pH and calcium regulate the water permeability of aquaporin 0

Aquaporins increase the water permeability in many cell types across many species. We investigated the effects of external pH and Ca2+ on water permeability of Xenopus oocytes injected with aquaporin cRNA by measuring the rate of swelling in hypotonic solutions. Lowering pH to 6.5 increased the water permeability of aquaporin (AQP0) 3.4 +/- 0.4-fold. Diethylpyrocarbonate pretreatment increased water permeability 4.2 +/- 0.5-fold and abolished pH sensitivity, suggesting that the pH regulation is mediated by an external histidine. Lowering Ca2+ increased mater permeability 4.1 +/- 0.1-fold. The effects of Ca2+ and pH each required the presence of histidine 40, indicating a critical role of this amino acid in facilitating the modulation of water permeability. Clamping intracellular Ca2+ at high or low values abolished sensitivity to external Ca2+, suggesting that Ca2+ acts at an internal site. Three different calmodulin inhibitors each increased AQP0 water permeability, suggesting that Ca2+ may act through calmodulin. None of the above altered the water permeability induced by AQP1 or AQP4. Because the greatest change in AQP0 water permeability is in the normal pH range found in the lens (7.2-6.5), this paper provides evidence for regulation of an aquaporin by pH under physiological conditions.