Enhancement of HCO(3)(-) permeability across the apical membrane of bovine corneal endothelium by multiple signaling pathways.

Purpose: In this study, the involvement of signaling pathways in the regulation of HCO(3)(-) permeability across the apical membrane of the corneal endothelium was examined.
Methods: Cultured bovine corneal endothelial cells (CBCECs) were grown to confluence on permeable membranes. Apical and basolateral sides were perfused with a HCO(3)(-)-rich Cl(-)-free Ringer's solution (28.5 mM; pH 7.5). Relative changes in apical HCO(3)(-) permeability were assayed by pulsing the apical perfusion bath with a low-HCO(3)(-) Cl(-)-free Ringer's solution (2.85 mM; pH 6.5), in the presence or absence of agonists or inhibitors, and comparing the rates of change in intracellular pH (pH(i)), as measured with a pH-sensitive dye. Ca(2+)-activated signaling was measured with the Ca(2+)-sensitive dye Fura-2. Qualitative changes in membrane potential (E(m)) were measured with a voltage-sensitive dye. RT-PCR using calcium-activated chloride channel (CLCA)-specific primers was used to examine the expression of CLCA in the corneal endothelium.
Results: The adenoceptor agonist adenosine (20 M) enhanced HCO(3)(-) permeability by a factor of 2. Forskolin (40 microM) exerted a 6.3-fold increase of HCO(3)(-) permeability, which was inhibited by the Cl(-) channel blockers, glibenclamide (50 microM) and niflumic acid (100 microM). Adenosine triphosphate (ATP) and ATPgammaS, P(2) receptor agonists that increased intracellular Ca(2+) in corneal endothelium, enhanced HCO(3)(-) permeability by 87% and 79%, respectively. ATPgammaS induced depolarization of the E(m), consistent with anion channel activation, rather than activation of Ca(2+)-dependent K(+) channels, which could secondarily increase extrusion of anions by E(m) hyperpolarization. Cyclopiazonic acid (CPA), an endoplasmic reticulum (ER) Ca(2+)-pump inhibitor that increased [Ca(2+)](i), also enhanced HCO(3)(-) permeability by 95%. Both the calmodulin kinase II (CaMKII) inhibitor KN-62 and the PKC inhibitor bisindolylmaleimide I (BIMI), decreased HCO(3)(-) permeability induced by ATPgammaS. The PKC activator PMA also increased HCO(3)(-) permeability by a factor of 1.8. RT-PCR using CLCA-specific primers showed the expression of CLCA1 in both fresh and cultured BCECs.
Conclusions: Activation of adenoceptors and purinoceptors enhances HCO(3)(-) permeability across the apical membrane of the cultured corneal endothelium. Multiple signaling pathways (PKA, PKC, and Ca(2+)/CaMKII) contribute to the HCO(3)(-) transport in cultured corneal endothelium. Both cAMP and Ca(2+)-activated Cl(-) channels (possibly CLCA) may be involved in HCO(3)(-) transport.