Imbalance of regional cerebral blood flow and oxygen consumption: Effect of vascular alpha adrenoceptor blockade

Abstract

This study tested the hypothesis that local cerebral O 2 supply and consumption are not precisely balanced and that vascular alpha adrenoceptors affect this equilibrium. Twenty-two male Long-Evans rats were used in this study. Cerebral blood flow was determined in six regions, using 4-iodo [ N-methyl- 14C]antipyrine. Oxygen saturation was determined microspectrophotometrically in small veins draining three regions of the brain of the rat. Comparisons were made between a control group and a group given N-methyl chlorpromazine, an alpha adrenergic blocker that does not affect central neuronal alpha adrenoceptors. Under control conditions, no regional differences in flow were found. With N-methyl chlorpromazine, cerebral blood flow in the basal ganglia was greater than all other regions, except the frontal cortex. Cerebral O 2 consumption was higher in the frontal cortex than the medulla under control conditions and this difference was not statistically significant after N-methyl chlorpromazine. Cerebral venous O 2 saturations (a measure of cerebral O 2 supply/consumption balance) were found to be significantly heterogeneous under control conditions. The coefficient of variation (CV = 100 × SD/mean) averaged 18%. The average cerebral venous O 2 saturation was 59 ± 11%. Administration of N-methyl chlorpromazine significantly reduced this heterogeneity through a reduction in the number of veins with low O 2 saturations (CV = 11%). The average value increased slightly but significantly to 62 ± 8%. Thus, N-methyl chlorpromazine eliminated many microregions of high O 2 extraction. This indicated that vascular alpha adrenoceptors limit cerebral blood flow to some of the brain regions.