title: Fluid transport by the cornea endothelium is dependent on buffering lactic acid efflux.
abstract: Maintenance of corneal hydration is dependent on the active transport properties of the corneal endothelium. We tested the hypothesis that lactic acid efflux, facilitated by buffering, is a component of the endothelial fluid pump. Rabbit corneas were perfused with bicarbonate-rich (BR) or bicarbonate-free (BF) Ringer of varying buffering power, while corneal thickness was measured. Perfusate was collected and analyzed for lactate efflux. In BF with no added HEPES, the maximal corneal swelling rate was 30.0 ± 4.1 μm/h compared with 5.2 ± 0.9 μm/h in BR. Corneal swelling decreased directly with [HEPES], such that with 60 mM HEPES corneas swelled at 7.5 ± 1.6 μm/h. Perfusate [lactate] increased directly with [HEPES]. Similarly, reducing the [HCO3 (-)] increased corneal swelling and decreased lactate efflux. Corneal swelling was inversely related to Ringer buffering power (β), whereas lactate efflux was directly related to β. Ouabain (100 μM) produced maximal swelling and reduction in lactate efflux, whereas carbonic anhydrase inhibition and an monocarboxylic acid transporter 1 inhibitor produced intermediate swelling and decreases in lactate efflux. Conversely, 10 μM adenosine reduced the swelling rate to 4.2 ± 0.8 μm/h and increased lactate efflux by 25%. We found a strong inverse relation between corneal swelling and lactate efflux (r = 0.98, P < 0.0001). Introducing lactate in the Ringer transiently increased corneal thickness, reaching a steady state (0 ± 0.6 μm/h) within 90 min. We conclude that corneal endothelial function does not have an absolute requirement for bicarbonate; rather it requires a perfusing solution with high buffering power. This facilitates lactic acid efflux, which is directly linked to water efflux, indicating that lactate flux is a component of the corneal endothelial pump.
authors.affiliation: School of Optometry, Indiana University, Bloomington, Indiana.
authors.forename: Shimin
authors.initials: S
authors.lastname: Li
authors.affiliation: School of Optometry, Indiana University, Bloomington, Indiana.
authors.forename: Edward
authors.initials: E
authors.lastname: Kim
authors.affiliation: School of Optometry, Indiana University, Bloomington, Indiana jbonanno@indiana.edu.
authors.forename: Joseph A
authors.identifier: 0000-0002-3842-7646
authors.initials: JA
authors.lastname: Bonanno
mesh_terms: D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D001693:Biological Transport, Active; D002021:Buffers; D002257:Carbonic Anhydrase Inhibitors / Q000494:pharmacology; D004728:Endothelium, Corneal / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D006531:HEPES / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007552:Isotonic Solutions / Q000378:metabolism; D019344:Lactic Acid / Q000378:metabolism*; D008297:Male; D008954:Models, Biological; D027501:Monocarboxylic Acid Transporters / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010477:Perfusion; D011817:Rabbits; D000077325:Ringer's Lactate; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors / Q000378:metabolism; D027981:Symporters / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013997:Time Factors; D000099233:Monocarboxylate Transport Protein 1
chemical_list: D001639:Bicarbonates; D002021:Buffers; D002257:Carbonic Anhydrase Inhibitors; D006531:HEPES; D007552:Isotonic Solutions; D019344:Lactic Acid; D027501:Monocarboxylic Acid Transporters; D000077325:Ringer's Lactate; D000254:Sodium-Potassium-Exchanging ATPase; D027981:Symporters; D000099233:Monocarboxylate Transport Protein 1
keywords: buffering power; corneal endothelium; lactate flux; monocarboxylic acid transporters; water transport