Decrease in Infarct-Limiting Effect of Chronic Normobaric Hypoxia in Rats with Induced Metabolic Syndrome Is Associated with Disturbances of Carbohydrate and Lipid Metabolism

We studied the infarct-limiting effect of adaptation to chronic normobaric hypoxia in rats with induced metabolic syndrome and the relationship between disturbances of adaptive cardioprotection and disorders of carbohydrate and lipid metabolism. Adaptation to chronic normobaric hypoxia was carried out for 21 days at 12% O₂ and 0.3% CO₂. The metabolic syndrome was modeled with a high-carbohydrate high-fat diet for 84 days with replacement of drinking water with a 20% fructose solution. The infarct size in rats exposed to chronic normobaric hypoxia was 38% smaller than in control animals. In rats with induced metabolic syndrome, hypertension, obesity, decreased glucose tolerance, increased serum triglyceride, and no infarction-limiting effect of chronic normobaric hypoxia were observed. Infarct size showed a direct correlation with impaired glucose tolerance and serum triglyceride levels. The study allows us to conclude that the lack of cardioprotection in chronic normobaric hypoxia in rats with induced metabolic syndrome is associated with impaired carbohydrate and lipid metabolism.

The Role of Mitochondrial KATP Channels in the Infarct-Reducing Effect of Normobaric Hypoxia

We studied the role of K_ATP channels in the infarct-limiting effect of short-term normobaric hypoxia. Male Wistar rats were subjected to a 45-min coronary artery occlusion followed by a 120-min reperfusion. Normobaric hypoxia was simulated 30 min before coronary artery occlusion: 6 sessions of hypoxia (8% O₂, 10 min) and reoxygenation (21% O₂, 10 min). The following drugs were administered to rats: glibenclamide, 5-hydroxydecanoate, and HMR1098. It was found that normobaric hypoxia contributes to a decrease in myocardial infarct size by 36%. Preliminary administration of glibenclamide or 5-hydroxydecanoate eliminated the infarct-reducing effect of normobaric hypoxia. Activator of mitochondrial K_ATP channel diazoxide limited the infarct size. These findings suggest that mitochondrial K_ATP channels are involved into the cardioprotective effect of normobaric hypoxia.

Signaling Role of Mitochondrial Enzymes and Ultrastructure in the Formation of Molecular Mechanisms of Adaptation to Hypoxia

This study was the first comprehensive investigation of the dependence of mitochondrial enzyme response (catalytic subunits of mitochondrial complexes (MC) I-V, including NDUFV2, SDHA, Cyt b, COX1 and ATP5A) and mitochondrial ultrastructure in the rat cerebral cortex (CC) on the severity and duration of in vivo hypoxic exposures. The role of individual animal's resistance to hypoxia was also studied. The respiratory chain (RC) was shown to respond to changes in environmental [O2] as follows: (a) differential reaction of mitochondrial enzymes, which depends on the severity of the hypoxic exposure and which indicates changes in the content and catalytic properties of mitochondrial enzymes, both during acute and multiple exposures; and (b) ultrastructural changes in mitochondria, which reflect various degrees of mitochondrial energization. Within a specific range of reduced O2 concentrations, activation of the MC II is a compensatory response supporting the RC electron transport function. In this process, MC I develops new kinetic properties, and its function recovers in hypoxia by reprograming the RC substrate site. Therefore, the mitochondrial RC performs as an in vivo molecular oxygen sensor. Substantial differences between responses of rats with high and low resistance to hypoxia were determined.

Coronary and cerebral metabolism-blood flow coupling and pulmonary alveolar ventilation-blood flow coupling may be disabled during acute carbon monoxide poisoning

Current evidence indicates that the toxicity of carbon monoxide (CO) poisoning results from increases in reactive oxygen species (ROS) generation plus tissue hypoxia resulting from decreases in capillary Po₂ evoked by effects of increases in blood [carboxyhemoglobin] on the oxyhemoglobin dissociation curve. There has not been consideration of how increases in Pco could influence metabolism-blood flow coupling, a physiological mechanism that regulates the uniformity of tissue Po₂, and alveolar ventilation-blood flow coupling, a mechanism that increases the efficiency of pulmonary O₂ uptake. Using published data, I consider hypotheses that these coupling mechanisms, triggered by O₂ and CO sensors located in arterial and arteriolar vessels in the coronary and cerebral circulations and in lung intralobar arteries, are disrupted during acute CO poisoning. These hypotheses are supported by calculations that show that the Pco in these vessels can reach levels during CO poisoning that would exert effects on signal transduction molecules involved in these coupling mechanisms.NEW & NOTEWORTHY This article introduces and supports a postulate that the tissue hypoxia component of carbon monoxide poisoning results in part from impairment of physiological adaptation mechanisms whereby tissues can match regional blood flow to O₂ uptake, and the lung can match regional blood flow to alveolar ventilation.

Genome-wide adaptive evolution to underground stresses in subterranean mammals: Hypoxia adaption, immunity promotion, and sensory specialization

Life underground has provided remarkable examples of adaptive evolution in subterranean mammals; however, genome-wide adaptive evolution to underground stresses still needs further research. There are approximately 250 species of subterranean mammals across three suborders and six families. These species not only inhabit hypoxic and dark burrows but also exhibit evolved adaptation to hypoxia, cancer resistance, and specialized sensory systems, making them an excellent model of evolution. The adaptive evolution of subterranean mammals has attracted great attention and needs further study. In the present study, phylogenetic analysis of 5,853 single-copy orthologous gene families of five subterranean mammals (Nannospalax galili, Heterocephalus glaber, Fukomys damarensis, Condylura cristata, and Chrysochloris asiatica) showed that they formed fou distinct clusters. This result is consistent with the traditional systematics of these species. Furthermore, comparison of the high-quality genomes of these five subterranean mammalian species led to the identification of the genomic signatures of adaptive evolution. Our results show that the five subterranean mammalian did not share positively selected genes but had similar functional enrichment categories, including hypoxia tolerance, immunity promotion, and sensory specialization, which adapted to the environment of underground stresses. Moreover, variations in soil hardness, climate, and lifestyles have resulted in different molecular mechanisms of adaptation to the hypoxic environment and different degrees of visual degradation. These results provide insights into the genome-wide adaptive evolution to underground stresses in subterranean mammals, with special focus on the characteristics of hypoxia adaption, immunity promotion, and sensory specialization response to the life underground.

Oxygen sensing by the carotid body: mechanisms and role in adaptation to hypoxia

Oxygen (O2) is fundamental for cell and whole-body homeostasis. Our understanding of the adaptive processes that take place in response to a lack of O2(hypoxia) has progressed significantly in recent years. The carotid body (CB) is the main arterial chemoreceptor that mediates the acute cardiorespiratory reflexes (hyperventilation and sympathetic activation) triggered by hypoxia. The CB is composed of clusters of cells (glomeruli) in close contact with blood vessels and nerve fibers. Glomus cells, the O2-sensitive elements in the CB, are neuron-like cells that contain O2-sensitive K(+)channels, which are inhibited by hypoxia. This leads to cell depolarization, Ca(2+)entry, and the release of transmitters to activate sensory fibers terminating at the respiratory center. The mechanism whereby O2modulates K(+)channels has remained elusive, although several appealing hypotheses have been postulated. Recent data suggest that mitochondria complex I signaling to membrane K(+)channels plays a fundamental role in acute O2sensing. CB activation during exposure to low Po2is also necessary for acclimatization to chronic hypoxia. CB growth during sustained hypoxia depends on the activation of a resident population of stem cells, which are also activated by transmitters released from the O2-sensitive glomus cells. These advances should foster further studies on the role of CB dysfunction in the pathogenesis of highly prevalent human diseases.

Cardioprotective and nonprotective regimens of chronic hypoxia diversely affect the myocardial antioxidant systems

It has been documented that adaptation to hypoxia increases myocardial tolerance to ischemia-reperfusion (I/R) injury depending on the regimen of adaptation. Reactive oxygen species (ROS) formed during hypoxia play an important role in the induction of protective cardiac phenotype. On the other hand, the excess of ROS can contribute to tissue damage caused by I/R. Here we investigated the relationship between myocardial tolerance to I/R injury and transcription activity of major antioxidant genes, transcription factors, and oxidative stress in three different regimens of chronic hypoxia. Adult male Wistar rats were exposed to continuous normobaric hypoxia (FiO2 0.1) either continuously (CNH) or intermittently for 8 h/day (INH8) or 23 h/day (INH23) for 3 wk period. A control group was kept in room air. Myocardial infarct size was assessed in anesthetized open-chest animals subjected to 20 min coronary artery occlusion and 3 h reperfusion. Levels of mRNA transcripts and the ratio of reduced and oxidized glutathione (GSH/GSSG) were analyzed by real-time RT-PCR and by liquid chromatography, respectively. Whereas CNH as well as INH8 decreased infarct size, 1 h daily reoxygenation (INH23) abolished the cardioprotective effect and decreased GSH/GSSG ratio. The majority of mRNAs of antioxidant genes related to mitochondrial antioxidant defense (manganese superoxide dismutase, glutathione reductase, thioredoxin/thioredoxin reductase, and peroxiredoxin 2) were upregulated in both cardioprotective regimens (CNH, INH8). In contrast, INH23 increased only PRX5, which was not sufficient to induce the cardioprotective phenotype. Our results suggest that the increased mitochondrial antioxidant defense plays an important role in cardioprotection afforded by chronic hypoxia.

Immediate and long-term responses of the carotid body to high altitude

High altitude and the decreased environmental oxygen pressure have both immediate and chronic effects on the carotid body. An immediate effect is to limit the oxygen available for mitochondrial oxidative phosphorylation, and this leads to increased activity on the afferent nerves leading to the brain. In the isolated carotid body preparation, the afferent nerve activity depends on the ratio of carbon monoxide (CO), an inhibitor of respiratory chain function, to oxygen. The CO-induced increase in afferent neural activity is reversed by light, and the wavelength dependence of this reversal shows that the site of CO (and therefore oxygen) interaction is cytochrome a3 of the mitochondrial respiratory chain. Thus, primary sensing of ambient oxygen pressure is through the oxygen dependence of mitochondrial oxidative phosphorylation. The conductance of ion channels in the cellular membranes may also be sensitive to oxygen pressure and, through this, modulate the sensitivity to oxygen pressure. Longer-term exposure to high altitude results in progressive changes in the carotid body that involve several mechanisms, including cellular energy metabolism and hypoxia inducible factor-1alpha (HIF-1alpha). These changes begin within minutes of exposure, but progress such that chronic exposure results in morphological and biochemical alterations in the carotid body, including enlarged cells, increased catecholamine levels, altered cellular appearance, and others. In the chronically adapted carotid body, responses to acute changes in oxygen pressure are enhanced. The adaptive changes due to chronic hypoxia are largely reversed upon return to lower altitudes.