Properties of lipid bilayer membranes separating two aqueous phases: water permeability

Water permeability of phospholipid vesicles

The water permeability of phospholipid vesicles 0.5 to 10 μ in diameter bounded by one or by several lipid bilayers was measured by following the change in turbidity of a suspension after mixing in a stopped flow apparatus. A semi-empirical formulation for evaluating volume changes of vesicles with a broad size range by measurement of turbidity is developed. The rate of flow is analyzed in terms of reaction rate theory. The water permeability coefficients for phosphatidylcholine vesicles were approximately 44 μ/sec at 25°C and 70 μ/sec at 37°C. The activation energy for water transport was 8.25 kcal/mole. The results were consistent with the view that water permeates by dissolution and diffusion in the membrane.

AQP1 gene expression is upregulated by arginine vasopressin and cyclic AMP agonists in trophoblast cells

Aquaporins (AQPs) are water channels that regulate water flow in many tissues. As AQP1 is a candidate to regulate placental fluid exchange, we sought to investigate the effect of arginine vasopressin (AVP) and cAMP agonists on AQP1 gene expression in first trimester-derived extravillous cytotrophoblasts (HTR-8/Svneo) and two highly proliferative carcinoma trophoblast-like cell lines but with a number of functional features of the syncytiotrophoblast namely; JAR and JEG-3 cells. Our data demonstrated that AVP (0.1 nM) significantly increased the expression of AQP1 mRNA at 10 h in HTR-8/SVneo and JEG-3 cells (P<0.05). Both SP-cAMP, a membrane-permeable and phosphodiesterase resistant cAMP, and forskolin, an adenylate cyclase stimulator significantly increased AQP1 mRNA expression in all cell lines after 2 h in a dose-dependent manner (P<0.05) with a parallel increase in protein expression. In the time course study, 5 microM of either SP-cAMP or forskolin significantly stimulated AQP1 mRNA expression after 2 h in HTR-8/SVneo cells and after 10 h in JAR and JEG-3 cells. AQP1 protein expression was highest after 20 h in both HTR-8/SVneo and JEG-3 cells (P<0.05). AVP-stimulated cAMP elevation was blocked in the presence of 9-(tetrahydro-2'-furyl) adenine (SQ22536) (100 microM), a cell-permeable adenylate cyclase inhibitor (P<0.05). These results indicate that in trophoblasts-like cells AQP1 gene expression is upregulated by both AVP and cAMP agonists. Furthermore, our data demonstrate that a cAMP-dependent pathway is responsible for the AVP effect on AQP1. Thus, modulation of AQP1 expression by maternal hormones may regulate invasion and fetal-placental-amnion water homeostasis during gestation.

A comparative study of diffusive and osmotic water permeation across bilayers composed of phospholipids with different head groups and fatty acyl chains

Osmotic and diffusive water permeability coefficients Pf and Pd were measured for lipid vesicles of 100-250 nm diameter composed of a variety of phospholipids with different head groups and fatty acyl chains. Two different methods were applied: the H2O/D2O exchange technique for diffusive water flow, and the osmotic technique for water flux driven by an osmotic gradient. For phosphatidylcholines in the liquid-crystalline state at 70 degrees C, permeability constants Pd between 3.0 and 5.2.10(-4) cm/s and ratios Pf/Pd 7 and 23 were observed. The observation of a permeability maximum in the phase transition region and the fact that osmotically driven water flux is higher than diffusive water exchange suggest that water is diffusing through small transient pores arising from density fluctuations in the bilayers. The Pd values depend on the nature of the head group, on the chemical structure of the chains, and on the type of chain linkage. In the case of charged lipids, the ionic strength of the solution has a strong influence. For phosphatidylethanolamines, phosphatidic acids, and ether phosphatidylcholines, permeability constants Pd were considerably lower (2-4.10(-6) cm/s at 70 degrees C). For liquid-crystalline phosphatidylcholines, a strong reduction of Pd after addition of ethanol was observed (2-4.10(-6) cm/s at 70 degrees C). The experimental values are discussed in connection with different permeation models.

Water transport across biological membranes

The rate of the lateral diffusion of straight-chain phospholipids predicts the rate of water diffusion through bilayers. A new model of lipid dynamics integrates these processes. Substances such as cholesterol that reduce water diffusion proportionally reduce lateral diffusion. The model yields a number of predictions about the dynamics of the lipids at the Tm and suggests different mechanisms for how water diffuses across bilayers of other-than-straight-chain lipids, and how proteins bind to membranes. A second recent development in water transport across biological membranes is the discovery of a ubiquitous family of water transport proteins that facilitate large-volume water translocation. Like water diffusion through lipid bilayers, water transport by these proteins is directed by osmosis and is therefore under the control of ATP and ion pumps. The presence of water transport proteins in membranes is often regulated by hormones.

Water balance in developing eggs of the codGadus morhua L

Measurements of yolk osmolality from the embryo of codGadus morhua L. revealed significant variations in the hyposmolality during the embryonic development. The embryo proved to have an extremely low water permeability, protecting it from dehydration in the hyperosmotic seawater. The effect of temperature on the water permeability is high, expressed by an exceptionally high activation energy for water transfer. The agreement between embryonic volume decrease and diffusion permeability during the first 8-10 days after fertilization indicates that no water uptake mechanism is present at this time, thus leaving the embryo fully dependent on internal water stores. The cod egg is buoyant in seawater throughout development. The mechanism for providing hydrostatic lift is the large volume of diluted tissue water located in the yolk and subdermal spaces.

Serial permeability barriers to water transport in human placental vesicles

Microvillous vesicles were prepared from term human placenta by shearing, differential centrifugation and Mg2+ precipitation. Vesicles were purified further on a sucrose density gradient producing two bands with densities of 1.16 to 1.18 g/ml (C1) and 1.13 to 1.15 g/ml (C2). The C2 fraction, which had a 24-fold enrichment of alkaline phosphatase and a three-fold reduction in Na+, K+-ATPase activity compared to homogenates, was used to measure osmotic water (Pf) permeability. Pf was measured from the time course of scattered light intensity following exposure of vesicles to specified gradients of impermeant solutes. Pf decreased from 3.0 X 10(-3) to 0.6 X 10(-3) cm/sec with increasing gradient size (65 to 730 mM; 23 degrees C). Four possible causes of this behavior were examined theoretically and experimentally: an unstirred layer, saturation of water transport, large changes in the vesicle surface area with changes in volume and a structural restriction to vesicle volume change. The measured dependence of Pf on gradient size and the effect of the channel-forming ionophore gramicidin on Pf fit best to the theoretical dependences predicted by a structural restriction mechanism. This finding was supported by experiments involving the effects on Pf of increased solution viscosity, initial vesicle volume, the magnitude of transmembrane volume flow, and the effects of gradient size on activation energy (Ea) for Pf. The decreased Pf resulting from a structural restriction limiting vesicle volume change was modeled mathematically as a second barrier in series with the vesicle membrane. Ea measured using a 250-mM inwardly directed sucrose gradient was 5.4 +/- 0.6 kcal/mol (T greater than 27 degrees C) and 10.0 +/- 0.6 kcal/mol (T less than 27 degrees C). Ea above 27 degrees C is in the range normally associated with transmembrane passage of water via aqueous channels. Water transport was not inhibited by p-chloromercuribenzenesulfonate.

Permeability of lipid bilayers to water and ionic solutes

The lipid bilayer moiety of biological membranes is considered to be the primary barrier to free diffusion of water and solutes. This conclusion arises from observations of lipid bilayer model membrane systems, which are generally less permeable than biological membranes. However, the nature of the permeability barrier remains unclear, particularly with respect to ionic solutes. For instance, anion permeability is significantly greater than cation permeability, and permeability to proton-hydroxide is orders of magnitude greater than to other monovalent inorganic ions. In this review, we first consider bilayer permeability to water and discuss proposed permeation mechanisms which involve transient defects arising from thermal fluctuations. We next consider whether such defects can account for ion permeation, including proton-hydroxide flux. We conclude that at least two varieties of transient defects are required to explain permeation of water and ionic solutes.

Thermodynamic modeling and cryomicroscopy of cell-size, unilamellar, and paucilamellar liposomes

Cryomicroscope studies of large unilamellar liposomes indicate that liposomes are an excellent model for studying membrane response to freezing and thawing. Liposomes are attractive for such use because they can be custom-manufactured for a particular investigation. In addition, liposome responses to freezing and thawing mimic real cell behavior in a number of significant ways. Analogous behavior includes osmotic shrinkage at slow cooling rates, internal ice formation at fast cooling rates, comparable nucleation temperatures, and a variety of comparable thawing responses. Experimental determination has been made of the equilibrium osmotic properties and the nonequilibrium water transport properties of the egg lecithin liposomes used in the freezing studies. These properties have been used in a computer model to simulate volume changes resulting from water transport during freezing and thawing. Comparison between computer model predictions and experimental data for the liposome volume response during freezing indicates reasonable agreement whereas computer simulations of volume response during thawing do not match experimental data well.

Phospholipid surface bilayers at the air-water interface. II. Water permeability of dimyristoylphosphatidylcholine surface bilayers

Dispersions of dimyristoylphosphatidylcholine (DMPC) in water have been reported to form a structure at 29 degrees C at the equilibrium air/water surface with a molecular density equal to that of a typical bilayer. In this study, the water permeability of this structure has been evaluated by measuring the rate of water evaporation from DMPC dispersions in water in the temperature range where the surface film density exceeds that of a monolayer. Evaporation rates for the lipid dispersions did not deviate from those for lipid-free systems throughout the entire temperature range examined (20-35 degrees C) except at 29 degrees C, where a barrier to evaporation was detected. This strengthens the view that the structure that forms at this temperature has the properties of a typical bilayer.

Water permeation through the lipid bilayer membrane. Test of the liquid hydrocarbon model

According to the liquid hydrocarbon model, the lipid bilayer is viewed simply as a thin slice of bulk hydrocarbon liquid. This allows the water permeability of the bilayer to be calculated from bulk properties. In this paper the prediction of the liquid hydrocarbon model is compared with the known water permeability coefficient of the glycerol monoolein/n-hexadecane bilayer (Fettiplace, R. (1978) Biochim. Biophys. Acta 513, 1-10). As the alkyl chain of glycerol monoolein is equivalent to 8-heptadecene, the water permeability coefficient of 8-heptadecene/n-hexadecane mixtures was measured for temperatures between 20 and 35 degrees C. The mole fraction of n-hexadecane in the bulk liquid was chosen at each temperature to match the known mole fraction of n-hexadecane in the bilayer (White, S. (1976) Nature 262, 421-422). The predicted water permeability coefficient agrees with the measured value at 32 degrees C but is 40% above the measured value at 20 degrees C. The apparent activation energy predicted by the liquid hydrocarbon model is 9.0 +/- 0.3 kcal/mol, while the measured value is 14.2 +/- 1.0 kcal/mol. The failure of the liquid hydrocarbon model probably results from a different molecular organization of the hydrocarbon chains in the bilayer and in the bulk liquid.

The influence of the lipid on the water permeability of artificial membranes

The water permeabilities of artificial membranes formed formed from various monoglycerides or phospholipids in alkane solvents have been measured using an osmotic method, and it has been shown that the permeability depends upon the type of lipid used. For monoglycerides, the permeability was found to increase with the unsaturation and decrease with the length of the acyl chain. Membranes formed from either egg phosphatidylcholine or dioleyl phosphatidylcholine had an osmotic permeability coefficient of approx. 35--40 micrometer/s at 25 degrees C; with sphingomyelin as the membrane lipid, the permeability was an order of magnitude lower than that for phosphatidylcholine. It is suggested that the water permeabilities of biological membranes might be partly controlled by the types of lipid present.

The effect of synthetic polymers on the electrical and permeability properties of lipid membranes

1. The effect of two series of hydrophilic and hydrophobic polymers on the stability, conductivity and permeability towards water and leucine of black lipid membranes and liposomes is reported. 2. The changes in properties of these membrane preparations is related to bulk phase viscosity and dielectric measurements together with monolayer studies. 3. The hydrophobic polymers dramatically increase membrane stability, had no effect on conductivity, but increased the permeability coefficient of leucine. 4. The hydrophilic polymers produced minor, but significant changes to membrane properties. 5. It is concluded that not only basic polymers but also neutral and acidic macromolecules can interact strongly with lipid membranes.

The effect of antidiuretic hormone on solute flows in mammalian collecting tubules

These experiments were intended to evaluate the antidiuretic hormone (ADH)-dependent reflection coefficients of urea, sucrose, and NaCl in cortical and outer medullary collecting tubules isolated from mammalian kidney. In one group of experiments, the ADH-dependent osmotic water flows, when the perfusing solutions contained hypotonic NaCl solutions, were indistinguishable from control observations when either urea or sucrose replaced, in part, NaCl in isotonic bathing solutions (cortical collecting tubules). Similarly, both in cortical and outer medullary collecting tubules exposed to ADH, there was zero net osmotic volume flow when a portion of the NaCl in the bathing and/or perfusing solutions was replaced by either sucrose or urea, so long as the perfusing and bathing solutions were isosmolal. Taken together, these observations suggest that the ADH-dependent reflection coefficients of NaCl, urea, and sucrose, in these tubules, were identical. Since the effective hydrodynamic radii of urea and sucrose are, respectively, 1.8 and 5.2 A, it is likely that sigma(i), for urea, sucrose, and NaCl, was unity. In support of this, the diffusion permeability coefficient (P(Di) cm sec(-1)) of urea was indistinguishable from zero. Since the limiting sites for urea penetration were the luminal interfaces of the tubules, these data are consistent with the view that ADH increases diffusional water flow across such interfaces.

A cytological study of artificial parthenogenesis in the sea urchin Arbacia punctulata

Eggs of the sea urchin Arbacia punctulata were artificially activated with hypertonic seawater. The artificially activated eggs undergo the cortical reaction which is not distinguished by a wavelike progression as in the case of inseminated eggs. The cortical granules are released at random loci at the surface of the egg and result in spaces separated by large cytoplasmic projections. Unreacted cortical granules and ribosomes are found within the matrix comprising the large cytoplasmic projections. No "fertilization cone" is formed. The subsequent release of additional cortical granules results in the formation of a continuous perivitelline space, 15 min following activation. 85 min postactivation, an organization of annulate lamellae, endoplasmic reticulum of the smooth variety, and microtubules around a centriole is observed prior to nuclear division. Before the breakdown of the nuclear envelope a streak stage is formed. The streak is composed of a central core of annulate lamellae and is encompassed by endoplasmic reticulum and vesicular components. Condensation of chromatin is followed by the establishment of the mitotic apparatus. Centrioles were not found in the mature egg; however, they are present after activation prior to the first nuclear division, in the four-cell embryo, multicellular embryo, and at blastula. Artificially activated eggs have been observed to develop to the pluteus stage in more than 50% of the eggs treated.

The ion permeability induced in thin lipid membranes by the polyene antibiotics nystatin and amphotericin B

Characteristics of nystatin and amphotericin B action on thin (<100 A) lipid membranes are: (a) micromolar amounts increase membrane conductance from 10(-8) to over 10(-2) Omega(-1) cm(-2); (b) such membranes are (non-ideally) anion selective and discriminate among anions on the basis of size; (c) membrane sterol is required for action; (d) antibiotic presence on both sides of membrane strongly favors action; (e) conductance is proportional to a large power of antibiotic concentration; (f) conductance decreases approximately 10(4) times for a 10 degrees C temperature rise; (g) kinetics of antibiotic action are also very temperature sensitive; (h) ion selectivity is pH independent between 3 and 10, but (i) activity is reversibly lost at high pH; (j) methyl ester derivatives are fully active; N-acetyl and N-succinyl derivatives are inactive; (k) current-voltage characteristic is nonlinear when membrane separates nonidentical salt solutions. These characteristics are contrasted with those of valinomycin. Observations (a)-(g) suggest that aggregates of polyene and sterol from opposite sides of the membrane interact to create aqueous pores; these pores are not static, but break up (melt) and reform continuously. Mechanism of anion selectivity is obscure. Observations (h)-(j) suggest-NH(3) (+) is important for activity; it is probably not responsible for selectivity, particularly since four polyene antibiotics, each containing two-NH(3) (+) groups, induce ideal cation selectivity. Possibly the many hydroxyl groups in nystatin and amphotericin B are responsible for anion selectivity. The effects of polyene antibiotics on thin lipid membranes are consistent with their action on biological membranes.

The effect of amphotericin B on the water and nonelectrolyte permeability of thin lipid membranes

This paper reports the effects of amphotericin B, a polyene antibiotic, on the water and nonelectrolyte permeability of optically black, thin lipid membranes formed from sheep red blood cell lipids dissolved in decane. The permeability coefficients for the diffusion of water and nonelectrolytes (P(DDi)) were estimated from unidirectional tracer fluxes when net water flow (J(w)) was zero. Alternatively, an osmotic water permeability coefficient (P(f)) was computed from J(w) when the two aqueous phases contained unequal solute concentrations. In the absence of amphotericin B, when the membrane solutions contained equimolar amounts of cholesterol and phospholipid, P(f) was 22.9 +/- 4.6 microsec(-1) and P(DDHDH2O) was 10.8 +/- 2.4 microsec(-1). Furthermore, P(DDi) was < 0.05 microsec(-1) for urea, glycerol, ribose, arabinose, glucose, and sucrose, and sigma(i), the reflection coefficient of each of these solutes was one. When amphotericin B (10(-6)M) was present in the aqueous phases and the membrane solutions contained equimolar amounts of cholesterol and phospholipid, P(DDHDH2O) was 18.1 +/- 2.4 microsec(-1); P(f) was 549 +/- 143 microsec(-1) when glucose, sucrose, and raffinose were the aqueous solutes. Concomitantly, P(DDi) varied inversely, and sigma(i) directly, with the effective hydrodynamic radii of the solutes tested. These polyene-dependent phenomena required the presence of cholesterol in the membrane solutions. These data were analyzed in terms of restricted diffusion and filtration through uniform right circular cylinders, and were compatible with the hypothesis that the interactions of amphotericin B with membrane-bound cholesterol result in the formation of pores whose equivalent radii are in the range 7 to 10.5 A.