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RT Mathias, K Varadaraj, S Kumari, GJ Baldo, A Shiels, S Bassnett; Water Transport by Aquaporin 0 in Lens . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4643.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Recently, Cahalan and Hall (J. Biol. Chem., 2000, 275:6777) reported that water permeability (Pw) of AQP0 in the Xenopus oocytes was influenced by pH and calcium. The present study was conducted to: 1. Determine the water permeability of AQP0 in lens fiber cell membrane using wild type and AQP0 knockout mice and 2. To find out the effect of pH and calcium on endogenous AQP0 Pw. Methods: Lens cortical fiber cell vesicles were prepared from mice and rabbit, and Pw was determined from the rate of volume change when the vesicle was immersed in hyper- or hypo- tonic bathing solution. Calcium uptake was studied using Fluo 3-AM and Calcium Green 2-AM. Results: Fiber cell membrane vesicles of wild type mouse lenses had a Pw of 39 ± 11 µm/s. Pw of fiber cell vesicles prepared from AQP0 knockout heterozygous mice was 18 ± 5 µm/s and that of homozygous knockout, 8 ± 3 µm/s. In rabbit lens membrane, Pw at pH 6.5 was 133 ± 31 µm/s and at pH 7.5 was 36 ± 14 µm/s. At zero calcium at pH 7.5, the Pw was 36 ± 14 µm/s. At high calcium (5-10 mM) at a pH of 7.5, Pw was 107 ± 64 µm/s and at high calcium at pH 6.5 Pw was 145 ± 29 µm/s. Calmodulin inhibitors decreased the Pw in high Ca2+ solution. Conclusion: The data from AQP0 knockout model suggest that AQP0 is critical for fiber cell water transport. Acidic pH and/or high calcium increased the fiber cell Pw by 3-4 fold. The pH effect is the same as seen for exogeneously expressed AQP0 in oocytes, whereas the Ca2+-effect is the opposite. In the intact lens, [H+] and [Ca2+] are higher in central fiber cells than cortical, suggesting AQP0 is regulated differently at different regions of the lens.
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