pH gating of lens fibre connexins

Chemical gating of gap junction channels by intracellular pH may be an important mechanism for the physiological regulation of cell-cell coupling. In the ocular lens, pH gating of gap junction channels has been implicated as a possible cause of cataract in diabetics. To address this question further, I determined the pH dependence of the rat connexin (Cx)-46 and ovine Cx49 in transfected HeLa cells using the pH-clamp technique during dual whole-cell recording. pH gating for both connexins was fast and reversible. The apparent pK(a) (pK(a,app)) was 6.66+/-0.01 and the Hill coefficient (n) 6.8+/-1.8 for Cx49, and for Cx46 6.8+/-0.01 and 2.2+/-0.15, respectively. C-terminal truncation of Cx46 by 163 aa did not abolish the pH sensitivity but shifted the pK(a,app) to 6.6+/-0.01. This finding is inconsistent with the ball-and-chain model proposed for Cx43. Voltage gating of Cx46 channels was also not altered by truncation or acidic pH, indicating that the two gating mechanisms are functionally and possibly structurally separate. The data also imply a significant role of pH gating for lens pathophysiology. For the normal pH range in the lens cortex (pH 6.8-7.2) most gap junction channels will be open. However, mild acidification will reduce gap junctional coupling significantly, especially for Cx50 channels. Localized closure of gap junction channels will disrupt lens transport and thus may contribute to the tissue damage observed in diabetic lenses.