Differences in Sleep-induced Hypoxia between A/J and DBA/2J Mouse Strains

Genetic depletion of 5-HT increases central apnea frequency and duration and dampens arousal but does not impact the circadian modulation of these variables

We examined the impact of serotonin (5-HT) on the frequency and duration of central apneic events and the frequency of accompanying arousals during nonrapid and rapid eye movement (NREM and REM, respectively) sleep across the light/dark cycle. Electroencephalography, electromyography, core body temperature, and activity were recorded for 24 h following implantation of telemeters in wild-type (Tph2+/+) and tryptophan hydroxylase 2 knockout (Tph2−/−) male mice. The frequency and duration of central apneic events were increased, the number of apneic events coupled to an arousal was decreased, and the ventilatory sensitivity to hypoxia and hypercapnia was decreased in the Tph2−/− compared with the Tph2+/+ mice during NREM sleep. Apnea frequency and duration were similar in the Tph2−/− and Tph2+/+ mice during REM sleep. The duration of apneic events during REM compared with NREM sleep was similar in the Tph2−/− mice. In contrast, the duration was greater during REM sleep in the Tph2+/+ mice. Our results also revealed that apnea frequency was greater during the light compared with the dark cycle. Circadian modulation of this variable was evident in both the Tph2−/− and Tph2+/+ mice during NREM and REM sleep. We conclude that depletion of 5-HT increases the frequency and duration of central apneic events, dampens arousal, and blunts the ventilatory response to hypoxia and hypercapnia during NREM sleep but is not essential for the circadian modulation of these variables.

NEW & NOTEWORTHY The presence of serotonin (5-HT) in the central nervous system diminishes the frequency of central apneic events. This neuromodulator also moderates the duration of central apneic events and promotes arousal from central events if they occur during nonrapid eye movement (NREM) sleep. However, 5-HT is not responsible for the circadian modulation of apnea frequency, which we found was greater during NREM sleep in the light compared with the dark cycle.

A phenotype of increased sleepiness in a mouse model of pulmonary hypertension and right ventricular hypertrophy

The relationship between cardiovascular disease and abnormalities in sleep architecture is complex and bi-directional. Sleep disordered breathing (SDB) often confounds human studies examining sleep in the setting of heart failure, and the independent impact of isolated right or left heart failure on sleep is difficult to assess. We utilized an animal model of right heart failure using pulmonary artery banding (PAB) in mice to examine the causal effect of right heart failure on sleep architecture. Four weeks after PAB or sham (control) surgery, sleep was measured by polysomnography for 48 hours and right ventricular (RV) hypertrophy confirmed prior to sacrifice. PAB resulted in right ventricular hypertrophy based on a 30% increase in the Fulton Index (p < 0.01). After PAB, mice spent significantly more time in NREM sleep compared to the control group over a 24 hour period (53.5 ± 1.5% vs. 46.6 ± 1.4%; p < 0.01) and exhibited an inability to both cycle into REM sleep and decrease delta density across the light/sleep period. Our results support a phenotype of impaired sleep cycling and increased ‘sleepiness’ in a mouse model of RV dysfunction.

Sleep phenotype in the Townes mouse model of sickle cell disease

Purpose

Patients with sickle cell disease (SCD) regularly experience abnormal sleep, characterized by frequent arousals and reduced total sleep time. However, obstructive sleep apnea syndrome (OSAS) is a common comorbidity of SCD, making it unclear whether the disease per se is impacting sleep, or sleep disruption is secondary to the presence of OSAS. Thus, we assessed sleep, independent of OSAS, using a mouse model of SCD.

Methods

Sleep was compared between 10-to-12-week-old Townes knockout-transgenic mice with the sickle cell phenotype SS (n = 6) and Townes mice with sickle cell trait AS (n = 6; control). The mice underwent chronic polysomnographic electrode implantation (4EEG/2EMG) to assess sleep architecture.

Results

The SS mice had significantly lower hemoglobin concentration compared to control AS mice (7.3 ± 1.3 vs. 12.9 ± 1.7 g/dL; p < 0.01), consistent with the expected SCD phenotype. SS mice exhibited significantly decreased total NREM sleep time (45.0 ± 0.7 vs. 53.0 ± 1.3% 24 h sleep time; p < 0.01), but no change in total REM sleep time compared to the AS mice. The SS mice took longer to resume sleep after a wake period compared to the AS mice (3.2 ± 0.3 min vs. 1.9 ± 0.2 min; p < 0.05). Unexpectedly, SS mice experienced fewer arousals compared to AS mice (19.0 ± 0.9 vs. 23.3 ± 2.1 arousals/h of sleep; p = 0.031).

Conclusions

The presence of decreased total NREM sleep associated with reduced arousals, in the absence of OSAS, suggests a distinctive underlying sleep phenotype in a mouse model of SCD.

Abnormal Growth and Feeding Behavior in Upper Airway Obstruction in Rats

Pediatric obstructive sleep apnea (OSA) is a syndrome manifesting with snoring and increased respiratory effort due to increased upper airway resistance. In addition to cause the abnormal sleep, this syndrome has been shown to elicit either growth retardation or metabolic syndrome and obesity. Treating OSA by adenotonsillectomy is usually associated with increased risk for obesity, despite near complete restoration of breathing and sleep. However, the underlying mechanism linking upper airways obstruction (AO) to persistent change in food intake, metabolism, and growth remains unclear. Rodent models have examined the impact of intermittent hypoxia on metabolism. However, an additional defining feature of OSA that is not related to intermittent hypoxia is enhanced respiratory loading leading to increased respiratory effort and abnormal sleep. The focus of this mini review is on recent evidence indicating the persistent abnormalities in endocrine regulation of feeding and growth that are not fully restored by the chronic upper AO removal in rats. Here, we highlight important aspects related to abnormal regulation of metabolism that are not related to intermittent hypoxia per se, in an animal model that mimics many of the clinical features of pediatric OSA. Our evidence from the AO model indicates that obstruction removal may not be sufficient to prevent the post-removal tendency for abnormal growth.

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.

Role of BK Channels in Murine Carotid Body Neural Responses in vivo

The aim of this study was to explore the role of BK channels in the hypoxic sensitivity of the in vivo murine carotid body (CB). Four strains of mice (DBA/2J, A/J, BKα1 knockout and BKα1 wild type - FVB background) were used. The mice were anesthetized, paralyzed and mechanically ventilated (PaCO(2) ~ 35 mmHg, PO(2) > 300 mmHg). We measured carotid sinus nerve (CSN) activity during three gas challenges (F(I)O(2): 0.21, 0.15 and 0.10). CSN activity was analyzed with time-variant spectral analysis with frequency domain conversion (Fast Fourier Transforms). Afferent CSN activity increased with lowering F(I)O(2) in the DBA/2J, BKKO and BKWT mice with the most robust response in 600-800 frequencies. No substantial changes were observed in the A/J mice. Although maximal neural output was similar between the BKKO and BKWT mice, the BKWT had a higher early response compared to BKKO. Thus, BK channels may play a role in the initial response of the CB to hypoxia. The contribution of BKβ subunits or the importance of frequency specific responses was unable to be determined by the current study.

Mild Transient Hypercapnia as a Novel Fear Conditioning Stimulus Allowing Re-Exposure during Sleep

Introduction

Studies suggest that sleep plays a role in traumatic memories and that treatment of sleep disorders may help alleviate symptoms of posttraumatic stress disorder. Fear-conditioning paradigms in rodents are used to investigate causal mechanisms of fear acquisition and the relationship between sleep and posttraumatic behaviors. We developed a novel conditioning stimulus (CS) that evoked fear and was subsequently used to study re-exposure to the CS during sleep.

Methods

Experiment 1 assessed physiological responses to a conditioned stimulus (mild transient hypercapnia, mtHC; 3.0% CO₂; n = 17)+footshock for the purpose of establishing a novel CS in male FVB/J mice. Responses to the novel CS were compared to tone+footshock (n = 18) and control groups of tone alone (n = 17) and mild transient hypercapnia alone (n = 10). A second proof of principle experiment re-exposed animals during sleep to mild transient hypercapnia or air (control) to study sleep processes related to the CS.

Results

Footshock elicited a response of acute tachycardia (30–40 bpm) and increased plasma epinephrine. When tone predicted footshock it elicited mild hypertension (1–2 mmHg) and a three-fold increase in plasma epinephrine. When mtHC predicted footshock it also induced mild hypertension, but additionally elicited a conditioned bradycardia and a smaller increase in plasma epinephrine. The overall mean 24 hour sleep–wake profile was unaffected immediately after fear conditioning.

Discussion

Our study demonstrates the efficacy of mtHC as a conditioning stimulus that is perceptible but innocuous (relative to tone) and applicable during sleep. This novel model will allow future studies to explore sleep-dependent mechanisms underlying maladaptive fear responses, as well as elucidate the moderators of the relationship between fear responses and sleep.

Rodent models of sleep apnea

Rodent models of sleep apnea have long been used to provide novel insight into the generation and predisposition to apneas as well as to characterize the impact of sleep apnea on cardiovascular, metabolic, and psychological health in humans. Given the significant body of work utilizing rodent models in the field of sleep apnea, the aims of this review are three-fold: first, to review the use of rodents as natural models of sleep apnea; second, to provide an overview of the experimental interventions employed in rodents to simulate sleep apnea; third, to discuss the refinement of rodent models to further our understanding of breathing abnormalities that occur during sleep. Given mounting evidence that sleep apnea impairs cognitive function, reduces quality of life, and exacerbates the course of multiple chronic diseases, rodent models will remain a high priority as a tool to interrogate both the pathophysiology and sequelae of breathing related abnormalities during sleep and to improve approaches to diagnosis and therapy.

ATP release from the carotid bodies of DBA/2J and A/J inbred mouse strains

The purposes of this study were to: (1) establish an effective method to measure the release of ATP from the mouse carotid body (CB) and (2) determine the release of ATP from the CB of the DBA/2 J (high hypoxic responder) and A/J (low hypoxic responder) mouse in response to hypoxia and hypercapnia. An incubation chamber was constructed utilizing a Costar® Spin-X Centrifuge Tube Filter. The filter was coated with low melting point agarose to hold 4 CBs or 4 superior cervical ganglia (SCG). Hypoxia did not increase ATP release from the CB of either strain. ATP increased in response to a normoxic/hypercapnic challenge in the DBA/2 J's CB but not in the A/J's CB. ATP release from the SCG was affected by neither hypoxia nor hypercapnia in both strains. Thus, we have concluded: (1) we successfully established a chamber system to measure ATP released from the mouse CB; (2) ATP may not be an excitatory neurotransmitter in the CB of these mice under hypoxia; (3) ATP may be a neurotransmitter in the CB of the DBA/2 J mouse strain during hypercapnia.

Carotid chemoreceptor development in mice

Mice are the most suitable species for understanding genetic aspects of postnatal developments of the carotid body due to the availability of many inbred strains and knockout mice. Our study has shown that the carotid body grows differentially in different mouse strains, indicating the involvement of genes. However, the small size hampers investigating functional development of the carotid body. Hypoxic and/or hyperoxic ventilatory responses have been investigated in newborn mice, but these responses are indirect assessment of the carotid body function. Therefore, we need to develop techniques of measuring carotid chemoreceptor neural activity from young mice. Many studies have taken advantage of the knockout mice to understand chemoreceptor function of the carotid body, but they are not always suitable for addressing postnatal development of the carotid body due to lethality during perinatal periods. Various inbred strains with well-designed experiments will provide useful information regarding genetic mechanisms of the postnatal carotid chemoreceptor development. Also, targeted gene deletion is a critical approach.

Divergent postnatal development of the carotid body in DBA/2J and A/J strains of mice

We have previously shown that the adult DBA/2J and A/J strains of mice differ in carotid body volume and morphology. The question has arisen whether these differences develop during the prenatal or postnatal period. Investigating morphological development of the carotid body and contributing genes in these mice can provide further understanding of the appropriate formation of the carotid body. We examined the carotid body of these mice from 1 day to 4 wk old for differences in volume, morphology, and gene expression of Gdnf family, Dlx2, Msx2, and Phox2b. The two strains showed divergent morphology starting at 1 wk old. The volume of the carotid body increased from 1 wk up to 2 wk old to the level of 4 wk old in the DBA/2J mice but not in the A/J mice. This corresponds with immunoreactivity of LC3, an autophagy marker, in A/J tissues at 10 days and 2 wk. The differences in gene expression were examined at 1 wk, 10 days, and 2 wk old, because divergent growth occurred during this period. The DBA/2J's carotid body at 1 wk old showed a greater expression of Msx2 than the A/J's carotid body. No other candidate genes showed consistent differences between the ages and strains. The difference was not seen in sympathetic cervical ganglia of 1 wk old, suggesting that the difference is carotid body specific. The current study indicates the critical postnatal period for developing distinctive morphology of the carotid body in these mice. Further studies are required to further elucidate a role of Msx2 and other uninvestigated genes.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Lack of association of the APOE epsilon 4 allele with the risk of obstructive sleep apnea: meta-analysis and meta-regression

STUDY OBJECTIVES

Reports on the association of polymorphisms in the gene encoding apolipoprotein E (APOE)--a vital macromolecule in cholesterol metabolism--with obstructive sleep apnea (OSA) have provided conflicting results. Our objective was to meta-analytically synthesize the existing evidence for the association of the APOE epsilon4 allele with the risk of OSA.

DESIGN

Random effects meta-analysis and meta-regression.

SETTING

Genetic epidemiological studies reporting the association of APOE epsilon4 allele with OSA susceptibility.

PATIENTS OR PARTICIPANTS

Synthesis of APOE epsilon4 allele data from 6,508 subjects including 1,901 cases of OSA and 4,607 controls.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Eight studies were included in the random effects meta-analysis; the summary effect size measured as odds ratio (OR) for association of the APOE epsilon4 allele with the risk of OSA was found to be 1.13 (95% confidence interval 0.86-1.47). There was a statistically significant heterogeneity (I2 = 72%, P = 0.001) across study results that was not explained by the mean age, proportion of males, or the proportion possessing the APOE epsilon4 allele or when grouped based on the geographic location of the study.

CONCLUSIONS

The hypothesis that the APOE epsilon4 allele may be causally associated with OSA cannot be supported on the basis of published literature.

Behavioral and respiratory characteristics during sleep in neonatal DBA/2J and A/J mice

The ventilatory response to hypoxia depends on the carotid body function and sleep-wake states. Therefore, the response must be measured in a consistent sleep-wake state. In mice, EMG with behavioral indices (coordinated movements, CMs; myoclonic twitches, MTs) has been used to assess sleep-wake states. However, in neonatal mice EMG instrumentation could induce stress, altering their behavior and ventilation. Accordingly, we examined: (1) if EMG can be eliminated for assessing sleep-wake states; and (2) behavioral characteristics and carotid body-mediated respiratory control during sleep with EMG (EMG+) or without EMG (EMG-). Seven-day-old DBA/2J and A/J mice were divided into EMG+ and EMG- groups. In both strains, CMs occurred when EMG was high; MTs were present during silent/low EMG activity. The durations of high EMG activity and of CMs were statistically indifferent. Thus, CMs can be used to indicate wake state without EMG. The stress caused by EMG instrumentation may be distinctively manifested based on genetic background. Prolonged agitation was observed in some EMG+ DBA/2J (5 of 13), but not in A/J mice. The sleep time and MT counts were indifferent between the groups in DBA/2J mice. The EMG+ A/J group showed longer sleep time and less MT counts than the EMG- A/J group. Mean respiratory variables (baseline, hyperoxic/hypoxic responses) were not severely influenced by EMG+ in either strain. Individual values were more variable in EMG+ mice. Carotid body-mediated respiratory responses (decreased ventilation upon hyperoxia and increased ventilation upon mild hypoxia) during sleep were clearly observed in these neonatal mice with or without EMG instrumentation.

Repeated inspiratory occlusions acutely impair myocardial function in rats

Repeated episodes of hypoxia and sympathetic activation during obstructive sleep apnoea are implicated in the initiation and progression of cardiovascular diseases, but the acute effects are unknown. We hypothesized that repeated inspiratory occlusions cause acute myocardial dysfunction and injury. In 22 spontaneously breathing pentobarbital-anaesthetized rats, inspiration was occluded for 30 s every 2 min for 3 h. After approximately 1.5 h, mean arterial pressure started to fall; heart rate between occlusions was stable throughout, consistent with only transient increases in sympathetic activity during each occlusion. Three hours of occlusions resulted in ventricular diastolic dysfunction (reduced peak rate of change of ventricular pressure and slower relaxation). Post-occlusions, the left ventricular contractile response to dobutamine was blunted. After 1 h of recovery, left ventricular pressure generation had returned to values no different from those in sham animals in 5 of 9 of the animals. Cardiac myofibrils from rats subjected to occlusions had depressed calcium-activated myosin ATPase activity, indicating myofilament contractile dysfunction that was not due to breakdown of contractile proteins. Haematoxylin and eosin-stained cross-sections revealed multifocal areas of necrosis within the septum and both ventricles. Repeated inspiratory occlusions, analogous to moderately severe obstructive sleep apnoea, acutely cause global cardiac dysfunction with multifocal myocardial infarcts.

A search for genes that may confer divergent morphology and function in the carotid body between two strains of mice

The carotid body (CB) is the primary hypoxic chemosensory organ. Its hypoxic response appears to be genetically controlled. We have hypothesized that: 1) genes related to CB function are expressed less in the A/J mice (low responder to hypoxia) compared with DBA/2J mice (high responder to hypoxia); and 2) gene expression levels of morphogenic and trophic factors of the CB are significantly lower in the A/J mice than DBA/2J mice. This study utilizes microarray analysis to test these hypotheses. Three sets of CBs were harvested from both strains. RNA was isolated and used for global gene expression profiling (Affymetrix Mouse 430 v2.0 array). Statistically significant gene expression was determined as a minimum six counts of nine pairwise comparisons, a minimum 1.5-fold change, and P ≤ 0.05. Our results demonstrated that 793 genes were expressed less and that 568 genes were expressed more in the A/J strain vs. the DBA/2J strain. Analysis of individual genes indicates that genes encoding ion channels are differentially expressed between the two strains. Genes related to neurotransmitter metabolism, synaptic vesicles, and the development of neural crest-derived cells are expressed less in the A/J CB vs. the DBA/2J CB. Through pathway analysis, we have constructed a model that shows gene interactions and offers a roadmap to investigate CB development and hypoxic chemosensing/chemotransduction processes. Particularly, Gdnf, Bmp2, Kcnmb2, Tph1, Hif1a, and Arnt2 may contribute to the functional differences in the CB between the two strains. Bmp2, Phox2b, Dlx2, and Msx2 may be important for the morphological differences.

Role of acetylcholine in neurotransmission of the carotid body

Acetylcholine (ACh) has been considered an important excitatory neurotransmitter in the carotid body (CB). Its physiological and pharmacological effects, metabolism, release, and receptors have been well documented in several species. Various nicotinic and muscarinic ACh receptors are present in both afferent nerve endings and glomus cells. Therefore, ACh can depolarize or hyperpolarize the cell membrane depending on the available receptor type in the vicinity. Binding of ACh to its receptor can create a wide variety of cellular responses including opening cation channels (nicotinic ACh receptor activation), releasing Ca(2+) from intracellular storage sites (via muscarinic ACh receptors), and modulating activities of K(+) and Ca(2+) channels. Interactions between ACh and other neurotransmitters (dopamine, adenosine, nitric oxide) have been known, and they may induce complicated responses. Cholinergic biology in the CB differs among species and even within the same species due to different genetic composition. Development and environment influence cholinergic biology. We discuss these issues in light of current knowledge of neuroscience.

The effects of dietary iron deprivation on murine circadian sleep architecture

OBJECTIVES

Iron deficiency is considered a putative cause for restless legs syndrome (RLS), a human sensorimotor disorder characterized by a circadian presentation of symptoms during the evening hours that disrupts one's ability to sleep. We sought to evaluate the sleep-wake effects of diet-induced iron deficiency in mice as an animal model of RLS. To this end, we hypothesized that the iron-deprived mice would exhibit a sleep-wake circadian pattern characteristic of the human syndrome: increased wakefulness during the hours immediately preceding the sleep-predominant period.

METHODS

Following weaning at post-natal day (PND) 21, C57BL/6J mice were assigned to one of two dietary treatments: iron-deficient (ID, n=7) or iron-adequate (i.e., control, CTL, n=6). At PND 44, the mice were surgically instrumented for polysomnographic (PSG) recording, and data were collected at young adulthood: PNDs 59 and 60. Sleep-wake architecture was characterized for the 12-h light and dark periods and also for six consecutive 4-h blocks comprising a 24-h day.

RESULTS

The ID mice showed marked increases in wake time in the 4-h period prior to lights-on; both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep were reduced. In contrast, sleep-wake activity did not differ across the 12-h light period.

CONCLUSIONS

Dietary iron deficiency in mice elicited increases in wakefulness during a particular circadian time point that corresponds to the period during which RLS symptoms would maximally disturb sleep onset and progression in humans. These data indicate that iron-deficient mice may provide a potentially useful animal model for RLS.

Hyperlipidemia and lipid peroxidation are dependent on the severity of chronic intermittent hypoxia

Obstructive sleep apnea (OSA) is characterized by chronic intermittent hypoxia (CIH) and associated with dysregulation of lipid metabolisms and atherosclerosis. Causal relationships between OSA and metabolic abnormalities have not been established because of confounding effects of underlying obesity. The goal of the study was to determine if CIH causes lipid peroxidation and dyslipidemia in the absence of obesity and whether the degrees of dyslipidemia and lipid peroxidation depend on the severity of hypoxia. Lean C57BL/6J mice were exposed to CIH for 4 wk with a fractional inspired O2 (FI(O2)) nadir of either 10% (moderate CIH) or 5% (severe CIH). Mice exposed to severe CIH exhibited significant increases in fasting serum levels of total cholesterol (129 +/- 2.9 vs. 113 +/- 2.8 mg/dl in control mice, P < 0.05) and low-density lipoprotein cholesterol (85.7 +/- 8.9 vs. 56.4 +/- 9.7 mg/dl, P < 0.05) in conjunction with a 1.5- to 2-fold increase in lipoprotein secretion, and upregulation of hepatic stearoyl coenzyme A desaturase 1 (SCD-1). Severe CIH also markedly increased lipid peroxidation in the liver (malondialdehyde levels of 94.4 +/- 5.4 vs. 57.4 +/- 5.2 nmol/mg in control mice, P < 0.001). In contrast, moderate CIH did not induce hyperlipidemia or change in hepatic SCD-1 levels but did cause lipid peroxidation in the liver at a reduced level relative to severe CIH. In conclusion, CIH leads to hypercholesterolemia and lipid peroxidation in the absence of obesity, and the degree of metabolic dysregulation is dependent on the severity of the hypoxic stimulus.

Chronic intermittent hypoxia upregulates genes of lipid biosynthesis in obese mice

Obstructive sleep apnea (OSA), a condition tightly linked to obesity, leads to chronic intermittent hypoxia (CIH) during sleep. There is emerging evidence that OSA is independently associated with insulin resistance and fatty liver disease, suggesting that OSA may affect hepatic lipid metabolism. To test this hypothesis, leptin-deficient obese (ob/ob) mice were exposed to CIH during the light phase (9 AM-9 PM) for 12 wk. Liver lipid content and gene expression profile in the liver (Affymetrix 430 GeneChip with real-time PCR validation) were determined on completion of the exposure. CIH caused a 30% increase in triglyceride and phospholipid liver content (P < 0.05), whereas liver cholesterol content was unchanged. Gene expression analysis showed that CIH upregulated multiple genes controlling 1) cholesterol and fatty acid biosynthesis [malic enzyme and acetyl coenzyme A (CoA) synthetase], 2) predominantly fatty acid biosynthesis (acetyl-CoA carboxylase and stearoyl-CoA desaturases 1 and 2), and 3) triglyceride and phospholipid biosynthesis (mitochondrial glycerol-3-phosphate acyltransferase). A majority of overexpressed genes were transcriptionally regulated by sterol regulatory element-binding protein (SREBP) 1, a master regulator of lipogenesis. A 2.8-fold increase in SREBP-1 gene expression in CIH was confirmed by real-time PCR (P = 0.001). Expression of major genes of cholesterol biosynthesis, SREBP-2 and 3-hydroxy-3-methylglutaryl-CoA reductase, was unchanged. In conclusion, we have shown that CIH may exacerbate preexisting fatty liver of obesity via upregulation of the pathways of lipid biosynthesis in the liver.

Acute and chronic cardiovascular effects of intermittent hypoxia in C57BL/6J mice

We investigated the effects of 1) acute hypoxia and 2) 5 wk of chronic intermittent hypoxia (IH) on the systemic and pulmonary circulations of C57BL/6J mice. Mice were chronically instrumented with either femoral artery or right ventricular catheters. In response to acute hypoxia (4 min of 10% O2; n = 6), systemic arterial blood pressure fell (P < 0.005) from 107.7 +/- 2.5 to 84.7 +/- 6.5 mmHg, whereas right ventricular pressure increased (P < 0.005) from 11.7 +/- 0.8 to 14.9 +/- 1.3 mmHg. Another cohort of mice was then exposed to IH for 5 wk (O2 nadir = 5%, 60-s cycles, 12 h/day) and then implanted with catheters. In response to 5 wk of chronic IH, mice (n = 8) increased systemic blood pressure by 7.5 mmHg, left ventricle + septum weight by 32.2 +/- 7.5 x 10(-2) g/100 g body wt (P < 0.015), and right ventricle weight by 19.3 +/- 3.2 x 10(-2) g/100 g body wt (P < 0.001), resulting in a 14% increase in the right ventricle/left ventricle + septum weight (P < 0.005). We conclude that in C57BL/6J mice 1) acute hypoxia causes opposite effects on the pulmonary and systemic circulations, leading to preferential loading of the right heart; and 2) chronic IH in mice results in mild to moderate systemic and pulmonary hypertension, with resultant left- and right-sided ventricular hypertrophy.