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M. Shahidullah, C.-H. To, N. A. Delamere; Studies on the Role of Carbonic Anhydrase and Bicarbonate in Aqueous Humor Secretion in the Isolated Porcine Eye and pH Regulation in the Non-Pigmented Ciliary Epithelium. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3163.
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To examine the mechanisms which underlie the effect of carbonic anhydrase inhibitors on aqueous humor (AH) secretion.
AH secretion rate was measured in a porcine isolated whole-eye preparation using a fluorescein dilution technique. Cytoplasmic pH (pHi) was studied in porcine non-pigmented ciliary epithelium (NPE) cultured using a modification of our previous technique (Shahidullah et al 2007, Curr. Eye Res. 32: 511) and loaded with the fluorescent dye BCECF.
The carbonic anhydrase inhibitor (CAI), acetazolamide (500µM), and the anion transport inhibitor, DIDS (100µM), reduced AH secretion in the isolated porcine eye by 44% and 25% respectively. The baseline pHi of porcine cultured NPE in HCO3-/CO2-free HEPES buffer was 7.25±0.13 (n=10). Addition of Krebs’ solution buffered with HCO3-/CO2 initially reduced pHi by ~0.4±0.02 units, then the cells gradually alkalinized toward baseline. Subsequent removal of HCO3-/CO2 and return to HEPES-buffered Krebs’ solution increased pHi by 0.4±0.04 units. Then the cells gradually acidified toward baseline. The phase of gradual alkalinization after addition of HCO3-/CO2 buffer was completely inhibited by Na-free solution and DIDS (100 µM). This alkalinization was also significantly inhibited by carbonic anhydrase inhibitors, acetazolamide (500µM) and methazolamide (100 and 500 µM), but not by low chloride solution. The phase of gradual acidification after removal of HCO3-/CO2 buffer was significantly inhibited by the two carbonic anhydrase inhibitors and by DIDS as well as when the cells were exposed to low chloride solution.
These findings support the notion that porcine NPE uses a Na+/HCO3- co-transporter for HCO3- entry to alkalinize the cell. On the other hand, Cl-/HCO3- exchange appears to export HCO3- to acidify the cells. The ability of acetazolamide and DIDS to modify HCO3- transport may explain the ability of these drugs to reduce AH secretion.
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