Individual variations and sex differences in hemodynamics with percutaneous arterial oxygen saturation (SpO2) in young Andean highlanders in Bolivia

Recent updates on cold adaptation in population and laboratory studies, including cross-adaptation with nonthermal factors

This review aims to update our understanding of human cold adaptation. First, an overview of the thermoregulatory response to cold is provided, with some recent updates in human brown adipose tissue (BAT). Variation in BAT activity and multiorgan contributions to cold-induced thermogenesis were introduced. We found that individuals with less BAT activity rely more on shivering to compensate for less non-shivering thermogenesis (NST). The mechanisms of cold-induced vasoconstriction are summarized, including the role of arteriovenous anastomoses, adrenergic neural function, and inhibition of the nitric oxide vasodilator pathway. In addition, cold-induced vasodilation (CIVD) during cold immersion of the distal extremities is summarized with some recent updates in physiological mechanism. Furthermore, the cold shock response at the onset of cold immersion is introduced. Next, categorization of cold acclimatization/acclimation into habituation of shivering and metabolic and insulative adaptation are provided, with some recent updates. Especially, the rediscovery of human BAT has clarified metabolic acclimation, where increased NST replace shivering. Then, a greater CIVD response in populations in cold regions has been reported, whereas recent laboratory studies suggest no increase in CIVD after repeated cold exposure. To prevent cold injuries, individuals should not rely on habituation through repeated cold exposure. In addition, habituation to the cold shock response after repeated cold water immersion could help reduce the number of drownings. Furthermore, cross-adaptation between cold and nonthermal factors in the thermoregulatory response is summarized. Recent studies explored the relationship between exercise training and BAT activity, although this remains unresolved, depending on the exercise intensity and environmental conditions. The effects of exercise with cold exposure on the thermoregulatory response to cold are summarized in studies including divers working in cold water. We investigated the effect of exercise training in cold water, which resulted in increased muscle deoxygenation during submaximal exercise and greater anerobic power. Moreover, the effects of a hypoxic environment on cold adaptation are summarized. Elevated basal metabolism and higher distal skin temperature in highlanders could improve their cold tolerance. Finally, factors affecting cold adaptation are discussed. The type of cold adaptation may depend on the specific thermoregulatory responses repeated during the adaptation process.

Sex differences in cardiorespiratory control under hypoxia: the roles of oxygen desaturation and hypoxic exposure time

Introduction

Males and females differ anatomically and functionally in cardiorespiratory regulation, with males tending to experience greater oxygen desaturation under hypoxia. Therefore, sex might moderate cardiorespiratory responses to acute hypoxia exposure. Accordingly, we hypothesized that sex differences in cardiovascular and ventilatory responses would be more pronounced with equal hypoxia duration (iso-time) but less pronounced at similar oxygen desaturation levels (iso-saturation).

Methods

Twenty-two (12 females) healthy individuals were exposed to normoxia (10 min at FiO2 = 0.21) and hypoxia (10 min at FiO2 = 0.115), respectively. Pulse oxygen saturation (SpO2), R-R intervals, cardiac output, blood pressure (BP), and ventilatory data were continuously recorded during spontaneous breathing. Spectral analysis of R-R intervals and systolic BP revealed cardiovascular autonomic modulation in the low- (LF; 0.04-0.15 Hz) and high-frequency (HF; 0.15-0.40 Hz) bands and alpha-index (α-LF) assessed spontaneous baroreflex sensitivity (BRS). Sex differences were compared in iso-saturation and iso-time analyses.

Results

At 10 min of hypoxia (iso-time), males desaturated more than females (interaction: p = 0.004), and hypoxia-induced tachycardia in both groups (p < 0.001), but no "sex-time" interaction was found for cardiovascular data. In contrast, only males responded with ventilatory responses during iso-time hypoxia, decreasing breathing frequency (interaction: p = 0.022) and increasing tidal volume (Vt) (interaction: p = 0.036). Otherwise, during iso-saturation (SpO2-matched ∼91%), heart rate and LF of R-R intervals increased more in females than in males (interaction: p = 0.049). However, only males increased Vt (interaction; p = 0.037).

Conclusion

Our data indicate that females counterbalance hypoxia mainly by systemic circulatory adjustments, while males use both, circulatory and ventilatory adjustments.

[Body Function Changes and Prevention Strategies in High-Altitude Environment]

In high-altitude regions, the unique challenges posed by the low-oxygen environment exert significant stress on the physiological functions of the human body. Currently, there is an urgent need to develop a deeper understanding of the pathogenesis, prevention and treatment strategies, as well as effective interventions, of altitude sickness. In this paper, we explored the self-regulatory mechanisms of the human body in such extreme environments, elaborating on the adaptive mechanisms at the molecular level and the process of acclimatization to high altitudes. Furthermore, we summarized the historical background and distinctive advantages of hypoxic preconditioning in the prevention and treatment of altitude diseases. Additionally, we reviewed various adaptation strategies employed by populations in different high-altitude regions, along with the extensive use of Western medication, single-herb traditional Chinese medicine remedies, and compound formulas and composite formulations of traditional Chinese medicine in these regions. Through this paper, we intend to provide a theoretical foundation for the health maintenance of high-altitude populations and to improve survival strategies under extreme environmental conditions. Our goal is also to provide a theoretical reference for improving the physiological adaptability of newcomers to high altitude areas and for deepening our understanding of the mechanisms of human adaptation to high-altitude environments.

Endocrine, inflammatory and immune responses and individual differences in acute hypobaric hypoxia in lowlanders

When lowlanders are exposed to environments inducing hypobaric hypoxia (HH) such as high mountains, hemodynamic changes occur to maintain oxygen levels in the body. However, changes to other physiological functions under such conditions have yet to be clarified. This study investigated changes in endocrine, inflammatory and immune parameters and individual differences during acute HH exposure using a climatic chamber (75 min of exposure to conditions mimicking 3500 m) in healthy lowlanders. Aldosterone and cortisol were significantly decreased and interleukin (IL)-6, IL-8 and white blood cell (WBC) counts were significantly increased after HH. Lower peripheral oxygen saturation (SpO2) was associated with higher IL-6 and WBC counts, and higher IL-8 was associated with higher cortisol. These findings suggest that endocrine, inflammatory and immune responses are evoked even with a short 75-min exposure to HH and individuals with lower SpO2 seemed to show more pronounced responses. Our results provide basic data for understanding the physiological responses and interactions of homeostatic systems during acute HH.

Individual variations and sex differences in hemodynamics and percutaneous arterial oxygen saturation (SpO2) in Tibetan highlanders of Tsarang in the Mustang district of Nepal

Background

Many studies have indicated specific low-hemoglobin (Hb) adaptation to high altitude in the Tibetan population, but studies focusing on physiological variations within this population are limited. This study aimed to investigate the relationships between SpO₂ and related factors, including individual variations and sex differences, to assess the generality of high-altitude adaptation in the Tibetan population of Tsarang.

Methods

The participants were 31 male and 41 female community-dwelling people aged ≥18 years living in Tsarang, in the Mustang district of Nepal. Height, weight, SpO₂, Hb concentration, finger temperature, heart rate, and blood pressure were measured. Lifestyle information was obtained by interview.

Results

Men had significantly higher systolic blood pressure (p = 0.002) and Hb (p < 0.001) than women. There was no significant correlation between SpO₂ and other parameters in men. In women, SpO₂ was negatively correlated with heart rate (p = 0.036), Hb (p = 0.004), and finger temperature (p = 0.037). In multiple regression analysis, a higher SpO₂ was marginally correlated with lower age (β = −0.109, p = 0.086) and higher Hb (β = 0.547, p = 0.053) in men. In women, higher SpO₂ was significantly correlated with lower heart rate (β = −0.045, p = 0.036) and Hb (β = −0.341, p = 0.018). Mean hemoglobin (95% confidence interval) was 13.6 g/dl (13.1–14.0 g/dl), which is lower than that found previously in Andeans and almost equal to that in Japanese lowlanders measured using the same device. In some participants of both sexes, hemoglobin was >17.0 g/dl.

Conclusion

Higher SpO₂ was marginally correlated with younger age and higher Hb in men and with lower heart rate and lower Hb in women. Hemoglobin concentration was similar to that found previously in lowlanders, but higher in some individuals. These results indicate individual variation and sex differences in the hemodynamics of high-altitude adaptation in Tibetan highlanders of Tsarang, as well as low-Hb adaptation to high altitude equal to that of other Tibetans.

Sexual Dimorphism of Dexamethasone as a Prophylactic Treatment in Pathologies Associated With Acute Hypobaric Hypoxia Exposure

Dexamethasone can be taken prophylactically to prevent hypobaric hypoxia-associated disorders of high-altitude. While dexamethasone-mediated protection against high-altitude disorders has been clinically evaluated, detailed sex-based mechanistic insights have not been explored. As part of our India-Leh-Dexamethasone-expedition-2020 (INDEX 2020) programme, we examined the phenotype of control (n = 14) and dexamethasone (n = 13) groups, which were airlifted from Delhi (∼225 m elevation) to Leh, Ladakh (∼3,500 m), India, for 3 days. Dexamethasone 4 mg twice daily significantly attenuated the rise in blood pressure, heart rate, pulmonary pressure, and drop in SaO₂ resulting from high-altitude exposure compared to control-treated subjects. Of note, the effect of dexamethasone was substantially greater in women than in men, in whom the drug had relatively little effect. Thus, for the first time, this study shows a sex-biased regulation by dexamethasone of physiologic parameters resulting from the hypoxic environment of high-altitude, which impacts the development of high-altitude pulmonary hypertension and acute mountain sickness. Future studies of cellular contributions toward sex-specific regulation may provide further insights and preventive measures in managing sex-specific, high-altitude–related disorders.

Seasonality and Sex-Biased Fluctuation of Birth Weight in Tibetan Populations

Birth weight (BW) is a key determinant of infant mortality. Previous studies have reported seasonal fluctuation of BW. However, the responsible environmental factors remain disputable. High-altitude environment provides a great opportunity to test the current hypotheses due to its distinctive climate conditions. We collected BW data of  ~ 9000 Tibetan singletons born at Lhasa (elevation: 3660 m) from 2014 to 2018. Using regression models, we analyzed BW seasonality of highland Tibetans. Multivariate models with meteorological factors as independent variables were employed to examine responsible environmental factors accounting for seasonal variation. We compared BW, low-BW prevalence and sex ratio between highland and lowland populations, and we observed a significant seasonal pattern of BW in Tibetans, with a peak in winter and a trough in summer. Notably, there is a marked sex-biased pattern of BW seasonality (more striking in males than in females). Sunlight exposure in the 3rd trimester and barometric pressure exposure in the 2nd trimester are significantly correlated with BW, and the latter can be explained by seasonal change of oxygen partial pressure. In particular, due to the male-biased BW seasonality, we found a more serious BW reduction and higher prevalence of low-BW in males, and a skewed sex ratio in highlanders. The infant BW of highland Tibetans has a clear pattern of seasonality. The winter BW is larger than the summer BW, due to the longer sunlight exposure during the late-trimester. Male infants are more sensitive to hypoxia than female infants during the 2nd trimester, leading to more BW reduction and higher mortality.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s43657-021-00038-7.

Effect of EGLN1 Genetic Polymorphisms on Hemoglobin Concentration in Andean Highlanders

The physiological characteristics of Andean natives living at high altitudes have been investigated extensively, with many studies reporting that Andean highlanders have a higher hemoglobin (Hb) concentration than other highlander populations. It has previously been reported that positive natural selection has acted independently on the egl-9 family hypoxia inducible factor 1 (EGLN1) gene in Tibetan and Andean highlanders and is related to Hb concentration in Tibetans. However, no study has yet revealed the genetic determinants of Hb concentration in Andeans even though several single-nucleotide polymorphisms (SNPs) in EGLN1 have previously been examined. Therefore, we explored the relationship between hematological measurements and tag SNPs designed to cover the whole EGLN1 genomic region in Andean highlanders living in Bolivia. Our findings indicated that haplotype frequencies estimated from the EGLN1 SNPs were significantly correlated with Hb concentration in the Bolivian highlanders. Moreover, we found that an Andean-dominant haplotype related to high Hb level may have expanded rapidly in ancestral Andean highlander populations. Analysis of genotype data in an ~436.3 kb genomic region containing EGLN1 using public databases indicated that the population structure based on EGLN1 genetic markers in Andean highlanders was largely different from that in other human populations. This finding may be related to an intrinsic or adaptive physiological characteristic of Andean highlanders. In conclusion, the high Hb concentrations in Andean highlanders can be partly characterized by EGLN1 genetic variants.