Influence of calcium, cholesterol, and unsaturation on lecithin monolayers

Interfacial properties as stability predictors of lecithin-stabilized perfluorocarbon emulsions

The purpose of this study was to determine whether the addition of small quantities of minor lecithin components (phosphatidylinositol, phosphatidic acid, lysophosphatidylethanolamine, and cholesterol) and Pluronic F68 to lecithin could improve the stability of lecithin-stabilized perfluorocarbon emulsions. Attempts were made to correlate emulsion stability with interfacial properties (tension and charge). Dynamic interfacial tension was determined using a Teflon Wilhelmy plate method [reported previously (1)]. Emulsions were prepared by microfluidization. Microelectrophoresis was used to measure emulsion droplet charge, and photon correlation spectroscopy and Coulter analysis were used to determine emulsion stability as a function of droplet size. Thermal kinetic accelerated stability testing was conducted. Various droplet size parameters were used to compare emulsion stabilities, and an overall stability ranking, based on these parameters, was obtained for each emulsion. Small quantities of additives altered emulsion stability and these data were correlated with interfacial properties and initial droplet diameters. The addition of cholesterol to lecithin resulted in the most stable perfluorocarbon emulsion.

Effect of pulmonary surfactant protein A (SP-A) and calcium on the adsorption of cholesterol and film stability

The effect of exogenous cholesterol on the stability of surface films at 37 degrees C from various surfactants was studied with the pulsating bubble surfactometer. Addition of cholesterol (5%, w/w) to bovine lipid extract surfactant (bLES) or mixtures of dipalmitoylphosphatidylcholine/1-palmitoyl-2-oleoyl-phosphatidylglycerol /SP-B (7:3:1%) dispersed in 1.5 mM CaCl2/0.9% NaCl resulted in unstable surface films. Although 10% cholesterol only partially impaired the surface activity of bLES, it virtually abolished that of the reconstituted surfactant. The inhibitory effects of cholesterol were significantly repressed by SP-A (10%, w/w of lipid) and 3 mM CaCl2 or 5 mM CaCl2 without SP-A. Adsorption of cholesterol from various surfactants into the air/water interface was examined by measuring the surface radioactivity of [14C]cholesterol. Cholesterol alone dispersed in 1.5 mM CaCl2/0.9% NaCl could not adsorb to the interface, but it adsorbed readily when mixed with bLES. Cholesterol adsorption was markedly suppressed by SP-A in 3 mM CaCl2/0.9% NaCl or 5 mM CaCl2/0.9% NaCl without SP-A. Electron microscopy revealed striking ultrastructural differences between bLES/5% cholesterol/10% SP-A in 3 mM CaCl2/0.9% NaCl and bLES/5% cholesterol in 3 or 5 mM CaCl2/0.9% NaCl. The former exhibited large multilayer and small unilamellar vesicles, while the latter displayed condensed patches of aggregates. Adsorption studies showed aggregated patches adsorbed more rapidly than vesicles but attained lower equilibrium surface pressures. These results indicate SP-A and calcium limit the adsorption of surfactant cholesterol to the air-water interface.

Acyl structure regulates galactosylceramide's interfacial interactions

Galactosylceramides (GalCer) with homogeneous acyl chains containing zero, one, or two cis double bonds have been synthesized and characterized at an argon-aqueous buffer interface using a Langmuir film balance. Both surface pressure and surface potential were measured as a function of molecular area at 24 degrees C. N-Lignoceroylgalactosylsphingosine (N-24:0-GalSph), N-stearoylgalactosylsphingosine (N-18:0-GalSph), and N-palmitoylgalactosylsphingosine (N-16:0-GalSph) form condensed films that are similar to that of bovine brain GalCer, which contains long saturated and mono-unsaturated acyl chains, almost half being hydroxylated. In contrast, a bovine brain GalCer subfraction (NFA-GalCer) that is devoid of the hydroxylated acyl chains displays an apparent two-dimensional phase transition near 9.0 mN/m at 54 A2/molecule. To determine the role of acyl unsaturation in regulating NFA-GalCer's surface behavior, GalCer derivatives containing different mono-unsaturated acyl residues were investigated. N-Nervonoyl-galactosylsphingosine (N-24:1 delta 15-GalSph) and N-docosenoylgalactosylsphingosine (N-22:1 delta 13-GalSph) show liquid-expanded to -condensed phase transitions in their force-area isotherms at 10 and 35 mN/m, respectively. Introduction of acyl chains that are short and saturated [e.g., N-decanoylgalactosylsphingosine (N-10:0-GalSph)] or that are long but contain two cis double bonds [e.g., N-linoleoylgalactosylsphingosine (N-18:2 delta 9,12-GalSph)] causes GalCer to display only liquid-expanded behavior at 24 degrees C. The surface potentials (delta V) of the condensed GalCer derivatives with long saturated acyl residues were quite similar and were over 100 mV higher than that of bovine brain GalCer.(ABSTRACT TRUNCATED AT 250 WORDS)

Adsorption, compression and stability of surface films from natural, lipid extract and reconstituted pulmonary surfactants

A pulsating bubble surfactometer was used to study the surface activities and surface film stabilities of bovine pulmonary surfactants (10 mg/ml) and a reconstituted surfactant (10 mg/ml). Pulmonary surfactants were natural surfactant (NS), lipid extract surfactant [LES(chol)] and lipid extract surfactant without neutral lipids (LES). NS is composed of phospholipids, neutral lipids and surfactant-associated proteins (SP-A, SP-B and SP-C). Both LES(chol) and LES are organic solvent extracts of NS. LES(chol) retains all the components of NS except SP-A. Reconstituted surfactant was dipalmitoylphosphatidylcholine (DPPC): 1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG): SP-B/7:3:1%. All three pulmonary surfactants attained the equilibrium surface tension almost instantaneously at 37 degrees C. The adsorption rates of NS and LES(chol) at 24 degrees C were similar to those at 37 degrees C, while LES exhibited a lower adsorption rate at 24 degrees C. Reconstituted surfactant adsorbed slower than any of the pulmonary surfactants. Film stability was studied by recording the spontaneous increase in the pressure gradient of a static bubble at the minimum size (Rmin) once near zero surface tension was attained. The order of surface film stabilities were: reconstituted surfactant > > NS > LES > LES(chol). Surface films of NS and LES could be stabilized by prolonged pulsation, while film stability of LES(chol) was only moderately affected by pulsation. These results indicate that SP-A in NS promotes formation of some unique structure, possibly tubular myelin, which induces selective adsorption of lipids into the surface.

Phospholipids at the oil/water interface: adsorption and interfacial phenomena in an electric field

Interfacial effects produced in an immiscible liquid system by the action of an external electric field have been considered. The addition of small amounts of neutral phospholipids to the nonaqueous phase has been shown to result in a marked increase in the sensitivity of the interfacial boundary to the voltage applied, which is manifested by: (i) an accelerated decrease of the interfacial tension after the two immiscible liquid phases have been brought into contact; (ii) reduced interfacial tension, by 20-30 mN/m, at the oil/water interface at field strengths of 1-10 kV/m (the interfacial tension drop in the absence of phospholipids does not exceed 5 mN/m); (iii) development of electrohydrodynamic instability at the planar dividing surface between phases; and (iv) dispersion of water into the nonaqueous phase at smaller field strengths by a factor of about 100 as compared to those normally required in the absence of phospholipids. In order to gain a deeper insight into the mechanisms of interfacial phenomena, mainly exemplified by the n-heptane/water system containing phosphatidylcholine, three major issues have been considered: (1) Kinetics of the adsorption of phospholipid at the oil/water interface from the nonaqueous phase, and effects produced by exposure to an external electric field; also, the adsorption under equilibrium conditions, and the structure of the adsorption layer formed. (2) Interactions between neutral phospholipid and inorganic or organic ions at the interfacial boundary under the voltage applied. (3) Conditions for the occurrence of electrohydrodynamic instability at the dividing surface between oil and water and the formation of a water-in-oil emulsion; also aggregation and gelation processes induced in the nonaqueous phospholipid solution bulk by the action of a weak external electric field. Throughout the present paper, an attempt has been made to relate the microscopic behaviour of phospholipids under an external electric field to macroscopically observable properties at the movable interfacial boundaries. The adsorption studies have shown that phosphatidylcholine is prone to self-organization into a liquid-crystalline state at an immiscible liquid interface. The disintegration of the interfacial lipid film thus formed by the action of a weak electric field has been explained as due to an enhanced electrohydrodynamic instability of liquid crystals. This results in the formation of either an emulsion, or a microemulsion in the nonaqueous solution bulk. The formation of a microemulsion is manifested by the appearance of an optically anisotropic gel, stable only under an external applied electric field, in the nonaqueous solution bulk.(ABSTRACT TRUNCATED AT 400 WORDS)

Phospholipid monolayers at the triolein-saline interface: production of microemulsion particles and conversion of monolayers to bilayers

Interfacial tensions of phospholipid monolayer at the triolein (TO)-saline interface were measured. The adsorption isotherms and the interfacial pressure-molecular area curves were evaluated on the basis of the measurements. Phosphatidylcholine (PC) forms a highly condensed monolayer, with a large lateral attractive interaction; phosphatidylethanolamine (PE) and phosphatidylserine (PS) form expanded monolayers with smaller lateral interaction energies. At the lowest interfacial tension (the highest interfacial pressure), the mole fractions of PC, PE, and PS in the monolayers are estimated as 0.95, 0.73, and 0.88, respectively. Therefore, PC forms the most stable monolayer at the interface. These results are consistent with the finding that the stable TO particles in aqueous solution were produced by using PC as an emulsifier, and PE and PS did not stabilize the particles. The phase diagram of TO and PC mixtures in saline obtained from theoretical considerations predicts the equilibrium conversion of the monolayers on TO particles to bilayers. This process may be closely related to the transformations of very low density lipoproteins and chylomicrons to high-density lipoproteins in plasma. The particle sizes of the emulsion are calculated theoretically as a function of PC mole fraction in the TO-PC mixture and compared with the experimental values obtained from quasi-elastic light scattering (QLS) measurements.

The monensin-mediated transport of Na+ and K+ through phospholipid bilayers studied by 23Na- and 39K-NMR

Addition of monesin to preparations of large unilamellar vesicles made from egg yolk phosphatidylcholine (EPC) in sodium or potassium chloride solution and from dioleoylphosphatidylcholine (DOPC) in sodium chloride solutions gives rise to dynamic 23Na- and 39K-NMR spectra. The dynamic spectra arise from the monensin-mediated transport of the metal ions through the membrane. The kinetics of the transport are followed as a function of monensin and metal ion concentrations and are compatible with a model in which one monensin molecule transports one metal ion. Rate constants for the association and dissociation of the monensin-metal complex in the membrane/water interface are extracted and the stability constants for complex formation are evaluated. The rate constants in DOPC are similar to those in EPC, confirming that diffusion is not rate-limiting in the transport process and that dissociation of the complex is the rate-limiting step. Although potassium on its own is transported more rapidly, sodium forms the more stable complex and is therefore transported preferentially in competition with potassium.

Effect of sterol incorporation on head group separation in liposomes

Electrophoretic mobilities of multilamellar liposomes of varying composition have been measured to determine the effect of incorporated sterols on surface charge density. Liposomes made from mixtures of zwitterionic egg phosphatidylcholine (PC) and anionic egg phosphatidylglycerol (PG) in varying proportions were shown to have electrophoretic mobilities consistent with the anticipated surface charge density. Incorporation of cholesterol up to 50 mole per cent in the bilayer produced no detectable change in surface charge density. Similar results were obtained for lanosterol and epicoprostanol. These results are interpreted to mean that incorporation of the sterols into the bilayers produced no detectable change (less than 3%) in the spacing of charged phospholipids. It is inferred that sterols are incorporated among the fatty acyl chains of these phospholipid bilayers with little or no displacement of the head groups at the surface.

Effect of lipid composition changes on carbocyanine dye fluorescent response

Egg yolk phosphatidyl choline liposomes containing variable amounts of phosphatidyl ethanolamine, phosphatidyl inositol or phosphatidyl serine demonstrated important variations in the fluorescence of 3.3' dipropylthiodicarbocyanine. When the membrane contained no cholesterol, fluorescence was not correlated with membrane fluidity as measured by diphenyl hexatriene polarization. Increasing cholesterol concentration in valinomycin containing liposome membranes decreased the potassium induced apparent membrane potential and prevented sorption of dye to the membrane. Discontinuity in the apparent potential occurred at 30 mol% cholesterol but could not be correlated with changes in microviscosity. These results indicate that great care should be taken when correlating rapid variations of fluorescence to changes in membrane potential. We propose that changes in phospholipid metabolism could well explain fluorescent changes when monitoring the fluorescence of cyanine dye molecules sorbed to biological membranes.

Stellate ganglion stimulation with alpha and beta adrenergic blockade: effect on lung lipids and compliance in cats

Stellate ganglion stimulation (SGS) can alter lung lipids and reduce static lung compliance, although the mechanisms remain unclear. Phentolamine and propranolol were administered to anesthetized cats prior to stimulation in order to investigate SGS effects on lung lipids and compliance mediated via alpha and beta adrenergic pathways. Analysis of lung lavage revealed that SGS alone decreased cholesterol and the cholesterol/DSPC ratio which might be expected to decrease lung compliance. Alpha and beta blockade alone resulted in no changes from control in cholesterol or DSPC. Alpha blockade plus SGS yielded increased rather than decreased cholesterol and DSPC, while beta blockade prevented any change. A reduction in both static and dynamic lung compliance caused by SGS also was blocked by both alpha and beta blockade. Thus both the alpha and beta blockade prevented the SGS-induced decreases in cholesterol, cholesterol/DSPC ratio, and lung compliance. Furthermore, alpha blockade plus SGS resulted in increased TPL as well as cholesterol and DSPC. The data are consistent with the view that DSPC and cholesterol are released into the subphase by beta adrenergic mechanisms, and that their relative amounts may influence surface properties.

On the mechanism of vesicle release from ATP-depleted human red blood cells

The release of spectrin-free vesicles from ATP-depleted human red blood cells (Lutz et al. (1977) J. Cell. Biol. 73, 548) can be considered the final step of a shape change from discocytes to echinocytes. The study of physical and chemical properties of released vesicles suggests that vesicle release is not merely a consequence of charge alterations within either monolayer of the budding membrane. Fresh membranes and released vesicles have within experimental error the same sialic acid content per surface area and the same electrophoretic mobilities. Vesicle release cannot be stimulated by doubling the charge density on the outer monolayer by means of a phospholipase D-treatment, but correlates with a breakdown of polyphosphoinositides to diacylglycerol on the inner monolayer. This breakdown does not lead to a significant change in the negative charge density on the inner monolayer, because an increased phosphatidate content compensates for this alteration. Furthermore, polyphosphoinositide breakdown and diacylglycerol production are not the rate-limiting step in vesicle release from ATP-depleting red blood cells. This is evident from the fact that 10 mM EDTA inhibits vesicle release to 75% without affecting polyphosphoinositide breakdown and diacylglycerol production. Hence, diacylglycerol formation may be sufficient for membrane budding as suggested earlier (Allan et al. (1976) Nature 261, 58), but vesicle release requires a second, as yet unidentified process.

Stellate ganglion influence on static compliance and surface layer lipids in cat lungs

In order to determine the effects of stellate ganglion stimulation (SGS) on static lung compliance, pressure-volume relationships were measured on lungs in situ in cats anesthetized with ketamine. The lungs were inflated with 100% oxygen to 100, 93.75, 87.5, 81.25, 75, 50, and 25% TLC as determined at 20 cm H2O transpulmonary pressure. After a maximal filling, lungs were inflated twice to each volume, alternating unstimulated and SGS conditions. Static lung compliance was decreased by SGS at 93.75 (5.7 +/- 0.4 (SE) vs. 5.4 +/- 0.4 ml/cm H2O; P less than 0.05) and 87.5% TLC (6)6 +/- 0.6 vs 6.0 +/- 0.5 ml/cm H2O; P less than 0.05). In order to assess possible effects of SGS on surfactant that might be related to the loss of compliance, lung lipids and proteins obtained during SGS were quantitated. Total phospholipid (TPL) increased from 3.22 +/- 0.33 to 4.16 +/- 0.23 mg/g lung after 1 min 7V SGS (P less than 0.05) and to 4.58 +/- 0.61 mg/g lung after 4 min 20V (P less than 0.05). The ratio of cholesterol (chol) to disaturated phosphatidylcholine (DSPC) decreased after 4 and 6 min, and may have resulted in increased surface tension forces at high lung volumes. The data from this study lend support to previous findings which showed that the compliance decrease during SGS may be related to increased surface tension forces.

Viability control and special properties of isolated rat hepatocytes

The need for quick viability tests is stressed. Aas these should achieve more than statically categorizing dead or non-dead cells, several procedures are suggested that picture the energetic state of the cells. The almost classical criterion of this category, namely stimulation of respiration by succinate, must be questioned on the basis of the present results. It is shown, that restricted respiration by succinate is not due to limited permeability of the plasma membrane, but to competition by endogenous substrates for uptake into mitochondria. Distribution equilibria for succinate appear to be according to (delta pH)2 with regard to cytoplasm. They are attained within 5-20 s or faster. Uptake is in part regulated by the surface charge density. Permeability changes caused by effectors of surface charge, such as amphiphilic ions, are examplified for succinate, chloride, phosphate, Na+, K+, and Ca2+. Such changes repeatedly also occur after pulses of BSP. They are counterregulated by the cell within a minute in a manner dependent on BSP concentration and the state of the cells. During the preincubation phase, that is the time of readaptation after transfer of cells from 0 degree C to higher temperature, a special labile state transiently occurs, where cyclic permeability changes for Ca2+, Na+, K+ can be caused by substrate addition, especially succinate, and/or ATP. The extent of these changes and their sequence again depend on the energetic state of the cells. In a probably narrow energetic window a sequence of cation movements reminding of that after depolarization of an excitable cell, is observed. Manipulation of the Na+/K+-ratio by variation of preincubation time and by ouabain shows that this is not simply the denominator for reversible calcium uptake. As the surface charge appears to reflect the energetic state, ANS fluorescence is applied to monitor the state of the plasma membrane, though difficulties arising from a slow ANS permeation are not yet solved.

Effect of substrate properties on the activity of lysosomal cholesteryl ester hydrolase

The effects of the substrate properties on the catalytic activity of lysosomal cholesteryl ester hydrolase from rat liver have been examined with three standard substrate types: vesicle, micelle and emulsion. The pH optimum of the enzyme coincided to 4.5--5.0 with the substrate types employed. The apparent Km values were 15.3, 14.3 and 7.3 microM for vesicle, micelle and emulsion substrates, respectively. In the systems used in this study reaction products, cholesterol and oleic acid, and the nonionic surfactant Tween 80 and Triton X-100 Had an inhibitory effect. The emulsifier phosphatidylcholine and the charged phospholipid phosphatidic acid stimulated the activity. The mixed micelle of sodium taurocholate and phosphatidylcholine was the most potent substrate vehicle. With dipalmitoyl phosphatidylcholine vesicles the enzyme showed maximal activity at the gel-liquid-crystalline transition temperature of the phospholipid. The possible physiological significance of the lysosomal cholesteryl ester hydrolase is discussed with special reference to the form of the substrate.

Laser Raman studies of molecular interactions with phosphatidylcholine multilayers. II. Effects of mono- and divalent ions on bilayer structure

Laser Raman spectroscopy was applied to characterize structural behavior of dipalmitoyl phosphatidylcholine multibilayer systems in the presence of several cations (K+, Na+, Cs+, Rb+, Ca2+, Mg2+, Cd2+, Ba2+) and anions Cl-, Br-, I-, NO-3, SO2-3, SO2-4, S2O2-3, S2O2-8). To evaluate the Raman-spectroscopical data quantitatively, characteristic intensity ratios, lateral and trans order parameters were used and compared. It was shown that the different trans order parameters are rather sensitive to ion-polar head group interactions and thus, they cannot give unequivocal information on the trans-gauche isomerization of hydrocarbon chains of phospholipids. The observed effects of ions on Raman spectra of phospholipid multilayers could not be explained simply on the basis of electrostatic interactions. The possible involvement of other factors (changes in polarizability, hydrogen bonds, etc.) is also discussed. It was demonstrated that the order parameters defined in different ways may result in different effectiveness sequences of ions. Of monopositive ions Na+ was found to be the most effective to influence the bilayer structure. For dipositive ions, of which Ca2+ proved to be the most effective, concentration-dependent effectiveness sequences were obtained. A plausible interpretation and some consequences of the concentration-dependent two-step binding of divalent cations were also outlined. Bilayered phospholipid structures turned out to be more responsive to anions than to most cations investigated. Interdependent actions of cations and anions, as well as the possible relevance of the charge distribution on anions are postulated.