Does Intermittent Hypoxia Training Augment Antioxidant and Anti-Glycation Enzymes in Rat Brain?

Hypothesis: Intermittent hypoxia training (IHT) has been found to minimize damage in the brain of rats subjected to ischemic stroke and alcohol intoxication-withdrawal, but the neuroprotective mechanisms are unclear. The finding that antioxidant treatments during IHT blunt the protection identifies a pivotal role of reactive oxygen species (ROS). Because ROS activate expression of antioxidant and anti-glycation enzymes, this study addressed the hypothesis that IHT augments these enzymes in rat brain. Specifically, the cerebral activities of anti-glycation (glyoxalase-1, i.e. GLO1), anti-oxidant (glucose 6-phosphate dehydrogenase, i.e. G6PDH) and hypoxia-inert (lactate dehydrogenase, i.e. LDH) enzymes were analyzed in IHT and non-hypoxic rats. Material and Methods: Ten rats (5 males) completed a 20 day IHT program (5-8 daily cycles of 5-10 min exposures to 9.5-10% O2 followed by 4 min room air exposures), and another 10 rats (5 males) were sham-conditioned by cyclic exposures to 21% O2. One day after completing the IHT or sham programs, the rats were isoflurane-anesthetized and decapitated. The cerebra were harvested, flash-frozen in liquid N2, pulverized in a mortar under liquid N2, homogenized in phosphate buffer, and centrifuged. Enzyme activities in supernatants were analyzed by spectrophotometry, and total protein content by colorimetric Bradford assay. Results: Activities of GLO1 were 78 ± 8 and 62 ± 7 mU/mg protein in the IHT and sham groups, respectively (P = 0.23). G6PDH activities were 21 ± 2 and 24 ± 2 mU/mg protein in the IHT and sham groups (P = 0.32). As expected, LDH activities were similar in the two groups: 899 ± 49 mU/mg protein in the IHT rats, and 940 ± 58 mU/mg protein in the sham rats (P = 0.60). Thus, IHT did not produce a statistically significant treatment effect on these enzymes. Conclusions: The 20 day IHT program, which exerts robust cerebroprotection against ischemia-reperfusion and ethanol intoxication-withdrawal, did not augment activities of selective antioxidant and anti-glycation enzymes. The impact of IHT on other antioxidant (e.g. glutathione peroxidase, superoxide dismutase, catalase) and anti-glycation (e.g. glyoxalase-2) enzymes remains to be evaluated. It also is possible that IHT activates other cytoprotective mechanisms, including signaling cascades that ameliorate mitochondrial permeability transition, oxidative stress and other mechanisms of neural injury. Such alternative mechanisms merit investigation.