Regulation of aquaporin water permeability in the lens

To examine Ca(2+)- and pH-mediated regulation of water permeability of endogenously expressed aquaporin (AQP)0 in lens fiber cells and AQP1 in lens epithelial cells.

Large, right-side-out membrane vesicles were formed from freshly isolated groups of lens fiber cells. Osmotic shrinking or swelling of these vesicles was used to determine the water permeability of endogenously expressed AQP0. The results were compared with those in similar studies of freshly isolated lens epithelial cells, which endogenously expressed AQP1, and of oocytes, which exogenously expressed AQP0.

In the lens or in oocytes, decreasing external pH from 7.5 to 6.5 caused a two- to fourfold increase in the water permeability of mammalian AQP0. Several lines of evidence suggest that this effect is mediated by the binding of H(+) to a histidine in the first extracellular loop (His40). Lens AQP1 lacks His40 and also lacks pH sensitivity. Increasing Ca(2+) caused a two- to fourfold increase in the water permeability of endogenous AQP0. The Ca(2+) effect on mouse AQP0 was a 2.5-fold increase in the lens, whereas in oocytes, it was a 4-fold decrease. In either environment, the effect was mediated through calmodulin, most likely through its binding to the proximal domain of the C terminus. Lens AQP1 does not have a similar domain and does not have calcium sensitivity.

In either the lens or oocytes, Ca(2+) and H(+) appear to affect the same mechanism, probably either the open probability of the water channel, or open-channel permeability. The difference between calcium's effects in lens versus oocytes was remarkable and is not understood. However, in the lens, Ca(2+) and H(+) are both increased in inner fiber cells, and so in the physiologically relevant environment, both may act to increase the water permeability of AQP0.