Sleep-related hypoxaemia and excessive erythrocytosis in Andean high-altitude natives.

To determine whether nocturnal hypoxaemia contributes to the excessive erythrocytosis (EE) in Andean natives, standard polysomnographies were performed in 10 patients with EE and in 10 controls (mean haematocrit 76.6 +/- 1.3% and 5.4 +/- 0.8%, respectively) living at an altitude of 4,380 m. In addition, the effect of O2 administration for 1 h prior to sleep, and the relationship between the hypoxic/hypercapnic ventilatory response and the apnoea/hypopnoea index (AHI) during sleep were studied. Awake arterial oxygen saturation (Sa,O2) was significantly lower in patients with EE than in controls (83.7 +/- 0.3% versus 85.6 +/- 0.4%). In both groups, the mean Sa,O2 significantly decreased during sleep (to 80.0 +/- 0.8% in EE and to 82.8 +/- 0.5% in controls). The mean Sa,O2 values remained significantly lower in patients with EE than in controls at all times of the night, and patients with EE spent significantly more time than the controls with an Sa,O2 of <80%. There were no differences between the two groups in the number and duration of the apnoeas/hypopnoeas. None of these variables were affected by O2 administration. In both groups the AHI positively correlated with the hypercapnic ventilatory response. Andean natives undergo minor respiratory disorders during sleep. The reduction in oxygen saturation found in subjects with excessive erythrocytosis was small, yet consistent and potentially important, as it remained below the threshold known for the increase in erythropoietin stimulation. This may be an important factor promoting erythropoiesis, but its relevance needs to be further explored.     

From the Cover: Contribution of a mutational hot spot to hemoglobin adaptation in high-altitude Andean house wrens

A key question in evolutionary genetics is why certain mutations or certain types of mutation make disproportionate contributions to adaptive phenotypic evolution. In principle, the preferential fixation of particular mutations could stem directly from variation in the underlying rate of mutation to function-altering alleles. However, the influence of mutation bias on the genetic architecture of phenotypic evolution is difficult to evaluate because data on rates of mutation to function-altering alleles are seldom available. Here, we report the discovery that a single point mutation at a highly mutable site in the β^A-globin gene has contributed to an evolutionary change in hemoglobin (Hb) function in high-altitude Andean house wrens (Troglodytes aedon). Results of experiments on native Hb variants and engineered, recombinant Hb mutants demonstrate that a nonsynonymous mutation at a CpG dinucleotide in the β^A-globin gene is responsible for an evolved difference in Hb–O₂ affinity between high- and low-altitude house wren populations. Moreover, patterns of genomic differentiation between high- and low-altitude populations suggest that altitudinal differentiation in allele frequencies at the causal amino acid polymorphism reflects a history of spatially varying selection. The experimental results highlight the influence of mutation rate on the genetic basis of phenotypic evolution by demonstrating that a large-effect allele at a highly mutable CpG site has promoted physiological differentiation in blood O₂ transport capacity between house wren populations that are native to different elevations.An important question in evolutionary genetics is whether certain mutations or certain types of mutation make disproportionate contributions to phenotypic evolution (1–6). Within a given gene, the mutations that contribute to evolutionary changes in phenotype may represent a biased, nonrandom subset of all possible mutations that are capable of producing the same functional effect. The preferential fixation of particular mutations (substitution bias) could have several causes. Most theoretical and empirical attention has focused on causes of fixation bias, i.e., mutations have different probabilities of being fixed once they arise, due to differences in dominance coefficients or the magnitude of deleterious pleiotropy (1, 2, 4, 7–9). In principle, substitution bias can also stem directly from mutation bias (some sites have higher rates of mutation to alleles that produce the change in phenotype) (4, 9–11). However, empirical evidence for the importance of mutation bias is scarce for an obvious reason: even in rare cases where it is possible to document the contributions of individual point mutations to evolutionary changes in phenotype, data on rates of mutation to function-altering alleles are typically lacking. Rare exceptions include cases where loss-of-function deletion mutations can be traced to hot spots of chromosomal instability or highly mutable changes in the copy number of repetitive elements (12). Documenting cases where genetic changes at highly mutable loci contribute to phenotypic divergence is therefore important for elucidating the evolutionary significance of mutation bias. This is especially true for cases where mutations cause fine-tuned modifications of protein activity rather than simple losses of function.Here, we report the discovery that a single amino acid replacement at a mutational hot spot in the avian β^A-globin gene has contributed to an evolutionary change in hemoglobin (Hb) function that has likely adaptive significance. By conducting experiments on native Hb variants and engineered recombinant Hb mutants, we demonstrate that a nonsynonymous mutation at a CpG dinucleotide in the β^A-globin gene of Andean house wrens (Troglodytes aedon) has contributed to an evolved difference in Hb–O₂ affinity between high- and low-altitude populations. In mammalian genomes, point mutations at CpG sites occur at a rate that is over an order of magnitude higher than the average for all other nucleotide sites (13, 14), and available data suggest a similar discrepancy in avian genomes (15, 16).Andean house wrens are compelling subjects for studies of Hb function because this passerine bird species has an exceptionally broad and continuous elevational distribution, ranging from sea level to elevations >4,500 m (17). At 4,500-m elevation, the standard barometric pressure is ∼450 torr, so O₂ partial pressure (PO₂) is <60% that at sea level (∼96 torr compared to ∼160 torr). Under such conditions, enhancements of pulmonary O₂ uptake and blood O₂ transport capacity are required to sustain O₂ flux to the tissue mitochondria in support of aerobic ATP synthesis (18). To complement changes in the cardiorespiratory system and microcirculation, changes in the O₂-binding affinity and cooperativity of Hb can enhance the O₂ capacitance of the blood (the total amount of O₂ unloaded for a given arteriovenous difference in O₂ tension). Because the optimal Hb–O₂ affinity is expected to vary according to the ambient PO₂, genetic variation in oxygenation properties of Hb may be subject to spatially varying selection between populations that inhabit different elevations. House wrens colonized South America in the late Pliocene or early Pleistocene via the newly formed Panamanian land bridge (19, 20), so the species may have been resident in the Andean highlands for up to ∼3 million years.The Hb tetramer is composed of two semirigid α₁β₁ and α₂β₂ dimers that undergo a mutual rotation during the oxygenation-linked transition in quaternary structure between the deoxy (low-affinity “T”) conformation and the oxy (high-affinity “R”) conformation (21). This oxygenation-linked structural transition between the T and R states is the basis for cooperative O₂ binding, and is central to the allosteric function of Hb as an O₂ transport molecule. Our analysis of house wren Hb highlights the influence of mutation rate on the genetic basis of phenotypic divergence by demonstrating that mutation at a CpG dinucleotide produced a large-effect amino acid replacement at an α₁β₁ intradimer contact (β55Val→Ile)—a replacement that produced a significant increase in Hb–O₂ affinity.     

Peripheral arterial vascular function at altitude: sea-level natives versus Himalayan high-altitude natives

OBJECTIVES: Regulation of the vascular system may limit physical performance and contribute to adaptation to high altitude. We evaluated vascular function in 10 Himalayan high-altitude natives and 10 recently acclimatized sea-level natives at an altitude of 5,050 m. METHODS: We registered electrocardiogram, blood flow velocity in the common femoral artery, and blood pressure in the radial artery using non-invasive methods under baseline conditions, and during maximal vasodilation after 2 min leg occlusion. Vascular mechanics were characterized by estimating pulse wave velocity and input impedance. RESULTS: Pulse wave velocity and parameters of input impedance did not differ between groups under baseline conditions. In the post-ischemic period, the ratio between maximal hyperemic and baseline blood flow velocity was significantly higher in the high-altitude than in the sea-level natives (5.7 +/- 2.5 versus 3.8 +/- 1.2, P < 0.05). The leg vascular resistance decreased in the post-occlusive period without differences between groups. Characteristic impedance decreased in the post-ischemic period by about one third of the baseline level without differences between groups. The post-ischemic decrease of input impedance modulus was more marked in the high-altitude than in the sea-level natives at low frequencies (28 +/- 12 versus 6.4 +/- 20% at 2 Hz, P < 0.01). CONCLUSIONS: Our results demonstrate a superior ability to increase blood flow velocity as a response to muscular ischemia in high-altitude natives compared to sea-level natives. This phenomenon may be associated with a more effective coupling between blood pressure and blood flow which is probably caused by differences in conduit vessel function.     

Overall and regional lung function in Andean natives after descent to low altitude.

Overall lung volumes, regional residual volume to total lung capacity ratio (RVr/TLCr), regional ventilation (V/V) and perfusion (Q/V) were measured at 670 m in six Quechua Indians on days 2 and 37 after leaving their high-altitude homes (3500-4500 m). On day 2 the lung volumes averaged between 124 and 137% of those predicted for low-altitude residents (LAR) and there were no significant changes on day 37. Although overall RV/TLC was not different from the predicted value for LAR, RVr/TLCr on day 2 was higher at the top and lower at the bottom of the lungs compared to LAR. Regional Q/V and V/V were not different from LAR on day 2, or on day 37. However, the ratio of Q/V at the bottom to Q/V at the top was 2.36 on day 2 and 2.84 on day 37 (P less than 0.05). On day 2 hemoglobin- and volume-corrected diffusing capacity was 145% of the value predicted for LAR and this fell to 135% predicted on day 37 (P less than 0.05). Natives of high altitude reportedly have more alveoli that LAR and this could explain the greater vital capacity toward the bottom of the lung if the alveolar proliferation is concentrated there. This might also lower pulmonary vascular resistance at the bottom which would explain the normal Q/V distribution, even though pulmonary artery pressure may be increased.     

Evolutionary and functional insights into the mechanism underlying high-altitude adaptation of deer mouse hemoglobin

Adaptive modifications of heteromeric proteins may involve genetically based changes in single subunit polypeptides or parallel changes in multiple genes that encode distinct, interacting subunits. Here we investigate these possibilities by conducting a combined evolutionary and functional analysis of duplicated globin genes in natural populations of deer mice (Peromyscus maniculatus) that are adapted to different elevational zones. A multilocus analysis of nucleotide polymorphism and linkage disequilibrium revealed that high-altitude adaptation of deer mouse hemoglobin involves parallel functional differentiation at multiple unlinked gene duplicates: two α-globin paralogs on chromosome 8 and two β-globin paralogs on chromosome 1. Differences in O₂-binding affinity of the alternative β-chain hemoglobin isoforms were entirely attributable to allelic differences in sensitivity to 2,3-diphosphoglycerate (DPG), an allosteric cofactor that stabilizes the low-affinity, deoxygenated conformation of the hemoglobin tetramer. The two-locus β-globin haplotype that predominates at high altitude is associated with suppressed DPG-sensitivity (and hence, increased hemoglobin-O₂ affinity), which enhances pulmonary O₂ loading under hypoxia. The discovery that allelic differences in DPG-sensitivity contribute to adaptive variation in hemoglobin–O₂ affinity illustrates the value of integrating evolutionary analyses of sequence variation with mechanistic appraisals of protein function. Investigation into the functional significance of the deer mouse β-globin polymorphism was motivated by the results of population genetic analyses which revealed evidence for a history of divergent selection between elevational zones. The experimental measures of O₂-binding properties corroborated the tests of selection by demonstrating a functional difference between the products of alternative alleles.     

An Ethiopian pattern of human adaptation to high-altitude hypoxia

We describe, in Ethiopia, a third successful pattern of human adaptation to high-altitude hypoxia that contrasts with both the Andean "classic" (erythrocytosis with arterial hypoxemia) and the more recently identified Tibetan (normal venous hemoglobin concentration with arterial hypoxemia) patterns. A field survey of 236 Ethiopian native residents at 3,530 m (11,650 feet), 14-86 years of age, without evidence of iron deficiency, hemoglobinopathy, or chronic inflammation, found an average hemoglobin concentration of 15.9 and 15.0 gdl for males and females, respectively, and an average oxygen saturation of hemoglobin of 95.3%. Thus, Ethiopian highlanders maintain venous hemoglobin concentrations and arterial oxygen saturation within the ranges of sea level populations, despite the unavoidable, universal decrease in the ambient oxygen tension at high altitude.     

The treatment of hypertension by adaptation to high-altitude hypoxia]

Ethnically close male populations aged 30-59 years who reside in high mountains (2800-3600 m above the sea level) and in low lands (800-900 m above the sea level) of the Tien Shan and the Pamirs were screened. The incidence of essential hypertension was found to be significantly lower (4.2%) among the highlanders than in the lowlanders (15.4%). In highlanders, hypertension is characterized by a high concurrence with high-altitude arterial hypertension and right ventricular hypertrophy. Daily urinary aldosterone excretion is substantially lower in residents of high mountains and natriuresis is higher than that in those from low-lands. Sixty-eight patients with mild hypertension took daily treatment as pressure chamber uplifting (3200 m above the sea level; pO2 112 mm Hg) for 15 days and 45 patients with moderate hypertension were treated with medium-land (1600 m; pO2 134 mm Hg) adaptation for 24 days. Pressure chamber hypoxic training and medium-land adaptation in 69% of patients with mild hypertension and 64.4% with moderate hypertension without signs of target organ lesions produced steady antihypertensive effects, a steady-state (for a 1.5-year follow-up) decrease in cardiac mechanical work, peripheral vascular resistance, arterial impedance, and an improvement of physical fitness. It was proposed to use hypoxic (pressure chamber and high-altitude) training for the treatment of early hypertensive disease and its secondary prevention.     

Two routes to emotional memory: distinct neural processes for valence and arousal

Prior investigations have demonstrated that emotional information is often better remembered than neutral information, but they have not directly contrasted effects attributable to valence and those attributable to arousal. By using functional MRI and behavioral studies, we found that distinct cognitive and neural processes contribute to emotional memory enhancement for arousing information versus valenced, nonarousing information. The former depended on an amygdalar-hippocampal network, whereas the latter was supported by a prefrontal cortex-hippocampal network implicated in controlled encoding processes. A behavioral companion study, with a divided-attention paradigm, confirmed that memory enhancement for valenced, nonarousing words relied on controlled encoding processes: concurrent task performance reduced the enhancement effect. Enhancement for arousing words occurred automatically, even when encoding resources were diverted to the secondary task.     

Activation of resistance arteries with endothelin-1: from vasoconstriction to functional adaptation and remodeling

Remodeling of resistance arteries is a key feature in hypertension. We studied the transition of vasoconstriction to remodeling in isolated rat skeletal muscle arterioles. Arterioles activated with 10 nM endothelin-1 showed functional adaptation when kept at low distension in a wire myograph setup, where contractile properties shifted towards a smaller lumen diameter after 1 day. Pressurized arteries kept in organoid culture showed physical inward remodeling after 3-day activation with 10 nM endothelin-1, characterized by a reduction in relaxed diameter without a change in the wall cross-sectional area (eutrophic remodeling). The relaxed lumen diameter (at 60 mm Hg) decreased from 169 +/- 5 (day 0) to 155 +/- 4 microm (day 3). An antibody directed to the beta(3)-integrin subunit (but not one directed to the beta(1)-integrin subunit) enhanced remodeling, from a reduction in relaxed diameter at 60 mm Hg of 15 +/- 2.4 to 22 +/- 1.8 microm (both on day 3). Collagen gel contraction experiments showed that the antibody directed to the beta(3)-integrin subunit enhanced the compaction of collagen by smooth muscle cells, from 83 +/- 1.5 to 68 +/- 1.5% of the initial gel diameter. In conclusion, these data show that inward eutrophic remodeling is a response to sustained contraction, which may involve collagen reorganization through beta(3)-integrins.     

Augmented sympathetic activation during short-term hypoxia and high-altitude exposure in subjects susceptible to high-altitude pulmonary edema

BACKGROUND: Pulmonary hypertension is a hallmark of high-altitude pulmonary edema and may contribute to its pathogenesis. Cardiovascular adjustments to hypoxia are mediated, at least in part, by the sympathetic nervous system, and sympathetic activation promotes pulmonary vasoconstriction and alveolar fluid flooding in experimental animals. METHODS AND RESULTS: We measured sympathetic nerve activity (using intraneural microelectrodes) in 8 mountaineers susceptible to high-altitude pulmonary edema and 7 mountaineers resistant to this condition during short-term hypoxic breathing at low altitude and at rest at a high-altitude laboratory (4559 m). We also measured systolic pulmonary artery pressure to examine the relationship between sympathetic activation and pulmonary vasoconstriction. In subjects prone to pulmonary edema, short-term hypoxic breathing at low altitude evoked comparable hypoxemia but a 2- to 3-times-larger increase in the rate of the sympathetic nerve discharge than in subjects resistant to edema (P<0.001). At high altitude, in subjects prone to edema, the increase in the mean+/-SE sympathetic firing rate was >2 times larger than in those resistant to edema (36+/-7 versus 15+/-4 bursts per minute, P<0.001) and preceded the development of lung edema. We observed a direct relationship between sympathetic nerve activity and pulmonary artery pressure measured at low and high altitude in the 2 groups (r=0.83, P<0.0001). CONCLUSIONS: With the use of direct measurements of postganglionic sympathetic nerve discharge, these data provide the first evidence for an exaggerated sympathetic activation in subjects prone to high-altitude pulmonary edema both during short-term hypoxic breathing at low altitude and during actual high-altitude exposure. Sympathetic overactivation may contribute to high-altitude pulmonary edema.     

Global analysis of predicted proteomes: functional adaptation of physical properties

The physical characteristics of proteins are fundamentally important in organismal function. We used the complete predicted proteomes of >100 organisms spanning the three domains of life to investigate the comparative biology and evolution of proteomes. Theoretical 2D gels were constructed with axes of protein mass and charge (pI) and converted to density estimates comparable across all types and sizes of proteome. We asked whether we could detect general patterns of proteome conservation and variation. The overall pattern of theoretical 2D gels was strongly conserved across all life forms. Nevertheless, coevolved replicons from the same organism (different chromosomes or plasmid and host chromosomes) encode proteomes more similar to each other than those from different organisms. Furthermore, there was disparity between the membrane and nonmembrane subproteomes within organisms (proteins of membrane proteomes are on the average more basic and heavier) and their variation across organisms, suggesting that membrane proteomes evolve most rapidly. Experimentally, a significant positive relationship independent of phylogeny was found between the predicted proteome and Biolog profile, a measure associated with the ecological niche. Finally, we show that, for the smallest and most alkaline proteomes, there is a negative relationship between proteome size and basicity. This relationship is not adequately explained by AT bias at the DNA sequence level. Together, these data provide evidence of functional adaptation in the properties of complete proteomes.     

Prostacyclin-thromboxane imbalance after adrenergic damage of the heart and aorta and its correction with calcium antagonist finoptin and the adaptation to high-altitude climate]

Adrenergic cardiac and aortic lesion was reproduced by using intramuscularly a stress-necrogenic epinephrine dose of 2 mg/kg. Following 24 hours, the levels of malonic dialdehyde (MDA) as a TxA2 marker, were measured in the platelets. The antiaggregability of the aortic wall where PGI2 is synthesized was also examined. The adrenergic lesion was found to impair the platelet function-vascular wall balance, creating the condition for enhancing spontaneous platelet aggregation. This led to 50% death of experimental "lowland" rats. The short-term high-altitude adaptation failed to substantially modify the death rates in adrenergic lesion, though the platelets showed a 30% decrease in MDA generation. The combination of high-altitude adaptation and the calcium antagonist finoptin completely prevented the stress-induced increase in the platelet generation of MDA, a TxA2 marker, which was followed by a drastic reduction (16.6% versus 50%) in sudden death cases among the rats during adrenergic cardiac and aortic lesion.     

Unattenuated structural and biochemical alterations in the rat lung during functional adaptation to ozone

Acute ozone (O3) exposure in humans produces changes in pulmonary function that attenuate with repeated exposure. This phenomenon, termed adaptation, has been produced in unanesthetized rats. Rats exposed to O3 (0, 0.35, 0.5, or 1.0 ppm) for 2.25 h for 5 consecutive days showed an increased frequency of breathing and a decreased tidal volume on Days 1 and 2 of exposure at all O3 concentrations. However, by Day 5 these breathing responses to O3 were diminished in rats exposed to 0.35 and 0.5 ppm, but not in rats exposed to 1.0 ppm. In addition, a flow limitation in smaller airways was observed after the second day of exposure to 0.5 ppm O3 that initially attenuated and then disappeared by the fifth day of exposure. In contrast to these findings, a light microscopic examination of fixed lung tissue sections from rats exposed to 0.5 ppm indicated a 5-day progressive pattern of epithelial damage and inflammation in the terminal bronchiolar region. A sustained 37% increase in lavageable protein was also observed over the course of the 5-day exposure regimen to 0.5 ppm. Lung glutathione increased initially, but it was within the control range on Days 4 and 5. Lung ascorbate was significantly elevated above control levels on Days 3 and 5. These data suggest that attenuation of the pulmonary function response to O3 occurs in laboratory rats with repeated exposure while biochemical and morphologic aspects of the tissue response continue to progress.