Correlation of membrane anisotropy with function in subpopulations of human blood platelets

Platelet sialic acid as a potential pathogenic factor in coronary heart disease

It was previously reported that, compared to healthy individuals, patients with coronary heart disease (CHD) exhibit a higher proportion of platelets with lower densities and higher propensity to aggregate. Reasons for this increased tendency to aggregate were unknown but appeared to be independent of the patient's age, gender, or smoking habits. Sialic acid (N-acetyl-neuraminic acid), a negatively charged sugar and constituent of many glycoproteins and gangliosides, is known to confer the bulk of negative charges to mammalian cell surfaces. These negatively charged surfaces can help cells of the bloodstream to maintain a relative distance from each other due to repulsion of the same (negative) charges. In this study, we examined whether differences in platelet sialic acid are a potential pathogenic factor in patients with coronary heart disease. Upon isolating platelets, we found a significantly higher (p < 0.05) proportion of low density platelets in patients compared to healthy controls, which is in accordance with previously published data. We found significantly less (p < 0.05) sialic acid in platelets from patients compared to the control. Most of the platelet total sialic acid was susceptible to cleavage by neuraminidase, demonstrating sialic acid to be preferably localized at the outer platelet surface. We conclude that the lower sialic acid content found in platelets from CHD patients could represent a contributing factor for the observed higher aggregability of platelets from these patients. Due to the lower sialic acid content and resulting lower negative surface charge, less repulsion between the platelets could facilitate aggregation.

Fluorescence spectroscopic analysis of circulating platelet activation during coronary angioplasty

BACKGROUND AND OBJECTIVE

Platelet activation during percutaneous transluminal coronary angioplasty (PTCA) initiates thrombus formation and plaque regrowth at sites of arterial injury, limiting procedure efficacy. We have developed a simple assay for circulating platelet activation based on fluorescence analysis of membrane fluidity and intracellular calcium concentration and light scattering analysis of platelet aggregation.

STUDY DESIGN/MATERIALS AND METHODS

Platelet activation state was measured in 45 patients undergoing angioplasty, before and after treatment with platelet inhibitors.

RESULTS

PTCA alone produced a decrease in pyrene dimer formation (P0.0083) and an increase in light scattering at 650 nm (P0.0128). Treatment with ADP and GPIIb/IIIa receptor antagonists reduced PTCA induced changes in pyrene dimer formation. An unexpected decrease in pyrene dimer formation (P0.05) was detected when the GPIIb/IIIa receptor antagonist was given together with an ADP receptor antagonist.

CONCLUSIONS

1) Analysis of membrane fluidity provides a sensitive marker for platelet activation state. 2) Reduced membrane fluidity after combined platelet inhibitor treatments suggests reduced antiplatelet efficacy.

Platelet serotonin and interleukin-1 beta after sleep deprivation and recovery sleep in humans

Sleep deprivation (SD) represents a well-established therapy for major depression. Recent findings suggest that the antidepressive effects of sleep deprivation are mediated at least in part by pro-serotoninergic mechanisms. Furthermore, SD has been demonstrated to modify different host defense activities. We therefore investigated the serotonin (5-HT) content in platelets, platelet density distribution and 5-HT-induced IL-1 beta release from platelets in 10 healthy men before and after total SD (TSD) as well as after recovery sleep. Blood samples were drawn on 3 consecutive days at 7.00 h, 13.00 h, and 19.00 h, respectively. In addition, the psychophysiological parameters tiredness and wakefulness were assessed. After TSD the normal daily variation of IL-1 beta release with high morning levels and low evening levels was found to be significantly inverted. The release of IL-1 beta corresponded positively to the subjectively experienced tiredness of the probands. Analysis of platelet density distribution indicated a significant daily variation of low density platelets with low levels in the morning and high levels in the evening, which was absent after TSD. Our findings favour an increased pro-serotoninergic effect after TSD, which comprises respective variations of the host defense system, but is abolished by consecutive recovery sleep.

Analysis of GTP-binding proteins, phosphoproteins, and cytosolic calcium in functional heterogeneous human blood platelet subpopulations

The biochemical basis for the functional heterogeneity of human blood platelets was investigated in terms of protein phosphorylation, cytoplasmic calcium ([Ca2+]i), the ratio of 46 and 50 kDa vasodilator-stimulated protein (VASP), and GTP-binding proteins (G-proteins). Platelets were fractionated by density. Comparing resting low-density platelets (LDP) to high-density platelets (HDP) revealed higher phosphorylation of proteins in the 47, 31, and 24 kDa ranges. A higher phosphorylation of the 20 kDa protein in LDP compared to HDP was related to an enhanced [Ca2+]i, an increased ADP-ribosylation of the inhibitory G-protein (G(i alpha1-3)) and rhoA, and a decreased ADP-ribosylation of the stimulatory G-protein (G(s alpha)). The differences in the ribosylation patterns of the subpopulations were not influenced by thrombin stimulation or exposure to prostaglandin E1 (PGE1). An 18 kDa phosphoprotein was more highly phosphorylated in resting HDP than in LDP. Thrombin exposure caused dephosphorylation of the 18 kDa phosphoprotein in the HDP, but generally increased phosphorylation of both HDP and LDP in the 47, 31, 24, and 20 kDa bands. Preincubation with prostaglandin E1 or sodium nitroprusside diminished the subsequent thrombin-induced increase in phosphorylation, particularly in HDP. In unstimulated HDP, the 50 kDa VASP phospho form was enhanced, whereas in unstimulated LDP the 46 kDa VASP dephospho form was increased. Our findings suggest that the functional heterogeneity of platelets is partly derived from differences in signal transduction mechanisms reflected in varying phosphoprotein patterns and G-protein properties of platelet stimulatory and inhibitory pathways.

Decreased platelet membrane anisotropy in patients with adrenoleukodystrophy treated with erucic acid (22:1)-rich triglycerides

Low platelet count and bleeding diathesis have been observed in patients with adrenoleukodystrophy (ALD) treated with erucic acid (22:1)-rich triglycerides ("Lorenzo's oil'). To investigate possible alterations of biophysical membrane properties, we measured platelet membrane anisotropy, which is inversely related to membrane fluidity, in 16 patients with and in 3 patients without treatment. In patients on treatment, platelet membrane anisotropy was significantly decreased. Additionally, we found increased platelet concentrations of 22:1 and compromised in vitro platelet aggregation response. The decrease of platelet membrane anisotropy is probably a main cause of bleeding diathesis. Long-term haematological side-effects must be considered in ALD patients treated with Lorenzo's oil.

Involvement of guanylate cyclase and phosphodiesterases in the functional heterogeneity of human blood platelet subpopulations

Cyclic nucleotides are involved in the regulation of platelet activation, shape change, and aggregation. In this study we have investigated the role of guanylate cyclase and phosphodiesterase in three functional heterogeneous human blood platelet subpopulations separated according to density. In low-density platelets aggregation was enhanced and inhibited less when cyclic GMP was increased by sodium nitroprusside, compared to high-density platelets. Low-density platelets possessed a lower basal level of cyclic GMP and exhibited a small increase in cyclic GMP after stimulation with sodium nitroprusside. Cyclic GMP-dependent phosphodiesterase activity was similar in high, low, and intermediate-density platelets. In contrast, the activity of the cyclic AMP-dependent phosphodiesterase was higher in low-density compared to high and intermediate-density platelets. These results suggest that regulation of cyclic GMP and cyclic AMP levels plays an important role in the functional heterogeneity of human blood platelets.

Lowering of body temperature affects human platelet functions and norepinephrine release

The effect of lowering body temperature on plasma epinephrine, norepinephrine, and platelet density distribution and volume was studied in a placebo-controlled double-blind study. Lowering of body core temperature was induced by either exposure to a cold environment at a temperature of 5 degrees C (CT) or by a single dose of the 5-HT1A agonist ipsapirone (IPS). A third group exposed to an ambient temperature of 28 degrees C was given placebo (PLAC). All of the three groups were investigated in a climate chamber. In the CT group the density distribution of blood platelet subpopulations was shifted to an increase in less dense platelets that were more sensitive towards aggregation-inducing agents. The mean platelet volume in this subpopulation was decreased. Epinephrine was not affected, whereas the increase of norepinephrine was correlated with an increase of platelets that were more sensitive to aggregation-inducing agents in the CT group but not in the PLAC and IPS groups.

Increased number of high sensitive platelets in hypercholesterolemia, cardiovascular diseases, and after incubation with cholesterol

The number of low density platelets was found to be increased in patients with hypercholesterolemia, as compared with the number in controls. The percentage increase of the low density platelet subpopulation was even more pronounced in patients with hypercholesterolemia when compared with that in patients suffering from myocardial infarction or angina. In vitro studies with control platelets incubated with cholesterol rich liposomes showed also an increase in the subpopulation of low density platelets. After incubation of control platelets with cholesterol rich liposomes, a higher membrane anisotropy and a higher cholesterol to phospholipid (C/P) molar ratio of the plasma membrane were found. Furthermore, cholesterol-enriched platelets were more sensitive upon thrombin stimulation. The results suggest that a shift of platelet subpopulations to a higher number of low density platelets could be caused by either the level of plasma cholesterol or an in-vitro incubation with cholesterol rich liposomes.

Daily variations of functional parameters and density distribution in human blood platelets

Blood platelets play a critical role in the onset of myocardial infarction, which has been shown to have a circadian rhythmicity with a peak incidence in the morning. In an attempt to correlate platelet parameters with the outcome of cardiovascular diseases, we studied the daily (24-h) variation of the following platelet parameters: distribution pattern of functional heterogeneous platelet subpopulations; serotonin uptake; ketanserin binding; aggregation upon thrombin, serotonin, and ADP stimulation; and platelet count. Furthermore, we analyzed the tryptophan and serotonin concentrations in the blood samples. The percentage of less dense platelets, which represent the subpopulation with the highest preactivation, showed a rhythmicity period of 24 h and an acrophase at 21:18 h. The time course of intermediate and high density platelets was inverse to that of low density platelets. The serotonin uptake exhibited also a rhythmicity with a 24-h period. The acrophase was at 13:50 h. The aggregation curves were inverse to the ketanserin binding curves. The serotonin concentration exhibited a 12-h rhythmicity. The results obtained suggest that (a) changes in platelet activity are reflected by several parameters of platelet function that underlie daily variations; (b) the aggregation curves show a peak in the morning, with an additional peak in the afternoon; and (c) changes in the distribution pattern occur independently from variations in platelet functions like aggregation and serotonin binding.

Plasma membrane properties in heterogeneous human blood platelet subfractions modulate the cellular response at the second messenger level

Three subfractions of human blood platelets differing in density and function exhibited also differences when EPR (electron paramagnetic resonance) spin label measurements and fluorescence polarization were performed. The membrane anisotropy in the plasma membranes was higher in low density platelets as compared with that of platelets of intermediate and higher densities. This higher plasma membrane anisotropy correlated with a significantly higher cholesterol-phospholipid (C:P) molar ratio in the plasma membranes of low density platelets. As compared with the other platelet subfractions the low density platelets exhibited the smallest cAMP increase after activation with the aggregation inhibitor prostaglandin (PGE1), and the highest percentage of inositol phosphate accumulation after thrombin stimulation. The results suggest a high correlation between functional parameters and the plasma membrane architecture of human blood platelets.

Modulation of serotonin binding sites in the brain of the Djungarian hamster, Phodopus sungorus, during adaptation to a short photoperiod

During the physiological adaptation of the Djungarian hamster, Phodopus sungorus, to a short photoperiod in autumn the modulation of specific serotonin (5-HT) binding sites of synaptic membranes was investigated in two brain regions, i.e. cerebral cortex and basal brain (CNS without cerebral cortex, cerebellum, pineal gland, and spinal cord). The radioligands [3H]5-HT and [3H]ketanserin were used to characterize total 5-HT1 and 5-HT2 binding sites, respectively. An increase of 5-HT1 and 5-HT2 binding sites was observed in both brain regions within 14 days after reduction of the photoperiod from a 14:10 h light/dark (l/d) cycle to an 8:16 h l/d cycle. The increase was still present after 56 days of the short photoperiod. Binding kinetics assayed after 4 days of the short photoperiod show that maximal specific binding of [3H]5-HT and [3H]ketanserin was increased, while dissociation constants (KD) were not changed. The membrane anisotropy of synaptic membranes, measured by fluorescence polarization, was reduced transiently during the early part of the adaptation. Neither the phospholipids nor the mole ratio of cholesterol to phospholipids were significantly affected by adaptation to short photoperiod. The results suggest an important role of the central nervous 5-HT system in the physiological adaptation of the Djungarian hamster to a short photoperiod.

Prostaglandin-E1-binding sites in rabbit erythrocyte membranes

Prostaglandin E1 (PGE1) binding sites have been identified on rabbit erythrocyte membranes. The binding of PGE1 to the membranes was found to be highly specific, reversible, and saturable. The high-affinity binding sites had a dissociation constant (Kd.1) of 5.6 +/- 1.2 nM with a binding capacity of 210 +/- 51 fmol/mg protein, whereas the low-affinity binding sites had a dissociation constant (Kd.2) of 22 +/- 6.4 microM, and a binding capacity of 321 +/- 78 pmol/mg protein. Incubation with PGE1 did not activate adenylate cyclase in the membranes. Preincubation of rabbit erythrocyte membranes with physiological amounts of insulin (1.5 nM) resulted in an increase of PGE1 binding to the membranes from 241 +/- 65 to 429 +/- 85 fmol/mg protein. The insulin-induced increase in PGE1 binding was due to an increase in binding sites (both high-affinity and low-affinity binding sites) rather than to an increase in the affinity of the binding sites. Treatment of erythrocyte membranes with PGE1 at concentrations (4.0-7.5 nM) which were within the Kd.1 value of the high-affinity binding sites, resulted in a significant reduction in membrane fluorescence anisotropy (0.27 +/- 0.005-0.21 +/- 0.003). Use of higher concentrations (> 15 nM) of PGE1 reversed the effect of its lower concentration on the membrane anisotropy.

Adhesion efficiency, platelet density and size

We have previously shown that adhesion of human platelets to immobilized collagen is extremely rapid, with initial rates approaching 3% of single particles adhering per 10 ms. Here, we have investigated adhesion efficiency to collagen as a function of platelet density. Platelet subpopulations: low-density (1.040 < d < 1.065 g/ml), intermediate-density (1.065 < d < 1.070 g/ml) and high-density (1.070 < d < 1.080 g/ml) were separated by Percoll density gradient centrifugation. They constituted 24%, 47% and 29% of the total platelet population and had mean volumes of 6.01, 7.37 and 8.21 fl, respectively. Using a continuous-flow, micro-affinity column, we found that the most dense (large) platelets exhibited initial rate of adhesion 4 times greater than the least dense (small) platelets. They were also less sensitive to inhibition by prostacyclin (PGI2). In contrast, there was no significant difference in aggregation induced by high doses of ADP and collagen, indicating that the most dense platelets were not preferentially involved in aggregation induced by high doses of agonists. These results suggest that normal circulating platelets can be distinctly heterogeneous in their ability to adhere to collagen under arterial-flow conditions. The greater efficiency of high-density platelets may be related to increased content of the glycoprotein Ia/IIa (GPIa/IIa) complex.

An independent haemostatic mechanism: shear induced platelet aggregation

We have published (1) evidence indicating that high shearing forces alone applied to platelets expose and activate a unique domain on glycoprotein IIb/IIIa (GPIIb/IIIa) at the platelet surface. In the presence of von Willebrand's factor (vWf) and divalent cations the platelets will aggregate. This paper reviews the extensive literature on high shear effects. It describes a device in which high shear produced by forcing heparinised whole blood through a complex filter normally results in platelet activation; the platelets aggregate and then block the filter. This system is inhibited by antibodies to GPIIb/IIIa and to vWf: fibrinogen is apparently not involved. The same antibodies to GPIIb/IIIa and vWf prevent high shear induced thrombosis occurring in vivo in animal models. The filter blockage is not influenced by aspirin, heparin and ticlopidine and so involves a different mechanism from the aspirin sensitive mechanisms involved in clinical thrombosis prevention in vivo in man. While there are a number of unexplained phenomena in this global test nevertheless this filter model is a simple way of studying a recently recognised pathway which is almost certainly involved in thrombogenesis in man.