Preconditioning with prolonged oxygen exposure induces ischemic tolerance in the brain via oxygen free radical formation.

PURPOSE

To determine if 100% oxygen (O2) inhalation induces ischemic tolerance to focal cerebral ischemia and if the effect is induced via O2 free radical formation.

METHODS

Experiment 1: 36 rats were randomly assigned to four groups (n = 9 each): Group A, control rats inhaled air for 24 hr; Groups B, C and D animals inhaled 100% O2 for six hours, 12 hr and 24 hr respectively. Experiment 2: 32 rats were randomly assigned to four groups (n = 8 each): Groups E and F rats received normal saline (5 mL.kg(-1) intraperitoneally) and then inhaled air (Group E) or 100% O2 (Group F) for 24 hr; Groups G and H animals received 10% dimethylthiourea (500 mg.kg(-1) intraperitoneally) and then inhaled 100% O2 (Group G) or air (Group H) for 24 hr. Twenty-four hours after the treatments, the right middle cerebral artery was occluded in all rats for 120 min. The neurologic deficit scores (NDS) and brain infarct volumes were evaluated at 24 hr after reperfusion.

RESULTS

Experiment 1: the infarct volume and NDS of Group D were smaller than in controls (P = 0.004 and 0.042 respectively). The infarct volume was reduced by 47% in Group D. There was no statistical difference among Groups A, B and C. Experiment 2: the infarct volume and NDS in Group F were less than in controls (Group E; P = 0.001 and 0.036 respectively). The infarct volume was reduced by 60% in Group F. There was no difference among Groups E, G and H.

CONCLUSION

Our study demonstrates that preconditioning with 100% O2 for 24 hr can induce ischemic tolerance via formation of O2 free radicals in transient focal cerebral ischemia in rats.