Relative role of environmental and genetic factors in respiratory adaptation to high altitude

Recovery of ventilatory and metabolic responses to hypoxia in neonatal rats after chronic hypoxia

Chronic hypoxia (CH) during postnatal development attenuates the hypoxic ventilatory response (HVR) in mammals, but there are conflicting reports on whether this plasticity is permanent or reversible. This study tested the hypothesis that CH-induced respiratory plasticity is reversible in neonatal rats and investigated whether the initial plasticity or recovery differs between sexes. Rat pups were exposed to 3 d of normobaric CH (12 % O2) beginning shortly after birth. Ventilation and metabolic CO2 production were then measured in normoxia and during an acute hypoxic challenge (12 % O2) immediately following CH and after 1, 4-5, and 7 d in room air. CH pups hyperventilated when returned to normoxia immediately following CH, but normoxic ventilation was similar to age-matched control rats within 7 d after return to room air. The early phase of the HVR (minute 1) was only blunted immediately following the CH exposure, while the late phase of the HVR (minute 15) remained blunted after 1 and 4-5 d in room air; recovery appeared complete by 7 d. However, when normalized to CO2 production, the late phase of the hypoxic response recovered within only 1 d. The initial blunting of the HVR and subsequent recovery were similar in female and male rats. Carotid body responses to hypoxia (in vitro) were also normal in CH pups after approximately one week in room air. Collectively, these data indicate that ventilatory and metabolic responses to hypoxia recover rapidly in both female and male neonatal rats once normoxia is restored following CH.

Comparing high versus low-altitude populations to test human adaptations for increased ventilation during sustained aerobic activity

Despite aerobic activity requiring up to tenfold increases in air intake, human populations in high-altitude hypoxic environments can sustain high levels of endurance physical activity. While these populations generally have relatively larger chest and lung volumes, how thoracic motions actively increase ventilation is unknown. Here we show that rib movements, in conjunction with chest shape, contribute to ventilation by assessing how adulthood acclimatization, developmental adaptation, and population-level adaptation to high-altitude affect sustained aerobic activity. We measured tidal volume, heart rate, and rib-motion during walking and running in lowland individuals from Boston (~ 35 m) and in Quechua populations born and living at sea-level (~ 150 m) and at high altitude (> 4000 m) in Peru. We found that Quechua participants, regardless of birth or testing altitudes, increase thoracic volume 2.0-2.2 times more than lowland participants (p < 0.05). Further, Quechua individuals from hypoxic environments have deeper chests resulting in 1.3 times greater increases in thoracic ventilation compared to age-matched, sea-level Quechua (p < 0.05). Thus, increased thoracic ventilation derives from a combination of acclimatization, developmental adaptation, and population-level adaptation to aerobic demand in different oxygen environments, demonstrating that ventilatory demand due to environment and activity has helped shape the form and function of the human thorax.

Methamphetamine exacerbates pathophysiology of traumatic brain injury at high altitude. Neuroprotective effects of nanodelivery of a potent antioxidant compound H-290/51

Military personnel are often exposed to high altitude (HA, ca. 4500-5000m) for combat operations associated with neurological dysfunctions. HA is a severe stressful situation and people frequently use methamphetamine (METH) or other psychostimulants to cope stress. Since military personnel are prone to different kinds of traumatic brain injury (TBI), in this review we discuss possible effects of METH on concussive head injury (CHI) at HA based on our own observations. METH exposure at HA exacerbates pathophysiology of CHI as compared to normobaric laboratory environment comparable to sea level. Increased blood-brain barrier (BBB) breakdown, edema formation and reductions in the cerebral blood flow (CBF) following CHI were exacerbated by METH intoxication at HA. Damage to cerebral microvasculature and expression of beta catenin was also exacerbated following CHI in METH treated group at HA. TiO2-nanowired delivery of H-290/51 (150mg/kg, i.p.), a potent chain-breaking antioxidant significantly enhanced CBF and reduced BBB breakdown, edema formation, beta catenin expression and brain pathology in METH exposed rats after CHI at HA. These observations are the first to point out that METH exposure in CHI exacerbated brain pathology at HA and this appears to be related with greater production of oxidative stress induced brain pathology, not reported earlier.

Ventilatory and carotid body responses to acute hypoxia in rats exposed to chronic hypoxia during the first and second postnatal weeks

Chronic hypoxia (CH) during postnatal development causes a blunted hypoxic ventilatory response (HVR) in neonatal mammals. The magnitude of the HVR generally increases with age, so CH could blunt the HVR by delaying this process. Accordingly, we predicted that CH would have different effects on the respiratory control of neonatal rats if initiated at birth versus initiated later in postnatal development (i.e., after the HVR has had time to mature). Rats had blunted ventilatory and carotid body responses to hypoxia whether CH (12 % O₂) occurred for the first postnatal week (P0 to P7) or second postnatal week (P7 to P14). However, if initiated at P0, CH also caused the HVR to retain the "biphasic" shape characteristic of newborn mammals; CH during the second postnatal week did not result in a biphasic HVR. CH from birth delayed the transition from a biphasic HVR to a sustained HVR until at least P9-11, but the HVR attained a sustained (albeit blunted) phenotype by P13-15. Since delayed maturation of the HVR did not completely explain the blunted HVR, we tested the alternative hypothesis that the blunted HVR was caused by an inflammatory response to CH. Daily administration of the anti-inflammatory drug ibuprofen (4 mg kg^-1, i.p.) did not alter the effects of CH on the HVR. Collectively, these data suggest that CH blunts the HVR in neonatal rats by impairing carotid body responses to hypoxia and by delaying (but not preventing) postnatal maturation of the biphasic HVR. The mechanisms underlying this plasticity require further investigation.

PHD2 inactivation in Type I cells drives HIF-2α-dependent multilineage hyperplasia and the formation of paraganglioma-like carotid bodies

KEY POINTS

The carotid body is a peripheral arterial chemoreceptor that regulates ventilation in response to both acute and sustained hypoxia. Type I cells in this organ respond to low oxygen both acutely by depolarization and dense core vesicle secretion and, over the longer term, via cellular proliferation and enhanced ventilatory responses. Using lineage analysis, the present study shows that the Type I cell lineage itself proliferates and expands in response to sustained hypoxia. Inactivation of HIF-2α in Type I cells impairs the ventilatory, proliferative and cell intrinsic (dense core vesicle) responses to hypoxia. Inactivation of PHD2 in Type I cells induces multilineage hyperplasia and ultrastructural changes in dense core vesicles to form paraganglioma-like carotid bodies. These changes, similar to those observed in hypoxia, are dependent on HIF-2α. Taken together, these findings demonstrate a key role for the PHD2-HIF-2α couple in Type I cells with respect to the oxygen sensing functions of the carotid body.

ABSTRACT

The carotid body is a peripheral chemoreceptor that plays a central role in mammalian oxygen homeostasis. In response to sustained hypoxia, it manifests a rapid cellular proliferation and an associated increase in responsiveness to hypoxia. Understanding the cellular and molecular mechanisms underlying these processes is of interest both to specialized chemoreceptive functions of that organ and, potentially, to the general physiology and pathophysiology of cellular hypoxia. We have combined cell lineage tracing technology and conditionally inactivated alleles in recombinant mice to examine the role of components of the HIF hydroxylase pathway in specific cell types within the carotid body. We show that exposure to sustained hypoxia (10% oxygen) drives rapid expansion of the Type I, tyrosine hydroxylase expressing cell lineage, with little transdifferentiation to (or from) that lineage. Inactivation of a specific HIF isoform, HIF-2α, in the Type I cells was associated with a greatly reduced proliferation of Type I cells and hypoxic ventilatory responses, with ultrastructural evidence of an abnormality in the action of hypoxia on dense core secretory vesicles. We also show that inactivation of the principal HIF prolyl hydroxylase PHD2 within the Type I cell lineage is sufficient to cause multilineage expansion of the carotid body, with characteristics resembling paragangliomas. These morphological changes were dependent on the integrity of HIF-2α. These findings implicate specific components of the HIF hydroxylase pathway (PHD2 and HIF-2α) within Type I cells of the carotid body with respect to the oxygen sensing and adaptive functions of that organ.

[Obstructive sleep apnea at high altitude]

Obstructive sleep apnea (OSA) is an important and socially relevant problem of modern medicine, which is referred to as a most common pathological condition. The problem of OSA is especially urgent for inhabitants of high mountainous regions, as a combination of climatic, social, and cultural factors can significantly affect the course of the disease in both indigenous highlanders and people temporarily residing at high altitude. The paper reviews the current literature covering the problem of OSA at high altitude. It gives the data of Russian and foreign literature on the pathogenesis and clinical presentation of OSA. The author also analyzes an update on the impact of high altitude on the course of OSA in indigenous highlanders and people temporarily living at high altitude. She emphasizes the role of hypobaric hypocapnia as the most important factor for the development of central sleep apnea in the presence of conditions that are obstructive and aggravating the course of the disease.

Cortical Thickness of Native Tibetans in the Qinghai-Tibetan Plateau

BACKGROUND AND PURPOSE

High-altitude environmental factors and genetic variants together could have exerted their effects on the human brain. The present study was designed to investigate the cerebral morphology in high-altitude native Tibetans.

MATERIALS AND METHODS

T1-weighted brain images were obtained from 77 Tibetan adolescents on the Qinghai-Tibetan Plateau (altitude, 2300-5300 m) and 80 matched Han controls living at sea level. Cortical thickness, curvature, and sulcus were analyzed by using FreeSurfer.

RESULTS

Cortical thickness was significantly decreased in the left posterior cingulate cortex, lingual gyrus, superior parietal cortex, precuneus, and rostral middle frontal cortex and the right medial orbitofrontal cortex, lateral occipital cortex, precuneus, and paracentral lobule. Curvature was significantly decreased in the left superior parietal cortex and right superior marginal gyrus; the depth of the sulcus was significantly increased in the left inferior temporal gyrus and significantly decreased in the right superior marginal gyrus, superior temporal gyrus, and insular cortex. Moreover, cortical thickness was negatively correlated with altitude in the left superior and middle temporal gyri, rostral middle frontal cortex, insular cortex, posterior cingulate cortex, precuneus, lingual gyrus, and the right superior temporal gyrus. Curvature was positively correlated with altitude in the left rostral middle frontal cortex, insular cortex, and middle temporal gyrus. The depth of the sulcus was negatively correlated with altitude in the left lingual gyrus and right medial orbitofrontal cortex.

CONCLUSIONS

Differences in cortical morphometry in native Tibetans may reflect adaptations related to high altitude.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

Lung Structure and the Intrinsic Challenges of Gas Exchange

Structural and functional complexities of the mammalian lung evolved to meet a unique set of challenges, namely, the provision of efficient delivery of inspired air to all lung units within a confined thoracic space, to build a large gas exchange surface associated with minimal barrier thickness and a microvascular network to accommodate the entire right ventricular cardiac output while withstanding cyclic mechanical stresses that increase several folds from rest to exercise. Intricate regulatory mechanisms at every level ensure that the dynamic capacities of ventilation, perfusion, diffusion, and chemical binding to hemoglobin are commensurate with usual metabolic demands and periodic extreme needs for activity and survival. This article reviews the structural design of mammalian and human lung, its functional challenges, limitations, and potential for adaptation. We discuss (i) the evolutionary origin of alveolar lungs and its advantages and compromises, (ii) structural determinants of alveolar gas exchange, including architecture of conducting bronchovascular trees that converge in gas exchange units, (iii) the challenges of matching ventilation, perfusion, and diffusion and tissue-erythrocyte and thoracopulmonary interactions. The notion of erythrocytes as an integral component of the gas exchanger is emphasized. We further discuss the signals, sources, and limits of structural plasticity of the lung in alveolar hypoxia and following a loss of lung units, and the promise and caveats of interventions aimed at augmenting endogenous adaptive responses. Our objective is to understand how individual components are matched at multiple levels to optimize organ function in the face of physiological demands or pathological constraints.

Persistent structural adaptation in the lungs of guinea pigs raised at high altitude

Laboratory guinea pigs raised at high altitude (HA, 3800 m) for up to 6 mo exhibit enhanced alveolar growth and remodeling (Hsia et al., 2005. Resp. Physiol. Neurobiol. 147, 105-115). To determine whether initial HA-induced structural enhancement persists following return to intermediate altitude (IA), we raised weanling guinea pigs at (a) HA for 11-12 mo, (b) IA (1200 m) for 11-12 mo, and (c) HA for 4 mo followed by IA for 7-8 mo (HA-to-IA). Morphometric analysis was performed under light and electron microscopy. Body weight and lung volume were similar among groups. Prolonged HA residence increased alveolar epithelium and interstitium volumes while reducing alveolar-capillary blood volume. The HA-induced gains in type-1 epithelium volume and alveolar surface area were no longer present following return to IA whereas volume increases in type-2 epithelium and interstitium and the reduction in alveolar duct volume persisted. Results demonstrate persistent augmentation of some but not all aspects of lung structure throughout prolonged HA residence, with partial reversibility following re-acclimatization to IA.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Genetic inheritance effects on endurance and muscle strength: an update

Top-level sport seems to play a natural Darwinian stage. The most outstanding athletes appear to emerge as a result of exogenous influences of nature and/or coincidence, namely, the contingency of practicing certain sport for which their talents best fit. This coincidence arises because certain individuals possess anatomical, metabolic, functional and behavioural characteristics that are precisely those required to excel in a given sport. Apart from the effects of training, there is strong evidence of genetic influence upon athletic performance. This article reviews the current state of knowledge regarding heritable genetic effects upon endurance and muscle strength, as reported by several twin and family studies. Due, probably, to the inaccuracy of the measurement procedures and sampling error, heritability estimates differ widely between studies. Even so, the genetic inheritence effects seem incontrovertible in most physical traits: ~40-70% for peak oxygen uptake and cardiac mass and structure, and ~30-90% for anaerobic power and capacity, ranging according to the metabolic category. Studies in development by several researchers at this present time seem to guarantee that future reviews will include twins and family studies concerning genes associated with the adaptive processes against hormetic agents, such as exercise, heat and oxidative stress.

Life-long consequences of postnatal normoxia exposure in rats raised at high altitude

We tested the hypothesis that exposure of high-altitude (HA) rats to a period of postnatal normoxia has long-term consequences on the ventilatory and hematological acclimatization in adults. Male and female HA rats (3,600 m, Po(2) ≃ 100 Torr; La Paz, Bolivia) were exposed to normal room air [HA control (HACont)] or enriched oxygen (32% O(2); Po(2) ≃ 160 Torr) from 1 day before to 15 days after birth [HA postnatal normoxia (HApNorm)]. Hematocrit and hemoglobin values were assessed at 2, 12, and 32 wk of age. Cardiac and lung morphology were assessed at 12 wk by measuring right ventricular hypertrophy (pulmonary hypertension index) and lung air space-to-tissue ratio (indicative of alveolarization). Respiratory parameters under baseline conditions and in response to 32% O(2) for 10 min (relieving the ambient hypoxic stimulus) were measured by whole body plethysmography at 12 wk. Finally, we performed a survival analysis up to 600 days of age. Compared with HACont, HApNorm rats had reduced hematocrit and hemoglobin levels at all ages (both sexes); reduced right ventricular hypertrophy (both sexes); lower air space-to-tissue ratio in the lungs (males only); reduced CO(2) production rate, but higher oxygen uptake (males only); and similar respiratory frequency, tidal volume, and minute ventilation. When breathing 32% O(2), HApNorm male rats had a stronger decrease of minute ventilation than HACont. HApNorm rats had a marked tendency toward longer survival throughout the study. We conclude that exposure to ambient hypoxia during postnatal development in HA rats has deleterious consequences on acclimatization to hypoxia as adults.

Differences in the control of breathing between Himalayan and sea-level residents

We compared the control of breathing of 12 male Himalayan highlanders with that of 21 male sea-level Caucasian lowlanders using isoxic hyperoxic ( = 150 mmHg) and hypoxic ( = 50 mmHg) Duffin's rebreathing tests. Highlanders had lower mean +/- s.e.m. ventilatory sensitivities to CO(2) than lowlanders at both isoxic tensions (hyperoxic: 2.3 +/- 0.3 vs. 4.2 +/- 0.3 l min(1) mmHg(1), P = 0.021; hypoxic: 2.8 +/- 0.3 vs. 7.1 +/- 0.6 l min(1) mmHg(1), P < 0.001), and the usual increase in ventilatory sensitivity to CO(2) induced by hypoxia in lowlanders was absent in highlanders (P = 0.361). Furthermore, the ventilatory recruitment threshold (VRT) CO(2) tensions in highlanders were lower than in lowlanders (hyperoxic: 33.8 +/- 0.9 vs. 48.9 +/- 0.7 mmHg, P < 0.001; hypoxic: 31.2 +/- 1.1 vs. 44.7 +/- 0.7 mmHg, P < 0.001). Both groups had reduced ventilatory recruitment thresholds with hypoxia (P < 0.001) and there were no differences in the sub-threshold ventilations (non-chemoreflex drives to breathe) between lowlanders and highlanders at both isoxic tensions (P = 0.982), with a trend for higher basal ventilation during hypoxia (P = 0.052). We conclude that control of breathing in Himalayan highlanders is distinctly different from that of sea-level lowlanders. Specifically, Himalayan highlanders have decreased central and absent peripheral sensitivities to CO(2). Their response to hypoxia was heterogeneous, with the majority decreasing their VRT indicating either a CO(2)-independent increase in activity of peripheral chemoreceptor or hypoxia-induced increase in [H(+)] at the central chemoreceptor. In some highlanders, the decrease in VRT was accompanied by an increase in sensitivity to CO(2), while in others VRT remained unchanged and their sub-threshold ventilations increased, although these were not statistically significant.

Ventilatory response to hypoxia in chicken hatchlings: a developmental window of sensitivity to embryonic hypoxia

We had reported previously [Szdzuy, K., Mortola, J.P., 2007b. Ventilatory chemosensitivity of the 1-day-old chicken hatchling after embryonic hypoxia. Am. J. Physiol. (Regul. Integr. Comp. Physiol.) 293, R1640-R1649] that hypoxia during incubation blunted ventilatory chemosensitivity in the hatchling. Because the carotid bodies become functional in the last portion of incubation, we asked whether these last days were the critical period for the effects of hypoxia on the development of ventilatory chemosensitivity. White Leghorn chicken eggs were incubated at 38 degrees C either in 21% O(2) (Controls) or in 15% O(2) for the whole 3-week incubation (HxTot) or for only the 1st (Hx1), 2nd (Hx2) or 3rd week of incubation (Hx3). Hatching time had a delay of half a day in HxTot, and was normal in the other groups. Body weight was similar in all hatchlings. Oxygen consumption ( [Formula: see text] ) and pulmonary ventilation (V e) were measured at about 20 h post-hatching. Ventilatory chemosensitivity was evaluated from the degree of hyperpnea (increase in V e) and hyperventilation (increase in [Formula: see text] ) during acute hypoxia (15 and 10% O(2), 20 min each) and acute hypercapnia (2 and 4% CO(2), 20 min each). The responses to hypoxia were similarly decreased in HxTot and in Hx3 compared to controls, and were normal in the other experimental groups; those to hypercapnia were blunted only in HxTot. The results are in agreement with the idea that prenatal hypoxia blunts V e chemosensitivity by interfering with the normal development of the carotid bodies.

Long-term effects of the perinatal environment on respiratory control

The respiratory control system exhibits considerable plasticity, similar to other regions of the nervous system. Plasticity is a persistent change in system behavior triggered by experiences such as changes in neural activity, hypoxia, and/or disease/injury. Although plasticity is observed in animals of all ages, some forms of plasticity appear to be unique to development (i.e., “developmental plasticity”). Developmental plasticity is an alteration in respiratory control induced by experiences during “critical” developmental periods; similar experiences outside the critical period will have little or no lasting effect. Thus complementary experiments on both mature and developing animals are generally needed to verify that the observed plasticity is unique to development. Frequently studied models of developmental plasticity in respiratory control include developmental manipulations of respiratory gas concentrations (O2and CO2). Environmental factors not specifically associated with breathing may also trigger developmental plasticity, however, including psychological stress or chemicals associated with maternal habits (e.g., nicotine, cocaine). Despite rapid advances in describing models of developmental plasticity in breathing, our understanding of fundamental mechanisms giving rise to such plasticity is poor; mechanistic studies of developmental plasticity are of considerable importance. Developmental plasticity may enable organisms to “fine tune” their phenotype to optimize the performance of this critical homeostatic regulatory system. On the other hand, developmental plasticity could also increase the risk of disease later in life. Future directions for studies concerning the mechanisms and functional implications of developmental plasticity in respiratory motor control are discussed.

Ventilatory chemosensitivity of the 1-day-old chicken hatchling after embryonic hypoxia

We investigated the effects of sustained embryonic hypoxia on the neonatal ventilatory chemosensitivity. White Leghorn chicken eggs were incubated at 38°C either in 21% O2 throughout incubation (normoxia, Nx) or in 15% O2 from embryonic day 5 (hypoxia, Hx), hatching time included. Hx embryos hatched ∼11 h later than Nx, with similar body weights. Measurements of gaseous metabolism (oxygen consumption, V̇o2) and pulmonary ventilation (V̇e) were conducted either within the first 8 h (early) or later hours (late) of the first posthatching day. In resting conditions, Hx had similar V̇o2 and body temperature (Tb) and slightly higher V̇e and ventilatory equivalent (V̇e/V̇o2) than Nx. Ventilatory chemosensitivity was evaluated from the degree of hyperpnea (increase in V̇e) and of hyperventilation (increase in V̇e/V̇o2) during acute hypoxia (15 and 10% O2, 20 min each) and acute hypercapnia (2 and 4% CO2, 20 min each). The chemosensitivity differed between the early and late hours, and at either time the responses to hypoxia and hypercapnia were less in Hx than in Nx because of a lower increase in V̇e and a lower hypoxic hypometabolism. In a second group of Nx and Hx hatchlings, the V̇e response to 10% O2 was tested in the same hatchlings at the early and late hours. The results confirmed the lower hypoxic chemosensitivity of Hx. We conclude that hypoxic incubation affected the development of respiratory control, resulting in a blunted ventilatory chemosensitivity.

Population genetic aspects and phenotypic plasticity of ventilatory responses in high altitude natives

Highland natives show unique breathing patterns and ventilatory responses at altitude, both at rest and during exercise. For many ventilatory traits, there is also significant variation between highland native groups, including indigenous populations in the Andes and Himalaya, and more recent altitude arrivals in places like Colorado. This review summarizes the literature in this area with some focus on partitioning putative population genetic differences from differences acquired through lifelong exposure to hypoxia. Current studies suggest that Tibetans have high resting ventilation (V (E)), and a high hypoxic ventilatory response (HVR), similar to altitude acclimatized lowlanders. Andeans, in contrast, show low resting V (E) and a low or "blunted" HVR, with little evidence that these traits are acquired via lifelong exposure. Resting V (E) of non-indigenous altitude natives is not well documented, but lifelong hypoxic exposure almost certainly blunts HVR in these groups through decreased chemosensitivity to hypoxia in a process known as hypoxic desensitization (HD). Together, these studies suggest that the time course of ventilatory response, and in particular the origin or absence of HD, depends on population genetic background i.e., the allele or haplotype frequencies that characterize a particular population. During exercise, altitude natives have lower V (E) compared to acclimatized lowland controls. Altitude natives also have smaller alveolar-arterial partial pressure differences P(AO2) - P(aO2) during exercise suggesting differences in gas exchange efficiency. Small P(AO2) - P(aO2) in highland natives of Colorado underscores the likely importance of developmental adaptation to hypoxia affecting structural/functional aspects of gas exchange with resultant changes in breathing pattern. However, in Andeans, at least, there is also evidence that low exercise V (E) is determined by genetic background affecting ventilatory control independent of gas exchange. Additional studies are needed to elucidate the effects of gene, environment, and gene-environment interaction on these traits, and these effects are likely to differ widely between altitude native populations.

Residence at 3,800-m altitude for 5 mo in growing dogs enhances lung diffusing capacity for oxygen that persists at least 2.5 years

Mammals native to high altitude (HA) exhibit larger lung volumes than their lowland counterparts. To test the hypothesis that adaptation induced by HA residence during somatic maturation improves pulmonary gas exchange in adulthood, male foxhounds born at sea level (SL) were raised at HA (3,800 m) from 2.5 to 7.5 mo of age and then returned to SL prior to somatic maturity while their littermates were simultaneously raised at SL. Following return to SL, all animals were trained to run on a treadmill; gas exchange and hemodynamics were measured 2.5 years later at rest and during exercise while breathing 21% and 13% O(2). The multiple inert gas elimination technique was employed to estimate ventilation-perfusion (Va/Q) distributions and lung diffusing capacity for O(2) (Dl(O(2))). There were no significant intergroup differences during exercise breathing 21% O(2). During exercise breathing 13% O(2), peak O(2) uptake and Va/Q distributions were similar between groups but arterial pH, base excess, and O(2) saturation were higher while peak lactate concentration was lower in animals raised at HA than at SL. At a given exercise intensity, alveolar-arterial O(2) tension gradient (A-aDo(2)) attributable to diffusion limitation was lower while Dlo(2) was 12-25% higher in HA-raised animals. Mean systemic arterial blood pressure was also lower in HA-raised animals; mean pulmonary arterial pressures were similar. We conclude that 5 mo of HA residence during maturation enhances long-term gas exchange efficiency and Dl(O(2)) without impacting Va/Q inequality during hypoxic exercise at SL.

Finding the genes underlying adaptation to hypoxia using genomic scans for genetic adaptation and admixture mapping

The complete sequencing the human genome and recent analytical advances have provided the opportunity to perform genome-wide studies of human variation. There is substantial potential for such population-genomic approaches to assist efforts to uncover the historical and demographic histories of human populations. Additionally, these genome-wide datasets allow for investigations of variability among genomic regions. Although all genomic regions in a population have experienced the same demographic events, they have not been affected by these events in precisely the same way. Much of the variability among genomic regions is simply the result of genetic drift (i.e., gene frequency changes resulting from the effects of small breeding-population size), but some is also the result of genetic adaptation, which will only affect the gene under selection and nearby regions. We have used a new DNA typing assay that allows for the genotyping of thousands of SNPs on hundreds of samples to identify regions most likely to have been affected by genetic adaptation. Populations that have inhabited different niches (e.g., high-altitude regions) can be used to identify genes underlying the physiological differences. We have used two methods (admixture mapping and genome scans for genetic adaptation) founded on the population-genomic paradigms to search for genes underlying population differences in response to chronic hypoxia. There is great promise that together these methods will facilitate the discovery of genes influencing hypoxic response.

Respiratory control in residents at high altitude: physiology and pathophysiology

Highland population (HA) from the Andes, living above 3000 m, have a blunted ventilatory response to increasing hypoxia, breathe less compared to acclimatized newcomers, but more, compared to sea-level natives at sea level. Subjects with chronic mountain sickness (CMS) breathe like sea-level natives and have excessive erythrocytosis (EE). The respiratory stimulation that arises through the peripheral chemoreflex is modestly less in the CMS group when compared with the HA group at the same P(ET(O2)). With regard to CO(2) sensitivity, CMS subjects seem to have reset their central CO(2) chemoreceptors to operate around the sea-level resting P(ET(CO2)). Acetazolamide, an acidifying drug that increases the chemosensitivity of regions in the brain stem that contain CO(2)/H(+) sensitive neurons, partially reverses this phenomenon, thus, providing CMS subjects with the possibility to have high CO(2) changes, despite small changes in ventilation. However, the same type of adjustments of the breathing pattern established for Andeans has not been found necessarily in Asian humans and/or domestic animals nor in the various high altitude species studied. The differing time frames of exposure to hypoxia among the populations, as well as the reversibility of the different components of the respiratory process at sea level, provide key concepts concerning the importance of time at high altitude in the evolution of an appropriate breathing pattern.

Growth and development of Andean high altitude residents

Growth and development under conditions of chronic hypoxia result in a different pattern of growth in Andean highlanders than in lowlanders. Growth at high altitude results in a small (1 to 4 cm) delay in linear growth, with most, if not all, of the delay probably established at or soon after birth. It also results in an enhancement of lung volumes, particularly residual volume, which is 70%-80% larger in highland than lowland children, on average, with the magnitude of the increase being positively related to age. In addition, growth and development under conditions of chronic hypoxia result in a blunted ventilatory response to hypoxia, a 4% to 5% reduction in Sa(O2), and a substantial increase in pulmonary diffusing capacity. Andean highlanders have V(O2 max) similar to that of lowlanders at low altitude, suggesting that they have successfully adapted to their hypoxic environment. It is likely that both developmental and genetic factors influence most, if not all, components of the cardiorespiratory system of Andean highlanders, but the relative importance of each is not clear.

Developmental plasticity of the hypoxic ventilatory response after perinatal hyperoxia and hypoxia

Both genetic and environmental factors influence the normal development of the respiratory control system. This review examines the role perinatal O2 plays in the development of normoxic breathing and the hypoxic ventilatory response in mammals. Hyperoxia and hypoxia elicit plasticity in respiratory control that is unique to development and may persist weeks to years after return to normoxia. Specifically, both hyperoxia and hypoxia during early postnatal development attenuate the adult hypoxic ventilatory response, but the underlying mechanisms for this plasticity differ. Hyperoxia attenuates the hypoxic ventilatory response through potentially life-long changes in carotid body function. Neonatal hypoxia appears to have short-term effects on carotid body function, but persistent changes in the hypoxic ventilatory response may instead reflect changes in respiratory mechanics or related neural pathways. Overall, it appears that a relatively narrow range of environmental O2 is consistent with "normal" postnatal respiratory control development, predisposing animals to potentially maladaptive plasticity in the face of disease or atypical environmental conditions.

Long-term enhancement of pulmonary gas exchange after high-altitude residence during maturation

In a previous study, our laboratory showed that young dogs born at sea level (SL) and raised from 2.5 mo of age to beyond somatic maturity at a high altitude (HA) of 3,100 m show enhanced resting lung function (Johnson RL Jr, Cassidy SS, Grover RF, Schutte JE, and Epstein RH. J Appl Physiol 59: 1773-1782, 1985). To examine whether HA-induced adaptation improves pulmonary gas exchange during exercise and whether adaptation is reversible when animals return to SL before somatic maturity, we raised 2.5-mo-old foxhounds at HA (3,800 m) for 5 mo (to age 7.5 mo) before returning them to SL. Lung function was measured under anesthesia 1 mo and 2 yr after return to SL and during exercise approximately 1 yr after return. In animals exposed to HA relative to simultaneous litter-matched SL controls, resting circulating blood and erythrocyte volumes, lung volumes, septal volume estimated by a rebreathing technique, and lung tissue volume estimated by high-resolution computed tomography scan were persistently higher. Lung diffusing capacity, membrane diffusing capacity, and pulmonary capillary blood volume estimated at a given cardiac output were significantly higher in animals exposed to HA, whereas maximal oxygen uptake and hematocrit were similar between groups. We conclude that relatively short exposure to HA during somatic maturation improves long-term lung function into adulthood.

Enhanced alveolar growth and remodeling in Guinea pigs raised at high altitude

To examine the effects of chronic high altitude (HA) exposure on lung structure during somatic maturation, we raised male weanling guinea pigs at HA (3800m) for 1, 3, or 6 months, while their respective male littermates were simultaneously raised at low altitude (LA, 1200m). Under anaesthesia, airway pressure was measured at different lung volumes. The right lung was fixed at a constant airway pressure for morphometric analysis under light and electron microscopy. In animals raised at HA for 1 month, lung volume, alveolar surface area and alveolar-capillary blood volume (V(c)) were elevated above LA control values. Following 3-6 months of HA exposure, increases in lung volume and alveolar surface area persisted while the initial increase in V(c) normalized. Additional adaptation occurred, including a higher epithelial cell volume, septal tissue volume and capillary surface area, a lower alveolar duct volume and lower harmonic mean diffusion barrier resulting in higher membrane and lung diffusing capacities. These data demonstrate enhanced alveolar septal growth and progressive acinar remodeling during chronic HA exposure with long-term augmentation of alveolar dimensions as well as functional compensation in lung compliance and diffusive gas transport.

Ancestry explains the blunted ventilatory response to sustained hypoxia and lower exercise ventilation of Quechua altitude natives

Andean high-altitude (HA) natives have a low (blunted) hypoxic ventilatory response (HVR), lower effective alveolar ventilation, and lower ventilation (VE) at rest and during exercise compared with acclimatized newcomers to HA. Despite blunted chemosensitivity and hypoventilation, Andeans maintain comparable arterial O(2) saturation (Sa(O(2))). This study was designed to evaluate the influence of ancestry on these trait differences. At sea level, we measured the HVR in both acute (HVR-A) and sustained (HVR-S) hypoxia in a sample of 32 male Peruvians of mainly Quechua and Spanish origins who were born and raised at sea level. We also measured resting and exercise VE after 10-12 h of exposure to altitude at 4,338 m. Native American ancestry proportion (NAAP) was assessed for each individual using a panel of 80 ancestry-informative molecular markers (AIMs). NAAP was inversely related to HVR-S after 10 min of isocapnic hypoxia (r = -0.36, P = 0.04) but was not associated with HVR-A. In addition, NAAP was inversely related to exercise VE (r = -0.50, P = 0.005) and ventilatory equivalent (VE/Vo(2), r = -0.51, P = 0.004) measured at 4,338 m. Thus Quechua ancestry may partly explain the well-known blunted HVR (10, 35, 36, 57, 62) at least to sustained hypoxia, and the relative exercise hypoventilation at altitude of Andeans compared with European controls. Lower HVR-S and exercise VE could reflect improved gas exchange and/or attenuated chemoreflex sensitivity with increasing NAAP. On the basis of these ancestry associations and on the fact that developmental effects were completely controlled by study design, we suggest both a genetic basis and an evolutionary origin for these traits in Quechua.

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.

The effects of flight and altitude

Increasing numbers of infants and children journey by aeroplane, or travel to high altitude destinations, for example, on holiday or as part of a population migration. Most are healthy, although increasingly children may be transported by aeroplane or helicopter specifically to obtain treatment for severe illness or injury. It is therefore useful to review the effects of altitude, and their relevance to children who undertake flights or travel to, or at high altitudes, particularly those with acute and chronic medical conditions.

Developmental plasticity of the hypoxic ventilatory response in rats induced by neonatal hypoxia

Neonatal hypoxia alters the development of the hypoxic ventilatory response in rats and other mammals. Here we demonstrate that neonatal hypoxia impairs the hypoxic ventilatory response in adult male, but not adult female, rats. Rats were raised in 10% O(2) for the first postnatal week, beginning within 12 h after birth. Subsequently, ventilatory responses were assessed in 7- to 9-week-old unanaesthetized rats via whole-body plethysmography. In response to 12% O(2), male rats exposed to neonatal hypoxia increased ventilation less than untreated control rats (mean +/-s.e.m. 35.2 +/- 7.7%versus 67.4 +/- 9.1%, respectively; P= 0.01). In contrast, neonatal hypoxia had no lasting effect on hypoxic ventilatory responses in female rats (67.9 +/- 12.6%versus 61.2 +/- 11.7% increase in hypoxia-treated and control rats, respectively; P > 0.05). Normoxic ventilation was unaffected by neonatal hypoxia in either sex at 7-9 weeks of age (P > 0.05). Since we hypothesized that neonatal hypoxia alters the hypoxic ventilatory response at the level of peripheral chemoreceptors or the central neural integration of chemoafferent activity, integrated phrenic responses to isocapnic hypoxia were investigated in urethane-anaesthetized, paralysed and ventilated rats. Phrenic responses were unaffected by neonatal hypoxia in rats of either sex (P > 0.05), suggesting that neonatal hypoxia-induced plasticity occurs between the phrenic nerve and the generation of airflow (e.g. neuromuscular junction, respiratory muscles or respiratory mechanics) and is not due to persistent changes in hypoxic chemosensitivity or central neural integration. The basis of sex differences in this developmental plasticity is unknown.

Cardiopulmonary transition in the high altitude infant

The perinatal cardiopulmonary transition at high altitude differs from that at sea level because oxygen plays a fundamental role in the developmental changes from fetus to newborn infant. Under conditions of high altitude hypoxia, arterial oxygen saturations are lower, breathing patterns and maturation of respiratory control reflexes differ, and regression of fetal characteristics of the pulmonary vasculature proceeds more slowly. Several aspects of transition vary not only with postnatal age and altitude, but also with population group, suggesting an effect of genetic adaptation on perinatal physiology. Exposure to chronic high altitude hypoxia during the perinatal transition also results in apparent lifelong alterations in respiratory reflex responses and pulmonary vasoreactivity. Disruption of the normal process of cardiopulmonary transition can result in symptomatic high altitude pulmonary hypertension. The exaggerated hypoxemia associated with acute respiratory infections in young infants still undergoing transition contributes to infant mortality at high altitude.

Hematological differences during growth among Tibetans and Han Chinese born and raised at high altitude in Qinghai, China

This study describes the hemoglobin concentration ([Hb]) and hematocrit (HCT) of over 1,000 Tibetan and Han children, adolescents, and young adults who were born and raised at 3,200 m, 3,800 m, or 4,300 m in Qinghai Province, western China. At 3,200 m, no altitude effect is evident in the hematological characteristics of either group. At 3,800 m and 4,300 m, both groups show [Hb] and HCT values that are above low-altitude norms. At both altitudes, Tibetan and Han children show no differences in the pattern of hematological response up to age 13. Among adolescents and young adults, however, the [Hb] and HCT of Han males and females are elevated compared to Tibetans. This indicates that the adolescent period may involve a divergence in the responses to hypoxia made by some individuals in these two groups. Also, many other adolescents and young adults in both groups show similar hematological characteristics, indicating that many Tibetans and Han share similar hematological responses to hypoxia.

Polymorphisms of the tyrosine hydroxylase gene in subjects susceptible to high-altitude pulmonary edema

STUDY OBJECTIVES

A blunted hypoxic ventilatory response (HVR) has been observed in some sufferers of high-altitude pulmonary edema (HAPE), and was proposed as a potential mechanism in its pathogenesis. Tyrosine hydroxylase (TH) is a rate-limiting enzyme in the carotid body responding to hypoxia to synthesize dopamine neurotransmitter to heighten ventilation. The association of constitutional susceptibility to HAPE regarding the blunted HVR aspect with polymorphisms of the TH gene was examined.

DESIGN

A cross-sectional case control study.

SETTING

Shinshu University Hospital, Matsumoto, Japan.

PARTICIPANTS

Forty-three subjects with a history of HAPE (HAPE group) and 51 healthy climbers without a history of HAPE (control group).

MEASUREMENTS

The (TCAT)n tetranucleotide microsatellite repeats within intron 1 and Met81Val variant in exon 2 of the TH gene were investigated by polymerase chain reaction following either direct sequencing or restriction fragment length polymorphism. The HVR in 21 subjects among the HAPE group was also measured.

RESULTS

No significant frequency differences could be found in terms of either of the two polymorphisms between the HAPE and control groups. Meanwhile, no relationships were observed between the HVR values of HAPE subjects and the individual alleles in both polymorphisms of the TH gene.

CONCLUSION

The genetic susceptibility of HAPE, specifically the blunted HVR in HAPE, is probably not associated with the mutations of the TH gene, implying that these two polymorphisms may not be a sufficient genetic marker for predicting a predisposition to the susceptibility to HAPE.

Chronic hypoxia-induced morphological and neurochemical changes in the carotid body

The carotid body (CB) plays an important role in the control of ventilation. Type I cells in CB are considered to be the chemoreceptive element which detects the levels of PO(2), PCO(2), and [H(+)] in the arterial blood. These cells originate from the neural crest and appear to retain some neuronal properties. They are excitable and produce a number of neurochemicals. Some of these neurochemicals, such as dopamine and norepinephrine, are considered to be primarily inhibitory to CB function and others, such as adenosine triphosphate, acetylcholine, and endothelin, are thought to be primarily excitatory. Chronic hypoxia (CH) induces profound morphological as well as neurochemical changes in the CB. CH enlarges the size of CB and causes hypertrophy and mitosis of type I cells. Also, CH changes the vascular structure of CB, including inducing marked vasodilation and the growth of new blood vessels. Moreover, CH upregulates certain neurochemical systems within the CB, e.g., tyrosine hydroxylase and dopaminergic activity in type I cells. There is also evidence that CH induces neurochemical changes within the innervation of the CB, e.g., nitric oxide synthase. During CH the sensitivity of the CB chemoreceptors to hypoxia is increased but the mechanisms by which the many CH-induced structural and neurochemical changes affect the sensitivity of CB to hypoxia remains to be established.

Lessons from chronic intermittent and sustained hypoxia at high altitudes

Recurrent sleep apnea (RSA), mimicking chronic intermittent hypoxia (CIH), may trigger unique adaptations in oxygen sensing in the carotid body, and consequent cellular functions unlike the effects of sustained hypoxia (SH). As a mechanism, an augmented generation of reactive oxygen species (ROS) in CIH has been invoked at the exclusion of SH effects. The ROS might act at hypoxia inducible factors (HIF-1s), giving rise to various genes whose function is to restore the tissue P(O(2)) close to the original. In a spate, review articles on the CIH effects at sea level have appeared but little on high altitude (HA). Their views have been reexamined with the primary focus on the peripheral chemoreception. At HA, RSA is more common in the lowlanders because of a high ventilatory sensitivity to hypoxia (with the consequent effects) unlike the high altitude natives (HAN). Undoubtedly, the HIF-1s play a central role at HA, the mechanisms of which are unknown and explorable.

Sleep and Breathing in Recreational Climbers at an Altitude of 4200 and 6400 Meters: Observational Study of Sleep and Patterning of Respiration During Sleep in a Group of Recreational Climbers

Background: The increasing popularity of mountain climbing will result in greater numbers of the general population being at risk for the disturbances known to occur with altitude exposure. Methods: Observations of sleep and breathing were made in 6 healthy travellers (5 males and 1 female, 38 to 62 years of age) before, during, and after a recreational climb. We modified a portable seven channel polygraph to record sleep state, oxygen saturation, respiratory movements, body position, and oronasal airflow 4 weeks prior to the expedition at home (500m), at base camp (4200m) and in 3 climbers at 6400m. All had a repeat study at 500m altitude 4 weeks after the expedition. Results: For the group, the total number of obstructive apneas and hypopneas (OA/H) at night increased from 36 at home to 68 at base camp over a one night recording. Separately counted central apneas and hypopneas (CA/CS) increased from 6.7 to 45. In one climber, who had a history of recurrent snoring and observed apneas at home, the number of apneas increased from 201 at 4200m to 322 at 6400m, whereas in 2 climbers measured at 6400m, all apneas decreased. The total sleep time (TST) increased in all 6 climbers by 10% at base camp in comparison to home records. In the 3 climbers attaining an altitude of 6400m, the REM (rapid eye movement) sleep declined by 10% compared to the record at 4200m. Conclusion: Respiratory disturbances at low altitude are amplified by exposure to high and extreme altitude. In those without symptoms of sleep apnea, significant physiologic alterations will occur at high altitude but at extreme altitude regular ventilation is re-established.

Reserved higher vagal tone under acute hypoxia in Tibetan adolescents with long-term migration to sea level

Tibetans are known as one of the largest and oldest high-altitude natives in the world and are among the best high-altitude-adapted ethnic groups. They exhibit greater vagal tone and less sympathetic stimulation than acclimatized lowlanders at high altitudes. Whether young native Tibetans who had spent long-term residence (more than 3 years) at sea level still reserved their unique autonomic characteristics was the main aim of this study. Heart rate variability (HRV) of 10 native young Tibetan male students and 12 Han counterparts were measured at resting supine position at sea level and 1 h after ascent to 3,700 m in a hypobaric chamber (PO(2) = 13.4 kPa). At sea level, Tibetans showed lower heart rate (HR) and greater HRV. At 3,700 m, the increase of HR was greater in the Hans than in the Tibetans, and the HRV was significantly diminished in the Han group but not in the Tibetan group. The results suggested that Tibetans had a greater parasympathetic dominance over the heart at rest, and acute moderate (3,700 m) hypoxia did not influence their HRV significantly, but it did on the Han subjects. We concluded that the long-term residence of the Tibetans at sea level did not change their unique characteristics of the autonomic systems.

Ventilatory and cardiovascular responses to hypoxia and exercise in Andean natives living at sea level

This study was designed to determine in subjects born at high altitude who move to sea level (HA-SL: born at 3500 m or above; n = 25) whether their cardiorespiratory responses to hypoxia and exercise are similar to those of sea level natives (SL,n = 25). The average age (39 +/- 7.3 yr), weight (72 +/- 7.3 kg), and height (1.71 +/- 0.01 m) did not differ between the SL and HA-SL subjects. All subjects were studied at rest or during exercise (60 W on cycle ergometer) while breathing room air (F(IO2) = 0.21 and P(B) = 760) or hypoxia (F(IO2) = 0.115 and PB = 760) in the following order: (1) normoxia at rest (NX-Rs), (2) hypoxia at rest (HX-Rs, 11.5% O(2)), hypoxia at exercise (HX-Ex), and normoxia at exercise (NX-Ex). Each period lasted 5 min. In absolute values, HA-SL showed significantly higher ventilation (V(E), L/min) during exercise in both normoxia and hypoxia and higher oxygen saturation (Sa(O2), %) during hypoxia both at rest and in exercise. They also had lower end-tidal CO(2) values (P(ETCO2), torr) at rest in both normoxia and hypoxia, but a higher P(ETCO2) in hypoxic exercise. Heart rate (HR, beats/min) was lower at rest in both normoxia and hypoxia, but higher in exercise. With acute hypoxia, Sa(O2) decreased less in the HA-SL than in the SL at rest (HA-SL, 9.2 +/- 0.8; SL, 12.0 +/- 0.82) and during exercise (HA-SL, 18.3 +/- 1.1; SL, 21.2 +/- 1.2). In conclusion, this study shows that HA-SL natives have increased ventilation and heart rate during exercise once their lifelong hypoxia is relieved.

The evidence for hereditary factors contributing to high altitude adaptation in Andean natives: a review

Humans have occupied the high plateaus and mountain valleys of the Andes and the Himalayas for thousands of years. Although sea level natives can, and often do, travel in these rarefied reaches, there is little doubt that natives born and raised in the "thin" air are better equipped to deal with the reduced availability of oxygen at altitude. What fraction of the hypoxia defense response of high altitude native populations is due to developmental adaptations acquired during growth and what fraction is due to a genetic component reflecting the effects of selective transmission of beneficial genetic variants through hundreds of generations of antecedents is as yet unresolved. This paper summarizes some of the studies that have been undertaken to address this issue in Andean indigenous populations, primarily with respect to those adaptations thought to be involved in the uptake, distribution and utilization of oxygen in children and adults. Specifically, it focuses on changes in chest morphology, pulmonary function, metabolism and hematology. Space constraints preclude extending this review to the large body of literature concerning prenatal and maternal adaptations although this critical stage in development has likely been subject to significant selective pressures. It is apparent that both nature and nurture influence the acquisition of a high altitude phenotype in humans and while there is some evidence for genetic adaptation in Andean highlanders, it is evident that these characteristics are expressed in concert with substantial environment-dependent developmental adjustments.

Altered structure and function of the carotid body at high altitude and associated chemoreflexes

The ventilatory response to hypoxia is complex. First contact with hypoxia causes an increase in ventilation within seconds that reaches full intensity within minutes because of an increase in carotid sinus nerve (CSN) input to the brain stem. With continued exposure, ventilation increases further over days (ventilatory acclimatization). Initially, it was hypothesized that ventilatory acclimatization arose from a central nervous system (CNS) mechanism. Compensation for alkalosis in the brain and restoration of pH in the vicinity of central chemoreceptors was believed to cause the secondary increase in ventilation. However, when this hypothesis could not be substantiated, attention was turned to the peripheral chemoreceptors. With the lowering of arterial PO2 at high altitude, there is an immediate increase in firing of afferents from chemoreceptors in the carotid body. After peaking over the next few minutes, the firing rate of afferents begins to rise again within hours until a steady state is reached. This secondary increase occurs along with increase in neurotransmitter synthesis and release and altered gene expression followed by hypertrophy of carotid body glomus cells. Further exposure to hypoxia eventually leads to blunting of the CSN output and ventilatory response in some species. This mini review is about the altered structure and function of the carotid body at high altitude and the associated blunting of the chemoreceptor and ventilatory responses observed in some species.

Developmental components of resting ventilation among high- and low-altitude Andean children and adults

This paper evaluates the age-associated changes of resting ventilation of 115 high- and low-altitude Aymara subjects, of whom 61 were from the rural Aymara village of Ventilla situated at an average altitude of 4,200 m and 54 from the rural village of Caranavi situated at an average altitude of 900 m. Comparison of the age patterns of resting ventilation suggests the following conclusions: 1) the resting ventilation (ml/kg/min) of high-altitude natives is markedly higher than that of low-altitude natives; 2) the age decline of ventilation is similar in both lowlanders and highlanders, but the starting point and therefore the age decline are much higher at high altitude; 3) the resting ventilation that characterizes high-altitude Andean natives is developmentally expressed in the same manner as it is at low altitude; and 4) the resting ventilation (ml/kg/min) of Aymara high-altitude natives is between 40-80% lower than that of Tibetans.