Nitric Oxide In Skeletal Muscle Research Articles

Exercise Training Decreases Nitrite Concentration in the Heart and Locomotory Muscles of Rats Without Changing the Muscle Nitrate Content.

Skeletal muscles are postulated to be a potent regulator of systemic nitric oxide homeostasis. In this study, we aimed to evaluate the impact of physical training on the heart and skeletal muscle nitric oxide bioavailability (judged on the basis of intramuscular nitrite and nitrate) in rats. Rats were trained on a treadmill for 8 weeks, performing mainly endurance running sessions with some sprinting runs. Muscle nitrite (NO2-) and nitrate (NO3-) concentrations were measured using a high-performance liquid chromatography-based method, while amino acids, pyruvate, lactate, and reduced and oxidized glutathione were determined using a liquid chromatography coupled with tandem mass spectrometry technique. The content of muscle nitrite reductases (electron transport chain proteins, myoglobin, and xanthine oxidase) was assessed by western immunoblotting. We found that 8 weeks of endurance training decreased basal NO2- in the locomotory muscles and in the heart, without changes in the basal NO3-. In the slow-twitch oxidative soleus muscle, the decrease in NO2- was already present after the first week of training, and the content of nitrite reductases remained unchanged throughout the entire period of training, except for the electron transport chain protein content, which increased no sooner than after 8 weeks of training. Muscle NO2- level, opposed to NO3-, decreases in the time course of training. This effect is rapid and already visible in the slow-oxidative soleus after the first week of training. The underlying mechanisms of training-induced muscle NO2- decrease may involve an increase in the oxidative stress, as well as metabolite changes related to an increased muscle anaerobic glycolytic activity contributing to (1) direct chemical reduction of NO2- or (2) activation of muscle nitrite reductases.

The Effect of Doxorubicin and Exercise on the Formation of Nitric Oxide in Skeletal Muscle

Doxorubicin (DOX) is an effective anti-tumor agent which leaves patients with debilitating side effects such as fatigue and muscle weakness. The purpose of this study was to examine the influence of both DOX and acute exercise on the formation of nitric oxide (NO) in skeletal muscle. Thirty male rats were randomly assigned to 5 groups (A through E, n=6 per group). All rats received an intraperitoneal injection of 4.5 mg/kg DOX. Rats in group A and E did not perform any exercise and were sacrificed 24 and 48 hours post injection respectively. Groups B and C swam for 60 minutes 24 hours post injection, and were sacrificed at 25 and 48 hours respectively. Group D swam for 60 minutes at 24 and 48 hours post injection and were sacrificed at 49 hours. The red and white gastrocnemius, soleus and plantaris muscles were immediately excised at each end point. Overall, no differences (P>0.05) in NO levels were observed in any of the muscles between any of the groups. However, consistently lower NO concentrations were observed in all muscles in the exercised versus non-exercised groups. For example, NO concentration in the plantaris muscles were 18.4 ± 1.6, 17.5 ± 1.4 and 17.0 ± 1.3 μmol/mg in groups B, C and D respectively versus 31.5 ± 9.7 and 25.3 ± 4.5 μmol/mg for groups A and E respectively. These data suggest that acute exercise may play an important role in regulating NO metabolism post DOX administration, however exercise intensity and duration need to be better elucidated to maximize its effects.

It's well and truly time to stop stating that AMPK regulates glucose uptake and fat oxidation during exercise.

None for Perspective.

Managing the power grid: how myoglobin can regulate PO2 and energy distribution in skeletal muscle.

Managing the power grid: how myoglobin can regulate PO2 and energy distribution in skeletal muscle.

Nitric oxide is required for the insulin sensitizing effects of contraction in mouse skeletal muscle.

People with insulin resistance or type 2 diabetes can substantially increase their skeletal muscle glucose uptake during exercise and insulin sensitivity after exercise. Skeletal muscle nitric oxide (NO) is important for glucose uptake during exercise, although how prior exercise increases insulin sensitivity is unclear. In the present study, we examined whether NO is necessary for normal increases in skeletal muscle insulin sensitivity after contraction ex vivo in mouse muscle. The present study uncovers, for the first time, a novel role for NO in the insulin sensitizing effects of ex vivo contraction, which is independent of blood flow. The factors regulating the increase in skeletal muscle insulin sensitivity after exercise are unclear. We examined whether nitric oxide (NO) is required for the increase in insulin sensitivity after ex vivo contractions. Isolated C57BL/6J mouse EDL muscles were contracted for 10min or remained at rest (basal) with or without the NO synthase (NOS) inhibition (NG -monomethyl-l-arginine; l-NMMA; 100μm). Then, 3.5h post contraction/basal, muscles were exposed to saline or insulin (120μUml-1 ) with or without l-NMMA during the last 30min. l-NMMA had no effect on basal skeletal muscle glucose uptake. The increase in muscle glucose uptake with insulin (57%) was significantly (P<0.05) greater after prior contraction (140% increase). NOS inhibition during the contractions had no effect on this insulin-sensitizing effect of contraction, whereas NOS inhibition during insulin prevented the increase in skeletal muscle insulin sensitivity post-contraction. Soluble guanylate cyclase inhibition, protein kinase G (PKG) inhibition or cyclic nucleotide phosphodiesterase inhibition each had no effect on the insulin-sensitizing effect of prior contraction. In conclusion, NO is required for increases in insulin sensitivity several hours after contraction of mouse skeletal muscle via a cGMP/PKG independent pathway.

Effects of Endurance Training on the Skeletal Muscle Nitric Oxide Metabolism in Insulin-Independent Type 2 Diabetic Men-A Pilot Study.

Increases in the amount of inducible nitric oxide synthase (iNOS) protein and abnormal production of nitric oxide (NO) in skeletal muscle have been suggested to be associated with peripheral insulin resistance in patients with type 2 diabetes mellitus (T2DM). This pilot study analyzed whether a 3-month endurance training can affect iNOS protein and NO metabolite levels in the vastus lateralis muscle of insulin-independent T2DM men, thereby affecting the patients` glycemic control. Furthermore, serum molecules, which have been shown to activate iNOS protein expression in in vitro experiments, were quantified. Eight overweight/obese T2DM men (years = 61 ± 10) participated in the study. Muscle biopsies and venous blood collections were performed at T1 (6 weeks before training), T2 (1 week before training), and T3 (3 to 4 days after training). Protein contents (iNOS) were determined by Western blotting, nitrite concentrations by chemiluminescence, and serum molecule levels by enzyme-linked immunosorbent assay kits. The training reduced iNOS protein contents significantly (T2-T3: approximately -31%, P = 0.018). Nitrite concentrations as well as fasting glucose and HbA1c decreased, but not significantly. Serum tumor necrosis factor-α, thiobarbituric acid-reactive substances (lipid peroxidation as an indirect measure of reactive oxygen species), lipopolysaccharide binding protein, interferon-γ, and interleukin-1β showed no significant changes. The data indicate that the endurance training performed in the present study can reduce iNOS protein contents in insulin-independent T2DM men. Future studies should identify key molecules in iNOS regulation in vivo and fully clarify whether iNOS downregulation can help improve insulin sensitivity in T2DM patients in the long term.

Doxorubicin chemotherapy affects intracellular and interstitial nitric oxide concentrations in skeletal muscle : Effect of doxorubicin on intracellular and interstitial NO in skeletal muscle.

The role of nitric oxide (NO) in doxorubicin (DOX; cancer chemotherapeutic)-induced cardiotoxicity is well established. In skeletal muscle (SM), NO regulation plays a critical role in health, biogenesis, and function. Despite the increasing evidence that indicates the negative impact of DOX on SM function, the effect of DOX on NO production in SM has yet to be examined. The purpose of the current study was to simultaneously examine intracellular and interstitial NO concentrations in the SM following the administration of DOX. A single dose of 1.5 or 4.5mg/kg was administered intraperitoneally to male Sprague Dawley rats, and interstitial (IS) and intracellular (IC) NO was quantified every 24 up to 192h post-injection. There was no significant difference in IC NO following the injection of 1.5mg/kg DOX when compared to the control; however, the administration of 4.5mg/kg DOX resulted in lower (P < 0.05) concentrations of NO in the IC. Interestingly, a consistently higher (P < 0.05) concentration of NO in the IS was established following the administration of 1.5mg/kg compared to the control while no significant changes in IS NO resulted from the administration of the 4.5mg/kg dose. The fluctuation of IS and IC NO was not a result of substrate availability as arginine concentrations remained stable throughout the experiment. By utilizing the microdialysis technique, we have simultaneously quantified for the first time the IS and IC concentrations of NO in SM following DOX administration. These data provide important insight in the possible mechanisms leading to DOX-related SM dysfunction.

Nitric Oxide in Skeletal Muscle: Role on Mitochondrial Biogenesis and Function

Nitric oxide (NO) has been implicated in several cellular processes as a signaling molecule and also as a source of reactive nitrogen species (RNS). NO is produced by three isoenzymes called nitric oxide synthases (NOS), all present in skeletal muscle. While neuronal NOS (nNOS) and endothelial NOS (eNOS) are isoforms constitutively expressed, inducible NOS (iNOS) is mainly expressed during inflammatory responses. Recent studies have demonstrated that NO is also involved in the mitochondrial biogenesis pathway, having PGC-1α as the main signaling molecule. Increased NO synthesis has been demonstrated in the sarcolemma of skeletal muscle fiber and NO can also reversibly inhibit cytochrome c oxidase (Complex IV of the respiratory chain). Investigation on cultured skeletal myotubes treated with NO donors, NO precursors or NOS inhibitors have also showed a bimodal effect of NO that depends on the concentration used. The present review will discuss the new insights on NO roles on mitochondrial biogenesis and function in skeletal muscle. We will also focus on potential therapeutic strategies based on NO precursors or analogs to treat patients with myopathies and mitochondrial deficiency.

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.

Nitric Oxide in Myogenesis and Therapeutic Muscle Repair

Nitric oxide is a short-lived intracellular and intercellular messenger. The first realisation that nitric oxide is important in physiology occurred in 1987 when its identity with the endothelium-derived relaxing factor was discovered. Subsequent studies have shown that nitric oxide possesses a number of physiological functions that are essential not only to vascular homeostasis but also to neurotransmission, such as in the processes of learning and memory and endocrine gland regulation, as well as inflammation and immune responses. The discovery in 1995 that a splice variant of the neuronal nitric oxide synthase is localised at the sarcolemma via the dystrophin-glycoprotein complex and of its displacement in Duchenne muscular dystrophy has stimulated a host of studies exploring the role of nitric oxide in skeletal muscle physiology. Recently, nitric oxide has emerged as a relevant messenger also of myogenesis that it regulates at several key steps, especially when the process is stimulated for muscle repair following acute and chronic muscle injuries. Here, we will review briefly the mechanisms and functions of nitric oxide in skeletal muscle and discuss its role in myogenesis, with specific attention to the promising nitric oxide-based approaches now being explored at the pre-clinical and clinical level for the therapy of muscular dystrophy.

Obesity, insulin resistance, and skeletal muscle nitric oxide synthase

The molecular mechanisms responsible for impaired insulin action have yet to be fully identified. Rodent models demonstrate a strong relationship between insulin resistance and an elevation in skeletal muscle inducible nitric oxide synthase (iNOS) expression; the purpose of this investigation was to explore this potential relationship in humans. Sedentary men and women were recruited to participate (means ± SE: nonobese, body mass index = 25.5 ± 0.3 kg/m(2), n = 13; obese, body mass index = 36.6 ± 0.4 kg/m(2), n = 14). Insulin sensitivity was measured using an intravenous glucose tolerance test with the subsequent modeling of an insulin sensitivity index (S(I)). Skeletal muscle was obtained from the vastus lateralis, and iNOS, endothelial nitric oxide synthase (eNOS), and neuronal nitric oxide synthase (nNOS) content were determined by Western blot. S(I) was significantly lower in the obese compared with the nonobese group (~43%; P < 0.05), yet skeletal muscle iNOS protein expression was not different between nonobese and obese groups. Skeletal muscle eNOS protein was significantly higher in the nonobese than the obese group, and skeletal muscle nNOS protein tended to be higher (P = 0.054) in the obese compared with the nonobese group. Alternative analysis based on S(I) (high and low tertile) indicated that the most insulin-resistant group did not have significantly more skeletal muscle iNOS protein than the most insulin-sensitive group. In conclusion, human insulin resistance does not appear to be associated with an elevation in skeletal muscle iNOS protein in middle-aged individuals under fasting conditions.

The regulation of skeletal-muscle specific nNOSmu by calmodulin binding and phosphorylation

Nitric oxide (NO) in muscle has been reported to affect the force-generating capacity and regulate appropriate blood delivery to active muscles during exercise. A major source of nitric oxide in skeletal muscle is a muscle specific isoform of neuronal NO synthase (nNOSmu) that contains extra-insertion of 34 amino acids in the autoinhibitory helix (AH). nNOS is thought to be conformationally locked by AH and the Ca 2+ /calmodulin (CaM) binding induces a conformational rearrangement to facilitate electron transfer required for NO formation. To investigate the effects of AH and the extra-insertion, we prepared and purified rat nNOSalpha, a major isoform of nNOS in brain), nNOSdelta40 (a deletion mutant of AH), nNOSmu and CaM from Escherichia coli expression systems. The interaction between each nNOS isoform and CaM was analyzed using surface plasmon resonance system (Biacore T100). CaM in the presence of sufficient Ca 2+ had a greater affinity for nNOSmu than nNOSalpha, while it showed slightly reduced affinity for nNOSdelta40. These results suggest that the longer AH insertion resulted in the higher affinity of CaM in nNOS. We also analyzed the phosphorylation site of endogenous nNOSmu isolated from rat skeletal muscle using TOF-MS analysis and generated a pseudo-phosphorylation mutant to investigate NO formation activity and interaction with CaM. We will discuss the regulation mechanism of nNOSmu isoform by CaM binding and phosphorylation.

Differential nitric oxide levels in the blood and skeletal muscle of type 2 diabetic subjects may be consequence of adiposity: a preliminary study

Background and AimsNitric oxide (NO·) exerts key regulatory functions including vasodilation and glucose uptake. Thus reduced NO· levels are associated with insulin resistance and hypertension. In this preliminary work we aimed to measure the levels of NO· metabolites in serum and skeletal muscle of obese and non-obese subjects, with or without type 2 diabetes mellitus (T2DM). MethodsFifteen sedentary male participants [7 obese controls (C) vs 5 obese and 3 non-obese T2DM; age 54±9 years] were selected according to their BMI (>30 kg/m2 for obese and 23–27 kg/m2 for non-obese participants) and evaluated for fasted values of blood glucose, HbA1c, lipid profile, serum CRP (C-reactive protein), erythrocyte glutathione (GSH) metabolism, plasma adiponectin, leptin and cytokines (TNF-α and INFγ), serum and skeletal muscle nitric oxide metabolites (nitrite and nitrates; tNOx) and skeletal muscle nNOS and iNOS expression. Body composition was measured by whole body DEXA and muscle microbiopsy was performed in the vastus lateralis. ResultsWe found that serum tNOx (total nitrite/nitrate; μmol/L) was lower in obese T2DM group (12.7±3.5) when compared with their controls (21.1±2.4), although the non-obese group presented higher concentration of tNOx (33.8±7.2). Skeletal muscle nNOS was higher in obese controls, lower in non-obese T2DM and undetected in obese T2DM. On the other hand, expression of iNOS had an inverse relationship with nNOS, showing higher expression in obese T2DM, decrease in non-obese T2DM and absence in obese control group. tNOx levels (μmol/mg protein) were decreased in the non-obese T2DM group (12.07±0.59) when compared with the obese control (21.68±6.2) and the obese T2DM group (26.3±7.26). ConclusionWe conclude that the decreased serum NO∙ production in obese T2DM patients seems to be associated with adipose mass as lower adiposity was associated with normal NO∙ which was reduced in the skeletal muscle of the non-obese T2DM patients. We suggest that the lower adiposity (and higher adiponectin) in non-obese T2DM could be responsible for differential levels of NO∙ production and insulin resistance.

Skeletal muscle nitric oxide signaling and exercise: a focus on glucose metabolism

Nitric oxide (NO) is an important vasodilator and regulator in the cardiovascular system, and this link was the subject of a Nobel prize in 1998. However, NO also plays many other regulatory roles, including thrombosis, immune function, neural activity, and gastrointestinal function. Low concentrations of NO are thought to have important signaling effects. In contrast, high concentrations of NO can interact with reactive oxygen species, causing damage to cells and cellular components. A less-recognized site of NO production is within skeletal muscle, where small increases are thought to have beneficial effects such as regulating glucose uptake and possibly blood flow, but higher levels of production are thought to lead to deleterious effects such as an association with insulin resistance. This review will discuss the role of NO in skeletal muscle during and following exercise, including in mitochondrial biogenesis, muscle efficiency, and blood flow with a particular focus on its potential role in regulating skeletal muscle glucose uptake during exercise.

Hypoadiponectinemia is closely associated with impaired nitric oxide synthase activity in skeletal muscle of type 2 diabetic subjects.

In vitro studies suggest that adiponectin plays an important role in nitric oxide (NO) generation. We studied the relationship between plasma adiponectin and skeletal muscle nitric oxide synthase (NOS) activity in type 2 diabetic (T2DM) patients. We determined NOS activity in skeletal muscle of 7 T2DM and 8 nondiabetic control subjects under basal conditions and after a 4-h euglycemic insulin (80 mU/m2 x min) clamp. Insulin-stimulated glucose disposal (Rd) (5.2 +/-0.4 vs. 9.0 +/-0.9 mg/kg-min, P < 0.01) and basal NOS activity (107 +/-45 vs. 459 +/- 100 pmol/min-mg protein, P < 0.05) were reduced in T2DM versus controls. In response to hyperinsulinemia, NOS activity increased approximately two-fold in controls (757 +/- 244, P < 0.05 vs basal) but failed to increase in T2DM (105 +/- 38, P < 0.01 vs. T2DM). Basal NOS protein content was similar in controls and T2DM and did not change following insulin. Plasma adiponectin was decreased in T2DM (4.5 +/- 0.8 vs. 7.0 +/-1.0 microg/mL, P < 0.02) and correlated with insulin-stimulated NOS activity (r = 0.49, P < 0.05) and with Rd (r = 0.50, P < 0.05). In controls and T2DM collectively, Rd correlated with insulin-stimulated NOS activity (r = 0.48, P < 0.05). Decreased plasma adiponectin correlates with impaired insulin-stimulated NOS activity and severity of insulin resistance in T2DM. Because impaired NO generation plays a central role in endothelial dysfunction and development of atherosclerosis, our results may provide a link between reduced plasma adiponectin levels and accelerated atherosclerosis in T2DM.

Impact of acute caffeine administration on skeletal muscle oxidative stress and antioxidant status

Caffeine is a pharmacological agent that has been shown to enhance muscle contraction. The stimulatory effects of caffeine may be, in part, due to alterations in the skeletal muscle redox status. Specifically, caffeine administration to rats has been shown to rapidly, but transiently, decrease levels of skeletal muscle nitric oxide (NO) synthase. Decreased levels of NO could lead to elevated oxidative stress, since NO is a potent superoxide radical scavenger. We tested the hypothesis that acute caffeine administration would decrease muscle nitrosative stress, increase oxidative stress, and upregulate the expression of key antioxidant enzymes. Male Sprague Dawley rats received i.p. administration of caffeine (16 mg/kg; n=8) or vehicle (saline; n=8). Thirty minutes post injection the rats were anesthetized and the soleus muscles were harvested for analysis. No differences in oxidative injury (protein carbonyls) or nitrosative injury (3‐nitrotyrosine) were detected (p&gt;0.05). Caffeine administration did not alter manganese superoxide dismutase or copper‐zinc superoxide dismutase protein levels, but resulted in a 50% increase in catalase protein levels (p&lt;0.05). In summary, acute caffeine administration may upregulate skeletal muscle catalase expression by a mechanism independent of oxidative stress. Supported by Syracuse University SOE and APS UGSRF

Skeletal muscle nitric oxide concentrations in normotensive and spontaneously hypertensive rats

Skeletal muscle nitric oxide concentrations in normotensive and spontaneously hypertensive rats

Nitric Oxide in Skeletal Muscle: Inhibition of Nitric Oxide Synthase Inhibits Walking Speed in Rats

Nitric oxide (NO·) is a multifunctional messenger molecule generated by a family of enzymes called the nitric oxide synthases (NOSs). Although NOSs have been identified in skeletal muscle, specifically brain NOS (bNOS) and endothelial NOS (eNOS), their role has not been well clarified. The goals of this investigation were to (1) characterize the immunoreactivity, Ca2+ dependence, and activity of NOS in human and rat skeletal muscle and (2) using a rat model, investigate the effect of chronic blockade of NOS on skeletal muscle structure and function. Our results showed that both human and rodent skeletal muscle had NOS activity. This NOS activity was similar to that of the endothelial and brain NOS isoforms in that it was calcium-dependent. However, Western blot analysis consistently showed that a polyclonal antibody raised against a peptide sequence of human inducible NOS (iNOS) reacted with a protein with a molecular weight (95 kDa) that was different from that of other NOS isoforms. RT-PCR analysis identified the mRNA expression of not only eNOS and bNOS but also iNOS in human and rat muscle. Inhibition of nitric oxide synthase in rats with Nω-nitro-l-arginine methyl ester (l-NAME) resulted in a progressive, severe reduction in walking speed (30-fold reduction in walking velocity at day 22, P < 0.001), muscle fiber cross-sectional area (40% reduction at day 22, P < 0.001), and muscle mass (40% reduction in dry weight at day 22, P < 0.01). Rats fed the same regimen of the enantiomer of l-NAME (d-NAME) had normal motor function, muscle fiber morphology, and muscle mass. Taken together, these results imply that there may be a novel nitric oxide synthase in muscle and that NO· generated from muscle may be important in muscle function.

Physiology of nitric oxide in skeletal muscle.

In the past five years, skeletal muscle has emerged as a paradigm of "nitric oxide" (NO) function and redox-related signaling in biology. All major nitric oxide synthase (NOS) isoforms, including a muscle-specific splice variant of neuronal-type (n) NOS, are expressed in skeletal muscles of all mammals. Expression and localization of NOS isoforms are dependent on age and developmental stage, innervation and activity, history of exposure to cytokines and growth factors, and muscle fiber type and species. nNOS in particular may show a fast-twitch muscle predominance. Muscle NOS localization and activity are regulated by a number of protein-protein interactions and co- and/or posttranslational modifications. Subcellular compartmentalization of the NOSs enables distinct functions that are mediated by increases in cGMP and by S-nitrosylation of proteins such as the ryanodine receptor-calcium release channel. Skeletal muscle functions regulated by NO or related molecules include force production (excitation-contraction coupling), autoregulation of blood flow, myocyte differentiation, respiration, and glucose homeostasis. These studies provide new insights into fundamental aspects of muscle physiology, cell biology, ion channel physiology, calcium homeostasis, signal transduction, and the biochemistry of redox-related systems.

Roles of adenosine and nitric oxide in skeletal muscle in acute and chronic hypoxia.

In experiments on anaesthetised rats, the roles played by adenosine and nitric oxide (NO) were determined in resting skeletal muscle in acute systemic hypoxia and during acclimation to chronic systemic hypoxia. It is concluded that adenosine acting on A1 receptors, at least in part in an NO-dependent manner, plays essential roles in causing the dilation of proximal and terminal arterioles that helps to maintain muscle O2 consumption when O2 delivery is reduced by acute systemic hypoxia. It is proposed that adenosine and NO are similarly responsible for causing the tonic vasodilation that gradually wanes in the first 7 days of chronic hypoxia and that concomitantly, adenosine and hypoxia stimulate VEGF expression, so increasing venular permeability and triggering angiogenesis. By 7 days of chronic hypoxia, arteriolar remodelling is well established and within 18-21 days, substantial capillary angiogenesis alleviates tissue hypoxia. At this time, vasoconstrictor responses to the sympathetic transmitter norepinephrine are reduced, but dilator responses to adenosine released by acute hypoxia are enhanced, as may be explained by increased sensitivity to NO. Thus, preservation of tissue oxygenation is apparently associated with impaired ability to regulate arterial pressure and vulnerability to further hypoxia.