Movement of CO2 and HCO-3 from blood to brain in dogs

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Abstract
The ratio of CO2 to HCO3- is important in acid-base homeostasis and to the central chemical drive to ventilation. The entry of HCO3- from blood into the CNS was controversial, and the entry of CO2, assumed to be rapid, has not been separated from HCO3- entry. Therefore the rates of movement of CO2 and HCO3- from blood to CNS were evaluated. The 1st-pass brain uptake of 11C-labeled CO2-HCO3- was studied under conditions with and without carbonic anhydrase inhibition (CAI), with the isotope injected either as CO2 (acid injectate) or as HCO3- (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 HCO3-. The uptake of 11C under conditions of cerebroventricular administration of acetazolamine was 61.5 .+-. 2.0% after injection as CO2 and 56.7 .+-. 8.3% after injection as HCO3-. 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 HCO3-. The uptakes were comparable for the combination of cerebroventricular and i.v. acetapolamide. From the 11C uptake with systemic CAI and the uncatalyzed reaction rates for interconversion of CO2 and HCO3-, the 1st-pass brain uptake was claculated to be 87.7 .+-. 7.8% for CO2 and 16.3 .+-. 1.8% for HCO3-. Thus there is a very rapid diffusion of CO2 from blood to brain and a significant movement of HCO3- from blood to brain.