Time-dependent changes in glucose and insulin regulation during intermittent hypoxia and continuous hypoxia

Hypoxia impairs blood glucose homeostasis in naked mole-rat adult subordinates but not queens

Naked mole-rats (NMRs) are among the most hypoxia-tolerant mammals and metabolize only carbohydrates in hypoxia. Glucose is the primary building block of dietary carbohydrates, but how blood glucose is regulated during hypoxia has not been explored in NMRs. We hypothesized that NMRs mobilize glucose stores to support anaerobic energy metabolism in hypoxia. To test this, we treated newborn, juvenile and adult (subordinate and queen) NMRs in normoxia (21% O2) or hypoxia (7, 5 or 3% O2), while measuring metabolic rate, body temperature and blood [glucose]. We also challenged animals with glucose, insulin or insulin-like growth factor-1 (IGF-1) injections and measured the rate of glucose clearance in normoxia and hypoxia. We found that: (1) blood [glucose] increases in moderate hypoxia in queens and pups, but only in severe hypoxia in adult subordinates and juveniles; (2) glucose tolerance is similar between developmental stages in normoxia, but glucose clearance times are 2- to 3-fold longer in juveniles and subordinates than in queens or pups in hypoxia; and (3) reoxygenation accelerates glucose clearance in hypoxic subordinate adults. Mechanistically, (4) insulin and IGF-1 reduce blood [glucose] in subordinates in both normoxia but only IGF-1 impacts blood [glucose] in hypoxic queens. Our results indicate that insulin signaling is impaired by hypoxia in NMRs, but that queens utilize IGF-1 to overcome this limitation and effectively regulate blood glucose in hypoxia. This suggests that sexual maturation impacts blood glucose handling in hypoxic NMR queens, which may allow queens to spend longer periods of time in hypoxic nest chambers.

The Cardiovascular and Metabolic Effects of Chronic Hypoxia in Animal Models: A Mini-Review

Animal models are useful to understand the myriad physiological effects of hypoxia. Such models attempt to recapitulate the hypoxemia of human disease in various ways. In this mini-review, we consider the various animal models which have been deployed to understand the effects of chronic hypoxia on pulmonary and systemic blood pressure, glucose and lipid metabolism, atherosclerosis, and stroke. Chronic sustained hypoxia (CSH)-a model of chronic lung or heart diseases in which hypoxemia may be longstanding and persistent, or of high altitude, in which effective atmospheric oxygen concentration is low-reliably induces pulmonary hypertension in rodents, and appears to have protective effects on glucose metabolism. Chronic intermittent hypoxia (CIH) has long been used as a model of obstructive sleep apnea (OSA), in which recurrent airway occlusion results in intermittent reductions in oxyhemoglobin saturations throughout the night. CIH was first shown to increase systemic blood pressure, but has also been associated with other maladaptive physiological changes, including glucose dysregulation, atherosclerosis, progression of nonalcoholic fatty liver disease, and endothelial dysfunction. However, models of CIH have generally been implemented so as to mimic severe human OSA, with comparatively less focus on milder hypoxic regimens. Here we discuss CSH and CIH conceptually, the effects of these stimuli, and limitations of the available data.

NOX4, MDA, IMA and oxidative DNA damage: can these parameters be used to estimate the presence and severity of OSA?

PURPOSE

Obstructive sleep apnoea (OSA) involves recurrent obstructive apnoeas and hypopnoeas which cause cyclic hypoxia, reoxygenation and formation of reactive oxygen species (ROS). We aimed to investigate a member of the nicotinamide adenine dinucleotide phosphate oxidase (NOX) family of enzymes, specifically (NOX4), not previously studied in humans, as well as 8-OHdG/106dG, MDA and IMA, which are known to be associated with oxidative stress. We also evaluated these parameters in predicting the presence and severity of OSA.

METHODS

All 120 subjects (90 with OSA, 30 healthy controls) underwent polysomnography and had blood serum samples taken at the same time of day. Subjects were grouped by presence and severity of OSA, and serum markers were compared among groups.

RESULTS

Age and body mass index were not significantly different among groups. In the OSA group, the levels of NOX4, IMA, MDA and 8-OHdG/106dG were significantly higher than in the healthy control group. NOX4 and other parameters were positively correlated with the severity of OSA. For all parameters, the highest levels were detected in patients with severe OSA.

CONCLUSIONS

The repeated hypoxia of OSA is associated with increases in the serum levels of inflammatory mediators such as MDA, IMA and 8-OHdG/106dG and the ROS NOX4. In this study, NOX4 and other markers were associated with the presence and severity of OSA.

Increased Insulin Sensitivity by High-Altitude Hypoxia in Mice with High-Fat Diet-Induced Obesity Is Associated with Activated AMPK Signaling and Subsequently Enhanced Mitochondrial Biogenesis in Skeletal Muscles

BACKGROUND

This study aimed to investigate whether and how high altitude-associated ambient hypoxia affects insulin sensitivity in mice fed a high-fat diet (HFD).

METHODS

Mice were randomly divided into a control group (with normal diet feeding and low-altitude housing), LA/HFD group (with HFD feeding and low-altitude housing), and HA/HFD group (with HFD feeding and high-altitude housing).

RESULTS

After 8 weeks, mice in the HA/HFD group showed improved insulin sensitivity-related indices compared with the LA/HFD group. In mice residing in a low-altitude region, HFD significantly impaired mitochondrial respiratory function and mitochondrial DNA content in skeletal muscles, which was partially reversed in mice in the HA/HFD group. In addition, the fatty acid oxidation-related enzyme gene CPT1 (carnitine palmitoyltransferase 1) and genes related to mitochondrial biogenesis such as peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), nuclear respiratory factor 1 (NRF1), and mitochondrial transcription factor A (Tfam) were upregulated in the skeletal muscles of mice housed at high altitude, in comparison to in the LA/HFD group. Furthermore, AMPK (adenosine monophosphate-activated protein kinase) signaling was activated in the skeletal muscles, as evidenced by a higher expression of phosphorylated AMPK (p-AMPK) and protein kinase B (p-AKT) in the HA/HFD group than in the LA/HFD group.

CONCLUSION

Our study suggests that high-altitude hypoxia improves insulin sensitivity in mice fed an HFD, which is associated with AMPK activation in the skeletal muscle and consequently enhanced mitochondrial biogenesis and fatty acid oxidation. This work provides a molecular explanation for why high altitude is associated with a reduced incidence of insulin resistance in the obese population.

Effects of intermittent hypoxia training on leukocyte pyruvate dehydrogenase kinase 1 (PDK-1) mRNA expression and blood insulin level in prediabetes patients

PURPOSE

Intermittent hypoxia training/treatment (IHT) is an emerging therapeutic approach to alleviate chronic diseases, such as diabetes. The present study investigated the effects of IHT on blood leucocyte pyruvate dehydrogenase kinase 1 (PDK-1) mRNA expression and its relationship with the changes in blood insulin level.

METHODS

Seven adult healthy volunteers and 11 prediabetic patients participated in this study. A 3-week course of IHT consisted of a 40-min session of 4 cycles of 5-min 12% O₂ and 5-min room air breathing per day, 3 sessions per week for 3 weeks (i.e., total 9 sessions of IHT). Plasma insulin levels and leukocyte PDK-1 mRNA expression were determined at various time points either under fasting condition or following oral glucose tolerance test (OGTT). Correlation between the IHT-induced changes in PDK-1 mRNA and insulin or glucose levels in the same serological samples was analyzed.

RESULTS

At pre-IHT baseline, PDK-1 mRNA expression was two times higher in prediabetes than control subjects. IHT resulted in significant augmentation in PDK-1 mRNA expression (> twofold) in prediabetes at the end of 3-week IHT and remained elevated 1 month after IHT, which was correlated with a significantly reduced insulin release and lower blood glucose after glucose loading with OGTT.

CONCLUSION

IHT can trigger beneficial effects in normalizing blood insulin levels in prediabetic patients under oral glucose load, which were closely correlated with an enhanced mRNA expression of PDK-1 in leukocytes. Further clinical trials are warranted to validate the utility of IHT as a non-invasive complementary therapy against diabetes-associated pathologies.

Life Under Hypoxia Lowers Blood Glucose Independently of Effects on Appetite and Body Weight in Mice

Blood glucose and the prevalence of diabetes are lower in mountain than lowland dwellers, which could among other factors be due to reduced oxygen availability. To investigate metabolic adaptations to life under hypoxia, male mice on high fat diet (HFD) were continuously maintained at 10% O2. At variance to preceding studies, the protocol was designed to dissect direct metabolic effects from such mediated indirectly via hypoxia-induced reductions in appetite and weight gain. This was achieved by two separate control groups on normal air, one with free access to HFD, and one fed restrictedly in order to obtain a weight curve matching that of hypoxia-exposed mice. Comparable body weight in restrictedly fed and hypoxic mice was achieved by similar reductions in calorie intake (-22%) and was associated with parallel effects on body composition as well as on circulating insulin, leptin, FGF-21, and adiponectin. Whereas the effects of hypoxia on the above parameters could thus be attributed entirely to blunted weight gain, hypoxia improved glucose homeostasis in part independently of body weight (fasted blood glucose, mmol/l: freely fed control, 10.2 ± 0.7; weight-matched control, 8.0 ± 0.3; hypoxia, 6.8 ± 0.2; p < 0.007 each; AUC in the glucose tolerance test, mol/l*min: freely fed control, 2.54 ± 0.15; weight-matched control, 1.86 ± 0.08; hypoxia, 1.67 ± 0.05; p < 0.05 each). Although counterintuitive to lowering of glycemia, insulin sensitivity appeared to be impaired in animals adapted to hypoxia: In the insulin tolerance test, hypoxia-treated mice started off with lower glycaemia than their weight-matched controls (initial blood glucose, mmol/l: freely fed control, 11.5 ± 0.7; weight-matched control, 9.4 ± 0.3; hypoxia, 8.1 ± 0.2; p < 0.02 each), but showed a weaker response to insulin (final blood glucose, mmol/l: freely fed control, 7.0 ± 0.3; weight-matched control, 4.5 ± 0.2; hypoxia, 5.5 ± 0.3; p < 0.01 each). Furthermore, hypoxia weight-independently reduced hepatic steatosis as normalized to total body fat, suggesting a shift in the relative distribution of triglycerides from liver to fat (mg/g liver triglycerides per g total fat mass: freely fed control, 10.3 ± 0.6; weight-matched control, 5.6 ± 0.3; hypoxia, 4.0 ± 0.2; p < 0.0004 each). The results show that exposure of HFD-fed mice to continuous hypoxia leads to a unique metabolic phenotype characterized by improved glucose homeostasis along with evidence for impaired rather than enhanced insulin sensitivity.

Blockade of Endothelin-1 Receptor Type B Ameliorates Glucose Intolerance and Insulin Resistance in a Mouse Model of Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is associated with insulin resistance (IR) and glucose intolerance. Elevated endothelin-1 (ET-1) levels have been observed in OSA patients and in mice exposed to intermittent hypoxia (IH). We examined whether pharmacological blockade of type A and type B ET-1 receptors (ETA and ETB) would ameliorate glucose intolerance and IR in mice exposed to IH. Subcutaneously implanted pumps delivered BQ-123 (ETA antagonist; 200 nmol/kg/day), BQ-788 (ETB antagonist; 200 nmol/kg/day) or vehicle (saline or propyleneglycol [PG]) for 14 days in C57BL6/J mice (10/group). During treatment, mice were exposed to IH (decreasing the FiO2 from 20.9% to 6%, 60/h) or intermittent air (IA). After IH or IA exposure, insulin (0.5 IU/kg) or glucose (1 mg/kg) was injected intraperitoneally and plasma glucose determined after injection and area under glucose curve (AUC) was calculated. Fourteen-day IH increased fasting glucose levels (122 ± 7 vs. 157 ± 8 mg/dL, PG: 118 ± 6 vs. 139 ± 8; both p < 0.05) and impaired glucose tolerance (AUCglucose: 19,249 ± 1105 vs. 29,124 ± 1444, PG AUCglucose: 18,066 ± 947 vs. 25,135 ± 797; both p < 0.05) in vehicle-treated animals. IH-induced impairments in glucose tolerance were partially ameliorated with BQ-788 treatment (AUCglucose: 21,969 ± 662; p < 0.05). Fourteen-day IH also induced IR (AUCglucose: 7185 ± 401 vs. 8699 ± 401; p < 0.05). Treatment with BQ-788 decreased IR under IA (AUCglucose: 5281 ± 401, p < 0.05) and reduced worsening of IR with IH (AUCglucose: 7302 ± 401, p < 0.05). There was no effect of BQ-123 on IH-induced impairments in glucose tolerance or IR. Our results suggest that ET-1 plays a role in IH-induced impairments in glucose homeostasis.

Nocturnal Hypoxia Improves Glucose Disposal, Decreases Mitochondrial Efficiency, and Increases Reactive Oxygen Species in the Muscle and Liver of C57BL/6J Mice Independent of Weight Change

Although acute exposure to hypoxia can disrupt metabolism, longer-term exposure may normalize glucose homeostasis or even improve glucose disposal in the presence of obesity. We examined the effects of two-week exposure to room air (Air), continuous 10% oxygen (C10%), and 12 hr nocturnal periods of 10% oxygen (N10%) on glucose disposal, insulin responsiveness, and mitochondrial function in lean and obese C57BL/6J mice. Both C10% and N10% improved glucose disposal relative to Air in lean and obese mice without evidence of an increase in insulin responsiveness; however, only the metabolic improvements with N10% exposure occurred in the absence of confounding effects of weight loss. In lean mice, N10% exposure caused a decreased respiratory control ratio (RCR) and increased reactive oxygen species (ROS) production in the mitochondria of the muscle and liver compared to Air-exposed mice. In the absence of hypoxia, obese mice exhibited a decreased RCR in the muscle and increased ROS production in the liver compared to lean mice; however, any additional effects of hypoxia in the presence of obesity were minimal. Our data suggest that the development of mitochondrial inefficiency may contribute to metabolic adaptions to hypoxia, independent of weight, and metabolic adaptations to adiposity, independent of hypoxia.

Chronic Intermittent Hypoxia Impairs Insulin Sensitivity but Improves Whole-Body Glucose Tolerance by Activating Skeletal Muscle AMPK

Obstructive sleep apnea syndrome is a highly prevalent disease resulting in transient respiratory arrest and chronic intermittent hypoxia (cIH). cIH is associated with insulin resistance and impaired metabolic homeostasis in rodents and humans, but the exact underlying mechanisms remain unclear. In the current study, we investigated the effects of 2 weeks of cIH (1-min cycle, fraction of inspired oxygen 21-5%, 8 h/day) on whole-body insulin sensitivity and glucose tolerance in lean mice. Although food intake and body weight were reduced compared with normoxia, cIH induced systemic insulin resistance in a hypoxia-inducible factor 1-independent manner and impaired insulin signaling in liver, white adipose tissue, and skeletal muscle. Unexpectedly, cIH improved whole-body glucose tolerance independently of changes in body weight and glucose-induced insulin response. This effect was associated with elevated phosphorylation of Thr172-AMPK and Ser237-TBC1 domain family member 1 (TBC1D1) in skeletal muscle, suggesting a tissue-specific AMPK-dependent increase in TBC1D1-driven glucose uptake. Remarkably, although food intake, body weight, and systemic insulin sensitivity were still affected, the improvement in glucose tolerance by cIH was abolished in muscle-specific AMPKα1α2-deficient mice. We conclude that cIH impairs insulin sensitivity while improving whole-body glucose tolerance by promoting specific activation of the skeletal muscle AMPK pathway.

Transcriptomic analysis identifies a role of PI3K-Akt signalling in the responses of skeletal muscle to acute hypoxia in vivo

KEY POINTS

Changes in gene expression that occur within hours of exposure to hypoxia in in vivo skeletal muscles remain unexplored. Two hours of hypoxia caused significant down-regulation of extracellular matrix genes followed by a shift at 6 h to altered expression of genes associated with the nuclear lumen while respiratory and blood gases were stabilized. Enrichment analysis of mRNAs classified by stability rates suggests an attenuation of post-transcriptional regulation within hours of hypoxic exposure, where PI3K-Akt signalling was suggested to have a nodal role by pathway analysis. Experimental measurements and bioinformatic analyses suggested that the dephosphorylation of Akt after 2 h of hypoxic exposure might deactivate RNA-binding protein BRF1, hence resulting in the selective degradation of mRNAs.

ABSTRACT

The effects of acute hypoxia have been widely studied, but there are few studies of transcriptional responses to hours of hypoxia in vivo, especially in hypoxia-tolerant tissues like skeletal muscles. We used RNA-seq to analyse gene expression in plantaris muscles while monitoring respiration, arterial blood gases, and blood glucose in mice exposed to 8% O2 for 2 or 6 h. Rapid decreases in blood gases and a slower reduction in blood glucose suggest stress, which was accompanied by widespread changes in gene expression. Early down-regulation of genes associated with the extracellular matrix was followed by a shift to genes associated with the nuclear lumen. Most of the early down-regulated genes had mRNA half-lives longer than 2 h, suggesting a role for post-transcriptional regulation. These transcriptional changes were enriched in signalling pathways in which the PI3K-Akt signalling pathway was identified as a hub. Our analyses indicated that gene targets of PI3K-Akt but not HIF were enriched in early transcriptional responses to hypoxia. Among the PI3K-Akt targets, 75% could be explained by a deactivation of adenylate-uridylate-rich element (ARE)-binding protein BRF1, a target of PI3K-Akt. Consistent decreases in the phosphorylation of Akt and BRF1 were experimentally confirmed following 2 h of hypoxia. These results suggest that the PI3K-Akt signalling pathway might play a role in responses induced by acute hypoxia in skeletal muscles, partially through the dephosphorylation of ARE-binding protein BRF1.

Inhibition of Lipolysis Ameliorates Diabetic Phenotype in a Mouse Model of Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is associated with insulin resistance, glucose intolerance, and type 2 diabetes. Causal mechanisms mediating this association are not well defined; however, augmented lipolysis in adipose might be involved. Here, we investigated the effect of acipimox treatment (lipolysis inhibitor) on glucose tolerance and insulin sensitivity in mice exposed to intermittent hypoxia (IH). C57BL6/J mice were exposed for 14 days to IH or control conditions. IH was created by decreasing the fraction of inspired oxygen from 20.9 to 6.5%, 60 times/h. Control exposure was air (fraction of inspired oxygen, 20.9%) delivered at an identical flow rate. Acipimox was provided in drinking water (0.5 g/ml) during exposures. After exposures, intraperitoneal insulin (0.5 IU/kg) and glucose (1 g/kg) tolerance tests were performed, and primary adipocytes were isolated for lipolysis experiments. IH elevated fasting glucose by 51% and worsened glucose tolerance and insulin sensitivity by 33 and 102%, respectively. In parallel, IH increased spontaneous lipolysis by 264%, and reduced epididymal fat mass by 15% and adipocyte size by 8%. Acipimox treatment prevented IH-induced lipolysis and increased epididymal fat mass and adipocyte size by 19 and 10%, respectively. Acipimox fully prevented IH-induced impairments in fasting glycemia, glucose tolerance, and insulin sensitivity. For all reported results, P less than 0.05 was considered significant. Augmented lipolysis contributes to insulin resistance and glucose intolerance observed in mice exposed to IH. Acipimox treatment ameliorated the metabolic consequences of IH and might represent a novel treatment option for patients with obstructive sleep apnea.

Visceral White Adipose Tissue after Chronic Intermittent and Sustained Hypoxia in Mice

Angiogenesis, a process induced by hypoxia in visceral white adipose tissues (vWAT) in the context of obesity, mediates obesity-induced metabolic dysfunction and insulin resistance. Chronic intermittent hypoxia (IH) and sustained hypoxia (SH) induce body weight reductions and insulin resistance of different magnitudes, suggesting different hypoxia inducible factor (HIF)-1α-related activity. Eight-week-old male C57BL/6J mice (n = 10-12/group) were exposed to either IH, SH, or room air (RA). vWAT were analyzed for insulin sensitivity (phosphorylated (pAKT)/AKT), HIF-1α transcription using chromatin immunoprecipitation (ChIP)-sequencing, angiogenesis using immunohistochemistry, and gene expression of different fat cell markers and HIF-1α gene targets using quantitative polymerase chain reaction or microarrays. Body and vWAT weights were reduced in hypoxia (SH > IH > RA; P < 0.001), with vWAT in IH manifesting vascular rarefaction and increased proinflammatory macrophages. HIF-1α ChIP-sequencing showed markedly increased binding sites in SH-exposed vWAT both at 6 hours and at 6 weeks compared with IH, the latter also showing decreased vascular endothelial growth factor, endothelial nitric oxide synthase, P2RX5, and PAT2 expression, and insulin resistance (IH > > > SH = RA; P < 0.001). IH induces preferential whitening of vWAT, as opposed to prominent browning in SH. Unlike SH, IH elicits early HIF-1α activity that is unsustained over time and is accompanied by concurrent vascular rarefaction, inflammation, and insulin resistance. Thus, the dichotomous changes in HIF-1α transcriptional activity and brown/beige/white fat balance in IH and SH should enable exploration of mechanisms by which altered sympathetic outflow, such as that which occurs in apneic patients, results in whitening, rather than the anticipated browning of adipose tissues that occurs in SH.

PERK/eIF2α contributes to changes of insulin signaling in HepG2 cell induced by intermittent hypoxia

AIMS

Obstructive sleep apnea hypopnea syndrome (OSAHS) is associated with abnormal glucose metabolism. Nowadays, endoplasmic reticulum (ER) stress emerges as an important mechanism underlying the development of type 2 diabetes mellitus (T2DM). However, it remains unclear that intermittent hypoxia (IH) could induce ER stress, resulting in abnormality of glucose metabolism. Thus, in the current study we explore the changes of insulin signaling under IH and the role of ER stress underlying these changes.

MAIN METHODS

HepG2 cells were exposed to room air (RA) or IH for 8h, 16h and 24h respectively. Oxygen concentration in IH groups was in a dynamic cycle from 21% to 1% every 5min, while it remained at 21% in RA groups. Insulin was added into cell culture medium for AKT and p-AKT measurement. In another experiment set, HepG2 cells were pre-cultured with 4-PBA prior to IH or RA exposure. Expression of AKT, p-AKT, p-JNK, p-IRE1, p-PERK and p-eIF2α was examined by Western Blot.

KEY FINDINGS

Compared with RA, p-AKT expression in HepG2 cells under IH for 24h was significantly lower even with insulin treatment. Expression of p-JNK, p-IRE1, ATF6, p-PERK and p-eIF2α were upregulated. p-AKT level in HepG2 with 4-PBA preculture under IH was restored. p-PERK and p-eIF2α expression in HepG2 cells in IH groups with 4-PBA preculture were inhibited while levels of p-JNK and p-IRE1 remained unchanged.

SIGNIFICANCE

IH, the hallmarker of OSAHS, could disturb insulin signaling via activating PERK/eIF2α.

Effect of adrenal medullectomy on metabolic responses to chronic intermittent hypoxia in the frequently sampled intravenous glucose tolerance test

Obstructive sleep apnea is associated with type 2 diabetes. We have previously developed a mouse model of intermittent hypoxia (IH) mimicking oxyhemoglobin desaturations in patients with sleep apnea and have shown that IH increases fasting glucose, hepatic glucose output, and plasma catecholamines. We hypothesize that adrenal medulla modulates glucose responses to IH and that such responses can be prevented by adrenal medullectomy. We performed adrenal medullectomy or sham surgery in lean C57BL/6J mice, which were exposed to IH or intermittent air (control) for 4 wk followed by the frequently sampled intravenous glucose tolerance test (FSIVGTT) in unanesthetized unrestrained animals. IH was administered during the 12-h light phase (9 AM to 9 PM) by decreasing inspired oxygen from 21 to 6.5% 60 cycles/h. Insulin sensitivity (S_I), insulin independent glucose disposal [glucose effectiveness (S_G)], and the insulin response to glucose (AIR_G) were determined using the minimal model method. In contrast to our previous data obtained in restrained mice, IH did not affect fasting blood glucose and plasma insulin levels in sham-operated mice. IH significantly decreased S_G but did not affect S_I and AIR_G Adrenal medullectomy decreased fasting blood glucose and plasma insulin levels and increased glycogen synthesis in the liver in hypoxic mice but did not have a significant effect on the FSIVGTT metrics. We conclude that, in the absence of restraints, IH has no effect on glucose metabolism in lean mice with exception of decreased S_G, whereas adrenal medullectomy decreases fasting glucose and insulin levels in the IH environment.NEW & NOTEWORTHY To our knowledge, this is the first study examining the role of adrenal catecholamines in glucose metabolism during intermittent hypoxia (IH) in unanesthetized unrestrained C57BL/6J mice. We report that IH did not affect fasting glucose and insulin levels nor insulin sensitivity and insulin secretion during, whereas glucose effectiveness was decreased. Adrenal medullectomy decreased fasting blood glucose and insulin levels in mice exposed to IH but had no effect on glucose metabolism, insulin secretion, and insulin sensitivity.

HMGB1 facilitates hypoxia-induced vWF upregulation through TLR2-MYD88-SP1 pathway

Increased plasma level of von Willebrand Factor (vWF) is associated with major cardiovascular diseases. We previously reported that multimeric vWF binds to NO synthase and inhibits insulin-induced production of NO, thus promoting insulin resistance during acute hypoxia (AH). However, the transcriptional regulation of vWF during AH is not clearly understood. Here, we investigated the mechanisms underlying the upregulation of vwf in mice. AH significantly upregulates the tlr2, tlr3, myd88, and vwf expression and phosphorylation of specificity protein 1 (SP1). Furthermore, AH significantly upregulates high mobility group box-1 (HMGB1) in a time-dependent manner. Moreover, a TLR2 agonist upregulates vWF but a TLR3 agonist does not. Pretreatment with an HMGB1 inhibitor, TLR2-immunoneutralizing antibody, or SP1 inhibitor significantly inhibits vWF expression. Furthermore, Tlr2 silencing completely inhibited MYD88, vWF expression, and SP1 phosphorylation. However, pretreatment with glycyrrhizic acid or silencing of Tlr2 completely blocks binding of Sp1 to the Vwf promoter, thus inhibiting its expression, and enhances insulin resistance during AH. Patients with type 2 diabetes mellitus also showed significantly elevated levels of HMGB1, TLR2, SP1, and vWF, thereby supporting the results of the murine model of AH. Taken together, HMGB1 upregulates vWF in vivo through the TLR2-MYD88-SP1 pathway in mice.

Biological plausibility linking sleep apnoea and metabolic dysfunction

Obstructive sleep apnoea (OSA) is a very common disorder that affects 10-25% of the general population. In the past two decades, OSA has emerged as a cardiometabolic risk factor in both paediatric and adult populations. OSA-induced metabolic perturbations include dyslipidaemia, atherogenesis, liver dysfunction and abnormal glucose metabolism. The mainstay of treatment for OSA is adenotonsillectomy in children and continuous positive airway pressure therapy in adults. Although these therapies are effective at resolving the sleep-disordered breathing component of OSA, they do not always produce beneficial effects on metabolic function. Thus, a deeper understanding of the underlying mechanisms by which OSA influences metabolic dysfunction might yield improved therapeutic approaches and outcomes. In this Review, we summarize the evidence obtained from animal models and studies of patients with OSA of potential mechanistic pathways linking the hallmarks of OSA (intermittent hypoxia and sleep fragmentation) with metabolic dysfunction. Special emphasis is given to adipose tissue dysfunction induced by sleep apnoea, which bears a striking resemblance to adipose dysfunction resulting from obesity. In addition, important gaps in current knowledge and promising lines of future investigation are identified.

Intermittent hypoxia maintains glycemia in streptozotocin-induced diabetic rats

Increasing studies have shown protective effects of intermittent hypoxia on brain injury and heart ischemia. However, the effect of intermittent hypoxia on blood glucose metabolism, especially in diabetic conditions, is rarely observed. The aim of this study was to investigate whether intermittent hypoxia influences blood glucose metabolism in type 1 diabetic rats. Streptozotocin-induced diabetic adult rats and age-matched control rats were treated with intermittent hypoxia (at an altitude of 3 km, 4 h per day for 3 weeks) or normoxia as control. Fasting blood glucose, body weight, plasma fructosamine, plasma insulin, homeostasis model assessment of insulin resistance (HOMA-IR), pancreas β-cell mass, and hepatic and soleus glycogen were measured. Compared with diabetic rats before treatment, the level of fasting blood glucose in diabetic rats after normoxic treatment was increased (19.88 ± 5.69 mmol/L vs. 14.79 ± 5.84 mmol/L, p < 0.05), while it was not different in diabetic rats after hypoxic treatment (13.14 ± 5.77 mmol/L vs. 14.79 ± 5.84 mmol/L, p > 0.05). Meanwhile, fasting blood glucose in diabetic rats after hypoxic treatment was also lower than that in diabetic rats after normoxic treatment (13.14 ± 5.77 mmol/L vs. 19.88 ± 5.69 mmol/L, p<0.05). Plasma fructosamine in diabetic rats receiving intermittent hypoxia was significantly lower than that in diabetic rats receiving normoxia (1.28 ± 0.11 vs. 1.39 ± 0.11, p < 0.05), while there were no significant changes in body weight, plasma insulin and β-cell mass. HOMA-IR in diabetic rats after hypoxic treatment was also lower compared with diabetic rats after normoxic treatment (3.48 ± 0.48 vs. 3.86 ± 0.42, p < 0.05). Moreover, intermittent hypoxia showed effect on the increase of soleus glycogen but not hepatic glycogen. We conclude that intermittent hypoxia maintains glycemia in streptozotocin-induced diabetic rats and its regulation on muscular glycogenesis may play a role in the underlying mechanism.

Effect of Intermittent Hypoxia and Rimonabant on Glucose Metabolism in Rats: Involvement of Expression of GLUT4 in Skeletal Muscle

Background

Obstructive sleep apnea (OSA) and its main feature, chronic intermittent hypoxia (IH) during sleep, is closely associated with insulin resistance (IR) and diabetes. Rimonabant can regulate glucose metabolism and improve IR. The present study aimed to assess the effect of IH and rimonabant on glucose metabolism and insulin sensitivity, and to explore the possible mechanisms.

Material/Methods

Thirty-two rats were randomly assigned into 4 groups: Control group, subjected to intermittent air only; IH group, subjected to IH only; IH+NS group, subjected to IH and treated with normal saline; and IH+Rim group, subjected to IH and treated with 10 mg/kg/day of rimonabant. All rats were killed after 28 days of exposure. Then, the blood and skeletal muscle were collected. We measured fasting blood glucose levels, fasting blood insulin levels, and the expression of glucose transporter 4 (GLUT4) in both mRNA and protein levels in skeletal muscle.

Results

IH can slow weight gain, increase serum insulin level, and reduce insulin sensitivity in rats. The expressions of GLUT4 mRNA, total GLUT4, and plasma membrane protein of GLUT4 (PM GLUT4) in skeletal muscle were decreased. Rimonabant treatment was demonstrated to improve weight gain and insulin sensitivity of the rats induced by IH. Rimonabant significantly upregulated the expression of GLUT4 mRNA, PM GLUT4, and total GLUT4 in skeletal muscle.

Conclusions

The present study demonstrates that IH can cause IR and reduced expression of GLUT4 in both mRNA and protein levels in skeletal muscle of rats. Rimonabant treatment can improve IH – induced IR, and the upregulation of GLUT4 expression may be involved in this process.

Insulin resistance, glucose intolerance and diabetes mellitus in obstructive sleep apnoea

Obstructive sleep apnoea (OSA) is a highly prevalent disorder, which conveys an increased risk of cardiovascular disease and death. Type 2 diabetes mellitus (T2DM), glucose intolerance and insulin resistance (IR) are common in subjects with OSA, but a shared intimate relationship with obesity makes discerning an independent link challenging. Nonetheless, mechanistic studies suggest that OSA could contribute to impaired glucose metabolism via the effects of sleep fragmentation, sympathetic excitation and intermittent hypoxia (IH) on pancreatic B-cell function, insulin sensitivity, and systemic inflammation. In particular, emerging data suggest that IH may have an important detrimental effect on adipose tissue function and inflammation. Similarly, data from population-and clinic-level studies suggest that OSA is independently related with the prevalence and incidence of T2DM and IR, and may also lead to worse glycaemic control in diabetics. However, the ability of continuous positive airway pressure (CPAP) therapy to make a meaningful impact on T2DM or IR remains uncertain. In this review we explore the available evidence linking OSA with IR, glucose intolerance and T2DM, and discuss potential pathobiological mechanisms by which sleep disordered breathing can affect metabolic health.

Translational approaches to understanding metabolic dysfunction and cardiovascular consequences of obstructive sleep apnea

Obstructive sleep apnea (OSA) is known to be independently associated with several cardiovascular diseases including hypertension, myocardial infarction, and stroke. To determine how OSA can increase cardiovascular risk, animal models have been developed to explore the underlying mechanisms and the cellular and end-organ targets of the predominant pathophysiological disturbance in OSA-intermittent hypoxia. Despite several limitations in translating data from animal models to the clinical arena, significant progress has been made in our understanding of how OSA confers increased cardiovascular risk. It is clear now that the hypoxic stress associated with OSA can elicit a broad spectrum of pathological systemic events including sympathetic activation, systemic inflammation, impaired glucose and lipid metabolism, and endothelial dysfunction, among others. This review provides an update of the basic, clinical, and translational advances in our understanding of the metabolic dysfunction and cardiovascular consequences of OSA and highlights the most recent findings and perspectives in the field.

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.

Intermittent hypoxemia and OSA: implications for comorbidities

OSA is a common chronic disorder that is associated with significant morbidity and mortality including cardiovascular, metabolic, and neurocognitive disease and increased cancer-related deaths. OSA is characterized by recurrent episodes of apneas and hypopneas associated with repetitive episodes of intermittent hypoxemia, intrathoracic pressure changes, and arousals. Intermittent hypoxemia (IH) is now being recognized as a potential major factor contributing to the pathogenesis of OSA-related comorbidities. OSA-related high-frequency IH is characterized by cycles of hypoxemia with reoxygenation that is distinctly different than sustained low-frequency hypoxia and contributes to ischemia-reperfusion injury. Data from both animal and human studies support mechanistic links between IH and its adverse impact at the tissue level. IH promotes oxidative stress by increased production of reactive oxygen species and angiogenesis, increased sympathetic activation with BP elevation, and systemic and vascular inflammation with endothelial dysfunction that contributes to diverse multiorgan chronic morbidity and mortality affecting cardiovascular disease, metabolic dysfunction, cognitive decline, and progression of cancer. Data from observational studies in large population groups also support the role for hypoxia in the pathogenesis of OSA comorbidity. Treatment with CPAP to reverse OSA-related symptoms and comorbidities has been shown to provide variable benefit in some but not all patient groups. Early treatment with CPAP makes intuitive sense to promote maximal functional recovery and minimize residual injury. More studies are needed to determine the interacting effects of IH and obesity, differential effects of both short-term and long-term hypoxemia, and the effect of CPAP treatment.

Inactivation of corticotropin-releasing hormone-induced insulinotropic role by high-altitude hypoxia

We have shown that hypoxia reduces plasma insulin, which correlates with corticotropin-releasing hormone (CRH) receptor 1 (CRHR1) in rats, but the mechanism remains unclear. Here, we report that hypobaric hypoxia at an altitude of 5,000 m for 8 h enhances rat plasma CRH, corticosterone, and glucose levels, whereas the plasma insulin and pancreatic ATP/ADP ratio is reduced. In islets cultured under normoxia, CRH stimulated insulin release in a glucose- and CRH-level-dependent manner by activating CRHR1 and thus the cAMP-dependent protein kinase pathway and calcium influx through L-type channels. In islets cultured under hypoxia, however, the insulinotropic effect of CRH was inactivated due to reduced ATP and cAMP and coincident loss of intracellular calcium oscillations. Serum and glucocorticoid-inducible kinase 1 (SGK1) also played an inhibitory role. In human volunteers rapidly ascended to 3,860 m, plasma CRH and glucose levels increased without a detectable change in plasma insulin. By contrast, volunteers with acute mountain sickness (AMS) exhibited a marked decrease in HOMA insulin sensitivity (HOMA-IS) and enhanced plasma CRH. In conclusion, hypoxia may attenuate the CRH-insulinotropic effect by reducing cellular ATP/ADP ratio, cAMP and calcium influx, and upregulated SGK1. Hypoxia may not affect HOMA-IS in healthy volunteers but reduces it in AMS volunteers.

Resveratrol attenuates intermittent hypoxia-induced macrophage migration to visceral white adipose tissue and insulin resistance in male mice

Chronic intermittent hypoxia during sleep (IH), as occurs in sleep apnea, promotes systemic insulin resistance. Resveratrol (Resv) has been reported to ameliorate high-fat diet-induced obesity, inflammation, and insulin resistance. To examine the effect of Resv on IH-induced metabolic dysfunction, male mice were subjected to IH or room air conditions for 8 weeks and treated with either Resv or vehicle (Veh). Fasting plasma levels of glucose, insulin, and leptin were obtained, homeostatic model assessment of insulin resistance index levels were calculated, and insulin sensitivity tests (phosphorylated AKT [also known as protein kinase B]/total AKT) were performed in 2 visceral white adipose tissue (VWAT) depots (epididymal [Epi] and mesenteric [Mes]) along with flow cytometry assessments for VWAT macrophages and phenotypes (M1 and M2). IH-Veh and IH-Resv mice showed initial reductions in food intake with later recovery, with resultant lower body weights after 8 weeks but with IH-Resv showing better increases in body weight vs IH-Veh. IH-Veh and IH-Resv mice exhibited lower fasting glucose levels, but only IH-Veh had increased homeostatic model assessment of insulin resistance index vs all 3 other groups. Leptin levels were preserved in IH-Veh but were significantly lower in IH-Resv. Reduced VWAT phosphorylated-AKT/AKT responses to insulin emerged in both Mes and Epi in IH-Veh but normalized in IH-Resv. Increases total macrophage counts and in M1 to M2 ratios occurred in IH-Veh Mes and Epi compared all other 3 groups. Thus, Resv ameliorates food intake and weight gain during IH exposures and markedly attenuates VWAT inflammation and insulin resistance, thereby providing a potentially useful adjunctive therapy for metabolic morbidity in the context of sleep apnea.

Intermittent hypoxia-induced glucose intolerance is abolished by α-adrenergic blockade or adrenal medullectomy

Obstructive sleep apnea causes intermittent hypoxia (IH) during sleep and is associated with dysregulation of glucose metabolism. We developed a novel model of clinically realistic IH in mice to test the hypothesis that IH causes hyperglycemia, glucose intolerance, and insulin resistance via activation of the sympathetic nervous system. Mice were exposed to acute hypoxia of graded severity (21, 14, 10, and 7% O2) or to IH of graded frequency [oxygen desaturation index (ODI) of 0, 15, 30, or 60, SpO2 nadir 80%] for 30 min to measure levels of glucose fatty acids, glycerol, insulin, and lactate. Glucose tolerance tests and insulin tolerance tests were then performed under each hypoxia condition. Next, we examined these outcomes in mice that were administered phentolamine (α-adrenergic blockade) or propranolol (β-adrenergic blockade) or that underwent adrenal medullectomy before IH exposure. In all experiments, mice were maintained in a thermoneutral environment. Sustained and IH induced hyperglycemia, glucose intolerance, and insulin resistance in a dose-dependent fashion. Only severe hypoxia (7% O2) increased lactate, and only frequent IH (ODI 60) increased plasma fatty acids. Phentolamine or adrenal medullectomy both prevented IH-induced hyperglycemia and glucose intolerance. IH inhibited glucose-stimulated insulin secretion, and phentolamine prevented the inhibition. Propranolol had no effect on glucose metabolism but abolished IH-induced lipolysis. IH-induced insulin resistance was not affected by any intervention. Acutely hypoxia causes hyperglycemia, glucose intolerance, and insulin resistance in a dose-dependent manner. During IH, circulating catecholamines act upon α-adrenoreceptors to cause hyperglycemia and glucose intolerance.

Metabolic dysfunction in obstructive sleep apnea: A critical examination of underlying mechanisms

It has recently become clear that obstructive sleep apnea (OSA) is an independent risk factor for the development of metabolic syndrome, a disorder of defective energy storage and use. Several mechanisms have been proposed to explain this finding, drawing upon the characteristics that define OSA. In particular, intermittent hypoxia, sleep fragmentation, elevated sympathetic tone, and oxidative stress - all consequences of OSA - have been implicated in the progression of poor metabolic outcomes in OSA. In this review we examine the evidence to support each of these disease manifestations of OSA as a unique risk for metabolic dysfunction. Tissue hypoxia and sleep fragmentation are each directly connected to insulin resistance and hypertension, and each of these also may increase sympathetic tone, resulting in defective glucose homeostasis, excessive lipolysis, and elevated blood pressure. Oxidative stress further worsens insulin resistance and in turn, metabolic dysfunction also increases oxidative stress. However, despite many studies linking each of these individual components of OSA to the development of metabolic syndrome, there are very few reports that actually provide a coherent narrative about the mechanism underlying metabolic dysfunction in OSA.

The effect of adrenal medullectomy on metabolic responses to chronic intermittent hypoxia

Obstructive sleep apnea causes intermittent hypoxia (IH) and is associated with insulin resistance and type 2 diabetes. IH increases plasma catecholamine levels, which may increase insulin resistance and suppress insulin secretion. The objective of this study was to determine if adrenal medullectomy (MED) prevents metabolic dysfunction in IH. MED or sham surgery was performed in 60 male C57BL/6J mice, which were then exposed to IH or control conditions (intermittent air) for 6 weeks. IH increased plasma epinephrine and norepinephrine levels, increased fasting blood glucose and lowered basal and glucose-stimulated insulin secretion. MED decreased baseline epinephrine and prevented the IH induced increase in epinephrine, whereas the norepinephrine response remained intact. MED improved glucose tolerance in mice exposed to IH, attenuated the impairment in basal and glucose-stimulated insulin secretion, but did not prevent IH-induced fasting hyperglycemia or insulin resistance. We conclude that the epinephrine release from the adrenal medulla during IH suppresses insulin secretion causing hyperglycemia.

Carotid body denervation prevents fasting hyperglycemia during chronic intermittent hypoxia

Obstructive sleep apnea causes chronic intermittent hypoxia (IH) and is associated with impaired glucose metabolism, but mechanisms are unknown. Carotid bodies orchestrate physiological responses to hypoxemia by activating the sympathetic nervous system. Therefore, we hypothesized that carotid body denervation would abolish glucose intolerance and insulin resistance induced by chronic IH. Male C57BL/6J mice underwent carotid sinus nerve dissection (CSND) or sham surgery and then were exposed to IH or intermittent air (IA) for 4 or 6 wk. Hypoxia was administered by decreasing a fraction of inspired oxygen from 20.9% to 6.5% once per minute, during the 12-h light phase (9 a.m.-9 p.m.). As expected, denervated mice exhibited blunted hypoxic ventilatory responses. In sham-operated mice, IH increased fasting blood glucose, baseline hepatic glucose output (HGO), and expression of a rate-liming hepatic enzyme of gluconeogenesis phosphoenolpyruvate carboxykinase (PEPCK), whereas the whole body glucose flux during hyperinsulinemic euglycemic clamp was not changed. IH did not affect glucose tolerance after adjustment for fasting hyperglycemia in the intraperitoneal glucose tolerance test. CSND prevented IH-induced fasting hyperglycemia and increases in baseline HGO and liver PEPCK expression. CSND trended to augment the insulin-stimulated glucose flux and enhanced liver Akt phosphorylation at both hypoxic and normoxic conditions. IH increased serum epinephrine levels and liver sympathetic innervation, and both increases were abolished by CSND. We conclude that chronic IH induces fasting hyperglycemia increasing baseline HGO via the CSN sympathetic output from carotid body chemoreceptors, but does not significantly impair whole body insulin sensitivity.

Quantitative and temporal control of oxygen microenvironment at the single islet level

Simultaneous oxygenation and monitoring of glucose stimulus-secretion coupling factors in a single technique is critical for modeling pathophysiological states of islet hypoxia, especially in transplant environments. Standard hypoxic chamber techniques cannot modulate both stimulations at the same time nor provide real-time monitoring of glucose stimulus-secretion coupling factors. To address these difficulties, we applied a multilayered microfluidic technique to integrate both aqueous and gas phase modulations via a diffusion membrane. This creates a stimulation sandwich around the microscaled islets within the transparent polydimethylsiloxane (PDMS) device, enabling monitoring of the aforementioned coupling factors via fluorescence microscopy. Additionally, the gas input is controlled by a pair of microdispensers, providing quantitative, sub-minute modulations of oxygen between 0-21%. This intermittent hypoxia is applied to investigate a new phenomenon of islet preconditioning. Moreover, armed with multimodal microscopy, we were able to look at detailed calcium and KATP channel dynamics during these hypoxic events. We envision microfluidic hypoxia, especially this simultaneous dual phase technique, as a valuable tool in studying islets as well as many ex vivo tissues.

Intermittent hypoxia impairs glucose homeostasis in C57BL6/J mice: partial improvement with cessation of the exposure

OBJECTIVES

Obstructive sleep apnea is associated with insulin resistance, glucose intolerance, and type 2 diabetes mellitus. Although several studies have suggested that intermittent hypoxia in obstructive sleep apnea may induce abnormalities in glucose homeostasis, it remains to be determined whether these abnormalities improve after discontinuation of the exposure. The objective of this study was to delineate the effects of intermittent hypoxia on glucose homeostasis, beta cell function, and liver glucose metabolism and to investigate whether the impairments improve after the hypoxic exposure is discontinued.

INTERVENTIONS

C57BL6/J mice were exposed to 14 days of intermittent hypoxia, 14 days of intermittent air, or 7 days of intermittent hypoxia followed by 7 days of intermittent air (recovery paradigm). Glucose and insulin tolerance tests were performed to estimate whole-body insulin sensitivity and calculate measures of beta cell function. Oxidative stress in pancreatic tissue and glucose output from isolated hepatocytes were also assessed.

RESULTS

Intermittent hypoxia increased fasting glucose levels and worsened glucose tolerance by 67% and 27%, respectively. Furthermore, intermittent hypoxia exposure was associated with impairments in insulin sensitivity and beta cell function, an increase in liver glycogen, higher hepatocyte glucose output, and an increase in oxidative stress in the pancreas. While fasting glucose levels and hepatic glucose output normalized after discontinuation of the hypoxic exposure, glucose intolerance, insulin resistance, and impairments in beta cell function persisted.

CONCLUSIONS

Intermittent hypoxia induces insulin resistance, impairs beta cell function, enhances hepatocyte glucose output, and increases oxidative stress in the pancreas. Cessation of the hypoxic exposure does not fully reverse the observed changes in glucose metabolism.

Metabolic effects of intermittent hypoxia in mice: steady versus high-frequency applied hypoxia daily during the rest period

Intermittent hypoxia (IH) is a frequent occurrence in sleep and respiratory disorders. Both human and murine studies show that IH may be implicated in metabolic dysfunction. Although the effects of nocturnal low-frequency intermittent hypoxia (IH(L)) have not been extensively examined, it would appear that IH(L) and high-frequency intermittent hypoxia (IH(H)) may elicit distinct metabolic adaptations. To this effect, C57BL/6J mice were randomly assigned to IH(H) (cycles of 90 s 6.4% O(2) and 90 s 21% O(2) during daylight), IH(L) (8% O(2) during daylight hours), or control (CTL) for 5 wk. At the end of exposures, some of the mice were subjected to a glucose tolerance test (GTT; after intraperitoneal injection of 2 mg glucose/g body wt), and others were subjected to an insulin tolerance test (ITT; 0.25 units Humulin/kg body wt), with plasma leptin and insulin levels being measured in fasting conditions. Skeletal muscles were harvested for GLUT4 and proliferator-activated receptor gamma coactivator 1-α (PGC1-α) expression. Both IH(H) and IH(L) displayed reduced body weight increases compared with CTL. CTL mice had higher basal glycemic levels, but GTT kinetics revealed marked differences between IH(L) and IH(H), with IH(L) manifesting the lowest insulin sensitivity compared with either IH(H) or CTL, and such findings were further confirmed by ITT. No differences emerged in PGC1-α expression across the three experimental groups. However, while cytosolic GLUT4 protein expression remained similar in IH(L), IH(H), and CTL, significant decreases in GLUT4 membrane fraction occurred in hypoxia and were most pronounced in IH(L)-exposed mice. Thus IH(H) and IH(L) elicit differential glucose homeostatic responses despite similar cumulative hypoxic profiles.