Bicarbonate transport under nominally bicarbonate-free conditions in bovine corneal endothelium

Fluid transport by the corneal endothelium is supported in air equilibrated, nominally HCO3- free Ringer's, yet it is inhibited by carbonic anhydrase inhibitors indicating dependence on the presence of CO2. It is possible that the CO2/HCO3- contained in air and/or the CO2 derived from oxidative metabolism is sufficient to support HCO3- transport. This study investigates whether endothelial HCO3- transport mechanisms, Na+:HCO3- cotransport and Cl-/HCO3- exchange, are active in the nominal absence of HCO3-. In cultured bovine corneal endothelial cells the rate of intracellular pH (pHi) recovery following acid loading in air-equilibrated, nominally HCO(3-)-free solutions was slowed 75% by 0.5 mM amiloride (inhibitor of Na+/H+ exchange), yet 60% of the acid load was still pumped out. This residual acid efflux (or base influx) was N(+)-dependent, slowed 60 +/- 15% by equilibration of solutions with 100% N2 (to minimize CO2 availability) and totally blocked by pretreatment with 100 microM DIDS, an inhibitor of Na+:HCO3- cotransport. Resting cells were acidified 0.19 +/- 0.01 pH units by addition of 0.5 mM H2DIDS when perfused with air-equilibrated solutions, but only 0.03 +/- 0.02 when perfused with 100% N2 equilibrated solutions. Application of 500 microM acetazolamide (a carbonic anhydrase inhibitor) also acidified resting cells, 0.11 +/- 0.01 pH units, but the acidification was eliminated when cells were perfused with 100% N2-equilibrated solutions, indicating the presence of carbonic anhydrase activity under nominally HCO(3-)-free conditions. Cl-/HCO3- exchange activity was eliminated by 100% N2 perfusion and slowed 29% by acetazolamide.(ABSTRACT TRUNCATED AT 250 WORDS)