Fast bicarbonate-chloride exchange between plasma and brain extracellular fluid at maintained $$P_{CO_2 } $$

The aim of this paper was to investigate the kinetics and mechanism of bicarbonate exchange at the blood-brain ECF barrier. The experiments were performed on anaesthetized and artificially ventilated cats in such a way that acid-base parameters of the brain extracellular fluid were continuously measured while an approximately rectangular increase in the arterial plasma bicarbonate concentration was produced at maintained\(P_{CO_2 } \). The results from such a preparation were: 1. A rapid increase in the brain extracellular bicarbonate concentration was observed in response to an i.v. bolus injection of 3 ml molar NaHCO₃. The bolus was followed by a slow infusion of bicarbonate solution. The brain extracellular bicarbonate reached a new steady state within a minute. 2. This increase was almost simultaneously accompanied by a decrease in the extracellular chloride concentration. The HCO ⁻₃ −Cl⁻ exchange ratio very closely approached one. 3. The extracellular bicarbonate concentration in the brain, after an initial increase, returned towards control in spite of elevated arterial bicarbonate at maintained\(P_{CO_2 } \). 4. The results are discussed in terms of a 5-compartment model, where the extracellular fluid is interposed between the glial cells and the interstitial side of the endothelial cells, similar to the blood plasma being interposed between the red cells and the luminal side of the endothelial cells. 5. A non-electrogenic carrier-mediated HCO ⁻₃ −Cl⁻ exchange at the interphase of the blood brain barrier is postulated.