavier and displayed a lower acidosis. As lower specific forces were developed by SCD_4m compared with SCD_2m, greater force-normalized phosphocreatine consumption and oxidative and nonoxidative costs of contraction were also reported. HU-treated mice (SCD_4m-HU) displayed a significantly higher specific force production as compared with untreated mice (SCD_4m), whereas muscle energetics was unchanged. Overall, our results support a beneficial effect of HU on muscle function.NEW & NOTEWORTHY Our results highlighted that force production decreases between 2 and 4 mo of age in SCD mice thereby indicating a decrease of muscle function during this period. Of interest, HU treatment seemed to blunt the observed age effect given that SCD_4m-HU mice displayed a higher specific force production as compared with SCD_4m mice. In that respect, HU treatment would help to maintain a higher capacity of force production during aging in SCD.

Role of nitric oxide in convective and diffusive skeletal microvascular oxygen kinetics

Progress in understanding physiological mechanisms often consists of discrete discoveries made across different models and species. Accordingly, understanding the mechanistic bases for how altering nitric oxide (NO) bioavailability impacts exercise tolerance (or not) depends on integrating information from cellular energetics and contractile regulation through microvascular/vascular control of O₂ transport and pulmonary gas exchange. This review adopts state-of-the-art concepts including the intramyocyte power grid, the Wagner conflation of perfusive and diffusive O₂ conductances, and the Critical Power/Critical Speed model of exercise tolerance to address how altered NO bioavailability may, or may not, affect physical performance. This question is germane from the elite athlete to the recreational exerciser and particularly the burgeoning heart failure (and other clinical) populations for whom elevating O₂ transport and/or exercise capacity translates directly to improved life quality and reduced morbidity and mortality. The dearth of studies in females is also highlighted, and areas of uncertainty and questions for future research are identified.

A new perspective on cardiovascular drift during prolonged exercise

Prolonged exercise induces cardiovascular drift, which is characterized by decreasing mean arterial pressure (MAP), stroke volume and heart rate increase. Cardiovascular drift has been debated for a long time. Although the exact mechanisms underlying cardiovascular drift are still unknown, two theories have been proposed. The first is that increased skin blood flow displaces blood volume from central circulation to the periphery, which reduces stroke volume. According to this theory, the rise in heart rate is presumably responding to the drop in stroke volume and MAP. The alternative theory is that an increase in heart rate is due to an increase in sympathetic nervous activity causing reducing time at diastole, and therefore stroke volume. It may be difficult to determine a single robust factor accounting for cardiovascular drift, due to the broad range of circumstances. The primary focus of this review is to elucidate our understanding of cardiovascular drift during prolonged exercise through nitric oxide and force-frequency relationship. We highlight for the very first time that cardiovascular drift (in some conditions and within a specific time period) may be considered as a protective strategy against potential damage that could be induced by the intense and prolonged contraction of the myocardium.

Systemic NOS inhibition reduces contracting muscle oxygenation more in intact female than male rats

Females respond to baroreceptor stimulation with enhanced modulation of heart rate (HR) to regulate blood pressure and also express greater reliance on nitric oxide (NO) for vascular control compared to males. Sex differences in muscle oxygenation consequent to central hemodynamic challenge induced by systemic NO synthase (NOS) inhibition are unknown. We tested the hypotheses that systemic NOS inhibition would induce lower contracting skeletal muscle oxygenation in females compared to males. The spinotrapezius of Sprague-Dawley rats (females (♀) = 9, males (♂) = 9) was surgically exposed and contracted by electrical stimulation (180s, 1 Hz, ~6 V) under pentobarbital sodium anesthesia. Oxyphor G4 was injected into the muscle and phosphorescence quenching was used to measure the interstitial PO₂ (PO₂is, determined by O₂ delivery-to-utilization matching) under control (Krebs-Henseleit solution) and after intra-arterial infusion of nitro-l-arginine methyl ester (l-NAME; NOS blockade; 10 mg kg^-1). At rest, females showed a greater PO₂is increase (ΔPO₂is/ΔMAP) and HR (ΔHR/ΔMAP) reduction than males in response to the elevated MAP induced by systemic NOS inhibition (both p < 0.05). Following l-NAME, during the contracting steady-state, females exhibited lower PO₂is than males (♂: 17.1 ± 1.4 vs ♀: 10.8 ± 1.4 mmHg, p < 0.05). The rate pressure product was lower in females than males (♂: 482 ± 14 vs ♀: 392 ± 29, p < 0.05) and correlated with the steady-state PO₂is (r = 0.66, p < 0.05). These results support that females express greater reductions in HR than males in response to l-NAME-induced elevation of MAP via the baroreceptor reflex and provide new insights on how central hemodynamics affect skeletal muscle oxygenation in a sex-specific manner.

Light-emitting diode therapy (photobiomodulation) effects on oxygen uptake and cardiac output dynamics during moderate exercise transitions: a randomized, crossover, double-blind, and placebo-controlled study

Light-emitting diodes (LEDs) might have a beneficial impact on cytochrome-c oxidase enzyme activity. Thus, it was hypothesized that photobiomodulation by light-emitting diode therapy (LEDT) could influence aerobic metabolism dynamics. Possible LEDT-mediated aerobic improvements were investigated mainly by a precise characterization of the pulmonary O₂ uptake dynamics during moderate exercise transitions. Eight healthy young adults were enrolled in this randomized, double-blind, placebo-controlled, crossover study. A multi-diode array of LEDs was used for muscular pre-conditioning 30 min and 6 h before exercise testing. Pulmonary O₂ uptake, carbon dioxide output, cardiac output, heart rate, stroke volume, and total arteriovenous oxygen difference dynamics were evaluated by frequency domain analysis. Comparisons revealed no statistical (p > 0.05) differences between LEDT and placebo, suggesting no significant changes in aerobic system dynamics. These results challenge earlier publications that reported changes in pulmonary O₂ uptake during incremental exercise until exhaustion after LEDT. Perhaps, increments in peak pulmonary O₂ uptake after LEDT may be a consequence of higher exercise tolerance caused by non-aerobic-related factors as opposed to an improved aerobic response.

Muscle Strength, But Not Muscle Oxidative Capacity, Varies Between the Morning and the Afternoon in Patients with Multiple Sclerosis: A Pilot Study

Despite frequent muscle strength or muscle oxidative capacity (based on exercise-onset oxygen uptake [VO2] kinetics) assessments in patients with multiple sclerosis, the impact of time of day on these parameters is often not taken into account. Based on observations in healthy subjects, it remains to be studied whether muscle strength, and/or exercise-onset VO2 kinetics, varies between the morning and the afternoon in patients with multiple sclerosis. In this prospective observational pilot study, walking capacity, exercise-onset VO2 kinetics, isometric knee extension/flexion strength (dynamometry), and self-reported fatigue were measured in 11 patients with multiple sclerosis (age, 51.8 ± 9.3 yrs; body mass index, 24.7 ± 5.1 kg/m; Expanded Disability Status Scale, 3.5 ± 1.4; 3 men) in the morning and 5 hrs later (afternoon). In the afternoon, self-reported fatigue (1.9 ± 0.9 cm) and muscle strength (knee extension peak torque at 45 degrees, 84 ± 26 Nm) were significantly different (P < 0.05), than in the morning (self-reported fatigue, 1.2 ± 0.9 cm; knee extension peak torque at 45 degrees, 93 ± 32 Nm), but walking capacity and exercise-onset VO2 kinetics were similar at these two time points (P > 0.05). Consistent with observations in healthy subjects, muscle strength varies between the morning and the afternoon in patients with multiple sclerosis, under the conditions of the present study. These findings suggest that muscle strength assessments should be conducted at similar or nearly similar times of the day to minimize diurnal variation in these measures and hence insure correct interpretation.

Exercise and NO production: relevance and implications in the cardiopulmonary system

This article reviews the existing knowledge about the effects of physical exercise on nitric oxide (NO) production in the cardiopulmonary system. The authors review the sources of NO in the cardiopulmonary system; involvement of three forms of NO synthases (eNOS, nNOS, and iNOS) in exercise physiology; exercise-induced modulation of NO and/or NOS in physiological and pathophysiological conditions in human subjects and animal models in the absence and presence of pharmacological modulators; and significance of exercise-induced NO production in health and disease. The authors suggest that physical activity significantly improves functioning of the cardiovascular system through an increase in NO bioavailability, potentiation of antioxidant defense, and decrease in the expression of reactive oxygen species-forming enzymes. Regular physical exercises are considered a useful approach to treat cardiovascular diseases. Future studies should focus on detailed identification of (i) the exercise-mediated mechanisms of NO exchange; (ii) optimal exercise approaches to improve cardiovascular function in health and disease; and (iii) physical effort thresholds.

Dietary nitrate supplementation and exercise performance

Dietary nitrate is growing in popularity as a sports nutrition supplement. This article reviews the evidence base for the potential of inorganic nitrate to enhance sports and exercise performance. Inorganic nitrate is present in numerous foodstuffs and is abundant in green leafy vegetables and beetroot. Following ingestion, nitrate is converted in the body to nitrite and stored and circulated in the blood. In conditions of low oxygen availability, nitrite can be converted into nitric oxide, which is known to play a number of important roles in vascular and metabolic control. Dietary nitrate supplementation increases plasma nitrite concentration and reduces resting blood pressure. Intriguingly, nitrate supplementation also reduces the oxygen cost of submaximal exercise and can, in some circumstances, enhance exercise tolerance and performance. The mechanisms that may be responsible for these effects are reviewed and practical guidelines for safe and efficacious dietary nitrate supplementation are provided.

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Highly athletic terrestrial mammals: horses and dogs

Evolutionary forces drive beneficial adaptations in response to a complex array of environmental conditions. In contrast, over several millennia, humans have been so enamored by the running/athletic prowess of horses and dogs that they have sculpted their anatomy and physiology based solely upon running speed. Thus, through hundreds of generations, those structural and functional traits crucial for running fast have been optimized. Central among these traits is the capacity to uptake, transport and utilize oxygen at spectacular rates. Moreover, the coupling of the key systems--pulmonary-cardiovascular-muscular is so exquisitely tuned in horses and dogs that oxygen uptake response kinetics evidence little inertia as the animal transitions from rest to exercise. These fast oxygen uptake kinetics minimize Intramyocyte perturbations that can limit exercise tolerance. For the physiologist, study of horses and dogs allows investigation not only of a broader range of oxidative function than available in humans, but explores the very limits of mammalian biological adaptability. Specifically, the unparalleled equine cardiovascular and muscular systems can transport and utilize more oxygen than the lungs can supply. Two consequences of this situation, particularly in the horse, are profound exercise-induced arterial hypoxemia and hypercapnia as well as structural failure of the delicate blood-gas barrier causing pulmonary hemorrhage and, in the extreme, overt epistaxis. This chapter compares and contrasts horses and dogs with humans with respect to the structural and functional features that enable these extraordinary mammals to support their prodigious oxidative and therefore athletic capabilities.

Exercise: Kinetic considerations for gas exchange

The activities of daily living typically occur at metabolic rates below the maximum rate of aerobic energy production. Such activity is characteristic of the nonsteady state, where energy demands, and consequential physiological responses, are in constant flux. The dynamics of the integrated physiological processes during these activities determine the degree to which exercise can be supported through rates of O₂ utilization and CO₂ clearance appropriate for their demands and, as such, provide a physiological framework for the notion of exercise intensity. The rate at which O₂ exchange responds to meet the changing energy demands of exercise--its kinetics--is dependent on the ability of the pulmonary, circulatory, and muscle bioenergetic systems to respond appropriately. Slow response kinetics in pulmonary O₂ uptake predispose toward a greater necessity for substrate-level energy supply, processes that are limited in their capacity, challenge system homeostasis and hence contribute to exercise intolerance. This review provides a physiological systems perspective of pulmonary gas exchange kinetics: from an integrative view on the control of muscle oxygen consumption kinetics to the dissociation of cellular respiration from its pulmonary expression by the circulatory dynamics and the gas capacitance of the lungs, blood, and tissues. The intensity dependence of gas exchange kinetics is discussed in relation to constant, intermittent, and ramped work rate changes. The influence of heterogeneity in the kinetic matching of O₂ delivery to utilization is presented in reference to exercise tolerance in endurance-trained athletes, the elderly, and patients with chronic heart or lung disease.

Vascular effects of dietary nitrate (as found in green leafy vegetables and beetroot) via the nitrate-nitrite-nitric oxide pathway

The discovery that dietary (inorganic) nitrate has important vascular effects came from the relatively recent realization of the 'nitrate-nitrite-nitric oxide (NO) pathway'. Dietary nitrate has been demonstrated to have a range of beneficial vascular effects, including reducing blood pressure, inhibiting platelet aggregation, preserving or improving endothelial dysfunction, enhancing exercise performance in healthy individuals and patients with peripheral arterial disease. Pre-clinical studies with nitrate or nitrite also show the potential to protect against ischaemia-reperfusion injury and reduce arterial stiffness, inflammation and intimal thickness. However, there is a need for good evidence for hard endpoints beyond epidemiological studies. Whilst these suggest reduction in cardiovascular risk with diets high in nitrate-rich vegetables (such as a Mediterranean diet), others have suggested possible small positive and negative associations with dietary nitrate and cancer, but these remain unproven. Interactions with other nutrients, such as vitamin C, polyphenols and fatty acids may enhance or inhibit these effects. In order to provide simple guidance on nitrate intake from different vegetables, we have developed the Nitrate 'Veg-Table' with 'Nitrate Units' [each unit being 1 mmol of nitrate (62 mg)] to achieve a nitrate intake that is likely to be sufficient to derive benefit, but also to minimize the risk of potential side effects from excessive ingestion, given the current available evidence. The lack of data concerning the long term effects of dietary nitrate is a limitation, and this will need to be addressed in future trials.

Mitochondria as a source of reactive oxygen and nitrogen species: from molecular mechanisms to human health

Mitochondrially generated reactive oxygen species are involved in a myriad of signaling and damaging pathways in different tissues. In addition, mitochondria are an important target of reactive oxygen and nitrogen species. Here, we discuss basic mechanisms of mitochondrial oxidant generation and removal and the main factors affecting mitochondrial redox balance. We also discuss the interaction between mitochondrial reactive oxygen and nitrogen species, and the involvement of these oxidants in mitochondrial diseases, cancer, neurological, and cardiovascular disorders.

The NO donor sodium nitroprusside: evaluation of skeletal muscle vascular and metabolic dysfunction

The nitric oxide (NO) donor sodium nitroprusside (SNP) may promote cyanide-induced toxicity and systemic and/or local responses approaching maximal vasodilation. The hypotheses were tested that SNP superfusion of the rat spinotrapezius muscle exerts 1) residual impairments in resting and contracting blood flow, oxygen utilization (VO(2)) and microvascular O(2) pressure (PO(2)mv); and 2) marked hypotension and elevation in resting PO(2)mv. Two superfusion protocols were performed: 1) Krebs-Henseleit (control 1), SNP (300 μM; a dose used commonly in superfusion studies) and Krebs-Henseleit (control 2), in this order; 2) 300 and 1200 μM SNP in random order. Spinotrapezius muscle blood flow (radiolabeled microspheres), VO(2) (Fick calculation) and PO(2)mv (phosphorescence quenching) were determined at rest and during electrically-induced (1 Hz) contractions. There were no differences in spinotrapezius blood flow, VO(2) or PO(2)mv at rest and during contractions pre- and post-SNP condition (control 1 and control 2; p>0.05 for all). With regard to dosing, SNP produced a graded elevation in resting PO(2)mv (p<0.05) with a reduction in mean arterial pressure only at the higher concentration (p<0.05). Contrary to our hypotheses, skeletal muscle superfusion with the NO donor SNP (300 μM) improved microvascular oxygenation during the transition from rest to contractions (PO(2)mv kinetics) without precipitating residual impairment of muscle hemodynamic or metabolic control or compromising systemic hemodynamics. These data suggest that SNP superfusion (300 μM) constitutes a valid and important tool for assessing the functional roles of NO in resting and contracting skeletal muscle function without incurring residual alterations consistent with cyanide accumulation and poisoning.

Influence of nitrate supplementation on VO₂ kinetics and endurance of elite cyclists

The present study examined if an elevated nitrate intake would improve VO(2) kinetics, endurance, and repeated sprint capacity in elite endurance athletes. Ten highly trained cyclists (72 ± 4 mL O(2) /kg/min, mean ± standard deviation) underwent testing for VO(2) kinetics (3 × 6 min at 298 ± 28 W), endurance (120 min preload followed by a 400-kcal time trial), and repeated sprint capacity (6 × 20 s sprints, recovery 100 s) during two 6-day periods in randomized order with a daily ingestion of either 0.5 L beetroot (BR) juice to increase nitrate levels or a 0.5 L placebo (PLA) drink with blackcurrant juice. Plasma NOx (nitrate + nitrite) levels were higher (P < 0.01) in BR (147 ± 102 and 159 ± 103 μM after 4 and 6 days of beverage intake, respectively) compared with PLA (41 ± 10 and 40 ± 7 μM). VO(2) kinetics and exercise economy were the same in BR and PLA. Time-trial performance was similar with an average completion time of 18:20 and 18:37 min:s in BR and PLA, respectively, with average power outputs of 290 ± 43 W in BR and 285 ± 44 W in PLA. Peak and mean power during repeated sprinting were similar in BR and PLA. In contrast to observations in moderately trained subjects intake of BR juice had no effect on VO(2) kinetics and performance in elite cyclists.

Slowed Exercise-Onset Vo2 Kinetics During Submaximal Endurance Exercise in Subjects With Multiple Sclerosis

Background. Low physical activity levels in persons with multiple sclerosis (MS) may reduce skeletal muscle oxidative capacity. Rehabilitation strategies might be altered by a measure of capacity that did not require invasive techniques or maximal exercise testing. For this purpose, we measured exercise onset and offset oxygen uptake (Vo2) kinetics during endurance exercise. Objective. This study compared exercise-onset and -offset Vo2 kinetics in mildly affected persons with MS with healthy matched participants. Methods. From 38 MS patients who had a mean Expanded Disability Status Scale of 3.1 and 16 healthy participants, exercise-onset and -offset Vo2 kinetics (mean response time [MRT]) were determined during two 6-minute submaximal bouts of exercise separated by a 6-minute recovery interval. Blood lactate, heart rate, expiratory volume, and Borg ratings of perceived exertion were assessed during exercise and compared between groups. Relationships between clinical characteristics and MRT were assessed. Results. During exercise, blood lactate, heart rate, and expiratory volume did not differ between groups ( P > .05), but exercise-onset MRT was significantly slower in MS versus healthy participants ( P = .007). Exercise-onset MRT was independently related to having MS ( P = .02). Exercise-offset MRT was not different between groups or was independently related to having MS ( P > .05). No independent relationships between clinical characteristics of MS and exercise-onset or -offset MRT were found. Conclusions. Exercise-onset Vo2 kinetics during submaximal endurance exercise are significantly slowed in mildly disabled persons with MS, suggesting low skeletal muscle oxidative capacity. Using mean response time testing, rehabilitation interventions for this reduction in exercise capacity can be assessed and targeted.

Exercise training and muscle microvascular oxygenation: functional role of nitric oxide

Exercise training induces multiple adaptations within skeletal muscle that may improve local O(2) delivery-utilization matching (i.e., Po(2)mv). We tested the hypothesis that increased nitric oxide (NO) function is intrinsic to improved muscle Po(2)mv kinetics from rest to contractions after exercise training. Healthy young Sprague-Dawley rats were assigned to sedentary (n = 18) or progressive treadmill exercise training (n = 10; 5 days/wk, 6-8 wk, final workload of 60 min/day at 35 m/min, -14% grade) groups. Po(2)mv was measured via phosphorescence quenching in the spinotrapezius muscle at rest and during 1-Hz twitch contractions under control (Krebs-Henseleit solution), sodium nitroprusside (SNP, NO donor; 300 μM), and N(G)-nitro-L-arginine methyl ester (l-NAME, nonspecific NO synthase blockade; 1.5 mM) superfusion conditions. Exercise-trained rats had greater peak oxygen uptake (Vo(2 peak)) than their sedentary counterparts (81 ± 1 vs. 72 ± 2 ml · kg(-1) · min(-1), respectively; P < 0.05). Exercise-trained rats had significantly slower Po(2)mv fall throughout contractions (τ(1); time constant for the first component) during control (sedentary: 8.1 ± 0.6; trained: 15.2 ± 2.8 s). Compared with control, SNP slowed τ(1) to a greater extent in sedentary rats (sedentary: 38.7 ± 5.6; trained: 26.8 ± 4.1 s; P > 0.05) whereas l-NAME abolished the differences in τ(1) between sedentary and trained rats (sedentary: 12.0 ± 1.7; trained: 11.2 ± 1.4 s; P < 0.05). Our results indicate that endurance exercise training leads to greater muscle microvascular oxygenation across the metabolic transient following the onset of contractions (i.e., slower Po(2)mv kinetics) partly via increased NO-mediated function, which likely constitutes an important mechanism for training-induced metabolic adaptations.

Opposing effects of nitric oxide and prostaglandin inhibition on muscle mitochondrial Vo(2) during exercise

Nitric oxide (NO) and prostaglandins (PG) together play a role in regulating blood flow during exercise. NO also regulates mitochondrial oxygen consumption through competitive binding to cytochrome-c oxidase. Indomethacin uncouples and inhibits the electron transport chain in a concentration-dependent manner, and thus, inhibition of NO and PG synthesis may regulate both muscle oxygen delivery and utilization. The purpose of this study was to examine the independent and combined effects of NO and PG synthesis blockade (L-NMMA and indomethacin, respectively) on mitochondrial respiration in human muscle following knee extension exercise (KEE). Specifically, this study examined the physiological effect of NO, and the pharmacological effect of indomethacin, on muscle mitochondrial function. Consistent with their mechanism of action, we hypothesized that inhibition of nitric oxide synthase (NOS) and PG synthesis would have opposite effects on muscle mitochondrial respiration. Mitochondrial respiration was measured ex vivo by high-resolution respirometry in saponin-permeabilized fibers following 6 min KEE in control (CON; n = 8), arterial infusion of N(G)-monomethyl-L-arginine (L-NMMA; n = 4) and Indo (n = 4) followed by combined inhibition of NOS and PG synthesis (L-NMMA + Indo, n = 8). ADP-stimulated state 3 respiration (OXPHOS) with substrates for complex I (glutamate, malate) was reduced 50% by Indo. State 3 O(2) flux with complex I and II substrates was reduced less with both Indo (20%) and L-NMMA + Indo (15%) compared with CON. The results indicate that indomethacin reduces state 3 mitochondrial respiration primarily at complex I of the respiratory chain, while blockade of NOS by L-NMMA counteracts the inhibition by Indo. This effect on muscle mitochondria, in concert with a reduction of blood flow accounts for in vivo changes in muscle O(2) consumption during combined blockade of NOS and PG synthesis.

Speeding of V̇o2 kinetics during moderate-intensity exercise subsequent to heavy-intensity exercise is associated with improved local O2 distribution

The relationship between the adjustment of muscle deoxygenation (Δ[HHb]) and phase II VO2p during moderate-intensity exercise was examined before (Mod 1) and after (Mod 2) a bout of heavy-intensity “priming” exercise. Moderate intensity VO2p and Δ[HHb] kinetics were determined in 18 young males (26 ± 3 yr). VO2p was measured breath-by-breath. Changes in Δ[HHb] of the vastus lateralis muscle were measured by near-infrared spectroscopy. VO2p and Δ[HHb] response profiles were fit using a monoexponential model, and scaled to a relative % of the response (0–100%). The Δ[HHb]/V̇o2 ratio for each individual (reflecting the local matching of O2 delivery to O2 utilization) was calculated as the average Δ[HHb]/V̇o2 response from 20 s to 120 s during the exercise on-transient. Phase II τVO2p was reduced in Mod 2 compared with Mod 1 ( P < 0.05). The effective τ′Δ[HHb] remained the same in Mod 1 and Mod 2 ( P > 0.05). During Mod 1, there was an “overshoot” in the Δ[HHb]/V̇o2 ratio (1.08; P < 0.05) that was not present during Mod 2 (1.01; P > 0.05). There was a positive correlation between the reduction in the Δ[HHb]/V̇o2 ratio and the smaller τVO2p from Mod 1 to Mod 2 ( r = 0.78; P < 0.05). This study showed that a smaller τVO2p during a moderate bout of exercise subsequent to a heavy-intensity priming exercise was associated with improved microvascular O2 delivery during the on-transient of exercise, as suggested by a smaller Δ[HHb]/V̇o2 ratio.

Influence of phase I duration on phase II V̇o2 kinetics parameter estimates in older and young adults

Older adults (O) may have a longer phase I pulmonary O2 uptake kinetics (V̇o2p) than young adults (Y); this may affect parameter estimates of phase II V̇o2p. Therefore, we sought to: 1) experimentally estimate the duration of phase I V̇o2p (EE phase I) in O and Y subjects during moderate-intensity exercise transitions; 2) examine the effects of selected phase I durations (i.e., different start times for modeling phase II) on parameter estimates of the phase II V̇o2p response; and 3) thereby determine whether slower phase II kinetics in O subjects represent a physiological difference or a by-product of fitting strategy. V̇o2p was measured breath-by-breath in 19 O (68 ± 6 yr; mean ± SD) and 19 Y (24 ± 5 yr) using a volume turbine and mass spectrometer. Phase I V̇o2p was longer in O (31 ± 4 s) than Y (20 ± 7 s) ( P < 0.05). In O, phase II τV̇o2p was larger ( P < 0.05) when fitting started at 15 s (49 ± 12 s) compared with fits starting at the individual EE phase I (43 ± 12 s), 25 s (42 ± 10 s), 35 s (42 ± 12 s), and 45 s (45 ± 15 s). In Y, τV̇o2p was not affected by the time at which phase II V̇o2p fitting started (τV̇o2p = 31 ± 7 s, 29 ± 9 s, 30 ± 10 s, 32 ± 11 s, and 30 ± 8 s for fittings starting at 15 s, 25 s, 35 s, 45 s, and EE phase I, respectively). Fitting from EE phase I, 25 s, or 35 s resulted in the smallest CI τV̇o2p in both O and Y. Thus, fitting phase II V̇o2p from (but not constrained to) 25 s or 35 s provides consistent estimates of V̇o2p kinetics parameters in Y and O, despite the longer phase I V̇o2p in O.

Emergent role of gasotransmitters in ischemia-reperfusion injury

Nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S) are lipid-soluble, endogenously produced gaseous messenger molecules collectively known as gasotransmitters. Over the last several decades, gasotransmitters have emerged as potent cytoprotective mediators in various models of tissue and cellular injury. Specifically, when used at physiological levels, the exogenous and endogenous manipulation of these three gases has been shown to modulate ischemia/reperfusion injury by inducing a number of cytoprotective mechanisms including: induction of vasodilatation, inhibition of apoptosis, modulation of mitochondrial respiration, induction of antioxidants, and inhibition of inflammation. However, while the actions are similar, there are some differences in the mechanisms by which these gasotransmitters induce these effects and the regulatory actions of the enzyme systems can vary depending upon the gas being investigated. Furthermore, there does appear to be some crosstalk between the gases, which can provide synergistic effects and additional regulatory effects. This review article will discuss several models and mechanisms of gas-mediated cytoprotection, as well as provide a brief discussion on the complex interactions between the gasotransmitter systems.

No effect of glutamine ingestion on indices of oxidative metabolism in stable COPD

COPD patients have reduced muscle glutamate which may contribute to an impaired response of oxidative metabolism to exercise. We hypothesised that prior glutamine supplementation would enhance V(O2) peak, V(O2) at lactate threshold and speed pulmonary oxygen uptake kinetics in COPD. 13 patients (9 males, age 66±5 years, mean±SD) with severe COPD (mean FEV(1) 0.88±0.23l, 33±7% predicted) performed on separate days ramp cycle-ergometry (5-10 W min(-1)) to volitional exhaustion and subsequently square-wave transitions to 80% estimated lactate threshold (LT) following consumption of either placebo (CON) or 0.125 g kg bm(-1) of glutamine (GLN) in 5 ml kg bm(-1) placebo. Oral glutamine had no effect on peak or V(O2) at LT, {V(O2) peak: CON=0.70±0.1 l min(-1) vs. GLN=0.73±0.2 l min(-1); LT: CON=0.57±0.1 l min(-1) vs. GLN=0.54±0.1 lmin(-1)} or V(O2) kinetics {tau: CON=68±22 s vs. GLN=68±16 s}. Ingestion of glutamine before exercise did not improve indices of oxidative metabolism in this patient group.

Are the parameters of VO2, heart rate and muscle deoxygenation kinetics affected by serial moderate-intensity exercise transitions in a single day?

This study compared the parameter estimates of pulmonary oxygen uptake (VO(2p)), heart rate (HR) and muscle deoxygenation (Δ[HHb]) kinetics when several moderate-intensity exercise transitions (MODs) were performed during a single visit versus several MODs performed during separate visits. Nine subjects (24 ± 5 years, mean ± SD) each completed two successive cycling MODs on six occasions (1-6A and 1-6B) from 20 W to a work rate corresponding to 80% estimated lactate threshold with 6 min recovery at 20 W. During one visit, subjects completed two series of three MODs (6A-F), separated by 20 min rest. VO(2p) time constants (τVO(2p); 27 ± 10 s, 25 ± 12 s, 25 ± 11 s) were similar (p > 0.05) for MODs 1-6A, 1-6B and 6A-F, respectively. τVO(2p) had reproducibility 95% confidence intervals (CI(95)) of 8.3, 8.2, 4.7, 4.9 and 4.7 s when comparing single (1A vs. 2A), the average of two (1-2A vs. 3-4A), three (1-3A vs. 4-6A), four (1-2AB vs. 3-4AB) and six (1-3AB vs. 4-6AB) MODs, respectively. The effective Δ[HHb] response time (τ'Δ[HHb]) was unaffected across conditions (1-6A: 19 ± 2 s, 1-6B: 19 ± 3 s, 6A-F: 17 ± 4 s) with reproducibility CI(95) of 5.3, 4.5, 3.1, 2.9 and 3.3 s when a single, two, three, four and six MODs were compared, respectively. τHR was reduced in MODs 6A-F compared to 1-6A and 1-6B (23 ± 5 s, 25 ± 5 s, 27 ± 6 s, respectively). This study showed that parameter estimates of VO(2p), HR and Δ[HHb] kinetics are largely unaffected by data collection sequence, and the day-to-day reproducibility of τVO(2p) and τ'Δ[HHb] estimates, as determined by the CI(95), was appreciably improved by averaging of at least three MODs.

Nitric oxide bioavailability modulates the dynamics of microvascular oxygen exchange during recovery from contractions

AIM

lowered microvascular PO(2) (PO(2) mv) during the exercise off-transient likely impairs muscle metabolic recovery and limits the capacity to perform repetitive tasks. The current investigation explored the impact of altered nitric oxide (NO) bioavailability on PO(2) mv during recovery from contractions in healthy skeletal muscle. We hypothesized that increased NO bioavailability (sodium nitroprusside: SNP) would enhance PO(2) mv and speed its recovery kinetics while decreased NO bioavailability (l-nitro arginine methyl ester: l-NAME) would reduce PO(2) mv and slow its recovery kinetics.

METHODS

  PO(2) mv was measured by phosphorescence quenching during transitions (rest-1 Hz twitch-contractions for 3 min-recovery) in the spinotrapezius muscle of Sprague-Dawley rats under SNP (300 microm), Krebs-Henseleit (CONTROL) and l-NAME (1.5 mm) superfusion conditions.

RESULTS

relative to recovery in CONTROL, SNP resulted in greater overall microvascular oxygenation as assessed by the area under the PO(2) mv curve (PO(2 AREA) ;

CONTROL

3471 ± 292 mmHg s; SNP: 4307 ± 282 mmHg s; P < 0.05) and faster off-kinetics as evidenced by the mean response time (MRToff;

CONTROL

60.2 ± 6.9 s; SNP: 34.8 ± 5.7 s; P < 0.05), whereas l-NAME produced lower PO(2 AREA) (2339 ± 444 mmHg s; P < 0.05) and slower MRToff (86.6 ± 14.5s; P < 0.05).

CONCLUSION

no bioavailability plays a key role in determining the matching of O(2) delivery-to-O(2) uptake and thus the upstream O(2) pressure driving capillary-myocyte O(2) flux (i.e. PO(2) mv) following cessation of contractions in healthy skeletal muscle. Additionally, these data support a mechanistic link between reduced NO bioavailability and prolonged muscle metabolic recovery commonly observed in ageing and diseased populations.

Acute L-arginine supplementation reduces the O2 cost of moderate-intensity exercise and enhances high-intensity exercise tolerance

It has recently been reported that dietary nitrate (NO(3)(-)) supplementation, which increases plasma nitrite (NO(2)(-)) concentration, a biomarker of nitric oxide (NO) availability, improves exercise efficiency and exercise tolerance in healthy humans. We hypothesized that dietary supplementation with L-arginine, the substrate for NO synthase (NOS), would elicit similar responses. In a double-blind, crossover study, nine healthy men (aged 19-38 yr) consumed 500 ml of a beverage containing 6 g of l-arginine (Arg) or a placebo beverage (PL) and completed a series of "step" moderate- and severe-intensity exercise bouts 1 h after ingestion of the beverage. Plasma NO(2)(-) concentration was significantly greater in the Arg than the PL group (331 ± 198 vs. 159 ± 102 nM, P < 0.05) and systolic blood pressure was significantly reduced (123 ± 3 vs. 131 ± 5 mmHg, P < 0.01). The steady-state O(2) uptake (VO(2)) during moderate-intensity exercise was reduced by 7% in the Arg group (1.48 ± 0.12 vs. 1.59 ± 0.14 l/min, P < 0.05). During severe-intensity exercise, the Vo(2) slow component amplitude was reduced (0.58 ± 0.23 and 0.76 ± 0.29 l/min in Arg and PL, respectively, P < 0.05) and the time to exhaustion was extended (707 ± 232 and 562 ± 145 s in Arg and PL, respectively, P < 0.05) following consumption of Arg. In conclusion, similar to the effects of increased dietary NO(3)(-) intake, elevating NO bioavailability through dietary L-Arg supplementation reduced the O(2) cost of moderate-intensity exercise and blunted the VO(2) slow component and extended the time to exhaustion during severe-intensity exercise.

Dietary nitrate supplementation enhances muscle contractile efficiency during knee-extensor exercise in humans

The purpose of this study was to elucidate the mechanistic bases for the reported reduction in the O2cost of exercise following short-term dietary nitrate (NO3−) supplementation. In a randomized, double-blind, crossover study, seven men (aged 19–38 yr) consumed 500 ml/day of either nitrate-rich beetroot juice (BR, 5.1 mmol of NO3−/day) or placebo (PL, with negligible nitrate content) for 6 consecutive days, and completed a series of low-intensity and high-intensity “step” exercise tests on the last 3 days for the determination of the muscle metabolic (using31P-MRS) and pulmonary oxygen uptake (V̇o2) responses to exercise. On days 4–6, BR resulted in a significant increase in plasma [nitrite] (mean ± SE, PL 231 ± 76 vs. BR 547 ± 55 nM; P < 0.05). During low-intensity exercise, BR attenuated the reduction in muscle phosphocreatine concentration ([PCr]; PL 8.1 ± 1.2 vs. BR 5.2 ± 0.8 mM; P < 0.05) and the increase in V̇o2(PL 484 ± 41 vs. BR 362 ± 30 ml/min; P < 0.05). During high-intensity exercise, BR reduced the amplitudes of the [PCr] (PL 3.9 ± 1.1 vs. BR 1.6 ± 0.7 mM; P < 0.05) and V̇o2(PL 209 ± 30 vs. BR 100 ± 26 ml/min; P < 0.05) slow components and improved time to exhaustion (PL 586 ± 80 vs. BR 734 ± 109 s; P < 0.01). The total ATP turnover rate was estimated to be less for both low-intensity (PL 296 ± 58 vs. BR 192 ± 38 μM/s; P < 0.05) and high-intensity (PL 607 ± 65 vs. BR 436 ± 43 μM/s; P < 0.05) exercise. Thus the reduced O2cost of exercise following dietary NO3−supplementation appears to be due to a reduced ATP cost of muscle force production. The reduced muscle metabolic perturbation with NO3−supplementation allowed high-intensity exercise to be tolerated for a greater period of time.

Speeding of VO2 kinetics with endurance training in old and young men is associated with improved matching of local O2 delivery to muscle O2 utilization

The time course and mechanisms of adjustment of pulmonary oxygen uptake (V(O(2))) kinetics (time constant tauV(O(2p))) were examined during step transitions from 20 W to moderate-intensity cycling in eight older men (O; 68 +/- 7 yr) and eight young men (Y; 23 +/- 5 yr) before training and at 3, 6, 9, and 12 wk of endurance training. V(O(2p)) was measured breath by breath with a volume turbine and a mass spectrometer. Changes in deoxygenated hemoglobin concentration (Delta[HHb]) were measured by near-infrared spectroscopy. V(O(2p)) and Delta[HHb] were modeled with a monoexponential model. Training was performed on a cycle ergometer three times per week for 45 min at approximately 70% of peak V(O(2)). Pretraining tauV(O(2p)) was greater (P < 0.05) in O (43 +/- 10 s) than Y (34 +/- 8 s). tauV(O(2p)) decreased (P < 0.05) by 3 wk of training in both O (35 +/- 9 s) and Y (22 +/- 8 s), with no further changes thereafter. The pretraining overall adjustment of Delta[HHb] was faster than tauV(O(2p)) in both O and Y, resulting in Delta[HHb]/V(O(2p)) displaying an "overshoot" during the transient relative to the subsequent steady-state level. After 3 wk of training the Delta[HHb]/V(O(2p)) overshoot was attenuated in both O and Y. With further training, this overshoot persisted in O but was eliminated after 6 wk in Y. The training-induced speeding of V(O(2p)) kinetics in O and Y at 3 wk of training was associated with an improved matching of local O(2) delivery to muscle V(O(2)) (as represented by a lower Delta[HHb]/V(O(2p))). The continued overshoot in Delta[HHb]/V(O(2p)) in O may reflect a reduced vasodilatory responsiveness that may limit muscle blood flow distribution during the on-transient of exercise.

Low blood flow at onset of moderate-intensity exercise does not limit muscle oxygen uptake

The effect of low blood flow at onset of moderate-intensity exercise on the rate of rise in muscle oxygen uptake was examined. Seven male subjects performed a 3.5-min one-legged knee-extensor exercise bout (24 +/- 1 W, mean +/- SD) without (Con) and with (double blockade; DB) arterial infusion of inhibitors of nitric oxide synthase (N(G)-monomethyl-l-arginine) and cyclooxygenase (indomethacin) to inhibit the synthesis of nitric oxide and prostanoids, respectively. Leg blood flow and leg oxygen delivery throughout exercise was 25-50% lower (P < 0.05) in DB compared with Con. Leg oxygen extraction (arteriovenous O(2) difference) was higher (P < 0.05) in DB than in Con (5 s: 127 +/- 3 vs. 56 +/- 4 ml/l), and leg oxygen uptake was not different between Con and DB during exercise. The difference between leg oxygen delivery and leg oxygen uptake was smaller (P < 0.05) during exercise in DB than in Con (5 s: 59 +/- 12 vs. 262 +/- 39 ml/min). The present data demonstrate that muscle blood flow and oxygen delivery can be markedly reduced without affecting muscle oxygen uptake in the initial phase of moderate-intensity exercise, suggesting that blood flow does not limit muscle oxygen uptake at the onset of exercise. Additionally, prostanoids and/or nitric oxide appear to play important roles in elevating skeletal muscle blood flow in the initial phase of exercise.

Oxygen uptake kinetics: historical perspective and future directions

Oxygen uptake has been studied in the transitions between rest and exercise for more than 100 years, yet the mechanisms regulating the rate of increase in oxidative metabolism remain controversial. Some of the controversy is a consequence of incorrect interpretations of kinetic parameters describing amplitude and time constant relationships, whereas other factors relate to an incomplete framework for interpretation of experimental results. In this review, a new conceptual 3-dimensional model is proposed to explore the intracellular environment of skeletal muscle in the rest-to-exercise transition. The model incorporates the so-called “metabolic inertia” describing the increases in metabolic substrates and enzyme activation, along with the dynamic changes in intracellular partial pressure of oxygen (PO2). Considerable evidence exists during normal submaximal exercise challenges for an effect of changes in O2 delivery to working muscles affecting the intracellular PO2 (displayed on the x axis) and the high energy phosphate concentration (y axis) during steady-state exercise as well as the transitions from rest to exercise. The z axis incorporates a hypothetical description of metabolic inertia that is enhanced by increased enzyme activation and production of metabolic substrates. Specific examples are given that describe how this axis can affect oxygen uptake kinetics within the context of changing intracellular PO2 and energetic states. Oxidative metabolism at the onset of exercise is regulated by a dynamic balance of O2 transport and utilization mechanisms and is not limited solely by metabolic inertia.

Dietary nitrate supplementation reduces the O2 cost of low-intensity exercise and enhances tolerance to high-intensity exercise in humans

Pharmacological sodium nitrate supplementation has been reported to reduce the O2 cost of submaximal exercise in humans. In this study, we hypothesized that dietary supplementation with inorganic nitrate in the form of beetroot juice (BR) would reduce the O2 cost of submaximal exercise and enhance the tolerance to high-intensity exercise. In a double-blind, placebo (PL)-controlled, crossover study, eight men (aged 19-38 yr) consumed 500 ml/day of either BR (containing 11.2 +/- 0.6 mM of nitrate) or blackcurrant cordial (as a PL, with negligible nitrate content) for 6 consecutive days and completed a series of "step" moderate-intensity and severe-intensity exercise tests on the last 3 days. On days 4-6, plasma nitrite concentration was significantly greater following dietary nitrate supplementation compared with PL (BR: 273 +/- 44 vs. PL: 140 +/- 50 nM; P < 0.05), and systolic blood pressure was significantly reduced (BR: 124 +/- 2 vs. PL: 132 +/- 5 mmHg; P < 0.01). During moderate exercise, nitrate supplementation reduced muscle fractional O2 extraction (as estimated using near-infrared spectroscopy). The gain of the increase in pulmonary O2 uptake following the onset of moderate exercise was reduced by 19% in the BR condition (BR: 8.6 +/- 0.7 vs. PL: 10.8 +/- 1.6 ml.min(-1).W(-1); P < 0.05). During severe exercise, the O2 uptake slow component was reduced (BR: 0.57 +/- 0.20 vs. PL: 0.74 +/- 0.24 l/min; P < 0.05), and the time-to-exhaustion was extended (BR: 675 +/- 203 vs. PL: 583 +/- 145 s; P < 0.05). The reduced O2 cost of exercise following increased dietary nitrate intake has important implications for our understanding of the factors that regulate mitochondrial respiration and muscle contractile energetics in humans.

Control of oxygen uptake during exercise

Other than during sleep and contrived laboratory testing protocols, humans rarely exist in prolonged metabolic steady states; rather, they transition among different metabolic rates (V O2). The dynamic transition of V O2 (V O2 kinetics), initiated, for example, at exercise onset, provides a unique window into understanding metabolic control. This brief review presents the state-of-the art regarding control of V O2 kinetics within the context of a simple model that helps explain the work rate dependence of V O2 kinetics as well as the effects of environmental perturbations and disease. Insights emerging from application of novel approaches and technologies are integrated into established concepts to assess in what circumstances O2 supply might exert a commanding role over V O2 kinetics, and where it probably does not. The common presumption that capillary blood flow dynamics can be extrapolated accurately from upstream arterial measurements is challenged. From this challenge, new complexities emerge with respect to the relationships between O2 supply and flux across the capillary-myocyte interface and the marked dependence of these processes on muscle fiber type. Indeed, because of interfiber type differences in O2 supply relative to V O2, the presence of much lower O2 levels in the microcirculation supplying fast-twitch muscle fibers, and the demonstrated metabolic sensitivity of muscle to O2, it is possible that fiber type recruitment profiles (and changes thereof) might help explain the slowing of V O2 kinetics at higher work rates and in chronic diseases such as heart failure and diabetes.

The effects of exercise training on the kinetics of oxygen uptake in patients with chronic heart failure

BACKGROUND

Prolonged oxygen uptake kinetics (O2 kinetics), following the onset of a constant workload of exercise has been associated with a poor prognosis in patients with chronic heart failure. This study aimed to determine both continuous and interval training effects on the different O2-kinetics phases in these patients.

DESIGN

Twenty-one patients (60+/-8 years) with stable chronic heart failure participated in a 36-session exercise rehabilitation program (three times weekly). Patients were randomly assigned to interval training (n=11; 100% of peak work rate for 30 s, alternating with 30 s-rest) and to continuous training (n=10; 50% of peak work rate).

METHODS

Before and after the completion of the program, all patients performed both incremental symptom-limited and constant workload submaximal cardiopulmonary exercise tests. Phase I O2-kinetics was evaluated by time (t), from the start of exercise until the onset of decreased respiratory exchange ratio and phase II by the time constant (tau) of the response from the end of phase I until steady state.

RESULTS

After training, there was a significant increase in peak oxygen uptake and peak work rate in both continuous (15.3+/-4.4 vs. 16.6+/-4.5 ml/kg per min; P=0.03 and 81.8+/-40.1 vs. 94.7+/-46.1 W; P=0.03) and interval training groups (14.2+/-3.1 vs. 15.4+/-4.2 ml/kg per min; P=0.03 and 82.5+/-24.1 vs. 93.7+/-30.1 W; P=0.04). Patients who underwent interval training had a significant decrease in t (39.7+/-3.7 to 36.1+/-6.9 s; P=0.05), but not tau (59.6+/-9.4 to 58.9+/-8.5 s; P=ns), whereas those assigned to continuous training had a significant decrease in both t (40.6+/-6.1 to 36.4+/-5.4 s; P=0.01) and tau (63.3+/-23.6 to 42.5+/-16.7 s; P=0.03).

CONCLUSIONS

Exercise training improves O2 kinetics in chronic heart failure patients. Both continuous and interval training improve phase I O2-kinetics, but continuous training results in superior improvement of the phase II O2-kinetics, an indirect index of muscle oxidative capacity.

Oxygen uptake (VO2) kinetics in different species: a brief review

When a human begins to move or locomote, the energetic demands of its skeletal muscles increase abruptly and the oxygen (O2) transport system responds to deliver increased amounts of O2to the respiring mitochondria. It is intuitively reasonable that the rapidity with which O2transport can be increased to and utilized by (VO2) the contracting muscles would be greater in those species with a higher maximal VO2capacity (i.e., VO2max). This review explores the relationship between VO2maxand VO2dynamics or kinetics at across a range of species selected, in part, for their disparate VO2maxcapacities. In healthy humans there is compelling evidence that the speed of the VO2kinetics at the onset of exercise is limited by an oxidative enzyme inertia within the exercising muscles rather than by VO2delivery to those muscles. This appears true also for the horse and dog but possibly not for a certain species of frog. Whereas there is a significant correlation between VO2maxand the speed of VO2kinetics among different species, it is possible to identify species or individuals within a species that exhibit widely disparate mass-specific VO2maxcapacities but similar VO2kinetics (i.e., superlative human athlete and horse).

Oxygen exchange in muscle of young and old rats: muscle-vascular-pulmonary coupling

Sustained performance of muscular exercise is contingent upon increasing muscle O(2) delivery (Qo2; the product of blood flow and arterial O(2) content, i.e. Q X Cao2) and utilization (Vo2m ) rapidly at exercise onset and sustaining necessary conductive and diffusive O(2) fluxes throughout exercise. A tight co-ordination of pulmonary, cardiovascular and muscle system responses is therefore required to prevent muscle microvascular O(2) pressures (P(mvO(2))) from falling to levels that impair blood-muscle O(2) exchange and/or impact metabolic control and reduce exercise tolerance. Microvascular O(2) pressures are determined by the balance between and Qo2 and Vo2m, and emerging evidence indicates that this balance is regulated differently across muscle fibre types and also in aged muscle. Moreover, disease states such as diabetes (type I and II) and chronic heart failure (CHF) also impact P(mvO(2)). This brief review primarily examines evidence obtained in animals that ageing: (1) redistributes exercising away from highly oxidative muscles and muscle fibres; (2) alters muscle capillary haemodynamics; and (3) reduces the O(2) pressure head within the microcirculation (P(mvO(2))) that serves to facilitate blood-muscle O(2) transfer. In many respects, these alterations found in healthy ageing animals bear a striking resemblance to those present in some chronic diseases (e.g. diabetes, CHF) and may help explain the compromised exercise tolerance present in aged individuals. Putative mechanistic insights are explored within the context of current knowledge and future investigative approaches.

Effects of glutamine and hyperoxia on pulmonary oxygen uptake and muscle deoxygenation kinetics

The aim of the present study was to determine whether glutamine ingestion, which has been shown to enhance the exercise-induced increase in the tricarboxylic acid intermediate (TCAi) pool size, resulted in augmentation of the rate of increase in oxidative metabolism at the onset of exercise. In addition, the potential interaction with oxygen availability was investigated by completing exercise in both normoxic and hyperoxic conditions. Eight male cyclists cycled for 6 min at 70% VO2max following consumption of a drink (5 ml kg body mass(-1)) containing a placebo or 0.125 g kg body mass(-1) of glutamine in normoxic (CON and GLN respectively) and hyperoxic (HYP and HPG respectively) conditions. Breath-by-breath pulmonary oxygen uptake and continuous, non-invasive muscle deoxygenation (via near infrared spectroscopy: NIRS) data were collected throughout exercise. The time constant of the phase II component of pulmonary oxygen uptake kinetics was unchanged between trials (CON: 21.5 +/- 3.0 vs. GLN: 18.2 +/- 1.3 vs. HYP: 18.9 +/- 2.0 vs. HPG: 18.6 +/- 1.2 s). There was also no alteration of the kinetics of relative muscle deoxygenation as measured via NIRS (CON: 5.9 +/- 0.7 vs. GLN: 7.3 +/- 0.8 vs. HYP: 6.5 +/- 0.9 vs. HPG: 5.2 +/- 0.4 s). Conversely, the mean response time of pulmonary oxygen uptake kinetics was faster (CON: 33.4 +/- 1.2 vs. GLN: 29.8 +/- 2.3 vs. HYP: 33.2 +/- 2.6 vs. HPG: 31.6 +/- 2.6 s) and the time at which muscle deoxygenation increased above pre-exercise values was earlier (CON: 9.6 +/- 0.9 vs. GLN: 8.7 +/- 1.1 vs. HYP: 8.5 +/- 0.8 vs. HPG: 8.4 +/- 0.7 s) following glutamine ingestion. In normoxic conditions, plasma lactate concentration was lower following glutamine ingestion compared to placebo. Whilst the results of the present study provide some support for the present hypothesis, the lack of any alteration in the time constant of pulmonary oxygen uptake and muscle deoxygenation kinetics suggest that the normal exercise induced expansion of the TCAi pool size is not limiting to oxidative metabolism at the onset of cycle exercise at 70% VO2max.

Prior heavy exercise elevates pyruvate dehydrogenase activity and speeds O2 uptake kinetics during subsequent moderate-intensity exercise in healthy young adults

The adaptation of pulmonary oxygen uptake (.VO2) during the transition to moderate-intensity exercise (Mod) is faster following a prior bout of heavy-intensity exercise. In the present study we examined the activation of pyruvate dehydrogenase (PDHa) during Mod both with and without prior heavy-intensity exercise. Subjects (n = 9) performed a Mod(1)-heavy-intensity-Mod(2) exercise protocol preceded by 20 W baseline. Breath-by-breath .VO2 kinetics and near-infrared spectroscopy-derived muscle oxygenation were measured continuously, and muscle biopsy samples were taken at specific times during the transition to Mod. In Mod(1), PDHa increased from baseline (1.08 +/- 0.2 mmol min(-1) (kg wet wt)(-1)) to 30 s (2.05 +/- 0.2 mmol min(-1) (kg wet wt)(-1)), with no additional change at 6 min exercise (2.07 +/- 0.3 mmol min(-1) (kg wet wt)(-1)). In Mod(2), PDHa was already elevated at baseline (1.88 +/- 0.3 mmol min(-1) (kg wet wt)(-1)) and was greater than in Mod(1), and did not change at 30 s (1.96 +/- 0.2 mmol min(-1) (kg wet wt)(-1)) but increased at 6 min exercise (2.70 +/- 0.3 mmol min(-1) (kg wet wt)(-1)). The time constant of .VO2 was lower in Mod(2) (19 +/- 2 s) than Mod(1) (24 +/- 3 s). Phosphocreatine (PCr) breakdown from baseline to 30 s was greater (P < 0.05) in Mod(1) (13.6 +/- 6.7 mmol (kg dry wt)(-1)) than Mod(2) (6.5 +/- 6.2 mmol (kg dry wt)(-1)) but total PCr breakdown was similar between conditions (Mod(1), 14.8 +/- 7.4 mmol (kg dry wt)(-1); Mod(2), 20.1 +/- 8.0 mmol (kg dry wt)(-1)). Both oxyhaemoglobin and total haemoglobin were elevated prior to and throughout Mod(2) compared with Mod(1). In conclusion, the greater PDHa at baseline prior to Mod(2) compared with Mod(1) may have contributed in part to the faster .VO2 kinetics in Mod(2). That oxyhaemoglobin and total haemoglobin were elevated prior to Mod(2) suggests that greater muscle perfusion may also have contributed to the observed faster .VO2 kinetics. These findings are consistent with metabolic inertia, via delayed activation of PDH, in part limiting the adaptation of pulmonary .VO2 and muscle O2 consumption during the normal transition to exercise.

Muscle microvascular oxygenation in chronic heart failure: role of nitric oxide availability

AIM

To test the hypothesis that diminished vascular nitric oxide availability might explain the inability of individuals with chronic heart failure (CHF) to maintain the microvascular PO(2)'s (PO(2mv) proportional, variant O(2) delivery-to-uptake ratio) seen in healthy animals.

METHODS

We superfused sodium nitroprusside (SNP; 300 microm), Krebs-Henseleit (control, CON) and L-nitro arginine methyl ester (L-NAME; 1.5 mM) onto the spinotrapezius muscle and measured PO(2mv) by phosphorescence quenching in female Sprague-Dawley rats (n = 26) at rest and during twitch contractions (1 Hz). Seven rats served as controls (Sham) while CHF was induced by myocardial infarction. CHF rats were grouped as moderate (MOD; n = 15) and severe CHF (SEV; n = 4) according to morphological data and baseline PO(2mv).

RESULTS

In contrast to Sham and MOD, L-NAME did not affect the PO(2mv) response (dynamics and steady-state) of SEV when compared with CON. SNP restored the PO(2mv) profile of SEV to that seen in Sham animals during CON. Specifically, the effect of L-NAME expressed as Delta(L-NAME - CON) were: Baseline PO(2mv) [in mmHg, DeltaSham = -7.0 +/- 1.6 (P < 0.05); DeltaSEV =-1.2 +/- 2.1], end-contractions PO(2mv) [in mmHg, DeltaSham = -5.0 +/- 1.0 (P < 0.05); DeltaSEV = -2.5 +/- 0.5] and time constant of PO(2mv) decrease [in s, DeltaSham = -6.5 +/- 3.0 (P < 0.05); DeltaSEV = -3.2 +/- 1.8].

CONCLUSION

These data provide the first direct evidence that the pathological profiles of PO(2mv) associated with severe CHF can be explained, in part, by a diminished vascular NO availability.

Biochemical background of the VO2 on-kinetics in skeletal muscles

This review discusses the present knowledge on the oxygen uptake kinetics at the onset of exercise in skeletal muscle and the contribution of a previously developed computer model of oxidative phosphorylation in intact skeletal muscle to the understanding of the factors determining this kinetics on the biochemical level. It has been demonstrated recently that an increase in the total creatine pool [PCr + Cr] and in glycolytic ATP supply lengthen the half-transition time of the VO2 on-kinetics, while an increase in mitochondria content, in parallel activation of ATP supply and ATP usage, in muscle oxygen concentration, in proton leak, in resting energy demand, in resting cytosolic pH, and in initial alkalization diminish this parameter. It has also been shown that the half-transition time is near-linearly proportional to the absolute difference between the phosphocreatine concentration during work and at rest (deltaPCr). The present review discusses whether the V/O2 on-kinetics on the muscle level is strictly or only approximately exponential. Finally, it is postulated that a short transition time of the VO2 on-kinetics in itself does not need be profitable for the skeletal muscle functioning during exercise, but usually a short transition time is correlated with factors that improve exercise capacity. The transition time is a phenomenological parameter resulting from the biochemical properties of the system and not a physical factor that can cause anything in the system.

Effects of altered nitric oxide availability on rat muscle microvascular oxygenation during contractions

AIM

To explore the role of nitric oxide (NO) in controlling microvascular O2 pressure (P(O2)mv) at rest and during contractions (1 Hz). We hypothesized that at the onset of contractions sodium nitroprusside (SNP) would raise P(O2)mv and slow the kinetics of P(O2)mv change whereas l-nitro arginine methyl ester (L-NAME) would decrease P(O2)mv and speed its kinetics.

METHODS

We superfused the spinotrapezius muscle of female Sprague-Dawley rats (n = 7, body mass = 298 +/- 10 g) with SNP (300 microM) and L-NAME (1.5 mm) and measured P(O2)mv (phosphorescence quenching) during contractions.

RESULTS

SNP decreased mean arterial pressure (92 +/- 5 mmHg) below that of control (CON, 124 +/- 4 mmHg) and L-NAME (120 +/- 4 mmHg) conditions. SNP did not raise P(O2)mv at rest but it did elevate the P(O2)mv-to-MAP ratio (50% increase, P < 0.05) and slow the kinetics by lengthening the time-delay (TD, 14.0 +/- 5.0 s) and time constant (tau, 24.0 +/- 10.0 s) of the response compared with CON (TD, 8.4 +/- 3.3 s; tau, 16.0 +/- 4.5 s, P < 0.05 vs. SNP). L-NAME decreased P(O2)mv at rest and tended to speed tau (10.1 +/- 3.8 s, P = 0.1), while TD (8.1 +/- 1.0 s) was not significantly different. L-NAME also caused P(O2)mv to fall transiently below steady-state contracting values.

CONCLUSIONS

These results indicate that NO availability can significantly affect P(O2)mv at rest and during contractions and suggests that P(O2)mv derangements in ageing and chronic disease conditions may potentially result from impairments in NO availability.