Apical and basolateral CO2-HCO3

Bonanno, Joseph A., Yi Guan, Sergey Jelamskii, and Xiao Jun Kang. Apical and basolateral CO2-HCO3 2 permeability in cultured bovine corneal endothelial cells. Am. J. Physiol. 277 (Cell Physiol. 46): C545–C553, 1999.—Corneal endothelial function is dependent on HCO3 2 transport. However, the relative HCO3 2 permeabilities of the apical and basolateral membranes are unknown. Using changes in intracellular pH secondary to removing CO2-HCO3 2 (at constant pH) or removing HCO3 2 alone (at constant CO2) from apical or basolateral compartments, we determined the relative apical and basolateral HCO3 2 permeabilities and their dependencies on Na1 and Cl2. Removal of CO2-HCO3 2 from the apical side caused a steady-state alkalinization (10.08 pH units), and removal from the basolateral side caused an acidification (20.05 pH units). Removal of HCO3 2 at constant CO2 indicated that the basolateral HCO3 2 fluxes were about three to four times the apical fluxes. Reducing perfusate Na1 concentration to 10 mM had no effect on apical flux but slowed basolateral HCO3 2 flux by one-half. In the absence of Cl2, there was an apparent increase in apical HCO3 2 flux under constant-pH conditions; however, no net change could be measured under constant-CO2 conditions. Basolateral flux was slowed ,30% in the absence of Cl2, but the net flux was unchanged. The steady-state alkalinization after removal of CO2-HCO3 2 apically suggests that CO2 diffusion may contribute to apical HCO3 2 flux through the action of a membraneassociated carbonic anhydrase. Indeed, apical CO2 fluxes were inhibited by the extracellular carbonic anhydrase inhibitor benzolamide and partially restored by exogenous carbonic anhydrase. The presence of membrane-bound carbonic anhydrase (CAIV) was confirmed by immunoblotting. We conclude that the Na1-dependent basolateral HCO3 2 permeability is consistent with Na-nHCO3 2 cotransport. Changes in HCO3 2