Microsomal membrane fluidity and phosphatidylcholine synthesis in rabbit lung under high oxygen tension

Phosphatidylcholine metabolism and membrane fluidity were studied in microsomes isolated from rabbit lung, which had been exposed to high oxygen tension for 30 min. In these microsomes the incorporation of [3H]-palmitate into phosphatidylcholine increased whereas the incorporation of [14C]-glycerol and [14C]-choline from CDP-[methyl-14C]-choline remained unchanged in comparison to the control microsomes. The enhanced [3H]-palmitate incorporation may be explained by an increase of the specific activity of acyl-CoA:lysophosphatidylcholine acyltransferase which was measured in microsomes from hyperoxic lung. Although microsomal parameters influencing membrane fluidity, such as the cholesterol/phospholipid molar ratio, unsaturation degree of phospholipid acyl chains and lipid/protein ratio, are altered after oxygen treatment in vivo, no change of fluorescence polarization (PDPH) and lipid structural order parameter (SDPH) could be measured. Probably, the membrane maintains its fluidity by counteracting effects on different factors on which the fluidity depends.     

Effect of lysophosphatidylcholine on behavior and structure of phosphatidylcholine liposomes

Differential scanning calorimetry (DSC), fluorescence polarization and X-ray diffraction were per-formed to investigate the kinetics of the micellar to the lamellar phase transition of dipalmitoylphosphatidylcholine/1-palmitoylphosphatidylcholine (16:0 LPC/DPPC) liposomes at gel phase. With a 16:0 LPC concentration up to 27 mol% only the sharp main transition with relatively high enthalpy (△H) values of DPPC was observed. Increasing 16 : 0 LPC concentration, the phase transition was broadened and the transition enthalpy was decreased and finally totally disappeared. The fluorescence probes of 3AS, 9AS, 12AS, and 16AP were employed, respectively, to detect the mo-bility of various sites of carbon chains of DPPC or 16:0 LPC/DPPC liposomes. It was shown that DPPC liposomes formed in the absence of 16:0 LPC always had a fluidity gradient in both gel and liquid-crystalline phase, while in the presence of 14.1 mol% and 27.0 mol% 16:0 LPC in the mixtures, the fluidity gradient tended to disappear below 40℃:     

Membrane fluidity of Toxoplasma gondii: a fluorescence polarization study

Toxoplasma gondii membrane fluidity was investigated by fluorescence polarization. We used 1,6-diphenyl 1,3,5-hexatriene (DPH) as a fluorescent hydrophobic probe. Fluorescence anisotropy (r) and degree of order (s) showed high fluidity properties. Chemical analysis was performed on this parasite. We found a low cholesterol/phospholipid ratio, many unsaturated fatty acids chains, and high phosphatidylcholine and low sphingomyelin amounts. These results were in good agreement with the observed high fluidity. This may be related to the great adaptability of Toxoplasma gondii in infesting a wide variety of host cells.     

Effects of alcohols on ADP-induced aggregation and membrane fluidity of gel-filtered bovine blood platelets

Summary The effects of four alcohols—n-propyl,n-butyl,n-amyl andn-hexyl alcohol—on the ADP-induced aggregation of gel-filtered bovine platelets were examined. All four alcohols inhibited the aggregation, the order of their effects beingn-propyln-amyl<n-hexyl. Comparison of the inhibitory effects of the alcohols with their physico-chemical properties showed that their degrees of inhibition depended on their hydrophobicities. Moreover, it was suggested that their interaction with the lipid layer of the membrane was important for the inhibition. Studies on the effects of alcohols on the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene-labeled platelets showed that the membrane fluidity of the platelets increased in the same concentration range in which aggregation inhibition was observed. Since the alcohols inhibited aggregation without affecting Ca²⁺ mobilization in the platelets, as revealed in this study, it was concluded that inhibition of platelet aggregation was due to perturbation of membrane lipids by the alcohols. This hypothesis is supported by several recent studies on the effects of cholesterol and cations, which suggest that a relatively rigid membrane favors platelet aggregation.     

Lipid fluidity and composition of the erythrocyte membrane from healthy dogs and labrador retrievers with hereditary muscular dystrophy

Erythrocyte membranes and their liposomes were prepared from clinically normal dogs and Labrador retrievers with hereditary muscular dystrophy. The static and dynamic components of fluidity of each membrane were then assessed by steady-state fluorescence polarization techniques using limiting hindered fluorescence anisotropy and order parameter values of 1,6-diphenyl-1,3,5-hexatriene (DPH) and fluorescence anisotropy values ofdl-2-(9-anthroyl)-stearic acid anddl-12-(9-anthroyl)-stearic acid, respectively. Membrane lipids were extracted and analyzed by thin-layer chromatography and gas chromatography. The results of these studies demonstrated that the lipid fluidity of erythrocyte membranes, and their liposomes, prepared from dystrophic dogs were found to possess significantly lower static and dynamic components of fluidity than control counterparts. Analysis of the composition of membranes from dystrophic dogs revealed a higher ratio of saturated fatty acyl chain/unsaturated chains (w/w) and lower double-bond index. Alterations in the fatty acid composition such as decrease in levels of linoleic (18:2) and arachidonic (20:4) acids and increase in palmitic (16:0) and stearic (18:0) acids were also observed in the membranes of dystrophic animals. These associated fatty acyl alterations could explain, at least in part, the differences in membrane fluidity between dystrophic and control dogs.     

Implications of sterol structure for membrane lipid composition, fluidity and phospholipid asymmetry in Saccharomyces cerevisiae

Sterols are essential components of the plasma membrane in eukaryotic cells. Nystatin-resistant erg mutants were used in the present study to investigate the in vitro effects of altered sterol structure on membrane lipid composition, fluidity, and asymmetry of phospholipids. Quantitative analyses of the wild type and mutants erg2, erg3 and erg6 revealed that mutants have lower sterol (free)-to-phospholipid molar ratios than the wild type. Phosphatidylcholine content was decreased in erg2 and erg3 mutants; however, it was increased in erg6 strains as compared to normals. Phosphatidylserine content was increased in the erg6 mutant only. Fluorescence anisotropy decreased with temperature in both probes, and was lower for mutants than for the wild type, suggesting an increased freedom in rotational movement due to decreased membrane order. Investigation of changes in the aminophospholipid transbilayer distribution using two chemical probes, trinitrobenzene sulfonic acid and fluorescamine, revealed that the amounts of phosphatidylethanolamine derivatized by these probes were quite similar in both the wild type and various erg strains. The present findings suggest that adaptive responses in yeast cells with altered sterol structure are possibly manifested through changes in membrane lipid composition and fluidity, and not through transbilayer rearrangement of aminophospholipids.     

Influence of increased membrane cholesterol on membrane fluidity and cell function in human red blood cells

Cholesterol and phospholipid are the two major lipids of the red cell membrane. Cholesterol is insoluble in water but is solubilized by phospholipids both in membranes and in plasma lipoproteins. Morever, cholesterol exchanges between membranes and lipoproteins. An equilibrium partition is established based on the amount of cholesterol relative to phospholipid (C/PL) in these two compartments. Increases in the C/PL of red cell membranes have been studied under three conditions: First, spontaneous increases in vivo have been observed in the spur red cells of patients with severe liver disease; second, similar red cell changes in vivo have been induced by the administration of cholesterol-enriched diets to rodents and dogs; third, increases in membrane cholesterol have been induced in vitro by enriching the C/PL of the lipoprotein environment with cholesterol-phospholipid dispersions (liposomes) having a C/PL of >1.0. In each case, there is a close relationship between the C/PL of the plasma environment and the C/PL of the red cell membrane. In vivo, the C/PL mole ratio of red cell membranes ranges from a normal value of 0.9–1.0 to values which approach but do not reach 2.0. In vitro, this ratio approaches 3.0. Cholesterol enrichment of red cell membranes directly influences membrane lipid fluidity, as assessed by the rotational diffusion of hydrophobic fluorescent probes such as diphenyl hexatriene (DPH). A close correlation exists between increases in red cell membrane C/PL and decreases in membrane fluidity over the range of membrane C/PL from 1.0 to 2.0; however, little further change in fluidity occurs when membrane C/PL is increased to 2.0–3.0. Cholesterol enrichment of red cell membranes is associated with the transformation of cell contour to one which is redundant and folded, and this is associated with a decrease in red cell filterability in vitro. Circulation in vivo in the presence of the slpeen further modifies cell shape to a spiny, irregular (spur) form, and the survival of cholesterol-rich red cells is decreased in the presence of the spleen. Although active Na-K transport is not influenced by cholesterol enrichment of human red cells, several carrier-mediated transport pathways are inhibited. We have demonstrated this effect for the cotransport of Na + K and similar results have been obtained by others in studies of organic acid transport and the transport of small neutral molecules such as erythritol and glycerol. Thus, red cell membrane C/PL is sensitive to the C/PL of the plasma environment. Increasing membrane C/PL causes a decrease in membrane fluidity, and these changes are associated with a reduction in membrane permeability, a distortion of cell contour and filterability and a shortening of the survival of redcells in vivo.     

Nuclear magnetic resonance studies on liposomes: Effects of steroids on lecithin fatty acyl chain mobility

Summary The effects of fourteen sterols on the NMR spectra of liposomes derived from egg yolk phosphatidylcholines were studied by continuous-wave and Fourier-transform measurements at 60 MHz. Sterols were compared for their ability to broaden the acyl methylene resonances of phosphatidylcholine, when incorporated into liposomes at 25% molar ratio. The ratio of the phosphatidylcholine peak heights (acyl methylene: cholinen-methyl) was used as a criterion of the relative condensing activity for the different sterols. This ratio was inversely proportional to the molar volume of the incorporated sterol, as measured by the parachor of the compound. Small sterols had little condensing effect, and the larger sterols such as cholesterol and ergosterol had maximum condensing effects. The study confirmed the importance of the sterol side-chain at C-17 as a requirement for sterol-phospholipid interaction.     

Membrane fluidity changes in goat sperm induced by cholesterol depletion using beta-cyclodextrin

Cholesterol efflux from membranes promotes acrosome reaction in goat spermatozoa. In 1 h of incubation of sperm in the presence of beta-cyclodextrin (βCD), all the interchangeable cholesterol is desorbed from sperm membranes, although acrosome reaction is fully accomplished only after 3-4 h of incubation, as previously published. In the present paper we investigate the effect of cholesterol removal from mature goat spermatozoa on the overall membrane “fluidity” of live cell membranes and of liposomes from sperm lipid extracts. Using steady state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), we studied the average thermotropic behaviour of membrane lipids, after incubation of live sperm for 1 h in BSA-free medium with the presence/absence of 8 mM β-cyclodextrin, as a cholesterol acceptor. Unimodal and bimodal theoretical sigmoids fitted best to the experimental thermotropic profiles of liposomes and whole cells, respectively. In the case of whole sperm, two phase transitions, attributable to different lipid domains, were clearly separated by using the fitting parameters. After cholesterol removal, important changes in the relative anisotropy range of the two transitions were found, indicating an increase in the “fluidity” of some of the lipid microdomains of sperm membranes. These changes in sperm lipid dynamics are produced before the onset of sperm acrosome reaction.     

Electron paramagnetic resonance studies on the fluidity and surface dynamics of egg phosphatidylcholine vesicles containing gangliosides

The influence of different gangliosides (G_M1, G_D1a, G_T1b) on the fluidity and surface dynamics of phosphatidylcholine small unilamellar vesicles was studied by electron paramagnetic resonance. 5-and 16-nitroxystearic acid, sounding respectively the region close to the surface and that close to the hydrophobic core of the vesicle, were employed as spin-label probes. The signals released by 5-nitroxystearic acid showed that the presence of gangliosides reduced the mobility of the hydrocarbon chains around the probe. The effect increased by increasing ganglioside concentration, and diminished from G_M1 to G_D1a and G_T1b. The decrease of membrane fluidity was also monitored by the 16-nitroxystearic acid probe. On addition of Ca²⁺ the fluidity of ganglioside-containing vesicles (as signalled by the 5-nitroxystearic acid probe) promptly decreased, thereafter returning slowly to the original value. It is suggested that gangliosides cause strong side-side head group interactions on the bilayer surface -between ganglioside oligosaccharide chains and between ganglioside and phosphatidylcholine polar portions - which lead the lipid chains to assembly in a more rigid fashion. The influence of Ca²⁺ is interpreted as due to lateral phase separation in the vesicle membrane. This phenomenon can be related to the formation or stabilization of ganglioside clusters on the vesicle surface.     

Chondroitin 6-sulfate and dextran sulfate promote hypochlorite-induced peroxidation of phosphatidylcholine liposomes

In this work, we studied whether chondroitin sulfates and dextran sulfates (DXSs) can influence hypochlorite-induced peroxidation of phosphatidylcholine (PC) liposomes. Multilamellar liposomes (2 mg lipid/ml) were prepared in phosphate buffer, pH 7.4, with NaCl or not and exposed to reagent HOCl/ClO⁻ (1 mM) at 37 °C in the presence of different concentrations of chondroitin 6-sulfate (C6S), chondroitin 4-sulfate (C4S), DXS 8000, DXS 40,000, and DXS 500,000. Lipid peroxidation was assessed by thiobarbituric acid-reactive substance (TBARS) production. DXSs and C6S enhanced TBARS production in a dose-dependent manner. The decline in TBARS production at the relatively high C6S concentrations may be attributed to C4S present in C6S, since in contrast to C6S, C4S is known to react with hypochlorite. Dextrans, nonsulfated analogues of DXS, failed to modulate TBARS production. This fact indicates the important role of negatively charged sulfate groups for DXS to facilitate hypochlorite-induced peroxidation of PC liposomes. The electrostatic nature of the mechanism providing for the pro-oxidative effect of DXS was also supported by the influence of liposome surface charge and solution ionic strength on the extent of liposome peroxidation. The addition of calcium ions to the incubation mixture did not prevent the pro-oxidative action of DXS. The relevance of the results to atherogenesis is discussed.     

Increase in lipid microviscosity of unilamellar vesicles upon the creation of transmembrane potential

Summary Diffusion potential of potassium ions was formed in unilamellar vesicles of phosphatidyl choline. The vesicles, which included potassium sulfate buffered with potassium phosphate, were diluted into an analogous salt solution made of sodium sulfate and sodium phosphate. The diffusion potential was created by the addition of the potassium-ionophore, valinomycin. The change in lipid microviscosity, ensuing the formation of membrane potential, was measured by the conventional method of fluorescence depolarization with 1,6-diphenyl-1,3,5-hexatriene as a probe. Lipid microviscosity was found to increase with membrane potential in a nonlinear manner, irrespective of the potential direction. Two tentative interpretations are proposed for this observation. The first assumes that the membrane potential imposes an energy barrier on the lipid flow which can be treated in terms of Boltzmann-distribution. The other interpretation assumes a decrease in lipid-free volume due to the pressure induced by the electrical potential. Since increase in lipid viscosity can reduce lateral and rotational motions, as well as increase exposure of functional membrane proteins, physiological effects induced by transmembrane potential could be associated with such dynamic changes.     

Antioxidant effects of water- and lipid-soluble nitroxide radicals in liposomes

Liposomes are today useful tools in different fields of science and technology. A lack of stability due to lipid peroxidation is the main problem in the extension of the use of these formulations. Recent investigative works have reported the protective effects of stable nitroxide radicals against oxidative processes in different media and under different stress conditions. Our group has focused its attention on the natural aging of liposomes and the protection provided by the water- and lipid-soluble nitroxide radicals 2,2,6,6-tetramethylpiperdine-1-oxyl (TEMPO) and doxylstearic acids (5-DSA, 12-DSA, and 16-DSA), respectively. Unilamellar liposomes were incubated under air atmosphere at 37°C, both in the absence and in the presence of these radicals. Conjugated dienes, lipid hydroperoxides, TBARS, membrane fluidity, and nitroxide ESR signal intensity were followed as a function of time. Our results demonstrated that doxylstearic acids were more efficient than TEMPO in retarding lipid peroxidation at all the concentrations tested. The inhibition percentages, depending on the total nitroxide concentration, were not proportional to the lipid–water partition coefficient. Furthermore, time-course ESR signals showed a slower decrease for doxylstearic acids than for TEMPO. No significant differences were found among 5-DSA, 12-DSA, and 16-DSA. We concluded that the nitroxide radical efficiency as antioxidant directly depends on both nitroxide concentration and lipophilicity.     

Mg-nucleotides induced dissociation of liposome-bound creatine kinase: reversible changes in its secondary structure and in the fluidity of the bilayer

MgADP binding to mitochondrial creatine kinase (mtCK) adsorbed on liposomes was induced by the photorelease of caged ADP. The nucleotide binding produced two types of structural changes. One was related to the well-established release of mtCK from the liposomes. The other corresponded to reversible structural changes induced by nucleotide binding to mtCK as demonstrated here. Infrared spectroscopy data show that the MgADP-induced desorption of mtCK from vesicles led to a slight increase in &#102 -helix structures in mtCK at the expense of a small decrease in &#103 -sheet structures and a concomitant increase in the fluidity of the membranes. The desorption of mtCK induced by MgADP and MgATP was almost complete, as shown by centrifugation and enzymatic activity measurements. The photorelease of MgADP in a reactive medium containing phosphocreatine and mtCK associated with liposomes led to nucleotide binding and to the formation of MgATP and creatine. Addition of phosphocreatine also desorbed mtCK from liposomes, while addition of creatine did not. Interpretation of these results would suggest that ADP, ATP or phosphocreatine induce the release of mtCK from membranes, increase the phospholipid bilayer fluidity, and may also decrease the number of contact sites between inner and outer mitochondrial membranes, thus affecting the activity of other mitochondrial enzymes. It is tempting to propose that membrane mtCK binding regulation by nucleotide and PCr concentrations may serve as a physiological adaptation for energy supply.     

Changes in fluidity of erythrocyte membranes after storage of erythrocytes and regeneration of cellular ATP level

The membrane fluidity of freshly collected human erythrocytes, of erythrocytes stored for 3–4 weeks and of stored erythrocytes rejuvenated with glucose and inosine was investigated by measuring polarization of fluorescence emission of 1,6-diphenyl-1,3,5-hexatriene and $\text{N-}\text{phenyl}\text{-1-}\text{naphthylamine}$. The fluidity of membranes prepared from stored erythrocytes was higher than that of fresh erythrocytes. After rejuvenation of erythrocytes with glucose and with or without inosine the membrane fluidity decreased. These changes were probably due to variations of ATP levels in the erythrocytes.     

Delivery of fluorescent-labeled cyclodextrin by liposomes: role of transferrin modification and phosphatidylcholine composition

Drug-in-CD-in-liposome (DCL) systems which encapsulate the drug/CD inclusion complexes into inner aqueous phase of liposomes have been applied as a novel strategy to improve efficacy of lipophilic antitumor drugs. The aim of this work was to assess the role of transferrin (Tf) modification and phosphatidylcholine (PC) composition on the properties of liposomes containing hydroxypropyl-β-cyclodextrin (HP-β-CD). Fluorescence dye, FITC, was conjugated with HP-β-CD to facilitate the analysis. The resulting FITC-HP-β-CD was further encapsulated into liposomes and then the liposomes were modified with Tf. The FITC-HP-β-CD-loaded liposomes with different PC compositions were compared in terms of particle size, zeta potential, FITC content, FITC-HP-β-CD leakage, phase transition temperature (T_m) and cellular uptake. The apparent partition coefficient values of different PCs were also determined. Compared to PEGylated liposomes, FITC-HP-β-CD-loaded liposomes modified with Tf had been proved to significantly increase vesicle stability and specific cellular uptake. Moreover, PC composition affected the properties of liposomes. Soybean phosphatidylcholine (SPC) liposomes modified with Tf were found to be more easily internalized into tumor cells than 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and hydrogenated soybean phosphatidylcholine (HSPC) while Tf density on the liposomal surface was similar. And the lipophilicity of SPC was found to be much higher than DPPC and HSPC. Collectively, by the optimization of PC composition, the development of DCL modified with Tf might represent a potential strategy for the antitumor application of lipophilic drugs.     

Asymmetry of membrane fluidity in the lipid bilayer of blood platelets: Fluorescence study with diphenylhexatriene and analogs

Summary Membrane fluidity of bovine platelets was examined with diphenylhexatriene (DPH), its cationic trimethylammonium derivative (TMA-DPH) and anionic propionic acid derivative (DPH-PA). After addition of these probes to platelet suspensions at 37°C, the fluorescence intensity of DPH-PA reached equilibrium within 2 min, whereas those of DPH and TMA-DPH increased gradually. With increase in the fluorescence intensity of TMA-DPH, its fluorescence anisotropy decreased significantly, but the fluorescence anisotropies of DPH-PA and DPH did not change during incubation. The gradual increase of fluorescence intensity of TMA-DPH was due to its penetration into the cytoplasmic side of the platelet membrane, as shown quantitatively by monitoring decrease in its extractability with albumin. Transbilayer movement of TMA-DPH was markedly temperature-dependent, and was scarcely observed at 15°C. The fluorescence intensity of TMA-DPH was much higher in platelet membranes and vesicles of extracted membrane lipids than the initial intensity in intact platelets. Moreover, the fluorescence anisotropy of TMA-DPH was much lower in the former preparations than the initial value in intact platelets. These results suggest that binding sites for TMA-DPH in the cytoplasmic side of the platelet membrane are more fluid than those in the outer leaflet of the plasma membrane. Platelet activation by ionomycin induced specific change in the fluorescence properties of TMA-DPH without causing transbilayer incorporation of the probe.     

Vesicular and nonvesicular transport of phosphatidylcholine in polarized HepG2 cells

We have investigated the transport and canalicular enrichment of fluorescent phosphatidylcholine (PC) in HepG2 cells using the fluorescent analogs of PC C6-NBD-PC and β-BODIPY-PC. Fluorescent PC was efficiently transported to the biliary canaliculus (BC) and became enriched on the lumenal side of the canalicular membrane as shown for C6-NBD-PC. Some fluorescent PC was transported in vesicles to a subapical compartment (SAC) or apical recycling compartment (ARC) in polarized HepG2 cells as shown by colocalization with fluorescent sphingomyelin (C6-NBD-SM) and fluorescent transferrin, respectively. Extensive trafficking of vesicles containing fluorescent PC between the basolateral domain, the SAC/ARC and the BC as well as endocytosis of PC analogs from the canalicular membrane were found. Evidence for nonvesicular transport included enrichment of the PC-analog β-BODIPY-PC in the BC (t_1/2 = 3.54 min) prior to its accumulation in the SAC/ARC (t_1/2 = 18.5 min) at 37 °C. Transport of fluorescent PC to the canalicular membrane also continued after disruption of the actin or microtubule cytoskeleton and at 2 °C. These results indicate that: (i) a nonvesicular transport pathway significantly contributes to the canalicular enrichment of PC in hepatocytic cells, and (ii) vesicular transport of fluorescent PC occurs from both membrane domains via the SAC/ARC.     

The effect of cholesterol on the structure of phosphatidylcholine bilayers

The effect of cholesterol on the structure of phosphatidylcholine bilayers was investigated by X-ray diffraction methods. Electron density profiles at 5 Å resolution along with chain tilt and chain packing parameters were obtained and compared for phosphatidylcholine/cholesterol bilayers and for pure phosphatidylcholine bilayers in both the gel and liquid crystalline states. The cholesterol in the bilayer was localized by noting the position of discrete elevations in the electron density profiles. Cholesterol can either increase or decrease the width of the bilayer depending on the physical state and chain length of the lipid before the introduction of cholesterol. For saturated phosphatidylcholines containing 12–16 carbons per chain, cholesterol increases the width of the bilayer as it removes the chain tilt from gel state lipids or increases the trans conformations of the chains for liquid crystalline lipids. However, cholesterol reduces the width of 18 carbon chain bilayers below the phase transition temperature as the long phospholipid chains must deform or kink to accomodate the significantly shorter cholesterol molecule. Although cholesterol has a marked effect on hydrocarbon chain organization, it was found that, within the resolution limits of the data, the phosphatidylcholine head group conformation is unchanged by the addition of cholesterol to the bilayer. The head group is oriented parallel to the plane of the bilayer for phosphatidylcholine in the gel and liquid crystalline states and this orientation is not changed by the addition of cholesterol.     

Differential effect of triiodothyronine and thyroxine on the liposomal membrane in liquid-crystalline and gel state

The effect of thyroid hormones on the degree of order or fluidity of dimyristoyl, dipalmitoyl or egg yolk phosphatidyl choline liposomes was evaluated by fluorescence spectroscopy methods. The freedom of molecular motion above the phase transition temperature was decreased, while below the transition, the mobility was actually increased by the incorporation of triiodothyronine to liposomes. While thyroxine decreases the fluidity in the liquid crystalline state, it cannot increase the fluidity in the gel state.A differential effect of triiodothyronine and thyroxine on the release of the liposomal content was found, depending on the liquid crystalline or gel state of the liposomes. These facts were correlated with the differential incorporation of the hormones to liposomes above and below the phase transition temperature of dimyristoyl and dipalmitoyl phospholipid choline. In gel state, a low incorporation of thyroxine compared with triiodothyronine was found.This work was supported by Grants PID 3-013800/89 from Consejo National de Investigaciones Científicas y Técnicas (CONICET), Fundación Antorchas A-12576/1-000065 and Consejo de Investigaciones de la Universidad National de Tucumán (CIUNT). We thank Dr. G. Rotillo for the space filling models.