Water permeability of asymmetric planar lipid bilayers: Leaflets of different composition offer independent and additive resistances to permeation

To understand how plasma membranes may limit water flux, we have modeled th e apical membrane of MDCK type I cells. Previous experiments demonstrated t hat liposomes designed to mimic the inner and outer leaflet of this membran e exhibited 18-fold lower water permeation for outer leaflet lipids than in ner leaflet lipids (Hill, W.G., and M.L. Zeidel. 2000. J. Biol. Chem. 275:3 0176-30185), confirming that the outer leaflet is the primary barrier to pe rmeation. If leaflets in a bilayer resist permeation independently, the fol lowing equation estimates single leaflet permeabilities: 1/P-AB = 1/P-A + 1 /P-B (Eq. 1), where P-AB is the permeability of a bilayer composed of leafl ets A and B, P-A is the permeability of leaflet A, and P-B is the permeabil ity of leaflet B. Using Eq. 1 for the MDCK leaflet-specific liposomes gives an estimated value for the osmotic water permeability (P-f) of 4.6 x 10(-4 ) cm/s (at 25 degreesC) that correlated well with experimentally measured v alues in intact cells. We have now constructed both symmetric and asymmetri c planar lipid bilayers that model the MDCK apical membrane. Water permeabi lity across these bilayers was monitored in the immediate membrane vicinity using a Na+-sensitive scanning microelectrode and an osmotic gradient indu ced by addition of urea. The near-membrane concentration distribution of so lute was used to calculate the velocity of water flow (Pohl, P., S.M. Sapar ov, and Y.N. Antonenko. 1997. Biophys. J. 72:1711-1718). At 36 degreesC, P- f was 3.44 +/-6.35 x 10(-3) cm/s for symmetrical. inner leaflet membranes a nd 3.40 +/-0.34 x 10(-4) cm/s for symmetrical exofacial membranes. From Eq. 1, the estimated permeability of an asymmetric membrane is 6.2 x 10(-4) cm /s. Water permeability measured For the asymmetric planar bilayer was 6.7 /-0.7 x 10(-4) cm/s, which is within 10% of the calculated value. Direct ex perimental measurement of P-f for an asymmetric planar membrane confirms th at leaflets in a bilayer offer independent and additive resistances to wate r permeation and validates the use of Eq. 1.