Johnson D C, Hoop B, Kazemi H
J Appl Physiol Respir Environ Exerc Physiol. 1983 Apr;54(4):989-96. doi: 10.1152/jappl.1983.54.4.989.
The ratio of carbon dioxide (CO2) to bicarbonate (HCO-3) is important in acid-base homeostasis and to the central chemical drive to ventilation. The entry of HCO-3 from blood into the central nervous system (CNS) has been controversial, and the entry of CO2, assumed to be rapid, has not been separated from HCO-3 entry. Therefore the rates of movement of CO2 and of HCO-3 from blood to CNS were evaluated. The first-pass brain uptake of 11C-labeled CO2-HCO-3 was studied under conditions with and without carbonic anhydrase inhibition (CAI), with the isotope injected either as CO2 (acid injectate) or as HCO-3 (alkaline injectate) into the aortic arch of anesthetized dogs. The uptake of 11C under conditions without CAI was about 80% and remained the same whether the isotope was injected as CO2 or as HCO-3. The uptake of 11C under conditions of cerebroventricular administration of acetazolamide was 61.5 +/- 2.0% after injection as CO2 and 56.7 +/- 8.3% after injection as HCO-3. The uptake of 11C under conditions of systemic CAI was 50.3 +/- 3.0% after injection as CO2 and 19.3 +/- 1.1% after injection as HCO-3. The uptakes were comparable for the combination of cerebroventricular and intravenous acetazolamide. From the values for 11C uptake with systemic CAI and the uncatalyzed reaction rates for interconversion of CO2 and HCO-3, the first-pass brain uptake was calculated to be 87.7 +/- 7.8% for CO2 and 16.3 +/- 1.8% for HCO-3. Thus there is a very rapid diffusion of CO2 from blood to brain and a significant movement of HCO-3 from blood to brain.
二氧化碳（CO₂）与碳酸氢盐（HCO₃⁻）的比例在酸碱平衡及对通气的中枢化学驱动中很重要。血液中的HCO₃⁻进入中枢神经系统（CNS）一直存在争议，而被认为快速进入的CO₂进入过程尚未与HCO₃⁻的进入区分开来。因此，评估了CO₂和HCO₃⁻从血液到CNS的移动速率。在有和没有碳酸酐酶抑制（CAI）的条件下，研究了¹¹C标记的CO₂-HCO₃的首过脑摄取，将同位素以CO₂（酸性注射剂）或HCO₃⁻（碱性注射剂）的形式注入麻醉犬的主动脉弓。在没有CAI的条件下，¹¹C的摄取约为80%，无论同位素是以CO₂还是HCO₃⁻的形式注入，摄取量都保持不变。在脑室注射乙酰唑胺的条件下，注入CO₂后¹¹C的摄取为61.5±2.0%，注入HCO₃⁻后为56.7±8.3%。在全身CAI的条件下，注入CO₂后¹¹C的摄取为50.3±3.0%，注入HCO₃⁻后为19.3±1.1%。脑室和静脉联合注射乙酰唑胺时的摄取情况相当。根据全身CAI时¹¹C摄取的值以及CO₂和HCO₃⁻相互转化的非催化反应速率，计算出CO₂的首过脑摄取为87.7±7.8%，HCO₃⁻为16.3±1.8%。因此，CO₂从血液到脑的扩散非常迅速，HCO₃⁻从血液到脑也有显著移动。
