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Marc H. Levin, A. S. Verkman; Aquaporin-Dependent Water Permeation at the Mouse Ocular Surface: In Vivo Microfluorimetric Measurements in Cornea and Conjunctiva. Invest. Ophthalmol. Vis. Sci. 2004;45(12):4423-4432. doi: 10.1167/iovs.04-0816.
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purpose. Fluorescence methods were developed to quantify membrane and tissue water permeabilities at the ocular surface and to compare water transport in wild-type mice versus transgenic mice lacking each of the water channels, aquaporin (AQP)-1, -3, and -5, normally expressed in cornea or conjunctiva.
methods. Membrane water permeabilities (P f mem) of calcein-stained surface epithelial cells were measured from the kinetics of fluorescence quenching in response to rapid (<0.2 seconds) changes in extraocular fluid osmolarity. Tissue water permeabilities (P f tiss) across intact cornea and conjunctiva—the relevant parameters describing water movement into the hyperosmolar tear film in vivo—were determined by a dye-dilution method from the fluorescence of Texas red-dextran in an anisosmolar solution in a microchamber at the ocular surface.
results. Osmotic equilibration occurred with an exponential time constant (τ) of 1.3 ± 0.2 seconds (P f mem = 0.045 cm/s) in calcein-loaded corneal epithelial cells of wild-type mice, slowing 2.1 ± 0.4-fold in AQP5-deficient mice; τ was 2.4 ± 0.1 seconds in conjunctiva (P f mem = 0.025 cm/s), slowing 3.6 ± 0.7-fold in AQP3-deficient mice. In dye-dilution experiments, P f tiss of cornea was 0.0017 cm/s and decreased by greater than fivefold in AQP5-deficient mice. P f tiss in AQP5-null mice was restored to 0.0015 cm/s after removal of the epithelium. P f tiss of conjunctiva was 0.0011 cm/s and was not sensitive to AQP3 deletion.
conclusions. These results define for the first time the water-transporting properties of the two principal ocular surface barriers in vivo. The permeability data were incorporated into a mathematical model of tear film osmolarity, providing insights into the pathophysiology of dry eye disorders.
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