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PHYSIOLOGY, ENDOCRINOLOGY, AND REPRODUCTION |
College of Animal Science and Technology, China Agricultural University, Beijing 100094, China
1 Corresponding author: chxwu{at}public.bta.net.cn
| ABSTRACT |
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Key Words: mitochondrial respiratory function Tibet chicken Silky chicken embryonic brain hypoxia
| INTRODUCTION |
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In general, highland native animals have genetically adapted themselves to hypobaric hypoxia. Significant differences of mitochondrial volume density were found between highland and lowland native humans (Kayser et al., 1991, 1996), but very little was known about differences in mitochondrial respiratory function between highland and lowland native animals of the same species. The Tibet chicken lives in Qinghai-Tibet Plateau in the west of China and adapted itself well to hypoxia, which is mainly demonstrated by the observation that the native chicken has higher hatchability at high altitude than lowland chicken breeds (Zhang et al., 2005, 2006). The Silky chicken is a lowland chicken from Jiangxi province of China, with an altitude of 750 m. The hatchabilities of the Tibet chicken and Silky chicken were about 85% in a normoxic hatching condition (21% oxygen concentration). However, in a simulated hypoxic hatchibator (designed by our laboratory) with 13% oxygen concentration, the hatchabilities of Tibet chicken and Silky chicken were about 40 and 2%, respectively (data not shown). The objective of the present study was to investigate whether there was any difference in brain mitochondrial respiratory function between Tibet chicken and Silky chicken embryos incubated in a normoxic (21%) or simulated hypoxic (13%) hatchibator.
| MATERIALS AND METHODS |
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Silky chicken hens and Tibet chicken hens of the same age were raised in the Experimental Chicken Farm of China Agricultural University with the same management procedure. At the end of wk 40, they were fertilized by artificial insemination. A total of 300 Silky chicken and 160 Tibet chicken breeding eggs were collected within 3 d. Sixty Silky chicken and 52 Tibet chicken eggs were incubated in a normoxic hatchibator and 240 Silky chicken and 108 Tibet chicken eggs were incubated in a simulated hypoxic hatchibator with 13% oxygen concentration.
Sampling Procedure
On d 16 of incubation, eggshells were opened at the air-cell and living embryos were pulled out with a nipper, and brains were obtained and immediately put into 2-mL microcentrifuge tubes in an ice bath for isolation of mitochondria.
Mitochondrial Isolation
Brain mitochondria were prepared by differential centrifugation, as described by Clayton and Doda (2001) with a little modification. Briefly, brains were immediately excised and finely minced in ice-cold isolation medium (225 mM mannitol, 75 mM sucrose, 1 mM EDTA, 50 mM Tris-HCl, and 0.2% BSA, pH 7.4). The sample was then carefully homogenized in a Potter-Elvenhjem vessel with a Teflon pestle of 0.1-mm clearance. After homogenization, 3 volumes of isolation medium were added to the homogenate, which was then fractionated by centrifugation at 1,300 x g for 10 min. The supernatant was decanted into a new tube placed in ice. The pellet was gently resuspended in 2 volumes of isolation medium and centrifuged at 1,300 x g for 10 min; then the supernatant was decanted into the new tube and mixed with the first supernatant and the pellet was discarded. The total supernatant was centrifuged at 17,000 x g for 15 min. The supernatant was discarded, and the pellet was gently resuspended in isolation medium (0.5 mL/100 mg of initial tissue) and centrifuged at 17,000 x g for 10 min. The supernatant was discarded, and the final pellet, containing the mitochondrial fraction, was gently resuspended (2 µL/mg of initial tissue) in reaction buffer (225 mM mannitol, 75 mM sucrose, 1 mM EDTA, 10 mM KH2PO4, and 0.1% BSA, pH 7.4). All mitochondrial isolation procedures were performed at 0 to 4°C. Mitochondrial protein concentration was spectrophotometrically estimated with the Bradford method using bovine serum albumin as standard. The mitochondrial suspensions were used within 3.5 h after the isolation and were maintained on ice (0 to 4°C) throughout this period. There was no decline in mitochondrial respiratory function in mitochondria isolated at the beginning and end of the experiment within each group.
Determination of Mitochondrial Function
Mitochondrial respiratory function was measured polarographically with Strathkelvin 782 Oxygen System (Strathkelvin Instruments Ltd., Glasgow, UK) according to the protocols described by Zhou et al. (2003) and Beijing SBS Biotechnology Company, Beijing, China. The respiratory control ratio (RCR) and the adenosine 5'-diphosphate: oxyen ratio (ADP/O) are 2 main parameters of mitochondrial respiratory function. The RCR, an index of electron transport chain coupling, is calculated as state 3 divided by state 4 respiration rate. The ADP/O is the amount of ADP utilized per nanomole of oxygen consumed during state 3 respiration and is an index of the ability of mitochondria to carry out oxidative phosphorylation of ATP (Cawthon et al., 1999). Reactions were conducted in 1.5-mL closed thermostated (25°C) glass chambers equipped with magnetic stirring. Aliquots (0.1 mL,
1.0 mg of protein) of the brain mitochondria were added to the reaction vessel containing 0.4 mL of the reaction buffer. The substrates tested in this study were glutamate-malate (2:2 mM) and succinate (4 mM), which donate electrons to the respiratory chain at complex I and complex II, respectively. After 2 min equilibration, mitochondrial respiration was initiated by adding glutamate-malate or succinate. State 3 respiration was started after adding ADP (200 µM, final concentration), followed by state 4 respiration when ADP was consumed.
Complex I Activity
Complex I [the reduced state of ß-nicotinamide adenine dinucleotide (NADH) ubiquinone: oxidoreductase] activity was assessed by ultraviolet spectrophotometry. Briefly, complex I activity was measured by following the oxidation of NADH (Bottje et al., 2002). Mitochondria (200 µL, 100 µg of protein) were added to a solution containing 50 mM Tris-HCl and 0.1 mM 2,6-dichloroindophenol in a final volume of 3.25 mL. The reaction was initiated with addition of 50 µL of 14.1 mM NADH. Absorbance at 600 nm was monitored for 10 min to follow the rate of oxidation of NADH, and the enzyme activity was determined by the change of absorbency. Values for complex I were expressed in units of changes of absorbency per minute per gram of mitochondrial protein.
Statistical Analysis
Means and standard errors were calculated for all variables in all groups, and means were separated by t-tests of Excel.xp (Microsoft Corp., Redmond, WA) and Duncan test of SPSS 13.0 (SPSS Inc., Chicago, IL). The significance levels were set at P < 0.05 and P < 0.01.
| RESULTS AND DISCUSSION |
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In the present study, hypoxia affected both the mitochondrial phosphorylation efficiency and the coupling between respiration and ATP synthesis. The RCR and the ADP/O were higher in brain mitochondria of embryos of both breeds in the simulated hypoxic hatchibator with either energy substrate (Figure 2
). Combining this result with state 4 and state 3 respiration rates (Table 1
), we suggested that the increases in the RCR with glutamate-malate and succinate as energy substrates (Figure 2
) were mainly due to a decrease in state 4 respiration and an increase in state 3 respiration in hypoxic embryos, respectively. State 4 respiration and the ADP/O are associated in theory with the intrinsic and extrinsic uncoupling of oxidative phosphorylation (Hoch, 1992; Kadenbach, 2003), and we hypothesized that the decrease in state 4 respiration and the increase in ADP/O may imply a reduction of cytochrome c oxidase slip (Kadenbach, 2003), a change of proton permeability of inner mitochondrial membrane (Hoch, 1992), or both. With regard to state 3 respiration in brain mitochondria provided with glutamate-malate as energy substrate, our data also showed that hypoxic embryos possessed higher values compared with normoxic embryos of the same breed, despite the fact that differences were not significant (Table 1
; P = 0.38 in Silky chicken; P = 0.09 in Tibet chicken). Because a decrease in state 3 respiration can be induced by the inhibition of complex I (Martínez et al., 2005) or ATPase (Wei et al., 1985) activity, we hypothesized that the increase in state 3 respiration in this study may imply an increased activity of ATPase or complex I, or both, of chicken embryos incubated in the hypoxic environment.
Under the normoxic incubation condition, there were no significant differences between Tibet chicken and Silky chicken embryonic brain mitochondria for all parameters in the present study. The result suggested that embryonic brain mitochondria isolated from Tibet and Silky chicken had the same mitochondrial phosphorylation efficiency and the same coupling ability when incubated in a normoxic environment, which agrees with the observation that Tibet and Silky chicken have the same hatchability in the normoxic hatchibator (our unpublished data). Under the hypoxic incubation condition, our result confirmed the difference in complex I activity between embryos of the 2 studied chicken breeds (Figure 3
). The increase in complex I activity observed in hypoxia-exposed Silky chicken embryos may be a kind of physiological compensation and may be due to one or both of the following causes: 1) increased expression of complex I; 2) change of the state of reversible phosphorylation in complex I subunits (Scacco et al., 2000; Papa et al., 2002). Unlike Silky chicken embryos, Tibet chicken embryos had a relatively stable value of complex I activity whether they were incubated in the hypoxic or in the normoxic hatchibator (Figure 3
). The Tibet chicken is a kind of highland native animal and has adapted itself to hypoxia. This result may reflect the difference in the relative hypoxic degree between Tibet chicken and Silky chicken embryos; that is, Silky chicken embryos were more sensitive to hypoxia than Tibet chicken embryos when they were incubated in the same simulated hypoxic hatchibator.
In summary, the results of this study provided evidence that electron transport in brain mitochondria of Tibet chicken embryos was more tightly coupled than that of lowland chicken (Silky chicken) embryos in a hypoxic incubation environment when glutamate-malate was provided as energy substrate. The higher RCR was associated with the difference in complex I activity between embryos of the 2 chicken breeds. Further studies are planned to investigate whether the differences in complex I activity in the embryonic brain between the Tibet chicken and the Silky chicken are caused by genetic variation of complex I subunit genes and to analyze the association of mitochondrial respiratory function with genetic polymorphism of complex I subunits.
| ACKNOWLEDGMENTS |
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Received for publication April 12, 2007. Accepted for publication June 15, 2007.
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