Bidirectional water fluxes and specificity for small hydrophilic moleculesin aquaporins 0-5

The dimensions of the aqueous pore in aquaporins (AQP) 0, 1, 2, 3, 4, and 5 expressed in Xenopus laevis oocytes were probed by comparing the ability o f various solutes to generate osmotic flow. By improved techniques, volume flows were determined from initial rates of changes. Identical values for t he osmotic water permeability (L-p) were obtained in swelling as in shrinka ge experiments demonstrating, for the first time, that aquaporins are bidir ectional. The reflection coefficients (sigma) of urea, glycerol, acetamide, and formamide at 23 degrees C were: AQP0: 1, 1, 0.8, 0.6; AQP1: 1, 0.8, 1, 1; AQP2: 1, 0.8, 1, 1; AQP3: 1, 0.2, 0.7, 0.4; AQP4: 1, 0.9, 1, 1; and AQP 5: 1, 1, 1, 0.8. As seen there is no clear connection between solute size a nd permeation. At 13 degrees C the sigma s for AQP3 were 1, 0.4, 1, and 0.5 ; functionally, this pore narrows at lower temperatures. HgCl2 reversibly r educed the L-p of AQP3 and increased sigma(glyc) to 1 and sigma(form) to 0. 6. We conclude that the pore of the various aquaporins are structurally dif ferent and that a simple steric model is insufficient to explain solute-por e interactions.