Comparison Of Protective Effects Of Short-term Hypobaric And Normobaric Hypoxic Preconditioning On Acute Altitude Hypoxia In Mice

Altitude hypoxia is one of the most common challenges that high-altitude operators meet with during flight.It often happens when the cabin suddenly broken down or other reasons that can destroy the cabin airtightness of aircraft.Altitude hypoxia prevention plays an important role to ensure flight safety.However,when the body is exposed to altitude hypoxia environment,physical labor ability is weakened with increasing altitude and mental work ability is also gradually affected,especially the complex and fine work ability are declined obviously.What's more,altitude hypoxia happens almost without warning,so the body has little time to compensate or take effective protective measures.Therefore,once the body got into altitude hypoxia environment,it could be a very dangerous situation.As a consequence,both altitude hypoxia protection and improvement of hypoxia tolerance have been the important issues of aviation medicine until now.Previous studies have found that a certain intensity of hypoxic training in the plain area in advance can significantly improve the early labor ability when people are exposed to acute altitude hypoxia,namely the hypoxia acclimatization ability can be reinforced by hypoxia beforehand and it is called as hypoxic preconditioning(HPC).Previous studies have adopted different methods to achieve HPC,including hypobaric HPC and normobaric HPC.However,whether hypobaric or normobaric HPC exerts similar protective effects on acute altitude hypoxia is still unknow.Our studies were aimed at comparing the similarities and differences of different preconditioning types,different preconditioning time in the protective effects of acute altitude hypoxia exposure and discuss its underlying mechanisms briefly.We hoped that our studies could shine light on how to choose economic and effective hypoxia preconditioning protocols in practical work.Male Kunming mice were randomly divided into the following groups: hypobaric hypoxic preconditioning(HHPC)in a hypobaric chamber(405 mm Hg,5 000 m of altitude),1 h/d(H1 group),2 h/d(H2 group)and 4 h/d(H4 group)for 7 d;normobaric hypoxic preconditioning(NHPC)in a normobaric chamber(10.6 % O2 ~ 89.4 % N2 gas mixture),1 h/d(N1 group),2 h/d(N2 group)and 4 h/d(N4 group)for 7 d;the control group(CON).After HPC periods,survival time of the mice from different groups was recorded when they were exposed to acute altitude hypoxia.Motor coordination was measured by coat-hanger test both at sea level and 5 000 m altitude exposure and arterial blood gas analysis including arterial blood oxygen saturation(SaO2),oxygen partial pressure(PaO2),carbon dioxide partial pressure(PaCO2),pH,hematocrit(Hct)and hemoglobin(Hb)was tested as well.The expression and localization of hypoxia inducible factor 1?(HIF-1?),prolyl hydroxylase 2(PHD2),vascular endothelia growth factor(VEGF)and glucose transporter 1(Glut1)in the brain and lung tissues were determined by Western blot and immunohistochemistry.Furthermore,the mice from each group were rapidly ascended to simulated 5 000 m altitude and stayed for 36 h after HPC training.After that,the brain and lung tissues were removed to assay the brain water content(BWC)and lung water content(LWC),respectively.Hematoxylin-eosin staining(HE staining)was used to contrast of histomorphology changes before and after continuous hypoxia exposure.The main findings of the present study are as follows:(1)When mice were exposed to acute altitude hypoxia,both HHPC and NHPC exerted protective effects by prolonging survival time,improving motor coordination,increasing SaO2 and eliminating high altitude pulmonary edema(HAPE)and high altitude cerebral edema(HACE).But there were differences between HHPC and NHPC groups.(1)Survival time: Compared with CON,survival time of H2,N2,H4 and N4 was significantly(P < 0.01)prolonged when mice were exposed to 5 % O2 ~ 95 % N2 gas mixture.Different HPC lasting time also had an impact on survival time.In HHPC groups,compared with H1,the survival time in H4 was significantly increased(P < 0.01).In NHPC groups,survival time of N2 and N4 was obviously(P < 0.01)extended compared with N1.As to HHPC and NHPC,survival time of N2 and N4 was longer than H2 and H4(P < 0.01),respectively.(2)Improving motor coordination: Coat-hanger test results were analyzed by Twoway ANOVA analysis.The score of body position revealed significant effects on altitude(both sea level and 5 000 m altitude),hypoxic preconditioning(HPC)and altitude × HPC interaction.Latency to reach the end of the horizontal bar showed significant effects on altitude and HPC but no significant effect on altitude × HPC interaction.Latency to fall off revealed a significant effect on altitude but no significant effect on neither HPC nor altitude × HPC interaction.Comparisons between groups revealed that the score of body position of H2,N2,H4 and N4 was notably higher(P < 0.05)compared with CON at 5 000 m altitude hypoxia;latency to reach the end of the horizontal bar was markedly prolonged(P < 0.01)in H4 and N2 compared with H1 and H2 respectively;latency to fall off was shortened(P < 0.01)in N4 compared to H4.(3)Increasing SaO2: Compared with CON,Sa O2 of N2 was significantly increased(P < 0.05).The blood pH during acute hypoxia exposure indicated that acidosis had existed but there was no significant difference between groups.Both PaO2 and PaCO2 were decreased at altitude hypoxia with no difference among groups.(4)Eliminating HAPE and HACE: After 36 h continuous hypoxia exposure,the body weight loss was distinctly reduced(P < 0.01)in H2,N2 and N4 compared with CON.BWC and LWC results were analyzed by Two-way ANOVA analysis.Both BWC and LWC revealed significant effects on hypoxia exposure but no significant effect on neither HPC nor hypoxia exposure × HPC interaction.Comparisons between groups indicated that both BWC and LWC were lower in N2(P < 0.01)compared to CON.HE staining showed that N2 mice were protected from HACE and HAPE.(2)Both HHPC and NHPC could increase the contents of Hct and Hb in blood.The protein expressions of HIF-1?,PHD2,VEGF and Glut1 were raised by HPC in both brain and lung tissues.But it existed differences in above mentioned physiological responses.(1)Hct and Hb: Compared with CON,both Hct and Hb in HPC groups were obviously increased(P < 0.01).Both Hct and Hb in N2 and N4 groups were higher than N1(P < 0.05)The Hct and Hb of N4 were also higher than H4(P < 0.01).(2)The protein expression of HIF-1?,PHD2,VEGF and Glut1: Western blot analysis showed that compared with CON,the expressions of HIF-1?,PHD2,VEGF and Glut1 were significantly upregulated(P < 0.05)in both brain and lung tissues in N2,H4 and N4.Both HIF-1? and PHD2 expression was increased over exposure time.These results indicated that 1)Both HHPC and NHPC exerted protective effects by prolonging survival time,improving motor coordination and eliminating HAPE and HACE when the mice were exposed to acute altitude hypoxia;2)Both HHPC and NHPC could result in Hct and Hb increasing to enhance capability to carry oxygen,and upregulate the protein expressions of HIF-1?,PHD2,VEGF and Glut1 in brain and lung tissues to strengthen physiological responses to hypoxia;3)Considering of the impact of different HPC protocols on hypoxia endurance and physiological responses,NHPC 2 h/d may be more efficient and economic to exert protective effects on improving tolerance to acute altitude hypoxia;4)The different protective effects on acute altitude hypoxia between HHPC and NHPC may be resulted from distinct physiological compensative responses and diverse hypoxia associated protein expression induced by HHPC/NHPC protocols.