Effects of hypoxia, hyperoxia and hypercapnia on graded cerebral ischemic responses in rabbits

This study was designed to determine how several factors interact to modify the cerebral ischemic pressor response (CIR) in anesthetized rabbits. After the carotid sinus and aortic nerves were bilaterally sectioned, blood flow through the left internal carotid artery (ICF), which was surgically restricted as the sole route of blood supply to the brain, was reduced by a servo-controller during ventilation with room air, and 8% and 90% O2 and 2 and 5% CO2 gas mixtures. Blood flow (MBF), tissue PO2, PCO2, and interstitial pH were measured in the rostral ventrolateral medulla. Internal carotid arterial pressure, tissue PO2, and MBF decreased proportionately as ICF decreased in the range from 4 to 0 ml/min. Hypoxia significantly increased the rise in renal nerve activity (RNA) and CIR caused by cerebral ischemia, while hyperoxia significantly decreased them. Hypercapnia had almost no influence on the increases in RNA and mean arterial pressure produced by cerebral ischemia. CIR showed a much higher correlation with changes in tissue PO2 than with the other factors. We examined how these factors interact to modify CIR and found that central hypoxia is the main factor in producing CIR.