Hypoxic Responsiveness Research Articles

Effect of Inspiratory Muscle-Loaded Exercise Training on Ventilatory Response and Intercostal Muscle Deoxygenation During Incremental Cycling Exercise

ABSTRACT Purpose: This study evaluated the effects of exercise training (ET) and inspiratory muscle-loaded exercise training (IMLET) on ventilatory response and intercostal muscle deoxygenation levels during incremental cycling exercise. Methods: Twenty-one male participants were randomly divided into IMLET (n = 10) or ET (n = 11) groups. All participants underwent a 4-week cycling exercise training at 60% peak oxygen uptake. IMLET loaded 50% of maximal inspiratory pressure (PImax). Respiratory muscle strength test, respiratory muscle endurance test (RMET), resting hypoxic ventilatory responsiveness (HVR) test, and incremental cycling test were performed pre- and post-training. Results: The extent of improvement in the PImax was significantly greater in the IMLET group (24%) than in the ET group (8%) (p = .018), and an extended RMET time was observed in the IMLET group (p < .001). Minute ventilation ( V ˙ E ) during exercise was unchanged in both groups before and after training, but tidal volume during exercise increased in the IMLET group. The increase in the exercise intensity threshold for muscle deoxygenation was similar in both groups (p < .001). HVR remained unchanged in both groups post-training. The exercise duration for the incremental exercise until reaching fatigue increased by 7.9% after ET and 6.9% after IMLET (p < .001). Conclusion: The 4-week IMLET improved respiratory muscle strength and endurance but did not alter HVR. Respiratory muscle deoxygenation was alleviated by exercise training, with a limited impact of inspiratory load training.

A fibroblastic foci-targeting and hypoxia-cleavable delivery system of pirfenidone for the treatment of idiopathic pulmonary fibrosis

The progressive formation of fibroblastic foci characterizes idiopathic pulmonary fibrosis (IPF), and excessive oral doses of approved pirfenidone (PFD) always cause gastrointestinal side effects. The fibrotic response driven by activated fibroblasts could perpetuate epithelial damage and promote abnormal extracellular matrix (ECM) deposition. When modified nanoparticles reach their target, it is important to ensure a responsive release of PFD. Hypoxia is a determining factor in IPF, leading to alveolar dysfunction and deeper cellular fibrosis. Herein, a fibroblastic foci-targeting and hypoxia-cleavable drug delivery system (Fn-Azo-BSA@PEG) was established to reprogram the fibrosis in IPF. We have modified the FnBAP5 peptide to enable comprehensive fibroblastic foci targeting, which helps BSA nanoparticles recognize and accumulate at fibrotic sites. Meantime, the hypoxia-responsive azobenzene group allowed for efficient and rapid drug diffusion, while the PEGylated BSA reduced system toxicity and increased circulation in vivo. As expected, the strategy of the fibronectin-targeting-modification and hypoxia-responsive drug release synergistically inhibited activated fibroblasts and reduced the secretion of the fibrosis-related protein. Fn-Azo-BSA@PEG could accumulate in pulmonary tissue and prolong the survival time in bleomycin-induced pulmonary fibrosis mice. Together, the multivalent BSA nanoparticles offered an efficient approach for improving lung architecture and function by regulating the fibroblastic foci and hypoxia. Statement of significanceWe established fibroblastic foci-targeting and hypoxia-cleavable bovine serum albumin (BSA) nanoparticles (Fn-Azo-BSA@PEG) to reprogramme the fibroblastic foci in idiopathic pulmonary fibrosis (IPF). Fn-Azo-BSA@PEG was designed to actively target fibroblasts and abnormal ECM with the FnBPA5 peptide, delivering more FDA-approved pirfenidone (PFD) to the cross-talk within the foci. Once the drug reached fibroblastic foci, the azobenzene group acted as a hypoxia-responsive linker to trigger effective and rapid drug release. Hypoxic responsiveness and FnBAP5-modification of Fn-Azo-BSA@PEG synergistically inhibited the secretion of proteins closely related to fibrogenesis. BSA's inherent transport and metabolic pathways in the pulmonary reduced the side effects of the main organs. The multivalent BSA nanoparticles efficiently inhibited IPF-fibrosis progress and preserved the lung architecture by regulating the fibroblastic foci and hypoxia.

Discovery of estrogen receptor α targeting caged hypoxia-responsive PROTACs with an inherent bicyclic skeleton for breast cancer treatment

In view of the potential off-target effects of antitumor drugs, including proteolysis targeting chimera (PROTAC), certain toxic effects may be caused in normal tissues. Herein, based on the characteristics of the tumor microenvironment, we reported the first estrogen receptor α (ERα) targeting hypoxia-responsive PROTACs in order to improve their safety in breast cancer treatment by introducing two hypoxia-activated groups, nitroimidazole and nitrobenzene, into the ER ligand or E3 ligand of an active PROTAC, which has certain cytotoxicity in normal cells. Bioactivity studies showed that these hypoxia-responsive PROTACs exhibited excellent hypoxic responsiveness and ERα degradation activity under hypoxic conditions, and thus improved the toxic effects of the active PROTAC in normal cells. It is expected that our caged compounds provide a new strategy for precise functional control of PROTAC drugs for breast cancer treatment.

Ventilatory Effects of Acute Intermittent Hypoxia in Conscious Dystrophic Mice.

Exposure to acute intermittent hypoxia (AIH) elicits a form of respiratory plasticity known as long-term facilitation (LTF). Interest has grown in developing AIH interventions to treat ventilatory insufficiency, with promising results in spinal cord injury and amyotrophic lateral sclerosis. Therapeutic AIH may have application in neuromuscular disorders including muscular dystrophies. We sought to establish hypoxic ventilatory responsiveness and the expression of ventilatory LTF in X-linked muscular dystrophy (mdx) mice.Experiments were performed in 15 male wild-type (BL10) and 15 male mdx mice at 4 months of age. Ventilation was assessed using whole-body plethysmography. Baseline measures of ventilation and metabolism were established. Mice were exposed to 10 successive bouts of hypoxia, each lasting 5min, interspersed with 5-min bouts of normoxia. Measurements were taken for 60 min following termination of AIH.In mdx mice, ventilation was significantly increased 60 min post-AIH compared to baseline. However, metabolic CO2 production was also increased. Therefore, ventilatory equivalent was unaffected by AIH exposure, i.e., no ventilatory LTF manifestation. In wild-type mice, ventilation and metabolism were not affected by AIH.Eliciting ventilatory LTF is dependent on many factors and may require concomitant isocapnia or hypercapnia during AIH exposures and/or repeated daily AIH exposures, which is worthy of further pursuit.

HIF-1α-Dependent Induction of Carboxypeptidase A4 and Carboxypeptidase E in Hypoxic Human Adipose-Derived Stem Cells.

Hypoxia induces the expression of several genes through the activation of a master transcription factor, hypoxia-inducible factor (HIF)-1α. This study shows that hypoxia strongly induced the expression of two carboxypeptidases (CP), CPA4 and CPE, in an HIF-1α-dependent manner. The hypoxic induction of CPA4 and CPE gene was accompanied by the recruitment of HIF-1α and upregulation in the active histone modification, H3K4me3, at their promoter regions. The hypoxic responsiveness of CPA4 and CPE genes was observed in human adipocytes, human adipose-derived stem cells, and human primary fibroblasts but not mouse primary adipocyte progenitor cells. CPA4 and CPE have been identified as secreted exopeptidases that degrade and process other secreted proteins and matrix proteins. This finding suggests that hypoxia changes the microenvironment of the obese hypoxic adipose tissue by inducing the expression of not only adipokines but also peptidases such as CPA4 and CPE.

Hypoxic tachycardia is not a result of increased respiratory activity in healthy subjects.

What is the central question of this research? Does increased ventilation contribute to the increase in heart rate during transient exposure to hypoxia in humans? What is the main finding and its importance? Voluntary suppression of the ventilatory response to transient hypoxia does not affect the magnitude of the heart rate response to the stimulus. This indicates that hypoxic tachycardia is not secondary to hyperpnoea in humans. Better understanding of the physiology underlying the cardiovascular response to hypoxia might help in identification of new markers of elevated chemoreceptor activity, which has been proposed as a target in treatment of sympathetically mediated diseases. Animal data suggest that hypoxic tachycardia is secondary to hyperpnoea, and for years this observation has been extrapolated to humans, despite a lack of experimental evidence. We addressed this issue in 17 volunteers aged 29±7(SD) years. A transient hypoxia test, comprising several nitrogen-breathing episodes, was performed twice in each subject. In the first test, the subject breathed spontaneously (spontaneous breathing). In the second test, the subject was repeatedly asked to adjust his or her depth and rate of breathing according to visual (real-time inspiratory flow) and auditory (metronome sound) cues, respectively (controlled breathing), to maintain respiration at the resting level during nitrogen-breathing episodes. Hypoxic responsiveness, including minute ventilation [Hyp-VI; in liters per minute per percentage of blood oxygen saturation ( )], tidal volume [Hyp-VT; in litres per ], heart rate [Hyp-HR; in beats per minute per ], systolic [Hyp-SBP; in millimetres of mercury per ] and mean blood pressure [Hyp-MAP; in millimetres of mercury per ] and systemic vascular resistance [Hyp-SVR; in dynes seconds (centimetres)-5 per ] was calculated as the slope of the regression line relating the variable to , including pre- and post-hypoxic values. The Hyp-VI and Hyp-VT were reduced by 69±25 and 75±10%, respectively, in controlled versus spontaneous breathing (Hyp-VI, -0.30±0.15 versus -0.11±0.09; Hyp-VT, -0.030±0.024 versus -0.007±0.004; both P<0.001). However, the cardiovascular responses did not differ between spontaneous and controlled breathing (Hyp-HR, -0.62±0.24 versus -0.71±0.33; Hyp-MAP, -0.43±0.19 versus -0.47±0.21; Hyp-SVR, 9.15±5.22 versus 9.53±5.57; all P≥0.22), indicating that hypoxic tachycardia is not secondary to hyperpnoea. Hyp-HR was correlated with Hyp-SVR (r=-074 and -0.80 for spontaneous and controlled breathing, respectively; both P<0.05) and resting barosensitivity assessed with the sequence technique (r=-0.60 for spontaneous breathing; P<0.05). This might suggest that the baroreflex mechanism is involved.

Impaired hypoxic ventilatory response and related factors in Han and Uygur patients with obstructive sleep apnea-hypopnea syndrome

Objective: To investigate the respiratory central hypoxia response and its related factors in Han and Uygur patients with obstructive sleep apnea hypopnea syndrome (OSAHS). Methods: One hundred and sixty six OSAHS patients were selected from Jan. 2016 to Dec. 2016 in Department of Respiratory and Critical Care Medicine, Kelamayi Central Hospital, including 69 cases of Han nationality and 97 cases of Uygur nationality. Seventy-three healthy subjects of Uygur nationality were enrolled as the control group. All of them under went sleep monitoring, nocturnal oxygen saturation (SaO(2)), pulmonary function and respiratory central hypoxia response. Results: The 3 groups were matched for age, gender, body mass index(BMI) and apnea-hypopnea index(AHI). The Uygur patients had a higher oxygen desaturation index (ODI4) [(30±22) per hour vs (18±17) per hour ] than Han patients of the same age and BMI. Compared to Han patients, Uygur patients had weaker hypoxic responsiveness [(-0.41±0.23) L·min(-1)·%(-1) vs (-0.36±0.22) L·min(-1)·%(-1,) P<0.05], and the difference still existed after adjusting for AHI [(-0.31±0.21) L·min(-1)·%(-1) vs (-0.41±0.22) L·min(-1)·%(-1,) P<0.05] in mild OSAHS, but this difference was not significant in severe OSAHS. Conclusions: The central hypoxic response in Uygur OSAHS patients was lower than that in Han OSAHS patients and normal controls.

In adult female hamsters hypothyroidism stimulates D1 receptor-mediated breathing without altering D1 receptor expression

Hypothyroidism affects cardiopulmonary regulation and function of dopaminergic receptors. Here we evaluated effects of 5 months of hypothyroidism on dopamine D1 receptor modulation of breathing in female hamsters using a D1 receptor antagonist SCH 23390. Euthyroid hamsters (EH) served as controls. Results indicated that hypothyroid female hamsters (HH) exhibited decreased body weights and minute ventilation (VE) following hypoxia due to decreased frequency of breathing (F). Moreover, SCH 23390 administration in HH increased VE by increasing tidal volume during exposure to air, hypoxia and following hypoxia. Relative to vehicle, SCH 23390 treatment decreased body temperature and hypoxic VE responsiveness in both groups. In EH, SCH 23390 decreased F in air, hypoxia and post hypoxia, and VE during hypoxia trended to decrease (P=0.053). Finally, expression of D1 receptor protein was not different between the two groups in any region evaluated. Thus, hypothyroidism in older female hamsters affected D1 receptor modulation of ventilation differently relative to euthyroid animals, but not expression of D1 receptors.

Developmental Expression and Hypoxic Induction of Hypoxia Inducible Transcription Factors in the Zebrafish.

The hypoxia inducible transcription factor (HIF) has been shown to coordinate the hypoxic response of vertebrates and is expressed in three different isoforms, HIF-1α, HIF-2α and HIF-3α. Knock down of either Hif-1α or Hif-2α in mice results in lethality in embryonic or perinatal stages, suggesting that this transcription factor is not only controlling the hypoxic response, but is also involved in developmental phenomena. In the translucent zebrafish embryo the performance of the cardiovascular system is not essential for early development, therefore this study was designed to analyze the expression of the three Hif-isoforms during zebrafish development and to test the hypoxic inducibility of these transcription factors. To complement the existing zfHif-1α antibody we expressed the whole zfHif-2α protein and used it for immunization and antibody generation. Similarly, fragments of the zfHif-3α protein were used for immunization and generation of a zfHif-3α specific antibody. To demonstrate presence of the Hif-isoforms during development [between 1 day post fertilization (1 dpf) and 9 dpf] affinity-purified antibodies were used. Hif-1α protein was present under normoxic conditions in all developmental stages, but no significant differences between the different developmental stages could be detected. Hif-2α was also present from 1 dpf onwards, but in post hatching stages (between 5 and 9 dpf) the expression level was significantly higher than prior to hatching. Similarly, Hif-3α was expressed from 1 dpf onwards, and the expression level significantly increased until 5 dpf, suggesting that Hif-2α and Hif-3α play a particular role in early development. Hypoxic exposure (oxygen partial pressure = 5 kPa) in turn caused a significant increase in the level of Hif-1α protein even at 1 dpf and in later stages, while neither Hif-2α nor Hif-3α protein level were affected. In these early developmental stages Hif-1α therefore appears to be more important for the coordination of hypoxic responsiveness.

The effect of prenatal maternal infection on respiratory function in mouse offspring: evidence for enhanced chemosensitivity.

Systemic maternal inflammation is implicated in preterm birth and bronchopulmonary dysplasia (BPD) and may induce morbidities including reduced pulmonary function, sleep-disordered breathing, and cardiovascular disorders. Here we test the hypothesis that antenatal maternal inflammation per se causes altered alveolar development and increased chemoreflex sensitivity that persists beyond infancy. Pregnant C57BL/6 mice were administered lipopolysaccharide (LPS) (150 μg/kg ip) to induce maternal inflammation or saline (SHAM) at embryonic day 16 (randomized). Pups were weighed daily. On days 7, 28, and 60 (D07, D28, and D60), unrestrained wholebody plethysmography quantified ventilation and chemoreflex responses to hypoxia (10%), hypercapnia (7%), and asphyxia (hypoxic hypercapnia). Lungs were harvested to quantify alveolar number, size, and septal thickness. LPS pups had reduced baseline ventilation per unit bodyweight (∼40%, P < 0.001) vs. SHAM. LPS increased ventilatory responses to hypoxia (D07: 66% vs. 28% increase in ventilation; P < 0.001) hypercapnia (170% vs. 88%; P < 0.001), and asphyxia (249% vs. 154%; P < 0.001); hypersensitive hypoxic responsiveness persisted until D60 (P < 0.001). LPS also increased apnea frequency (P < 0.01). LPS caused thicker alveolar septae (D07, P < 0.001), diminished alveolar number (D28, P < 0.001) vs. SHAM, but effects were minimal by D60. Pups delivered from mothers exposed to antenatal inflammation exhibit deficits in lung structure and hypersensitive responses to respiratory stimuli that persist beyond the newborn period. Antenatal inflammation may contribute to impaired gas exchange and unstable breathing in newborn infants and adversely affect long-term health.

D1 Dopamine Receptor Modulation of Breathing in Young Hypothyroid Female Hamsters

Previously we noted in 8 month old female hamsters hypothyroid (H) for 5 months that administration of the dopamine D1 receptor antagonist SCH 23390 (SCH) in saline increased ventilation (VE) while animals were exposed to air, hypoxia and air following hypoxia (PH) relative to saline. These ventilatory responses to SCH were dissimilar to those observed in euthyroid age‐matched females (E). The increased VE's were due to increased tidal volumes (VT), but not frequency of breathing (F). In the present study effects of SCH on VE were studied in 15 four month old female hamsters rendered H for two months by administration of propylthiouracil in drinking water. Fifteen age and gender matched female E hamsters served as controls. Ventilation was measured in conscious hamsters using plethysmography. Relative to saline, 0.025 mg/kg SCH decreased VE during exposure of the H hamsters to air, PH and hypercapnia by decreasing F (due to increases of inspiratory &amp; expiratory times), without affecting VT. Similar decreases in VE occurred in E hamsters exposed to air and hypercapnia. Hypoxic responsiveness, lower in H saline‐treated hypothyroid hamsters, was increased by SCH to levels comparable to those in E hamsters. No effects of SCH were noted on hypercapnic responsiveness. Thus, age and the length of hypothyroidism affect D1 receptor modulation of ventilation in female hamsters.Funding: COBRE Pilot Grant

Respiratory Toxicity of Dimethyl Sulfoxide.

Dimethyl sulfoxide (DMSO) is one of the most commonly used solvents for hydrophobic substances in biological experiments. In addition, the compound exhibits a plethora of bioactivities, which makes it of potential pharmacological use of its own. The influence on respiration, and thus on arterial blood oxygenation, of DMSO is unclear, contentious, and an area of limited study. Thus, in the present investigation we set out to determine the influence on lung ventilation of cumulated doses of DMSO in the amount of 0.5, 1.5, 3.5, 7.5, and 15.5 g/kg; each dose given intraperitoneally at 1 h interval in conscious mice. Ventilation and its responses to 7 % hypoxia (N(2) balanced) were recorded in a whole body plethsymograph. We demonstrate a dose-dependent inhibitory effect of DMSO on lung ventilation and its hypoxic responsiveness, driven mostly by changes in the tidal component. The maximum safe dose of DMSO devoid of meaningful consequences for respiratory function was 3.5 g/kg. The dose of 7.5 g/kg of DMSO significantly dampened respiration, with yet well preserved hyperventilatory response to hypoxia. The highest dose of 15.5 g/kg severely impaired ventilation and its responses. The study delineates the safety profile of DMSO regarding the respiratory function which is essential for maintaining proper tissue oxygenation. Caution should be exercised concerning dose concentration of DMSO.

Travelling to High Altitudes Lead to Difficult Sleeping- Review

Background: High-altitude (HA) environments have adverse effects on the normal physiological functioning of body in the people who are accustomed to living at low altitudes. New arrivals to altitude commonly experience decline in quality sleep. Most people don’t sleep well at altitude. Sojourns commonly report vivid dreams, feelings of being suffocated and wake up in the morning feeling un-refreshed. These complaints are commonly associated with increased fragmentation of sleep by frequent brief arousals, which are in turn linked to periodic breathing. Findings: Changes in sleep architecture include a shift toward lighter sleep stages, with noticeable decrements in slow wave sleep and with variable decreases in REM sleep. Increased hypoxic ventilatory responsiveness and loss of regularization of breathing during sleep contribute to the occurrence of periodicity. Conclusions: One of the immediate effects of altitude exposure is to cause a general reduction in sleep quality. The purpose of this review was to consolidate the findings of the significant studies that examined the effects of HA on the sleep disturbances so far, so that further study in this regard can take new dimensions.

Decreased hypoxic ventilatory chemoresponsiveness and clinical features in patients with narcolepsy cataplexy

To explore the relationship between hypoxic responsiveness of the patients with narcolepsy-cataplexy and their clinical features. A total of 113 patients with narcolepsy-cataplexy (narcolepsy group) at Peking University People's Hospital from June 2007 to May 2008 and 128 gender-age matched volunteers (control group) were recruited. And their status of human leukocyte antigen (HLA)-DQB1*0602 was examined to differentiate hypercapnic and hypoxic responsiveness. Among them, 93 patients with severe hypersomnolence had hypercapnic and hypoxic responsiveness tested before and after the treatment of methylphenidate and another 20 with severe cataplexy did the same before and after the treatment of chlorimiopramine. Compared with the control group, the narcolepsy group had depressed hypoxic responsiveness ((-0.135 ± 0.105) vs (-0.223 ± 0.136) L×min⁻¹ × %SpO₂⁻¹, P < 0.001). After the treatment of methylphenidate, sleepiness improved significantly in all 93 patients, but their low hypoxic responsiveness did not change ((-0.151 ± 0.111) vs (-0.149 ± 0.105) L×min⁻¹× % SpO₂⁻¹, P = 0.780). After the treatment of chlorimiopramine, cataplexy also improved in 20 patients.However their low hypoxic responsiveness had no change ((-0.114 ± 0.054) vs (-0.115 ± 0.065) L×min⁻¹ × %SpO₂⁻¹, P = 0.949). Lower hypoxic responsiveness in narcolepsy group is not related with the clinical features of disease.

Effects of hypothyroidism in female hamsters on D1 receptor modulated breathing and expression of D1 receptors (1178.2)

The D1 receptor antagonist, SCH23390 (SCH), decreased ventilation (VE) in euthyroid (E) male hamsters exposed to air and hypoxia, but increased VE in hypothyroid (H) hamsters exposed to air, but not during hypoxic exposures. In H relative to E males, D1 receptor expression was decreased in the carotid body (CB), striatum, and paraventricular nucleus of the hypothalamus (PVN), but not in the nucleus tractus solitarius (NTS). For the present study we hypothesized that in H female hamsters D1 receptor expression in the CB, PVN, NTS and striatum and modulation of breathing would be dissimilar to that observed in H males. H and E female hamsters received SCH or saline. Physiological parameters were evaluated in awake animals exposed to air, 5 minutes of hypoxia (10% O2 in N2) and then air. SCH decreased body temperatures in both groups, had no effect on metabolism, but VE was increased in the H hamsters relative to E hamsters’ responses during air and hypoxic exposures due to increased tidal volumes. SCH relative to saline decreased breathing frequency, but increased tidal volume in the E hamsters during baseline and post‐hypoxic air exposures. In both groups SCH decreased hypoxic responsiveness. Expression of D1 receptors was similar in both groups in all regions. Thus, the ventilatory responses in H females were similar to SCH relative to H males (but not in E males), expression of D1 receptors was dissimilar in 3 regions.Grant Funding Source: Internal Sanford School of Medicine Grant

Peripheral Chemoreceptor Responsiveness and Hypoxic Pulmonary Vasoconstriction in Humans

Studies in animals have shown that interruption of carotid body afferent hypoxic signaling or efferent CNS activity to the lung enhances hypoxic pulmonary vasoconstriction (HPV). Whether a similar influence of the CNS on HPV strength is present in humans has never been studied, owing to the invasive nature of physical neural ablation or nonspecific systemic effects of pharmacological blockade of putative neural pathways. In order to demonstrate a peripheral chemoreceptor-mediated modulation of HPV in man, we hypothesized that individuals with high hypoxic ventilatory responsiveness, indicative of strong peripheral hypoxic chemosensitivity, should have less HPV in response to inspired hypoxia. In 15 healthy men and women, we measured the normobaric poikilocapnic hypoxic ventilatory response (HVR; L min(-1) % SPo2(-1)) during 15 min of hypoxia (FIo2=0.12). On the following day, we then measured pulmonary artery systolic pressure (PASP) using echosonography while subjects randomly breathed 0.21, 0.18, 0.15, and 0.12 FIo2, each for periods of 15 min. We chose this strategy to obtain an equivalent stimulus for HPV in all subjects, using SPo2 as a surrogate for alveolar Po2. HPV was assessed as PASP at a common interpolated arterial oxygen saturation (SPo2) of 85%. We recorded a sufficient six-fold range of HVR (0.05-0.30, mean 0.13 L min(-1) % SPo2(-1)) similar to previously published data on normobaric, poikilocapnic HVR. HPV at SPo2 of 85% was 28.5 mmHg (range 21.7-41.3). There was a significant inverse relationship between poikilocapnic HVR and HPV (p=0.006, R(2)=0.38). Previous studies of individuals with susceptibility to high altitude pulmonary edema (HAPE) have suggested that both low HVR and high HPV are important risk factors. We show that these two responses are inversely correlated and conclude that a greater magnitude of peripheral chemoreceptor response to hypoxia limits hypoxic pulmonary vasoconstriction in healthy subjects.

Expression of Prolyl Hydroxylases (PHDs) Is Selectively Controlled by HIF-1 and HIF-2 Proteins in Nucleus Pulposus Cells of the Intervertebral Disc: DISTINCT ROLES OF PHD2 AND PHD3 PROTEINS IN CONTROLLING HIF-1α ACTIVITY IN HYPOXIA

Adaptive response to hypoxia in nucleus pulposus cells of the intervertebral disc is regulated by the hypoxia-inducible factors, HIF-1α and HIF-2α. Moreover, oxygen-dependent turnover of HIF-1α in these cells is controlled by the prolyl-4-hydroxylase domain (PHD) family of proteins. Whether HIF homologues control expression of PHDs and whether PHDs control hypoxia-inducible factor (HIF) turnover and/or activity under hypoxia is not known. Here, we show that in nucleus pulposus cells, hypoxia robustly induces PHD3 expression and, to a lesser extent, of PHD2 and PHD1. Reporter analysis shows that the hypoxic induction of the PHD2 promoter is HIF-1α dependent, whereas PHD3 promoter/enhancer activity is dependent on both HIF-1α and HIF-2α. Lentiviral delivery of HIF-1α, ShHIF-1α, and ShHIF-1β confirmed these observations. Noteworthy, HIF-1α maintains basal expression of PHD1 in hypoxia at the posttranscriptional level. Finally, loss of function studies using lentiviral transduction of ShPHDs clearly shows that even at 1% O(2), PHD2 selectively degrades HIF-1α. In contrast, in hypoxia, PHD3 enhances HIF-1α transcriptional activity without affecting protein levels. To correlate these observations with disc disease, a condition characterized by tissue vascularization, we analyzed human tissues. Increased PHD1 mRNA expression but decreased PHD2 and PHD3 expression is observed in degenerate tissues. Interestingly, the hypoxic responsiveness of all the PHDs is maintained in isolated nucleus pulposus cells regardless of the disease state. We propose that PHD2 and PHD3 can be used as a biomarker of tissue oxygenation in the disc and that, as such, it may have important clinical implications.

Developmental Changes in Mesenteric Artery Reactivity in Fetal and Perinatal Chicks

Background and aims: The fetal intestine presents a low-flow, high-resistance vascular bed. Birth is accompanied by a dramatic increase in intestinal blood flow. The vascular changes accompanying this transition have been partly characterized. Here, we aimed to investigate the developmental changes in chick mesenteric artery (MA) reactivity during the fetal and perinatal period. Methods: Rings of the distal segment of the cranial MA from 15-d (F15) and 19-d (F19) chicken fetuses (total incubation: 21d) as well as non-fed 0-3-h-old (NH3h) and first-fed 1-d-old (NH1d) newly hatched (NH) chicks were mounted in a wire myograph. Results: F15 MA rings responded to KCl-evoked depolarization and to norepinephrine (NE), thromboxane A2 (U46619), endothelin (ET)-1, acetylcholine (ACh), sodium nitroprusside (SNP), and forskolin indicating the presence of electro- and pharmaco-mechanical coupling as well as cGMP- and cAMP-mediated relaxation. The contractile efficacy of KCl underwent a linear increase (F15< F19< NH3h< NH1d). The efficacy of NE and U46619 increased in ovo but not ex ovo (F15< F19=NH3h=NH1d) and the efficacy of ET-1 peaked at F19. The relaxations elicited by ACh, SNP, and forskolin did not undergo dramatic developmental changes. Hypoxia induced contraction in MA rings. F15 MAs did not respond to hypoxia and the hypoxic responsiveness did not change ex-ovo (F15=0< F19< NH3h=NH1d). Conclusions: The ability of chick MAs to constrict in response to pharmacological and physical stimuli increases during the fetal period but no dramatic changes are induced by hatching or the first feeding. Maturation of vasodilator mechanisms precedes that of vasoconstrictor mechanisms.

Effects of Hydrogen Sulfide Synthesis Inhibitors on Posthypoxic Ventilatory Behavior in the C57BL/6J Mouse

Background: H₂S synthesis inhibitors (HSSI) have been shown to impact respiratory control. For instance, the HSSI hydroxylamine (HA) decreases the respiratory discharge rate from isolated medullary sections, and HA in addition to other HSSIs propargylglycine and amino-oxyacetic acid (AOAA) have been found to reduce hypoxic responsiveness. Objectives: The aim of this study was to determine if administration of HSSIs could improve respiratory stability in an intact organism prone to recurrent central apneas. Methods: Saline and HSSI compounds were administered to C57BL/6J mice (n = 24), a strain predisposed to recurrent central apneas, prior to measurement of hypoxic and posthypoxic ventilatory behavior. Results: Administration of HA and AOAA resulted in a significantly smaller percentage of animals expressing one or more apneas during reoxygenation compared to saline control, and animals given AOAA demonstrated a smaller coefficient of variation for frequency during reoxygenation, a marker suggesting greater respiratory stability. This occurred despite varying effects of the three HSSI compounds on hypoxic ventilatory response. Conclusions: Instability and pause expression are improved by targeting H₂S synthesis, an effect not predicted by effects on hypoxic responsiveness.

Hypoxic Ventilatory Response Does Not Predict Performance At Moderate Altitude In Elite Distance Runners

Research suggests that individuals with strong hypoxic ventilatory responsiveness are better suited for mountain climbing performance at high altitudes. We asked if this relationship between hypoxic ventilatory response (HVR) and endurance exercise performance would extend to racing performance of elite distance runners at moderate altitude. PURPOSE: To test the hypothesis that endurance performance at moderate altitude is related to initial levels of and/or changes in HVR in a group of elite distance runners. METHODS: Twenty-one national class U.S. distance runners (13 M, 8 F) completed a 28-day altitude training intervention in Deer Valley, Utah, elevation 2500m, following a "Live High - Train Low" training model. Isocapnic HVR was measured in the one week prior to and within 72 hours upon return from altitude. Endurance performance was measured using 3km time trials completed on 400m all-weather tracks at sea-level (SL) one day prior to departure for altitude, and twice at an altitude of 2100m - 48 hours after arrival to altitude (ALT1), and on day 23 of the altitude intervention (ALT4). RESULTS: HVR was significantly increased from pre- to post-altitude (0.21±0.04 vs. 0.40±0.06 ΔL·min-1·Δ%SaO2-1, p<0.05). ALT1 3km performance time was significantly slower than that measured at SL (9.53±0.68 vs. 8.76±0.63 min). ALT4 3km time was significantly faster than ALT1 3km time (9.34 ± 0.68 min but remained significantly slower than SL 3km time. HVR was not significantly correlated with 3km performance time at either ALT1 (r = -0.25 M; r = -0.14 F) or ALT4 (r = 0.05 M; r = -0.32 F). In addition, pre-altitude HVR was not significantly related to the change in 3km time from SL to ALT1 (r = -0.28 M; r = 0.34 F). The change in HVR after chronic altitude exposure (pre- to post-altitude) was not significantly correlated with the change in 3km performance time from ALT1 to ALT4 (r = 0.24). CONCLUSIONS: In contrast to the positive HVR-performance relationship seen in mountain climbers at high altitude, it appears that the performance of elite distance runners at moderate altitude does not depend on initial levels of or changes in hypoxic ventilatory response. Supported by US Olympic Committee grant SST97-ATH-007, a grant from USA Track and Field, and institutional support from Presbyterian Hospital of Dallas and Indiana University.