Lipid Bilayer Of Erythrocyte Membrane Research Articles

Peculiarities of phospholipiid changes in erythrocyte membranes in parturient women with COVID-19-associated community-acquired pneumonia

Aim. To evaluate the phospholipid composition of erythrocyte membranes in parturient women who had COVID-19-associated community-acquired pneumonia (CAP) in the third trimester.Materials and methods. The material for the study was erythrocytes of peripheral blood of 65 parturient women diagnosed with COVID-19, moderate/severe course, CAP of viral etiology (main group). Patients of the main group, depending on the severity of CAP, were divided into two subgroups: subgroup 1 – moderate course of pneumonia (n=33), subgroup 2 – severe course of pneumonia (n=32). The control group consisted of 35 healthy parturient women. The quantitative composition of phospholipids was studied by two-dimensional thin-layer chromatography according to Kirchner.Results. In subgroup 1, the concentration of phosphatidylethanolamine and phosphatidylcholine in erythrocyte membranes was below the standard values by 38% and 29%, respectively (p<0.001), in subgroup 2, these indicators decreased by 32% and 48%, respectively (p<0.001). At the same time, a significant increase in the concentration of lysophosphatidylcholine was found in patients of subgroup 1 by 92% (p<0.001) and in patients of subgroup 2 by 110% (p<0.001), compared with the group of healthy individuals. In addition, structural changes in the lipid bilayer of erythrocyte membranes under conditions of COVID-19associated CAP were characterized by a pronounced increase in the concentration of minor fractions of phospholipids: phosphatidylserine and phosphatidylinositol in subgroup 1 by 63% and 53%, respectively (p<0.001), in subgroup 2 by 79% and 68%, respectively (p<0.001), compared with similar indicators in the control group.Conclusion. With COVID19-associated CAP in maternity women, structural disorganization of the phospholipid components of erythrocyte membranes is determined, manifested by a decrease in the concentration of phosphatidylethanolamine and phosphatidylcholine with a simultaneous increase in the level of lysophosphatidylcholine, phosphatidylserine and phosphatidylinositol. These disorders increase with increasing severity of pulmonary inflammation. The revealed changes in the lipid spectrum of peripheral blood and the composition of erythrocyte membrane phospholipids in COVID-19-associated CAP indicate the need to develop methods for their correction.

Correction of Cold Damage to Mammalian Erythrocytes by Chlorpromazine to Influence the Dynamic Structure of a Membrane

Abstract—The resistance of mammalian erythrocytes (human, dog, horse, bull, and rabbit) to cold shock in the presence of chlorpromazine was studied; its effect on the state of erythrocyte membranes was investigated. The bond strength between membrane phospholipids of mammalian erythrocytes and the characteristics of the cold shock of these cells was determined with the correlation analysis. It has been established that erythrocytes of mammals whose membranes contain a high content of sphingomyelin and phosphatidylethanolamine and a low content of phosphatidylcholines are more resistant to cold shock. Chlorpromazine showed high efficiency in protecting mammalian erythrocytes against cold shock. The dynamic structure of membranes of human erythrocytes treated with the chlorpromazine was studied by EPR of spin-probes. The use of a set of spin-labeled probes: palmitic acid amide, 5-DOXYL stearic acid, and 16-DOXYL stearic acid, which allow studying the micro-viscosity parameters of the lipid bilayer of erythrocyte membranes in the transmembrane direction, made it possible to establish the locations of chlorpromazine action, namely, in the region of polar heads and hydrophobic tails of membrane phospholipids.

Proteomics-based identification of hypoxia-sensitive membrane-bound proteins in rat erythrocytes

This study examines the action of hypoxia on integrity, fluidity and protein composition of red blood cell (RBC) membrane. Twenty-min exposure to oxygen-free environment decreases rat RBC integrity documented by 3-fold elevation of hemoglobin release without any action on the membrane fluidity estimated by electron magnetic resonance spectroscopy of spin-labeled stearic acid analogues. The proteomics technology in combination with relative label free quantification analysis revealed a dozen of membrane-bound proteins, including elevated content of hemoglobin, reproducibly affected by hypoxia. Mapping the identified proteins in the KEGG pathway database we found that the proteins of multi subunit Cullin-Rbx E3 ubiquitin ligase complex are presented in normoxic RBC ghosts but not in the hypoxic samples. Our results suggest that Cullin-Rbx E3 complex, associated with RBC membrane in normoxia, provides detection and deletion of membrane proteins damaged by reactive oxygen species. In hypoxic conditions, deoxy-Hb binds to band 3 protein, resulting in dissociation of Cullin-Rbx E3 complex from RBC membrane and impaired clearance of damaged cytoskeleton proteins. These rearrangements of membrane proteins might be involved in attenuated membrane integrity revealed in hypoxic RBC. SignificanceThis study demonstrate that sustained deoxygenation of rat erythrocytes alters the composition of membrane-bound proteins including elevation of the content of hemoglobin without any changes in the viscosity of erythrocyte membrane lipid bilayer. These results suggest that the changes the composition of membrane proteins result in attenuated membrane integrity and contribute to augment release of hemoglobin seen in hypoxic conditions.

Experiment study and FEM simulation on erythrocytes under linear stretching of optical micromanipulation

The elasticity of erythrocytes is an important criterion to evaluate the quality of blood. This paper presents a novel research on erythrocytes’ elasticity with the application of optical tweezers and the finite element method (FEM) during blood storage. In this work, the erythrocytes with different in vitro times were linearly stretched by trapping force using optical tweezers and the time dependent elasticity of erythrocytes was investigated. The experimental results indicate that the membrane shear moduli of erythrocytes increased with the increasing in vitro time, namely the elasticity was decreasing. Simultaneously, an erythrocyte shell model with two parameters (membrane thickness h and membrane shear modulus H) was built to simulate the linear stretching states of erythrocytes by the FEM, and the simulations conform to the results obtained in the experiment. The evolution process was found that the erythrocytes membrane thicknesses were decreasing. The analysis assumes that the partial proteins and lipid bilayer of erythrocyte membrane were decomposed during the in vitro preservation of blood, which results in thin thickness, weak bending resistance, and losing elasticity of erythrocyte membrane. This study implies that the FEM can be employed to investigate the inward mechanical property changes of erythrocyte in different environments, which also can be a guideline for studying the erythrocyte mechanical state suffered from different diseases.

An Unrecognized Function of Cholesterol: Regulating the Mechanism Controlling Membrane Phospholipid Asymmetry.

An asymmetric distribution of phospholipids in the membrane bilayer is inseparable from physiological functions, including shape preservation and survival of erythrocytes, and by implication other cells. Aminophospholipids, notably phosphatidylserine (PS), are confined to the inner leaflet of the erythrocyte membrane lipid bilayer by the ATP-dependent flippase enzyme, ATP11C, counteracting the activity of an ATP-independent scramblase. Phospholipid scramblase 1 (PLSCR1), a single-transmembrane protein, was previously reported to possess scrambling activity in erythrocytes. However, its function was cast in doubt by the retention of scramblase activity in erythrocytes of knockout mice lacking this protein. We show that in the human erythrocyte PLSCR1 is the predominant scramblase and by reconstitution into liposomes that its activity resides in the transmembrane domain. At or below physiological intracellular calcium concentrations, total suppression of flippase activity nevertheless leaves the membrane asymmetry undisturbed. When liposomes or erythrocytes are depleted of cholesterol (a reversible process in the case of erythrocytes), PS quickly appears at the outer surface, implying that cholesterol acts in the cell as a powerful scramblase inhibitor. Thus, our results bring to light a previously unsuspected function of cholesterol in regulating phospholipid scrambling.

ATP11C Encodes a Major Flippase in Human Erythrocyte and Its Genetic Defect Causes Congenital Non-Spherocytic Hemolytic Anemia

Flippases are members of the P-IV ATPase family of proteins, and contribute to localization of phosphatidylserine (PS) in inner leaflet by its ATP-dependent active transport from outer to inner leaflet of lipid bilayer of erythrocyte membranes. It is critical that erythrocytes maintain PS in the inner monolayer to ensure their 120-day survival in circulation since externalization of PS will be recognized as an eat-me signal, resulting in phagocytosis by splenic macrophages. In the present study, we identified that ATP11C gene encodes a major flippase in human erythrocytes. A 13 years-old boy was referred to our hospital for consultation for work up of an undiagnosed congenital hemolytic anemia. Since extensive biochemical and molecular analysis failed to identify hemoglobin, erythrocyte membranes and enzyme abnormalities for the pathogenesis of hemolysis, we performed the whole exome analysis by massively parallel sequencing. We identified that the proband is hemizyogous, and the mother is heterozygous for a missense mutation of ATP11C, c.1253C>A, corresponding to a single amino acid substitution, p.Thr418Asn. Flipping activity as measured by PS internalization was decreased to 10% in the red cells of the proband compared to a normal control, clearly demonstrating that ATP11C encodes a major flippase in the human erythrocyte membranes. The PS-positive erythrocytes were not significantly increased in the whole blood but only in the most dense senescent cells, suggesting that PS exposure did not occur until very late stages of lifespan. We showed that PS exposure mediated by Ca2+-stimulated phospholipid scrambling was not different between red cells of the proband and control. Taken together, our findings imply that suppressed scrambling activity rather than flippase activity is the major contributor to maintainance of PS in inner leaflet of normal red cells during their 120-day lifespan, and that PS exposure to cell surface as an 'eat-me' signal depends primarily on scramblase activity at the end of lifespan. Importantly, our study has enables us to identify the major flippase of human erythrocyte membrane. DisclosuresNo relevant conflicts of interest to declare.

Interaction of double-chained cationic surfactants, dimethyldialkylammoniums, with erythrocyte membranes: stabilization of the cationic vesicles by phosphatidylcholines with unsaturated fatty acyl chains.

We studied the interaction of double-chained cationic surfactants, dimethyldialkylammoniums, (CH3)2N+(CnH2n+1)2, with the lipid bilayer of guinea-pig erythrocytes by observing the haemolysis, aggregation and shape change in the erythrocytes. In the presence of sonicated dispersions of the five dimethyldialkylammoniums tested (n = 10, 12, 14, 16 and 18), haemolysis was induced dose dependently, and at 0.1 mM or higher concentrations, haemolysis was induced more rapidly by dimethyldialkylammoniums with shorter alkyl chains. The cationic surfactants with longer alkyl chains, such as dimethyldipalmitylammonium, induced aggregation of the erythrocytes before haemolysis fully progressed. The vesicles of these long-chain dimethyldialkylammoniums in the presence of phosphatidylcholines with unsaturated fatty acyl chains markedly reduced the haemolysis rates. Furthermore, in the presence of phosphatidylcholines with unsaturated acyl chains the formation of tightly aggregated structures of several erythrocytes was observed. These findings, and analysis by spin label 5-doxylstearic acid, indicate that phosphatidylcholines enriched with unsaturated acyl chains stabilize the cationic vesicles of long-chain dimethyldialkylammoniums and the interaction with the lipid bilayer of erythrocyte membranes as cationic vesicles became prominent.

STRUCTURAL EFFECTS OF VERAPAMIL ON CELL MEMBRANES AND MOLECULAR MODELS

ABSTRACT Verapamil is one of the frequently prescribed calcium channel blockers used in the treatment of hypertension and angina pectoris . Results of evaluations of the therapy have led to reports of toxic effects. This study presents several evidences that verapamil affects human cells. Scanning electron microscopy observations of intact human erythrocytes indicated that they underwent morphological alterations as increasing verapamil concentrations starting from 5 μM changed their discoid normal shape, and finally to hemolysis. Fluorescence spectroscopy on isolated unsealed human erythrocyte membranes confirmed these outcomes. In fact, the assays showed that verapamil induced a significant increase of the anisotropy parameters and a moderate one of the generalized polarization, indicative of enhanced order at the acyl chain and polar head regions of the erythrocyte membrane lipid bilayer. X-ray diffraction experiments on dimyristoylphosphatidylcholine and dimyristoylphosphatidylethanolamine bilayers, classes of the major phospholipids present in both outer and inner sides of the erythrocyte membrane, respectively showed that verapamil perturbed the polar head and acyl chain regions of both lipid bilayers. These interactions were found to be stronger with DMPC bilayers. On the other hand, human SH-SY5Y neuroblastoma cells incubated with verapamil suffered a sharp decrease of cell viability.

Correction of metabolic disorders in patients with chronic obstructive pulmonary disease treated with Troventol

The study was aimed to investigate efficacy, safety, and ability of correction some metabolic disturbances in patients with chronic obstructive pulmonary diseases (COPD) at the North during therapy with Russian anticholinergic drug Troventol. Thirty-nine moderate COPD patients and 11 mild non-atopic asthma patients were observed. Good clinical efficiency was noted in 72.3 % and satisfactory effect was in 20.8 % of the patients after the Troventol alone therapy. The treatment resulted in normalization of reduced blood antioxidant potential due to increase in intercellular superoxiddysmutase erythrocyte level, ceruloplasmin and transferrin plasma levels. Troventol provided normalization of increased fluidity of the erythrocyte membrane lipid bilayer. Therapeutic doses of Troventol were well-tolerated and did not cause severe adverse effects.

Lead‐induced changes in human erythrocytes and lymphocytes

In the present work we studied, by chemiluminescence measurements, the influence of lead on the production of reactive oxygen species (ROS) in haemolysates obtained from human erythrocytes incubated in the presence of different concentrations of lead acetate. Moreover, we evaluated the modification of proteins and lipids in human erythrocyte and lymphocyte membranes by using the fluorescence probes N-(1-pyrene)maleimide (PM), laurdan and pyrene. No significant changes in chemiluminescence were detected for erythrocytes incubated with 1-10 microM lead acetate for 3 h at 37 degrees C. By increasing the lead acetate concentration in cell suspensions up to 50 microM for the same incubation time, the percentage of chemiluminescence inhibition was ca. 20%. It was shown that, after incorporating fluorescence probes in the membrane lipid bilayer of erythrocytes and lymphocytes treated with 10 and/or 50 microM lead acetate, the total fluorescence intensity and the excimer to monomer intensity ratio of PM decreased and the generalized fluorescence polarization of laurdan decreased by 10-15%. The pyrene excimerization coefficient (kappa(ex)) increased by 20% (in comparison with a magnitude of kappa(ex) for white membranes isolated from intact erythrocytes) with 6-10 microM lead acetate for 3 h at 37 degrees C. The data obtained suggest that the effect of low concentrations of lead acetate does not cause production of ROS in erythrocytes in vitro, but can change the physicochemical state of proteins and lipids in erythrocyte and lymphocyte membranes. This effect is important because it influences the enzymatic activity and the functionality of receptors and channels present at the plasma membrane level, thus modulating the molecular composition of the intracellular space and cell functions.

The Antiepileptic Drug Diphenylhydantoin Affects the Structure of the Human Erythrocyte Membrane

Phenytoin (diphenylhydantoin) is an antiepileptic agent effective against all types of partial and tonic-clonic seizures. Phenytoin limits the repetitive firing of action potentials evoked by a sustained depolarization of mouse spinal cord neurons maintained in vitro. This effect is mediated by a slowing of the rate of recovery of voltage activated Na+ channels from inactivation. For this reasons it was thought of interest to study the binding affinities of phenytoin with cell membranes and their perturbing effects upon membrane structures. The effects of phenytoin on the human erythrocyte membrane and molecular models have been investigated in the present work. This report presents the following evidence that phenytoin interacts with cell membranes: a) X-ray diffraction and fluorescence spectroscopy of phospholipid bilayers showed that phenytoin perturbed a class of lipids found in the outer moiety of cell membranes; b) in isolated unsealed human erythrocyte membranes (IUM) the drug induced a disordering effect on the polar head groups and acyl chains of the erythrocyte membrane lipid bilayer; c) in scanning electron microscopy (SEM) studies on human erythrocytes the formation of echinocytes was observed, due to the insertion of phenytoin in the outer monolayer of the red cell membrane. This is the first time that an effect of phenytoin on the red cell shape is described. However, the effects of the drug were observed at concentrations higher than those currently found in plasma when phenytoin is therapeutically administered.

Structure and properties of lipid bilayer of erythrocyte membranes in patients with malignant tumors.

The study of erythrocyte membranes in untreated patients with malignant tumors of different location (lung cancer, tumors of the head and neck, stomach and colorectal cancer) revealed changes in the fatty-acid spectrum of phospholipid fraction paralleled by an increase in the viscosity of the lipid bilayer, including the area of protein-lipid contacts. The degree of changes depended on tumor location.

The reaction of lanthanide ions with n-doxyl stearic acids and its utilization for the ESR study on the permeability of lipid-bilayer of erythrocyte membrane to gadolinium ions

The reaction of lanthanide ions with n-doxyl stearic acid ( nDS) spin labels ( n=5,7,12,16) was investigated by the electron spin resonance technique in aqueous solution. Among the lanthanides, the Gd 3+, Tb 3+, Tm 3+ and Ce 3+ ions strongly quenched the ESR signal of spin labels, but the effects of La 3+, Eu 3+ and Lu 3+ are very weak. The quenching effects are featured by: (1) the dependence on the concentration of lanthanide ions; (2) no obvious changes of the ESR line shape in the presence of lanthanide ions; (3) the quenching constant decreases in the order: Gd 3+ > Tb 3+ > Tm 3+ > Ce 3+; (4) the quenching effects of lanthanide ions are found to strikingly correlate with their magnetic properties. These findings indicate that the interaction of lanthanide ions with nitroxide oxygen leading to the reduction of ESR signal amplitude is dominated by their magnetic characteristics rather than the coordination effect. By labeling erythrocyte membrane with nDS, n=5,7,12,16 at different depths, we studied the diffusion of Gd 3+ into the lipid-bilayer of erythrocyte membrane by monitoring the reduction processes of the ESR signals of nitroxide spin labels located at different depths of membrane lipid-bilayer after addition of Gd 3+. These results revealed that the Gd 3+ ions can penetrate into the lipid-bilayer, though the entry rate is slow. It was shown that the Gd 3+ ions bind to the membrane and enhance the permeability of extracellular ascorbate into erythrocyte membrane. The transport mechanism of Gd 3+ ions through the lipid-bilayer might be involved in the Gd 3+ cation-induced pore formation in the surface of membrane.

Effects of dehydroabietic acid on the physical state of cytoskeletal proteins and the lipid bilayer of erythrocyte membranes

Dehydroabietic acid (DHAA) is a major aquatic toxic resin acid usually found in unbleached pulp mill effluents. This compound has been reported to accumulate in the red cells of rainbow trout and to cause hemolysis. To elucidate further understanding to the mechanism of action of this resin, the interaction of DHAA with human erythrocyte membranes has been monitored by electron paramagnetic resonance techniques of spin labeling. Results presented in this paper indicate that DHAA, in a concentration-dependent manner, significantly altered both the motion and order of the lipid bilayer and the physical state of cytoskeletal proteins, while DHAA had no effect on isolated lipids. It is proposed that the increase in the ‘fluidity’ of the lipid bilayer induced by DHAA is a secondary effect of primary changes in the physical state of the cytoskeletal proteins of the membrane, and that the latter effect is critically associated with the toxicity of DHAA.

Phospholipid dynamics and function in cell membrane of blood cells. With reference to mode of action of certain amphiphilic drugs

An asymmetric modification of the cell membrane lipid bilayer of erythrocytes by means of exogenous phospholipase (PL) treatment or of phospholipid addition induced an echinocyte- or stomatocyte-type shape change of the cells. Such membrane shape change was demonstrated to be due to asymmetric swelling or shrinkage of the lipid bilayer halves, from the simultaneous determination of the change or intra-bilayer transfer of the phospholipid molecules. In the collagen-induced activation of blood platelets, the importance of PLA2 activation as the initial reaction of the signaling system was demonstrated, and the regulatory effects of the membrane fluidity change on the enzyme activation were also shown. In the antigen-induced activation of peritoneal mast cells, a pathway through the PLD activation appeared to be more important. Phosphatidic acid produced from the membrane phospholipids through the PLD activation was shown to act as an intracellular mediator to stimulate the PLA2 activation. Various amphiphilic drugs, including benzylisoquinoline alkaloids such as cepharanthine, were shown to inhibit the platelet activation and the mechanism was studied. The anti-platelet activity appeared to be mainly due to inhibition of the PLA2 activation.

Effect of some nickel compounds on red blood cell characteristics

The effect of certain inorganic and coordinated nickel compounds on the resistance to different destructive substances, rheological properties, and functional activity of healthy human red blood cells (RBC), was investigated. It is shown that nickel compounds affect the erythrocyte membrane lipid bilayer, as well as membrane proteins to various extents, depending on the type of compounds used. In general, the acceleration of erythrocyte aging was observed to be more pronounced in young erythrocytes. The observed results suggest that nickel compounds decrease water permeability across erythrocyte membranes. Almost all the investigated nickel compounds decrease erythrocyte thermostability, deformability, and the rate of O2 release by erythrocytes.

Mechanism of polymyxin B-mediated lysis of lipopolysaccharide-treated erythrocytes.

A novel system was used previously to characterize the dynamic interaction of a polysaccharide-deficient, lipid-rich lipopolysaccharide (LPS) with rabbit erythrocytes (RaRBC). Exposure of the RaRBC to the LPS rendered them sensitive to induction of hemolysis by the cationic antibiotic polymyxin B (PB) in a time- and temperature-independent manner. Subsequent decay in the response of LPS-sensitized cells to PB was shown to be critically dependent on both the time and temperature of incubation of RaRBC with LPS and to be independent of a change in LPS binding (Carr and Morrison, Infect. Immun. 43:600-606, 1984). In the present study, we performed experiments designed to define the mechanism by which PB mediates hemolysis of LPS-sensitized RaRBC. Experiments were performed to examine the molecular requirements of the LPS and the PB that were essential for hemolytic activity. The capacity of various cations to mediate hemolysis of LPS-sensitized RaRBC or to block PB-mediated hemolysis and the temperature dependence of the PB lytic reaction were investigated. The results of these experiments suggest that PB-mediated hemolysis of LPS-treated erythrocytes is dependent upon an initial ionic association of PB with erythrocyte membrane-bound LPS, followed by hydrophobic insertion of the PB fatty acid into the erythrocyte membrane lipid bilayer.

Metabolic dependence of the fluidity of intact erythrocyte membrane

The motion of spin labeled phosphatidylcholine embedded in the lipid bilayer of erythrocyte membrane increased in association with metabolic ATP-depletion. The fluidity change was reversed by subsequent ATP-replenishment. However, levels of cellular 1,2-diacylglycerol, GSH or intracellular Ca 2+ contributed little to the fluidity change. Temperature dependence of the fluidity indicated the disappearance of inflection points in ATP-depleted cells, thereby suggesting alterations in protein-lipid interactions. The fluidity change was also reproduced with oxidizing agents which cross-link spectrin. We suggest that the lipid phase state of membrane is maintained by protein-lipid interactions which depend on the metabolic state of the cells, particularly ATP levels.

Enveloped virus- and lectin-induced structural change in erythrocyte lipid bilayer: Dependence on species of erythrocyte and on the multivalence of the agglutinin

Attachment of influenza virus or concanavalin A (Con A) to avian or amphibian erythrocytes causes the erythrocyte membrane lipid bilayer to become more fluid, as detected by spin-label electron spin resonance. No change in bilayer fluidity occurs upon agglutination of avian erythrocytes pretreated with the microtubule-disrupting drug colchicine or agglutination of mammalian erythrocytes, which lack microtubules. The change in bilayer fluidity upon agglutination of chicken erythrocytes appears to require crosslinking of receptor molecules. The hemagglutinin glycoprotein isolated from influenza virus by treatment with bromelain and the succinylated derivative of Con A bind to receptors, but do not crosslink them and do not cause a change in lipid bilayer fluidity. Multivalent aggregates of influenza virus hemagglutinin, isolated by treatment with Triton X-100 followed by removal of the detergent, are able to induce an increase in erythrocyte lipid bilayer fluidity. Erythrocytes pretreated with succinyl-Con A are agglutinated by antibody to Con A, resulting in an increase in membrane bilayer fluidity. These results support the hypothesis that the fluidity change in the avian erythrocyte membrane induced by virus and lectin attachment involves lateral rearrangement of receptor glycoproteins in the plane of the membrane mediated by microtubule-like systems.

Sendai virus-induced hemolysis: reduction in heterogeneity of erythrocyte lipid bilayer fluidity.

Hemolysis of human or chicken erythrocytes by Sendai virus causes a change in the structure of the erythrocyte membrane lipid bilayer that can be detected by spin label electron spin resonance. In the intact erythrocyte, the phosphatidylcholine derivative spin label exists in a more rigid environment than the corresponding phosphatidylethanolamine label. Virus-induced hemolysis tends to abolish this difference in fluidity, i.e., the region of the phosphatidylcholine spin label becomes more fluid and that of the phosphatidylethanolamine spin label becomes more rigid. Fatty acid derivative spin labels, which may detect some "average" environment, show no change in fluidity. The fluidity change is detected at several different positions in the fatty acyl chain of the phosphatidylcholine spin label. Sendai virions grown in Madin-Darby bovine kidney (MDBK) cells or grown in eggs and harvested early, which lack hemolytic activity, cause no significant change in bilayer structure. Hemolytic activity and the ability to alter erythrocyte bilayer fluidity can be activated in MDBK-grown Sendai virions by trypsin treatment in vitro and in early-harvest egg-grown Sendai virions by freezing and thawing. Erythrocyte ghosts prepared by osmotic hemolysis and resealed by treatment with Mg2+ or elevated ionic strength exhibit a difference in fluidity between phosphatidylcholine and phosphatidylethanolamine spin labels, although less than that observed in whole cells. Incubation of resealed ghosts with Sendai virus abolishes the difference in fluidity. Unsealed ghosts that have been extensively washed show no heterogeneity in membrane bilayer fluidity, and incubation with Sendai virus causes no further fluidity change. Virus-induced hemolysis as measured by hemoglobin release is more sensitive to inhibition by Ca2+ than is the associated fluidity change in the bilayer.