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K. Varadaraj, S.S. Kumari, A. Shiels, R.T. Mathias; Effect of pH and Calcium on Frog Lens AQP0 . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4477.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Cahalan and Hall (J. Biol. Chem., 2000, 275:6777) reported that water permeability (Pw) of bovine AQP0, exogenously expressed in Xenopus oocytes, was increased by a reduction in pH or calcium; the pH effect was eliminated by mutation of an extracellular histidine (His-40) and the calcium effect was eliminated by calmodulin inhibitors. Previously, we reported that endogenous AQP0 Pw in mouse or rabbit was increased by a reduction in pH or an increase in calcium. The present study was conducted to determine the effect of pH, Ca, and calmodulin inhibition on the Pw of frog lens endogenous AQP0, which lacks His-40. Methods: AQP0 amino acid sequences for different species were compared using data in the GenBank and a protein-protein Blast method. Frog lens cortical fiber cell vesicles were prepared and Pw was determined from the rate of volume change when the vesicle was immersed in hyper- or hypo-tonic bathing solution. Results: Amino acid His-40 is asparagine in frog. Pw of fiber cell membrane vesicles of frog lenses at zero calcium, at a pH of 7.5 was 19 ± 2 µm/s and at pH 6.5 was 19 ± 2 µm/s. At high calcium (5 mM) at a pH of 7.5, Pw was 30 ± 6 µm/s and at high calcium at pH 6.5 Pw was 27 ± 5 µm/s. Calmodulin inhibitor trifluoperazine (75 µM) at high calcium (5 mM) at pHs 7.5 and 6.5, decreased the Pw to 20 ± 3 µm/s and 19 ± 2 µm/s, respectively. Conclusions: Comparison of data from mouse, rabbit and frog lens fiber cell membranes suggest that pH effects on endogenous AQP0 are mediated by His-40, consistent with results from exogenous expression of bovine AQP0. In contrast, high calcium increased the Pw of endogenous AQP0 in frog, rabbit and mouse, but decreased it in exogenously expressed bovine AQP0. Interestingly, calmodulin inhibition eliminates either the increase or decrease. Whether the difference in calcium effect is due to species or exogenous expression is not yet determined, but these data suggest that AQP0 in the lens may be regulated differently in different species.
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