The release of histamine and 5-hydroxytryptamine (serotonin) from platelets by antigen-antibody reactions (in vitro)

Immune Thrombosis: Exploring the Significance of Immune Complexes and NETosis

Neutrophil extracellular traps (NETs) are major contributors to inflammation and autoimmunity, playing a key role in the development of thrombotic disorders. NETs, composed of DNA, histones, and numerous other proteins serve as scaffolds for thrombus formation and promote platelet activation, coagulation, and endothelial dysfunction. Accumulating evidence indicates that NETs mediate thrombosis in autoimmune diseases, viral and bacterial infections, cancer, and cardiovascular disease. This article reviews the role and mechanisms of immune complexes in NETs formation and their contribution to the generation of a prothrombotic state. Immune complexes are formed by interactions between antigens and antibodies and can induce NETosis by the direct activation of neutrophils via Fc receptors, via platelet activation, and through endothelial inflammation. We discuss the mechanisms by which NETs induced by immune complexes contribute to immune thrombotic processes and consider the potential development of therapeutic strategies. Targeting immune complexes and NETosis hold promise for mitigating thrombotic events and reducing the burden of immune thrombosis.

The Fyn-Stat5 cascade is required for Fcγ receptor-mediated mast cell function

Mast cells, well established effectors in allergic disease, can be activated by numerous stimuli. We previously found that the Fyn-Stat5B pathway is critical for FcεRI-stimulated mast cell function. Because IgG receptors employ similar signaling pathways, we investigated Fyn-Stat5B function downstream of FcγR. We report that FcγR elicits Fyn-dependent Stat5B tyrosine phosphorylation in mast cells. As we previously found for Fyn kinase, Stat5B is indispensable for IgG-mediated mast cell cytokine expression and secretion. However, Stat5B KO macrophages responded normally to FcγR signaling, indicating a lineage-restricted role for Stat5B. This was consistent in vivo, since passive FcγR activation induced anaphylaxis in a macrophage-dominated response even when Stat5B was deleted. We further investigated this lineage restriction using the K/BxN model of inflammatory arthritis. This model exhibits a rapid and transient mast cell-dependent joint inflammation followed days later by a macrophage- and neutrophil-dependent response. Consistent with our hypothesis, Fyn or Stat5B deficiency did not protect mice from late joint swelling, but greatly reduced the early mast cell-dependent response. This was associated with decreased joint and plasma histamine. We conclude that Fyn-Stat5B is a linage-restricted pathway critical for IgG-mediated mast cell responses.

Inflammatory events induced by brown spider venom and its recombinant dermonecrotic toxin: a pharmacological investigation

Accidents involving Brown spider (Loxosceles sp.) venom produce a massive inflammatory response in injured region. This venom has a complex mixture of different toxins, and the dermonecrotic toxin is the major contributor to toxic effects. The ability of Loxosceles intermedia venom and a recombinant isoform of dermonecrotic toxin to induce edema and increase in vascular permeability was investigated. These toxins were injected into hind paws and caused a marked dose and time-dependent edema and increase in vascular permeability in mice. Furthermore, the enzymatic activity of venom toxins may be primal for these effects. A mutated recombinant isoform of dermonecrotic toxin, that has only residual enzymatic activity, was not able to induce these inflammatory events. Besides the previous heating of toxins markedly reduced the paw edema and vascular permeability showing that thermolabile constituents can trigger these effects. In addition, the ability of these venom toxins to evoke inflammatory events was partially reduced in compound 48/80-pretreated animals, suggesting that mast cells may be involved in these responses. Pretreating mice with histamine (prometazine and cetirizine) and serotonin (methysergide) receptor antagonists significantly attenuated toxins induced edema and vascular permeability. Moreover, HPLC analysis of whole venom showed the presence of histamine sufficient to induce inflammatory responses. In conclusion, these inflammatory events may result from the activation of mast cells, which in turn release bioamines and may be related to intrinsic histamine content of venom.

The interaction of bacterial pathogens with platelets

In recent years, the frequency of serious cardiovascular infections such as endocarditis has increased, particularly in association with nosocomially acquired antibiotic-resistant pathogens. Growing evidence suggests a crucial role for the interaction of bacteria with human platelets in the pathogenesis of cardiovascular infections. Here, we review the nature of the interactions between platelets and bacteria, and the role of these interactions in the pathogenesis of endocarditis and other cardiovascular diseases.

Studies on the mechanism of hypersensitivity phenomena. III. The participation of complement in the formation of anaphylatoxin

The interaction of fresh rat and guinea pig serum with washed preformed immune aggregates has been studied with respect to the loss in hemolytic potency of the serum, diminution of the C'₃; activity, and appearance of anaphylatoxin. It has been found that the formation of anaphylatoxin, as judged by its effect on capillary permeability and smooth muscle contraction, is coincident with or subsequent to the fixation of all the known C' components. Less anaphylatoxin is formed by aggregates formed with excess antigen than those in equivalence ratio combination. C' fixation, as well as anaphylatoxin production, may be inhibited by chelation of the divalent cations, presumably by interfering with the fixation of C'₁, C'₄, and C'₂. Phlorizin suppresses the utilization of C'₃ in immune hemolysis, C' fixation by antigen-antibody aggregates, and the production of anaphylatoxin. The biological activities associated with the fixation of C' are not manifest unless C'₃ participates in this process at 37°C. It is concluded that the formation of anaphylatoxin may be regarded as a product of C' fixation.

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia with or without thrombosis has been recognized increasingly as a serious complication of heparin use. This article reviews type II heparin-induced thrombocytopenia, which is mediated by an antibody that in most cases has specificity for a complex between heparin and platelet factor 4, a secreted platelet alpha-granule protein. The antibody-heparin-platelet factor 4 complex can activate platelets and endothelial cells, thereby initiating thrombosis. Clinical thrombosis in this syndrome may be arterial or venous. Treatment of the syndrome requires discontinuation of heparin and institution of an alternative anticoagulant.

Serotonin unmasks functional NK-2 receptors in vagal sensory neurones of the guinea-pig

1. The regulation of substance P (SP) responsiveness in acutely isolated nodose neurones from adult guinea-pigs was investigated using standard intracellular recording techniques. 2. In control neurones, SP produced no measurable electrophysiological effects. However, following incubation with serotonin (5-HT, 10 microM), 64% of neurones were depolarized by 10 +/- 0.6 mV (n = 84 of 132 neurones) by SP (100 nM). 5-HT-induced SP responses were inhibited by SR48968 (100 nM, n = 6), a neurokinin 2 (NK-2) receptor antagonist, but were unaffected by CP99,994 and SR142801, NK-1 and NK-3 receptor antagonists (n = 3 each), respectively. 3. 5-HT-induced unmasking of SP responses was maximal within 5 min. Increasing the 5-HT incubation time up to 120 min did not increase the mean response amplitude or the percentage of SP responsive neurones (P = 0.611 and 0.867, respectively). 4. 5-HT-induced unmasking of SP responses was dose dependent (EC50 = 14 nM). A 5-HT3 receptor agonist CPBG (1 microM), mimicked the unmasking effects of 5-HT (n = 10 of 19 neurones), while 5-CT (10 microM), a non-selective 5-HT agonist devoid of action at 5-HT3 receptors, did not (n = 18). ICS205-930 (1 microM), a 5-HT3 receptor antagonist, completely blocked the 5-HT-induced unmasking of SP responses (n = 10 of 10 neurones). 5. In 68% of the neurones tested, bath-applied 5-HT (10 microM) evoked a 178 +/- 29.5 nM increase in [Ca2+]i (n = 16), which was blocked by nominally zero [Ca2+]o (n = 4) or by ICS205-930 (1 microM, n = 4). Nodose neurones incubated with 5-HT in the presence of nominally zero [Ca2+]o did not respond to SP (n = 12 of 13 neurones) in Locke solution containing normal [Ca2+]o, indicating that the 5-HT-mediated elevation of [Ca2+]i is required for unmasking of SP responses. Calmidazolium (100 nM), a calmodulin inhibitor, inhibited the unmasking effects of 5-HT (n = 5 of 5 neurones). 6. Incubating neurones with the nitric oxide (NO) donors papaNONOate (1 mM, 15-30 min) or SNAP (50 microM, 30-60 min) unmasked depolarizing SP responses in 71% and 45% of the neurones studied, respectively. L-NMMA (30 microM), a NO synthase inhibitor, blocked 5-HT-induced unmasking of SP responses (n = 10 of 10 neurones). 7. In sum, these results suggest that stimulation of 5-HT3 receptors activates an intracellular signalling cascade that couples calcium-calmodulin and NO activation to NK-2 receptor unmasking in sensory neurones.

Anaphylaxis in chopped guinea pig lung. I. Effect of peptidase substrates and inhibitors

The quantitative release of histamine by specific antigen from perfused, chopped, sensitized guinea pig lung has been used to study the effect of peptidase substrates and inhibitors on the anaphylactic reaction. The anaphylactic release of histamine is prevented by chymotrypsin substrates and inhibitors but not by trypsin, carboxypeptidase, or leucine aminopeptidase substrates or the soybean trypsin inhibitor. The chymotrypsin substrates and inhibitors appear to be acting on an antigen-antibody-activated step because these substances fail to inhibit if the tissue is washed free of them prior to antigen addition, and because there is complete desensitization of the tissue without histamine release when the antigen is added in the presence of these inhibitors. The inhibitors work equally well in tissue from passively sensitized animals or in tissue from animals actively sensitized with either ovalbumin or bovine gamma globulin. These observations suggest that activation of a chymotrypsin-like enzyme is a necessary condition for the anaphylactic release of histamine in guinea pig lung. Diisopropylfluophosphate is inhibitory when present at the time of antigen addition but not when the tissue is washed free of unfixed diisopropylfluophosphate prior to adding antigen. This indicates that diisopropylfluophosphate must be acting exclusively on an enzyme which exists in lung tissue in a precursor form resistant to diisopropylfluophosphate until activated by the antigen-antibody interaction. Thiol alkylating or oxidizing agents also prevent the anaphylactic release of histamine, but in contrast to the situation with diisopropylfluophosphate and the other chymotrypsin inhibitors, the phase of the anaphylactic reaction inhibited by N-ethylmaleimide is available prior to the antigen-antibody interaction. The similarities and differences between immune hemolysis and anaphylaxis in chopped guinea pig lung are considered in detail.

Effects of bacterial endotoxin on rabbit platelets. I. Platelet aggregation and release of platelet factors in vitro

Incubation of platelet-rich rabbit plasma with E. coli endotoxin at 37°C results in platelet aggregation and transfer of platelet 5-hydroxytryptamine to plasma. Release of 5HT is influenced by dose of endotoxin, type of anticoagulant, and temperature of incubation. A heat-labile plasma factor is necessary for the platelet-endotoxin interaction. Additional studies have shown that incubation of endotoxin with platelet-rich rabbit plasma also results in release of platelet phospholipid and bactericidins active against B. subtilis.

Vespula vulgaris venom: role of kinins and release of 5-hydroxytryptamine from skin mast cells

Wasp venoms contain several active components, among them kinin-related peptides. Like bradykinin and [Thr6]bradykinin, Vespula vulgaris venom caused paw oedema following subplantar injection in anaesthetized rats. The oedema was partly inhibited by the bradykinin B2 receptor antagonist icatibant (Hoe 140); the remaining part was abolished by additional pretreatment with 5-hydroxytryptamine (5-HT) receptor antagonists or mast cell depletion. Histamine receptor antagonists were ineffective. Capsaicin pretreatment attenuated oedema formation indicating a neurogenic sensory component. Nociceptive behavioural responses induced by the venom in unanaesthetized rats were abolished by icatibant. It is concluded that kinins, either contained in the venom or released from the tissue, play the predominant role in the inflammatory and algesic effects. The inflammatory effects only partly rely on direct, bradykinin receptor-mediated mechanisms while the remaining part depends on the release of 5-HT from skin mast cells. The algesic effects of the venom are entirely due to direct B2 receptor activation.

EFFECTS OF BACTERIAL ENDOTOXIN ON RABBIT PLATELETS. 3. COMPARISON OF PLATELET INJURY INDUCED BY THROMBIN AND BY ENDOTOXIN

The platelet injury produced by bacterial endotoxin and thrombin have been compared in studies utilizing citrated rabbit platelet-rich plasma. Endotoxin-induced platelet injury is characterized by a lag period, is progressive, and does not produce gross clot formation. Thrombin-induced platelet injury is immediate, non-progressive, and is associated with clot formation. The quantity of thrombin required to produce clot formation in this citrated system is less than that required to produce release of platelet 5-hydroxytryptamine. The endotoxin-induced platelet injury required extremely large quantities of heparin for inhibition. The platelet injury induced by thrombin can be inhibited by small quantities of heparin. It is concluded that the injurious effects of endotoxin on platelets is mediated through some mechanism other than thrombin formation.

Effect of intravenous immunoglobulin on inhibition of fibrinogen binding to platelets by sera from patients with immune thrombocytopenia

We previously described an ELISA to measure the inhibition of platelet glycoprotein IIb/IIIa (GPIIb/IIIa) binding to fibrinogen due to immune complexes and/or anti-platelet antibodies from patients with immune thrombocytopenia (ITP) or HIV-related ITP. Circulating immune complexes (CIC) were the main factor in the inhibition of GPIIb/IIIa binding to fibrinogen in HIV-related ITP, whereas in non-HIV ITP, inhibition was only partially due to CIC; anti-platelet antibodies specific to GPIIIa were also shown to play a role. In this study, we correlated the rise in the platelet count after intravenous immunoglobulin (IVIG) infusion with the decrease in inhibition of fibrinogen binding to GPIIb/IIIa by the sera of patients with ITP and HIV-related ITP. In the majority of the patients' sera tested, as the platelet count increased following the administration of IVIG, the degree of inhibition of GPIIb/IIIa binding to fibrinogen decreased. We also observed a decrease and/or disappearance of the antibodies specific to GPIIb and/or GPIIIa after IVIG administration. In HIV-seronegative ITP patients, the decrease or disappearance of anti-platelet antibodies directly correlated with the decreased inhibition of GPIIb/IIIa binding to fibrinogen by the 2% PEG supernatants of sera which contained anti-platelet antibodies. These findings suggest that IVIG directly affects the binding of CIC and anti-platelet antibodies to platelets and thereby improves platelet survival. Our results also suggest that the anti-idiotypic effect may contribute to IVIG's therapeutic action. In contrast, in the HIV-seropositive group, the decreased inhibition by PEG precipitates after IVIG administration was more strongly associated with an increase in the platelet count.

Rabbit Platelets do not Express Fc Receptors for IgG

Human platelets are activated by IgG immune complexes and undergo platelet aggregation and the release of granular contents. Recently, the receptor for IgG on human platelets has been identified and characterised. In contrast, it is not known if rabbit platelets carry Fc receptors. In this report we describe investigations to detect the presence of an Fc receptor on rabbit platelets. The Fc receptor was investigated using functional assays (platelet aggregation and (14)C-serotonin release), a direct binding assay, and an affinity precipitation assay. All of these methods gave consistently positive results using human platelets and consistently negative results using rabbit platelets, indicating that rabbit platelets do not carry Fc receptors. This inconsistency in the interactions of immune complexes with rabbit and human platelets indicates that rabbits are not ideal models for studying a number of human diseases that could be caused by immune complexes, including heparin-induced thrombocytopenia and certain immunological disorders such as systemic lupus erythematosus.

Characterization of the Fc gamma receptor on human platelets

IgG-containing immune complexes may play a role in the immune destruction of human platelets by interacting with an Fc gamma receptor on the platelet surface. We studied the platelet Fc gamma receptor and characterized its interaction with IgG ligand and anti-Fc gamma receptor monoclonal antibodies. Oligomers of IgG, but not monomeric IgG, bound to platelets and the number of binding sites was significantly increased at low ionic strength. Ligand-binding studies indicated that normal human platelets express a single Fc gamma receptor (Fc gamma RII) with 8559 +/- 852 sites per cell, Kd = 12.5 +/- 1.7 X 10(-8) M using trimeric IgG. Results of studies with bivalent and Fab monoclonal anti-Fc gamma RII were consistent with each Fc gamma receptor expressing two epitopes recognized by the antibody. The number of Fc gamma binding sites and affinity of binding were unchanged by the presence of 2.0 mM Mg2+ or 10 micrograms/ml cytochalasin B. Platelet stimulation with thrombin or ADP in the presence of fibrinogen also did not alter the number of Fc gamma binding sites or the affinity of binding. However, platelets preincubated with 5 microM dexamethasone expressed a decreased number of Fc gamma binding sites as well as decreased IgG-dependent platelet aggregation. Platelets from patients with Glanzmann's thrombasthenia and from patients with the Bernard Soulier syndrome expressed a normal number and affinity of Fc gamma binding sites. The data suggest that platelet Fc gamma RII binding of trimeric IgG occurs independent of actin filament interaction, Mg2+, ADP, or thrombin and does not require GPIIb/IIIa or GPIIb/IIIa-fibrinogen interaction. Furthermore, this receptor appears to be normally expressed on GPIb-deficient platelets and susceptible to modulation by glucocorticoids. Finally, the Fc gamma-binding protein was isolated from whole platelets as a 220-kDa protein which upon reduction dissociates into 50,000 Mr subunits.

Increased platelet thromboxane synthesis in renal glomerular diseases

Washed platelets were prepared from healthy children and adults, and patients with renal glomerular diseases, and incubated with [1-14C] arachidonate to measure the generation capacities of thromboxane (Tx) A2 and 12-hydroxyeicosatetraenoate (12-HETE). Tx generation capacity of platelets was significantly higher in patients with chronic glomerulonephritis, purpura nephritis and lupus nephritis than in healthy control subjects. There was no significant increase in minimal change nephrotic syndrome. 12-HETE showed a decreasing tendency in the glomerular diseases, which was restored to normal level by in vitro addition of indomethacin. Such increased Tx generation capacity of platelets may cause abnormal enhancement of platelet functions and conceivably constitute an aggravating factor of glomerular and microvascular damage in the affected kidney.

Platelet IgG Fc receptor

The glycoprotein localization of the platelet binding site for the Fc IgG has been the subject of debate. We attempted to resolve this issue by relating the binding of radiolabeled IgG immune complexes composed of heat-aggregated IgG to platelets from healthy individuals; an individual with Bernard-Soulier syndrome lacking glycoproteins IIb and IX; and a patient with Glanzmann's thrombasthenia lacking glycoproteins IIb and IIIa. The binding of IgG complexes to platelets was determined by measuring the specific binding of radiolabeled heat-aggregated IgG to washed platelets in a plasma-free mileu. 125I aggregated IgG bound to normal platelets in a saturable and concentration-dependent fashion. Specific binding could be inhibited by a 50-fold excess of purified Fc, but not by F(ab')2. Identical binding curves were obtained by using platelets from a patient with Glanzmann's thrombasthenia and a patient with Bernard-Soulier syndrome, indicating that the platelet Fc receptor is not carried on glycoproteins Ib, IIb, IIIa, or IX. We then measured the binding of radiolabeled detergent-solubilized platelets to IgG fixed to a solid matrix. A 40-kD platelet fragment bound to the immobilized IgG following passage across a density gradient. Confirmation of the Fc specificity of the interaction was shown by inhibition of platelet glycoprotein binding by excess IgG or purified Fc but not F(ab')2. The electrophoretic mobility decreased slightly after reduction, which indicated the existence of at least one intrachain disulfide bond. Treatment with high salt solutions or urea did not solubilize the receptor, which indicated that it was an integral protein. Enzyme studies showed that the platelet Fc receptor was not digested by neuraminidase, but neuraminidase treatment altered mobility by about 3%. In addition, treatment of platelets with trypsin or pronase did not affect its function as measured by the binding of 125I-IgG aggregates to enzyme-treated platelets, but did prevent its detection when using radioimmunoprecipitation studies. The platelet Fc receptor is a 40-kD, integral protein without interchain disulfide bonds.

[Reciprocal potentiation of the bronchoconstrictor activity of histamine and 5-hydroxytryptamine in the guinea pig]

Bronchoconstriction due to aerosolized histamine (H), 5-hydroxytryptamine (5-HT) and combined administration of both substances has been studied on 53 guinea-pigs. The results are consistant with: a major bronchoconstrictor effect obtained with H than with 5-HT; a potentiation of the individual effects of H and 5-HT in two thirds of the guinea-pigs, by the combined administration of the two drugs simultaneously; a more potent antagonistic effect of Ketanserin against 5-HT than against H; potentiation due to the combined administration of 5-HT and H is suppressed by Ketanserin; neither vagal reflexes nor cholinergic receptors seem to interfere with the bronchospastic response of the guinea-pig to H and 5-HT.

Beta-thromboglobulin in inflammatory synovial fluid

Activated platelets release substances which potentially can contribute to joint lesions in inflammatory arthritides. To elucidate a possible participation of platelets in inflammatory joint reactions, the concentrations of the platelet protein beta-thromboglobulin (beta-TG) were measured in 90 inflammatory synovial fluids. Seven percent of the patients with rheumatoid arthritis and none of the patients with other inflammatory joint diseases (e.g., Reiter's disease, reactive or crystal arthritides) had beta-TG concentrations in synovial fluid exceeding the upper normal range of plasma beta-TG. The absent or very modest signs of local platelet activation were contrasted by the pronounced neutrophilic and monocytic activation, as assessed by the measurements of some granule proteins: lactoferrin, myeloperoxidase, lysozyme, and ferritin. No correlation was found between these inflammatory cell markers and beta-TG. A positive correlation (p less than 0.001) was noted between beta-TG and beta 2-microglobulin, which appeared in particularly high amounts in rheumatoid arthritis. This correlation may reflect a disturbed permeability of synovial membrane for LMW proteins or a related activation of platelets and lymphocytes. The present results do not give any evidence of platelet activation playing a major role in proliferative or destructive processes in arthritis.

Endothelial and platelet reactions in the idiopathic nephrotic syndrome: an ultrastructural study

In this ultrastructural study we have critically examined blood vascular reactions in the glomerular microcirculation of 14 patients with idiopathic membranous nephropathy and 11 patients with minimal change disease. In all patients, bullous intracellular edema was observed in the glomerular endothelium, a change interpreted as reflecting endothelial injury. Platelets in various stages of degranulation were seen in relation to the endothelium. In ten biopsy specimens platelet aggregates were observed. Platelets with exhausted secretory products ("ghosts") were frequently identified in areas contiguous with other obvious degranulating platelets. In some cases, pseudopodia of degranulating platelets were observed in direct contact with the glomerular basement membrane in areas denuded of endothelium. Tactoids of fibrin were seen in nine cases in relation to degranulating platelets or edematous endothelium, or both. Our data suggest that these endothelial and platelet reactions play a role in the pathophysiology oi the nephrotic syndrome in patients with minimal change disease or idiopathic membranous nephropathy.

The role of circulating immune complexes in the pathogenesis of diabetes mellitus

We have studied soluble immune complexes (IC) in the sera of both insulin-dependent and non-insulin-dependent diabetics by a variety of non-specific techniques and also by a method that detects specifically insulin-anti-insulin IC. Our screening studies, detailed in the first section of this work, showed that insulin-anti-insulin IC appear not to be the only type of IC present in diabetics. Non-specific screening tests gave practically identical percentages of positive results in insulin-dependent and non-insulin-dependent diabetics. However, the agreement between different screening tests was poor. We propose the use of 'IC scores' (numerical expressions of the general trend of several screening tests performed with one given serum sample) for the statistical analysis of correlations between the presence of soluble IC and clinical evidence of diabetic microangiopathy. As expected, the use of 'scores' minimized false positive or negatives and considerably enhanced statistical correlations between levels of IC and proteinuria, nephropathy, retinopathy, peripheral neuropathy, and peripheral vasculopathy. The second section of this report describes our isolation studies, which provided definitive proof for the existence of soluble insulin-anti-insulin IC and allowed us to carry out the first successful studies of the biological properties of soluble IC purified from the sera of diabetic patients, as detailed in the third section of this report. Such IC-induced platelet aggregation and activation which in vivo could lead to the development of microvascular lesions could explain, at least in part, the abnormalities in platelet function seen in diabetics. Although the precise mechanisms by which soluble IC could induce pathological damage in diabetics have not been totally clarified, we have obtained sufficient evidence to prove that antigen-antibody complexes exist in diabetics, are associated with higher frequencies of complications, and have the capacity to interact with cells in a potentially pathogenic fashion.

Proliferation of cultured rabbit renal glomerular cells stimulated by platelet factor

Platelet contents stimulated the proliferation of cultured rabbit renal glomerular (probably mesangial) cells. Growth stimulatory effect of platelet factor was heat-stable at 56 degrees C for 30 min. These results suggest that platelets play an important role in glomerular lesions in glomerulonephritis.

Divalent cation-induced interaction of phospholipid vesicle and monolayer membranes

The effects of phospholipid vesicles and divalent cations in the subphase solution on the surface tension of phospholipid monolayer membranes were studied in order to elucidate the nature of the divalent cation-induced vesicle membrane interaction. The monolayers were formed at the air/water interface. Various concentrations of unilamellar phospholipid (phosphatidylserine, phosphatidylcholine and their mixtures) vesicles and divalent cations (Mg2+, Ca2+, Mn2+, etc.) were introduced into the subphase solution of the monolayers. The changes of surface tension of monolayers were measured by the Wilhelmy plate (Teflon) method with respect to divalent ion concentrations and time. When a monolayer of phosphatidylserine and vesicles of phosphatidylserine/phosphatidylcholine (1 : 1) were used, there were critical concentrations of divalent cations to produce a large reduction in surface tension of the monolayer. These concentrations were 16 mM for Mg2+, 7 mM for Sr2+, 6 mM for Ca2+, 3.5 mM for Ba2+ and 1.8 mM for Mn2+. On the other hand, for a phosphatidylcholine monolayer and phosphatidylcholine vesicles, there was no change in surface tension of the monolayer up to 25 mM of any divalent ion used. When a phosphatidylserine monolayer and phosphatidylcholine vesicles were used, the order of divalent ions to effect the large reduction of surface tension was Mn2+ greater than Ca2+ greater than Mg2+ and their critical concentrations were in upon vesicle concentrations as well as the area/molecule of monolayers. For phosphatidylserine monolayers and phosphatidylserine/phosphatidylcholine : 1) vesicles, above the critical concentrations of Mn2+ and Ca2+, the surface tension decreased to a value close to the equilibrium pressure of the monolayers within 0.5 h. This decrease in surface tension of the monolayers is interpreted partly as the consequence of fusion of the vesicles with the monolayer membranes. The order and magnitude of divalent cation concentrations at which phosphatidylserine/phosphatidylcholine (1 : 1) and phosphatidylserine vesicle suspensions showed a large increase in turbidity were similar to those obtained in the above mentioned experiments.