Effect of 3-Day and 21-Day Hypoxic Preconditioning on Recovery Following Cerebral Ischemia in Rats.

Abstract

We have previously reported that in a rat model of chronic hypoxia, HIF-1α and its target genes have significantly accumulated by 3days of exposure, whereas no significant increase in capillary density has occurred; there is a significant increase in capillary density at 21days of chronic hypoxic exposure. In this study we hypothesize that by utilizing 3days and 21days of hypoxic preconditioning, we would distinguish between the relative neuroprotective contributions of the accumulation of HIF-1α and its target genes and angiogenic adaptation in a rat middle cerebral artery occlusion (MCAO) model. Rats were randomly assigned to either hypoxic precondition groups (3-day and 21-day hypoxia) or normoxic control group. Hypoxic animals were kept in a hypobaric chamber at a constant pressure of 0.5 atmosphere (380mmHg, equivalent to 10% normobaric oxygen at sea level) for either 3 or 21days. Normoxic controls were housed in the same room next to the hypobaric chamber. Erythropoietin (EPO) was measured at 3 and 21days of hypoxia using Western blotting analysis. Infarct volumes were measured following 24hours of permanent MCAO. We found that EPO is upregulated at 3days of hypoxia and returns to baseline by 21days of hypoxia. The infarct volumes following 24-hour MCAO were significantly reduced with 3-day hypoxic preconditioning when compared to normoxic controls (%, 31.8±5, n=9 vs. 50.1±10.9, n=7). No significant differences in infarct volume were seen between the normoxic controls and 21-day hypoxic preconditioned rats. We have shown that a 3-day hypoxic preconditioning, but not 21-day hypoxic preconditioning, provides significant neuroprotection against focal ischemia in rats, supporting a larger role for the accumulations of HIF-1α and upregulation of its target genes in the neuroprotection against focal ischemia.