Cardiovascular responses to progressive hypoxia in ducks native to high altitude in the Andes.

The cardiovascular system is critical for delivering O2 to tissues. Here we examine the cardiovascular responses to progressive hypoxia in four high-altitude Andean duck species compared to four related low-altitude populations in North America, tested at their native altitude. Ducks were exposed to stepwise decreases in inspired partial pressure of O2 while we monitored heart rate, O2 consumption rate, blood O2 saturation, haematocrit (Hct), and blood haemoglobin concentration [Hb]. We calculated O2 pulse (the product of stroke volume and the arterial-venous O2 content difference), blood O2 concentration, and heart rate variability. Regardless of altitude, all eight populations maintained O2 consumption rate with minimal change in heart rate or O2 pulse, indicating that O2 consumption was maintained by either a constant arterial-venous O2 content difference (an increase in the relative O2 extracted from arterial blood) or by a combination of changes in stroke volume and the arterial-venous O2 content difference. Three high-altitude taxa (yellow-billed pintails, cinnamon teal, and speckled teal) had higher Hct and [Hb], increasing the O2 content of arterial blood, and potentially providing a greater reserve for enhancing O2 delivery during hypoxia. Hct and [Hb] between low- and high-altitude populations of ruddy duck were similar, representing a potential adaptation to diving life. Heart rate variability was generally lower in high-altitude ducks, concurrent with similar or lower heart rates than low-altitude ducks, suggesting a reduction in vagal and sympathetic tone. These unique features of the Andean ducks differ from previous observations in both Andean geese and bar-headed geese, neither of which exhibit significant elevations in Hct or [Hb] compared to their low-altitude relatives, revealing yet another avian strategy for coping with high altitude.

Cardiovascular responses to progressive hypoxia in ducks native to high altitude in the Andes.

The cardiovascular system is critical for delivering O2 to tissues. Here, we examined the cardiovascular responses to progressive hypoxia in four high-altitude Andean duck species compared with four related low-altitude populations in North America, tested at their native altitude. Ducks were exposed to stepwise decreases in inspired partial pressure of O2 while we monitored heart rate, O2 consumption rate, blood O2 saturation, haematocrit (Hct) and blood haemoglobin (Hb) concentration. We calculated O2 pulse (the product of stroke volume and the arterial-venous O2 content difference), blood O2 concentration and heart rate variability. Regardless of altitude, all eight populations maintained O2 consumption rate with minimal change in heart rate or O2 pulse, indicating that O2 consumption was maintained by either a constant arterial-venous O2 content difference (an increase in the relative O2 extracted from arterial blood) or by a combination of changes in stroke volume and the arterial-venous O2 content difference. Three high-altitude taxa (yellow-billed pintails, cinnamon teal and speckled teal) had higher Hct and Hb concentration, increasing the O2 content of arterial blood, and potentially providing a greater reserve for enhancing O2 delivery during hypoxia. Hct and Hb concentration between low- and high-altitude populations of ruddy duck were similar, representing a potential adaptation to diving life. Heart rate variability was generally lower in high-altitude ducks, concurrent with similar or lower heart rates than low-altitude ducks, suggesting a reduction in vagal and sympathetic tone. These unique features of the Andean ducks differ from previous observations in both Andean geese and bar-headed geese, neither of which exhibit significant elevations in Hct or Hb concentration compared with their low-altitude relatives, revealing yet another avian strategy for coping with high altitude.

Control of breathing and respiratory gas exchange in high-altitude ducks native to the Andes.

We examined the control of breathing and respiratory gas exchange in six species of high-altitude duck that independently colonized the high Andes. We compared ducks from high-altitude populations in Peru (Lake Titicaca at ∼3800 m above sea level; Chancay River at ∼3000-4100 m) with closely related populations or species from low altitude. Hypoxic ventilatory responses were measured shortly after capture at the native altitude. In general, ducks responded to acute hypoxia with robust increases in total ventilation and pulmonary O2 extraction. O2 consumption rates were maintained or increased slightly in acute hypoxia, despite ∼1-2°C reductions in body temperature in most species. Two high-altitude taxa - yellow-billed pintail and torrent duck - exhibited higher total ventilation than their low-altitude counterparts, and yellow-billed pintail exhibited greater increases in pulmonary O2 extraction in severe hypoxia. In contrast, three other high-altitude taxa - Andean ruddy duck, Andean cinnamon teal and speckled teal - had similar or slightly reduced total ventilation and pulmonary O2 extraction compared with low-altitude relatives. Arterial O2 saturation (SaO2 ) was elevated in yellow-billed pintails at moderate levels of hypoxia, but there were no differences in SaO2  in other high-altitude taxa compared with their close relatives. This finding suggests that improvements in SaO2  in hypoxia can require increases in both breathing and haemoglobin-O2 affinity, because the yellow-billed pintail was the only high-altitude duck with concurrent increases in both traits compared with its low-altitude relative. Overall, our results suggest that distinct physiological strategies for coping with hypoxia can exist across different high-altitude lineages, even among those inhabiting very similar high-altitude habitats.

Negligible differences in metabolism and thermal tolerance between diploid and triploid Atlantic salmon (Salmo salar).

The mechanisms that underlie thermal tolerance in aquatic ectotherms remain unresolved. Triploid fish have been reported to exhibit lower thermal tolerance than diploids, offering a potential model organism to better understand the physiological drivers of thermal tolerance. Here, we compared triploid and diploid juvenile Atlantic salmon (Salmo salar) in freshwater to investigate the proposed link between aerobic capacity and thermal tolerance. We measured specific growth rates (SGR) and resting (aerobic) metabolic rates (RMR) in freshwater at 3, 7 and 9 weeks of acclimation to 10, 14 and 18°C. Additionally, maximum metabolic rates (MMR) were measured at 3 and 7 weeks of acclimation, and critical thermal maxima (CTmax) were measured at 9 weeks. Mass, SGR and RMR differed between ploidies across all temperatures at the beginning of the acclimation period, but all three metrics were similar across ploidies by week 7. Aerobic scope (MMR-RMR) remained consistent across ploidies, acclimation temperatures and time. At 9 weeks, CTmax was independent of ploidy, but correlated positively with acclimation temperature despite the similar aerobic scope between acclimation groups. Our findings suggest that acute thermal tolerance is not modulated by aerobic scope, and the altered genome of triploid Atlantic salmon does not translate to reduced thermal tolerance of juvenile fish in freshwater.

Divergent respiratory and cardiovascular responses to hypoxia in bar-headed geese and Andean birds.

Many high-altitude vertebrates have evolved increased capacities in their oxygen transport cascade (ventilation, pulmonary diffusion, circulation and tissue diffusion), enhancing oxygen transfer from the atmosphere to mitochondria. However, the extent of interspecies variation in the control processes that dictate hypoxia responses remains largely unknown. We compared the metabolic, cardiovascular and respiratory responses to progressive decreases in inspired oxygen levels of bar-headed geese (Anser indicus), birds that biannually migrate across the Himalayan mountains, with those of Andean geese (Chloephaga melanoptera) and crested ducks (Lophonetta specularioides), lifelong residents of the high Andes. We show that Andean geese and crested ducks have evolved fundamentally different mechanisms for maintaining oxygen supply during low oxygen (hypoxia) from those of bar-headed geese. Bar-headed geese respond to hypoxia with robust increases in ventilation and heart rate, whereas Andean species increase lung oxygen extraction and cardiac stroke volume. We propose that transient high-altitude performance has favoured the evolution of robust convective oxygen transport recruitment in hypoxia, whereas life-long high-altitude residency has favoured the evolution of structural enhancements to the lungs and heart that increase lung diffusion and stroke volume.

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

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.

Hypoxic acclimation leads to metabolic compensation after reoxygenation in Atlantic salmon yolk-sac alevins.

Hypoxia is common in aquatic environments and has substantial effects on development, metabolism and survival of aquatic organisms. To understand the physiological effects of hypoxia and its dependence on temperature, metabolic rate ( [Formula: see text] ) and cardiorespiratory function were studied in response to acute hypoxia (21→5kPa) at different measurement temperatures (Ta; 4, 8 and 12°C) in Salmo salar alevins that were incubated under normoxic conditions (PO2=21kPa) or following hypoxic acclimation (PO2=10kPa) as well as two different temperatures (4°C or 8°C). Hypoxic acclimation lead to a developmental delay manifested through slower yolk absorption. The general response to acute hypoxia was metabolic depression (~60%). Hypoxia acclimated alevins had higher [Formula: see text] s when measured in normoxia than alevins acclimated to normoxia. [Formula: see text] s were elevated to the same degree (~30% per 4°C change) irrespective of Ta. Under severe, acute hypoxia (~5kPa) and irrespective of Ta or acclimation, [Formula: see text] s were similar between most groups. This suggests that despite different acclimation regimes, O2 transport was limited to the same degree. While cardiorespiratory function (heart-, ventilation rate) was unchanged in response to acute hypoxia after normoxic acclimation, hypoxic acclimation led to cardiorespiratory changes predominantly in severe hypoxia, indicating earlier onset and plasticity of cardiorespiratory control mechanisms. Although [Formula: see text] in normoxia was higher after hypoxic acclimation, at the respective acclimation PO2, [Formula: see text] was similar in normoxia and hypoxia acclimated alevins. This is indicative of metabolic compensation to an intrinsic [Formula: see text] at the acclimation condition in hypoxia-acclimated alevins after re-exposure to normoxia.

How bar-headed geese fly over the Himalayas.

Bar-headed geese cross the Himalayas on one of the most iconic high-altitude migrations in the world. Heart rates and metabolic costs of flight increase with elevation and can be near maximal during steep climbs. Their ability to sustain the high oxygen demands of flight in air that is exceedingly oxygen-thin depends on the unique cardiorespiratory physiology of birds in general along with several evolved specializations across the O2 transport cascade.

α4-Containing nicotinic receptors contribute to the effects of perinatal nicotine on ventilatory and metabolic responses of neonatal mice to ambient cooling.

Among numerous studies, perinatal nicotine exposure (PN) has had variable effects on respiratory control in the neonatal period. The effects of acute nicotine exposure on breathing are largely mediated by α4-containing nicotine acetylcholine receptors (nAChRs). These receptors are also involved in thermoregulatory responses induced by both acetylcholine and nicotine. We therefore hypothesized that α4-containing nAChRs would mediate the effects of PN on the metabolic and ventilatory responses of neonates to modest cold exposure. Wild-type (WT) and α4 knockout (KO) mice were exposed to 6 mg·kg-1·day-1 nicotine or vehicle from embryonic day 14 At postnatal day (P) 7 mice were cooled from an ambient temperature (TA) of 32 to 20°C. Body temperature (TB), rate of O2 consumption (V̇o2), ventilation (V̇e), respiratory frequency (FB), and tidal volume (VT) were continually monitored. An absence of α4 had no effect on the metabolic response to ambient cooling. Surprisingly, PN selectively increased the metabolic response of KO pups to cooling. Regardless, KO pups became hypothermic to the same degree as WT pups, and for both genotypes the drop in TB was exacerbated by PN. PN led to hyperventilation in WT pups caused by an increase in VT, an effect that was absent in α4 KO littermates. We show that PN interacts with α4-containing nAChRs in unique ways to modulate the control of breathing and thermoregulation in the early postnatal period.

Pregnancy limits lung function during exercise and depresses metabolic rate in the skink Tiliqua nigrolutea.

High gestational loads have been associated with a range of ecological costs, such as decreased locomotor ability; however, the physiological mechanisms that underpin these changes are poorly understood. In this study, breathing patterns, metabolic rates, lung volume and lung diffusing capacity were measured at rest and during exercise in the pregnant skink Tiliqua nigrolutea. Breathing patterns were largely unaffected by gestation; however, decreases in metabolic rate (rate of oxygen consumption) in the late stages of pregnancy induced a relative hyperventilation. The reductions in metabolic rate during late pregnancy prevent the calculation of the maintenance cost of pregnancy based on post-partum and neonatal metabolic rates. Despite the high relative litter mass of 38.9±5.3%, lung diffusing capacity was maintained during all stages of pregnancy, suggesting that alterations in diffusion at the alveolar capillary membrane were not responsible for the relative hyperventilation. Lung volume was increased during pregnancy compared with non-pregnant females, but lung volume was significantly lower during pregnancy compared with post-partum lung volume. Pregnant females were unable to produce the same metabolic and ventilatory changes induced by exercise in non-pregnant females. This lack of ability to respond to increased respiratory drive during exercise may underpin the locomotor impairment measured during gestation in previous studies.

Fluctuations in oxygen influence facultative endothermy in bumblebees.

Bumblebees are facultative endotherms, having the ability to elevate thorax temperature above ambient temperature by elevating metabolism. Here, we investigated the influence of hypoxia on metabolic demands and thermoregulatory capabilities of the bumblebee Bombus terrestris. We measured thorax temperature, rates of oxygen consumption and carbon dioxide production, and abdominal pumping rates of bees randomly exposed to oxygen levels of 20, 15, 10 and 5 kPa at 26°C. Under normoxia, bumblebees maintained an elevated mean thorax temperature of 35.5°C. There was no significant change in thorax temperature at 15 kPa O2 (33.4°C). Mean thorax temperature decreased significantly at 10 kPa O2 (31.6°C) and 5 kPa O2 (27.3°C). Bees were able to maintain an elevated metabolic rate at 15 and 10 kPa O2. In normoxia, endothermic bees exhibited periods of rapid abdominal pumping (327 min(-1)) interspaced by periods of no abdominal pumping. At 10 kPa O2, abdominal pumping rate decreased (255 min(-1)) but became more continuous. Upon exposure to 5 kPa, metabolic rate and abdominal pumping rate (152 min(-1)) decreased, although the animals continued abdominal pumping at the reduced rate throughout the exposure period. Bumblebees are able to meet the energetic demands of endothermy at 15 kPa O2, but become compromised at levels of 10 kPa O2 and below.

Growth hormone transgenesis and polyploidy increase metabolic rate, alter the cardiorespiratory response and influence HSP expression in response to acute hypoxia in Atlantic salmon (Salmo salar) yolk-sac alevins.

Growth hormone (GH)-transgenic Atlantic salmon display accelerated growth rates compared with non-transgenics. GH-transgenic fish also display cardiorespiratory and metabolic modifications that accompany the increased growth rate. An elevated routine metabolic rate has been described for pre- and post-smolt GH-transgenic salmon that also display improvements in oxygen delivery to support the increased aerobic demand. The early ontogenic effects of GH transgenesis on the respiratory and cellular physiology of fish, especially during adverse environmental conditions, and the effect of polyploidy are unclear. Here, we investigated the effects of GH transgenesis and polyploidy on metabolic, heart and ventilation rates and heat shock protein (HSP) levels after exposure to acute hypoxia in post-hatch Atlantic salmon yolk-sac alevins. Metabolic rate decreased with decreasing partial pressures of oxygen in all genotypes. In normoxia, triploid transgenics displayed the highest mass-specific metabolic rates in comparison to diploid transgenics and non-transgenic triploids, which, in contrast, had higher rates than diploid non-transgenics. In hypoxia, we observed a lower mass-specific metabolic rate in diploid non-transgenics compared with all other genotypes. However, no evidence for improved O2 uptake through heart or ventilation rate was found. Heart rate decreased in diploid non-transgenics while ventilation rate decreased in both diploid non-transgenics and triploid transgenics in severe hypoxia. Regardless of genotype or treatment, inducible HSP70 was not expressed in alevins. Following hypoxia, the constitutive isoform of HSP70, HSC70, decreased in transgenics and HSP90 expression decreased in all genotypes. These data suggest that physiological changes through GH transgenesis and polyploidy are manifested during early ontogeny in Atlantic salmon.

The trans-Himalayan flights of bar-headed geese (Anser indicus).

Birds that fly over mountain barriers must be capable of meeting the increased energetic cost of climbing in low-density air, even though less oxygen may be available to support their metabolism. This challenge is magnified by the reduction in maximum sustained climbing rates in large birds. Bar-headed geese (Anser indicus) make one of the highest and most iconic transmountain migrations in the world. We show that those populations of geese that winter at sea level in India are capable of passing over the Himalayas in 1 d, typically climbing between 4,000 and 6,000 m in 7-8 h. Surprisingly, these birds do not rely on the assistance of upslope tailwinds that usually occur during the day and can support minimum climb rates of 0.8-2.2 km·h(-1), even in the relative stillness of the night. They appear to strategically avoid higher speed winds during the afternoon, thus maximizing safety and control during flight. It would seem, therefore, that bar-headed geese are capable of sustained climbing flight over the passes of the Himalaya under their own aerobic power.

Metabolic cold adaptation in fishes occurs at the level of whole animal, mitochondria and enzyme.

Metabolic cold adaptation (MCA), the hypothesis that species from cold climates have relatively higher metabolic rates than those from warm climates, was first proposed nearly 100 years ago and remains one of the most controversial hypotheses in physiological ecology. In the present study, we test the MCA hypothesis in fishes at the level of whole animal, mitochondria and enzyme. In support of the MCA hypothesis, we find that when normalized to a common temperature, species with ranges that extend to high latitude (cooler climates) have high aerobic enzyme (citrate synthase) activity, high rates of mitochondrial respiration and high standard metabolic rates. Metabolic compensation for the global temperature gradient is not complete however, so when measured at their habitat temperature species from high latitude have lower absolute rates of metabolism than species from low latitudes. Evolutionary adaptation and thermal plasticity are therefore insufficient to completely overcome the acute thermodynamic effects of temperature, at least in fishes.

An information-theoretic approach to evaluating the size and temperature dependence of metabolic rate.

The effects of body mass and temperature on metabolic rate (MR) are among the most widely examined physiological relationships. Recently, these relationships have been incorporated into the metabolic theory of ecology (MTE) that links the ecology of populations, communities and ecosystems to the MR of individual organisms. The fundamental equation of MTE derives the relation between mass and MR using first principles and predicts the temperature dependence of MR based on biochemical kinetics. It is a deliberately simple, zeroth-order approximation that represents a baseline against which variation in real biological systems can be examined. In the present study, we evaluate the fundamental equation of MTE against other more parameter-rich models for MR using an information-theoretic approach to penalize the inclusion of additional parameters. Using a comparative database of MR measurements for 1359 species, from 11 groups ranging from prokaryotes to mammals, and spanning 16 orders of magnitude in mass and a 59°C range in body temperature, we show that differences between taxa in the mass and temperature dependence of MR are sufficiently large as to be retained in the best model for MR despite the requirement for estimation of 22 more parameters than the fundamental equation of MTE.

The accessory role of the diaphragmaticus muscle in lung ventilation in the estuarine crocodile Crocodylus porosus.

Crocodilians use a combination of three muscular mechanisms to effect lung ventilation: the intercostal muscles producing thoracic movement, the abdominal muscles producing pelvic rotation and gastralial translation, and the diaphragmaticus muscle producing visceral displacement. Earlier studies suggested that the diaphragmaticus is a primary muscle of inspiration in crocodilians, but direct measurements of the diaphragmatic contribution to lung ventilation and gas exchange have not been made to date. In this study, ventilation, metabolic rate and arterial blood gases were measured from juvenile estuarine crocodiles under three conditions: (i) while resting at 30°C and 20°C; (ii) while breathing hypercapnic gases; and (iii) during immediate recovery from treadmill exercise. The relative contribution of the diaphragmaticus was then determined by obtaining measurements before and after transection of the muscle. The diaphragmaticus was found to make only a limited contribution to lung ventilation while crocodiles were resting at 30°C and 20°C, and during increased respiratory drive induced by hypercapnic gas. However, the diaphragmaticus muscle was found to play a significant role in facilitating a higher rate of inspiratory airflow in response to exercise. Transection of the diaphragmaticus decreased the exercise-induced increase in the rate of inspiration (with no compensatory increases in the duration of inspiration), thus compromising the exercise-induced increases in tidal volume and minute ventilation. These results suggest that, in C. porosus, costal ventilation alone is able to support metabolic demands at rest, and the diaphragmaticus is largely an accessory muscle used at times of elevated metabolic demand.

The ventilatory response to hypoxia and hypercapnia is absent in the neonatal fat-tailed dunnart.

At birth, the newborn fat-tailed dunnart relies on cutaneous gas exchange to meet metabolic demands, with continuous lung ventilation emerging several days later. We hypothesised that the delayed expression of lung ventilation (V(E)) in these animals is in part due to a low responsiveness of the respiratory control system to blood gas perturbations. To address this hypothesis, we assessed the ventilatory and metabolic response to hypoxia (10% O(2)) and hypercapnia (5% CO(2)) using closed-system respirometry from birth to 23 days postpartum (P). Neonatal fat-tailed dunnarts displayed no significant hypoxic or hypercapnic ventilatory responses at any age. Regardless, significant hyperventilation through a suppression of metabolic rate (V(O(2))) was observed at birth in response to hypercapnia and in response to hypoxia at all ages, except P12. Therefore, reliance on cutaneous gas exchange during early life may be partially attributed to reduced chemosensitivity or a lack of central integration of chemosensitive afferent information. This may be in part due to the relative immaturity of this species at birth, compared with other mammals.

Use beer to calibrate your CO2 analyser: celebrate!

Bottled beer is used to calibrate a CO2 analyser by measuring the dilution of O2 when gas collected in the headspace of the bottle is diluted with atmospheric air. The method is simple and provides an accurate calibration of a CO2 analyser in the field without the need or hassle of transporting expensive calibration gas.

Breath-to-breath hypercapnic response in neonatal rats: temperature dependency of the chemoreflexes and potential implications for breathing stability.

The breathing of newborns is destabilized by warm temperatures. We hypothesized that in unanesthetized, intact newborn rats, body temperature (T(B)) influences the peripheral chemoreflex response (PCR response) to hypercapnia. To test this, we delivered square-wave challenges of 8% CO(2) in air to postnatal day 4-5 (P4-P5) rats held at a T(B) of 30 degrees C (Cold group, n = 11), 33 degrees C (Cool group, n = 10), and 35 degrees C thermoneutral zone group [thermoneutral zone (TNZ) group, n = 11], while measuring ventilation (Ve) directly with a pneumotach and mask. Cool animals were challenged with 8% CO(2) balanced in either air or hyperoxia (n = 10) to identify the PCR response. Breath-to-breath analysis was performed on 30 room air breaths and every breath of the 1-min CO(2) challenge. As expected, warmer T(B) was associated with an unstable breathing pattern in room air: TNZ animals had a coefficient of variation in Ve (Ve CV%) that was double that of animals held at cooler T(B) (P < 0.001). Hyperoxia markedly suppressed the hypercapnic ventilatory response over the first 10 breaths (or approximately 4 s), suggesting that this domain is dominated by the PCR response. The PCR response (P = 0.03) and total response (P = 0.04) were significantly greater in TNZ animals compared with hypothermic animals. The total response had a significant, negative relationship with Vco(2) (R(2) = 0.53; P < 0.001). Breathing stability was positively related to the total response (R(2) = 0.36; P < 0.001) and to a lesser extent, the PCR response (R(2) = 0.19; P = 0.01) and was negatively related to Vco(2) (R(2) = 0.34; P < 0.001). ANCOVA confirmed a significant effect of T(B) alone on breathing stability (P < 0.01), with no independent effects of Vco(2) (P = 0.41), the PCR response (P = 0.82), or the total Ve response (P = 0.08). Our data suggest that in early postnatal life, the chemoreflex responses to CO(2) are highly influenced by T(B), and while related to breathing stability, are not predictors of stability after accounting for the independent effect of T(B).