Regulation of Water Permeability in the Rabbit Conjunctival Epithelium by Anisotonic Conditions

The effects of unilateral exposure to anisotonic conditions on the diffusional water permeability of the isolated rabbit conjunctiva were determined. For these experiments, a segment of the bulbar‐palpebral conjunctiva was mounted between Ussing‐type hemichambers under short‐circuit conditions. Unidirectional water fluxes ( J dw ) were measured in either direction by adding 3 H 2 O to one hemichamber and sampling from the other. Electrical parameters were measured simultaneously. J dw were determined under control isosmotic conditions and after the introduction of either hyper‐ or hypotonic solutions against the tear‐ or stromal‐sides of the preparations. In each of these 4 separate conditions, the anisotonic medium produced a 20–30% reduction in J dw across the tissue, with the exception that in order to obtain such reduction with increased tonicity from the stromal side (medium osmolality increased by adding sucrose), conditions presumptively inhibiting RVI mechanisms (e.g., pretreatment with amiloride and bumetanide) were also required. All reductions in J dw elicited by the various anisotonic conditions were reversible upon restoring the control tonicity. In experiments in which the preparations were pretreated with the protein cross‐linking agent, glutaraldehyde, the anisotonic‐elicited reductions in J dw were not observed. From concomitant changes in transepithelial electrical resistance, as well as, 14 C‐mannitol fluxes, 14 C‐butanol fluxes, and Arrhenius plots, the above effects are best explained as a cell‐regulated reduction in membrane water permeability that occurs at the level of water‐transporting channels. Presumably both apical and basolateral membranes can down‐regulate their water permeabilities as part of a protective mechanism to help maintain cell volume.