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

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.