Mechanisms And Improvement Of Intermittent Short-duration Reoxygenation For The Protection Against Hypobaric Hypoxia Induced Pulmonary Hypertension

With the increasing demand for border defense and economic development,there is an increasing demand for people to migrate to the plateau in short period.However,the migration is accompanied with many factors that are harmful to human health,such as low air pressure,strong ultraviolet,and large temperature difference between day and night.Among these factors,the most important factor is hypobaric hypoxia.Short term hypoxia exposure may cause acute high-altitude diseases including pulmonary edema and cerebral edema,which can severely reduce work ability and even potentially threaten lives.Recent studies have found that increasing altitude step by step or using hypoxic preconditioning can effectively reduce the incidence of acute high-altitude diseases and significantly improve the work ability.However,the above methods have no significant effect on pulmonary hypertension and right ventricular hypertrophy caused by chronic exposure to high altitude.Moreover,due to the complex mechanism of hypoxic pulmonary hypertension,there is currently no effective measure to totally prevent the occurrence of hypoxic pulmonary hypertension and right ventricular hypertrophy.The key mechanism of hypoxic pulmonary hypertension is the remodeling of distal pulmonary arterioles,which results in increased pulmonary vascular resistance.Due to the complex mechanism involved in hypoxic vascular remodeling,there is currently no mechanism that can perfectly explain the occurrence of pulmonary vascular remodeling.Therefore,currently it is not feasible to prevent hypoxic pulmonary hypertension targeting single mechanism.We were inspired by our previous study about preventing arterial remodeling caused by simulated weightlessness by intermittent artificial gravity,and speculated that intermittent short-duration reoxygenation can prevent hypoxic pulmonary hypertension,then we found that three times a day,one hour of reoxygenation to sea level each time,can effectively alleviate pulmonary hypertension and right ventricular hypertrophy caused by two weeks of 5000 m hypoxia in rats.In addition,we also found the main factors affecting intermittent short-duration reoxygenation are reoxygenation frequency,single reoxygenation duration,and physiological equivalent height of reoxygenation.Under other unchanged conditions,increasing the reoxygenation frequency from 3 to 6 times,or extending the single reoxygenation time to 1.5 hours,both protocols showed a lower protective effect than the protocol of 3 times a day and one hour each time.In addition,increasing the frequency of reoxygenation or prolonging the duration of a single reoxygenation significantly increased the level of oxidative stress in lung tissue.This suggests that oxidative stress may play an important role in the protective effect of intermittent short-duration reoxygenation.It has been reported that elevated tissue oxidative stress was closely related to inflammatory response.In addition,oxidative stress and inflammatory response are also important participants in hypoxic pulmonary hypertension.Based on the existing evidence,we propose a hypothesis:since increasing oxygen consumption can lead to an increase in oxidative stress levels,which is harmful to the protective effects,the level of oxidative stress and inflammatory response in I3 group might be higher than the continuous hypoxia group.And the role of oxidative stress and inflammatory response in the protective effect of I3 need to be further explored.Based on the above hypothesis,we designed this study and used a simulated 5000 m altitude hypobaric hypoxic rat model.We simulated high-altitude oxygen supply by descending altitude of the chambers,and used measures such as hemodynamic monitoring,ultrasound measurement,tissue morphology analysis,and molecular biology detection to explore the changes in some oxidative stress and inflammatory pathways in I3.And we intervened targeting the changed pathways to explore their roles in the protective effect of I3 from 2 aspects:pulmonary arteriole remodeling and pulmonary microvascular density.Then we found 2 methods that can improve the protective effect of intermittent short-duration reoxygenation.The main results of this study are as follows:1.The overall oxidative stress level of lung tissue in I3 group is comparable to that of H group,but the level of H₂O₂ in I3 group is obviously reduced.The axis of NOX4/H₂O₂/PPAR-γplays a role in the protection against hypoxic pulmonary hypertension by I3.（1）I3 did not reduce the overall oxidative stress levels of MDA and8-OHG in lung tissue,but the H₂O₂ levels were significantly lower than that of H group;（2）I3 significantly reversed the increase in NOX4 expression and the reduction in PPAR-γexpression in lung tissue caused by chronic hypoxia;（3）Intervention on the axis of NOX4/H₂O₂/PPAR-γin PASMC in vitro counteracted the protective effect of I3 on the proliferation of PASMC.（4）Inhibition of NOX1/4 or activation of PPAR-γin combined with I3 failed to further improve the protective effect against pulmonary hypertension and right heart hypertrophy in rats.2.Reducing the level of ROS derived from mitochondria improved the protective effect.（1）Mitochondrial antioxidant Mito Q in combined with I3 further reduced the degree of pulmonary hypertension and right ventricular hypertrophy.（2）After reducing the dosage of Mito Q from 10 mg/kg/d to 5 mg/kg/d,the additional protective effect of Mito Q disappeared.（3）The combination of 3000 m for resting time and single daily reoxygenation for 1 h can achieve better protective effects than I3.3.I3 restored pulmonary vascular density by upregulating the expression of NLRP3inflammasome and ICAM-1.（1）I3 obviously increased the expression of NLRP3inflammasome and ICAM-1 in lung tissues.（2）Continuous hypoxia caused a decrease in vascular density at the edge of lung tissue,and I3 obviously restored vascular density.（3）The use of NLRP3 inflammasome inhibitor MCC950 partially counteracted the protective effect of I3 and reduced pulmonary vascular density.（4）The use of ICAM-1/LFA-1combination inhibitor Lifitegrast completely counteracted the protective effect of I3 and significantly reduced pulmonary vascular density.4.Alveolar macrophage plays a positive role in I3.（1）The expression of NLRP3inflammasome in alveolar macrophage in I3 group was elevated both in vivo and in vitro.（2）The alveolar macrophage cultured in vitro in I3 group promoted the expression of ICAM-1 in pulmonary microvascular endothelial cell and enhance their tube formation ability.（3）After depletion of alveolar macrophage by chlorophosphonate liposomes,the pulmonary hypertension and right ventricular hypertrophy of rats in the I3 group significantly deteriorated.Conclusion:1.I3 inhibited the pulmonary vascular remodeling caused by continuous hypoxia by inhibiting the axis of NOX4/H₂O₂/PPAR-γ.2.MitoQ enhanced the protective effect of I3 by reducing ROS from mitochondria.3.The combination of reducing altitude in resting time and single daily reoxygenation achieved more practical and effective protective effects.4.The upregulation of NLRP3 inflammasome and ICAM-1 during intermittent short-duration reoxygenation plays a positive role in restoring pulmonary vascular density.5.Alveolar macrophage plays a positive role in the protective effect of I3.