Journal
JOURNAL OF GENERAL PHYSIOLOGY
Volume 118, Issue 4, Pages 333-339Publisher
ROCKEFELLER UNIV PRESS
DOI: 10.1085/jgp.118.4.333
Keywords
MDCK cells; apical membrane; fluidity; cell membrane permeability; barrier function
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Funding
- NIDDK NIH HHS [DK43955, R01 DK043955] Funding Source: Medline
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To understand how plasma membranes may limit water flux, we have modeled the apical membrane of MDCK type I cells. Previous experiments demonstrated that liposomes designed to mimic the inner and outer leaflet of this membrane exhibited 18-fold lower water permeation for outer leaflet lipids than inner leaflet lipids (Hill, W.G., and M.L. Zeidel. 2000. J. Biol. Chem. 275:30176-30185), confirming that the outer leaflet is the primary barrier to permeation. If leaflets in a bilayer resist permeation independently, the following 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 leaflets A and B, P-A is the permeability of leaflet A, and P-B is the permeability 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 values in intact cells. We have now constructed both symmetric and asymmetric planar lipid bilayers that model the MDCK apical membrane. Water permeability across these bilayers was monitored in the immediate membrane vicinity using a Na+-sensitive scanning microelectrode and an osmotic gradient induced by addition of urea. The near-membrane concentration distribution of solute was used to calculate the velocity of water flow (Pohl, P., S.M. Saparov, 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 and 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 experimental measurement of P-f for an asymmetric planar membrane confirms that leaflets in a bilayer offer independent and additive resistances to water permeation and validates the use of Eq. 1.
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