Comparative ventilatory strategies of acclimated rats and burrowing plateau pika (Ochotona curzoniae) in response to hypoxic-hypercapnia Aur´elien Pichon, Nicolas Voituron, Zhenzhong Bai, Florine Jeton, Wuren Tana, Dominique Marchant, Guoen Jin, Jean-Paul Richalet, Ri-Li Ge PII: DOI: Reference:
S1095-6433(15)00117-8 doi: 10.1016/j.cbpa.2015.05.004 CBA 9889
To appear in:
Comparative Biochemistry and Physiology, Part A
Received date: Revised date: Accepted date:
12 December 2014 16 April 2015 7 May 2015
Please cite this article as: Pichon, Aur´elien, Voituron, Nicolas, Bai, Zhenzhong, Jeton, Florine, Tana, Wuren, Marchant, Dominique, Jin, Guoen, Richalet, Jean-Paul, Ge, RiLi, Comparative ventilatory strategies of acclimated rats and burrowing plateau pika (Ochotona curzoniae) in response to hypoxic-hypercapnia, Comparative Biochemistry and Physiology, Part A (2015), doi: 10.1016/j.cbpa.2015.05.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title: Comparative ventilatory strategies of acclimated rats and burrowing plateau pika
PT
(Ochotona curzoniae) in response to hypoxic-hypercapnia
Authors: Aurélien PICHONa,b,c,#*, Nicolas VOITURONb,c,*, Zhenzhong BAId, Florine JETONb,c,
SC RI
Wuren TANAd, Dominique MARCHANTb, Guoen JINd, Jean-Paul RICHALETb,c, Ri-Li GEd,# Affiliations:
Present address: Laboratory MOVE (EA 6314), University of Poitiers, 4 Allée Jean Monnet,
NU
a,
86000 Poitiers, France
Université Paris 13, Sorbonne Paris Cité, Laboratory «Hypoxia & Lung» EA2363, Bobigny,
France
MA
b,
Laboratory of Excellence GR-Ex
d,
Qinghai University Medical College, Research Centre for High Altitude Medicine, Xining,
CE
Qinghai, R. P. China
PT
ED
c,
AC
Author contributions: * AP and NV equally contributed to this work Running title: Ventilatory response to hypoxic-hypercapnia in pikas
#
Corresponding author:
Aurélien Pichon, Ph.D. Laboratory «Hypoxia & Lung» EA2363, UFR SMBH, 74 rue Marcel Cachin, 93017 BOBIGNY Cedex, FRANCE. E-mail: [email protected]
1
ACCEPTED MANUSCRIPT Authorship: AP & NV provided part of the funding, designed and performed the experiments, analyzed the data and prepared the manuscript; FJ, WT & DM analyzed the data and corrected the manuscript; ZB collected the pikas, analyzed the data and assisted in
PT
the manuscript preparation; GJ collected the pikas and help during experiments; The project
SC RI
was supported by grants received by RLG & JPR which included equipment purchase. They
AC
CE
PT
ED
MA
NU
have also reviewed and assisted with the manuscript writing.
2
ACCEPTED MANUSCRIPT Abstract: The objective of this study was to compare the different ventilatory strategies that helps in
PT
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
SC RI
adjusted to high altitude. Arterial blood samples taken at 4100m of elevation in acclimatized rats and adapted pikas revealed inter-species differences with lower haemoglobin and hematocrit and higher blood pH in pikas. A linear and significant increase in minute
NU
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
MA
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
ED
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
PT
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
CE
strategies help maintaining a better blood oxygenation despite high CO2 concentration in
AC
burrows at high altitude.
Keywords: Hypoxia – Hypercapnia – Ventilation – Adaptation – Burrow
3
ACCEPTED MANUSCRIPT 1. Introduction: It is well established that chronic exposure to hypoxic-hypercapnia, such as conditions
PT
encountered in burrows, have led to adaptive changes in the ventilation and ventilatory response to hypoxia and/or hypercapnia in mammals (Dempsey and Forster, 1982).
SC RI
Atmospheric dry air contains 20.93% oxygen (O2) and 0.039% carbon dioxide (CO2) outside the burrows. In burrows, air movement is reduced, thus the ventilation of burrowing animals leads to a depletion of oxygen well below what is observed outside burrows and to increased
NU
level of CO2. Indeed, in burrows the O2 concentration may vary from 17% to 10% while CO2 concentration can rise from 3% to as high as 10% (Boggs et al., 1984; Kuhnen, 1986;
MA
Lechner, 1976; Williams and Rausch, 1973). Burrowing animals such as mole rats, hamsters, squirrels or woodchucks exhibit a small or blunted ventilatory response to hypoxia
ED
and/or hypercapnia (Arieli and Ar, 1979; Barros et al., 2001; Boggs and Birchard, 1989; Walker et al., 1985). These respiratory adaptations preserve energy and facilitate survival in
PT
the extreme conditions that are present in the burrows. In the spiny rat (Clyomys bishopi), that lives at low altitude in tropical forest of Brazil (Barros et al., 2004), the effects of hypoxic-
CE
hypercapnia is the addition of the independent effects of hypoxia and hypercapnia. Indeed,
AC
the ventilatory response showed an initial hyperventilation that could be linked to both hypoxic and hypercapnic stimuli followed by a relative ventilatory roll off that could be due both to inhibition of hypoxic ventilatory drive and hypoxia-induced hypometabolism (Barros et al., 2004). In addition, the hypoxic conditions encountered in burrows worsen at high altitude because of a marked decrease in O2 partial pressure. The plateau pika or black-lipped pika (Ochotona curzoniae) is a small diurnal and nonhibernating lagomorph endemic to alpine meadows of the Qinghai-Tibetan plateau (People’s Republic of China) living between 3200 to 5300 m above sea level, in an area of extreme cold (down to -40 °C annual minimum temperature) and dry climate conditions (Manabe and Broccoli, 1990; Yang et al., 2014). Some pika fossil samples found on the north edge of the Qinghai-Tibetan plateau are around 37 million years old (Heath and Williams, 1981) and
4
ACCEPTED MANUSCRIPT those found in Tibetan plateaus are up to 20 to 30 millions years old (Mason, 2003; Wang et al., 2008; Wang et al., 2012; Zhu et al., 2005) suggesting that this species is well adapted to high altitude (Ge et al., 1998) (Ge et al. 1998). Plateau pikas present an adapted strategy to
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cope with the cold and hypoxic (partial pressure of inspired O2, PIO2 86 mmHg, 4100m)
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environment. Indeed, plateau pikas exhibit high resting metabolic rate, non-shivering thermogenesis, low pulmonary arterial pressure and high rate of O2 utilization (Boggs et al., 1984; Du and Li, 1982; Du et al., 1984; Ge et al., 1998; Li et al., 2001; Sheafor, 2003). The
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Tibetan plateau pikas live in family groups within interconnected underground burrow system in the alpine meadows of the Tibetan plateau. These living conditions represent both hypoxic
MA
and hypercapnic environments (Kuhnen, 1986). This lifestyle led to particular cardiorespiratory adaptation with a better ventilation efficiency and a greater cardiac output to improve oxygen transport as compared to moderately acclimated animals (Pichon et al.,
ED
2009; Pichon et al., 2013). However, to our knowledge, no study has concerned the analysis
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of breathing regulation during hypoxic-hypercapnia exposure in plateau pikas. N-Methyl-D-Aspartate receptors (NMDA-R) and neuronal nitric oxide synthase (nNOS)
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pathway is involved in the ventilatory response to hypoxia. Indeed, NMDA-R are crucial for
AC
the hypoxic ventilatory response and ventilatory acclimatization to hypoxia (El HasnaouiSaadani et al., 2007; Mizusawa et al., 1994; Voituron et al., 2014). Previous studies suggest an important role of NMDA-R in case of hypoxic-hypercapnia exposure in piglets (Fanous et al., 2006; Machaalani and Waters, 2002). Moreover, nNOS mechanisms could contribute to peripheral chemoreception and decrease the sensitivity to severe acute hypoxia in plateau pikas (Pichon et al., 2009). The objective of the present study was to validate the hypothesis that plateau pikas have developed some ventilatory adaptive strategies to its fossorial lifestyle at high altitude. We also hypothesized that Nitric Oxyde (NO) central pathway, through NMDA-R1 subunit, plays a role in these adaptive strategies. We compared plateau pikas, living at an altitude of
5
ACCEPTED MANUSCRIPT 4100m, to a more common species living at lower altitudes (Sprague Dawley rats)
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CE
PT
ED
MA
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moderately acclimated to hypoxia.
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ACCEPTED MANUSCRIPT 2. Material and methods 2.1. Ethical considerations
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Work was carried out under permit from Qinghai University ethics committee (agreement
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number QHU-RCHAMD-2013-04-441 for Research Center for High Altitude Medicine, Xining) and under the Guiding Principles in the Care and Use of Animals (Institute of Laboratory Animal Research 1996). Wild pikas (n=15), weighing 153 ± 24g, were captured at an altitude between 4050 and 4150 m using traps in the Tianjun private area of Haixi Mongol
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and Tibetan Autonomous Prefecture on the Qinghai-Tibetan Plateau. The owners of the
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2.2. Experimental animals
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capture zone had given permission to capture a maximum of 50 pikas for scientific purposes.
After being captured, pikas were transported from the Tianjun private area (4100m PIO2-
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86 mmHg, partial pressure of inspired CO2 (PICO2)-0.15 mmHg) to Xining (2262m PIO2=111 mmHg, Qinghai Province, People’s Republic of China) by car and were maintained between
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10°C and 15°C. The animals had food and water available ad libitum during the travel and each cage contained 1 animal. The experiments were carried out in a hypobaric chamber
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within the 2 days following capture simulating an altitude of 4100m. The hypobaric chamber (Model: DYC-3000, FENGLEI, CHINA) dimensions were 3.3m x 2m x 17m and depression was created by 2 water pumps and one air-proof pump (possible ascent speed of 15 m/s). We compared male plateau pikas to male wistar rats (n=13). The rats were obtained from crossing animals from Lanzhou University Medical School Animal Center. The rats (weighing 304± 30 g) were born and raised at Xining and used in this experiment at two months of age. For seven days prior to the experiment the rats were placed in the simulated hypobaric chamber (4100m) to acclimate them to high altitude. Then the animals (pikas and rats) were divided in 2 groups. The first group was used for plethysmography and biochemical analysis
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ACCEPTED MANUSCRIPT of blood gas parameters and the second group was used for the biological analysis
2.3 Biochemical analysis of blood gas parameters
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performed on brain.
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After application of anesthesia (xylazine 10 mg.kg-1 + ketamine 50 mg.kg-1 intra-peritoneal), arterial blood samples from rats and pikas were collected from the group used for the measurement of ventilation at 4100m (n = 4 in each group). Arterial blood samples were
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quickly collected via left heart puncture (verified after post-mortem examination) with a heparinized syringe and analyzed using an automated blood gas analyzer (i-STAT Portable
MA
Clinical Analyzer and CHIPS i-STAT EG7, Abbott, USA) to obtain values of haemoglobin concentration (g.dl-1), hematocrit (%), oxygen saturation (SaO2, %), partial pressure of O2
ED
and CO2 (PaO2, mmHg and PaCO2, mmHg), blood pH and bicarbonate concentration (HCO3, mmol.L-1). Most values obtained by the i-STAT system were measured directly (ex: pH,
( )
to
the
following
CE
according
PT
PCO2). PO2 and PCO2 values were corrected for the real temperature (Tp) of each animal
(
)
0.019 T p -37
( )
PO2 Tp = PO2 ´ 10
9.72´10 -9 PO2 3.88 +2.30
(T p -37)
and
. The device calculated other data as haemoglobin
AC
PCO2 Tp = PCO2 ´ 10
formulas:
5.49´10 -11 PO2 3.88 +0.071
concentration, HCO3- and SaO2. However, as automated blood gas analyzers can induce substantial bias for PaO2 and HCO3- values due to inappropriate blood temperature corrections or differences in hematocrit values these results should be used with caution (Bleul and Gotz, 2014; Malte et al., 2014). Nevertheless, our blood gas analyzer was calibrated regularly to ensure continuously high levels of reliability and accuracy.
2.4. Whole body plethysmography Ventilation was measured in non-anesthetized and unrestrained animals (7 rats and 9 pikas) using a whole body plethysmography approach with one-liter experimental chambers
8
ACCEPTED MANUSCRIPT ventilated with air (3.0 L.min-1) (Pichon et al., 2009). Briefly, measurement of respiratory parameters was performed using an adaptation of the technique developed by Drorbaugh and Fenn (1955) and modified by Bartlett and Tenney (1970). We used a recording chamber
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equipped with a temperature probe. This one was connected to a differential pressure
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transducer that measured pressure fluctuations within the closed chamber and compared that to a reference chamber of the same volume. Ventilation was measured with differential pressure transducer (Biopac TSD 160A), amplified using Biopac DA100c and then recorded
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(Biopac MP150, BIOPAC System Inc., Santa Barbara, California, USA). A calibrating syringe was connected to the system. Calibrations were recorded at the end of each recording
MA
session by injecting a volume of air in the recording chamber. During each recording session, the chamber was hermetically sealed and temperature was continuously recorded and maintained around 21°C. Rectal temperatures of the animals were recorded before and after
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each experiment in order to assure that there is no change in body temperature. As the
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measurements were conducted in the hypobaric chamber, temperature and humidity were maintained constant at 21°C and 45% of relative humidity.
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To reduce stress, each animal (n=7 rats and n=9 pikas) was habituated in the chamber for
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at least 30 minutes prior to each measurement. Only periods of breathing were analyzed when the animals were in a stationary condition. We measured the mean respiratory frequency (fR in breath.min-1), tidal volume normalized by body weight (VT, µl.g-1), minute ventilation normalized by body weight (VE, ml.g-1.min-1, with VE = fR x VT/1000), the inspiratory time (Ti), the total time of the respiratory cycle (Ttot), and the irregularity score (IS, variability in duration of respiratory cycles (Viemari et al., 2005; Voituron et al., 2009)). The absolute values of VT was calculated using the formula derived from the Drorbaugh and Fenn equation (1955) that takes into account the difference between the body temperature of the animal and the recording chamber temperature and the calibration. For VT measurements, volume calibrations were performed by injecting a known volume of air in recording chambers (500 µl). IS was defined for each respiratory cycle by applying the formula for
9
ACCEPTED MANUSCRIPT consecutive respiratory cycle period values, 100 x ABS (Pn x Pn-1)/Pn-1, with P being the period (Ttot) of the nth respiratory cycle. The Ti/Ttot and VT/Ti ratio were determined in order measure the respiratory timing and an index of inspiratory drive respectively (Milic-Emili and
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Grunstein, 1976). To assess ventilatory responses to hypoxic-hypercapnia, air was replaced
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by a gas mixture of 16% O2, 5% CO2 and balanced N2 (PIO2-66 mmHg, PICO2-23 mmHg) for a short period of time (5 min). The ventilatory parameters were analyzed at 2 and 5 min after
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2.5. Protein and mRNA measurements
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the onset of exposure to the hypoxic-hypercapnia conditions.
Animals from the second group (n=6 rats and n=6 pikas) were anesthetized by intraperitoneal injection of xylazine (10 mg.kg-1) and ketamine (50 mg.kg-1) and were then
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sacrificed by cervical dislocation. Animals were then decapitated, the brain was rapidly removed and the medulla were quickly dissected, frozen in liquid nitrogen and stored at -
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80°C, until further use either to quantify protein (n = 6 for each group and each protein) or
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perform PCR analyses. The medulla sample was obtained after a caudal cut above the first cervical spinal roots and a rostral cut at the level of the VIII cranial nerve exit points. All
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procedures were done on ice to avoid protein degradation.
2.5.1. NOS and NMDA-NR1 Western Blot Analysis To compare nNOS and NMDA-R1 protein expression between different groups, equal amount of protein (40µg assessed by bicinchoninic acid (BCA) assay) from animals of the same species were loaded on the same gel, and Western blotting was carried out as described below. Experiments were performed using the medulla of Plateau pika or rats. Samples were weighed and homogenized at low speed on ice-cold RIPA protein lysis buffer (100mg tissue was homogenized using 1 ml of extraction buffer). Homogenates were then directly centrifuged at 12000 rpm for 20 min at 4°C for nNOS and for 50 min at 12000 rpm at
10
ACCEPTED MANUSCRIPT 4°C for NMDA-R1 after the incubation of homogenates at 37°C for 30 min to solubilize and denature the NMDA receptor complexes. Then supernatants for nNOS quantification were directly stored at -80°C until further use; supernatants of NMDA-R1 were diluted using equal
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volume of extraction buffer containing 0.1% Triton X-100. Protein concentrations in the
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supernatant were determined by Thermo Pierce protein assay (USA) using BSA as standard. Proteins (15µg for nNOS and 20µg for NMDA-NR1) were separated by electrophoresis using 12% SDS-polyacrylamide gel and transferred to PVDF membranes (Millipore, USA).
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Membranes were incubated at room temperature for two hours in 5% non-fat milk powder with Tris-buffered saline-0.5% Tween 20 (TBS-T) to block non-specific binding. Membranes
MA
were incubated overnight at 4°C with a rabbit polyclonal antibody against nNOS (Abcam, UK) or NMDA-R1 (Abcam, UK). Antibodies for nNOS was diluted 1:1,500 and antibodies for
ED
NMDA-NR1 was diluted 1:500 using 1% BSA/TBS-T. Membranes were washed using TBS-T and incubated for 2 hours at room temperature in 1:3000 anti-rabbit IgG antibody-
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horseradish peroxidase conjugate (Santa Cruz Biotechnology). Anti-rabbit IgG was also diluted in 1% BSA/TBS-T. Immunodetection was accomplished using ECL Western blotting
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kit detection (Pierce, Thermo USA). Anti-GAPDH (Sigma, USA) was used as a loading
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control. Bio-Rad (USA) Gel-DOX XR with Image Lab software was used to perform densitometric scanning to attain proteins quantification per sample. The density of each sample was reported to the density of the corresponding GAPDH sample. Data were then expressed as the ratio of the control and experimental density to that of the GAPDH density. 2.5.2. Real-time PCR assay To assess the differences between the relative genes expression we extracted total RNA from brainstem tissues of pikas (n=6) and rats (n=6) using Trizol reagent (Life tech. USA). 5ug of total RNA was used to synthesize cDNA using the RT2 first strand cDNA synthesis kit (Qiagen USA). Real time RT-PCR was performed on ABI Prism 7500 (Applied Biosystems, Foster City, CA, USA), using QuantiFast SYBR Green PCR Kit (Qiagen USA). For accurate
11
ACCEPTED MANUSCRIPT and reliable gene expression analysis, quantitative RT-PCR was performed using Qiagen Primer Assays, which were specifically designed and experimentally verified. The Primers are the following: nNOS (QT00186340), NMDA-NR1 (QT00182287) and 18s rRNA
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(QT00199374). PCR reactions (25l reaction volume) were performed at 95°C for 15 s and
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60°C for 60 s for 40 cycles. 18s ribosome RNA (18s rRNA) was used as an internal standard to normalize mRNA levels for differences in sample concentration and loading. Fold changes in the expression of each target mRNA relative to 18s rRNA were calculated based on the
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threshold cycle (Ct) as 2-ΔΔCt[1, 2], where ΔCt = Ct(target)-Ct(18s rRNA) and ΔΔCt = ΔCt(pika)- ΔCt(rat). Quantitative PCRs were performed in triplicates. The REST2009
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software (Qiagen USA) was used to analyse real-time PCR results. The sequencing of PCR products showed that amplicons are 100% homologous between rat
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and pika for the NMDA receptor gene, and 99% homologous for the nNOS gene (1 mismatch
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far from end of the primer) (figure 1).
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2.6. Statistical analysis
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Values are presented as mean ± standard deviation (SD). For all statistics, differences were considered significant when p < 0.05. The normality of distribution was assessed by the Kolmogorov-Smirnov test. Comparison of the blood-gas parameters was assessed by Student’s unpaired ‘t’ test. The effects of hypoxic-hypercapnia on each ventilatory parameter were assessed by two-way ANOVAs for rats and pikas as independent factor and time as dependant factor. Newman-Keuls test was used for post-hoc test. All statistical analyses were done using the Statistica software (StatSoft, Inc, Tulsa, USA).
12
ACCEPTED MANUSCRIPT 3. Results
3.1. Biochemical analysis of blood-gas parameters
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Arterial blood samples taken at simulated altitude (4100m) in moderately acclimated rats
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and adapted pikas revealed inter-species differences (Table 1). The arterial PaO2, PaCO2 and SaO2 were not significantly different in both species. Haemoglobin concentration and hematocrit were lower in pikas compared to rats. Blood pH was higher in pikas compared to
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rats, while bicarbonate values of pikas were not different from rats.
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3.2. Comparison between rats and pikas in normoxic-normocapnia conditions In normoxic-normocapnia conditions (control conditions), fR, VT and VE values were higher in pikas than in rats (table 2). The irregularity score (IS, table 2), which was not different
ED
between rats and pikas, was low in the two species meaning that ventilation was regular and
PT
stable. Ti/Ttot ratio was lower in pikas compared to rats indicating that duration of inspiration was shorter in pikas. VT/Ti ratio was higher in pikas than in rats, suggesting that inspiratory
CE
drive was larger in pikas. Rectal temperature was higher in pikas than in rats in normoxic-
AC
normocapnia conditions.
3.3. Effect of acute exposure to hypoxic-hypercapnia In pikas, hypoxic-hypercapnia exposure led to progressive and significant increase in VE at 2 and 5 minutes after the onset of exposure. These increases were due to an increase in fR and VT early during hypoxic-hypercapnic stimulation (table 2). On the other hand, rats did not change their VE in these conditions. The variability of breathing tended to decrease in rats after exposure to the stimulus whereas IS did not change in pikas. The respiratory timing decreased in rats during hypoxic-hypercapnia exposure, whereas it remained unchanged in pikas. The index of inspiratory drive increased in rats and pikas during hypoxic-hypercapnic challenges (figure 2). There was no modification of rectal temperature 5 minutes after the onset of stimulation and body temperature remained higher in pikas compared to rats after
13
ACCEPTED MANUSCRIPT the hypoxic-hypercapnia challenge.
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3.4. Protein and mRNA measurements
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Expressions of NMDA-NR1 and nNOS genes were 5-fold and 13-fold up regulated in pikas when compared to rats (p
S1095-6433(15)00117-8 doi: 10.1016/j.cbpa.2015.05.004 CBA 9889
To appear in:
Comparative Biochemistry and Physiology, Part A
Received date: Revised date: Accepted date:
12 December 2014 16 April 2015 7 May 2015
Please cite this article as: Pichon, Aur´elien, Voituron, Nicolas, Bai, Zhenzhong, Jeton, Florine, Tana, Wuren, Marchant, Dominique, Jin, Guoen, Richalet, Jean-Paul, Ge, RiLi, Comparative ventilatory strategies of acclimated rats and burrowing plateau pika (Ochotona curzoniae) in response to hypoxic-hypercapnia, Comparative Biochemistry and Physiology, Part A (2015), doi: 10.1016/j.cbpa.2015.05.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title: Comparative ventilatory strategies of acclimated rats and burrowing plateau pika
PT
(Ochotona curzoniae) in response to hypoxic-hypercapnia
Authors: Aurélien PICHONa,b,c,#*, Nicolas VOITURONb,c,*, Zhenzhong BAId, Florine JETONb,c,
SC RI
Wuren TANAd, Dominique MARCHANTb, Guoen JINd, Jean-Paul RICHALETb,c, Ri-Li GEd,# Affiliations:
Present address: Laboratory MOVE (EA 6314), University of Poitiers, 4 Allée Jean Monnet,
NU
a,
86000 Poitiers, France
Université Paris 13, Sorbonne Paris Cité, Laboratory «Hypoxia & Lung» EA2363, Bobigny,
France
MA
b,
Laboratory of Excellence GR-Ex
d,
Qinghai University Medical College, Research Centre for High Altitude Medicine, Xining,
CE
Qinghai, R. P. China
PT
ED
c,
AC
Author contributions: * AP and NV equally contributed to this work Running title: Ventilatory response to hypoxic-hypercapnia in pikas
#
Corresponding author:
Aurélien Pichon, Ph.D. Laboratory «Hypoxia & Lung» EA2363, UFR SMBH, 74 rue Marcel Cachin, 93017 BOBIGNY Cedex, FRANCE. E-mail: [email protected]
1
ACCEPTED MANUSCRIPT Authorship: AP & NV provided part of the funding, designed and performed the experiments, analyzed the data and prepared the manuscript; FJ, WT & DM analyzed the data and corrected the manuscript; ZB collected the pikas, analyzed the data and assisted in
PT
the manuscript preparation; GJ collected the pikas and help during experiments; The project
SC RI
was supported by grants received by RLG & JPR which included equipment purchase. They
AC
CE
PT
ED
MA
NU
have also reviewed and assisted with the manuscript writing.
2
ACCEPTED MANUSCRIPT Abstract: The objective of this study was to compare the different ventilatory strategies that helps in
PT
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
SC RI
adjusted to high altitude. Arterial blood samples taken at 4100m of elevation in acclimatized rats and adapted pikas revealed inter-species differences with lower haemoglobin and hematocrit and higher blood pH in pikas. A linear and significant increase in minute
NU
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
MA
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
ED
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
PT
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
CE
strategies help maintaining a better blood oxygenation despite high CO2 concentration in
AC
burrows at high altitude.
Keywords: Hypoxia – Hypercapnia – Ventilation – Adaptation – Burrow
3
ACCEPTED MANUSCRIPT 1. Introduction: It is well established that chronic exposure to hypoxic-hypercapnia, such as conditions
PT
encountered in burrows, have led to adaptive changes in the ventilation and ventilatory response to hypoxia and/or hypercapnia in mammals (Dempsey and Forster, 1982).
SC RI
Atmospheric dry air contains 20.93% oxygen (O2) and 0.039% carbon dioxide (CO2) outside the burrows. In burrows, air movement is reduced, thus the ventilation of burrowing animals leads to a depletion of oxygen well below what is observed outside burrows and to increased
NU
level of CO2. Indeed, in burrows the O2 concentration may vary from 17% to 10% while CO2 concentration can rise from 3% to as high as 10% (Boggs et al., 1984; Kuhnen, 1986;
MA
Lechner, 1976; Williams and Rausch, 1973). Burrowing animals such as mole rats, hamsters, squirrels or woodchucks exhibit a small or blunted ventilatory response to hypoxia
ED
and/or hypercapnia (Arieli and Ar, 1979; Barros et al., 2001; Boggs and Birchard, 1989; Walker et al., 1985). These respiratory adaptations preserve energy and facilitate survival in
PT
the extreme conditions that are present in the burrows. In the spiny rat (Clyomys bishopi), that lives at low altitude in tropical forest of Brazil (Barros et al., 2004), the effects of hypoxic-
CE
hypercapnia is the addition of the independent effects of hypoxia and hypercapnia. Indeed,
AC
the ventilatory response showed an initial hyperventilation that could be linked to both hypoxic and hypercapnic stimuli followed by a relative ventilatory roll off that could be due both to inhibition of hypoxic ventilatory drive and hypoxia-induced hypometabolism (Barros et al., 2004). In addition, the hypoxic conditions encountered in burrows worsen at high altitude because of a marked decrease in O2 partial pressure. The plateau pika or black-lipped pika (Ochotona curzoniae) is a small diurnal and nonhibernating lagomorph endemic to alpine meadows of the Qinghai-Tibetan plateau (People’s Republic of China) living between 3200 to 5300 m above sea level, in an area of extreme cold (down to -40 °C annual minimum temperature) and dry climate conditions (Manabe and Broccoli, 1990; Yang et al., 2014). Some pika fossil samples found on the north edge of the Qinghai-Tibetan plateau are around 37 million years old (Heath and Williams, 1981) and
4
ACCEPTED MANUSCRIPT those found in Tibetan plateaus are up to 20 to 30 millions years old (Mason, 2003; Wang et al., 2008; Wang et al., 2012; Zhu et al., 2005) suggesting that this species is well adapted to high altitude (Ge et al., 1998) (Ge et al. 1998). Plateau pikas present an adapted strategy to
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cope with the cold and hypoxic (partial pressure of inspired O2, PIO2 86 mmHg, 4100m)
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environment. Indeed, plateau pikas exhibit high resting metabolic rate, non-shivering thermogenesis, low pulmonary arterial pressure and high rate of O2 utilization (Boggs et al., 1984; Du and Li, 1982; Du et al., 1984; Ge et al., 1998; Li et al., 2001; Sheafor, 2003). The
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Tibetan plateau pikas live in family groups within interconnected underground burrow system in the alpine meadows of the Tibetan plateau. These living conditions represent both hypoxic
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and hypercapnic environments (Kuhnen, 1986). This lifestyle led to particular cardiorespiratory adaptation with a better ventilation efficiency and a greater cardiac output to improve oxygen transport as compared to moderately acclimated animals (Pichon et al.,
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2009; Pichon et al., 2013). However, to our knowledge, no study has concerned the analysis
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of breathing regulation during hypoxic-hypercapnia exposure in plateau pikas. N-Methyl-D-Aspartate receptors (NMDA-R) and neuronal nitric oxide synthase (nNOS)
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pathway is involved in the ventilatory response to hypoxia. Indeed, NMDA-R are crucial for
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the hypoxic ventilatory response and ventilatory acclimatization to hypoxia (El HasnaouiSaadani et al., 2007; Mizusawa et al., 1994; Voituron et al., 2014). Previous studies suggest an important role of NMDA-R in case of hypoxic-hypercapnia exposure in piglets (Fanous et al., 2006; Machaalani and Waters, 2002). Moreover, nNOS mechanisms could contribute to peripheral chemoreception and decrease the sensitivity to severe acute hypoxia in plateau pikas (Pichon et al., 2009). The objective of the present study was to validate the hypothesis that plateau pikas have developed some ventilatory adaptive strategies to its fossorial lifestyle at high altitude. We also hypothesized that Nitric Oxyde (NO) central pathway, through NMDA-R1 subunit, plays a role in these adaptive strategies. We compared plateau pikas, living at an altitude of
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ACCEPTED MANUSCRIPT 4100m, to a more common species living at lower altitudes (Sprague Dawley rats)
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moderately acclimated to hypoxia.
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ACCEPTED MANUSCRIPT 2. Material and methods 2.1. Ethical considerations
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Work was carried out under permit from Qinghai University ethics committee (agreement
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number QHU-RCHAMD-2013-04-441 for Research Center for High Altitude Medicine, Xining) and under the Guiding Principles in the Care and Use of Animals (Institute of Laboratory Animal Research 1996). Wild pikas (n=15), weighing 153 ± 24g, were captured at an altitude between 4050 and 4150 m using traps in the Tianjun private area of Haixi Mongol
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and Tibetan Autonomous Prefecture on the Qinghai-Tibetan Plateau. The owners of the
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2.2. Experimental animals
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capture zone had given permission to capture a maximum of 50 pikas for scientific purposes.
After being captured, pikas were transported from the Tianjun private area (4100m PIO2-
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86 mmHg, partial pressure of inspired CO2 (PICO2)-0.15 mmHg) to Xining (2262m PIO2=111 mmHg, Qinghai Province, People’s Republic of China) by car and were maintained between
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10°C and 15°C. The animals had food and water available ad libitum during the travel and each cage contained 1 animal. The experiments were carried out in a hypobaric chamber
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within the 2 days following capture simulating an altitude of 4100m. The hypobaric chamber (Model: DYC-3000, FENGLEI, CHINA) dimensions were 3.3m x 2m x 17m and depression was created by 2 water pumps and one air-proof pump (possible ascent speed of 15 m/s). We compared male plateau pikas to male wistar rats (n=13). The rats were obtained from crossing animals from Lanzhou University Medical School Animal Center. The rats (weighing 304± 30 g) were born and raised at Xining and used in this experiment at two months of age. For seven days prior to the experiment the rats were placed in the simulated hypobaric chamber (4100m) to acclimate them to high altitude. Then the animals (pikas and rats) were divided in 2 groups. The first group was used for plethysmography and biochemical analysis
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ACCEPTED MANUSCRIPT of blood gas parameters and the second group was used for the biological analysis
2.3 Biochemical analysis of blood gas parameters
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performed on brain.
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After application of anesthesia (xylazine 10 mg.kg-1 + ketamine 50 mg.kg-1 intra-peritoneal), arterial blood samples from rats and pikas were collected from the group used for the measurement of ventilation at 4100m (n = 4 in each group). Arterial blood samples were
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quickly collected via left heart puncture (verified after post-mortem examination) with a heparinized syringe and analyzed using an automated blood gas analyzer (i-STAT Portable
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Clinical Analyzer and CHIPS i-STAT EG7, Abbott, USA) to obtain values of haemoglobin concentration (g.dl-1), hematocrit (%), oxygen saturation (SaO2, %), partial pressure of O2
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and CO2 (PaO2, mmHg and PaCO2, mmHg), blood pH and bicarbonate concentration (HCO3, mmol.L-1). Most values obtained by the i-STAT system were measured directly (ex: pH,
( )
to
the
following
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according
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PCO2). PO2 and PCO2 values were corrected for the real temperature (Tp) of each animal
(
)
0.019 T p -37
( )
PO2 Tp = PO2 ´ 10
9.72´10 -9 PO2 3.88 +2.30
(T p -37)
and
. The device calculated other data as haemoglobin
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PCO2 Tp = PCO2 ´ 10
formulas:
5.49´10 -11 PO2 3.88 +0.071
concentration, HCO3- and SaO2. However, as automated blood gas analyzers can induce substantial bias for PaO2 and HCO3- values due to inappropriate blood temperature corrections or differences in hematocrit values these results should be used with caution (Bleul and Gotz, 2014; Malte et al., 2014). Nevertheless, our blood gas analyzer was calibrated regularly to ensure continuously high levels of reliability and accuracy.
2.4. Whole body plethysmography Ventilation was measured in non-anesthetized and unrestrained animals (7 rats and 9 pikas) using a whole body plethysmography approach with one-liter experimental chambers
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ACCEPTED MANUSCRIPT ventilated with air (3.0 L.min-1) (Pichon et al., 2009). Briefly, measurement of respiratory parameters was performed using an adaptation of the technique developed by Drorbaugh and Fenn (1955) and modified by Bartlett and Tenney (1970). We used a recording chamber
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equipped with a temperature probe. This one was connected to a differential pressure
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transducer that measured pressure fluctuations within the closed chamber and compared that to a reference chamber of the same volume. Ventilation was measured with differential pressure transducer (Biopac TSD 160A), amplified using Biopac DA100c and then recorded
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(Biopac MP150, BIOPAC System Inc., Santa Barbara, California, USA). A calibrating syringe was connected to the system. Calibrations were recorded at the end of each recording
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session by injecting a volume of air in the recording chamber. During each recording session, the chamber was hermetically sealed and temperature was continuously recorded and maintained around 21°C. Rectal temperatures of the animals were recorded before and after
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each experiment in order to assure that there is no change in body temperature. As the
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measurements were conducted in the hypobaric chamber, temperature and humidity were maintained constant at 21°C and 45% of relative humidity.
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To reduce stress, each animal (n=7 rats and n=9 pikas) was habituated in the chamber for
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at least 30 minutes prior to each measurement. Only periods of breathing were analyzed when the animals were in a stationary condition. We measured the mean respiratory frequency (fR in breath.min-1), tidal volume normalized by body weight (VT, µl.g-1), minute ventilation normalized by body weight (VE, ml.g-1.min-1, with VE = fR x VT/1000), the inspiratory time (Ti), the total time of the respiratory cycle (Ttot), and the irregularity score (IS, variability in duration of respiratory cycles (Viemari et al., 2005; Voituron et al., 2009)). The absolute values of VT was calculated using the formula derived from the Drorbaugh and Fenn equation (1955) that takes into account the difference between the body temperature of the animal and the recording chamber temperature and the calibration. For VT measurements, volume calibrations were performed by injecting a known volume of air in recording chambers (500 µl). IS was defined for each respiratory cycle by applying the formula for
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ACCEPTED MANUSCRIPT consecutive respiratory cycle period values, 100 x ABS (Pn x Pn-1)/Pn-1, with P being the period (Ttot) of the nth respiratory cycle. The Ti/Ttot and VT/Ti ratio were determined in order measure the respiratory timing and an index of inspiratory drive respectively (Milic-Emili and
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Grunstein, 1976). To assess ventilatory responses to hypoxic-hypercapnia, air was replaced
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by a gas mixture of 16% O2, 5% CO2 and balanced N2 (PIO2-66 mmHg, PICO2-23 mmHg) for a short period of time (5 min). The ventilatory parameters were analyzed at 2 and 5 min after
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2.5. Protein and mRNA measurements
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the onset of exposure to the hypoxic-hypercapnia conditions.
Animals from the second group (n=6 rats and n=6 pikas) were anesthetized by intraperitoneal injection of xylazine (10 mg.kg-1) and ketamine (50 mg.kg-1) and were then
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sacrificed by cervical dislocation. Animals were then decapitated, the brain was rapidly removed and the medulla were quickly dissected, frozen in liquid nitrogen and stored at -
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80°C, until further use either to quantify protein (n = 6 for each group and each protein) or
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perform PCR analyses. The medulla sample was obtained after a caudal cut above the first cervical spinal roots and a rostral cut at the level of the VIII cranial nerve exit points. All
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procedures were done on ice to avoid protein degradation.
2.5.1. NOS and NMDA-NR1 Western Blot Analysis To compare nNOS and NMDA-R1 protein expression between different groups, equal amount of protein (40µg assessed by bicinchoninic acid (BCA) assay) from animals of the same species were loaded on the same gel, and Western blotting was carried out as described below. Experiments were performed using the medulla of Plateau pika or rats. Samples were weighed and homogenized at low speed on ice-cold RIPA protein lysis buffer (100mg tissue was homogenized using 1 ml of extraction buffer). Homogenates were then directly centrifuged at 12000 rpm for 20 min at 4°C for nNOS and for 50 min at 12000 rpm at
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ACCEPTED MANUSCRIPT 4°C for NMDA-R1 after the incubation of homogenates at 37°C for 30 min to solubilize and denature the NMDA receptor complexes. Then supernatants for nNOS quantification were directly stored at -80°C until further use; supernatants of NMDA-R1 were diluted using equal
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volume of extraction buffer containing 0.1% Triton X-100. Protein concentrations in the
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supernatant were determined by Thermo Pierce protein assay (USA) using BSA as standard. Proteins (15µg for nNOS and 20µg for NMDA-NR1) were separated by electrophoresis using 12% SDS-polyacrylamide gel and transferred to PVDF membranes (Millipore, USA).
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Membranes were incubated at room temperature for two hours in 5% non-fat milk powder with Tris-buffered saline-0.5% Tween 20 (TBS-T) to block non-specific binding. Membranes
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were incubated overnight at 4°C with a rabbit polyclonal antibody against nNOS (Abcam, UK) or NMDA-R1 (Abcam, UK). Antibodies for nNOS was diluted 1:1,500 and antibodies for
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NMDA-NR1 was diluted 1:500 using 1% BSA/TBS-T. Membranes were washed using TBS-T and incubated for 2 hours at room temperature in 1:3000 anti-rabbit IgG antibody-
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horseradish peroxidase conjugate (Santa Cruz Biotechnology). Anti-rabbit IgG was also diluted in 1% BSA/TBS-T. Immunodetection was accomplished using ECL Western blotting
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kit detection (Pierce, Thermo USA). Anti-GAPDH (Sigma, USA) was used as a loading
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control. Bio-Rad (USA) Gel-DOX XR with Image Lab software was used to perform densitometric scanning to attain proteins quantification per sample. The density of each sample was reported to the density of the corresponding GAPDH sample. Data were then expressed as the ratio of the control and experimental density to that of the GAPDH density. 2.5.2. Real-time PCR assay To assess the differences between the relative genes expression we extracted total RNA from brainstem tissues of pikas (n=6) and rats (n=6) using Trizol reagent (Life tech. USA). 5ug of total RNA was used to synthesize cDNA using the RT2 first strand cDNA synthesis kit (Qiagen USA). Real time RT-PCR was performed on ABI Prism 7500 (Applied Biosystems, Foster City, CA, USA), using QuantiFast SYBR Green PCR Kit (Qiagen USA). For accurate
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ACCEPTED MANUSCRIPT and reliable gene expression analysis, quantitative RT-PCR was performed using Qiagen Primer Assays, which were specifically designed and experimentally verified. The Primers are the following: nNOS (QT00186340), NMDA-NR1 (QT00182287) and 18s rRNA
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(QT00199374). PCR reactions (25l reaction volume) were performed at 95°C for 15 s and
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60°C for 60 s for 40 cycles. 18s ribosome RNA (18s rRNA) was used as an internal standard to normalize mRNA levels for differences in sample concentration and loading. Fold changes in the expression of each target mRNA relative to 18s rRNA were calculated based on the
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threshold cycle (Ct) as 2-ΔΔCt[1, 2], where ΔCt = Ct(target)-Ct(18s rRNA) and ΔΔCt = ΔCt(pika)- ΔCt(rat). Quantitative PCRs were performed in triplicates. The REST2009
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software (Qiagen USA) was used to analyse real-time PCR results. The sequencing of PCR products showed that amplicons are 100% homologous between rat
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and pika for the NMDA receptor gene, and 99% homologous for the nNOS gene (1 mismatch
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far from end of the primer) (figure 1).
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2.6. Statistical analysis
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Values are presented as mean ± standard deviation (SD). For all statistics, differences were considered significant when p < 0.05. The normality of distribution was assessed by the Kolmogorov-Smirnov test. Comparison of the blood-gas parameters was assessed by Student’s unpaired ‘t’ test. The effects of hypoxic-hypercapnia on each ventilatory parameter were assessed by two-way ANOVAs for rats and pikas as independent factor and time as dependant factor. Newman-Keuls test was used for post-hoc test. All statistical analyses were done using the Statistica software (StatSoft, Inc, Tulsa, USA).
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ACCEPTED MANUSCRIPT 3. Results
3.1. Biochemical analysis of blood-gas parameters
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Arterial blood samples taken at simulated altitude (4100m) in moderately acclimated rats
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and adapted pikas revealed inter-species differences (Table 1). The arterial PaO2, PaCO2 and SaO2 were not significantly different in both species. Haemoglobin concentration and hematocrit were lower in pikas compared to rats. Blood pH was higher in pikas compared to
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rats, while bicarbonate values of pikas were not different from rats.
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3.2. Comparison between rats and pikas in normoxic-normocapnia conditions In normoxic-normocapnia conditions (control conditions), fR, VT and VE values were higher in pikas than in rats (table 2). The irregularity score (IS, table 2), which was not different
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between rats and pikas, was low in the two species meaning that ventilation was regular and
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stable. Ti/Ttot ratio was lower in pikas compared to rats indicating that duration of inspiration was shorter in pikas. VT/Ti ratio was higher in pikas than in rats, suggesting that inspiratory
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drive was larger in pikas. Rectal temperature was higher in pikas than in rats in normoxic-
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normocapnia conditions.
3.3. Effect of acute exposure to hypoxic-hypercapnia In pikas, hypoxic-hypercapnia exposure led to progressive and significant increase in VE at 2 and 5 minutes after the onset of exposure. These increases were due to an increase in fR and VT early during hypoxic-hypercapnic stimulation (table 2). On the other hand, rats did not change their VE in these conditions. The variability of breathing tended to decrease in rats after exposure to the stimulus whereas IS did not change in pikas. The respiratory timing decreased in rats during hypoxic-hypercapnia exposure, whereas it remained unchanged in pikas. The index of inspiratory drive increased in rats and pikas during hypoxic-hypercapnic challenges (figure 2). There was no modification of rectal temperature 5 minutes after the onset of stimulation and body temperature remained higher in pikas compared to rats after
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ACCEPTED MANUSCRIPT the hypoxic-hypercapnia challenge.
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3.4. Protein and mRNA measurements
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Expressions of NMDA-NR1 and nNOS genes were 5-fold and 13-fold up regulated in pikas when compared to rats (p
