title: Bicarbonate transport under nominally bicarbonate-free conditions in bovine corneal endothelium.
abstract: 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)
authors.affiliation: Morton D. Sarver Center for Cornea and Contact Lens Research, University of California School of Optometry, Berkeley 94720.
authors.forename: J A
authors.initials: JA
authors.lastname: Bonanno
mesh_terms: D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D000086:Acetazolamide / Q000494:pharmacology; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D001692:Biological Transport; D002245:Carbon Dioxide / Q000378:metabolism; D002417:Cattle; D002478:Cells, Cultured; D002713:Chlorine / Q000378:metabolism; D004728:Endothelium, Corneal / Q000378:metabolism*; D005470:Fluorometry; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D009584:Nitrogen / Q000494:pharmacology
chemical_list: D001639:Bicarbonates; D002245:Carbon Dioxide; D002713:Chlorine; C018564:dihydro-DIDS; D000584:Amiloride; D009584:Nitrogen; D000086:Acetazolamide; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid