Effect of hypoxia on glucose metabolism in human skeletal muscle during exercise

Docosahexaenoic Acid Counteracts the Hypoxic-Induced Inflammatory and Metabolic Alterations in 3T3-L1 Adipocytes

Background: Hypoxia is caused by the excessive expansion of the white adipose tissue (AT) and is associated with obesity-related conditions such as insulin resistance, inflammation, and oxidative stress. Docosahexaenoic acid (DHA) is an omega-3 fatty acid reported to have beneficial health effects. However, the effects of DHA in AT against hypoxia-induced immune-metabolic perturbations in adipocytes exposed to low O₂ tension are not well known. Consequently, this study aimed to evaluate the impact of DHA on markers of inflammation, metabolism, apoptosis, and oxidative stress in 3T3-L1 cell adipocytes exposed to low O₂ tension (1% O₂) induced hypoxia. Methods: The apoptosis and reactive oxygen species (ROS) rates were evaluated. Metabolic parameters such as lactate, FFA, glycerol release, glucose uptake, and ATP content were assessed by a fluorometer. The expression of HIF-1, GLUT1 and the secretion of adipocytokines such as leptin, adiponectin, and pro-inflammatory markers was evaluated. Results: DHA-treated hypoxic cells showed significantly decreased basal free fatty acid release, lactate production, and enhanced glucose consumption. In addition, DHA-treatment of hypoxic cells caused a significant reduction in the apoptosis rate and ROS production with decreased lipid peroxidation. Moreover, DHA-treatment of hypoxic cells caused a decreased secretion of pro-inflammatory markers (IL-6, MCP-1) and leptin and increased adiponectin secretion compared with hypoxic cells. Furthermore, DHA-treatment of hypoxic cells caused significant reductions in the expression of genes related to hypoxia (HIF-1, HIF-2), anaerobic metabolism (GLUT1 and Ldha), ATP production (ANT2), and fat metabolism (FASN and PPARY). Conclusion: This study suggests that DHA can exert potential anti-obesity effects by reducing the secretion of inflammatory adipokines, oxidative stress, lipolysis, and apoptosis.

Acute normobaric hypoxia blunts contraction-mediated mTORC1- and JNK-signaling in human skeletal muscle

Aim

Hypoxia has been shown to reduce resistance exercise‐induced stimulation of protein synthesis and long‐term gains in muscle mass. However, the mechanism whereby hypoxia exerts its effect is not clear. Here, we examine the effect of acute hypoxia on the activity of several signalling pathways involved in the regulation of muscle growth following a bout of resistance exercise.

Methods

Eight men performed two sessions of leg resistance exercise in normoxia or hypoxia (12% O₂) in a randomized crossover fashion. Muscle biopsies were obtained at rest and 0, 90,180 minutes after exercise. Muscle analyses included levels of signalling proteins and metabolites associated with energy turnover.

Results

Exercise during normoxia induced a 5‐10‐fold increase of S6K1^Thr389 phosphorylation throughout the recovery period, but hypoxia blunted the increases by ~50%. Phosphorylation of JNK^{Thr183/Tyr185} and the JNK target SMAD2^{Ser245/250/255} was increased by 30‐ to 40‐fold immediately after the exercise in normoxia, but hypoxia blocked almost 70% of the activation. Throughout recovery, phosphorylation of JNK and SMAD2 remained elevated following the exercise in normoxia, but the effect of hypoxia was lost at 90‐180 minutes post‐exercise. Hypoxia had no effect on exercise‐induced Hippo or autophagy signalling and ubiquitin‐proteasome related protein levels. Nor did hypoxia alter the changes induced by exercise in high‐energy phosphates, glucose 6‐P, lactate or phosphorylation of AMPK or ACC.

Conclusion

We conclude that acute severe hypoxia inhibits resistance exercise‐induced mTORC1‐ and JNK signalling in human skeletal muscle, effects that do not appear to be mediated by changes in the degree of metabolic stress in the muscle.

Exogenous glucose oxidation during endurance exercise in hypoxia

Purpose

Endurance exercise in hypoxia promotes carbohydrate (CHO) metabolism. However, detailed CHO metabolism remains unclear. The purpose of this study was to evaluate the effects of endurance exercise in moderate hypoxia on exogenous glucose oxidation at the same energy expenditure or relative exercise intensity.

Methods

Nine active healthy males completed three trials on different days, consisting of 30 min of running at each exercise intensity: (a) exercise at 65% of normoxic maximal oxygen uptake in normoxia [NOR, fraction of inspired oxygen (F_iO₂) = 20.9%, 10.6 ± 0.3 km/h], (b) exercise at the same relative exercise intensity with NOR in hypoxia (HYPR, F_iO₂ = 14.5%, 9.4 ± 0.3 km/h), and (c) exercise at the same absolute exercise intensity with NOR in hypoxia (HYPA, F_iO₂ = 14.5%, 10.6 ± 0.3 km/h). The subjects consumed ¹¹³C‐labeled glucose immediately before exercise, and expired gas samples were collected during exercise to determine ¹³C‐excretion (calculated by ¹³CO₂/¹²CO₂).

Results

The exercise‐induced increase in blood lactate was significantly augmented in the HYPA than in the NOR and HYPR (p = .001). HYPA involved a significantly higher respiratory exchange ratio (RER) during exercise compared with the other two trials (p < .0001). In contrast, exogenous glucose oxidation (¹³C‐excretion) during exercise was significantly lower in the HYPA than in the NOR (p = .03). No significant differences were observed in blood lactate elevation, RER, or exogenous glucose oxidation between NOR and HYPR.

Conclusion

Endurance exercise in moderate hypoxia caused a greater exercise‐induced blood lactate elevation and RER compared with the running exercise at same absolute exercise intensity in normoxia. However, exogenous glucose oxidation (¹³C‐excretion) during exercise was attenuated compared with the same exercise in normoxia.

Glucose homeostasis during short-term and prolonged exposure to high altitudes

Most of the literature related to high altitude medicine is devoted to the short-term effects of high-altitude exposure on human physiology. However, long-term effects of living at high altitudes may be more important in relation to human disease because more than 400 million people worldwide reside above 1500 m. Interestingly, individuals living at higher altitudes have a lower fasting glycemia and better glucose tolerance compared with those who live near sea level. There is also emerging evidence of the lower prevalence of both obesity and diabetes at higher altitudes. The mechanisms underlying improved glucose control at higher altitudes remain unclear. In this review, we present the most current evidence about glucose homeostasis in residents living above 1500 m and discuss possible mechanisms that could explain the lower fasting glycemia and lower prevalence of obesity and diabetes in this population. Understanding the mechanisms that regulate and maintain the lower fasting glycemia in individuals who live at higher altitudes could lead to new therapeutics for impaired glucose homeostasis.

Differentiation of human adipocytes at physiological oxygen levels results in increased adiponectin secretion and isoproterenol-stimulated lipolysis

Adipose tissue (AT) hypoxia occurs in obese humans and mice. Acute hypoxia in adipocytes causes dysregulation of adipokine secretion with an increase in inflammatory factors and diminished adiponectin release. O₂ levels in humans range between 3 and 11% revealing that conventional in vitro culturing at ambient air and acute hypoxia treatment (1% O₂) are performed under non-physiological conditions. In this study, we mimicked physiological conditions by differentiating human primary adipocytes under 10% or 5% O₂ in comparison to 21% O₂. Induction of differentiation markers was comparable between all three conditions. Adipokine release by adipocytes differentiated at lower oxygen levels was altered, with a marked upregulation of adiponectin, IL-6 and DPP4 secretion, and reduced leptin levels compared with adipocytes differentiated at 21% O₂. Isoproterenol-induced lipolysis was significantly elevated in adipocytes differentiated at 10% and 5% compared with 21% O₂. This effect was accompanied by increased protein expression of β-1 and -2 adrenergic receptor, HSL and perilipin. Conditioned medium (CM) of adipocytes differentiated at the three different conditions was generated for stimulation of human skeletal muscle cells (SkMC) or smooth muscle cells (SMC). CM-induced insulin resistance in SkMC was comparable for the different CMs. However, the SMC proliferative effect of CM from adipocytes differentiated at 10% O₂ was significantly reduced compared with 21% O₂. This study demonstrates that oxygen levels during adipogenesis are important factors altering adipocyte functionality such as adipokine release, in particular adiponectin secretion, as well as the hormone-induced lipolytic pathway.

Effect of exercise intensity and hypoxia on skeletal muscle AMPK signaling and substrate metabolism in humans

We compared in human skeletal muscle the effect of absolute vs. relative exercise intensity on AMP-activated protein kinase (AMPK) signaling and substrate metabolism under normoxic and hypoxic conditions. Eight untrained males cycled for 30 min under hypoxic conditions (11.5% O(2), 111 +/- 12 W, 72 +/- 3% hypoxia Vo(2 peak); 72% Hypoxia) or under normoxic conditions (20.9% O(2)) matched to the same absolute (111 +/- 12 W, 51 +/- 1% normoxia Vo(2 peak); 51% Normoxia) or relative (to Vo(2 peak)) intensity (171 +/- 18 W, 73 +/- 1% normoxia Vo(2 peak); 73% Normoxia). Increases (P < 0.05) in AMPK activity, AMPKalpha Thr(172) phosphorylation, ACCbeta Ser(221) phosphorylation, free AMP content, and glucose clearance were more influenced by the absolute than by the relative exercise intensity, being greatest in 73% Normoxia with no difference between 51% Normoxia and 72% Hypoxia. In contrast to this, increases in muscle glycogen use, muscle lactate content, and plasma catecholamine concentration were more influenced by the relative than by the absolute exercise intensity, being similar in 72% Hypoxia and 73% Normoxia, with both trials higher than in 51% Normoxia. In conclusion, increases in muscle AMPK signaling, free AMP content, and glucose disposal during exercise are largely determined by the absolute exercise intensity, whereas increases in plasma catecholamine levels, muscle glycogen use, and muscle lactate levels are more closely associated with the relative exercise intensity.

Biochemical tissue monitoring during hypoxia and reoxygenation

Oxygen deficiency during critical illness may cause profound changes in cellular metabolism and subsequent tissue and organ dysfunction. Clinical treatment in these cases targets rapid reoxygenation to avoid a prolonged impaired synthesis of cellular high-energy phosphates (ATP). However, the effect of this therapeutic intervention on tissue metabolism has not been determined yet. Thus the present study was designed to determine the effects of hypoxia and reoxygenation with either room air or 100% oxygen on variables of interstitial metabolism in different tissues using in vivo microdialysis. Twenty-seven adult, male CD-rats (407-487 g; Ivanovas, Kisslegg, Germany) were studied during general anesthesia. Following preparation and randomization, rats were normoventilated for 45 min (FiO(2) 0.21), followed by induction of hypoxia (FiO(2) 0.1, 40 min) and reoxygenated for 50 min either with FiO(2) 1.0 (group 1, n=10) or FiO(2) 0.21 (group 2, n=10). Control animals (n=7) were ventilated with 21% oxygen during the observation period. Additional to invasive haemodynamic parameters, biochemical tissue monitoring was performed using CMA 20 microdialysis probes, inserted into muscle, subcutaneous space, liver, and the peritoneal cavity allowing analyses of lactate and pyruvate at short intervals. Hypoxia induced a significant reduction in mean arterial pressure (MAP) in group 1 and 2 compared with the control group (P<0.05) without any significant differences between both treatment groups. This was accompanied by a significant increase in blood lactate (10.5+/-3.1 mM (group 1) and 12.3+/-4.1 mM (group 2) vs. 1.5+/-0.3 mM (control); P<0.05) and severe metabolic acidosis (base excess (BE): -18.3+/-5 mM (1) and -17.3+/-7 mM (2) vs. -2.6+/-1.8 mM (control), P<0.05). During hypoxia, the interstitial lacate/pyruvate ratio in groups 1 and 2 increased to 455+/-199% (muscle), 468+/-148% (intraperitoneal), 770+/-218% (hepatic) and 855+/-432% (subcutaneous) (P<0.05 vs. control, respectively). No significant inter-organ or inter-group differences in interstitial dialysates were observed in the treatment groups, neither during hypoxia nor during reoxygenation. Our data suggest, that hypoxia induces comparable metabolic alterations in various tissues and that reoxygenation with 100% oxygen is not superior to 21% oxygen in restoring tissue metabolism after critical hypoxia.

Regulation of glycogen phosphorylase and PDH during exercise in human skeletal muscle during hypoxia

The present study examined the acute effects of hypoxia on the regulation of skeletal muscle metabolism at rest and during 15 min of submaximal exercise. Subjects exercised on two occasions for 15 min at 55% of their normoxic maximal oxygen uptake while breathing 11% O(2) (hypoxia) or room air (normoxia). Muscle biopsies were taken at rest and after 1 and 15 min of exercise. At rest, no effects on muscle metabolism were observed in response to hypoxia. In the 1st min of exercise, glycogenolysis was significantly greater in hypoxia compared with normoxia. This small difference in glycogenolysis was associated with a tendency toward a greater concentration of substrate, free P(i), in hypoxia compared with normoxia. Pyruvate dehydrogenase activity (PDH(a)) was lower in hypoxia at 1 min compared with normoxia, resulting in a reduced rate of pyruvate oxidation and a greater lactate accumulation. During the last 14 min of exercise, glycogenolysis was greater in hypoxia despite a lower mole fraction of phosphorylase a. The greater glycogenolytic rate was maintained posttransformationally through significantly higher free [AMP] and [P(i)]. At the end of exercise, PDH(a) was greater in hypoxia compared with normoxia, contributing to a greater rate of pyruvate oxidation. Because of the higher glycogenolytic rate in hypoxia, the rate of pyruvate production continued to exceed the rate of pyruvate oxidation, resulting in significant lactate accumulation in hypoxia compared with no further lactate accumulation in normoxia. Hence, the elevated lactate production associated with hypoxia at the same absolute workload could in part be explained by the effects of hypoxia on the activities of the rate-limiting enzymes, phosphorylase and PDH, which regulate the rates of pyruvate production and pyruvate oxidation, respectively.

Hyperglycemia suppresses the sympatho-adrenal response to hypoxia, but not to handling stress

We hypothesized that the ability of prior hyperglycemia to suppress the sympatho-adrenal response would depend on the type of stress. To test this hypothesis, hyperglycemia was induced in chronically catheterized rats, before submitting them to either hypoxia (7.5% O2) or handling stress. Central venous blood samples were drawn for the determination of plasma glucose, epinephrine (EPI), norepinephrine (NOR) and insulin concentrations. Hypoxia caused significant increases in plasma EPI and NOR concentrations (deltaEPI = + 2.95+/-0.68 nmol/l, deltaNOR = + 12.45+/-1.29 nmol/l). Hyperglycemia, antecedent to hypoxia, dose dependently reduced the sympatho-adrenal response. In contrast, the sympatho-adrenal response to handling stress was not affected by even marked antecedent hyperglycemia (deltaEPI = + 2.48+/-0.46 nmol/l, deltaNOR = + 3.12+/-0.69 nmol/l at glucose = 20.7+/-0.6 mmol/l; vs. deltaEPI = + 2.48 + 0.58 nmol/l, deltaNOR= +2.97+/-0.11 nmol/l at glucose = 6.77+/-0.17 mg/dl). Thus, antecedent hyperglycemia suppresses the hypoxia-induced activation of both the sympathetic nerves and the adrenal medulla, but not the activation induced by handling. We conclude that the ability of hyperglycemia to suppress sympathetic activation depends on the stress producing the activation. We therefore speculate that hypoxic stress has a metabolic component to its central activation that handling stress does not.

Skeletal muscle glucose uptake during short-term contractile activity in vivo: effect of prior contractions

The goal of the present study was to examine the time course of skeletal muscle glucose uptake and changes in intracellular metabolites occurring with the onset of in situ stimulation, and to assess the effect of a prior period of contractions on subsequent contraction-induced increases in glucose uptake. Hindlimb muscle in anesthetized rabbits was studied noninvasively using the positron-emitting glucose analog 18F-2-fluoro-deoxy-D-glucose (FDG). Fractional rates of FDG phosphorylation were measured on a minute-to-minute basis during rest, 3.5 minutes of priming exercise (PE), 15 or 30 minutes of PE recovery, and a subsequent 15-minute period of contractions. Muscles were electrically stimulated at 2 Hz, and force production was held constant during the contraction period(s). FDG uptake did not differ from control values either during PE or during 60 minutes of recovery from PE. In response to 15 minutes of contractions, muscle stimulated without PE demonstrated increased FDG uptake, but only after a delay of 5.0 +/- 0.7 minutes. Muscle with PE but rested 15 minutes had increased FDG uptake with a delay of 0.5 +/- 0.2 minutes, and muscle with PE but rested 30 minutes had increased FDG uptake after a delay of 8.0 +/- 0.9 minutes (P < .01 all groups). All groups reached similar levels of FDG uptake by the end of 15 minutes of contractions. Both groups with PE had control levels of adenosine triphosphate (ATP), phosphocreatine (PCr), and glucose-6-phosphate (G6P) after PE recovery, but glycogen level was lower than the control value (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Carbohydrates and exercise

Muscle glycogen and blood glucose are important substrates for contracting skeletal muscle during exercise and fatigue often coincides with depletion of these carbohydrate reserves. Carbohydrate utilization during exercise is influenced by several factors including exercise intensity and duration, training status, diet, environment and gender. In view of the importance of carbohydrates for exercise performance, active individuals should ensure their diet contains sufficient carbohydrate. For athletes engaged in heavy training the daily carbohydrate requirement may be as high as 9-10 g carbohydrate per kg body mass in order to guarantee adequate carbohydrate availability prior to and during exercise and to allow full recovery of carbohydrate reserves following exercise.

Regulation of glucose utilization in human skeletal muscle during moderate dynamic exercise

The effect of bicycle exercise (75% of maximal oxygen uptake) on glucose uptake by the inferior limb (LGU) and glycolysis in human skeletal muscle has been investigated. Biopsies were obtained from the quadriceps femoris muscle before exercise, after 5 and 40 min of exercise, and at fatigue [74.9 +/- 4.7 (SE) min]. LGU was 0.05 +/- 0.02 mmol/min at rest, increased approximately sevenfold after 5 min of exercise, and continued to increase linearly during the first 40 min of exercise. Thereafter LGU stabilized at approximately 1.4 mmol/min until fatigue. Intracellular glucose was low at rest but increased sixfold after 5 min of exercise (P less than 0.01 vs. rest); thereafter, intracellular glucose decreased and was not significantly different from the value at rest after 40 min or at fatigue (P greater than 0.05). D-Glucose 6-phosphate (G-6-P) and alpha-D-glucose 1,6-bisphosphate (G-1,6-P2) (inhibitors of hexokinase) increased significantly after 5 min of exercise (approximately 300% G-6-P; approximately 25% G-1,6-P2) and then decreased continuously. The muscle glycolytic rate (glycogenolysis + glucose uptake) averaged 7.7 mmol.kg dry wt-1.min-1 during the first 40 min of exercise and 3.7 mmol.kg dry wt-1.min-1 during the last 35 min of exercise. The contribution of extracellular glucose to muscle glycolysis was estimated to be only 5 and 19% during the initial and latter phases of exercise, respectively. It is concluded that, during the initial phase of exercise, glucose utilization is limited by phosphorylation, probably due to G-6-P-dependent inhibition of hexokinase.(ABSTRACT TRUNCATED AT 250 WORDS)