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Alteration of vasorelaxation in response to reduced pO2 in aortas of diabetic rats - effects of hyperbaric oxygenation

Various metabolites of arachidonic acid, such as prostaglandins, EETs and HETEs contribute to vascular responses to different physiological stimuli, eg. hypoxia. Alternative vasodilatation pathways may be crucial to maintain vascular relaxation response in various physiological and pathological conditions, such as diabetes mellitus (1). The aim of this study was to evaluate the effects of HBO2 on hypoxia-induced relaxation in aortic rings of healthy and diabetic rats and to elucidate the mechanisms of these responses at functional level. Twenty nine 12-14 weeks old male Sprague-Dawley rats were divided in four groups (7 rats/per group): healthy controls (CTR), diabetic rats (DM, 8 weeks of diabetes) and control and diabetic rats that underwent hyperbaric oxygenation (HBO2 ; CTR+HBO2 and DM+HBO2). HBO2 was administered for four consecutive days, single 120 min session of 100% O2 at 2.0 bars. On 5th day rats were anaesthetized by 75 mg/kg of ketamine and 0, 5 mg/kg of midazolam and decapitated. Hypoxia-induced vasorelaxation was measured in organ bath chamber (Experimetria Ltd.) in norepinefrine (NE) precontracted aortic rings with or without L-NAME (NOS inhibitor), indomethacine (non-selective COX inhibitor) or MS-PPOH (selective inhibitor of CYP450-epoxygenase that catalyzes EETs production). Data were analyzed by Student's t-test or Mann-Whitney Rank Sum tests where applicable ; p<0.05 was considered significant. All experimental procedures conformed to the European Guidelines for the Care and Use of Laboratory Animals (directive 86/609) and were approved by the local Ethical Committee. Response to hypoxia was significantly lower in DM+HBO2 group compared to all other groups. Relaxation in response to hypoxia in control rats was not affected by L-NAME but was partially blocked by both, indomethacine and MS-PPOH (70% and 67%, respectively). The similar was observed in CTR+HBO2 rats. In DM and DM-HBO2 rats, blockage of eNOS by L-NAME significantly reduced vasorelaxation in response to hypoxia (p=0.003 and p=0.018, respectively). In addition, vasorelaxation in response to hypoxia was partially blocked by MS-PPOH in all groups and did not differ among HBO2 untreated groups. Vasorelaxation was almost entirely eliminated with MS-PPOH in DM+HBO2-treated rats compared to untreated DM group (p=0, 037).NO and EETs are potentially involved in the mechanisms of hypoxic vasorelaxation in diabetic animals and DM+HBO2 rats (EETs more prominently in HBO2 treated diabetic rats). This finding suggests that HBO2 probably increases production and/or vascular sensitivity to EETs in diabetic rats that underwent hyperbaric oxygenation, in concordance to our previous studies (2).

diabetes mellitus; aorta; vascular function; hyperbaric oxigenation

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