Rabbit Corneal Hydration and the Bicarbonate Pump

Abstract

Experiments were conducted on the transport properties of the rabbit corneal endothelium at 22 °C, at which temperature the endothelium was able to stabilize the hydration of corneal stroma at physiological values. When bicarbonate was omitted from the bathing solution, the cornea swelled at 11 ± 1 μm.h⁻¹. The swelling was completely reversible upon the subsequent re-introduction of bicarbonate. Similar swelling rates were observed when the endothelial pump was irreversibly inhibited with ouabain. In an Ussing-type chamber, the endothelium developed an electrical resistance of 25.0 ± 1.0 Ω.cm² and a short circuit current (s.c.c.) of 6.0 ± 1.1 μA.cm⁻². Neither electrical resistance of the corneal endothelium nor its s.c.c. were changed significantly after exposure to 0.5 mM amiloride. Ouabain abolished the s.c.c. but had no significant effect on resistance. When paired preparations were short-circuited, the endothelium developed a net H[¹⁴C]O ⁻₃ flux of 0.24 ± 0.03 μmoles.cm⁻².h⁻¹ into the aqueous humour, which was close in magnitude and direction to the s.c.c. of 0.22 ± 0.01 μEq.cm⁻².h⁻¹. There was no significant net flux of ⁸⁶Rb (0.04 ± 0.03 μmoles.cm⁻².h⁻¹). Similar magnitude fluxes for both bicarbonate and rubidium were found with open-circuit preparations. It is suggested that a metabolically driven electrogenic bicarbonate current passing across the corneal endothelium is solely responsible for maintaining corneal hydration at 22 °C. Based on these and other studies, a model is proposed for active bicarbonate transport across corneal endothelium consisting of uphill entry into the cell through a baso-lateral membrane sodium/bicarbonate cotransporter (NBC) and downhill exit through an apical membrane anion channel. Studies on the transport properties of the endothelium at 35 °C are discussed and reasons suggested for the discrepancy between short circuit current and net bicarbonate flux at this closed eye temperature.