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Clio A. Rubinos, Miduturu Srinivas; Permeability of Lens Gap Junctions to Antioxidants. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1534.
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© ARVO (1962-2015); The Authors (2016-present)
Previous studies indicate there is a concentration gradient for the anionic tripeptide glutathione (GSH) in the lens; levels of GSH are high in outer cortex, where it is synthesized but low in the inner cortex and nucleus. This regional difference worsens with age, and is proposed to be a major factor in the formation of senile cataracts. Several mechanisms have been proposed to explain the regional difference in GSH levels. Because Cx46 and Cx50 gap junctions couple outer fiber cells to those in the inner cortex and nucleus, it is possible that the unique permeability properties of these channels may contribute to the regional difference in GSH levels. In this study, we compared the permeability of large anions including GSH, glutamate and ascorbate through Cx46 and Cx50 channels with that of Cx43 channels.
Charge selectivity was determined by 1) varying the composition of the principal anion or cation in the patch pipette and measuring the unitary conductances of the fully open state and by 2)measuring reversal potentials in bi-ionic conditions and estimating the relative permeability using the Goldman-Hodgkin-Katz equation. Junctional currents were measured using electrophysiological methods applied to N2A cells expressing connexins
Equimolar replacement of CsCl with chloride salts of monovalent cations reduced unitary conductance of Cx43, Cx46 and Cx50 as predicted by aqueous mobility of cations. In contrast, only the unitary conductance of Cx43 channels decreased in proportion to aqueous mobility when Cl- was replaced with larger anions. Cx43 unitary conductance was reduced from 94 ± 2 (n=6) in NaCl to 57 ± 3 (n=4) in Naglutamate and 38 ± 4 (n=4) in NaGSH. In contrast, the unitary conductance of Cx46 channels was reduced by only 7 ± 3 % (n=4) and 14 ± 4 % (n=5) when glutamate and GSH replaced Cl-, respectively, indicating that the conductance in Cx46 is largely dominated by cations whereas in Cx43 it is provided by both anions and cations. Cx50 channels appear to be more selective for cations than anions, but anionic flux is significant, because replacement of Cl- with glutamate or GSH produced a significant change in conductance (decreases of 15 ± 3 % and 32 ± 5 %). Similar results were obtained when patch pipettes contained Na-Ascorbate instead of NaCl. Reversal potential measurements also suggested that Cx43 is more permeable to large anionic molecules than Cx46 and Cx50.
These results indicate that connexin permeability of anions, including GSH, follows the sequence Cx43 >Cx50 >Cx46. Whether these unique permeability properties to large anions explains the regional difference in GSH levels remains to be determined.
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