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

Krylov AV, Pohl P, Zeidel ML, Hill WG - J. Gen. Physiol. (2001)

Bottom Line: Biophys.J. 72:1711-1718).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.

View Article: PubMed Central - PubMed

Affiliation: Nachwuchsgruppe Biophysik, Forschungsinstitut fuer Molekulare Pharmakologie, 13125 Berlin, Germany.

ABSTRACT
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.

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Schematic diagram of apparatus used to measure osmotically induced water transport. The sodium concentration in the vicinity of planar lipid membranes was monitored using the microelectrode technique. The membranes were formed by the apposition of two differently composed monolayers within the aperture (diam 150–250 μm) in a polytetrafluorethylene septum. The microelectrode (ME) was driven to the membrane with the help of a microdrive. The signal was amplified, digitized, and transferred to a personal computer.
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Figure 1: Schematic diagram of apparatus used to measure osmotically induced water transport. The sodium concentration in the vicinity of planar lipid membranes was monitored using the microelectrode technique. The membranes were formed by the apposition of two differently composed monolayers within the aperture (diam 150–250 μm) in a polytetrafluorethylene septum. The microelectrode (ME) was driven to the membrane with the help of a microdrive. The signal was amplified, digitized, and transferred to a personal computer.

Mentions: The sodium concentration distribution in the immediate membrane vicinity was measured using a scanning ion-sensitive microelectrode (Fig. 1). The latter was moved perpendicular to the surface of the membrane by a hydraulic microdrive manipulator (Narishige). The touching of the membrane was indicated by a steep potential change (Antonenko and Bulychev 1991). From the known velocity of microelectrode motion (1 μm s−1), the position of the microsensor relative to the membrane was determined at any instant of the experiment. The accuracy of the distance measurements was estimated to be ±5 μm. Electrodes with a 90% rise time below 0.6 s were selected. Artifacts due to a very slow electrode motion were therefore unlikely. Nevertheless, possible effects of time resolution or distortion of the stagnant near-membrane water layer were tested by making measurements while moving the microelectrode toward and away from the bilayer. Since no hysteresis was found, it can be assumed that an electrode of appropriate time resolution was driven at a rate that is slow relative to the rate at which any electrode-induced disturbance of the unstirred layer reaches a “stationary” state.


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

Krylov AV, Pohl P, Zeidel ML, Hill WG - J. Gen. Physiol. (2001)

Schematic diagram of apparatus used to measure osmotically induced water transport. The sodium concentration in the vicinity of planar lipid membranes was monitored using the microelectrode technique. The membranes were formed by the apposition of two differently composed monolayers within the aperture (diam 150–250 μm) in a polytetrafluorethylene septum. The microelectrode (ME) was driven to the membrane with the help of a microdrive. The signal was amplified, digitized, and transferred to a personal computer.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2233699&req=5

Figure 1: Schematic diagram of apparatus used to measure osmotically induced water transport. The sodium concentration in the vicinity of planar lipid membranes was monitored using the microelectrode technique. The membranes were formed by the apposition of two differently composed monolayers within the aperture (diam 150–250 μm) in a polytetrafluorethylene septum. The microelectrode (ME) was driven to the membrane with the help of a microdrive. The signal was amplified, digitized, and transferred to a personal computer.
Mentions: The sodium concentration distribution in the immediate membrane vicinity was measured using a scanning ion-sensitive microelectrode (Fig. 1). The latter was moved perpendicular to the surface of the membrane by a hydraulic microdrive manipulator (Narishige). The touching of the membrane was indicated by a steep potential change (Antonenko and Bulychev 1991). From the known velocity of microelectrode motion (1 μm s−1), the position of the microsensor relative to the membrane was determined at any instant of the experiment. The accuracy of the distance measurements was estimated to be ±5 μm. Electrodes with a 90% rise time below 0.6 s were selected. Artifacts due to a very slow electrode motion were therefore unlikely. Nevertheless, possible effects of time resolution or distortion of the stagnant near-membrane water layer were tested by making measurements while moving the microelectrode toward and away from the bilayer. Since no hysteresis was found, it can be assumed that an electrode of appropriate time resolution was driven at a rate that is slow relative to the rate at which any electrode-induced disturbance of the unstirred layer reaches a “stationary” state.

Bottom Line: Biophys.J. 72:1711-1718).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.

View Article: PubMed Central - PubMed

Affiliation: Nachwuchsgruppe Biophysik, Forschungsinstitut fuer Molekulare Pharmakologie, 13125 Berlin, Germany.

ABSTRACT
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.

Show MeSH