Catherine M. Ivy

High-altitude ancestry and hypoxia acclimation have distinct effects on exercise capacity and muscle phenotype in deer mice

The hypoxic and cold environment at high altitudes requires that small mammals sustain high rates of O2 transport for exercise and thermogenesis while facing a diminished O2 availability. We used laboratory-born and -raised deer mice (Peromyscus maniculatus) from highland and lowland populations to determine the interactive effects of ancestry and hypoxia acclimation on exercise performance. Maximal O₂consumption (V̇o(2max)) during exercise in hypoxia increased after hypoxia acclimation (equivalent to the hypoxia at ∼4,300 m elevation for 6-8 wk) and was consistently greater in highlanders than in lowlanders. V̇o(2max) during exercise in normoxia was not affected by ancestry or acclimation. Highlanders also had consistently greater capillarity, oxidative fiber density, and maximal activities of oxidative enzymes (cytochrome c oxidase and citrate synthase) in the gastrocnemius muscle, lower lactate dehydrogenase activity in the gastrocnemius, and greater cytochrome c oxidase activity in the diaphragm. Hypoxia acclimation did not affect any of these muscle traits. The unique gastrocnemius phenotype of highlanders was associated with higher mRNA and protein abundances of peroxisome proliferator-activated receptor γ (PPARγ). Vascular endothelial growth factor (VEGFA) transcript abundance was lower in highlanders, and hypoxia acclimation reduced the expression of numerous genes that regulate angiogenesis and energy metabolism, in contrast to the observed population differences in muscle phenotype. Lowlanders exhibited greater increases in blood hemoglobin content, hematocrit, and wet lung mass (but not dry lung mass) than highlanders after hypoxia acclimation. Genotypic adaptation to high altitude, therefore, improves exercise performance in hypoxia by mechanisms that are at least partially distinct from those underlying hypoxia acclimation.

Control of breathing and the circulation in high-altitude mammals and birds☆

Hypoxia is an unremitting stressor at high altitudes that places a premium on oxygen transport by the respiratory and cardiovascular systems. Phenotypic plasticity and genotypic adaptation at various steps in the O2 cascade could help offset the effects of hypoxia on cellular O2 supply in high-altitude natives. In this review, we will discuss the unique mechanisms by which ventilation, cardiac output, and blood flow are controlled in high-altitude mammals and birds. Acclimatization to high altitudes leads to some changes in respiratory and cardiovascular control that increase O2 transport in hypoxia (e.g., ventilatory acclimatization to hypoxia). However, acclimatization or development in hypoxia can also modify cardiorespiratory control in ways that are maladaptive for O2 transport. Hypoxia responses that arose as short-term solutions to O2 deprivation (e.g., peripheral vasoconstriction) or regional variation in O2 levels in the lungs (i.e., hypoxic pulmonary vasoconstriction) are detrimental at in chronic high-altitude hypoxia. Evolved changes in cardiorespiratory control have arisen in many high-altitude taxa, including increases in effective ventilation, attenuation of hypoxic pulmonary vasoconstriction, and changes in catecholamine sensitivity of the heart and systemic vasculature. Parallel evolution of some of these changes in independent highland lineages supports their adaptive significance. Much less is known about the genomic bases and potential interactive effects of adaptation, acclimatization, developmental plasticity, and trans-generational epigenetic transfer on cardiorespiratory control. Future work to understand these various influences on breathing and circulation in high-altitude natives will help elucidate how complex physiological systems can be pushed to their limits to maintain cellular function in hypoxia.

Control of breathing and ventilatory acclimatization to hypoxia in deer mice native to high altitudes

We compared the control of breathing and heart rate by hypoxia between high‐ and low‐altitude populations of Peromyscus mice, to help elucidate the physiological specializations that help high‐altitude natives cope with O2 limitation.

Mitochondrial physiology in the skeletal and cardiac muscles is altered in torrent ducks, Merganetta armata, from high altitudes in the Andes

ABSTRACT Torrent ducks inhabit fast-flowing rivers in the Andes from sea level to altitudes up to 4500 m. We examined the mitochondrial physiology that facilitates performance over this altitudinal cline by comparing the respiratory capacities of permeabilized fibers, the activities of 16 key metabolic enzymes and the myoglobin content in muscles between high- and low-altitude populations of this species. Mitochondrial respiratory capacities (assessed using substrates of mitochondrial complexes I, II and/or IV) were higher in highland ducks in the gastrocnemius muscle – the primary muscle used to support swimming and diving – but were similar between populations in the pectoralis muscle and the left ventricle. The heightened respiratory capacity in the gastrocnemius of highland ducks was associated with elevated activities of cytochrome oxidase, phosphofructokinase, pyruvate kinase and malate dehydrogenase (MDH). Although respiratory capacities were similar between populations in the other muscles, highland ducks had elevated activities of ATP synthase, lactate dehydrogenase, MDH, hydroxyacyl CoA dehydrogenase and creatine kinase in the left ventricle, and elevated MDH activity and myoglobin content in the pectoralis. Thus, although there was a significant increase in the oxidative capacity of the gastrocnemius in highland ducks, which correlates with improved performance at high altitudes, the variation in metabolic enzyme activities in other muscles not correlated to respiratory capacity, such as the consistent upregulation of MDH activity, may serve other functions that contribute to success at high altitudes. Summary: Torrent ducks display markedly different physiological and enzymatic specializations at high altitude, characterized by increased mitochondrial respiratory capacity, increased myoglobin content and reorganized enzymatic capacities.

Circulatory mechanisms underlying adaptive increases in thermogenic capacity in high-altitude deer mice

ABSTRACT We examined the circulatory mechanisms underlying adaptive increases in thermogenic capacity in deer mice (Peromyscus maniculatus) native to the cold hypoxic environment at high altitudes. Deer mice from high- and low-altitude populations were born and raised in captivity to adulthood, and then acclimated to normoxia or hypobaric hypoxia (simulating hypoxia at ∼4300 m). Thermogenic capacity [maximal O2 consumption (V̇O2,max), during cold exposure] was measured in hypoxia, along with arterial O2 saturation (SaO2) and heart rate (fH). Hypoxia acclimation increased V̇O2,max by a greater magnitude in highlanders than in lowlanders. Highlanders also had higher SaO2 and extracted more O2 from the blood per heartbeat (O2 pulse=V̇O2,max/fH). Hypoxia acclimation increased fH, O2 pulse and capillary density in the left ventricle of the heart. Our results suggest that adaptive increases in thermogenic capacity involve integrated functional changes across the O2 cascade that augment O2 circulation and extraction from the blood. Summary: Adaptive increases in thermogenic capacity in high-altitude deer mice involve integrated functional changes across the O2 cascade that augment O2 circulation and extraction from the blood.

Respiratory mechanics of eleven avian species resident at high and low altitude

ABSTRACT The metabolic cost of breathing at rest has never been successfully measured in birds, but has been hypothesized to be higher than in mammals of a similar size because of the rocking motion of the avian sternum being encumbered by the pectoral flight muscles. To measure the cost and work of breathing, and to investigate whether species resident at high altitude exhibit morphological or mechanical changes that alter the work of breathing, we studied 11 species of waterfowl: five from high altitudes (>3000 m) in Perú, and six from low altitudes in Oregon, USA. Birds were anesthetized and mechanically ventilated in sternal recumbency with known tidal volumes and breathing frequencies. The work done by the ventilator was measured, and these values were applied to the combinations of tidal volumes and breathing frequencies used by the birds to breathe at rest. We found the respiratory system of high-altitude species to be of a similar size, but consistently more compliant than that of low-altitude sister taxa, although this did not translate to a significantly reduced work of breathing. The metabolic cost of breathing was estimated to be between 1 and 3% of basal metabolic rate, as low or lower than estimates for other groups of tetrapods. Highlighted Article: Work and cost of breathing in 11 species of waterfowl are reported to be lower than previously predicted in birds, but without effect of altitudinal habitat despite mechanical differences in highland species.

Ventilatory acclimatization to hypoxia in mice: Methodological considerations

We examined ventilatory acclimatization to hypoxia (VAH) in CD1 mice, and contrasted results obtained using the barometric method on unrestrained mice with pneumotachography and pulse oximetry on restrained mice. Responses to progressive step reductions in O2 fraction (21%-8%) were assessed in mice acclimated to normoxia and hypobaric hypoxia (barometric pressure of 60kPa for 6-8 weeks). Hypoxia acclimation increased the hypoxic ventilatory response (primarily by increasing breathing frequency rather than tidal volume), arterial O2 saturation (SaO2) and heart rate in deep hypoxia, hypoxic chemosensitivity (ventilatory O2/CO2 equivalents versus SaO2), and respiratory water loss, and it blunted the hypoxic depression of metabolism and body temperature. Although some effects of hypoxia acclimation were qualitatively similar between methods, the effects were often greater in magnitude when assessed using pneumotachography. Furthermore, whereas hypoxia acclimation reduced ventilatory O2 equivalent and increased pulmonary O2 extraction in barometric experiments, it had the opposite effects in pneumotachography experiments. Our findings highlight the importance of considering the impact of how breathing is measured on the apparent responses to hypoxia.

Validation of a Pulse Oximetry System for High-Altitude Waterfowl by Examining the Hypoxia Responses of the Andean Goose (Chloephaga melanoptera)

Hypoxia at high altitudes constrains O2 supply to support metabolism, thermoregulation in the cold, and exercise. High-altitude natives that somehow overcome this challenge—who live, reproduce, and sometimes perform impressive feats of exercise at high altitudes—are a powerful group in which to study the evolution of physiological systems underlying hypoxia resistance. Here, we sought to determine whether a common pulse oximetry system for rodents (MouseOx Plus) can be used reliably in studies of high-altitude birds by examining the hypoxia responses of the Andean goose. We compared concurrent measurements of heart rate obtained using pulse oximetry versus electrocardiography. We also compared our measurements of peripheral arterial O2 saturation (SaO2) in uncannulated birds with published data collected from blood samples in birds that were surgically implanted arterial cannulae. Responses to acute hypoxia were measured during stepwise reductions in inspired partial pressure of O2. Andean geese exhibited very modest breathing and heart rate responses to hypoxia but were nevertheless able to maintain normal O2 consumption rates during severe hypoxia exposure down to 5 kPa O2. There were some minor quantitative differences between uncannulated and cannulated birds, which suggest that surgery, cannulation, and/or other sources of variability between studies had modest effects on the hypoxic ventilatory response, heart rate, blood hemoglobin, and hematocrit. Nevertheless, measurements of heart rate and SaO2 by pulse oximetry had small standard errors and were generally concordant and well correlated with measurements using other techniques. We conclude that the MouseOx Plus pulse oximetry system can be a valuable tool for studying the cardiorespiratory physiology of waterfowl without the deleterious effects of surgery/cannulation.

Maladaptive phenotypic plasticity in cardiac muscle growth is suppressed in high-altitude deer mice

How often phenotypic plasticity acts to promote or inhibit adaptive evolution is an ongoing debate among biologists. Recent work suggests that adaptive phenotypic plasticity promotes evolutionary divergence, though several studies have also suggested that maladaptive plasticity can potentiate adaptation. The role of phenotypic plasticity, adaptive, or maladaptive, in evolutionary divergence remains controversial. We examined the role of plasticity in evolutionary divergence between two species of Peromyscus mice that differ in native elevations. We used cardiac mass as a model phenotype, since ancestral hypoxia‐induced responses of the heart may be both adaptive and maladaptive at high‐altitude. While left ventricle growth should enhance oxygen delivery to tissues, hypertrophy of the right ventricle can lead to heart failure and death. We compared left‐ and right‐ventricle plasticity in response to hypoxia between captive‐bred P. leucopus (representing the ancestral lowland condition) and P. maniculatus from high‐altitude. We found that maladaptive ancestral plasticity in right ventricle hypertrophy is reduced in high‐altitude deer mice. Analysis of the heart transcriptome suggests that changes in expression of inflammatory signaling genes, particularly interferon regulatory factors, contribute to the suppression of right ventricle hypertrophy. We found weak evidence that adaptive plasticity of left ventricle mass contributes to evolution. Our results suggest that selection to suppress ancestral maladaptive plasticity plays a role in adaptation.

Evolved changes in breathing and CO2 sensitivity in deer mice native to high altitudes

We examined the control of breathing by O2 and CO2 in deer mice native to high altitude to help uncover the physiological specializations used to cope with hypoxia in high-altitude environments. Highland deer mice ( Peromyscus maniculatus) and lowland white-footed mice ( P. leucopus) were bred in captivity at sea level. The first and second generation progeny of each population was raised to adulthood and then acclimated to normoxia or hypobaric hypoxia (12 kPa O2, simulating hypoxia at ~4,300 m) for 6-8 wk. Ventilatory responses to poikilocapnic hypoxia (stepwise reductions in inspired O2) and hypercapnia (stepwise increases in inspired CO2) were then compared between groups. Both generations of lowlanders appeared to exhibit ventilatory acclimatization to hypoxia (VAH), in which hypoxia acclimation enhanced the hypoxic ventilatory response and/or made the breathing pattern more effective (higher tidal volumes and lower breathing frequencies at a given total ventilation). In contrast, hypoxia acclimation had no effect on breathing in either generation of highlanders, and breathing was generally similar to hypoxia-acclimated lowlanders. Therefore, attenuation of VAH may be an evolved feature of highlanders that persists for multiple generations in captivity. Hypoxia acclimation increased CO2 sensitivity of breathing, but in this case, the effect of hypoxia acclimation was similar in highlanders and lowlanders. Our results suggest that highland deer mice have evolved high rates of alveolar ventilation that are unaltered by exposure to chronic hypoxia, but they have preserved ventilatory sensitivity to CO2.