Secondary CO2 diffusion following HCO3- shift across the red blood cell membrane

In order to clarify the interaction between CO2 diffusion and HCO3- shift in the red blood cell (RBC), HCO3- shift was measured by using a stopped flow method combined with fluorometry. When HCO3- entered the RBC, the intracellular PCO2 increased, causing a secondary outflow of CO2. Conversely, when HCO3- ions flowed out of the RBC, the resulting decrease of PCO2 caused an inward CO2 diffusion. The PCO2 change caused by the inward HCO3- shift was about 3- to 4-fold that of the outward shift. During the respective in- and outward-shifts, the mean half-times of the extracellular pH changes were 0.15 and 0.13 sec. These were approximately twice as long as those of the primary CO2 diffusion. The permeability of HCO3- across the RBC membrane was obtained by comparing the experimental extracellular pH curve with a numerical solution for CO2 and HCO3- diffusions accompanied by the hydration and dehydration reactions. Thus the HCO3- permeability was determined to be 5 x 10-4 cm/sec, in the in- and outward-HCO3- shifts, respectively. The influence of Cl- concentration on HCO3- permeability was tested by reducing the initial Cl- gradient across the RBC membrane. In a physiological Cl- concentration range the HCO3- permeability was not affected by the Cl- gradient.