Measurement of the volume flow and hydraulic conductivity across the isolated dog retinal pigment epithelium.

The isolated dog retinal pigment epithelium (RPE)-choroid was gently stretched on the inner surface of a spherical stainless mesh, retinal side upward, and clamped between half-chambers made of Kel-F. The volume flow across the tissue was monitored by the movement of water in capillary tubes connected to both chambers. With zero pressure difference across the RPE-choroid, retina-to-choroid fluid flow was determined to be 6.4 microliters/hr/cm2 (absorption). Removal of HCO-3 from the solution did not affect the fluid flow. However, the flow was reduced 88% in Cl- -free medium, indicating a coupling between water and Cl- absorption. The flow was also inhibited by ouabain (10(-5) M) and furosemide (10(-4) M). Hydraulic conductivity (Lp) of the RPE-choroid was determined to be 0.0126 microliters/min/cm2/mm Hg which places the dog RPE-choroid in the category of a "leaky" epithelium.