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A.C. Zamudio, L.J. Alvarez, O.A. Candia; Regulation of Water Permeability in the Rabbit Conjunctival Epithelium by Anisotonic Conditions . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4721.
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Previous studies in frog corneal epithelium and toad bladder indicated that their water permeability changed when exposed to anisotonic conditions. We now determined the effects of unilateral exposure to anisotonic conditions on the diffusional water permeability of the isolated rabbit conjunctiva.
A segment of the bulbar–palpebral conjunctiva was mounted between Ussing–type hemichambers under short–circuit conditions. Unidirectional water fluxes (Jdw) were measured in either direction by adding 3H2O to one hemichamber and sampling from the other. Electrical parameters were measured simultaneously.
Jdw 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 Jdw 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 Jdw 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 Jdw were not observed. Nor were such reductions observed in the presence of HgCl2, a result implying the involvement of aquaporins. However, it is possible that the mercurial may be toxic to the epithelium preventing the tonicity response. Nevertheless, from concomitant changes in transepithelial electrical resistance, as well as, 14C–mannitol fluxes, 14C–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.
Both apical and basolateral membranes may down–regulate their water permeabilities as part of a protective mechanism to help maintain cell volume.
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