Production and storage of nitric oxide in adaptation to hypoxia.

Adaptation to hypobaric hypoxia is known to exert multiple protective effects related with nitric oxide (NO). However the effect of adaptation to hypoxia on NO metabolism has remained unclear in many respects. In the present work we studied the interrelation between NO production and storage in the process of adaptation to hypoxia. The NO production was determined by the total nitrite/nitrate concentration in rats plasma. The volume of NO store was evaluated in vitro by the magnitude of isolated aorta relaxation to diethyldithiocarbamate. It was shown that both the nitrite/nitrate level and the NO store increased as adaptation to hypoxia developed. Furthermore, the NO store volume significantly correlated with plasma nitrite/nitrate. Therefore, adaptation to hypoxia stimulates NO production and storage and these effects can potentially underlie NO-dependent beneficial effects of adaptation.     

Nitric oxide is involved in the protective effects of short-term adaptaion to hypoxia in the course of stress-induced disorders in Krushinsky-Molodkina rats

A possible involvement of nitric oxide in the protective effect of short-term adaptation of Krushinsky-Molodkina rats to mild hypoxia simulating 5000 m above sea level was studied. Nitric oxide proved to have a considerable protective effect on stress-induced disorders in Krushinsky-Molodkina rats as demonstrated using NO-synthase inhibitors and NO monitoring by electron spin resonance under different experimental conditions.     

Role of nitric oxide in cardiovascular adaptation to intermittent hypoxia

Hypoxia is one of the most frequently encountered stresses in health and disease. The duration, frequency, and severity of hypoxic episodes are critical factors determining whether hypoxia is beneficial or harmful. Adaptation to intermittent hypoxia has been demonstrated to confer cardiovascular protection against more severe and sustained hypoxia, and, moreover, to protect against other stresses, including ischemia. Thus, the direct and cross protective effects of adaptation to intermittent hypoxia have been used for treatment and prevention of a variety of diseases and to increase efficiency of exercise training. Evidence is mounting that nitric oxide (NO) plays a central role in these adaptive mechanisms. NO-dependent protective mechanisms activated by intermittent hypoxia include stimulation of NO synthesis as well as restriction of NO overproduction. In addition, alternative, nonenzymic sources of NO and negative feedback of NO synthesis are important factors in optimizing NO concentrations. The adaptive enhancement of NO synthesis and/or availability activates or increases expression of other protective factors, including heat shock proteins, antioxidants and prostaglandins, making the protection more robust and sustained. Understanding the role of NO in mechanisms of adaptation to hypoxia will support development of therapies to prevent and treat hypoxic or ischemic damage to organs and cells and to increase adaptive capabilities of the organism.     

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.     

Catecholamines,nitrogen oxide,and resistivity to stressor lesions: effect of adaptation to hypoxia

Pronouncement of stress-induced disturbance of searching behaviour (using "open field" test) and stomach ulceration were compared for the first time with activity of the catecholamine system in hypothalamus and striatum and also with activity of the stress-limiting system of nitric oxide (NO) in the rats of two strains August and Wistar, which differ in their resistance against stress-induced cardiovascular disorders. The effect of prior adaptation to hypobaric hypoxia on these disorders was also studied. August rats appeared to be more resistant than Wistar rats against stress-induced disturbance of the searching behaviour and stomach ulceration. Results of measuring the content of catecholamines in brain structures and the content of NO stable metabolites nitrate/nitrite in plasma suggested that these differences could be due to the stress activation of the nigro-striatal dopaminergic system in August rats, which was not observed in Wistar rats, and also to the higher production of NO in August than in Wistar rats. Adaptation to hypoxia considerably restricted these stress disorders in rats of both strains. Importantly, the protective effects were associated with activation of the nigro-striatal dopaminergic system in all the animals. In the result, adapted Wistar rats, as distinct from non-adapted Wistar rats, displayed a stress activation of this system. The protective effects of adaptation were also accompanied by an increased NO synthesis. Taken together, the data suggest an important role of the responsiveness of the brain dopaminergic system and NO system in the mechanism of resistance against stress-induced disturbances.     

Nitric oxide storage in rats of various genetic strains and its role in the antistressor effect of adaptation to hypoxia

Adaptation to hypobaric hypoxia induced a gradual increase in the NO production along with a progressive NO storage in vascular wall. Unadapted August rats were more resistant against stress-induced stomach ulceration than the Wistar rats. Following a 6-day adaptation rats of both strains revealed a protective antiulcerogenic effect. A long-term adaptation potentiated the stress damage of the stomach rather than protected against it. A higher basal NO production seems to provide a more efficient antistress defence in the August rats. An intense NO storage may create a relative NO shortage and thus predispose to stress-induced vasoconstriction and ulceration.     

NO-dependent mechanisms of adaptation to hypoxia.

In studying NO-dependent mechanisms of resistance to hypoxia, it was shown that (1) acute hypoxia induces NO overproduction in brain and leaves unaffected NO production in liver of rats; (2) adaptation to hypoxia decreases NO production in liver and brain; and (3) adaptation to hypoxia prevents NO overproduction in brain and potentiates NO synthesis in liver in acute hypoxia. Dinitrosyl iron complex (DNIC, 200 microg/kg, single dose, iv), a NO donor, decreases the resistance of animals to acute hypoxia by 30%. Nomega-nitro-L-arginine (L-NNA, 50 mg/kg, single dose, ip), a NO synthase inhibitor, and diethyl dithiocarbamate (DETC, 200 mg/kg, single dose, iv), a NO trap, increases this parameter 1.3 and 2 times, respectively. Adaptation to hypoxia developed against a background of accumulation of heat shock protein HSP70 in liver and brain. A course of DNIC reproduced the antihypoxic effect of adaptation. A course of L-NNA during adaptation hampered both accumulation of HSP70 and development of the antihypoxic effect. Therefore, NO and the NO-dependent activation of HSP70 synthesis play important roles in adaptation to hypoxia.     

Effects of modulation of nitric oxide on rat diaphragm isotonic contractility during hypoxia.

Nitric oxide (NO) is essential for optimal myofilament function of the rat diaphragm in vitro during active shortening. Little is known about the role of NO in muscle contraction under hypoxic conditions. Hypoxia might increase the NO synthase (NOS) activity within the rat diaphragm. We hypothesized that NO plays a protective role in isotonic contractile and fatigue properties during hypoxia in vitro. The effects of the NOS inhibitor N(G)-monomethyl-l-arginine (l-NMMA), the NO scavenger hemoglobin, and the NO donor spermine NONOate on shortening velocity, power generation, and isotonic fatigability during hypoxia were evaluated (Po(2) approximately 7 kPa). l-NMMA and hemoglobin slowed the shortening velocity, depressed power generation, and increased isotonic fatigability during hypoxia. The effects of l-NMMA were prevented by coadministration with the NOS substrate l-arginine. Spermine NONOate did not alter isotonic contractile and fatigue properties during hypoxia. These results indicate that endogenous NO is needed for optimal muscle contraction of the rat diaphragm in vitro during hypoxia.     

The Role of Preventing Nitric Oxide Deficiency in the Antihypertensive Effect of Adaptation to Hypoxia

Shortage of endothelial nitric oxide (NO) manifested as decreased daily urinary excretion of nitrate and nitrite as well as attenuated endothelium-dependent relaxation of conduit and resistance vessels progresses with age-related increase of blood pressure (BP) in stroke-prone spontaneously hypertensive rats (SHRSP). Simultaneous NO-dependent suppression of vascular contractions is, apparently, due to the inducible NO synthase activity in vascular smooth muscle specific for spontaneously hypertensive rat. The adaptation of rats to hypobaric hypoxia initiated at early hypertensive stage (at the age of 5–6 weeks) decelerates hypertension progress. The antihypertensive effect of the adaptation was accompanied by stimulation of endothelial NO synthesis and prevention of impaired NO-dependent response in isolated blood vessels. Nitric oxide stores were formed in the vascular wall of SHRSP and WKY rats at the same time. The obtained data indicate that the correction of endothelial NO deficiency plays a significant role in the antihypertensive effect of adaptation to hypoxia.     

Myocardial contraction in the reverse development of adaptation to short-term exposure to stress

Contractile function of an isolated right atrium was studied in short-term stressor effects-adapted male Wistar rats at different times after adaptation was completed. Adaptation to short-term stressor effects was shown to produce a restricted decrease of myocardial contractility shortly after adaptation was completed. At the 3d day another decrease of contractile function was noted. However, contractile function returned to the control level by the 5th day. At the same time adaptation completely prevented the impairment of myocardial contractility, induced by prolonged stress. The protective effect was seen immediately after adaptation, by days 3 and 5 after it, being reduced by day 10. It is assumed that at the 5th day after adaptation, the animals experience the post-adaptation state marked by disappearance of the negative adaptation effect and by remarkable protective effect of adaptation. As a result, all the characteristics of myocardial contractility evaluated after prolonged stress experienced by the animals at the 5th day following short-term stressor effects do not differ from control parameters.     

The effect of adaptation to stress exposures and to periodic hypoxia on the cardiomyocyte trigger activity of the papillary muscles in the rat]

Action potential of cardiomyocytes was recorded in experiments on isolated papillary muscle of the rat left ventricle. The effect was estimated of preliminary adaptation to intermittent hypobaric hypoxia or to short-term stress exposure on the incidence and the pronouncement of delayed after depolarization and of trigger activity induced by a high frequency stimulation against the background of isoproterenol (10(-8) M). It was shown that adaptation to hypoxia or to stress reduced the incidence of delayed after depolarization and of trigger activity, adaptation to stress exerting a more pronounced effect. Immobilization stress (6 hours) potentiated the trigger activity, adaptation to stress exerting a more pronounced effect. Immobilization stress (6 hours) potentiated the trigger activity and this potentiation was effectively prevented by either type of adaptation.     

Comparative assessment of the effect of adaptation to stress exposure and high altitude hypoxia on heart resistance to reperfusion injury after total ischemia]

Experiments on isolated Wistar rat hearts perfused according to Langendorff showed that adaptation to stress exposure limited the depression of contraction amplitude and contracture and possessed an antiarrhythmic effect in reperfusion. Furthermore, adaptation to stress exposure efficiently limited reperfusion damage to sarcolemma. It was shown that adaptation to hypoxia did not result in any increase in the heart resistance to reperfusion damage following total ischemia. Possible mechanisms of differences in the protective effects of adaptation to stress exposure and hypoxia are discussed.     

Effects of adaptation to intermittent high altitude hypoxia on ischemic ventricular arrhythmias in rats

We compared the effects of adaptation to intermittent high altitude (IHA) hypoxia of various degree and duration on ischemia-induced ventricular arrhythmias in rats. The animals were exposed to either relatively moderate hypoxia of 5000 m (4 or 8 h/day, 2-3 or 5-6 weeks) or severe hypoxia of 7000 m (8 h/day, 5-6 weeks). Ventricular arrhythmias induced by coronary artery occlusion were assessed in isolated buffer-perfused hearts or open-chest animals. In the isolated hearts, both antiarrhythmic and proarrhythmic effects were demonstrated depending on the degree and duration of hypoxic exposure. Whereas the adaptation to 5000 m for 4 h/day decreased the total number of premature ventricular complexes (PVCs), extending the daily exposure to 8 h and/or increasing the altitude to 7000 m led to opposite effects. On the contrary, the open-chest rats adapted to IHA hypoxia exhibited an increased tolerance to arrhythmias that was even more pronounced at the higher altitude. The distribution of PVCs over the ischemic period was not altered by any protocol of adaptation. It may be concluded that adaptation to IHA hypoxia is associated with enhanced tolerance of the rat heart to ischemic arrhythmias unless its severity exceeds a certain upper limit. The opposite effects of moderate and severe hypoxia on the isolated hearts cannot be explained by differences in the occluded zone size, heart rate or degree of myocardial fibrosis. The proarrhythmic effect of severe hypoxia may be related to a moderate left ventricular hypertrophy (27 %), which was present in rats adapted to 7000 m but not in those adapted to 5000 m. This adverse effect can be overcome by an unknown protective mechanism(s) that is absent in the isolated hearts.     

Participation of heart prostaglandins in cardioprotective cross-effects of adaptation to stress and hypoxia

An adaptation to stress and hypoxia was found to increase the baseline PGE content in the myocardium. The data obtained suggest that the activation of the myocardial system of protective PGs is an important link in the cardioprotective effects of adaptation.     

NO-dependent effects during adaptation of rats to intermittent hypoxia

In experiments on Wistar rats processes nitric oxide production on concentration of anions (NO2-, NO3-), carbamide and polyamines contents were investigated in processes of rats adaptation to acute hypoxia (7% O2 in N2, 30 min) and intermittent hypoxia training (10% O2 in N2, 15 min, 5 cycles daily) during 14 days. NO production by oxygen-dependent and oxygen-independent metabolites paths has been investigated. It is concluded that the disturbances in nitric oxide system induced by acute hypoxia by L-arginine injections may result in acute hypoxia.     

Nitric oxide and cell death in liver cancer cells

Nitric oxide (NO) is a lipophillic, highly diffusible, and short-lived physiological messenger which regulates a variety of physiopathological responses. NO may exert its cellular action through cGMP-dependent and cGMP-independent pathways which includes different postranslational modifications. The effect of NO in cancer depends on the activity and localization of NOS isoforms, concentration and duration of NO exposure, cellular sensitivity, and hypoxia/re-oxygenation process. NO regulates critical factors such as the hypoxia inducible factor-1 (HIF-1) and p53 generally leading to growth arrest, apoptosis or adaptation. NO sensitizes hepatoma cells to chemotherapeutic compounds probably through increased p53 and cell death receptor expressions.     

Intrinsic defensive mechanisms in the heart: a potential novel approach to cardiac protection against ischemic injury

Despite recent advances in pharmacotherapy of coronary artery disease and interventional cardiology, the management of myocardial ischemia still remains a major challenge for basic scientists and clinical cardiologists. An urgent need to combat ischemic heart disease, its forms, such as infarction, and complications including sudden cardiac death led to the development of an alternative strategy of myocardial protection based on the exploitation of the heart's own intrinsic protective mechanisms. A new concept relies on the evidence that the heart is able to protect itself by way of adaptation, either short-term or long-term, to transient episodes of stress (e.g., ischemia, hypoxia, free oxygen radicals, heat stress, etc.) preceding sustained ischemia. Preconditioning by brief episodes of ischemia (ischemic preconditioning, IP) represents the most powerful cardioprotective phenomenon. Apart from the short-lasting protection afforded by classical IP or its delayed ("second window") phase, adaptation to long-lasting physiological stimuli or pathological processes is also known to increase myocardial resistance to ischemic injury. Although molecular mechanisms of cardiac adaptation conferring a higher ischemic tolerance still remain not sufficiently elucidated, multiple cascades of intracellular signalization are suggested to be involved in this process. Experimental studies led to the observations that pharmacological modulations at different levels of signal transduction might mimic protective effects of the adaptive phenomena and thus provide a safer way of inducing cardioprotection in humans.     

间歇性低氧适应的心脏保护

间歇性低氧（intermittent hypoxia,IH）是指一定时间间断地暴露于低氧环境,而其余时间处于常氧环境。IH是机体某种生理和病理状态下的低氧形式。研究表明：间歇性低氧适应（IHadaptation）,类似缺血预适应（ischemic preconditioning,IPC）和长期高原低氧适应（long-termhigh-altitude hypoxic adaptation,LHA）,具有明显的心脏保护作用,表现为增强心肌对缺血／再灌注损伤的耐受性、限制心肌梗死面积和形态学改变、抗细胞凋亡、促进缺血／再灌注心脏舒缩功能的恢复,以及抗心律失常。尽管IH对心脏的保护作用不容质疑,但其作用机制远未阐明。IH心脏保护作用可能涉及氧的运输、能量代谢、神经体液调节、抗氧化酶、应激蛋白、腺苷系统、ATP敏感钾通道、线粒体及其钙调控、一氧化氮和蛋白激酶等多方面机制,并受低氧处理方式、动物年龄和性别等因素影响。IH心脏保护持续时间明显长于IPC,而对机体的不良影响远小于LHA,具有潜在的应用价值。     

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

Action potential (AP) of cardiomyocytes was recorded in experiments on isolated perfused according to Langendorf rat hearts. The effect was estimated of preliminary adaptation to intermittent hypobaric hypoxia or to repeated short-term stress exposure on the resting potential (RP) and the amplitude and duration of action potential (APD) in global ischemia and reperfusion. It was shown that adaptation to hypoxia is more effective in prevention of ischemic fall of RP, AP and APD. In reperfusion, the parameters enumerated restored more quickly and efficiently in hearts from adapted to stress animals.     

Hypoxia-induced pulmonary endothelin-1 expression is unaltered by nitric oxide.

Nitric oxide (NO) attenuates hypoxia-induced endothelin (ET)-1 expression in cultured umbilical vein endothelial cells. We hypothesized that NO similarly attenuates hypoxia-induced increases in ET-1 expression in the lungs of intact animals and reasoned that potentially reduced ET-1 levels may contribute to the protective effects of NO against the development of pulmonary hypertension during chronic hypoxia. As expected, hypoxic exposure (24 h, 10% O(2)) increased rat lung ET-1 peptide and prepro-ET-1 mRNA levels. Contrary to our hypothesis, inhaled NO (iNO) did not attenuate hypoxia-induced increases in pulmonary ET-1 peptide or prepro-ET-1 mRNA levels. Because of this surprising finding, we also examined the effects of NO on hypoxia-induced increases in ET peptide levels in cultured cell experiments. Consistent with the results of iNO experiments, administration of the NO donor S-nitroso-N-acetyl-penicillamine to cultured bovine pulmonary endothelial cells did not attenuate increases in ET peptide levels resulting from hypoxic (24 h, 3% O(2)) exposure. In additional experiments, we examined the effects of NO on the activity of a cloned ET-1 promoter fragment containing a functional hypoxia inducible factor-1 binding site in reporter gene experiments. Whereas moderate hypoxia (24 h, 3% O(2)) had no effect on ET-1 promoter activity, activity was increased by severe hypoxic (24 h, 0.5% O(2)) exposure. ET-1 promoter activity after S-nitroso-N-acetyl-penicillamine administration during severe hypoxia was greater than that in normoxic controls, although activity was reduced compared with that in hypoxic controls. These findings suggest that hypoxia-induced pulmonary ET-1 expression is unaffected by NO.