The corneal endothelium is the site the of a fairly intense fluid transport mechanism. This mechanism moves fluid out of the corneal stroma and into the aqueous, thereby maintaining the stroma's somewhat dehydrated state and providing optimal thickness

From: Ophthalmology Research: Ocular Transporters in Ophthalmic Diseases and Drug Delivery Edited by: J. Tombran-Tmk and C. J. Barnstable © Humana Press, Totowa, NJ

for transparency. Given the importance of the layer and its function, over the years, the molecular composition of corneal endothelium has attracted much attention. By now, many of the membrane proteins of the endothelium have been located and characterized (1-3). There are two major questions that remain unanswered; (i) what is the mode by which bicarbonate ions cross the apical membrane? (ii) what is the mechanism by which fluid is transported?

Still, the arrangement of the membrane proteins has allowed us to elaborate a descriptive model which includes eleven of them. In the model, these proteins are seen to be the site of fluxes of four ions of interest (Na+, K+, Cl-, and HCO3-) across the plasma membrane. The key to this development is that the ionic fluxes cannot assume arbitrary values, as they are all interdependent and linked by equations of conservation of mass and electrical charge. We have solved the system of 11 equations and 11 unknowns that allow one to find values for the 11 fluxes at any given time. As a result, we are in a position to sketch how each of the transporters and channels could be contributing to the task of transporting fluid and electrolytes across the layer. In addition, we are able to evaluate differing hypotheses to explain fluid transport. The assumption of solute-fluid coupling via electro-osmosis produces the closest match to experimental data. The characteristics and consequences of the arrangement of the endothelial transporters and channels are presented in the remainder of this chapter.

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