Renin–Angiotensin–Aldosterone System Is Not Involved in the Arterial Stiffening Induced by Acute and Prolonged Exposure to High Altitude

Impact of moderate altitude exposure on cardiovascular risk

Acute exposure to extreme altitude increases arterial stiffness by activation of the sympathetic and endothelin system as well as hypoxia-induced reactive oxygen species production. Beyond a certain individual threshold, these physiological adaptations represent a relevant cardiovascular risk factor. In this pilot study we investigated to what extent temporary exposure to moderate altitude, as present during hiking, skiing or in aeroplanes, leads to changes in vascular tone. Pulse wave parameters of 8 healthy individuals were assessed with a BR-102 plus pulse wave analyser (Schiller, Germany) at baseline (521 m) and after 24 h exposure to moderate altitude (2650 m). We identified a significant increase in heart rate (61 vs. 68/min, p = 0.021) as well as changes in central (35.6 vs. 41.4 mmHg, p = 0.024) and peripheral pulse pressure (44.7 vs. 52.6 mmHg, p = 0.006). Amplitudes of forward (21.6 vs. 25.4 mmHg, p = 0.012) and backward pulse waves (15.3 vs. 17.6 mmHg, p = 0.043) were significantly elevated. Pulse wave velocity showed no significant change from 5.8 m/s at baseline to 6.1 m/s at moderate altitude (p = 0.056). We show that temporary exposure to moderate altitude leads to mild changes in vascular tone reflected by pulse pressure and pulse wave amplitude in healthy adults. Although the observed effects were mild in our study, it indicates that adaptation capacity is of crucial importance and any restrictions by disease or simply with the process of ageing demand increased awareness, even in moderate altitude.

Kidney Renin Release under Hypoxia and Its Potential Link with Nitric Oxide: A Narrative Review

The renin-angiotensin system (RAS) and hypoxia have a complex interaction: RAS is activated under hypoxia and activated RAS aggravates hypoxia in reverse. Renin is an aspartyl protease that catalyzes the first step of RAS and tightly regulates RAS activation. Here, we outline kidney renin expression and release under hypoxia and discuss the putative mechanisms involved. It is important that renin generally increases in response to acute hypoxemic hypoxia and intermittent hypoxemic hypoxia, but not under chronic hypoxemic hypoxia. The increase in renin activity can also be observed in anemic hypoxia and carbon monoxide-induced histotoxic hypoxia. The increased renin is contributed to by juxtaglomerular cells and the recruitment of renin lineage cells. Potential mechanisms regulating hypoxic renin expression involve hypoxia-inducible factor signaling, natriuretic peptides, nitric oxide, and Notch signaling-induced renin transcription.

New strategy for rational use of antihypertensive drugs in clinical practice in high-altitude hypoxic environments

High-altitude hypoxic environments have critical implications on cardiovascular system function as well as blood pressure regulation. Such environments place patients with hypertension at risk by activating the sympathetic nervous system, which leads to an increase in blood pressure. In addition, the high-altitude hypoxic environment alters the in vivo metabolism and antihypertensive effects of antihypertensive drugs, which changes the activity and expression of drug-metabolizing enzymes and drug transporters. The present study reviewed the pharmacodynamics and pharmacokinetics of antihypertensive drugs and its effects on patients with hypertension in a high-altitude hypoxic environment. It also proposes a new strategy for the rational use of antihypertensive drugs in clinical practice in high-altitude hypoxic environments. The increase in blood pressure on exposure to a high-altitude hypoxic environment was mainly dependent on increased sympathetic nervous system activity. Blood pressure also increased proportionally to altitude, whilst ambulatory blood pressure increased more than conventional blood pressure, especially at night. High-altitude hypoxia can reduce the activities and expression of drug-metabolizing enzymes, such as CYP1A1, CYP1A2, CYP3A1, and CYP2E1, while increasing those of CYP2D1, CYP2D6, and CYP3A6. Drug transporter changes were related to tissue type, hypoxic degree, and hypoxic exposure time. Furthermore, the effects of high-altitude hypoxia on drug-metabolism enzymes and transporters altered drug pharmacokinetics, causing changes in pharmacodynamic responses. These findings suggest that high-altitude hypoxic environments affect the blood pressure, pharmacokinetics, and pharmacodynamics of antihypertensive drugs. The optimal hypertension treatment plan and safe and effective medication strategy should be formulated considering high-altitude hypoxic environments.

A preliminary study of the salivary microbiota of young male subjects before, during, and after acute high-altitude exposure

Background

The microbial community structure in saliva differs at different altitudes. However, the impact of acute high-altitude exposure on the oral microbiota is unclear. This study explored the impact of acute high-altitude exposure on the salivary microbiome to establish a foundation for the future prevention of oral diseases. Methods. Unstimulated whole saliva samples were collected from 12 male subjects at the following three time points: one day before entering high altitude (an altitude of 350 m, pre-altitude group), seven days after arrival at high altitude (an altitude of 4,500 m, altitude group) and seven days after returning to low altitude (an altitude of 350 m, post-altitude group). Thus, a total of 36 saliva samples were obtained. 16S rRNA V3-V4 region amplicon sequencing was used to analyze the diversity and structure of the salivary microbial communities, and a network analysis was employed to investigate the relationships among salivary microorganisms. The function of these microorganisms was predicted with a Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analysis.

Results

In total, there were 756 operational taxonomic units (OTUs) identified, with 541, 613, and 615 OTUs identified in the pre-altitude, altitude, and post-altitude groups, respectively. Acute high-altitude exposure decreased the diversity of the salivary microbiome. Prior to acute high-altitude exposure, the microbiome mainly consisted of Proteobacteria, Firmicutes, Bacteroidetes, Fusobacteria, and Actinobacteria. After altitude exposure, the relative abundance of Streptococcus and Veillonella increased, and the relative abundance of Prevotella, Porphyromonas, and Alloprevotella decreased. The relationship among the salivary microorganisms was also affected by acute high-altitude exposure. The relative abundance of carbohydrate metabolism gene functions was upregulated, while the relative abundance of coenzyme and vitamin metabolism gene functions was downregulated.

Conclusion

Rapid high-altitude exposure decreased the biodiversity of the salivary microbiome, changing the community structure, symbiotic relationships among species, and abundance of functional genes. This suggests that the stress of acute high-altitude exposure influenced the stability of the salivary microbiome.

Nocturnal hypoxemia, blood pressure, vascular status and chronic mountain sickness in the highest city in the world

INTRODUCTION

Chronic mountain sickness (CMS) is a condition characterized by excessive erythrocytosis in response to chronic hypobaric hypoxia. CMS frequently triggers cardiorespiratory diseases such as pulmonary hypertension and right or left heart failure. Ambient hypoxia might be further amplified night-time by intermittent hypoxia related to sleep-disordered breathing (SDB) so that sleep disturbance may be an important feature of CMS. Our aim was to characterize in a cross-sectional study nocturnal hypoxaemia, SDB, blood pressure (BP), arterial stiffness and carotid intima-media thickness (CIMT) in highlanders living at extreme altitude.

METHODS

Men aged 18 to 55 years were prospectively recruited. Home sleep apnoea test, questionnaires (short-form health survey; Montreal cognitive assessment; Pittsburgh Sleep Questionnaire Index and the Insomnia severity index), 24-h ambulatory BP monitoring, CIMT and arterial stiffness were evaluated in 3 groups: i) Andean lowlanders (sea-level); ii) highlanders living at 3,800 m and iii) highlanders living at 5,100 m. Analyses were conducted in sub-groups according to 1) CMS severity 2) healthy subjects living at the three different altitude.

RESULTS

Ninety-two males were evaluated at their living altitudes. Among the 54 highlanders living at 5,100 m, subjects with CMS showed lower mean nocturnal oxygen saturation (SpO₂), SpO₂ nadir, lower pulse wave velocity and higher nocturnal BP variability than those with no-CMS. Lower nocturnal SpO₂ nadir was associated with higher CMS severity (ß= -0.14, p=.009). Among the 55 healthy subjects, healthy highlanders at 5,100 m were characterized by lower scores on quality of life and sleep quality scales and lower mean SpO₂ compared to lowlanders.

CONCLUSIONS

Lower nocturnal SpO₂ and higher nocturnal BP variability are associated with CMS severity in individuals living permanently at high altitude. The role of lower SpO₂ and higher nocturnal BP variability in the cardiovascular progression of CMS and in the overall prognosis of the disease need to be evaluated in further studies.

Haemodynamic Adaptive Mechanisms at High Altitude: Comparison between European Lowlanders and Nepalese Highlanders

Background: Exposure to high altitudes determines several adaptive mechanisms affecting in a complex way the whole cardiovascular, respiratory, endocrine systems because of the hypobaric hypoxic condition. The aim of our study was to evaluate the circulatory adaptive mechanisms at high altitudes, during a scientific expedition in the Himalayas. Methods: Arterial distensibility was assessed measuring carotid-radial and carotid-femoral pulse wave velocity. Tests were carried out at several altitudes, from 1350 to 5050 m above sea level, on 8 lowlander European researchers and 11 highlander Nepalese porters. Results: In Europeans, systolic blood pressure and pulse pressure increased slightly but significantly with altitude (p < 0.05 and p < 0.001, respectively). Norepinephrine showed a significant increase after the lowlanders had spent some time at high altitude (p < 0.001). With increasing altitude, a progressive increase in carotid-radial and carotid-femoral pulse wave velocity values was observed in lowlanders, showing a particularly significant increase (p < 0.001) after staying at high altitude (carotid-radial pulse wave velocity, median value (interquartile range) from 9.2 (7.9−10.0) to 11.2 (10.9−11.8) m/s and carotid-femoral pulse wave velocity from 8.5 (7.9−9.0) to 11.3 (10.9−11.8) m/s). At high altitudes (3400 and 5050 m above sea level), no significant differences were observed between highlanders and lowlanders in hemodynamic parameters (blood pressure, carotid-radial and carotid-femoral pulse wave velocity). Conclusions: The progressive arterial stiffening with altitude observed in European lowlanders could explain the increase in systolic and pulse pressure values observed at high altitudes in this ethnic group. Further studies are needed to evaluate the role of aortic stiffening in the pathogenesis of acute mountain sickness.

Alternative RAS in Various Hypoxic Conditions: From Myocardial Infarction to COVID-19

Alternative branches of the classical renin–angiotensin–aldosterone system (RAS) represent an important cascade in which angiotensin 2 (AngII) undergoes cleavage via the action of the angiotensin-converting enzyme 2 (ACE2) with subsequent production of Ang(1-7) and other related metabolites eliciting its effects via Mas receptor activation. Generally, this branch of the RAS system is described as its non-canonical alternative arm with counterbalancing actions to the classical RAS, conveying vasodilation, anti-inflammatory, anti-remodeling and anti-proliferative effects. The implication of this branch was proposed for many different diseases, ranging from acute cardiovascular conditions, through chronic respiratory diseases to cancer, nonetheless, hypoxia is one of the most prominent common factors discussed in conjugation with the changes in the activity of alternative RAS branches. The aim of this review is to bring complex insights into the mechanisms behind the various forms of hypoxic insults on the activity of alternative RAS branches based on the different duration of stimuli and causes (acute vs. intermittent vs. chronic), localization and tissue (heart vs. vessels vs. lungs) and clinical relevance of studied phenomenon (experimental vs. clinical condition). Moreover, we provide novel insights into the future strategies utilizing the alternative RAS as a diagnostic tool as well as a promising pharmacological target in serious hypoxia-associated cardiovascular and cardiopulmonary diseases.

Transcriptional landscape in rat intestines under hypobaric hypoxia

Oxygen metabolism is closely related to the intestinal homeostasis environment, and the occurrence of many intestinal diseases is as a result of the destruction of oxygen gradients. The hypobaric hypoxic environment of the plateau can cause dysfunction of the intestine for humans, such as inflammation. The compensatory response of the small intestine cells to the harsh environment definitely changes their gene expression. How the small intestine cells response the hypobaric hypoxic environment is still unclear. We studied the rat small intestine under hypobaric hypoxic conditions to explore the transcriptional changes in rats under acute/chronic hypobaric hypoxic conditions. We randomly divided rats into three groups: normal control group (S), acute hypobaric hypoxia group, exposing to hypobaric hypoxic condition for 2 weeks (W2S) and chronic hypobaric hypoxia group, exposing to hypobaric hypoxic condition for 4 weeks (W4S). The RNA sequencing was performed on the small intestine tissues of the three groups of rats. The results of principal component analysis showed that the W4S and W2S groups were quite different from the control group. We identified a total of 636 differentially expressed genes, such as ATP binding cassette, Ace2 and Fabp. KEGG pathway analysis identified several metabolic and digestive pathways, such as PPAR signaling pathway, glycerolipid metabolism, fat metabolism, mineral absorption and vitamin metabolism. Cogena analysis found that up-regulation of digestive and metabolic functions began from the second week of high altitude exposure. Our study highlights the critical role of metabolic and digestive pathways of the intestine in response to the hypobaric hypoxic environment, provides new aspects for the molecular effects of hypobaric hypoxic environment on intestine, and raises further questions about between the lipid metabolism disorders and inflammation.