Cerebral circulatory responses to hypercapnia and hypoxia in the recovery period following complete and incomplete cerebral ischemia in the rat

Abstract

In this study we examined the reactions of cerebral vessels to hypercapnia and hypoxia during the recovery period following cerebral ischemia. We used ventilated, lightly anesthetized rats and induced complete ischemia by CSF compression, incomplete ischemia by bilateral carotid occlusion combined with hypotension. After 15 min of ischemia and 60 min of recirculation the animals were rendered hypercapnic or hypoxic for 2–3 min and local CBF was then measured autoradiographically with ¹⁴C-iodoantipyrine. Following complete ischemia vascular CO₂ responsiveness was abolished or attenuated in most structures analysed. However, there was a considerable interstructural heterogeneity. For example, in the cerebellum and the red nucleus flow rates were observed which approached values obtained in hypercapnic control animals, whereas CO₂ responsiveness was abolished in several cortical areas and hippocampus. The response to CO₂ following incomplete (“forebrain”) ischemia varied considerably. In the cerebral cortices areas with low flow rates were often mixed with hyperemic zones, and in most structures that had very low flow rates during ischemia, CO₂ responsiveness was lost or grossly attenuated. Structures that had suffered moderate or only mild ischemia had better retained or completely preserved CO₂ response. The cerebrovascular reaction to hypoxia was found to be attenuated in most, but not abolished in any of the structures examined. In general, the vascular response to hypoxia was better preserved than that to hypercapnia. Reactivity was similar following complete and incomplete ischemia. As observed during hypercapnia, there were pronounced interstructural variations with considerable increases in flow rates e.g. in the substantia nigra and the cerebellum.