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

二氧化碳是调节脑血流的最有效的生理代谢物之一。氟烷和巴比妥酸盐均可降低脑代谢率（CMR），导致CO_2产生减少，从而导致脑动脉收缩。然而，虽然巴比妥类药物似乎维持CBF和CMR之间的正常关系，但氟烷似乎具有增加CBF的能力。我们研究了麻醉药对低碳酸血症引起的离体狗大脑动脉收缩的影响。将大脑（大脑基底动脉和大脑中动脉）和肠系膜动脉的动脉环置于95% O_2和5% CO_2充气的克雷布斯林格溶液中。用2.5% CO_2和97.5% O_2置换曝气混合气可致低碳酸血症。脑动脉而非肠系膜动脉因低碳酸血症而收缩。从细胞外液中去除Ca^<2+>可消除收缩。内皮剥脱不影响收缩。氟烷浓度为0.4、0.7和戊巴比妥浓度为3 × 10^4 m时均能减弱低碳酸血症引起的收缩，而氯巴比妥对KCI 15mM引起的收缩的抑制作用优于氟巴比妥。2.0 MCA的异氟醚也能显著抑制低碳酸血症引起的收缩，而2 MCA的七氟醚则不能影响收缩。在挥发性麻醉药中，对低碳酸血症引起的收缩的抑制作用依次为氟烷b>异氟烷b>七氟烷。结果表明，与异氟醚和七氟醚麻醉相比，氟烷麻醉对代谢性脑动脉收缩的抑制作用更大，而戊巴比妥麻醉对脑动脉收缩的抑制作用则有所保留。

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.