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Obstructive sleep apnea (OSA), characterized by episodes of intermittent hypoxia (IH), is highly prevalent in patients with abdominal aortic aneurysm (AAA). However, whether IH serves as an independent risk factor for AAA development remains to be investigated. Here, we determined the effects of chronic (6 mo) IH on angiotensin (Ang II)-induced AAA development in C57BL/6J male mice and investigated the underlying mechanisms of IH in cultured vascular smooth muscle cells (SMCs). IH increased the susceptibility of mice to develop AAA in response to Ang II infusion by facilitating the augmentation of the abdominal aorta's diameter as assessed by transabdominal ultrasound imaging. Importantly, IH with Ang II augmented aortic elastin degradation and the expression of matrix metalloproteinases (MMPs), mainly MMP8, MMP12, and a disintegrin and metalloproteinase-17 (ADAM17) as measured by histology and immunohistochemistry. Mechanistically, IH increased the activities of MMP2, MMP8, MMP9, MMP12, and ADAM17, while reducing the expression of the MMP regulator reversion-inducing cysteine-rich protein with Kazal motifs (RECK) in cultured SMCs. Aortic samples from human AAA were associated with decreased RECK and increased expression of ADAM17 and MMPs. These data suggest that IH facilitates AAA development when additional stressors are superimposed and that this occurs in association with an increased presence of aortic MMPs and ADAM17, potentially due to IH-induced modulation of RECK expression. These findings support a plausible synergistic link between OSA and AAA and provide a better understanding of the molecular mechanisms underlying the pathogenesis of AAA. NEW & NOTEWORTHY: IH facilitates Ang II-induced abdominal aortic diameter expansion and AAA development in C57BL/6J male mice. IH upregulates the expression of specific MMPs such as MMP8, MMP12, and ADAM17. IH directly suppresses RECK expression and increases MMPs activity in SMCs. Human AAA tissues exhibit a downregulation of RECK and an upregulation of ADAM17 and MMPs. [ABSTRACT FROM AUTHOR]

Aging is associated with vascular endothelial dysfunction observed through a progressive loss of flow-mediated dilation caused partly by a decreased nitric oxide bioavailability. Intermittent hypoxia, consisting of alternating short bouts of breathing hypoxic and normoxic air, was reported to either maintain or improve vascular function in young adults. The aim of this study was to determine the impact of age on the vascular response to intermittent hypoxia. Twelve young adults and 11 older adults visited the laboratory on two occasions. Plasma nitrate concentrations and brachial artery flow-mediated dilation were assessed before and after exposure to either intermittent hypoxia or a sham protocol. Intermittent hypoxia consisted of eight 4-min hypoxic cycles at a targeted oxygen saturation of 80% interspersed with breathing room air to resaturation, and the sham protocol consisted of eight 4-min normoxic cycles interspersed with breathing room air. Vascular responses were assessed during intermittent hypoxia and the sham protocol. Intermittent hypoxia elicited a brachial artery vasodilation but did not change brachial artery shear rate in both young and older adults. Plasma nitrate concentrations were not significantly affected by intermittent hypoxia compared with the sham protocol in both groups. Brachial artery flow-mediated dilation was not acutely affected by intermittent hypoxia or the sham protocol in either young or older adults. In conclusion, the brachial artery vasodilatory response to intermittent hypoxia was not influenced by age. Intermittent hypoxia increased brachial artery diameter but did not acutely affect endothelium-dependent vasodilation in young or older adults. NEW & NOTEWORTHY: The objective of this study was to determine the impact of age on the vascular response to intermittent hypoxia. Eight 4-min bouts of hypoxia at a targeted oxygen saturation of 80% induced a brachial artery vasodilation in both young and older adults, indicating that age does not influence the vasodilatory response to intermittent hypoxia. Intermittent hypoxia did not acutely affect brachial artery flow-mediated dilation in young or older adults. [ABSTRACT FROM AUTHOR]

Acute intermittent hypoxia (AIH) can induce sustained facilitation of motor output in people with spinal cord injury (SCI). Most studies of corticospinal tract excitability in humans have used 9% fraction inspired oxygen (F I O 2 ) AIH (AIH-9%), with inconsistent outcomes. We investigated the effect of single sessions of 9% F I O 2 and 12% F I O 2 AIH (AIH-12%) on corticospinal excitability of a hand and leg muscle in able-bodied adults. Ten naïve participants completed three sessions on separate days comprising 15 epochs of 1 min of AIH-9%, AIH-12%, or sham (SHAM-21%) followed by 1 min of room air (21% F I O 2 ) in a randomized crossover design. Motor-evoked potentials (MEPs; n = 30, ∼1 mV) elicited at rest by transcranial magnetic stimulation and maximal M-waves (Mmax) evoked by peripheral nerve stimulation were measured from the first dorsal interosseous (FDI) and tibialis anterior (TA) muscles at baseline and at ∼0, 20, 40, and 60 min post intervention. AIH-9% induced the greatest reduction in peripheral oxygen saturation (to 85% vs. 93% and 100% in AIH-12% and SHAM-21%, respectively; P < 0.001) and the greatest increase in ventilation [by 22% vs. 12% and −3% in AIH-9%, AIH-12%, and SHAM-21%, respectively (P < 0.001)]. There was no difference in MEP amplitudes (%Mmax) after any of the three conditions (AIH-9%, AIH-12%, SHAM-21%) for both the FDI (P = 0.399) and TA (P = 0.582). Despite greater cardiorespiratory changes during AIH-9%, there was no evidence of corticospinal facilitation (tested with MEPs) in this study. Further studies could explore variability in response to AIH between individuals and other methods to measure motor facilitation in people with and without spinal cord injuries. NEW & NOTEWORTHY: This is the first study that tests whether acute intermittent hypoxia (AIH) induces motor output facilitation in humans after two different doses of AIH (9% and 12% F I O 2 ) and the reproducibility of participant responses after a repeat AIH intervention at 9% AIH. There was no motor output facilitation in response to either dose of AIH. The results question the effectiveness of a single 30-min session of AIH in inducing motor output facilitation, tested in this way. [ABSTRACT FROM AUTHOR]

Eight 4-min cycles of intermittent hypoxia represent the shortest hypoxic exposure to increase erythropoietin (EPO) levels in young adults. The impact of aging on the EPO response to a hypoxic stimulus remains equivocal. Thus, the objective of this study was to determine the effect of the same intermittent hypoxia protocol on EPO levels in older adults. Twenty-two participants (12 women, age: 53 ± 7 yr) were randomly assigned to an intermittent hypoxia group (IH, n = 11) or an intermittent normoxia group (IN, n = 11). Intermittent hypoxia consisted of eight 4-min cycles at a targeted oxygen saturation of 80% interspersed with normoxic cycles to resaturation. Air was made hypoxic by titrating nitrogen into a breathing circuit. Intermittent normoxia consisted of the same protocol, but nitrogen was not added to the breathing circuit. EPO levels were measured before and 4.5 h after the beginning of each protocol. Intermittent hypoxia lowered oxygen saturation to 82 ± 3%, which corresponded to a fraction of inspired oxygen of 10.9 ± 1.0%. There was a greater increase in EPO levels following intermittent hypoxia than intermittent normoxia (IH: 3.2 ± 2.2 vs. IN: 0.7 ± 0.8 mU/mL, P < 0.01). A single session of eight 4-min cycles of hypoxia increased EPO levels, the glycoprotein stimulating red blood cell production, in older adults. Exposure to intermittent hypoxia has therefore the potential to increase oxygen-carrying capacity in a population with reduced red blood cell volume. [ABSTRACT FROM AUTHOR]

Adults born preterm have an increased risk of pulmonary vascular disease. Extreme preterm infants often require supplemental oxygen but they also exhibit frequent intermittent hypoxemic episodes (IH). Here, we test the hypothesis that neonatal IH induces lung endothelial cell mitochondrial DNA (mitDNA) damage and contributes to long-term pulmonary vascular disease and pulmonary hypertension (PH). Newborn C57BL/6J mice were assigned to the following groups: 1) normoxia, 2) hyperoxia (O2 65%), 3) normoxia cycling with IH (O2 21% + O2 10%), and 4) hyperoxia cycling with IH (O2 65% + O2 10%) for 3 wk. IH episodes were initiated on postnatal day 7. Lung angiogenesis, PH, and mitDNA lesions were assessed at 3 wk and 3 mo. In vitro, the effect of IH on tubule formation and mitDNA lesions was evaluated in human pulmonary microvascular endothelial cells (HPMECs). Data were analyzed by ANOVA. In vitro, IH exposure reduced tubule formation and increased mitDNA lesions in HPMECs. This was most marked in HPMECs exposed to hyperoxia cycling with IH. In vivo, neonatal IH increased lung mitDNA lesions, impaired angiogenesis, and induced PH in 3-wk-old mice. These findings were pronounced in mice exposed to hyperoxia cycling with IH. At 3 mo follow-up, mice exposed to neonatal IH had persistently increased lung mitDNA lesions and impaired lung angiogenesis, even without concomitant hyperoxia exposure. Neonatal IH induces lung endothelial cell mitDNA damage and causes persistent impairment in lung angiogenesis. These findings provide important mechanistic insight into the pathogenesis of pulmonary vascular disease in preterm survivors. NEW & NOTEWORTHY Our current study demonstrates that neonatal intermittent hypoxia (IH) alters lung endothelial cell function, induces mitochondrial DNA lesions, and impairs lung vascular growth into adulthood. Moreover, when superimposed on hyperoxia, neonatal IH induces a severe lung vascular phenotype that is seen in preterm infants with PH. These findings suggest that neonatal IH contributes to PH in adults born preterm and importantly, that mitochondrial protection strategies may mitigate these deleterious effects. [ABSTRACT FROM AUTHOR]

Intermittent hypoxia, or intermittent low oxygen interspersed with normal oxygen levels, has differential effects that depend on the "dose" of hypoxic episodes (duration, severity, number per day, and number of days). Whereas "low dose" daily acute intermittent hypoxia (dAIH) elicits neuroprotection and neuroplasticity, "high dose" chronic intermittent hypoxia (CIH) similar to that experienced during sleep apnea elicits neuropathology. Sleep apnea is comorbid in >50% of patients with Alzheimer's disease--a progressive, neurodegenerative disease associated with brain amyloid and chronic Tau dysregulation (pathology). Although patients with sleep apnea present with higher Tau levels, it is unknown if sleep apnea through attendant CIH contributes to onset of Tau pathology. We hypothesized CIH characteristic of moderate sleep apnea would increase dysregulation of phosphorylated Tau (phospho-Tau) species in Sprague-Dawley rat hippocampus and prefrontal cortex. Conversely, we hypothesized that dAIH, a promising neurotherapeutic, has minimal impact on Tau phosphorylation. We report a dose-dependent intermittent hypoxia effect, with region-specific increases in 1) phospho-Tau species associated with human Tauopathies in the soluble form and 2) accumulated phospho-Tau in the insoluble fraction. The latter observation was particularly evident with higher CIH intensities. This important and novel finding is consistent with the idea that sleep apnea and attendant CIH have the potential to accelerate the progression of Alzheimer's disease and/or other Tauopathies. NEW & NOTEWORTHY Sleep apnea is highly prevalent in people with Alzheimer's disease, suggesting the potential to accelerate disease onset and/or progression. These studies demonstrate that intermittent hypoxia (IH) induces dose-dependent, region- specific Tau phosphorylation, and are the first to indicate that higher IH "doses" elicit both endogenous, (rat) Tau hyperphosphorylation and accumulation in the hippocampus. These findings are essential for development and implementation of new treatment strategies that minimize sleep apnea and its adverse impact on neurodegenerative diseases. [ABSTRACT FROM AUTHOR]

Ischemic preconditioning attenuates the reduction in brachial artery endothelial function following an ischemia-reperfusion injury. Brief bouts of systemic hypoxemia could similarly mitigate the blunted vasodilatory response induced by an ischemia-reperfusion injury. The aim of the present study was to determine whether an acute bout of intermittent hypoxia protects against an ischemia-reperfusion injury in young healthy individuals. Brachial artery endothelial function was assessed by flow-mediated dilation in 16 young healthy individuals before and after a 20-min upper arm blood flow occlusion to induce ischemia-reperfusion injury. Blood flow occlusion was preceded by either intermittent hypoxia or intermittent normoxia. Intermittent hypoxia consisted of three 4-min hypoxic cycles at an arterial oxygen saturation of 87 ± 3% separated by 4-min normoxic cycles. Intermittent hypoxia resulted in a lower arterial oxygen saturation than intermittent normoxia (hypoxia: 87 ± 3% vs. normoxia: 99 ± 1%, P < 0.01), which was equivalent to a lower fraction of inspired oxygen (hypoxia: 0.123 ± 0.013 and normoxia: 0.210 ± 0.003, P < 0.01). When preceded by intermittent normoxia, blood flow occlusion resulted in a blunted flow-mediated dilation. In contrast, the reduction in flow-mediated dilation following blood flow occlusion was attenuated by prior exposure to intermittent hypoxia (hypoxia: 6.4 ± 1.9 to 4.4 ± 2.3% and normoxia: 7.1 ± 2.5 to 4.0 ± 2.4%, time x condition interaction P = 0.048). Exposure to intermittent hypoxia did not affect mean arterial pressure (hypoxia: 92 ± 9mmHg and normoxia: 89 ± 8 mmHg, P = 0.19) or cardiac output (hypoxia: 5.8 ± 1.1 L·min-1 and normoxia: 5.3 ± 1.1 L·min-1, P = 0.29). In conclusion, hypoxic preconditioning attenuates the reduction in flow-mediated dilation induced by blood flow occlusion in young healthy individuals. Intermittent hypoxia represents a potential strategy to mitigate the effect of ischemia-reperfusion injury associated with ischemic events. [ABSTRACT FROM AUTHOR]

Obstructive sleep apnea (OSA), characterized by events of hypoxia-reoxygenation, is highly prevalent in pregnancy, negatively affecting the gestation process and particularly the fetus. Whether the consequences of OSA for the fetus and offspring are mainly caused by systemic alterations in the mother or by a direct effect of intermittent hypoxia in the fetus is unknown. In fact, how apnea-induced hypoxemic swings in OSA are transmitted across the placenta remains to be investigated. The aim of this study was to test the hypothesis, based on a theoretical background on the damping effect of oxygen transfer in the placenta, that oxygen partial pressure (PO2) swings resulting from obstructive apneas mimicking OSA are mitigated in the fetal circulation. To this end, four anesthetized ewes close to term pregnancy were subjected to obstructive apneas consisting of 25-s airway obstructions. Real-time PO2 was measured in the maternal carotid artery and in the umbilical vein with fast-response fiber-optic oxygen sensors. The amplitudes of PO2 swings in the umbilical vein were considerably smaller [3.1 ± 1.0 vs. 21.0 ± 6.1 mmHg (mean ± SE); P < 0.05]. Corresponding estimated swings in fetal and maternal oxyhemoglobin saturation tracked PO2 swings. This study provides novel insights into fetal oxygenation in a model of gestational OSA and highlights the importance of further understanding the impact of sleep-disordered breathing on fetal and offspring development. NEW & NOTEWORTHY This study in an airway obstruction sheep model of gestational sleep apnea provides novel data on how swings in oxygen partial pressure (PO2) translate from maternal to fetal blood. Real-time simultaneous measurement of PO2 in maternal artery and in umbilical vein shows that placenta transfer attenuates the magnitude of oxygenation swings. These data prompt further investigation of the extent to which maternal apneas could induce similar direct oxidative stress in fetal and maternal tissues. [ABSTRACT FROM AUTHOR]

Serotonin (5-HT) is a key regulator of spinal respiratory motor plasticity. For example, spinal 5-HT receptor activation is necessary for the induction of phrenic long-term facilitation (pLTF), a form of respiratory motor plasticity triggered by moderate acute intermittent hypoxia (mAIH). mAIH-induced pLTF is blocked by cervical spinal application of the broad-spectrum 5-HT-receptor antagonist, methysergide. However, methysergide does not allow distinctions between the relative contributions of different 5-HT receptor subtypes. Intravenous administration of the Gq protein-coupled 5-HT2A/2C receptor antagonist ketanserin blocks mAIH-induced pLTF when administered before, but not after, mAIH; thus, 5-HT2 receptor activation is necessary for the induction but not maintenance of mAIH-induced pLTF. However, systemic ketanserin administration does not identify the site of the relevant 5-HT2A/2C receptors. Furthermore, this approach does not differentiate between the roles of 5-HT2A versus 5-HT2C receptors, nor does it preclude involvement of other Gq protein-coupled metabotropic 5-HT receptors capable of eliciting long-lasting phrenic motor facilitation, such as 5-HT2B receptors. Here we tested the hypothesis that mAIH-induced pLTF requires cervical spinal 5-HT2 receptor activation and determined which 5-HT2 receptor subtypes are involved. Anesthetized, paralyzed, and ventilated adult male Sprague Dawley rats were pretreated intrathecally with cervical (~C3-C5) spinal injections of subtype selective 5-HT2A/2C, 5-HT2B, or 5-HT2C receptor antagonists before mAIH. Whereas cervical spinal 5-HT2C receptor inhibition had no impact on mAIH-induced pLTF, pLTF was no longer observed after pretreatment with either 5-HT2A/2C or 5-HT2B receptor antagonists. Furthermore, spinal pretreatment with an MEK/ERK MAPK inhibitor blocked phrenic motor facilitation elicited by intrathecal injections of 5-HT2A but not 5-HT2B receptor agonists. Thus, mAIH-induced pLTF requires concurrent cervical spinal activation of both 5-HT2A and 5-HT2B receptors. However, these distinct receptor subtypes contribute to phrenic motor facilitation via distinct downstream signaling cascades that differ in their requirement for ERK MAPK signaling. The demonstration that both 5-HT2A and 5-HT2B receptors make unique contributions to mAIH-induced pLTF advances our understanding of mechanisms that underlie 5-HT-induced phrenic motor plasticity. [ABSTRACT FROM AUTHOR]

Obstructive sleep apnea is characterized by intermittent hypoxia (IH) during sleep and predisposes to endothelial dysfunction. Obesity is a major risk factor for the occurrence of sleep apnea. The present study compared the functional impact of low- (IH10; 10 hypoxic events/h) and high-frequency (IH60; 60 hypoxic events/h) IH for 4 wk on endothelial function in male C57BL/6 mice with or without high-fat (HF) diet-induced obesity. Mean arterial blood pressure (tail cuff method) was increased in obese mice after IH60 exposure, i.e., HF + IH60 group. The serum levels of the oxidative stress marker malondialdehyde were augmented in lean IH60 and HF groups, with a further increase in HF + IH60 but a reduction in HF + IH10 mice compared with the HF group. Vascular responsiveness was assessed as changes in isometric tension in isolated arteries. Relaxations to the endothelium-dependent vasodilator acetylcholine were impaired in HF + IH60 aortae. Endothelium-dependent contractions (EDC; response to acetylcholine in the presence of the nitric oxide synthase inhibitor L-NAME) in carotid arteries were augmented in the HF group, but this HF-induced augmentation was suppressed by low-frequency IH exposure. The addition of apocynin (antioxidant) reduced EDC in HF and HF + IH60 groups but not in HF + IH10 group. In conclusion, these findings suggest that exposure of obese mice to mild IH exerts preconditioning-like suppression of endothelium-dependent and oxidative stress-mediated contractions. When IH severity increases, this suppression diminishes and endothelial dysfunction accelerates. NEW & NOTEWORTHY The present study demonstrates, for the first time, that low-frequency intermittent hypoxia may exert a preconditioning-like suppression of oxidative stress-induced endothelium-dependent contractions in mice with diet-induced obesity. This relative suppression was diminished as intermittent hypoxia became more severe, and a deleterious effect on endothelial function emerged. [ABSTRACT FROM AUTHOR]

Obstructive sleep apnea (OSA) is common in pregnancy and may compromise fetal and even postnatal development. We developed an animal model to determine if maternal OSA could have lasting effects in offspring. Pregnant Sprague-Dawley rats were exposed to reduced ambient O2 from 21 to 4-5%, approximately once per minute [chronic intermittent hypoxia (CIH)] for 8 h/day during gestation days 3-19. Similarly handled animals exposed to ambient air served as controls (HC). Offspring were studied for body growth and cardiovascular function for 8 postnatal weeks. Compared with HC, prenatal CIH led to growth restriction, indicated by smaller body weight and tibial length, and higher arterial blood pressure in both male and female offspring. Compared with same-sex HC, CIH males showed abdominal obesity (greater ratio of abdominal fat weight to body weight or tibial length), left ventricular (LV) hypertrophy (greater heart weight-to-tibial length ratio and LV posterior wall diastolic thickness), elevated LV contractility (increases in LV ejection fraction, end-systolic pressure-volume relations and preload recruitable stroke work), elevated LV and arterial stiffness (increased end-diastolic pressure-volume relationship and arterial elasticity), and LV oxidative stress (greater lipid peroxide content). Compared with female CIH offspring, male CIH offspring had more profound changes in blood pressure (BP), cardiac function, myocardial lipid peroxidase (LPO) content, and abdominal adiposity. Rodent prenatal CIH exposure, mimicking human maternal OSA, exerts detrimental morphological and cardiovascular effects on developing offspring; the model may provide useful insights of OSA effects in humans. NEW & NOTEWORTHY Obstructive sleep apnea is common in human pregnancy. Following maternal exposure to chronic intermittent hypoxia, a hallmark of sleep apnea, both sexes of rat offspring showed growth retardation, with males being more vulnerable to hypertension and dysfunctional left ventricular changes. This model is useful to study detrimental effects of maternal obstructive sleep apnea on developing offspring in humans. [ABSTRACT FROM AUTHOR]

Cerebral vasodilation and increased cerebral oxygen extraction help maintain cerebral oxygen uptake in the face of hypoxemia. This study examined cerebrovascular responses to intermittent hypoxemia in eight healthy men breathing 10% O2 for 5 cycles, each 6 min, interspersed with 4 min of room air breathing. Hypoxia exposures raised heart rate (P < 0.01) without altering arterial pressure, and increased ventilation (P < 0.01) by expanding tidal volume. Arterial oxygen saturation (SaO2 ) and cerebral tissue oxygenation (ScO2 ) fell (P < 0.01) less appreciably in the first bout (from 97.0 ± 0.3% and 72.8 ± 1.6% to 75.5 ± 0.9% and 54.5 ± 0.9%, respectively) than the fifth bout (from 94.9 ± 0.4% and 70.8 ± 1.0% to 66.7 ± 2.3% and 49.2 ± 1.5%, respectively). Flow velocity in the middle cerebral artery (VMCA) and cerebrovascular conductance increased in a sigmoid fashion with decreases in SaO2 and ScO2 . These stimulusresponse curves shifted leftward and upward from the first to the fifth hypoxia bouts; thus, the centering points fell from 79.2 ± 1.4 to 74.6 ± 1.1% (P < 0.01) and from 59.8 ± 1.0 to 56.6 ± 0.3% (P < 0.002), and the minimum VMCA increased from 54.0 ± 0.5 to 57.2 ± 0.5 cm/s (P < 0.0001) and from 53.9 ± 0.5 to 57.1 ± 0.3 cm/s (P < 0.0001) for the SaO2 -VMCA and ScO2 -VMCA curves, respectively. Cerebral oxygen extraction increased from prehypoxia 0.22 ± 0.01 to 0.25 ± 0.02 in minute 6 of the first hypoxia bout, and remained elevated between 0.25 ± 0.01 and 0.27 ± 0.01 throughout the fifth hypoxia bout. These results demonstrate that cerebral vasodilation combined with enhanced cerebral oxygen extraction fully compensated for decreased oxygen content during acute, cyclic hypoxemia. NEW & NOTEWORTHY Five bouts of 6-min intermittent hypoxia (IH) exposures to 10% O2 progressively reduce arterial oxygen saturation (SaO2 ) to 67% without causing discomfort or distress. Cerebrovascular responses to hypoxemia are dynamically reset over the course of a single IH session, such that threshold and saturation for cerebral vasodilations occurred at lower SaO2 and cerebral tissue oxygenation (ScO2 ) during the fifth vs. first hypoxia bouts. Cerebral oxygen extraction is augmented during acute hypoxemia, which compensates for decreased arterial O2 content. [ABSTRACT FROM AUTHOR]

The brain is a vital organ that relies on a constant and adequate blood flow to match oxygen and glucose delivery with the local metabolic demands of active neurons. Thus exquisite regulation of cerebral blood flow (CBF) is particularly important under hypoxic conditions to prevent a detrimental decrease in the partial pressure of oxygen within the brain tissues. Cerebrovascular sensitivity to hypoxia, assessed as the change in CBF during a hypoxic challenge, represents the capacity of cerebral vessels to respond to, and compensate for, a reduced oxygen supply, and has been shown to be impaired or blunted in a number of conditions. For instance, this is observed with aging, and in clinical conditions such as untreated obstructive sleep apnea (OSA) and in healthy humans exposed to intermittent hypoxia. This review will 1) provide a brief overview of cerebral blood flow regulation and results of pharmacological intervention studies which we have performed to better elucidate the basic mechanisms of cerebrovascular regulation in humans; and 2) present data from studies in clinical and healthy populations, using a translational physiology approach, to investigate human CBF control during hypoxia. Results from studies in patients with chronic obstructive pulmonary disease and OSA will be presented to identify the effects of the disease processes on cerebrovascular sensitivity to hypoxia. Data emerging from experimental human models of intermittent hypoxia during wakefulness will also be reviewed to highlight the effects of intermittent hypoxia on the brain. [ABSTRACT FROM AUTHOR]

Our purpose was to determine whether exposure to mild intermittent hypoxia leads to a reduction in the therapeutic continuous positive airway pressure required to eliminate breathing events. Ten male participants were treated with twelve 2-min episodes of hypoxia (PETCO2 ≈50 mmHg) separated by 2-min intervals of normoxia in the presence of PETCO2 that was sustained 3 mmHg above baseline. During recovery from the last episode, the positive airway pressure was reduced in a stepwise fashion until flow limitation was evident. The participants also completed a sham protocol under normocapnic conditions, which mimicked the time frame of the intermittent hypoxia protocol. After exposure to intermittent hypoxia, the therapeutic pressure was significantly reduced (i.e., 5 cmH2O) without evidence of flow limitation (103.4 ± 6.3% of baseline, P = 0.5) or increases in upper airway resistance (95.6 ± 15.0% of baseline, P = 0.6). In contrast, a similar decrease in pressure was accompanied by flow limitation (77.0 ± 1.8% of baseline, P = 0.001) and an increase in upper airway resistance (167.2 ± 17.5% of baseline, P = 0.01) after the sham protocol. Consistent with the initiation of long-term facilitation of upper airway muscle activity, exposure to intermittent hypoxia reduced the therapeutic pressure required to eliminate apneic events that could improve treatment compliance. This possibility, coupled with the potentially beneficial effects of intermittent hypoxia on comorbidities linked to sleep apnea, suggests that mild intermittent hypoxia may have a multipronged therapeutic effect on sleep apnea. NEW & NOTEWORTHY Our new finding is that exposure to mild intermittent hypoxia reduced the therapeutic pressure required to treat sleep apnea. These findings are consistent with previous results, which have shown that long-term facilitation of upper muscle activity can be initiated following exposure to intermittent hypoxia in humans. [ABSTRACT FROM AUTHOR]

Abdominal expiratory activity is absent at rest and is evoked during metabolic challenges, such as hypercapnia and hypoxia, or after the exposure to intermittent hypoxia (IH). The mechanisms engaged during this process are not completely understood. In this study, we hypothesized that serotonin (5-HT), acting in the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG), is able to generate active expiration. In anesthetized (urethane, ip), tracheostomized, spontaneously-breathing adult male Holtzman rats we microinjected a serotoninergic agonist and antagonist bilaterally in the RTN/pFRG and recorded diaphragm and abdominal muscle activities. We found that episodic (3 times, 5 min apart), but not single microinjections of 5-HT (1 mM) in the RTN/pFRG elicited an enduring (>30 min) increase in abdominal activity. This response was amplified in vagotomized rats and blocked by previous 5-HT receptor antagonism with ketanserin (10 µM). Episodic 5-HT microinjections in the RTN/pFRG also potentiated the inspiratory and expiratory reflex responses to hypercapnia. The antagonism of 5-HT receptors in the RTN/pFRG also prevented the long-term facilitation (>30 min) of abdominal activity in response to acute IH exposure (10 x 6-7% O for 45 s every 5 min). Our findings indicate the activation of serotoninergic mechanisms in the RTN/pFRG is sufficient to increase abdominal expiratory activity at resting conditions and required for the emergence of active expiration after IH in anesthetized animals. [ABSTRACT FROM AUTHOR]

The carotid body (CB) substantially influences breathing in premature infants by affecting the frequency of apnea and periodic breathing. In adult animals, inflammation alters the structure and chemosensitivity of the CB, yet it is not known if this pertains to neonates. We hypothesized that early postnatal inflammation leads to morphological and functional changes in the developing rat CB, which persists for 1 wk after the initial provoking insult. To test our hypothesis, we exposed rat pups at postnatal day 2 (P2) to lipopolysaccharide (LPS; 100 g/kg) or saline (SAL) intraperitoneally. At P9-10 (1 wk after treatment), LPS-exposed animals had significantly more spontaneous intermittent hypoxic (IH) events, attenuated ventilatory responses to changes in oxygen tension (measured by whole body plethysmography), and attenuated hypoxic chemosensitivity of the carotid sinus nerve (measured in vitro), compared with SAL-exposed controls. These functional changes were associated with the following: 1) increased inflammatory cytokine mRNA levels; 2) decreased volume of supportive type II cells; and 3) elevated dopamine levels (a major inhibitory neuromodulator) within the CB. These findings suggest that early postnatal inflammation in newborn rats adversely affects the structure and function of the CB and is associated with increased frequency of intermittent desaturations, similar to the phenomenon observed in premature infants. Furthermore, this is the first newborn model of spontaneous intermittent desaturations that may be used to understand the mechanisms contributing to IH events in newborns. [ABSTRACT FROM AUTHOR]

5-HT neurons contribute to autoresuscitation and survival during intermittent severe hypoxia (IsH). In adults, catecholaminergic neurons in the ventrolateral medulla (VLM) contribute to the autonomic response to hypoxia. We hypothesized that 1) catecholaminergic neurons in the neonatal VLM are activated following IsH, 2) this activation is compromised following an acute loss of brain stem 5-HT, and 3) IsH induces cellular and/or transcriptomic plasticity within catecholaminergic and serotonergic neurons that are within or project to the VLM, respectively. To test these hypotheses, we treated rat pups with 6-fluorotryptophan, a tryptophan hydroxylase (TPH) inhibitor, and then exposed treated and vehicle controls to IsH or air. Along with immunohistochemistry to detect tyrosine hydroxylase (TH)- or Fos-positive neurons, we used RNA sequencing to resolve the effects of IsH and 5-HT deficiency on the expression of serotonergic and catecholaminergic system genes in the VLM. 5-HT deficiency compromised autoresuscitation and survival. IsH significantly increased the number of identifiable TH-positive VLM neurons, an effect enhanced by 5-HT deficiency (P = 0.003). Contrary to our hypothesis, 5-HT-deficient pups had significantly more Fos-positive neurons following IsH (P = 0.008) and more activated TH-positive neurons following IsH or air (P = 0.04). In both groups the expression of the 5-HT transporter and TPH2 was increased following IsH. In 5-HT-deficient pups, the expression of the inhibitory 5-HT1A receptor was decreased following IsH, while the expression of DOPA decarboxylase was increased. These data show that the serotonergic and catecholaminergic systems in the VLM of the neonatal rat are dynamically upregulated by IsH, potentially adapting cardiorespiratory responses to severe hypoxia. [ABSTRACT FROM AUTHOR]

Obstructive sleep apnea (OSA) is a major risk factor for cardiovascular mortality, and apnea-induced intermittent hypoxia (IH) is known to promote various cardiovascular alterations such as vascular remodeling. However, the mechanisms that underlie IH remain incompletely investigated. We previously demonstrated that the hypoxia-inducible factor-1 (HIF-1) and endothelin-1 (ET-1) are involved in arterial hypertension and myocardial susceptibility to infarction induced by IH. Thus the objective of the present study was to investigate whether both ET-1 and HIF-1 were also involved in the vascular inflammatory remodeling induced by IH. Mice partially deficient for the Hif1α gene (HIF- 1α-/-) and their wild-type equivalents, as well as C57BL/6J mice, treated or not with bosentan, a dual endothelin receptor antagonist, were exposed to IH or normoxia for 2 wk, 8 h/day. Splenocyte proliferative and secretory capacities, aortic nuclear factor-κB (NF-κB) and HIF-1 activities, and expression of cytokines and intima-media thickness (IMT) were measured. IH induced a systemic and aortic inflammation characterized by an increase in splenocyte proliferative and secretory capacities, aortic NF-κB activity, and cytokine expression in the aortic wall. This was accompanied by an increase in IMT. These modifications were prevented in HIF-1α-/- and bosentan-treated mice. The results of this study suggest that ET-1 is a major contributor to the vascular inflammatory remodeling induced by OSA-related IH, probably through HIF-1-dependent activation of NF-κB. [ABSTRACT FROM AUTHOR]

In recent years, it has become clear that brief, repeated presentations of hypoxia [i.e., acute intermittent hypoxia (AIH)] can boost the efficacy of more traditional therapeutic strategies in certain cases of neurologic dysfunction. This hypothesis derives from a series of studies in animal models and human subjects performed over the past 35 yr. In 1980, Millhorn et al. (Millhorn DE, Eldridge FL, Waldrop TG. Respir Physiol 41: 87-103, 1980) showed that electrical stimulation of carotid chemoafferent neurons produced a persistent, serotonin-dependent increase in phrenic motor output that outlasts the stimulus for more than 90 min (i.e., a "respiratory memory"). AIH elicits similar phrenic "long-term facilitation" (LTF) by a mechanism that requires cervical spinal serotonin receptor activation and de novo protein synthesis. From 2003 to present, a series of studies demonstrated that AIH can induce neuroplasticity in the injured spinal cord, causing functional recovery of breathing capacity after cervical spinal injury. Subsequently, it was demonstrated that repeated AIH (rAIH) can induce recovery of limb function, and the functional benefits of rAIH are greatest when paired with task-specific training. Since uncontrolled and/or prolonged intermittent hypoxia can elicit pathophysiology, a challenge of intermittent hypoxia research is to ensure that therapeutic protocols are well below the threshold for pathogenesis. This is possible since many low dose rAIH protocols have induced functional benefits without evidence of pathology. We propose that carefully controlled rAIH is a safe and noninvasive modality that can be paired with other neurorehabilitative strategies including traditional activity-based physical therapy or cell-based therapies such as intraspinal transplantation of neural progenitors. [ABSTRACT FROM AUTHOR]

Sleep apnea (SA) leads to metabolic abnormalities and cardiovascular dysfunction. Rodent models of nocturnal intermittent hypoxia (IH) are used to mimic arterial hypoxemias that occur during SA. This mini-review focuses on our work examining central nervous system (CNS) mechanisms whereby nocturnal IH results in increased sympathetic nerve discharge (SND) and hypertension (HTN) that persist throughout the 24-h diurnal period. Within the first 1-2 days of IH, arterial pressure (AP) increases even during non-IH periods of the day. Exposure to IH for 7 days biases nucleus tractus solitarius (NTS) neurons receiving arterial chemoreceptor inputs toward increased discharge, providing a substrate for persistent activation of sympathetic outflow. IH HTN is blunted by manipulations that reduce angiotensin II (ANG II) signaling within the forebrain lamina terminalis suggesting that central ANG II supports persistent IH HTN. Inhibition of the hypothalamic paraventricular nucleus (PVN) reduces ongoing SND and acutely lowers AP in IH-conditioned animals. These findings support a role for the PVN, which integrates information ascending from NTS and descending from the lamina terminalis, in sustaining IH HTN. In summary, our findings indicate that IH rapidly and persistently activates a central circuit that includes the NTS, forebrain lamina terminalis, and the PVN. Our working model holds that NTS neuromodulation increases transmission of arterial chemoreceptor inputs, increasing SND via connections with PVN and rostral ventrolateral medulla. Increased circulating ANG II sensed by the lamina terminalis generates yet another excitatory drive to PVN. Together with adaptations intrinsic to the PVN, these responses to IH support rapid onset neurogenic HTN. [ABSTRACT FROM AUTHOR]

Obstructive sleep apnea (OSA) is characterized by repetitive episodes of upper airway obstruction during sleep. These obstructive episodes are characterized by cyclic intermittent hypoxia (CIH), by sleep fragmentation, and by hemodynamic instability, and they result in sustained sympathoexcitation and elevated arterial pressure that persist during waking, after restoration of normoxia. Early studies established that 1) CIH, rather than sleep disruption, accounts for the increase in arterial pressure; 2) the increase in arterial pressure is a consequence of the sympathoactivation; and 3) arterial hypertension after CIH exposure requires an intact peripheral chemoreflex. More recently, however, evidence has accumulated that sympathoactivation and hypertension after CIH are also dependent on altered central sympathoregulation. Furthermore, although many molecular pathways are activated in both the carotid chemoreceptor and in the central nervous system by CIH exposure, two specific neuromodulators-endothelin-1 and angiotensin II-appear to play crucial roles in mediating the sympathetic and hemodynamic response to intermittent hypoxia. [ABSTRACT FROM AUTHOR]

The reason for increased sleep-disordered breathing with a predominance of central apneas in the elderly is unknown. We speculate that ventilatory control instability may provide a link between aging and the onset of unstable breathing during sleep. We sought to investigate potential underlying mechanisms in healthy, elderly adults during sleep. We hypothesized that there is 1) a decline in respiratory plasticity or long-term facilitation (LTF) of ventilation and/or 2) increased ventilatory chemosensitivity in older adults during non-, this should be hyphenated, nonrapid rapid eye movement (NREM) sleep. Fourteen elderly adults underwent 15, 1-min episodes of isocapnic hypoxia (EH), nadir O2 saturation: 87.0 ± 0.8%. Measurements were obtained during control, hypoxia, and up to 20 min of recovery following the EH protocol, respectively, for minute ventilation (VI), timing, and inspiratory upper-airway resistances (RUA). The results showed the following. 1) Compared with baseline, there was a significant increase in VI (158 ± 11%, P < 0.05) during EH, but this was not accompanied by augmentation of VI during the successive hypoxia trials nor in VI during the recovery period (94.4 ± 3.5%, P = not significant), indicating an absence of LTF. There was no change in inspiratory RUA during the trials. This is in contrast to our previous findings of respiratory plasticity in young adults during sleep. Sham studies did not show a change in any of the measured parameters. 2)We observed increased chemosensitivity with increased isocapnic hypoxic ventilatory response and hyperoxic suppression of VI in older vs. young adults during NREM sleep. Thus increased chemosensitivity, unconstrained by respiratory plasticity, may explain increased periodic breathing and central apneas in elderly adults during NREM sleep. [ABSTRACT FROM AUTHOR]

Our investigation was designed to determine whether the time of day affects the carbon dioxide reserve and chemoreflex sensitivity during non-rapid eye movement (NREM) sleep. Ten healthy men with obstructive sleep apnea completed a constant routine protocol that consisted of sleep sessions in the evening (10 PM to 1 AM), morning (6 AM to 9 AM), and afternoon (2 PM to 5 PM). Between sleep sessions, the participants were awake. During each sleep session, core body temperature, baseline levels of carbon dioxide (PETCO2) and minute ventilation, as well as the PETCO2 that demarcated the apneic threshold and hypocapnic ventilatory response, were measured. The nadir of core body temperature during sleep occurred in the morning and was accompanied by reductions in minute ventilation and PETCO2 compared with the evening and afternoon (minute ventilation: 5.3 ± 0.3 vs. 6.2 ± 0.2 vs. 6.1 ± 0.2 l/min, P < 0.02; PETCO2: 39.7 ± 0.4 vs. 41.4 ± 0.6 vs. 40.4 ± 0.6 Torr, P < 0.02). The carbon dioxide reserve was reduced, and the hypocapnic ventilatory response increased in the morning compared with the evening and afternoon (carbon dioxide reserve: 2.1 ± 0.3 vs. 3.6 ± 0.5 vs. 3.5 ± 0.3 Torr, P < 0.002; hypocapnic ventilatory response: 2.3 ± 0.3 vs. 1.6 ± 0.2 vs. 1.8 ± 0.2 l·min-1·mmHg-1, P < 0.001). We conclude that time of day affects chemoreflex properties during sleep, which may contribute to increases in breathing instability in the morning compared with other periods throughout the day/night cycle in individuals with sleep apnea. [ABSTRACT FROM AUTHOR]

Obstructive sleep apnea (OSA) consists of sleep-related repetitive obstructions of upper airways that generate episodes of recurrent or intermittent hypoxia (IH). OSA commonly generates cardiovascular and metabolic pathologies defining the obstructive sleep apnea syndrome (OSAS). Literature usually links OSA-associated pathologies to IH episodes that would cause an oxidative status and a carotid body-mediated sympathetic hyperactivity. Because cardiovascular and metabolic pathologies in obese patients and those with OSAS are analogous, we used models (24-wk-old Wistar rats) of IH (applied from weeks 22 to 24) and diet-induced obesity (O; animals fed a high-fat diet from weeks 12 to 24) to define the effect of each individual maneuver and their combination on the oxidative status and sympathetic tone of animals, and to quantify cardiovascular and metabolic parameters and their deviation from normality. We found that IH and O cause an oxidative status (increased lipid peroxides and diminished activities of superoxide dismutases), an inflammatory status (augmented C-reactive protein and nuclear factor kappa-B activation), and sympathetic hyperactivity (augmented plasma and renal artery catecholamine levels and synthesis rate); combined treatments worsened those alterations. IH and O augmented liver lipid content and plasma cholesterol, triglycerides, leptin, glycemia, insulin levels, and HOMA index, and caused hypertension; most of these parameters were aggravated when IH and O were combined. IH diminished ventilatory response to hypoxia, and hypercapnia and O created a restrictive ventilatory pattern; a combination of treatments led to restrictive hypoventilation. Data demonstrate that IH and O cause comparable metabolic and cardiovascular pathologies via misregulation of the redox status and sympathetic hyperactivity. [ABSTRACT FROM AUTHOR]

Although rats are a frequent model for studies of plasticity in respiratory motor control, the relative capacity of rat accessory respiratory muscles to express plasticity is not well known, particularly in unanesthetized animals. Here, we characterized external intercostal (T2, T4, T5, T6, T7, T8, T9 EIC) and abdominal muscle (external oblique and rectus abdominis) electromyogram (EMG) activity in unanesthetized rats via radiotelemetry during normoxia (Nx: 21% O2) and following acute intermittent hypoxia (AIH: 10 × 5-min, 10.5% O2; 5-min intervals). Diaphragm and T2–T5 EIC EMG activity, and ventilation were also assessed during maximal chemoreceptor stimulation (MCS: 7% CO2, 10.5% O2) and sustained hypoxia (SH: 10.5% O2). In Nx, T2 EIC exhibits prominent inspiratory activity, whereas T4, T5, T6, and T7 EIC inspiratory activity decreases in a caudal direction. T8 and T9 EIC and abdominal muscles show only tonic or sporadic activity, without consistent respiratory activity. MCS increases diaphragm and T2 EIC EMG amplitude and tidal volume more than SH (0.94 ± 0.10 vs. 0.68 ± 0.05 ml/100 g; P < 0.001). Following AIH, T2 EIC EMG amplitude remained above baseline for more than 60 min post-AIH (i.e., EIC long-term facilitation, LTF), and was greater than diaphragm LTF (41.5 ± 1.3% vs. 19.1 ± 2.0% baseline; P < 0.001). We conclude that 1) diaphragm and rostral T2–T5 EIC muscles exhibit inspiratory activity during Nx; 2) MCS elicits greater ventilatory, diaphragm, and rostral T2–T5 EIC muscle activity vs. SH; and 3) AIH induces greater rostral EIC LTF than diaphragm LTF. [ABSTRACT FROM AUTHOR]

Our study was designed to determine if central nervous system (CNS) serotonin is required for the induction of ventilatory long-term facilitation (LTF) in intact, spontaneously breathing mice. Nineteen tryptophan hydrox-/ ylase 2-deficient (Tph2-/-) mice, devoid of serotonin in the CNS, and/ their wild-type counterparts (Tph2+/+) were exposed to intermittent hypoxia each day for 10 consecutive days. The ventilatory response to/ intermittent hypoxia was greater in the Tph2+/+ compared with the Tph2-/- mice (1.10 ± 0.10 vs. 0.77 ± 0.01 ml min-1 *percent-1 oxygen; P ≤ 0.04). Ventilatory LTF, caused by increases in breathing// frequency, was evident in Tph2+/+ and Tph2-/- mice following exposure to intermittent hypoxia each day; however, the magnitude of// the response was greater in the Tph2+/+ compared with the Tph2-/- mice (1.11 ± 0.02 vs. 1.05 ± 0.01 normalized to baseline on each day; P ≤ 0.01). The magnitude of ventilatory LTF increased significantly from the initial to the finals days of the protocol in the/ Tph2-/- (1.06 ± 0.02 vs. 1.11 ± 0.03 normalized to baseline on the initial days; P ≤ 0.004) but not in the Tph2+/+ mice. This enhanced response was mediated by increases in tidal volume. Body temperature and metabolic rate did not account for differences in the magnitude of ventilatory LTF observed between groups after acute and repeated daily exposure to intermittent hypoxia. We conclude that ventilatory LTF, after acute exposure to intermittent hypoxia, is mediated by increases in breathing frequency and occurs in the absence of serotonin, although the magnitude of the response is diminished. This weakened response is enhanced following repeated daily exposure to intermittent hypoxia, via increases in tidal volume, to a similar magnitude evident in Tph2+/+ mice. Thus the magnitude of ventilatory LTF following repeated daily exposure to intermittent hypoxia is not dependent on the presence of CNS serotonin. [ABSTRACT FROM AUTHOR]

Obstructive sleep apnea (OSA) causes intermittent hypoxia (IH) during sleep. Both obesity and OSA are associated with insulin resistance and systemic inflammation, which may be attributable to tissue hypoxia. We hypothesized that a pattern of hypoxic exposure determines both oxygen profiles in peripheral tissues and systemic metabolic outcomes, and that obesity has a modifying effect. Lean and obese C57BL6 mice were exposed to 12 h of intermittent hypoxia 60 times/h (IH60) [inspired O2 fraction (FIO2) 21-5%, 60/h], IH 12 times/h (FIO2 5% for 15 s, 12/h), sustained hypoxia (SH; FIO2 10%), or normoxia while fasting. Tissue oxygen partial pressure (PtiO2) in liver, skeletal muscle and epididymal fat, plasma leptin, adiponectin, insulin, blood glucose, and adipose tumor necrosis factor-or (TNF-α) were mea- sured. In lean mice, IH60 caused oxygen swings in the liver, whereas fluctuations of PtiO2 were attenuated in muscle and abolished in fat. In obese mice, baseline liver PtiO2 was lower than in lean mice, whereas muscle and fat PtiO2 did not differ. During IH, PtiO2 was similar in obese and lean mice. All hypoxic regimens caused insulin resistance. In lean mice, hypoxia significantly increased leptin, especially during SH (44-fold); IH60, but not SH, induced a 2.5- to 3-fold increase in TNF-α secretion by fat. Obesity was associated with striking increases in leptin and TNF-α, which overwhelmed effects of hypoxia. In conclusion, IH60 led to oxygen fluctuations in liver and muscle and steady hypoxia in fat. IH and SH induced insulin resistance, but inflammation was increased only by IH60 in lean mice. Obesity caused severe inflammation, which was not augmented by acute hypoxic regimens. [ABSTRACT FROM AUTHOR]

N-methyl-D-aspartate (NMDA) receptor antagonism in the phrenic motonucleus area eliminates phrenic long-term facilitation (pLTF; a persistent augmentation of phrenic nerve activity after episodic hypoxia) in anesthetized rats. However, whether NMDA antagonism can eliminate ventilatory LTF (vLTF) in awake rats is unclear. The role of non-NMDA receptors in LTF is also unknown. Serotonin receptor antagonism before, but not after, episodic hypoxia eliminates pLTF, suggesting that serotonin receptors are required for induction, but not maintenance, of pLTF. However, because NMDA and non-NMDA ionotropic glutamate receptors are directly involved in mediating the inspiratory drive to phrenic, hypoglossal, and intercostal motoneurons, we hypothesized that these receptors are required for both formation and maintenance of vLTF. vLTF, induced by five episodes of 5-mm poikilocapnic hypoxia (10% O2) with 5-mm normoxia intervals, was measured with plethysmography in conscious adult male Sprague-Dawley rats. Either (± )-2-amino-5-phosphonovaleric acid (APV; NMDA antagonist, 1.5 mg/kg) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; non-NMDA antagonist, 10 mg/kg) was systemically (ip) injected ∼30 mm before hypoxia. APV was also injected immediately after or 20 mm after episodic hypoxia in additional groups. As control, vehicle was similarly injected in each rat 1-2 days before. Regardless of being injected before or after episodic hypoxia, vehicle did not alter vLTF (∼-23%), whereas APV eliminated vLTF while having little effect on baseline ventilation or hypoxic ventilatory response. In contrast, CNQX enhanced vLTF (∼34%) while decreasing baseline ventilation. Collectively, these results suggest that activation of NMDA but not non-NMDA receptors is necessary for formation and maintenance of vLTF in awake rats. [ABSTRACT FROM AUTHOR]

Respiratory long-term facilitation (LTF) is a long-lasting (>1 h) augmentaton of respiratory motor output that occurs even after cessation of hypoxic stimuli, is serotonin-dependent, and is thought to prevent sleep-disordered breathing such as sleep apnea. Raphe nuclei, which modulate several physiological functions through serotonin, receive dense projections from orexin-containing neurons in the hypothalamus. We examined possible contributions of orexin to ventilatory LTF by measuring respiration in freely moving prepro-orexin knockout mice (ORX-KO) and wild-type (WT) littermates before, during, and after exposure to intermittent hypoxia (IH; 5 × 5 min at 10% O2), sustained hypoxia (SH; 25 min at 10% O2), or sham stimulation. Respiratory data during quiet wakefulness (QW), slow wave sleep (SWS), and rapid-eye-movement sleep were separately calculated. Baseline ventilation before hypoxic stimulation and acute responses during stimulation did not differ between the ORX-KO and WT mice, although ventilation depended on vigilance state. Whereas the WT showed augmented minute ventilation ( 20.0 ± 4.5% during QW and 26.5 ± 5.3% during SWS; n = 8) for 2 h following IH, ORX-KO showed no significant increase (by -31 ± 4.6% during QW and 0.3 ± 5.2% during SWS; n = 8). Both genotypes showed no LTF after SH or sham stimulation. Sleep apnea indexes did not change following IH, even when LTF appeared in the WT mice. We conclude that LTF occurs during both sleep and wake periods, that orexin is necessary for eliciting LTF, and that LTF cannot prevent sleep apnea, at least in mice. [ABSTRACT FROM AUTHOR]

We investigated the effects of nightly intermittent exposure to hypoxia and of training during intermittent hypoxia on both erythropoiesis and running economy (RE), which is indicated by the oxygen cost during running at submaximal speeds. Twenty-five college long- and middle- distance runners [maximal oxygen uptake (VO2max) 60.3 ± 4.7 ml·kg-1·min-1] were randomly assigned to one of three groups: hypoxic residential group (HypR, 11 h/night at 3,000 m simulated altitude), hypoxic training group (HypT), or control group (Con), for an intervention of 29 nights. All subjects trained in Tokyo (altitude of 60 m) but HypT had additional high-intensity treadmill running for 30 mm at 3,000 m simulated altitude on 12 days during the night intervention. VO2 was measured at standing rest during four submaximal speeds (12, 14, 16, and 18 km/h) and during a maximal stage to volitional exhaustion on a treadmill. Total hemoglobin mass (THb) was measured by carbon monoxide rebreathing. There were no significant changes in VO2max, THb, and the time to exhaustion in all three groups after the intervention. Nevertheless, HypR showed ~5% improvement of RE in normoxia (P < 0.01) after the intervention, reflected by reduced VO2 at 18 km/h and the decreased regression slope fitted to VO2 measured during rest position and the four submaximal speeds (P < 0.05), whereas no significant corresponding changes were found in HypT and Con. We concluded that our dose of intermittent hypoxia (3,000 m for ~11 h/night for 29 nights) was insufficient to enhance erythropoiesis or VO2max but improved the RE at race speed of college runners. [ABSTRACT FROM AUTHOR]

The cerebellar fastigial nuclei (FN) assist in regulating compensatory responses to large blood pressure changes and show structural injury and functional impairment to cardiovascular challenges in syndromes with sleep-disordered breathing. The patterned time course of FN responses to elevation or lowering of blood pressure and location of responsive regions within the nuclei are unclear. We evaluated FN neural activity in six anesthetized rats using optical imaging procedures during elevation and lowering of arterial pressure by phenylephrine and nitroprusside, respectively. Hypertension diminished optical correlates of FN neural activity, while measures of activity increased to hypotension, with peak neural responses occurring 5–10 s later than peak blood pressure changes. Blood pressure responses were followed by heart rate changes, and peak respiratory rates developed even later, in close temporal proximity to FN activity patterns. Although overall topographical response trends were similar, regional patterns of altered neural activity appeared to both hypertension and hypotension. The extent of neural change was greater during recovery from hypertension than for hypotension at high-dose levels. Blood pressure levels saturated with increasing phenylephrine doses, while FN activity continued to decline. No saturation appeared in heart or respiratory rate trends. The findings suggest that the FN compensate for large blood pressure changes by sympathoexcitatory and inhibitory processes, which accompany late-developing somatic or respiratory adjustments. [ABSTRACT FROM AUTHOR]

The efficiency of ‘living high, training low’ (LHTL) remains controversial, despite its wide utilization. This study aimed to verify whether maximal and/or submaximal aerobic performance were modified by LHTL and whether these effects persist for 15 days after returning to normoxia. Last, we tried to elucidate whether the mechanisms involved were only related to changes in oxygen-carrying capacity. Eleven elite middle-distance runners were tested before (Pre), at the end (Post1), and 15 days after the end (Post2) of an 18-day LHTL session. Hypoxic group (LHTL, n = 5) spent 14 h/day in hypoxia (6 nights at 2,500 m and 12 nights at 3,000 m), whereas the control group (CON. n = 6) slept in normoxia (1,200 m). Both LHTL and CON trained at 1,200 m. Maximal oxygen uptake and maximal aerobic power were improved at Post1 and Post2 for LHTL only (+7.1 and +3,4% for maximal oxygen uptake, +8.4 and +4.7% for maximal aerobic power, respectively), Similarly oxygen uptake and ventilation at ventilatory threshold increased in LHTL only (+18.1 and +12.2% at Post1, +15.9 and +15.4% at Post2, respectively). Heart rate during a 10-min run at 19.5 km/h decreased for LHTL at Post2 (-4.4%). Despite the stimulation of erythropoiesis in LHTL shown by the 27.4% increase in serum transferrin receptor and the 10.1% increase in total hemoglobin mass, red cell volume was not significantly increased at Post l (+9.2%, not significant). Therefore, both maximal and submaximal aerobic performance in elite runners were increased by LHTL mainly linked to an improvement in oxygen transport in early return to normoxia and probably to other process at Post2. [ABSTRACT FROM AUTHOR]

Repeated exposure to brief periods of hypoxia leads to pathophysiological changes in experimental animals similar to those seen in sleep apnea. To determine the effects of such exposure on oxygen levels in vivo, we used an optical method to measure Po2 in microcirculatory vessels and tissue of the rat cremaster muscle during a 1-min step reduction of inspired oxygen fraction from 0.21 to 0.07. Under control conditions, Po2 was 98.1 ± 1.9 Torr in arterial blood, 52.2 ± 2.8 Torr in 29.0 ± 2.7-μm arterioles, 26.8 ± 1.7 Torr in the tissue interstitium near venous capillaries, and 35.1 ± 2.6 Torr in 29.7 ± 1.9-μm venules. The initial fall in Po2 during hypoxia was significantly greater in arterial blood, being 93% complete in the first 10 s, whereas it was 68% complete in arterioles, 47% at the tissue sites, and 38% in venules. In the 10- to 30-s period, the fall in normalized tissue and venular Po2 was significantly greater than in arterial Po2. At the end of hypoxic exposure, Po2 at all measurement sites had fallen very nearly in proportion to that in the inspired gas, but tissue oxygen levels did not reach critical Po2. Significant differences in oxyhemoglobin desaturation rate were also observed between arterial and microcirculatory vessels during hypoxia. In conclusion, the fall in microcirculatory and tissue oxygen levels in resting skeletal muscle is significantly slower than in arterial blood during a step reduction to an inspired oxygen fraction of 0.07, and tissue Po2 does not reach anaerobic levels. [ABSTRACT FROM AUTHOR]

Reports on the hypothesis that episodic hypoxia (EH) promotes inflammation of the cerebral microcirculation and that nitric oxide (NO) produced by the endothelial and neuronal isoforms of NO synthase modulated this response. Association of obstructive sleep apnea, apnea of prematurity and sudden infant death syndrome with a high risk of morbidity and mortality secondary to the neuronal and cerebrovascular consequences of the intermittent hypoxia; Conclusion that NO derived from nNO synthase promotes an inflammatory response in the cerebrovascular microcirculation after short-term EH.

Focuses on previous and recent studies that examined the changes in the expression and posttranslational modification of proteins in response to chronic sustained and intermittent forms of hypoxia. Studies that corroborate findings using a global proteomics approach; Information from the analysis of expression and posttranslational modification of the novel protein targets.

Examines the effects of altitude exposure on running economy (RE) in elite distance runners. Hypothesis that RE would be improved after both natural moderate [live moderate-train moderate (LMTM)] and simulated altitude (LHTL); Assessment of RE during submaximal treadmill runs; Conclusion that 20 days of LHTL at simulated altitude improved the RE of elite distance runners.