Corneal hydration and metabolically dependent transcellular passive transfer of water

Abstract

A theoretical model of the fluid pump in the corneal endothelium of the rabbit is presented. It is proposed that the stromal and aqueous surfaces of the endothelial cells have different solute reflection coefficients and that an adequate solute concentration is maintained in the cells by a mechanism dependent on metabolic energy. The model does not require transendothelial active transport of either solute or water. However, it requires the net volume flow observed through the isolated endothelium from stroma to aqueous to occur across the cells. Two situations are possible: (1) that the osmolarity of the cells is lower than the aqueous solution in conjunction with the aqueous side of the cells having the larger reflection coefficient; (2) that the osmolarity of the cells is higher than that in the stromal solution in conjunction with the stromal side of the cells having the higher reflection coefficient. A consequence of these requirements is that in case 1 the hydrostatic pressure within the endothelial cell must be less than the hydrostatic pressure in the stromal solution. In case 2 the intracellular hydrostatic pressure must be greater than the hydrostatic pressure of the aqueous solution. In vivo when the eye is closed there is zero net volume flow across the endothelium. There will be an anteriorly directed volume flow through the intercellular channels equal in magnitude to the posteriorly directed flow across the cells. This will produce a circulation of volume flow across the endothelium. When the eye is opened evaporation will take place across the epithelium, resulting in a net volume flow from the aqueous fluid into the stromal fluid. While the volume flow through the intercellular channels should remain anteriorly directed, the volume flow across the endothelial cells may be either anteriorly or posteriorly directed, depending on the values of pressures, concentrations and reflection coefficients. Two independent theoretical estimates of the net volume flow across the isolated endothelium can be made and compared with experimental results. These estimates are 3·35 μl/hr/cm 2 and 2·56 μl/hr/cm 2. They are in good agreement with the average experimental value of 6·5 μl/hr/cm 2 (Maurice, 1972).