Aminophylline effects on ventilatory response to hypoxia and hyperoxia in normal adults

In 10 normal young adults, ventilation was evaluated with and without pretreatment with aminophylline, an adenosine blocker, while they breathed pure O2 1) after breathing room air and 2) after 25 min of isocapnic hypoxia (arterial O2 saturation 80%). With and without aminophylline, 5 min of hyperoxia significantly increased inspiratory minute ventilation (VI) from the normoxic base line. In control experiments, with hypoxia, VI initially increased and then declined to levels that were slightly above the normoxic base line. Pretreatment with aminophylline significantly attenuated the hypoxic ventilatory decline. During transitions to pure O2 (cessation of carotid bodies' output), VI and breathing patterns were analyzed breath by breath with a moving-average technique, searching for nadirs before and after hyperoxia. On placebo days, at the end of hypoxia, hyperoxia produced nadirs that were significantly lower than those observed with room-air breathing and also significantly lower than when hyperoxia followed normoxia, averaging, respectively, 6.41 +/- 0.52, 8.07 +/- 0.32, and 8.04 +/- 0.39 (SE) l/min. This hypoxic depression was due to significant decrease in tidal volume and prolongation of expiratory time. Aminophylline partly prevented these alterations in breathing pattern; significant posthypoxic ventilatory depression was not observed. We conclude that aminophylline attenuated hypoxic central depression of ventilation, although it does not affect hyperoxic steady-state hyperventilation. Adenosine may play a modulatory role in hypoxic but not in hyperoxic ventilation.     

Hydrogen peroxide production by alveolar macrophages is increased and its concentration is elevated in the breath of rats exposed to hypoxia: relationship to lung lipid peroxidation

Hypoxic exposure triggers a generation of reactive oxygen species that initiate free radical damage to the lung. Hydrogen peroxide is the product of alveolar macrophages detectable in the expired breath. We evaluated the significance of breath H(2)O(2) concentration for the assessment of lung damage after hypoxic exposure and during posthypoxic period. Adult male rats were exposed to normobaric hypoxia (10 % O(2)) for 3 hours or 5 days. Immediately after the hypoxic exposure and then after 7 days or 14 days of air breathing, H(2)O(2) was determined in the breath condensate and in isolated lung macrophages. Lipid peroxidation was measured in lung homogenates. Three-hour hypoxia did not cause immediate increase in the breath H(2)O(2); 5-day hypoxia increased breath H(2)O(2) level to 458 %. After 7 days of subsequent air breathing H2O2 was elevated in both groups exposed to hypoxia. Increased production of H(2)O(2) by macrophages was observed after 5 days of hypoxia and during the 7 days of subsequent air breathing. Lipid peroxidation increased in the periods of enhanced H(2)O(2) generation by macrophages. As the major increase (1040 %) in the breath H(2)O(2) concentration found 7 days after 3 hours of hypoxia was not accompanied by lipid peroxidation, it can be concluded that the breath H(2)O(2) is not a reliable indicator of lung oxidative damage.     

Hydrogen peroxide in the breath of rats: the effects of hypoxia and paraquat

The hypothesis that oxidative stress can be induced by hypoxia was tested by measuring the concentration of hydrogen peroxide by a luminometric technique in the breath samples of rats exposed to hypoxia and paraquat. The group of animals (n=15) exposed to normobaric hypoxia (10% O2) for three days had an increased amount of H2O2 (200%, P<0.001) in their breath in comparison to control animals. After 7 days of recovery in air, the exposed animals still produced significantly increased levels of H2O2 (152%, P<0.001). Paraquat administration was used as a positive control, since it is a redox cycling compound producing free radicals. In the animals treated with a toxic dose of paraquat, the peak H2O2 production was observed 5 h after i.p. injection (156%, P<0.02). Within the next 2 h it decreased to the control level and stayed constant for 48 h, when the animals began to die. It is suggested that H2O2, observed in the breath samples, is a product of a metabolic pathway that could itself be sensitive to oxidative damage.     

Effect of adaptation to hypoxia on the resistance of rats to the epileptogenic action of penicillin

The influence of adaptation to moderate hypoxia on anticonvulsive resistance of low tolerant rats has been investigated. Focal epilepsy was induced by penicillin application to sensorimotor cortex of the rat brain. Adaptation to hypoxia has been shown to increase the resistance of rats to epileptogenic penicillin effect which is manifested in the prolongation of the latent period of epileptiform discharges and less frequent epileptic fits. The mechanisms of the resistance increase remains to be investigated.     

Dynamics of the mass of the right ventricle of the heart and concentration of RNA in it during the process of "reverse" development of hypertrophy]

Experiments were conducted on male rats, 250-300 g in weight. Adaptation to high altitude hypoxia was created by placing the animals daily for 5 hours, into an altitude chamber, at an "altitude" of 6000 m. The degree of hypertrophy of the right ventricle and its RNA content was studied after 20 days of adaptation, as well as 2, 10, 20 and 40 days after cessation of hypoxia. Twenty days after the beginning of adaptation the muscle mass of the right ventricle the RNA concentration and amount in it was found to increase considerably. After cessation of hypoxia half of the acquired increase in the ventricle muscle mass was lost in 10 days, and half of the acquired increase in the RNA--as soon as in 2 days. Forty days after cessation of hypoxia the right ventricle mass and its RNA content in the adapted animals did not differ from the same indices in control rats.     

Adaptation to hypobaric hypoxia involves GABA A receptors in the pons

Survival in low-oxygen environments requires adaptation of sympathorespiratory control networks located in the brain stem. The molecular mechanisms underlying adaptation are unclear. In na?ve animals, acute hypoxia evokes increases in phrenic (respiratory) and splanchnic (sympathetic) nerve activities that persist after repeated challenges (long-term facilitation, LTF). In contrast, our studies show that conditioning rats to chronic hypobaric hypoxia (CHH), an environment characteristic of living at high altitude, diminishes the response to hypoxia and attenuates LTF in a time-dependent manner. Phrenic LTF decreases following 7 days of CHH, and both sympathetic and phrenic LTF disappear following 14 days of CHH. Previous studies demonstrated that GABA is released in the brain stem during hypoxia and depresses respiratory activity. Furthermore, the sensitivity of brain stem neurons to GABA is increased following prolonged hypoxia. In this study, we demonstrate that GABA(A) receptor expression changes along with the CHH-induced physiological changes. Expression of the GABA(A) receptor alpha4 subunit mRNA increases two-fold in animals conditioned to CHH for 7 days. In addition, de novo expression of delta and alpha6, a subunit normally found exclusively in the cerebellum, is observed after 14 days. Consistent with these changes, diazepam-insensitive binding sites, characteristic of GABA(A) receptors containing alpha4 and alpha6 subunits, increase in the pons. Immunohistochemistry revealed that CHH-induced GABA(A) receptor subunit expression is localized in regions of sympathorespiratory control within the pons. Our findings suggest that a GABA(A) receptor mediated-mechanism participates in adaptation of the sympathorespiratory system to hypobaric hypoxia.     

Mild sustained and intermittent hypoxia induce apoptosis in PC-12 cells via different mechanisms

Episodic hypoxia, a characteristic feature of obstructive sleep apnea, induces cellular changes and apoptosis in brain regions associated with neurocognitive function. To investigate whether mild, intermittent hypoxia would induce more extensive neuronal damage than would a similar degree of sustained hypoxia, rat pheochromocytoma PC-12 neuronal cells were subjected to either sustained (5% O₂) or intermittent (alternating 5% O₂ 35 min, 21% O₂ 25 min) hypoxia for 2 or 4 days. Quantitative assessment of apoptosis showed that while mild sustained hypoxia did not significantly increase cell apoptosis at 2 days (1.31 ± 0.29-fold, n = 8; P = NS), a significant increase in apoptosis occurred after 4 days (2.25 ± 0.4-fold, n = 8; P < 0.002), without increased caspase activation. Furthermore, caspase inhibition with the general caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (Z-VAD-FMK) did not modify sustained hypoxia-induced apoptosis. In contrast, mild, intermittent hypoxia induced significant increases in apoptosis at 2 days (3.72 ± 1.43-fold, n = 8; P < 0.03) and at 4 days (4.57 ± 0.82-fold, n = 8; P < 0.001) that was associated with enhanced caspase activity and attenuated by Z-VAD-FMK pretreatment. We conclude that intermittent hypoxia induces an earlier and more extensive apoptotic response than sustained hypoxia and that this response is at least partially dependent on caspase-mediated pathways. In contrast, caspases do not seem to play a role in sustained hypoxia-induced apoptosis. These findings suggest that different signaling pathways are involved in sustained and intermittent hypoxia-induced cell injury and may contribute to the understanding of differential brain susceptibility to sustained and intermittent hypoxia. episodic hypoxia; neuronal cell death; caspase; hypoxic adaptation     

Spleen and thymus lymphoid tissue in hypoxia (a biometric study)

The spleen and thymus have been studied macro- and microscopically in rats (180-200 g body mass) on the 1st, 3d, 5th, 7th, 14th and 28th days of adaptation to a decreased atmospheric pressure in the altitude chamber corresponding to lifting to 5,000 and 7,500 m (after a preliminary gradual acclimatization) and on the 14th, 28th, 42d, 56th days of readaptation. A relative mass of the organs, the white pulp section area--the transversal section area of the spleen ratio, the summation section area of its lymph nodules have been estimated. In the thymus the cortico-medullary index (CMI) has been defined. A relative mass of the spleen increases during the first week of hypoxia, and during adaptation period it somewhat decreases and stabilizes, remaining higher than in the control. At the altitude of 5,000 m the cross section area of the lymph nodules decreases by 17% on the 28th adaptation day and at the altitude of 7,500 m--by 27% beginning from the 14th up to the 28th adaptation days. In the thymus the CMI, after some decrease during the first days of hypoxia at the altitude of 5,000 m, increases and normalizes on the 28th adaptation day, and at the altitude of 7,500 m stabilizes on the 14th - 28th days of hypoxia. When the rats are at the altitudes of 5,000 and 7,500 m the thymus lymphoid tissue reacts more quickly to the hypoxia effect and much sooner normalizes during the readaptation period than does the white pulp of the spleen. The main changes in the lymphoid tissue of the spleen and thymus take place on the 7th - 28th days of hypoxia.     

Effect of adaptation to altitude hypoxia in early ontogeny on the indices of higher nervous activity]

Newborn male and female Wistar rats were adapted to hypoxia in a pressure chamber. Adaptation was started from the "altitude" of 1000 m, 1 hour daily, and then the time and the intensity of hypoxic actions were gradually increased: beginning from the 17th day the animals were subjected to adaptation to the "altitude" of 5000 m, for 5 hours, 5 days a week. The defence conditioned reflex of active avoidance was provoked in the animals after 2-month adaptation. A tendency to a more rapid elaboration of the reflex and a markedly enhanced degree of its retention in comparison with control was revealed in the adapted males. No significant changes in the elaboration and rentention of the reflex were found in the female animals adapted to hypoxia under analogous conditions.     

Tyrosine hydroxylase expression and activity in the rat brain: differential regulation after long-term intermittent or sustained hypoxia.

Tyrosine hydroxylase, a hypoxia-regulated gene, may be involved in tissue adaptation to hypoxia. Intermittent hypoxia, a characteristic feature of sleep apnea, leads to significant memory deficits, as well as to cortex and hippocampal apoptosis that are absent after sustained hypoxia. To examine the hypothesis that sustained and intermittent hypoxia induce different catecholaminergic responses, changes in tyrosine hydroxylase mRNA, protein expression, and activity were compared in various brain regions of male rats exposed for 6 h, 1 day, 3 days, and 7 days to sustained hypoxia (10% O(2)), intermittent hypoxia (alternating room air and 10% O(2)), or normoxia. Tyrosine hydroxylase activity, measured at 7 days, increased in the cortex as follows: sustained > intermittent > normoxia. Furthermore, activity decreased in the brain stem and was unchanged in other brain regions of sustained hypoxia-exposed rats, as well as in all regions from animals exposed to intermittent hypoxia, suggesting stimulus-specific and heterotopic catecholamine regulation. In the cortex, tyrosine hydroxylase mRNA expression was increased, whereas protein expression remained unchanged. In addition, significant differences in the time course of cortical Ser(40) tyrosine hydroxylase phosphorylation were present in the cortex, suggesting that intermittent and sustained hypoxia-induced enzymatic activity differences are related to different phosphorylation patterns. We conclude that long-term hypoxia induces site-specific changes in tyrosine hydroxylase activity and that intermittent hypoxia elicits reduced tyrosine hydroxylase recruitment and phosphorylation compared with sustained hypoxia. Such changes may not only account for differences in enzyme activity but also suggest that, with differential regional brain susceptibility to hypoxia, recruitment of different mechanisms in response to hypoxia will elicit region-specific modulation of catecholamine response.     

Dizocilpine pretreatment suppresses the action of hypoxia on hippocampal epileptic afterdischarges in immature rats

Effect of dizocilpine (0.5 mg/kg i.p.) on epileptic afterdischarges elicited by low-frequency electrical stimulation of the dorsal hippocampus was studied in rat pups aged 12 and 18 days. Repeated elicitation of afterdischarges (ADs) in control animals resulted in a progressive increase of the duration of ADs in both age groups. Dizocilpine (MK-801) injected after the first afterdischarge suppressed this prolongation in 12-day-old rats only. Hypobaric hypoxia (simulated altitude of 9000 m for one hour) led to a marked prolongation of the first afterdischarge in both age groups with a tendency to shorter ADs after repeated stimulations. Dizocilpine potentiated this tendency in 12-day-old rat pups so that it became statistically significant. Administration of dizocilpine before hypoxia prevented the increase in duration of the first afterdischarge in both age groups.     

Correction of NO-dependent cardiovascular disorders by adaptation to hypoxia

Spontaneously hypertensive rats (SHR-SP) were adapted to intermittent hypobaric hypoxia in an altitude chamber for 40 days. The adaptation to hypoxia prevented an excessive endothelium-dependent relaxation and hypotension characteristic of myocardial infarction. The adaptation also attenuated the increase in blood pressure and prevented impairment of the endothelium-dependent relaxation in SHR-SP. The universal nature of the adaptation allows to use it for correcting many cardiovascular disorders related to diverse alterations of NO metabolism.     

Role of nitric oxide in adaptation to hypoxia and adaptive defense

Adaptation to hypoxia is beneficial in cardiovascular pathology related to NO shortage or overproduction. However, the question about the influence of adaptation to hypoxia on NO metabolism has remained open. The present work was aimed at the relationship between processes of NO production and storage during adaptation to hypoxia and the possible protective significance of these processes. Rats were adapted to intermittent hypobaric hypoxia in an altitude chamber. NO production was determined by plasma nitrite/nitrate level. Vascular NO stores were evaluated by relaxation of the isolated aorta to diethyldithiocarbamate. Experimental myocardial infarction was used as a model of NO overproduction; stroke-prone spontaneously hypertensive rats (SHR-SP) were used as a model of NO shortage. During adaptation to hypoxia, the plasma nitrite/nitrate level progressively increased and was correlated with the increase in NO stores. Adaptation to hypoxia prevented the excessive endothelium-dependent relaxation and hypotension characteristic for myocardial infarction. At the same time, the adaptation attenuated the increase in blood pressure and prevented the impairment of endothelium-dependent relaxation in SHR-SP. The data suggest that NO stores induced by adaptation to hypoxia can either bind excessive NO to protect the organism against NO overproduction or provide a NO reserve to be used in NO deficiency.     

Inducible nitric oxide synthase expression and plasma bilirubin changes in rats under intermittent hypoxia treatment

It has been reported that intermittent hypoxia treatment prevents oxidative injuries to the brain and protects the heart against ischemia-reperfusion injury. Both anti-oxidative defensive systems and prevention of free intracellular calcium overload might be the result of intermittent hypoxia. Thus, the purpose of this study was to explore the effects of intermittent hypoxia (8 h at 12 % O2 per day) for 0, 7 or 14 days on inducible nitric oxide synthase (iNOS) expression in the spleen and on splenic calcium response to the mitogen phytohemagglutinin (PHA). The results demonstrated that administration of intermittent hypoxia for 7 days caused severe hemolysis of erythrocytes in the spleen and the hemolytic condition was ameliorated by intermittent hypoxia for 14 days. However, a significant decline in splenic weight and an increase in plasma total bilirubin levels appeared in rats after hypoxia for 14 days. No calcium response to PHA was observed in splenocytes obtained from rats after intermittent hypoxia for 7 days. After intermittent hypoxia for 14 days, the calcium response to PHA was restored to the level of the controls. Intermittent hypoxia for 7 days was able to induce higher iNOS expression in splenic tissues than hypoxia for 14 days. These results suggested that intermittent hypoxia for 14 days appeared to involve acclimatization that protects the rats from oxidative injury through less hemolysis and iNOS expression in splenic tissues and by the presence of more bilirubin in the plasma. The increase in plasma total bilirubin levels might be the cause of induced adaptation to chronic intermittent hypoxia.     

Increase of alpha 1-adrenoreactivity of the rat heart in adaptation to periodic hypoxia]

There is a possibility that the cardioprotective effect of adaptation to intermittent hypoxia is due to changes in receptors apparatus of the heart. In this connection the effect of preliminary adaptation to intermittent hypoxia (4 hours per day at the altitude of 4000 m during 40 days) on the state of beta-receptors-adenylate-cyclase system and same other receptors of the heart were studied. It was shown that at the end of the course of adaptation the number of beta-adrenoceptors in the heart was increased with simultaneous decrease in basal adenylate-cyclase activity, accompanied by the diminution of its response to beta-agonist. The number of beta-adrenoceptors was increased by 48% and their affinity to ligand was increased by almost 2 times. The revealed decrease in the reactivity of beta-receptor-adenylate-cyclase system and increase of alpha 1-adrenoreactivity can play a certain role in the mechanism of cardioprotective effect of adaptation to hypoxia.     

Prevention of the depression of normal killer activity in stress via adaptation to the periodic action of hypoxia

It has been demonstrated that progressive adaptation of BALB/c mice for 28 days to periodic action of pressure chamber hypoxia prevents the stress-induced depression of normal killer activity. Moreover, preadaptation to hypoxia reverses the stress-provoked inhibition of DNA synthesis in the thymic and spleen cells.     

Metabolic response of river birch and European birch and European birch roots to hypoxia

Flood tolerance of woody plants has been attributed to internal oxygen diffusion from shoot to root, metabolic adaptation within the root, or both. The purpose of this study was to compare several biochemical and physiological responses of birch roots to hypoxia in order to determine the nature of root metabolic adaptation to low oxygen tension. One-year-old seedlings of flood-tolerant river birch (Betula nigra L.) and flood-intolerant European birch (Betula pendula Roth) were transferred to solution culture, and the solutions were bubbled with air or nitrogen. After 18 days of hypoxia, total adenosine phosphate and ATP contents of river birch roots were 35% and 23% of controls, respectively, whereas those of European birch roots were 13% and 8%. Adenylate energy charge of river birch roots decreased between 6 and 12 days of hypoxia. In contrast, energy charge of European birch roots decreased after only 1 day of hypoxia. In vitro activity of cytochrome c oxidase and oxygen consumption capacity of excised roots from both birch species decreased under hypoxia. In vitro activity of alcohol dehydrogenase from roots of both species increased after 1 day of hypoxia. However, alcohol dehydrogenase activity from river birch roots increased 25-fold after 6 days of hypoxia, whereas that from European birch decreased back to control levels. Hypoxia decreased malate content of roots from both species. Metabolic adaptation within the root, rather than internal oxygen diffusion, appears to be responsible for the relative tolerance of river birch to hypoxia.     

Changes in conditioned reflex activity during adaptation of the rat to pressure-chamber and high-altitude hypoxia

A group of rats with a stereotype of conditioned reflexes was preliminarily trained to hypoxia effects during 30 days (at the "altitude" of 6000 m, time of exhibition--from 10 to 60 min, for 18 days--only 60 min). Adaptive changes in the process of training consisted in a weakening of differentiation inhibition, partial amnesia of the conditioned reaction of active avoidance and appearance of phasic states (equalization and paradoxical phases) in the cerebral cortex. The following adaptation of hypoxia "trained" rats to new natural conditions of Alpine altitude (3200 m) proceeded favourably, without disturbance of differentiation inhibition and without phasic states. Rats without preliminary training to altitude chamber hypoxia, in mountains (3200 m) were subjected to moderate tension resulting in protective inhibition, partial amnesia and transient disturbance of differentiation inhibition.     

慢性缺氧大鼠肺动脉内皮依赖性舒张反应与内皮结构的动态变化

本实验对慢性减压缺氧（５０００ｍ）过程中肺动脉ＡＣｈ内皮依赖性舒张反应作了动态观察,并结合分析了其与内皮超微结构和肺动脉压演变的关系。结果表明,缺氧３─２１ｄ,平均肺动脉压（ｍＰＡＰ）显著递增（Ｐ＜０．０５─０．００１）,而缺氧４０ｄ组基本与缺氧２１ｄ组持平,未再进一步升高。缺氧１ｄ组,各ＡＣｈ浓度（１０－１０、１０－９、１０－７、１０－６、１０－５ｍｏｌ／Ｌ）引起的内皮依赖性舒张反应明显受抑（Ｐ＜０．０５─０．００１）。缺氧７ｄ组,舒张反应的受抑程度与缺氧１ｄ组基本相同;但ＡＣｈ１０－５ｍｏｌ／Ｌ引发的反应则较缺氧１ｄ时更弱。缺氧２１ｄ和４０ｄ组,ＡＣｈ１０－６和１０－５ｍｏｌ／Ｌ引起的舒张反应,尽管仍显著低于对照,但却基本上高于缺氧１ｄ和７ｄ组。其余各浓度ＡＣｈ引发的反应则已趋于恢复至对照水平。电镜观察,缺氧１─１４ｄ肺动脉内皮呈逐渐加重的水肿变性;缺氧２１─４０ｄ内皮水肿消失,代之出现渐趋活跃的内皮增生。结果提示,随缺氧时间延长,因内皮从损伤逐渐加重到出现代偿适应,可能存在相应的内皮舒张因子由释放减少到有所恢复的动态变化过程,并对整体肺动脉压有一定程度的影响。     

Effects of adaptation to periodic hypoxia on bioelectric activity of cardiomyocytes of isolated heart in ischemia and reperfusion

Experiments on isolated Wistar rat heart perfused according to Langendorff showed that preliminary adaptation of rats to intermittent hypobaric hypoxia limited the fall of values of the resting potential and the amplitude and duration of action potential characteristic for ischemia. Under similar conditions, adaptation considerably reduced the increased time of impulse conduction along the myocardium. In reperfusion, the parameters enumerated restored much more efficiently in hearts from adapted animals than in controls. The role of these changes in the antiarrhythmic effect of adaptation to intermittent hypoxia is under discussion.