Volume regulation of non-pigmented cells from ciliary epithelium

We describe a new method for investigating ion and water transport in ciliary epithelium. A single ciliary process from the rabbit is isolated, placed in a chamber, and rapidly perfused with physiological Ringer. If this process is then viewed at its edge with a light microscope using Hoffman modulation contrast optics, it is possible to record the image of a single layer of non-pigmented epithelial cells. When these cells are exposed to hypotonic Ringer, they swell and then reduce their volume by extruding salt and water. We have used the rate of swelling to calculate the hydraulic conductivity of the non-pigmented cells. These measurements show that the aqueous humor could be secreted by the generation of a modest osmotic gradient across the non-pigmented cell basal membrane. We have also used this preparation to investigate the mechanism of the decrease in cell volume after swelling in hypotonic medium. This volume regulatory decrease can occur at a rate as large as the rate of normal fluid transport during the secretion of the aqueous. It appears to be caused in part by active Na extrusion, since it is partially inhibited by exposure to ouabain. In addition, there appears also to be a contribution to the regulatory response from a swelling-induced K efflux, since the rate of volume regulation can be altered by perfusion with Ba2+ and by changing the extracellular K concentration.