Krylov A V, Pohl P, Zeidel M L, Hill W G
Nachwuchsgruppe Biophysik, Forschungsinstitut fuer Molekulare Pharmakologie, 13125 Berlin, Germany.
J Gen Physiol. 2001 Oct;118(4):333-40. doi: 10.1085/jgp.118.4.333.
To understand how plasma membranes may limit water flux, we have modeled the apical membrane of MDCK type 1 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. l), 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 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 degrees C) 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 degrees C, P(f) was 3.44 +/- 0.35 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, 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.
为了理解质膜如何限制水通量，我们对1型MDCK细胞的顶端膜进行了建模。先前的实验表明，设计用来模拟该膜内外小叶的脂质体，其外小叶脂质的水渗透能力比内小叶脂质低18倍（希尔，W.G.，和M.L. 蔡德尔。2000年。《生物化学杂志》275:30176 - 30185），这证实了外小叶是渗透的主要屏障。如果双层膜中的小叶独立抵抗渗透，那么以下公式可估算单个小叶的渗透率：1/P(AB) = 1/P(A) + 1/P(B)（公式1），其中P(AB)是由小叶A和B组成的双层膜的渗透率，P(A)是小叶A的渗透率，P(B)是小叶B的渗透率。使用针对MDCK小叶特异性的脂质体得出的渗透水渗透率（P(f)）估计值为4.6×10⁻⁴厘米/秒（在25℃时），这与完整细胞中的实验测量值相关性良好。我们现在构建了模拟MDCK顶端膜的对称和不对称平面脂质双层。使用对Na⁺敏感的扫描微电极并通过添加尿素诱导渗透梯度，监测这些双层膜附近的水渗透率。溶质在膜附近的浓度分布用于计算水流速度（波尔，P.，S.M. 萨帕罗夫，和Y.N. 安东年科。1997年。《生物物理杂志》72:1711 - 1718）。在36℃时，对称内小叶膜的P(f)为3.44±0.35×10⁻³厘米/秒，对称外小叶膜的P(f)为3.
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