Role Of Nitric Oxide In Hypoxic Tolerance Induced By Hyperbaric Oxygen

Hypoxia is the major pathophysiological feature of various diseases such as chronic obstructive pulmaonary disease, cyanotic congenital heart defects and tumor progression. Hypoxia preconditioning is the classical method of hypoxia adaptation. Accumulated evidence shows that both HBO preconditioning and hypoxia preconditioning can enhance the anti-oxidant ability of the body, prevent the chronic hyperbaric oxygen toxicity of animals, lighten the ischemia damage in central nervous system, and improve the sensitiveness of central nervous system to HBO. Our study indicated that HBO can improve the ability to resist the hypoxic damages in mice, but the mechanism is still unclear.NO is a versatile diffusible signaling molecule with multiple physiological proceses involved in neuromodulation, vasodilatation and reproductive function, especially in cerebral blood-flow regulation and neuroprotection. NO may protect the brain from the injury caused by hypoxia or ischemia. NOS catalyse a reaction of L-arginine to NO and citrulline. Thus, the content and activity of NOS are involved in regulation of NO generation directly.The mechanism of protective effects of NO may be summarized as follows:(l)vasodilatation:NO has been shown to play a major role in vascular smooth muscle cells(VSMCs) by binding to soluble guanylate cylase, resulting in guanosin 3'5'-cyclic monophosphate (cGMP) production and the activation of signal transduction pathways leading to vasodilation. It can also prevent constriction responses to NE and 5-HT.(2) signal transmission: NO released from the periphery of nonadrenergic noncholinergic (NANC) neurons which control the brain arteries plays an important role in the signal transmission between NANC neurons and cerebral VSMCs.(3) maintance of the normal structure of vessels: NO also inhibits the proliferation of the underlying layer of VSMCs and the adherence of white blood cells and platelets to the blood vessel wall.(4) NO regulates HIF and its target gene by interplaying with HIF-la then suppresses its degradation. (5) NO can combine with COX and inhibit its activity, this inhibition is competitive with oxygen and reversible. NO-mediated cytochrome partial inhibition leads to a metabolic adaptation of the enzyme to the lower molecular oxygen availability this mechanism can be considered cellular response to hypoxia-induced adaptive changes.It is not well understand wheather HBO induces hypoxic tolerance through NO pathway. The objective of this work is to examine the effect of HBO exposures on NO concentration, NOS expression and activity, and the role of NO and NOS in hypoxic tolerance induced by HBO. For the aim, we have performed the following studies.-1. To investigate the hypoxic tolerance, the effects of HBO on survival time duing nomobaric hypoxia exposures were measured.2. To examine whether NO mediate the hypoxic tolerance induced by HBO, the NO concentration in hippocampus and hypothalamus was determined after HBO exposures.3. Employing NADPH-diaphorase staining and colorimetric method, we demonstrate changes of NOS activity in hypothalamus and hippocampus, and evaluate role of NOS in the changes of NO concentration after HBO exposures.4. Investigate the expression of eNOS> nNOS and their mRNA in hypothalamus and hippocampus during HBO by western blot analysis and RT-PCR.The main results are as follows:1. HBO exposure significantly prolongs the survival time during normobaric hypoxia exposure. And the effect is not associated with the increase of pressure.2. HBO exposure evokes NO concentration in hippocampus and hypothalamus. And NO concentration is at high level until 24 hours after HBO exposures. It shows that the effect is not respectively disappeared after HBO exposure.3. NOS activity is significantly increased after HBO exposures, in the meanwhile, the cNOS activity is increased, iNOS activity is decreased.4. The expression of eNOS, nNOS and their mRNA in hypothalamus and hippocampus during HBO is obviously increased.These results indicate that HBO can induce hypoxic tolerance, the mechanism may be related to NO accumulation resulted from HBO exposure. The accumulation might be accounted for the increase of constitutive NOS expression and activity.