Effects of Oxygen Depletion on Norepinephrine- and Carbachol-Stimulated Phosphoinositide Turnover in Rat Brain Slices

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Abstract
We examined the effects of in vitro anoxia and in vivo hypoxia (8% O2/92% N2) on norepinephrine (NE)- and carbachol-stimulated phosphoinositide (PI) turnover in rat brain slices. The formation of 3H-labeled polyPI in cortical slices was impaired by in vitro anoxia and fully restored by reoxygenation. Accumulation of 3H-labeled myo-inositol phosphates (3H-IPs) stimulated by 10-5 M NE was significantly reduced by anoxia (control at 60 min, 1,217 .+-. 86 cpm/mg of protein; anoxia for 60 min, 651 .+-. 82 cpm/mg; mean .+-. SEM; n = 5; p < 0.01), and reoxygenation following anoxia resulted in overshooting of the accumulation (control at 120 min 1,302 .+-. 70 cpm/mg; anoxia for 50 min plus oxgenation for 70 min, 1,790 .+-. 126 cpm/mg; n = 5; p < 0.01). The underlying mechanisms for the two phenomena-the decrease caused by anoxia and the overshooting caused by reoxygenation following anoxia-seemed to be completely different because of the following observations. (a) Although the suppression of NE-stimulated accumulation at low O2 tensions was also observed in Ca2+-free medium, the overshooting in response to reoxygenation was not. (b) Carbachol-stimulated accumulation was significantly reduced by anoxia and was restored by reoxygenation only to control levels. Thus, the postanoxic overshooting in accumulation of 3H-IPs seems to be a specific response of NE. (c) The decrease observed at low O2 tensions was due to a decrease in Emax value, whereas the postanoxic overshooting was due to a decrease in EC50 value. There was also a significant increase in NE-stimulated accumulation of 3H-IPs in cortical slices from rats exposed to in vivo hypoxia (8% O2/92% N2) for > 6 h, compared with those from rats exposed to room air. These results indicate that depletion of oxygen causes significant changes in receptor-mediated PI metabolism in brain slices and may provide a clue for understanding biochemical mechanisms for the electrophysiologically demonstrable effects of hypoxia/anoxia on synaptic transmission.