Najarian T, Marrache A M, Dumont I, Hardy P, Beauchamp M H, Hou X, Peri K, Gobeil F, Varma D R, Chemtob S
Department of Pharmacology and Therapeutics, McGill University, Montreal, Canada.
Circ Res. 2000 Dec 8;87(12):1149-56. doi: 10.1161/01.res.87.12.1149.
Mechanisms for secondary sustained increase in cerebral blood flow (CBF) during prolonged hypercapnia are unknown. We show that induction of endothelial NO synthase (eNOS) by an increase in prostaglandins (PGs) contributes to the secondary CBF increase during hypercapnic acidosis. Ventilation of pigs with 6% CO(2) (PaCO(2 approximately)65 mm Hg; pH approximately 7.2) caused a approximately 2.5-fold increase in CBF at 30 minutes, which declined to basal values at 3 hours and gradually rose again at 6 and 8 hours; the latter increase was associated with PG elevation, nitrite formation, eNOS mRNA expression, and in situ NO synthase (NOS) reactivity (NADPH-diaphorase staining). Subjecting free-floating brain sections to acidotic conditions increased eNOS expression, the time course of which was similar to that of CBF increase. Treatment of pigs with the cyclooxygenase inhibitor diclofenac or the NOS inhibitor Nomega-nitro-L-arginine blunted the initial rise and prevented the secondary CBF increase during hypercapnic acidosis; neuronal NOS blockers 1-(2-trifluoromethylphenyl) imidazole and 3-bromo-7-nitroindazole were ineffective. Diclofenac abolished the hypercapnia-induced rise in cerebrovascular nitrite production, eNOS mRNA expression, and NADPH-diaphorase reactivity. Acidosis (pH approximately 7.15, PCO(2 approximately )40 mm Hg; 6 hours) produced similar increases in prostaglandin E(2) (PGE(2)) and eNOS mRNA levels in isolated brain microvessels and in NADPH-diaphorase reactivity of brain microvasculature; these changes were prevented by diclofenac, by the receptor-operated Ca(2+) channel blocker SK&F96365, and by the K(ATP) channel blocker glybenclamide. Acidosis increased Ca(2+) transients in brain endothelial cells, which were blocked by glybenclamide and SK&F96365 but not by diclofenac. Increased PG-related eNOS mRNA and NO-dependent vasorelaxation to substance P was detected as well in rat brain exposed to 6 hours of hypercapnia. PGE(2) was the only major prostanoid that modulated brain eNOS expression during acidosis. Thus, in prolonged hypercapnic acidosis, the secondary CBF rise is closely associated with induction of eNOS expression; this seems to be mediated by PGE(2) generated by a K(ATP) and Ca(2+) channel-dependent process.
在长时间高碳酸血症期间，脑血流量（CBF）继发性持续增加的机制尚不清楚。我们发现，前列腺素（PGs）增加诱导内皮型一氧化氮合酶（eNOS），这有助于高碳酸性酸中毒期间CBF的继发性增加。用6%二氧化碳（PaCO₂约65 mmHg；pH约7.2）对猪进行通气，在30分钟时CBF增加约2.5倍，在3小时时降至基础值，并在6小时和8小时时再次逐渐上升；后者的增加与PG升高、亚硝酸盐形成、eNOS mRNA表达以及原位一氧化氮合酶（NOS）反应性（NADPH-黄递酶染色）有关。将游离的脑切片置于酸性条件下会增加eNOS表达，其时间进程与CBF增加的时间进程相似。用环氧化酶抑制剂双氯芬酸或NOS抑制剂Nω-硝基-L-精氨酸处理猪，可减弱高碳酸性酸中毒期间CBF的初始升高并阻止其继发性增加；神经元NOS阻滞剂1-（2-三氟甲基苯基）咪唑和3-溴-7-硝基吲唑无效。双氯芬酸消除了高碳酸血症诱导的脑血管亚硝酸盐生成增加、eNOS mRNA表达和NADPH-黄递酶反应性。酸中毒（pH约7.15，PCO₂约40 mmHg；6小时）使分离的脑微血管中前列腺素E₂（PGE₂）和eNOS mRNA水平以及脑微血管的NADPH-黄递酶反应性产生类似增加；双氯芬酸、受体操纵的Ca²⁺通道阻滞剂SK&F96365和KATP通道阻滞剂格列本脲可阻止这些变化。酸中毒增加了脑内皮细胞中的Ca²⁺瞬变，格列本脲和SK&F96365可阻断该瞬变，但双氯芬酸不能。在暴露于6小时高碳酸血症的大鼠脑中也检测到与PG相关的eNOS mRNA增加以及对P物质的NO依赖性血管舒张。PGE₂是酸中毒期间调节脑eNOS表达的唯一主要类前列腺素。因此，在长时间高碳酸性酸中毒中，CBF的继发性升高与eNOS表达的诱导密切相关；这似乎是由KATP和Ca²⁺通道依赖性过程产生的PGE₂介导的。
