Long-term ventilatory adaptation and ventilatory response to hypoxia in plateau pika (Ochotona curzoniae): role of nNOS and dopamine

Life-long exposure to hypoxia affects metabolism and respiratory physiology across life stages in high-altitude deer mice (Peromyscus maniculatus)

Hypoxia exposure can have distinct physiological effects between early developmental and adult life stages, but it is unclear how the effects of hypoxia may progress during continuous exposure throughout life. We examined this issue in deer mice (Peromyscus maniculatus) from a population native to high altitude. Mice were bred in captivity in one of three treatment groups: normoxia (controls), life-long hypoxia (∼12 kPa O₂ from conception to adulthood) and parental hypoxia (normoxia from conception to adulthood, but parents previously exposed to hypoxia). Metabolic, thermoregulatory and ventilatory responses to progressive stepwise hypoxia and haematology were then measured at post-natal day (P) 14 and 30 and/or in adulthood. Life-long hypoxia had consistent effects across ages on metabolism, attenuating the declines in O₂ consumption rate (V̇ _O2 ) and body temperature during progressive hypoxia compared with control mice. However, life-long hypoxia had age-specific effects on breathing, blunting the hypoxia-induced increases in air convection requirement (quotient of total ventilation and V̇ _O2 ) at P14 and P30 only, but then shifting breathing pattern towards deeper and/or less frequent breaths at P30 and adulthood. Hypoxia exposure also increased blood-O₂ affinity at P14 and P30, in association with an increase in arterial O₂ saturation in hypoxia at P30. In contrast, parental hypoxia had no effects on metabolism or breathing, but it increased blood-O₂ affinity and decreased red cell haemoglobin content at P14 (but not P30). Therefore, hypoxia exposure has some consistent effects across early life and adulthood, and some other effects that are unique to specific life stages.

Developmental and reproductive physiology of small mammals at high altitude: challenges and evolutionary innovations

High-altitude environments, characterized by low oxygen levels and low ambient temperatures, have been repeatedly colonized by small altricial mammals. These species inhabit mountainous regions year-round, enduring chronic cold and hypoxia. The adaptations that allow small mammals to thrive at altitude have been well studied in non-reproducing adults; however, our knowledge of adaptations specific to earlier life stages and reproductive females is extremely limited. In lowland natives, chronic hypoxia during gestation affects maternal physiology and placental function, ultimately limiting fetal growth. During post-natal development, hypoxia and cold further limit growth both directly by acting on neonatal physiology and indirectly via impacts on maternal milk production and care. Although lowland natives can survive brief sojourns to even extreme high altitude as adults, reproductive success in these environments is very low, and lowland young rarely survive to sexual maturity in chronic cold and hypoxia. Here, we review the limits to maternal and offspring physiology - both pre-natal and post-natal - that highland-adapted species have overcome, with a focus on recent studies on high-altitude populations of the North American deer mouse (Peromyscus maniculatus). We conclude that a combination of maternal and developmental adaptations were likely to have been critical steps in the evolutionary history of high-altitude native mammals.

Ontogenesis of evolved changes in respiratory physiology in deer mice native to high altitude

High-altitude environments are cold and hypoxic, and many high-altitude natives have evolved changes in respiratory physiology that improve O2 uptake in hypoxia as adults. Altricial mammals undergo a dramatic metabolic transition from ectothermy to endothermy in early post-natal life, which may influence the ontogenetic development of respiratory traits at high altitude. We examined the developmental changes in respiratory and haematological traits in deer mice (Peromyscus maniculatus) native to high altitude, comparing the respiratory responses to progressive hypoxia between highland and lowland deer mice. Among adults, highlanders exhibited higher total ventilation and a more effective breathing pattern (relatively deeper tidal volumes), for mice that were caught and tested at their native altitudes and those lab-raised in normoxia. Lab-raised progeny of each population were also tested at post-natal day (P)7, 14, 21 and 30. Highlanders developed an enhanced hypoxic ventilatory response by P21, concurrent with the full maturation of the carotid bodies, and their more effective breathing pattern arose by P14; these ages correspond to critical benchmarks in the full development of homeothermy in highlanders. However, highlanders exhibited developmental delays in ventilatory sensitivity to hypoxia, hyperplasia of type I cells in the carotid body and increases in blood haemoglobin content compared with lowland mice. Nevertheless, highlanders maintained consistently higher arterial O2 saturation in hypoxia across development, in association with increases in blood-O2 affinity that were apparent from birth. We conclude that evolved changes in respiratory physiology in high-altitude deer mice become expressed in association with the post-natal development of endothermy.

The role of gasotransmitters in neonatal physiology

The gasotransmitters, nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO), are endogenously-produced volatile molecules that perform signaling functions throughout the body. In biological tissues, these small, lipid-permeable molecules exist in free gaseous form for only seconds or less, and thus they are ideal for paracrine signaling that can be controlled rapidly by changes in their rates of production or consumption. In addition, tissue concentrations of the gasotransmitters are influenced by fluctuations in the level of O₂ and reactive oxygen species (ROS). The normal transition from fetus to newborn involves a several-fold increase in tissue O₂ tensions and ROS, and requires rapid morphological and functional adaptations to the extrauterine environment. This review summarizes the role of gasotransmitters as it pertains to newborn physiology. Particular focus is given to the vasculature, ventilatory, and gastrointestinal systems, each of which uniquely illustrate the function of gasotransmitters in the birth transition and newborn periods. Moreover, given the relative lack of studies on the role that gasotransmitters play in the newborn, particularly that of H₂S and CO, important gaps in knowledge are highlighted throughout the review.

Life Ascending: Mechanism and Process in Physiological Adaptation to High-Altitude Hypoxia

To cope with the reduced availability of O₂ at high altitude, air-breathing vertebrates have evolved myriad adjustments in the cardiorespiratory system to match tissue O₂ delivery with metabolic O₂ demand. We explain how changes at interacting steps of the O₂ transport pathway contribute to plastic and evolved changes in whole-animal aerobic performance under hypoxia. In vertebrates native to high altitude, enhancements of aerobic performance under hypoxia are attributable to a combination of environmentally induced and evolved changes in multiple steps of the pathway. Additionally, evidence suggests that many high-altitude natives have evolved mechanisms for attenuating maladaptive acclimatization responses to hypoxia, resulting in counter-gradient patterns of altitudinal variation for key physiological phenotypes. For traits that exhibit counteracting environmental and genetic effects, evolved changes in phenotype may be cryptic under field conditions and can only be revealed by rearing representatives of high-and low-altitude populations under standardized environmental conditions to control for plasticity.

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 (S _aO2 ) was elevated in yellow-billed pintails at moderate levels of hypoxia, but there were no differences in S _aO2  in other high-altitude taxa compared with their close relatives. This finding suggests that improvements in S _aO2  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.

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.

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

AIM

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.

METHODS

Deer mice (Peromyscus maniculatus) native to high altitude and congeneric mice native to low altitude (Peromyscus leucopus) were bred in captivity at sea level. The F1 progeny of each population were raised to adulthood and then acclimated to normoxia or hypobaric hypoxia (12 kPa, simulating hypoxia at ~4300 m) for 5 months. Responses to acute hypoxia were then measured during stepwise reductions in inspired O₂ fraction.

RESULTS

Lowlanders exhibited ventilatory acclimatization to hypoxia (VAH), in which hypoxia acclimation enhanced the hypoxic ventilatory response, made breathing pattern more effective (higher tidal volumes and lower breathing frequencies at a given total ventilation), increased arterial O₂ saturation and heart rate during acute hypoxia, augmented respiratory water loss and led to significant growth of the carotid body. In contrast, highlanders did not exhibit VAH - exhibiting a fixed increase in breathing that was similar to hypoxia-acclimated lowlanders - and they maintained even higher arterial O₂ saturations in hypoxia. However, the carotid bodies of highlanders were not enlarged by hypoxia acclimation and were similar in size to those of normoxic lowlanders. Highlanders also maintained consistently higher heart rates than lowlanders during acute hypoxia.

CONCLUSIONS

Our results suggest that highland deer mice have evolved high rates of alveolar ventilation and respiratory O₂ uptake without the significant enlargement of the carotid bodies that is typical of VAH in lowlanders, possibly to adjust the hypoxic chemoreflex for life in high-altitude hypoxia.

A Novel Point-of-Care Smartphone Based System for Monitoring the Cardiac and Respiratory Systems

Cardio-respiratory monitoring is one of the most demanding areas in the rapidly growing, mobile-device, based health care delivery. We developed a 12-lead smartphone-based electrocardiogram (ECG) acquisition and monitoring system (called “cvrPhone”), and an application to assess underlying ischemia, and estimate the respiration rate (RR) and tidal volume (TV) from analysis of electrocardiographic (ECG) signals only. During in-vivo swine studies (n = 6), 12-lead ECG signals were recorded at baseline and following coronary artery occlusion. Ischemic indices calculated from each lead showed statistically significant (p < 0.05) increase within 2 min of occlusion compared to baseline. Following myocardial infarction, spontaneous ventricular tachycardia episodes (n = 3) were preceded by significant (p < 0.05) increase of the ischemic index ~1–4 min prior to the onset of the tachy-arrhythmias. In order to assess the respiratory status during apnea, the mechanical ventilator was paused for up to 2 min during normal breathing. We observed that the RR and TV estimation algorithms detected apnea within 7.9 ± 1.1 sec and 5.5 ± 2.2 sec, respectively, while the estimated RR and TV values were 0 breaths/min and less than 100 ml, respectively. In conclusion, the cvrPhone can be used to detect myocardial ischemia and periods of respiratory apnea using a readily available mobile platform.

Naked mole rats exhibit metabolic but not ventilatory plasticity following chronic sustained hypoxia

Naked mole rats are among the most hypoxia-tolerant mammals identified and live in chronic hypoxia throughout their lives. The physiological mechanisms underlying this tolerance, however, are poorly understood. Most vertebrates hyperventilate in acute hypoxia and exhibit an enhanced hyperventilation following acclimatization to chronic sustained hypoxia (CSH). Conversely, naked mole rats do not hyperventilate in acute hypoxia and their response to CSH has not been examined. In this study, we explored mechanisms of plasticity in the control of the hypoxic ventilatory response (HVR) and hypoxic metabolic response (HMR) of freely behaving naked mole rats following 8-10 days of chronic sustained normoxia (CSN) or CSH. Specifically, we investigated the role of the major inhibitory neurotransmitter γ-amino butyric acid (GABA) in mediating these responses. Our study yielded three important findings. First, naked mole rats did not exhibit ventilatory plasticity following CSH, which is unique among adult animals studied to date. Second, GABA receptor (GABAR) antagonism altered breathing patterns in CSN and CSH animals and modulated the acute HVR in CSN animals. Third, naked mole rats exhibited GABAR-dependent metabolic plasticity following long-term hypoxia, such that the basal metabolic rate was approximately 25% higher in normoxic CSH animals than CSN animals, and GABAR antagonists modulated this increase.

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 (Sa_O2) and heart rate in deep hypoxia, hypoxic chemosensitivity (ventilatory O₂/CO₂ equivalents versus Sa_O2), 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.

Effect of Hypoxia on Ldh-c Expression in Somatic Cells of Plateau Pika

Sperm specific lactate dehydrogenases (LDH-C₄) is a lactate dehydrogenase that catalyzes the conversion of pyruvate to lactate. In mammals, Ldh-c was originally thought to be expressed only in testes and spermatozoa. Plateau pika (Ochotona curzoniae), which belongs to the genus Ochotona of the Ochotonidea family, is a hypoxia-tolerant mammal living 3000–5000 m above sea level on the Qinghai-Tibet Plateau, an environment which is strongly hypoxic. Ldh-c is expressed not only in testes and sperm, but also in the somatic tissues of plateau pika. To reveal the effect of hypoxia on pika Ldh-c expression, we investigated the mRNA and protein level of Ldh-c as well as the biochemical index of anaerobic glycolysis in pika somatic tissues at the altitudes of 2200 m, 3200 m and 3900 m. Our results showed that mRNA and protein expression levels of Ldh-c in the tissues of pika’s heart, liver, brain and skeletal muscle were increased significantly from 2200 m to 3200 m, but had no difference from 3200 m to 3900 m; the activities of LDH and the contents of lactate showed no difference from 2200 m to 3200 m, but were increased significantly from 3200 m to 3900 m. Hypoxia up-regulated and maintained the expression levels of Ldh-c in the pika somatic cells. Under the hypoxia condition, plateau pikas increased anaerobic glycolysis in somatic cells by LDH-C₄, and that may have reduced their dependence on oxygen and enhanced their adaptation to the hypoxic environment.

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.

Adenosine receptors mediate the hypoxic ventilatory response but not the hypoxic metabolic response in the naked mole rat during acute hypoxia

Naked mole rats are the most hypoxia-tolerant mammals identified; however, the mechanisms underlying this tolerance are poorly understood. Using whole-animal plethysmography and open-flow respirometry, we examined the hypoxic metabolic response (HMR), hypoxic ventilatory response (HVR) and hypoxic thermal response in awake, freely behaving naked mole rats exposed to 7% O₂ for 1 h. Metabolic rate and ventilation each reversibly decreased 70% in hypoxia (from 39.6 ± 2.9 to 12.1 ± 0.3 ml O₂ min(-1) kg(-1), and 1412 ± 244 to 417 ± 62 ml min(-1) kg(-1), respectively; p < 0.05), whereas body temperature was unchanged and animals remained awake and active. Subcutaneous injection of the general adenosine receptor antagonist aminophylline (AMP; 100 mg kg(-1), in saline), but not control saline injections, prevented the HVR but had no effect on the HMR. As a result, AMP-treated naked mole rats exhibited extreme hyperventilation in hypoxia. These animals were also less tolerant to hypoxia, and in some cases hypoxia was lethal following AMP injection. We conclude that in naked mole rats (i) hypoxia tolerance is partially dependent on profound hypoxic metabolic and ventilatory responses, which are equal in magnitude but occur independently of thermal changes in hypoxia, and (ii) adenosine receptors mediate the HVR but not the HMR.

Enzymatic Kinetic Properties of the Lactate Dehydrogenase Isoenzyme C₄ of the Plateau Pika (Ochotona curzoniae)

Testis-specific lactate dehydrogenase (LDH-C₄) is one of the lactate dehydrogenase (LDH) isozymes that catalyze the terminal reaction of pyruvate to lactate in the glycolytic pathway. LDH-C₄ in mammals was previously thought to be expressed only in spermatozoa and testis and not in other tissues. Plateau pika (Ochotona curzoniae) belongs to the genus Ochotona of the Ochotonidea family. It is a hypoxia-tolerant species living in remote mountain areas at altitudes of 3000-5000 m above sea level on the Qinghai-Tibet Plateau. Surprisingly, Ldh-c is expressed not only in its testis and sperm, but also in somatic tissues of plateau pika. To shed light on the function of LDH-C₄ in somatic cells, Ldh-a, Ldh-b, and Ldh-c of plateau pika were subcloned into bacterial expression vectors. The pure enzymes of Lactate Dehydrogenase A₄ (LDH-A₄), Lactate Dehydrogenase B₄ (LDH-B₄), and LDH-C₄ were prepared by a series of expression and purification processes, and the three enzymes were identified by the method of sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and native polyacrylamide gel electrophoresis (PAGE). The enzymatic kinetics properties of these enzymes were studied by Lineweaver-Burk double-reciprocal plots. The results showed the Michaelis constant (Km) of LDH-C₄ for pyruvate and lactate was 0.052 and 4.934 mmol/L, respectively, with an approximate 90 times higher affinity of LDH-C₄ for pyruvate than for lactate. At relatively high concentrations of lactate, the inhibition constant (Ki) of the LDH isoenzymes varied: LDH-A₄ (Ki = 26.900 mmol/L), LDH-B₄ (Ki = 23.800 mmol/L), and LDH-C₄ (Ki = 65.500 mmol/L). These data suggest that inhibition of lactate by LDH-A₄ and LDH-B₄ were stronger than LDH-C₄. In light of the enzymatic kinetics properties, we suggest that the plateau pika can reduce reliance on oxygen supply and enhance its adaptation to the hypoxic environments due to increased anaerobic glycolysis by LDH-C₄.

Testis-Specific Lactate Dehydrogenase (LDH-C4) in Skeletal Muscle Enhances a Pika's Sprint-Running Capacity in Hypoxic Environment

LDH-C4 is a lactate dehydrogenase that catalyzes the conversion of pyruvate to lactate. In mammals, ldh-c was originally thought to be expressed only in testis and spermatozoa. Plateau pika (Ochotona curzoniae), which belongs to the genus Ochotona of the Ochotonidea family, is a hypoxia tolerant mammal living 3000-5000 m above sea level on the Qinghai-Tibet Plateau, an environment which is strongly hypoxic. Ldh-c is expressed not only in testis and sperm but also in somatic tissues of plateau pika. In this study, the effects of N-propyl oxamate and N-isopropyl oxamate on LDH isozyme kinetics were compared to screens for a selective inhibitor of LDH-C4. To reveal the role and physiological mechanism of LDH-C4 in skeletal muscle of plateau pika, we investigated the effect of N-isopropyl oxamate on the pika exercise tolerance as well as the physiological mechanism. Our results show that Ki of N-propyl oxamate and N-isopropyl oxamate for LDH-A4, LDH-B4, and LDH-C4 were 0.094 mmol/L and 0.462 mmol/L, 0.119 mmol/L and 0.248 mmol/L, and 0.015 mmol/L and 0.013 mmol/L, respectively. N-isopropyl oxamate is a powerful selective inhibitor of plateau pika LDH-C4. In our exercise tolerance experiment, groups treated with inhibitors had significantly lower swimming times than the uninhibited control group. The inhibition rates of LDH, LD, and ATP were 37.12%, 66.27%, and 32.42%, respectively. Our results suggested that ldh-c is expressed in the skeletal muscle of plateau pika, and at least 32.42% of ATP in the skeletal muscle is catalyzed by LDH-C4 by anaerobic glycolysis. This suggests that pika has reduced dependence on oxygen and enhanced adaptation to hypoxic environment due to increased anaerobic glycolysis by LDH-C4 in skeletal muscle. LDH-C4 in plateau pika plays the crucial role in anaerobic glycolysis and generates ATP rapidly since this is the role of LDH-A4 in most species on plain land, which provide evidence that the native humans and animals in Qinghai-Tibet plateau can adapt to the hypoxia environment.

Comparative ventilatory strategies of acclimated rats and burrowing plateau pika (Ochotona curzoniae) in response to hypoxic-hypercapnia

The objective of this study was to compare the different ventilatory strategies that help in coping with hypoxic-hypercapnia environment among two species: use acclimated rats and plateau pikas (Ochotona curzoniae) that live in Tibetan plateaus, and have been well adjusted to high altitude. Arterial blood samples taken at 4100 m of elevation in acclimatized rats and adapted pikas revealed inter-species differences with lower hemoglobin and hematocrit and higher blood pH in pikas. A linear and significant increase in minute ventilation was observed in pikas, which help them to cope with hypoxic-hypercapnia. Pikas also displayed a high inspiratory drive and an invariant respiratory timing regardless of the conditions. Biochemical analysis revealed that N-methyl-D-aspartate receptor (NMDA) receptor gene and nNOS gene are highly conserved between rats and pikas, however pikas have higher expression of NMDA receptors and nNOS compared to rats at the brainstem level. Taken together, these results suggest that pikas have developed a specific ventilatory pattern supported by a modification of the NMDA/NO ventilatory central pathways to survive in extreme conditions imposed on the Tibetan plateaus. These physiological adaptive strategies help in maintaining a better blood oxygenation despite high CO2 concentration in burrows at high altitude.

Role of glutamate and serotonin on the hypoxic ventilatory response in high-altitude-adapted plateau Pika

The highland "plateau Pika" is considered to be adapted to chronic hypoxia. We hypothesized that glutamate N-methyl-D-aspartate (NMDA) and non-NMDA receptors, nitric oxide (NO) synthase, and serotonin are involved in hypoxic ventilatory response (HVR) in Pikas. We tested the effects of NMDA (memantine) and non-NMDA receptors (DNQX) antagonists, NO synthase inhibitor (L-NAME), and selective serotonin reuptake inhibitors (fluoxetine) on ventilation and HVR in Pikas. Ventilatory parameters were measured before and after drug (or vehicle) injections in conscious Pikas at their natural living altitude (PIO2 86 mmHg) and after a hypoxic challenge (PIO2 57 mmHg, 3 min) to assess the influence of peripheral chemoreceptor on HVR. Minute ventilation (VI) and tidal volume (Vt) increased during hypoxic challenge after vehicle injection, whereas the Ti/Ttot ratio remained unchanged. The increase in VI and Vt observed with vehicle at PIO2-57, when compared with PIO2-86, was inhibited after memantine and fluoxetine injection, whereas the DNQX injection increased HVR. At PIO2-57, L-NAME induced an increase in the Ti/Ttot ratio when compared with vehicle. Therefore, the glutamate through NMDA-R/AMPA receptor bindings and serotonin pathway are implicated at the peripheral chemoreceptor level in HVR in Pikas. However, NO influences the ventilatory pattern of Pikas at their habitual living altitude.

No evidence of a role for neuronal nitric oxide synthase in the nucleus tractus solitarius in ventilatory responses to acute or chronic hypoxia in awake rats

When exposed to a hypoxic environment, the body's first response is a reflex increase in ventilation, termed the hypoxic ventilatory response (HVR). With chronic sustained hypoxia (CSH), such as during acclimatization to high altitude, an additional time-dependent increase in ventilation occurs, which increases the HVR and is termed ventilatory acclimatization to hypoxia (VAH). This secondary increase persists after exposure to CSH and involves plasticity within the circuits in the central nervous system that control breathing. The mechanisms of HVR plasticity are currently poorly understood. We hypothesized that changes in neuronal nitric oxide synthase (nNOS) activity or expression in the nucleus tractus solitarius contribute to this plasticity and underlie VAH in rats. To test this, we treated rats held in normoxia or 10% O2 (CSH, PIO2 = 70 Torr) for 7-9 days and measured ventilation in conscious, unrestrained animals before and after microinjecting the general NOS antagonist L-NG-Nitroarginine methyl ester into the nucleus tractus solitarius (NTS) or systemically injecting the nNOS-specific antagonist S-methyl-l-thiocitrulline. Localization of injection sites in the NTS was confirmed by histology following the experiment. We found that 1) neither NTS-specific nor systemic nNOS antagonism had any effect on hypoxia-mediated changes in breathing or metabolism (P > 0.05), but 2) nNOS protein expression was increased in the middle and caudal NTS by CSH. A persistent HVR after nNOS blockade in the NTS contrasts with results in awake mice, and our findings do not support the hypotheses that nNOS in the NTS contribute to the HVR or VAH in awake rats.

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.

An optimized method for the estimation of the respiratory rate from electrocardiographic signals: implications for estimating minute ventilation

It is well-known that respiratory activity influences electrocardiographic (ECG) morphology. In this article we present a new algorithm for the extraction of respiratory rate from either intracardiac or body surface electrograms. The algorithm optimizes selection of ECG leads for respiratory analysis, as validated in a swine model. The algorithm estimates the respiratory rate from any two ECG leads by finding the power spectral peak of the derived ratio of the estimated root-mean-squared amplitude of the QRS complexes on a beat-by-beat basis across a 32-beat window and automatically selects the lead combination with the highest power spectral signal-to-noise ratio. In 12 mechanically ventilated swine, we collected intracardiac electrograms from catheters in the right ventricle, coronary sinus, left ventricle, and epicardial surface, as well as body surface electrograms, while the ventilation rate was varied between 7 and 13 breaths/min at tidal volumes of 500 and 750 ml. We found excellent agreement between the estimated and true respiratory rate for right ventricular (R(2) = 0.97), coronary sinus (R(2) = 0.96), left ventricular (R(2) = 0.96), and epicardial (R(2) = 0.97) intracardiac leads referenced to surface lead ECGII. When applied to intracardiac right ventricular-coronary sinus bipolar leads, the algorithm exhibited an accuracy of 99.1% (R(2) = 0.97). When applied to 12-lead body surface ECGs collected in 4 swine, the algorithm exhibited an accuracy of 100% (R(2) = 0.93). In conclusion, the proposed algorithm provides an accurate estimation of the respiratory rate using either intracardiac or body surface signals without the need for additional hardware.

Glutamate receptors in the nucleus tractus solitarius contribute to ventilatory acclimatization to hypoxia in rat

When exposed to a hypoxic environment the body's first response is a reflex increase in ventilation, termed the hypoxic ventilatory response (HVR). With chronic sustained hypoxia (CSH), such as during acclimatization to high altitude, an additional time-dependent increase in ventilation occurs, which increases the HVR. This secondary increase persists after exposure to CSH and involves plasticity within the circuits in the central nervous system that control breathing. Currently these mechanisms of HVR plasticity are unknown and we hypothesized that they involve glutamatergic synapses in the nucleus tractus solitarius (NTS), where afferent endings from arterial chemoreceptors terminate. To test this, we treated rats held in normoxia (CON) or 10% O2 (CSH) for 7 days and measured ventilation in conscious, unrestrained animals before and after microinjecting glutamate receptor agonists and antagonists into the NTS. In normoxia, AMPA increased ventilation 25% and 50% in CON and CSH, respectively, while NMDA doubled ventilation in both groups (P < 0.05). Specific AMPA and NMDA receptor antagonists (NBQX and MK801, respectively) abolished these effects. MK801 significantly decreased the HVR in CON rats, and completely blocked the acute HVR in CSH rats but had no effect on ventilation in normoxia. NBQX decreased ventilation whenever it was increased relative to normoxic controls; i.e. acute hypoxia in CON and CSH, and normoxia in CSH. These results support our hypothesis that glutamate receptors in the NTS contribute to plasticity in the HVR with CSH. The mechanism underlying this synaptic plasticity is probably glutamate receptor modification, as in CSH rats the expression of phosphorylated NR1 and GluR1 proteins in the NTS increased 35% and 70%, respectively, relative to that in CON rats.

Effects of hypoxia on the proliferation, mineralization and ultrastructure of human periodontal ligament fibroblasts in vitro

This study aimed to investigate the effects of hypoxia on the proliferation, mineralization and ultrastructure of human periodontal ligament fibroblasts (HPLFs) at various times in vitro in order to further study plateau-hypoxia-induced periodontal disease. HPLFs (fifth passage) cultured by the tissue culture method were assigned to the slight (5% O₂), middle (2% O₂), and severe hypoxia (1% O₂) groups and the control (21% O₂) group, respectively. At 12, 24, 48 and 72 h, the proliferation and alkaline phosphatase (ALP) activities were detected. The ultrastructure of the severe hypoxia group was observed. HPLFs grew more rapidly with an increase in the degree of hypoxia at 12 and 24 h, and significant levels of proliferation (P<0.05) were observed in the severe hypoxia group at 24 h. Cell growth was restrained with an increase in the degree of hypoxia at 48 and 72 h, and the restrictions were clear (P<0.05) in the middle and severe hypoxia groups. ALP activity was restrained with increasing hypoxia at each time point. The restrictions were marked (P<0.05) in the severe hypoxia group at 24 h and in the middle and severe hypoxia groups at 48 and 72 h. However, the restriction was more marked (P<0.05) in the severe hypoxia group at 72 h. An increase was observed in the number of mitochondria and rough endoplasmic reticula (RER), with slightly expanded but complete membrane structures, in the severe hypoxia group at 24 h. At 48 h, the number of mitochondria and RER decreased as the mitochondria increased in size. Furthermore, mitochondrial cristae appeared to be vague, and a RER structural disorder was observed. At 72 h, the number of mitochondria and RER decreased further when the mitochondrial cristae were broken, vacuolar degeneration occurred, and the RER particles were reduced while the number of lysosomes increased. HPLF proliferation and mineralization was restrained. Additionally, HPLF structure was broken for a relatively long period of time in the middle and severe hypoxia groups. This finding demonstrated that hypoxia was capable of damaging the metabolism, reconstruction and recovery of HPLFs. The poor state of HPLFs under hypoxic conditions may therefore initiate or aggravate periodontal disease.

Cardiac adaptation to high altitude in the plateau pika (Ochotona curzoniae)

The aim of this study was to assess maximal heart rate (HR) and heart morphological changes in high altitude living “plateau pikas” and rats bred at 2260 m. Rats and pikas were catheterized to measure HR (2260 m). After baseline measurements, 1 mg/kg of atropine (AT) and increasing doses of isoproterenol (IsoP) (0.1, 1, 10, and 100 μg kg) were injected into animals. Right (RV) and left ventricles (LV) were removed to calculate Fulton's ratio (LV + septum (S) to RV weights) and to assess mRNA expression level of β1- and β2-adrenoceptors, muscarinic m1 and m2 receptors, and vascular endothelial growth factor (VEGF). Resting HR was significantly lower in rats than in pikas and increased after AT injection only in rats. IsoP injection induced a significant increase in HR in rat for all doses, which was systematically greater than in pikas. In pikas HR was slightly increased only after the two highest concentrations of IsoP. Fulton's ratio was greater in rats compared with pikas but the LV + S adjusted for body weight was greater in pikas. Pikas showed lower β1-adrenoceptors and muscarinic m2 receptors mRNA expression but larger VEGF mRNA expression than rats both in RV and LV. These results suggest that pikas have a lower maximal HR compared with rats certainly due to a decrease in β-adrenergic and muscarinic receptors mRNA expression. However, the LV hypertrophy probably led to an increase in stroke volume to maintain cardiac output in response to the cold and hypoxic environment.

Phenotypic plasticity and genetic adaptation to high-altitude hypoxia in vertebrates

High-altitude environments provide ideal testing grounds for investigations of mechanism and process in physiological adaptation. In vertebrates, much of our understanding of the acclimatization response to high-altitude hypoxia derives from studies of animal species that are native to lowland environments. Such studies can indicate whether phenotypic plasticity will generally facilitate or impede adaptation to high altitude. Here, we review general mechanisms of physiological acclimatization and genetic adaptation to high-altitude hypoxia in birds and mammals. We evaluate whether the acclimatization response to environmental hypoxia can be regarded generally as a mechanism of adaptive phenotypic plasticity, or whether it might sometimes represent a misdirected response that acts as a hindrance to genetic adaptation. In cases in which the acclimatization response to hypoxia is maladaptive, selection will favor an attenuation of the induced phenotypic change. This can result in a form of cryptic adaptive evolution in which phenotypic similarity between high- and low-altitude populations is attributable to directional selection on genetically based trait variation that offsets environmentally induced changes. The blunted erythropoietic and pulmonary vasoconstriction responses to hypoxia in Tibetan humans and numerous high-altitude birds and mammals provide possible examples of this phenomenon. When lowland animals colonize high-altitude environments, adaptive phenotypic plasticity can mitigate the costs of selection, thereby enhancing prospects for population establishment and persistence. By contrast, maladaptive plasticity has the opposite effect. Thus, insights into the acclimatization response of lowland animals to high-altitude hypoxia can provide a basis for predicting how altitudinal range limits might shift in response to climate change.