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Shimin Li, Joseph A Bonanno; Maintenance of Corneal Thickness and Lactate Efflux Are Dependent on Perfusate Buffering Capacity. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1195.
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© ARVO (1962-2015); The Authors (2016-present)
To test the hypothesis that the corneal endothelium maintains corneal hydration by facilitating lactic acid flux via buffering capacity and is not simply dependent on the presence of HCO3-.
The thickness of mounted intact rabbit corneas was measured while being superfused on the endothelial surface with 28.5 mM HCO3-/5% CO2 buffered Ringer solution (BR) or HCO3-free Ringers (BF), buffered with 0, 10, 25, 40, or 60 mM HEPES, all with 1mM phosphate at pH 7.5, 37oC. Perfusate was collected over 30 minute intervals and assayed for [lactate].
Perfusion with BR resulted in 24.67 + 1.15 µm increase (n=3) in corneal thickness over 5 hours. BF solutions buffered with 0, 10, 25, 40, or 60 mM HEPES caused corneal swelling of 76.33 + 1.21, 57.33 + 1.15, 44.33 + 1.15, 34.33 + 1.15, and 29.67 + 0.58 µm (n=3) indicating that increasing buffering capacity in the absence of bicarbonate can support endothelial function, wherein 60mM HEPES was almost as good as BR. Overall, corneal lactate efflux rate decreased with perfusion time. The decrease in lactate efflux was the least with BR (60.02%) and 85.28%, 79.15%, 73.85%, 68.63% and 64.61% with HEPES at 0, 10, 25, 40, and 60mM, respectively, and after perfusion, corneal [lactate] was 11.97 + 1.33 nmols/mg dry tissue in BR cornea, and 21.08 + 1.85, 18.12 + 2.21, 16.03 + 1.52, 14.6 + 1.25 and 13.53 + 1.41 nmols/mg in BF with HEPES at 0, 10, 25, 40, and 60 mM, respectively.
Corneal endothelial function is not dependent on bicarbonate per se, but by the high buffering capacity that it provides. Bicarbonate and other buffers act by facilitating lactic acid efflux, which controls corneal hydration.
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