EFFECTS OF ANESTHETICS ON HYPOCAPNIA-INDUCED CONTRACTION OF ISOLATED CEREBRAL ARTERY

二氧化碳是调节脑血流最有效的生理代谢物之一。氟烷和巴比妥类药物都能降低大脑代谢速率，从而减少CO_2的产生，从而使脑动脉收缩。然而，巴比妥酸盐似乎维持了CBF和CMR之间的正常关系，而氟烷似乎具有增加CBF的能力。我们观察了麻醉剂对低碳酸血症引起的犬脑动脉收缩的影响。将来自大脑(大脑基底动脉和大脑中动脉)和肠系膜动脉的动脉环安装在充气95%O2和5%CO2的Krebs‘s Ringer液中，用2.5%CO2和97.5%O2代替充气的混合气体诱导低碳酸血症。低碳酸血症时，大脑动脉而不是肠系膜动脉收缩。从胞外液中去除Ca~(2+)和Gt~(2+)可取消收缩。内皮剥离不影响收缩。低碳酸血症引起的收缩可被0.4、0.7的氟烷和高浓度的戊巴比妥钠所减弱，而15 mM的KCl15 mM则优先被戊巴比妥所抑制。2.0MAC的异氟醚对低碳酸血症引起的收缩也有明显的抑制作用，而2MCA的七氟醚则不能抑制这种收缩。在挥发性麻醉药中，对低碳酸血症引起的收缩的抑制作用大小顺序为氟烷、异氟烷和七氟烷。提示氟烷麻醉对代谢性脑动脉收缩的抑制作用大于异氟醚和七氟烷麻醉，但在戊巴比妥钠麻醉时可被保留。

Carbon dioxide is one of the most potent physiologic metabolites for regulation of cerebral blood flow. Both halothane and barbiturates have been shown to decrease cerebral matabolic rate (CMR), resulting in reduction of CO_2 production and thereby contraction of cerebral arteries. However, while barbiturates appear to maintain the normal relationship between CBF and CMR, halothane appears to possess the capacity to increase CBF.Weinvestigated the effects of anesthetics on hypocapnia-induced contraction of isolated dog cerebral artery.Arterial rings from cerebrai (basilar and middle cerebral) and mesenteric arteries were mounted in Krebs' Ringer solution aerated with 95% O_2 and 5% CO_2. Hypocapnia was induced by replacing the aerating gas mixture with 2.5% CO_2 and 97.5% O_2. Cerebral artery but not mesenteric artery contracted in response to hypocapnia. Removal of Ca^<2+> from extracellular fluid abolished the contraction. Endothelial denudation did not affect the contraction. The hypocapnia-induced contraction was attenuated by ezposure to halothane at 0.4, 0.7 and by pentobarbital only at high concectrations up to 3x10^4 M.The contraction induced by KCI 15mM was preferentially inhibited by pentobarbital rather than halothane. Isoflurane at 2.0 MAC also inhibited hypocapnia-induced contraction significantly while sevoflurane at 2 MCA failed to sffect the contraction. Among volatile anesthetics, the inhibitory effect on hypocapnia-induced contraction was in order of halothane > isoflurane > sevoflurane. It is suggested that metabolically mediated contraction of cerebral artery is inhibited during halothane to a greater extent than isoflurane and sevoflurane anesthesia, but is preserved during pentobarbital anesthesia.