Showing papers on "Hypoxia (medical) published in 1994"

Hypoxia induces accumulation of p53 protein, but activation of a G1-phase checkpoint by low-oxygen conditions is independent of p53 status.

Abstract: It has been convincingly demonstrated that genotoxic stresses cause the accumulation of the tumor suppressor gene p53. One important consequence of increased p53 protein levels in response to DNA damage is the activation of a G1-phase cell cycle checkpoint. It has also been shown that G1-phase cell cycle checkpoints are activated in response to other stresses, such as lack of oxygen. Here we show that hypoxia and heat, agents that induce cellular stress primarily by inhibiting oxygen-dependent metabolism and denaturing proteins, respectively, also cause an increase in p53 protein levels. The p53 protein induced by heat is localized in the cytoplasm and forms a complex with the heat shock protein hsc70. The increase in nuclear p53 protein levels and DNA-binding activity and the induction of reporter gene constructs containing p53 binding sites following hypoxia occur in cells that are wild type for p53 but not in cells that possess mutant p53. However, unlike ionizing radiation, the accumulation of cells in G1 phase by hypoxia is not strictly dependent on wild-type p53 function. In addition, cells expressing the human papillomavirus E6 gene, which show increased degradation of p53 by ubiquitination and fail to accumulate p53 in response to DNA-damaging agents, do increase their p53 levels following heat and hypoxia. These results suggest that hypoxia is an example of a "nongenotoxic" stress which induces p53 activity by a different pathway than DNA-damaging agents.

Hypoxia and drug resistance

Abstract: Biologically and therapeutically important hypoxia occurs in many solid tumor masses. Hypoxia can be a direct cause of therapeutic resistance because some drugs and radiation require oxygen to be maximally cytotoxic. Cellular metabolism is altered under hypoxic conditions. Hypoxia can result in drug resistance indirectly if under this condition cells more effectively detoxify the drug molecules. Finally, there is evidence that hypoxia can enhance genetic instability in tumor cells thus allowing more rapid development of drug resistance cells. The current review describes the effects of hypoxia on tumor response to a variety of anticancer agents and also describes progress toward therapeutically useful methods of delivering oxygen to tumors in an effort to overcome therapeutic resistance due to hypoxia. Finally, the use of hypoxic cell selective cytotoxic agents as a means of addressing hypoxic ‘drug resistance’ is discussed.

Upregulation of vascular endothelial growth factor expression induced by myocardial ischaemia: implications for coronary angiogenesis.

Abstract: Objective: The process of coronary collateral development is poorly understood. It is assumed that particular angiogenic factors are upregulated during episodes of myocardial ischaemia and act as a trigger for neovascularisation. However, the identity of these factors is unknown. The angiogenic factor vascular endothelial growth factor (VEGF) has been shown to be hypoxia inducible, so this factor may mediate ischaemia induced angiogenesis in the heart. The aim of this study was to examine hypoxia inducibility of VEGF in cultured myocardial cells as well as in normally perfused and ischaemic porcine myocardium. Methods: (1) In vitro experiment: cultured rat myocardial cells were subjected to hypoxia, and steady state levels of VEGF mRNA were measured after 2 and 4 h of hypoxia. (2) In vivo experiment: myocardial ischaemia in pigs hearts was induced by repeated 2-10 min left anterior descending coronary artery occlusions, separated by 20 min of reperfusion. Hearts were retrieved after 6 h of intermittent ischaemia. Total RNA was extracted from normal and ischaemic zones of the heart and processed for RNA blot hybridisation analysis. Results: In vitro experiment: as soon as 2-4 h after exposure of cultures to hypoxia, VEGF mRNA levels were significantly raised (6-10-fold). In vivo experiment: VEGF expression was significantly augmented in the ischaemic territory of the myocardium (three- to fivefold induction). Furthermore, polymerase chain reaction amplification of the reverse transcribed mRNA showed increased production of multiple forms of differentially spliced VEGF mRNA in the ischaemic myocardium. Conclusions: VEGF production in the myocardium is significantly upregulated by hypoxia in vitro and by ischaemia in vivo. These results suggest that VEGF is a likely mediator in the natural process of ischaemia induced myocardial neovascularisation. Cardiovascular Research 1994; 28 :1176-1179

Regulation of breathing

Abstract: The Control of Breathing: An Uninhibited Survey from the Perspective of Comparative Physiology Part 1 Central Nervous Control: Neural Control of Respiratory Pattern in Mammals - An Overview Mechanisms of Respiratory Motor Output Glutamate, GABA, and Serotonin in Ventilatory Control Central Pathways of Pulmonary and Airway Vagal Afferents Mechanisms and Analysis of Ventilatory Stability Hypothalamic Involvement in Respiratory and Cardiovascular Regulation Cerebral Cortex and Respiration Part 2 Afferent Systems: Mechanisms of Carotid Body Chemoreception Central Chemo-reception Role of Airway Afferents on Upper Airway Muscle Activity Respiratory Control by Diaphragmatic and Respiratory Muscle Afferents Afferents from Limb Skeletal Muscle Peripheral and Central Effects of Hypoxia on the Control of Ventilation Respiratory Sinus Arrhythmia and Other Human Cardiovascular Neural Periodicities Part 3 Developmental and Hormonal Influences: Developmental and Hormonal Influences Unique Issues in Neonatal Respiratory Control lnfluences of Sex Steroids on Ventilation and Ventilatory Control Part 4 Loads, Sensations, and Failure Mechanisms of Respiratory Load Compensation Respiratory Sensations and Their Role in the Control of Breathing Causes of Respiratory Failure Metabolic-State Effects Changes in Respiratory Motor Activity During Rapid Eye Movement Sleep Interaction Between Metabolism and Ventilation - Effects of Respiratory Gases and Temperature Regulation of Hyperpnoea, Hyperventilation, and Respiratory Muscle Recruitment During Exercise.

Hypoxia inhibits expression of eNOS via transcriptional and posttranscriptional mechanisms

Abstract: Normal blood vessel tone is maintained by a balance of vasoconstrictors and vasodilators produced by endothelial cells in the vasculature. Nitric oxide (NO) is a potent vasodilator that causes vascular smooth muscle cell relaxation by elevating intracellular guanosine 3',5'-cyclic monophosphate (cGMP) levels. The physiological mechanisms regulating NO production in the vasculature are not completely understood. We report here that production of this vasodilator by vascular endothelial cells can be significantly suppressed by hypoxia. Exposing human endothelial cells to low PO2 results in 40-60% reduction in the steady-state mRNA levels of endothelial constitutive NO synthase (eNOS), the major enzyme responsible for NO production in these cells. The lower levels of eNOS mRNA result from decreased transcription of the gene as well as reduced message stability. In endothelial-smooth muscle cell co-culture experiments, hypoxic endothelial cells stimulated significantly less cGMP production by smooth muscle cells than the corresponding normoxic controls. This inhibitory effect of hypoxia on NOS production by endothelial cells occurs after 24 h of hypoxia and persists for at least 48 h. These new findings suggest that hypoxia might cause changes in blood vessel tone through compound mechanisms: by increasing the production of endothelium-derived vasoconstrictors and, as shown here, by suppressing the production of vasodilators like NO.

Hypoxic induction of interleukin-8 gene expression in human endothelial cells.

Abstract: Because leukocyte-mediated tissue damage is an important component of the pathologic picture in ischemia/reperfusion, we have sought mechanisms by which PMNs are directed into hypoxic tissue. Incubation of human endothelial cells (ECs) in hypoxia, PO2 approximately 14-18 Torr, led to time-dependent release of IL-8 antigen into the conditioned medium; this was accompanied by increased chemotactic activity for PMNs, blocked by antibody to IL-8. Production of IL-8 by hypoxic ECs occurred concomitantly with both increased levels of IL-8 mRNA, based on polymerase chain reaction analysis, and increased IL-8 transcription, based on nuclear run-on assays. Northern analysis of mRNA from hypoxic ECs also demonstrated increased levels of mRNA for macrophage chemotactic protein-1, another member of the chemokine superfamily of proinflammatory cytokines. IL-8 gene induction was associated with the presence of increased binding activity in nuclear extracts from hypoxic ECs for the NF-kB site. Studies with human umbilical vein segments exposed to hypoxia also demonstrated increased elaboration of IL-8 antigen compared with normoxic controls. In mice exposed to hypoxia (PO2 approximately 30-40 Torr), there was increased pulmonary leukostasis, as evidenced by increased myeloperoxidase activity in tissue homogenates. In parallel, increased levels of transcripts for IP-10, a murine homologue in the chemokine family related to IL-8, were observed in hypoxic lung tissue. Taken together, these data suggest that hypoxia constitutes a stimulus for leukocyte chemotaxis and tissue leukostasis.

Exposure to Alternating Hypoxia and Hyperoxia Causes Severe Proliferative Retinopathy in the Newborn Rat

Abstract: Exposure to variable hyperoxia has recently been shown to be much more effective at producing proliferative retinopathy in the newborn rat than exposure to constant hyperoxia. To incorporate a more clinically relevant oxygen-exposure paradigm in our studies, we have now used a cycle between 50 and 10% oxygen and have compared its effects with those found using new exposures to the previously used 80/40% cycle. Starting at birth and continuing for 14 d, rats were exposed to environments that cycled between 50 and 10% oxygen or 80 and 40% oxygen every 24 h. After exposure, some rats were killed for assessment of retinal vascular development. Others were removed to room air for 4 d before killing and evaluation for the presence of abnormal neovascularization--a clinical consequence believed to be promoted by termination of oxygen therapy. The 50/10% cycle resulted in greater retardation of retinal blood vessel development during oxygen than that found in the 80/40% exposure group. After 4 d postexposure in room air, the incidence of preretinal neovascularization was 97% in the 50/10% rats and 72% in the 80/40% group. Clearly, the overall amount of oxygen the subject receives is less critical than other parameters of its administration in producing proliferative retinopathy. Also, the range of variation (40% in both cases) is not the controlling characteristic. Our results suggest that consistency of oxygen level and avoidance of hypoxic levels should be important concerns in neonatal oxygen therapy.

Chronic hypoxia is associated with reduced delayed rectifier K+ current in rat pulmonary artery muscle cells

Abstract: Pulmonary hypertension due to long-term hypoxia occurs as a result of both chronic obstructive pulmonary disease and habitation at high altitudes. Studies in animal models of chronic hypoxia have demonstrated the development of a persistent depolarization of pulmonary artery (PA) smooth muscle cells (SMCs). In seeking to explain this effect, we compared under normoxic conditions the K+ currents in SMCs isolated from small PA of chronically hypoxic and normoxic rats. Chronic hypoxia was associated with a marked (40-50%) reduction in amplitude of a K+ current, which had the pharmacological and kinetic characteristics of a delayed rectifier. The resting potential of the isolated PA cells from chronically hypoxic animals was significantly more positive (-43.5 +/- 2 mV) than that of cells from normoxic animals (-54.3 +/- 2 mV), and this depolarization could be approximately mimicked in the cells from normoxic animals by application of 1 mM 4-aminopyridine, a blocker of the delayed rectifier K+ current. Glibenclamide (1 microM), a blocker of ATP-sensitive K+ (KATP) channels, also caused a substantial (14.5 +/- 2.2 mV) depolarization of the membrane. These results suggest that both delayed rectifier and ATP-dependent K+ currents contribute to setting the membrane potential in these cells and are consistent with the possibility that downregulation of the delayed rectifier K+ current contributes to the depolarization and altered responsiveness to vasoactive agents of PAs that occurs during long-term hypoxia.

Interleukin-1 receptor antagonist treatment reduces pulmonary hypertension generated in rats by monocrotaline.

Abstract: Chronic pulmonary hypertension is associated with significant vascular remodeling. We demonstrated recently in the monocrotaline (MCT) and chronic hypoxia rat models of pulmonary hypertension that treatment with platelet-activating factor (PAF) antagonists inhibited the development of chronic pulmonary hypertension. PAF and other lipid mediators interact with interleukin-1. We postulated that chronic treatment with a recombinant human interleukin-1 receptor antagonist (IL-1ra) would inhibit development of chronic pulmonary hypertension in animal models. Rats were either injected with (60 mg/kg) MCT or exposed to a stimulated high altitude of 16,000 feet; half of the animals were treated with twice-daily injections (2 mg/kg) of IL-1ra. At 3 wk after MCT injection or 3 wk of hypoxic exposure, pulmonary artery pressure and right heart ventricle weight/(left ventricle and septum weight), RV/(LV + S), were measured. IL-1ra treatment reduced pulmonary hypertension and right heart hypertrophy in the MCT model, but not in the chronic hypoxia model. Measurement of lung homogenate IL-1 alpha by radioimmunoassay showed elevated levels in the MCT-treated rats throughout the 3-wk observation period. IL-1ra treatment reduced the levels of IL-1 alpha in lung tissue in most of the MCT-treated rats. MCT treatment was also associated with an increase in lung mRNA for IL-1 alpha, IL-1 beta, and IL-1ra. Immunohistology, using an antibody against rat IL-1 alpha, revealed staining of alveolar structures and of vascular and bronchial smooth muscle. In situ hybridization using a human IL-1 alpha cDNA probe demonstrated increased expression of the IL-1 alpha gene in the lung cells after endotoxin or MCT treatment. Northern blot analysis demonstrated low-level expression of IL-1 alpha mRNA in extracts of normal rat lung and increased expression after endotoxin or MCT treatment. We conclude that chronic treatment with human IL-1ra inhibited the development of pulmonary hypertension in the inflammatory (MCT) model, but not in the chronically hypoxic rats. This result indicates that IL-1 participates in the pathogenesis of some forms of pulmonary hypertension.

Induction of manganese superoxide dismutase in rat cardiac myocytes increases tolerance to hypoxia 24 hours after preconditioning.

Abstract: Manganese superoxide dismutase (Mn-SOD) is induced in ischemic hearts 24 h after ischemic preconditioning, when tolerance to ischemia is acquired. We examined the relationship between Mn-SOD induction and the protective effect of preconditioning using cultured rat cardiac myocytes. Exposure of cardiac myocytes to brief hypoxia (1 h) decreased creatine kinase release induced by sustained hypoxia (3 h) that follows when the sustained hypoxia was applied 24 h after hypoxic preconditioning (57% of that in cells without preconditioning). The activity and content of Mn-SOD in cardiac myocytes were increased 24 h after hypoxic preconditioning (activity, 170%; content, 139% compared with cells without preconditioning) coincidentally with the acquisition of tolerance to hypoxia. Mn-SOD mRNA was also increased 20-40 min after preconditioning. Antisense oligodeoxyribonucleotides corresponding to the initiation site of Mn-SOD translation inhibited the increases in the Mn-SOD content and activity and abolished the expected decrease in creatine kinase release induced by sustained hypoxia after 24 h of hypoxic preconditioning. Sense oligodeoxyribonucleotides did not abolish either Mn-SOD induction or tolerance to hypoxia. These results suggest that the induction of Mn-SOD in myocytes by preconditioning plays a pivotal role in the acquisition of tolerance to ischemia at a later phase (24 h) of ischemic preconditioning.

Role of hypoxic pulmonary vasoconstriction in pulmonary gas exchange and blood flow distribution. 2. Pathophysiology.

Abstract: In this review, the second of a two part series, the analytic techniques introduced in the first part are applied to a broad range of pulmonary pathophysiologic conditions. The contributions of hypoxic pulmonary vasoconstriction to both homeostasis and pathophysiology are quantitated for atelectasis, pneumonia, sepsis, pulmonary embolism, chronic obstructive pulmonary disease and adult respiratory distress syndrome. For each disease state the influence of principle variables, including inspired oxygen concentration, cardiac output and severity of pathology are explored and the actions of selected drugs including inhaled nitric oxide and infused vasodilators are illustrated. It is concluded that hypoxic pulmonary vasoconstriction is often a critical determinant of hypoxemia and/or pulmonary hypertension. Furthermore this analysis demonstrates the value of computer simulation to reveal which of the many variables are most responsible for pathophysiologic results.

The cardiovascular effects of obstructive sleep apnoeas: analysis of pathogenic mechanisms

Abstract: Obstructive sleep apnoeas (OSA) exert immediate marked cardiovascular effects, and may favour the development of systemic and pulmonary hypertension in the long-term. As for the pathogenesis of the acute cardiovascular changes, the first studies high-lighted the role of OSA-induced hypoxia and mechanical changes. However, more recent work pointed to the role played by the arousal reaction terminating OSA, and to the activity of the autonomic nervous system during apnoea and inter-apnoeic phase. As for the pathogenesis of chronic cardiovascular changes, recent findings suggest that the link between OSA and systemic hypertension may be through an abnormal function of the carotid body and underline the importance of chronic intermittent hypoxia versus continuous hypoxia in the development of stable systemic hypertension. On the other hand, OSA do not appear to enhance strongly the development of stable pulmonary hypertension. In this review, we analyze OSA-induced cardiovascular changes with particular emphasis on to the interplay of the possible pathogenic mechanisms involved. Acute OSA-induced cardiovascular alterations during the apnoeic phase appear to result mainly from the mechanical effects of OSA, while during the interapnoeic phase they seem mostly determined by chemical factors (hypoxia, hypercapnia) and by the arousal reaction. In addition, the role of reflex changes elicited by resumption of ventilation should be reconsidered, since lung inflation seems to exert a positive effect on the cardiovascular changes occurring at the end of OSA. This would be in contrast with the inhibitory effects described as "lung inflation reflex", and deserves further study.

Hypoxic vasoconstriction in rat pulmonary and mesenteric arteries

Abstract: Hypoxic vasoconstriction was investigated in isolated pulmonary and mesenteric arteries of the rat. Experiments were performed on large (approximately 2 mm pulmonary, approximately 0.8 mm mesenteric) and small (100-350 microns) arteries. Hypoxia [oxygen partial pressure (PO2) approximately 33 mmHg] elicited a biphasic response in arteries precontracted with prostaglandin F2 alpha (10 microM). A transient contraction reaching a peak within 2-3 min was observed in both large and small pulmonary and mesenteric arteries (phase 1). In pulmonary arteries, this was followed by a slowly developing contraction over 45 min (phase 2). In mesenteric arteries, there was no phase 2 but instead a profound relaxation. Mechanical disruption of the endothelium had no significant effect on phase 1 in preconstricted large pulmonary arteries but reduced phase 1 in small arteries by 40%. Phase 2 was abolished in both large and small arteries. Inhibition of endothelium-derived relaxing factor synthesis or cyclooxygenase pathways had no effect on either phase. Verapamil substantially reduced phase 1 but abolished phase 2. In conclusion, we have found a clear biphasic response to hypoxia in pulmonary arteries of the rat, but, in contrast to some previous reports, phase 1 was only partially dependent on the endothelium, whereas phase 2 was entirely dependent on the endothelium. Small and large arteries had qualitatively similar responses. These results are consistent with the involvement of at least two mechanisms for hypoxic vasoconstriction, one of which may involve release of an as yet unidentified endothelium-derived constrictor factor.

Role of hypoxic pulmonary vasoconstriction in pulmonary gas exchange and blood flow distribution

Abstract: The regulation of the distribution of ventilation/perfusion ratios by hypoxic pulmonary vasoconstriction contributes to both the efficiency of gas exchange and to pulmonary hemodynamics. In this review, the first of a two part series, are summarized the physiologic principles on which the analysis of ventilation/perfusion ratios and of pressure — flow relationships are based. A new combined analysis is introduced that permits the important contributions of hypoxic pulmonary vasoconstriction to overall gas exchange to be demonstrated in the circumstances of clinical complexity.

Inhibitor of nitric oxide synthesis reduces hypoxic-ischemic brain damage in the neonatal rat.

Abstract: We evaluated the neuroprotective effect of the nitric oxide synthesis inhibitor, NG-nitro-L-arginine in a neonatal hypoxic-ischemic rat model. Unilateral hypoxic-ischemic injury was produced in the brain of 7-d-old rats using a combination of a common carotid artery ligation and a hypoxic (8% oxygen) exposure for 2.5 h. In our experimental condition, rectal temperatures did not differ between NG-nitro-L-arginine-treated and saline-injected pups. We killed the animals 72 h later and assessed the hypoxic-ischemic brain damage histologically. NG-nitro-L-arginine (2 mg/kg) administered intraperitoneally 1.5 h before hypoxia resulted in 77% reduction of the infarcted hemispheric volume and 87% reduction of the infarcted striatal volume compared to saline injected controls. NG-nitro-L-arginine given 1.5 h before the insult also significantly prevented hypoxic-ischemic damage in the five hip-pocampal structures examined, dentate gyrus, CA4, CA3, CA1, and subiculum. NG-nitro-L-arginine administered immediately after hypoxia did not prevent hypoxic-ischemic brain damage. These results indicate that nitric oxide plays a key role in producing neonatal hypoxic-ischemic brain damage.

Distribution of NOS in normoxic vs. hypoxic rat lung: upregulation of NOS by chronic hypoxia

Abstract: Expression and localization of nitric oxide synthase (NOS) in the lungs of chronically hypoxic and normoxic rats were studied using both immunohistochemistry and NADPH diaphorase (NADPH-d) staining techniques In the normoxic and in the hypoxic rat, NOS was detected by both methods in the endothelium of large pulmonary vessels and in the epithelium of bronchi and bronchioli NOS expression was not detected in the endothelium of normoxic pulmonary resistance vessels but was prominently expressed in the endothelium of these vessels after 2-4 wk of chronic hypoxia In contrast to small pulmonary vessels, the endothelium of small bronchial vessels exhibited NOS immunostaining in both normoxic and hypoxic lungs Hypoxia was also found to induce de novo NOS expression in the smooth muscle of large and small pulmonary vessels and in bronchial smooth muscle NOS enzyme activity in lung homogenates was assessed by [3H]arginine to [3H]citrulline conversion The activity of soluble NOS, but not particulate NOS, was increased in the hypoxic lungs These results demonstrate chronic hypoxia-induced upregulation of NOS protein expression and activity in the rat lung, suggesting a potentially important role of nitric oxide in adaptation of the pulmonary circulation to chronic hypoxia The lack of immunostaining in small pulmonary resistance vessels is also consistent with physiological studies suggesting that NO may not be involved in the mechanism for maintaining the normally low pulmonary vascular resistance

Continuous inhalation of nitric oxide protects against development of pulmonary hypertension in chronically hypoxic rats.

Abstract: Exposure to hypoxia and subsequent development of pulmonary hypertension is associated with an impairment of the nitric oxide (NO) mediated response to endothelium-dependent vasodilators. Inhaled NO may reach resistive pulmonary vessels through an abluminal route. The aim of this study was to investigate if continuous inhalation of NO would attenuate the development of pulmonary hypertension in rats exposed to chronic hypoxia. In conscious rats previously exposed to 10% O2 for 3 wk, short-term inhalation of NO caused a dose-dependent decrease in pulmonary artery pressure (PAP) from 44 +/- 1 to 32 +/- 1 mmHg at 40 ppm with no changes in systemic arterial pressure, cardiac output, or heart rate. In normoxic rats, acute NO inhalation did not cause changes in PAP. In rats simultaneously exposed to 10% O2 and 10 ppm NO during 2 wk, right ventricular hypertrophy was less severe (P < 0.01), and the degree of muscularization of pulmonary vessels at both alveolar duct and alveolar wall levels was lower (P < 0.01) than in rats exposed to hypoxia alone. Tolerance to the pulmonary vasodilator effect of NO did not develop after prolonged inhalation. Brief discontinuation of NO after 2 wk of hypoxia plus NO caused a rapid increase in PAP. These data demonstrate that prolonged inhalation of low concentrations of NO induces sustained pulmonary vasodilation and reduces pulmonary vascular remodeling in response to chronic hypoxia.

Increased endothelin receptor gene expression in hypoxic rat lung.

Abstract: Our previous studies demonstrated that exposure to hypoxia increases pulmonary artery pressure and plasma endothelin-1 (ET-1) levels and selectively enhances ET-1 gene expression in rat lung. The current study examined the effects of hypoxia (48 h, 10% O2, 1 atm) on ET-1 and endothelin A (ETA) and ETB receptor steady-state mRNA levels in lung, heart, pulmonary artery, thoracic aorta, superior vena cava, kidney, spleen, and liver of the rat. In lung, hypoxic exposure was associated with significant increases in ET-1 mRNA (4.1-fold), ET-1 peptide (1.5-fold) and ETA mRNA (2.3-fold) levels; ETB mRNA levels were unchanged. ET-1 mRNA was increased in response to hypoxia in pulmonary artery but not in aorta; both ETA and ETB receptor steady-state mRNA levels were increased in thoracic aorta, left atrium, and right ventricle, and tended to be increased in right atrium of hypoxia-exposed rats, compared with air controls. ETB but not ETA receptor steady-state mRNA levels were increased in pulmonary artery of hypoxia-exposed rats. No change in expression of either ET receptor steady-state mRNA levels was seen in organs perfused by the systemic vascular bed. In no case were ET receptor mRNA levels in hypoxic rats reduced below air control levels, despite elevations in local and/or circulating ET-1. These findings are consistent with a role for ET-1, acting through ETA receptors, in the pathogenesis of hypoxia-induced pulmonary hypertension.

Evidence for hypoxia‐induced, programmed cell death of cultured neurons

Abstract: Apoptosis, a form of cell death ("programmed" cell death) in which the nucleus and cytoplasm shrink and often fragment, serves to eliminate excessive or unwanted cells during remodeling of embryonic tissues, during organ involution, and in tumor regression. In acute pathological states, such as ischemia, the cells tend to swell and lyse--a process called necrosis. We hypothesize that the delayed neural death clinically associated with hypoxia may, in part, represent apoptosis. A tissue culture model of 24 hours of hypoxia was employed using sympathetic neurons. Pretreatment with an endonuclease inhibitor (aurintricarboxylic acid) decreased cell death by 53%, depolarizing conditions (55 mM potassium chloride) decreased cell death by 33%, and an RNA synthesis inhibitor (actinomycin D) by 26% (all have been shown to prevent apoptosis). Pretreatment with antisense c-myc had no effect. Fluorescent staining with propidium iodide (a DNA marker) demonstrated chromatin condensation and agarose gel electrophoresis demonstrated a DNA "ladder." These data suggest that apoptosis may play a role in hypoxic cell death and that in this paradigm, expression of c-myc is unnecessary. This would suggest a new approach to our understanding of hypoxia and open new strategies to lessen neuronal damage secondary to this process.

Hypoxia selectively excites vasomotor neurons of rostral ventrolateral medulla in rats

Abstract: Systemic hypoxia [PaO2 27.3 +/- 1.8 (SE) mmHg] in anesthetized paralyzed rats reversibly increased within seconds the arterial pressure and activities of the sympathetic nerves and the reticulospinal vasomotor neurons of the rostral ventrolateral medulla (RVL). After peripheral chemodenervation, hypoxia also increased activity of the sympathetic nerves and doubled discharges of the vasomotor neurons while inhibiting a majority of the RVL respiratory neurons. Systemic hypercapnia was not effective in eliciting sympathoexcitatory responses. Iontophoresis of sodium cyanide stimulated the vasomotor and inhibited the respiratory neurons. In contrast, iontophoreses of H+, HCO3-, and lactate were without effects on activity of the vasomotor neurons. We conclude 1) hypoxia excites the vasomotor neurons by activating the arterial chemoreceptors and by activating intrinsic cellular mechanisms probably unrelated to accumulation of metabolic byproducts; 2) hypoxia may be the adequate stimulus exciting the RVL-spinal vasomotor and inhibiting the respiratory neurons during the cerebral ischemic response; and 3) these vasomotor neurons may be central oxygen detectors.

Excess collagen in hypertensive pulmonary arteries decreases vascular distensibility.

Abstract: Increased vascular collagen content is a major feature of pulmonary vascular remodeling. The functional role of excess collagen in decreasing pulmonary vascular compliance has not been established. We determined whether there was a correlation between hydroxyproline content of rat pulmonary artery segments and elastance (EPA) of the pulmonary artery bed during development of hypoxic pulmonary hypertension (10% O2, 10 d) and normoxic recovery. EPA was measured by air-filled pressure-volume curves. After 10 d of hypoxia, hydroxyproline content increased approximately 2-fold in large segments (1,200-250 microns in diameter) but not significantly in small segments (> 250 microns). Elastance increased from 87 +/- 6 (SEM) to 145 +/- 8 mm Hg/ml (p < 0.05) within 5 d of hypoxia and returned to control value 3 wk after recovery. There was a correlation between collagen content and EPA in large segments during development of hypertension; no correlation was found during recovery from hypoxia. The ratio of hydroxyproline to total protein was unchanged in large segments after recovery from hypoxia but was increased in small segments after recovery. We conclude that increased collagen in large pulmonary arteries directly influences EPA during the development of hypoxic pulmonary hypertension.

A neurogenic basis for acute altitude illness

Abstract: Acute altitude illnesses include acute mountain sickness (AMS), a benign condition involving headache, nausea, vomiting, irritability, insomnia, dizziness, lethargy, and peripheral edema, and potentially lethal high-altitude cerebral edema and pulmonary edema (HAPE). Recent evidence is summarized that AMS is related to cerebral edema secondary at least in part to hypoxic cerebral vasodilation and elevated cerebral capillary hydrostatic pressure. This results in reduced brain compliance with compression of intracranial structures in the absence of altered global brain metabolism. It is postulated that these primary intracranial events elevate peripheral sympathetic activity that acts neurogenically in the lung possibly in concert with pulmonary capillary stress failure to cause HAPE and in the kidney to promote salt and water retention. The adrenergic responses are likely modulated by striking increases of aldosterone, vasopressin and atrial natriuretic peptide. The effects of exercise on altitude-induced illness and various therapeutic regimens (acetazolamide, CO2 breathing, dexamethasone, and alpha adrenergic inhibitors) are discussed in light of this hypothesis.

Hypoxia increases glucose transport at blood-brain barrier in rats

Abstract: Prolonged hypoxia causes several adaptive changes in systemic physiology and tissue metabolism. We studied the effects of hypobaric hypoxia on glucose transport at the blood-brain barrier (BBB) in the rat. We found that hypoxia increased the density of brain microvessels seen on immunocytochemical stains using an antibody to the glucose transporting protein GLUT. In addition, we found that hypoxia increased the density of GLUT in isolated cerebral microvessels as determined by specific cytochalasin B binding. The higher GLUT density in isolated cerebral microvessels was evident after 1 wk of hypoxia and was associated with decreased activity of gamma-glutamyltranspeptidase. Consistent with these findings, we also demonstrated that 3 wk of hypobaric hypoxia caused increased unidirectional transport of glucose at the BBB in several brain regions in vivo, as determined by the doubly labeled single-pass indicator-fractionation atrial bolus injection method in anesthetized rats. We conclude that chronic hypobaric hypoxia is associated with increased glucose transport at the BBB.

Hypoxic and ischemic hypoxia exacerbate brain injury associated with metabolic encephalopathy in laboratory animals.

Abstract: Hypoxemia is a major comorbid factor for permanent brain damage in several metabolic encephalopathies. To determine whether hypoxia impairs brain adaptation to hyponatremia, worsening brain edema, we performed in vitro and in vivo studies in cats and rats with hyponatremia plus either ischemic or hypoxic hypoxia. Mortality with hypoxic hypoxia was 0%; with hyponatremia, 22%; and with hyponatremia+hypoxia, 100%. Hyponatremia in cats produced brain edema, with a compensatory decrease of brain sodium. Ischemic hypoxia also resulted in brain edema, but with elevation of brain sodium. However, when ischemic hypoxia was superimposed upon hyponatremia, there was elevation of brain sodium with further elevation of water. Outward sodium transport in cat cerebral cortex synaptosomes was measured via three major pathways through which brain osmolality can be decreased. After hyponatremia, sodium transport was significantly altered such that brain cell osmolality would decrease: 44% increase in Na(+)-K(+)-ATPase transport activity (ouabain inhibitable); 26% decrease in amiloride-sensitive sodium uptake. The change in veratridine-stimulated sodium uptake was not significant (P > 0.05). When ischemic hypoxia was superimposed upon hyponatremia, all of the cerebral adaptive changes induced by hyponatremia alone were eliminated. Thus, hypoxia combined with hyponatremia produces a major increase in brain edema and mortality, probably by eliminating the compensatory mechanisms of sodium transport initiated by hyponatremia that tend to minimize brain swelling.