Relationships between membrane cholesterol, alpha-adrenergic receptors, and platelet function

Comparison of Thrombocyte Count Between Pediatric Patients With Migraine or Tension-Type Headache: A Retrospective Cohort Study

Hypercoagulability may explain the increased risk of thromboembolic cerebrovascular events in patients with migraine. Thrombocytes play a crucial part in the coagulation process, and some studies have demonstrated hyperaggregation of thrombocytes in adult migraineurs. We aimed to compare thrombocyte count between pediatric patients with migraine or tension-type headache and to evaluate the correlation of thrombocyte count with headache parameters. The electronic database of a tertiary pediatric headache clinic was retrospectively searched for all children and adolescents diagnosed with migraine or tension-type headache in 2016-2018. Data on thrombocyte counts were collected from the medical files and compared between the groups by parametric and nonparametric statistical tests. The cohort included 299 patients, 176 girls (59.0%) and 123 (412.0%) boys, of mean age 12.2 ± 3.4 years; 198 had migraine and 101 had tension-type headache. Among the laboratory parameters evaluated, a significantly lower mean thrombocyte number was found in the migraine group than in the tension-type headache group (282 ± 60 vs 304±71 ×10³/μL, P = .004). Within the migraine group, there was a significant negative correlation between the thrombocyte count and the duration of headache attacks in hours (P < .05). No significant between- or within-group differences were found in other laboratory parameters. The low relative thrombocyte count in pediatric headache clinic patients with migraine and its negative correlation with duration of migraine suggest that migraine may be associated with a different underlying pathogenesis from tension-type headache.

Platelet Function in Aging

Aging is associated with an increased incidence of cardiovascular disease and thrombosis. Platelets play a major role in maintaining hemostasis and in thrombus formation, making them a key player in thrombotic disorders. Whereas it is well-known that platelet aggregability is increased in vascular diseases, the contribution of age-related changes in platelet biology to cardiovascular risk is not well-understood. Several lines of evidence support that platelets from older subjects differ in their function and structure, making platelets more prone to activation and less sensitive to inhibition. These age-related changes could lead to platelet hyperactivity and to the development of a prothrombotic state in advanced age. This review will focus on platelet biochemical modifications during aging and on the mechanisms by which these alterations could lead to thrombotic disease.

Metabolic Profiling Comparison of Human Pancreatic Ductal Epithelial Cells and Three Pancreatic Cancer Cell Lines using NMR Based Metabonomics

Metabolic profiles of hydrophilic and lipophilic cell extracts from three cancer cell lines, Miapaca-2, Panc-1 and AsPC-1, and a non-cancerous pancreatic ductal epithelial cell line, H6C7, were examined by proton nuclear magnetic resonance spectroscopy. Over twenty five hydrophilic metabolites were identified by principal component and statistical significance analyses as distinguishing the four cell types. Fifteen metabolites were identified with significantly altered concentrations in all cancer cells, e.g. absence of phosphatidylgrycerol and phosphatidylcholine, and increased phosphatidylethanolamine and cholesterols. Altered concentrations of metabolites involved in glycerophospholipid metabolism, lipopolysaccharide and fatty acid biosynthesis indicated differences in cellular membrane composition between non-cancerous and cancer cells. In addition to cancer specific metabolites, several metabolite changes were unique to each cancer cell line. Increased N-acetyl groups in AsPC-1, octanoic acids in Panc-1, and UDP species in Miapaca-2 indicated differences in cellular membrane composition between the cancer cell lines. Induced glutamine metabolism and protein synthesis in cancer cells were indicated by absence of glutamine other metabolites such as acetate, lactate, serine, branched amino acids, and succinate. Knowledge of the specifically altered metabolic pathways identified in these pancreatic cancer cell lines may be useful for identifying new therapeutic targets and studying the effects of potential new therapeutic drugs.

Lipid rafts are critical membrane domains in blood platelet activation processes

Among the various hematopoi;etic cells, platelets are critical for maintaining the integrity of the vascular system. They must be rapidly activated by sequential and coordinated mechanisms in order to efficiently prevent haemorrhage upon vascular injury. Several signal transduction pathways lead to platelet activation in vitro and in vivo, among them, several are initiated via receptors or co-receptors containing immuno-receptor tyrosine-based activation motifs (ITAM) which trigger downstream signalling like the immune receptors in lymphocytes. However, in contrast to immune cells for which the role of lipid rafts in signalling has largely been described, the involvement of laterally segregated membrane microdomains in platelet activation has been investigated only recently. The results obtained until now strongly suggest that early steps of platelet activation via the collagen receptor GpVI or via FcgammaRIIa occur preferentially in these microdomains where specific proteins efficiently organize key downstream signalling pathways. In addition, lipid rafts also contribute to platelet activation via heterotrimeric G-protein-coupled receptors. They are sites where the phosphoinositide (PI) metabolism is highly active, leading to a local generation of lipid second messengers such as phosphatidylinositol 3,4,5-trisphosphate. Here, evidence is accumulating that cholesterol-enriched membrane microdomains are part of a general process that contributes to the efficiency and the coordination of platelet activation mechanisms. Here we will discuss the biochemical and functional characterizations of human platelet rafts and their potential impact in platelet physiopathology.

Age-related changes in platelet microviscosity and aggregation in rats

1. To study possible changes in platelet microviscosity in aged animals, 1,6-diphenyl-1,3,5-hexatriene (DPH) was used as a nonpolar probe embedded in thrice-washed platelets from young, adult and aged rats. With the known values of maximum limiting anisotropy and the structural parameter of DPH and by estimating the steady state of fluorescence anisotropy and the average fluorescence of lifetime, we applied the Perrin equation to calculate the microviscosity. 2. We measured platelet aggregation, platelet lipids and platelet polyunsaturated fatty acids to determine any causal relationship between these parameters. Platelet aggregation, the platelet molar ratio of cholesterol to phospholipid ([C]/[PL]) and platelet microviscosity increased with age (P < 0.05) and were correlated with one another (P < 0.05). 3. Age-dependent increases in the steady state of fluorescence anisotropy, order parameters and the short component of fluorescence lifetime of the fluorophore were expressed as functions of variables, such as microviscosity or the [C]/[PL] ratio. 4. Platelet concentrations of arachidonic and eicosapentaenoic acids increased with age, but were not associated with aggregation. Age-related changes in microviscosity and the [C]/[PL] ratio seemed to be determinants affecting biophysical properties of platelet aggregation.

Cholesterol and non-cardiovascular disease: basic science

Cholesterol metabolism is of fundamental biological importance. This review examines the role of cholesterol metabolism in relation to non-cardiovascular disease (non-CVD). Particular attention is paid to the question of whether or not low levels of cholesterol may have harmful effects on cell function or lead to pathological processes. Many in vitro phenomena have been demonstrated at levels of cholesterol which are very low in comparison to physiological conditions. Nevertheless, low cholesterol is more favourable than high cholesterol for most aspects of cell function. There is no evidence that any catastrophic cellular response or pathological process occurs due to exposure of organisms to low cholesterol. On the other hand, the inflammatory process is a powerful and consistent cause of decreased cholesterol levels. This, together with other confounding factors, appears to explain a major component of the association between low cholesterol levels and non-CVD.

Effects of hypercholesterolemia on the nucleotide content in human blood cells

In hypercholesterolemia significant changes in the nucleotide pattern of erythrocytes and lymphocytes as determined by high performance liquid chromatography were found. The decrease in ATP of lymphocytes in hypercholesterolemia from 10.4 +/- 0.3 to 7.0 +/- 0.4 nmol mg-1 protein (n = 8) was associated with an increase in ADP from 2.2 +/- 0.2 to 4.0 +/- 0.2 nmol mg-1 protein (P less than 0.005). The pattern of guanosine phosphates likewise was found to be changed in hypercholesterolemia. Akin to lymphocytes, red blood cells displayed marked changes in nucleotide levels. No such changes were observed in platelets. Cultured lymphocytes incubated with human plasma low density lipoproteins (LDL) (140 mg cholesterol dl-1) displayed a reversible fall in ATP and an increase in ADP by about 40% and 160%, respectively, with high density lipoproteins (HDL) or very low density lipoproteins (VLDL) being essentially ineffectual. It is concluded that in hypercholesterolemia a significant change in the nucleotide pattern of blood cells is exerted by the increase in LDL. Possible pathophysiological implications are discussed.

Modulation of cellular cholesterol and its effect on cornified envelope formation in cultured human epidermal keratinocytes

When cultured human epidermal keratinocytes (NHK) reach confluence they start to differentiate and an increase in the total cellular cholesterol content is observed. This increase parallels the appearance of a characteristic feature of terminal keratinocyte differentiation, the spontaneous formation of cornified envelopes (CE). Synthesis of CE is catalyzed by the plasma membrane-associated transglutaminase (TGm). Supplementation of the medium with inhibitors of cholesterologenesis suppressed increase in cholesterol levels and CE formation but did not interfere with TGm expression or TGm activity. Modulation of the plasma membrane cholesterol-phospholipid ratio of confluent NHK cultures using either pure phospholipid liposomes or liposomes enriched in cholesterol strongly affected spontaneous CE formation. Pure phospholipid liposomes completely inhibited CE formation, whereas cholesterol-enriched liposomes ensured envelope formation, even in the presence of inhibitors of cholesterol synthesis. From these results we conclude that in differentiating NHK an increase in the cellular cholesterol level is part of the differentiation program and is essential for the spontaneous CE formation.

Hemostatic variables in homozygous familial hypercholesterolemia. Effect of regular plasma cholesterol removal by low density lipoprotein apheresis

Plasma levels of tissue plasminogen activator (t-PA) and plasminogen activator inhibitor (PAI) and the in vitro ability of platelets to aggregate and of monocytes to express procoagulant (tissue factor) activity (PCA) were evaluated in five patients who are homozygous for familial hypercholesterolemia (FH) before and after a single and a regular 5-month cholesterol removal by low density lipoprotein (LDL) apheresis. The biweekly procedure resulted in a 25% to 30% reduction (approximately 150 mg/dl) in total and LDL cholesterol (both were greater than 550 mg/dl at the beginning of the study). The basal levels of t-PA antigen and fibrinolytic activity before and after 10 minutes of venous stasis, basal PAI activity, and PAI-1 antigen were comparable to controls and were not affected by LDL apheresis. Likewise, regardless of the cholesterol removal, the PCA of freshly isolated monocytes and that of monocytes incubated with lipopolysaccharide did not differ from control values. Finally, the pre-apheresis sensitivity of platelets to adenosine diphosphate, arachidonic acid, and collagen was 1.5 to 2 times the normal value. This ratio was unchanged throughout the 5-month procedure. We conclude that fibrinolysis and monocyte PCA are normal in FH patients, whereas platelet aggregation is abnormally high, and none of these parameters is significantly affected by a 25% to 30% reduction in total and LDL cholesterol by LDL apheresis. Furthermore, our data suggest that removal of cholesterol from plasma by LDL apheresis is important for gaining insight into the mechanisms involved in the ischemic complications of arteriosclerosis in FH patients.

Effects of the novel antidepressant S-adenosyl-methionine on alpha 1- and beta-adrenoceptors in rat brain

alpha 1- and beta-adrenoceptors were studied ex vivo in the brains of rats receiving repeated daily treatment with the standard antidepressant imipramine or the atypical antidepressant S-adenosyl-L-methionine (SAM), which has minimal effects on monoamine reuptake or turnover. Consistent with past studies, a decrease in the density of beta receptors at three weeks and an increase in the affinity of alpha 1 receptors for the agonist phenylephrine at one week of treatment was observed with imipramine. By comparison, an increase in the density of beta receptors and a decrease in the affinity of alpha 1 receptors for phenylephrine was observed at one week of treatment with SAM. These changes were no longer apparent at three weeks of treatment. The results suggest that treatment with SAM does lead to changes in adrenergic neurotransmission, but that down regulation of beta receptors or increased agonist affinity of alpha 1 receptors may not be necessary for the production of antidepressant effects.

Excess membrane cholesterol alters calcium channels in arterial smooth muscle

We studied the effects of cholesterol enrichment on arterial function by evaluating its effects on 45Ca2+ uptake and tension development in the carotid artery of the rabbit. Arterial segments were enriched with cholesterol in vitro, using media containing liposomes composed of free (unesterified) cholesterol (FC) and phospholipid (PL) in a 2:1 molar ratio. Control segments were simultaneously perfused with 0.5:1 liposomal medium to compare the possible effects of PL. Rings from these arteries were then tested for basal and activated Ca2+ uptake and for contractile responses to norepinephrine (NE) and KCl. We found elevated 45Ca2+ uptake under basal and NE-activated conditions along with an increased contractile sensitivity (4-fold) to NE. These alterations correlated with a 78% increase in the FC/PL ratio reflecting cholesterol enrichment of cellular membranes. Cholesterol enrichment did not alter resting or maximal tensions, K+-activated Ca2+ uptake, or contractile sensitivity to K+. Pretreatment with 1 microM diltiazem abolished the cholesterol-induced increase in basal as well as NE-activated 45Ca2+ uptake but had no effect on either uptake in control vessels. These studies suggest that excess membrane cholesterol selectively increases NE contractile sensitivity by increasing basal or NE-activated Ca2+ influx (or both) as a result of fundamental alteration in the calcium channels in arterial smooth muscle cell membrane.

Sterol depletion reduces receptor-mediated low-density lipoprotein binding in NS-1 mouse myeloma cells

NS-1 mouse myeloma cells, a cholesterol auxotrophic cell line with a lesion in the cholesterol biosynthetic pathway at the demethylation of lanosterol to C-29 sterol, were depleted of cholesterol by incubation in cholesterol-free medium for 24 to 48 h. The low-density lipoprotein receptor activities in untreated and in cholesterol-depleted cells were then compared. The cholesterol-depleted NS-1 cells consistently exhibited a 75 to 90% reduction in receptor-mediated low-density lipoprotein binding compared to untreated cells. The decline of the low-density lipoprotein binding of cholesterol-free medium-incubated NS-1 cells was prevented by addition of free cholesterol or its biosynthetic intermediate, demosterol, to the medium. The addition of lanosterol, an intermediate upstream to the lesion site in the cholesterol biosynthetic pathway, was completely ineffective. The results indicate that proper membrane cholesterol content is necessary for the maintenance of normal low-density lipoprotein receptor function in NS-1 cells.

Increased severity of acute myocardial ischemia in experimental atherosclerosis

We investigated the effects of acute myocardial ischemia (MI) in a rabbit model of atherosclerosis to determine whether atherosclerosis augments the severity of damage produced in the ischemic myocardium. Normal rabbits were fed a control rabbit chow diet or a diet enriched with either 2% cholesterol or 0.5% cholesterol for 10-12 weeks prior to induction of MI. Plasma cholesterol concentrations in the cholesterol-fed rabbits were 1697 +/- 70 mg/dl (2%) and 1056 +/- 51 mg/dl (0.5%) vs. 61 +/- 12 mg/dl for the non-cholesterol-fed rabbits. All rabbits were observed for 5 h following induction of MI or sham MI. At the conclusion of the experiment, tissue biopsies from the MI region and non-MI (NMI) regions were taken and analyzed for two indicators of the severity of MI--myocardial creatine kinase (CK) activity and free amino-nitrogen concentration. Atherosclerosis was confirmed histologically in coronary artery and aortic specimens. No difference was found among any group with respect to heart rate (HR), mean arterial blood pressure (MABP), or pressure-rate index (HR x MABP/1000, a measure of myocardial oxygen demand). Myocardial CK loss (NMI - MI) was significantly greater for the 2% and 0.5% cholesterol groups (7.3 +/- 1.3 and 4.9 +/- 0.7 IU/mg protein, respectively, P less than 0.05) than in the nonatherosclerotic group (2.5 +/- 0.4 IU/mg protein; P less than 0.001 for 2% and P less than 0.01 for 0.5%). Increased severity of MI was confirmed by a significantly greater myocardial loss of free amino-nitrogen (NMI - MI) in the two atherosclerotic groups.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of acute hypercholesterolemia on myocardial infarct size and the no-reflow phenomenon during coronary occlusion-reperfusion

The goal of this study was to determine the effects of acute hypercholesterolemia on the evolution of myocardial infarction in a preparation of coronary occlusion-reperfusion. New Zealand white rabbits were fed a 2% cholesterol-enriched diet for 3 days (plasma cholesterol 329 +/- 70 mg/dl), or maintained on the control diet (plasma cholesterol 67 +/- 12 mg/dl). Temporary (30 min) coronary artery occlusion was performed in open-chest rabbits with a suture snare. The snare was released to permit reperfusion. When the animals were killed 5.5 hr later, left ventricles were cut into 3 mm slices. Infarct size was determined by planimetry of tetrazolium-stained slices while the area at risk of infarction (hypoperfused zone) was determined by planimetry of the "cold spots" on autoradiograms of the slices that contained 99m Tc-labeled microspheres that had been injected 1 min after occlusion. Infarct size, expressed as percent of the hypoperfused zone, was 42.8 +/- 1.3% (n = 10) in the control group and was increased by approximately 100% in cholesterol-fed animals to 83.7 +/- 2.0% (n = 10, p less than .001). To test the hypothesis that vascular obstruction (no reflow) might account for the larger infarct size, thioflavin S was injected immediately before the animals were killed to demarcate perfused myocardium in three additional groups of animals: standard chow-fed rabbits (n = 5), cholesterol-fed rabbits (n = 5), and standard chow-fed rabbits that, in addition, received an infusion of isoproterenol (0.1 microgram/kg/min, n = 6), an intervention believed to increase infarct size through a mechanism not dependent on the no-reflow phenomenon.(ABSTRACT TRUNCATED AT 250 WORDS)

Megakaryopoiesis disturbances in atherosclerotic rabbits

Rabbits given a hypercholesterolemic diet (500 mg/day) for 6 months and then maintained for another 6 months on a normal diet were found to have developed fibrous lipidic lesions in the aorta. Although circulating platelet levels in these animals were normal there was a reduction in mean megakaryocyte ploidy. The high concentrations of megakaryoblasts in all the sedimentation fractions collected by the 'STAPUT' system suggested an increase in megakaryocyte turnover with activation of committed stem cells. In addition, other defects in maturation of megakaryocytes were observed, such as abnormalities in the demarcation membrane system and granule number. These data reveal that defects in megakaryocyte maturation and turnover may occur during the process of reparative fibrosis of the arterial tree following a period of moderate hypercholesterolemic diet in the rabbit.

Platelet activation in normo- and hyperlipoproteinemias

In the last few years it became obvious that platelets are involved in the development of atherosclerotic diseases. This involvement of platelets has been taken into account in the "response to injury" hypothesis of atherosclerosis. The hypothesis is based on the assumption that atherosclerotic lesions result from endothelial injury, followed by the interaction of vessel wall constituents with lipoproteins, macrophages, and platelets. In the first part of this review, general aspects of platelet activation are summarized and the pathways of platelet aggregation as well as their involvement in blood coagulation are discussed. The second part of this paper describes the influence of cholesterol, lipoproteins, and apolipoproteins upon the activation and metabolic behavior of platelets. Physiological and pathophysiological processes particularly occurring in different types of hyperlipoproteinemias and atherosclerotic disorders are discussed in this context.

Enzymatic reconstitution of brain membrane and membrane opiate receptors

A new method using lysophosphatide and acyl-CoA as detergents has been used to solubilize the rat brain opiate receptor. After solubilization, lysophosphatide and acyl-CoA can be almost completely removed by an enzymatic reaction that uses an acyltransferase from rat liver microsomes and reconstitutes the solubilized receptor in membranous vesicles. Morphological studies performed with negative staining and freeze-fracture electron microscopy revealed that the general appearance and intramembrane particle distribution of fracture faces in the reconstituted membrane are similar to those of the native membrane; this indicates that hydrophobic protein components of the original membrane were incorporated during reconstitution. Reconstituted membrane, however, contained higher levels of phosphatidylcholine and lower levels of cholesterol. The activities of the membrane-bound enzymes Na+, K+-ATPase and Ca2+, Mg2+-ATPase in the reconstituted system were 24 and 3%, respectively, those of the native membrane. Although binding of opiate ligands to the reconstituted membrane was stereospecific and saturable, higher concentrations of some of the unlabeled ligands were required to inhibit binding of the radiolabeled ligands. These changes in receptor characteristics are likely due to changes in lipid composition, physical state, and/or distribution of the lipids in the reconstituted membrane bilayer. This conclusion is supported by an increase in the affinity of opiate ligands for reconstituted membrane after adjustment of the latter's lipid composition to match more closely that of the original membrane. This was accomplished by treatment with phospholipid exchange protein to remove the excess phosphatidylcholine and by incorporation of cholesterol into the reconstituted membrane.

A cell membrane correlate of tardive dyskinesia in patients treated with phenothiazines

Phenothiazine administration to psychiatric patients is associated with an increase in the "structural order" of platelet membranes as determined by steady-state fluorescence polarization measurements with 1,6-diphenyl-1,3,5-hexatriene (DPH), a fluorescent probe that localizes preferentially in the hydrocarbon region of cell membranes (Zubenko and Cohen 1984, 1985a, b). In this study, platelet membranes prepared from a group of psychiatric patients who developed tardive dyskinesia following chronic treatment with phenothiazines exhibited a significant elevation in DPH fluorescence polarization when compared to similar preparations from an otherwise matched group of patients who had no symptoms or history of tardive dyskinesia. The distribution of polarization values obtained for the tardive dyskinesia group displayed minimal overlap with that of an unmedicated, psychiatrically-healthy control group matched for age and gender. The fluorescence polarization of DPH-labelled platelet membranes was not significantly correlated with phenothiazine daily dose or serum cholesterol concentration in the phenothiazine-treated patient groups, or with dyskinesia severity (AIMS rating) in the tardive dyskinesia group. Patient gender and the presence of an affective disorder did not significantly correlate with DPH fluorescence polarization. The potential physiological and clinical significance of these findings is discussed.

Aging and the biophysical properties of cell membranes

Fluorescence spectroscopy was used to examine the biophysical characteristics of human platelet membranes as a function of subject age. The structural order of membrane lipid domains was determined with the use of 1,6-diphenyl-1,3,5-hexatriene (DPH), a fluorescent probe that preferentially localizes in the hydrocarbon core of synthetic and biological membranes. Over the age range of subjects examined (17 to 86 years) the structural order of platelet membranes, as reflected by the steady-state fluorescence polarization of DPH, increased substantially. The magnitude of the observed increase in membrane structural order is sufficient to affect membrane-related cell functions including platelet aggregation. A major contributor to the increase in structural order of platelet membranes may have been an increase in the concentration of cholesterol in serum and tissue with age. The changes observed here in platelet membranes may be a general phenomenon of aging, as changes of similar type and magnitude have been observed in lymphocyte membranes and brain with age in other studies.

Cholesterol modulation of beta-adrenergic receptor characteristics

Cholesterol, a major structural component of plasma membranes, has a profound influence on cell surface receptor characteristics and on adenylate cyclase activity. beta-Adrenergic receptor number, adenylate cyclase activity, and receptor-cyclase coupling were assessed in rat lung membranes following preincubation with cholesteryl hemisuccinate. beta-Adrenergic receptor number increased by 50% without a change in antagonist affinity. However, beta-adrenergic receptor affinity for isoproterenol increased 2-fold as a result of an increase in the affinity of the isoproterenol high-affinity binding site. The increase in agonist affinity did not potentiate hormone-stimulated adenylate cyclase activity, which decreased 3-fold following cholesterol incorporation. However, the ratio of isoproterenol to GTP-stimulated activity was unchanged with cholesterol. Stimulation distal to the receptor by GTP, NaF, GppNHp, Mn2+ and forskolin also demonstrated 50-80% reduced enzyme activity following cholesterol incorporation. These data suggest that membrane cholesterol incorporation decreases catalytic unit activity without affecting transduction of the hormone signal.

The differentiation of avian skeletal muscle in culture: changes in responsiveness of adenylyl cyclase to prostaglandin E1 and adrenergic agonists

The responsiveness of adenylyl cyclase during avian myogenesis in vitro has been examined. Measurements of cyclic AMP generation in intact cells revealed that the precursor myoblast is highly responsive to prostaglandin E1 (11-fold maximum stimulation); whereas its response to isoproterenol is much smaller (2-fold). From the onset of terminal differentiation, responsiveness to the beta-adrenergic agonist increases progressively, reaching a 5.5-fold maximum response by 96 hr of culture. In contrast, there is little change in the cell population's responsiveness to prostaglandin E1. The rise in catecholamine responsiveness is consistent with previously reported increases in beta-receptors accompanying differentiation. DL-propranolol blocks the response of myoblasts and myotubes to 10(-6) M isoproterenol with the same half maximal inhibition value of 1 X 10(-8) mol. The results also suggest a change in the adrenergic character of the receptors and/or coupling to adenylyl cyclase as myoblasts differentiate. First the alpha-adrenergic antagonist phentolamine (10(-7)-10(-4) mol) inhibits the myoblast's response but enhances that of the myotube. Second, the potency ratios for the responses to isoproterenol, epinephrine, and norepinephrine shift from 1.1:1.0:1.0 in the myoblast to 3.3:2.1:1.0 in the myotube. The findings are discussed with reference to the role of prostaglandins in the positive control of muscle differentiation and the changes in the catecholamine-responsive adenylyl cyclase system as an aspect of the expression of the muscle phenotype.

Adenylate cyclase and a fatty acid spin probe detect changes in plasma membrane lipid phase separations induced by dietary manipulation of the cholesterol:phospholipid ratio

Rats fed with a cholesterol supplement to their diet exhibited an increase in their plasma membrane cholesterol phospholipid (C/P)-lipid molar ratio from 0.72 to 0.98, whereas those fed the hypocholesterolaemic drug clofibrate in their diet exhibited a decrease in this ratio to 0.62. The properties of these membranes were analysed with regard to ligand-stimulated adenylate cyclase activity and the mobility of a fatty acid spin probe which allowed lipid phase separations to be identified. Membranes with elevated C/P ratios exhibited two distinct lipid phase separations, one at around 36 degrees C that was attributed to the external half of the bilayer and one at around 22 degrees C which was attributed to the inner half of the bilayer. Membranes with lowered C/P ratios exhibited a single lipid phase separation occurring at around 21 degrees C which was attributed to the lipids of the inner half of the bilayer. These results were compared with those obtained by manipulation of C/P ratios in vitro using liposome-cholesterol exchange techniques. Dietary manipulation of the C/P ratio of plasma membranes in vivo led to alterations in the fold stimulation of adenylate cyclase by various stimulatory ligands.

Effects of phenothiazine treatment on the physical properties of platelet membranes from psychiatric patients

Exposure of normal platelet membranes in vitro to clinically relevant concentrations of phenothiazines has been reported to result in an increase in the structural order of the hydrocarbon region of these membranes (Zubenko and Cohen 1984b,c). In this study, the order parameters (reciprocal of fluidity) of platelet membranes prepared from psychiatric patients receiving phenothiazine antipsychotic drugs were compared to those of an otherwise matched population of psychiatric patients who were not receiving medications that significantly affect this cell membrane property in vitro (Zubenko and Cohen 1984b,c). Membrane order was determined ex vivo by steady-state fluorescence polarization measurements employing 1,6-diphenyl 1,3,5-hexatriene, a fluorescent probe that localizes preferentially in the hydrocarbon region of cell membranes. Phenothiazine administration was associated with a significant increase in platelet membrane order and most likely results from a direct action of these agents on cell membranes. The platelet membrane order parameters of phenothiazine-free patients did not differ significantly from those of a matched group of unmedicated, normal controls. Platelet membrane order was not correlated with daily phenothiazine dose or serum cholesterol concentration. Patient gender did not significantly affect platelet membrane order in any of the groups studied. The potential physiological and clinical significance of phenothiazine-induced increases in cell membrane order is discussed.

The effect of hypercholesterolemia on guinea pig platelets, erythrocytes and megakaryocytes

This study has examined the effect of diet-induced hypercholesterolemia on guinea pig platelets, erythrocytes, megakaryocytes and plasma. The cholesterol/phospholipid ratios of plasma and erythrocytes began to increase after one day on the diet and increased steadily for two weeks and more slowly thereafter until 30 days. In contrast, the cholesterol/phospholipid ratio of platelets remained constant for 4-5 days, then increased until reaching a maximum of about 0.85 in two weeks. Thus, the time-course for increase of the cholesterol/phospholipid ratio is different for platelets than for erythrocytes and plasma. The increase in the cholesterol/phospholipid ratio of megakaryocytes was small and not dependent on the degree of increase in the plasma cholesterol/phospholipid ratio. The cholesterol esters of both platelets and megakaryocytes increased with time for two weeks. The increase in megakaryocyte cholesterol esters appeared to precede that of platelets. The protein content of platelets and megakaryocytes and average megakaryocyte size were increased. Normal platelets incubated in plasma from hypercholesterolemic guinea pigs did not accumulate excess cholesterol, but erythrocyte cholesterol increased 45% in 6 h under the same conditions. Cholesterol synthesis in megakaryocytes was depressed 50-80% by cholesterol feeding and by in vitro incubation of the cells in hypercholesterolemic plasma. The data suggest that the platelets and erythrocytes may accumulate excess cholesterol by different mechanisms. The effects of cholesterol feeding on megakaryocytes and the lag in accumulation of cholesterol in platelets relative to erythrocytes and plasma suggest that a defect in the megakaryocyte may be a primary determinant of accumulation of cholesterol in platelets.

Decreased number of PGD2 binding sites on platelets from patients with type IIa hyperlipoproteinemia

Platelets from patients with familial hypercholesterolemia (type IIa hyperlipoproteinemia), a condition associated with a high prevalence of atherosclerosis and its ischemic complications, are claimed to be hyperresponsive to aggregating stimuli. We investigated the platelet responsiveness to and the binding of PGD2, a potent endogenous inhibitor of platelet aggregation via stimulation of adenylate cyclase, in a group of 7 patients affected by IIa hyperlipoproteinemia (IIa HLP) and in a control group of 10 healthy subjects. Inhibition by PGD2 of ADP-induced platelet aggregation was significantly lower in IIa HLP patients than in controls. The number of binding sites for PGD2 of platelets from IIa HLP patients was significantly reduced in comparison with that from controls (93 +/- 19 and 232 +/- 23 receptors/platelet, respectively), whereas the affinity for PGD2 was comparable to that of controls (Kd = 68.8 +/- 19.8 nM in patients and 66.1 +/- 15.9 nM in controls). The reduced number of platelet PGD2 binding sites in IIa HLP patients may account for the impaired sensitivity to PGD2 shown in vitro by platelets and may contribute to the increased tendency to thrombotic manifestations observed in IIa HLP.

Effects of psychotropic agents on the physical properties of platelet membranes in vitro

Normal platelet membranes were exposed in vitro to a variety of psychotropic medications commonly used in the treatment of patients with psychiatric disorders. Changes in structural order at the hydrocarbon region of the drug-exposed membranes were determined by steady-state fluorescence polarization measurements employing the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH). Chlorpromazine, an aliphatic phenothiazine, produced a significant increase in DPH fluorescence polarization at concentrations from 2-200 microM. Thioridazine, a piperidine phenothiazine, and three piperazine derivatives, perphenazine, trifluoperazine, and fluphenazine, produced significant increases in this parameter at concentrations from 20-200 microM. The other agents tested, including thiothixene, lithium, antidepressants, anxiolytics, and anticonvulsants, were without effect in the concentration ranges examined. The phenothiazine-induced increase in DPH fluorescence polarization apparently depended on the structure of the phenothiazine nucleus; changes in side-chain structure appeared to modulate this effect, most likely by altering the inherent membrane solubility of the agents.

The effect of age and cholesterol on the rat lung beta-adrenergic system

To assess the influence of membrane lipid composition on beta-adrenergic receptor number and adenylate cyclase activity in aging, we investigated the effect of cholesteryl hemisuccinate on these parameters in lung membranes of 3-, 12-, and 24-month-old CDF (F-344) rats. When cholesteryl hemisuccinate (0.5 mg/ml) was incubated with lung membranes, beta-adrenergic receptor density was increased by 70%. This effect was the same for each age group studied and indicated that the density of both basal and CHS-sensitive receptors is unaltered in rat lung with age. Forskolin, NaF, p[NH]ppG, and isoproteronol-stimulated adenylate cyclase activity is 30% lower in lung membranes from aged rats. Since enzyme activity is affected by the lipid environment and membrane composition often changes with age, we assessed adenylate cyclase activity following cholesteryl hemisuccinate incorporation. There was up to a 75% decrease in adenylate cyclase activity following cholesteryl hemisuccinate incorporation in lung membranes in each of the three age groups. In untreated membranes, there was no significant difference in cholesterol or lipid phosphate content with age. These data suggest that cholesterol content does not account for alterations in senescent rat lung adenylate-cyclase activity.

Effect of lithocholic acid on cholesterol synthesis and transport in the rat liver

The cellular origin of cholesterol which accumulates in liver cell plasma membrane fractions enriched in bile canalicular structures after lithocholic acid treatment was determined in vivo. Rats were given [3H]cholesterol followed 16 h later by [2-14C]mevalonic acid, [2-14C]acetic acid or lithocholic acid. Lithocholic acid injection enhanced the de novo synthesis of cholesterol in the microsomes and both compounds were transported to the bile canalicular membranes. However, in vitro studies demonstrated that lithocholic acid is capable of stripping cholesterol from microsomal membranes even in the absence of increased de novo synthesis. This suggests that transfer of cholesterol from subcellular organelles (microsomes) to bile canalicular membranes may be the initial step in the development of lithocholic acid-induced cholestasis.

The alpha 2-adrenergic system of the platelet in cystic fibrosis

The ability of norepinephrine to inhibit prostaglandin E1 (PGE1)-stimulated accumulation of adenosine 3':5' cyclic monophosphate (cyclic AMP) in intact washed platelets was determined in 12 patients with cystic fibrosis, 6 parents of patients with cystic fibrosis, and a total of 21 healthy age-matched controls. Patients with cystic fibrosis and their parents did not differ from their age-matched controls in basal or PGE1-stimulated levels of cyclic AMP, nor in the dose dependent inhibition of cAMP accumulation by norepinephrine. Moreover, binding sites for [3H]-dihydroergocryptine were present in normal numbers and had normal ligand affinity in platelet membranes from patients with cystic fibrosis. In all measures tested, the alpha 2-adrenergic system in the platelet was normal in cystic fibrosis.

The effect of low-density lipoproteins on the synthesis of cyclic nucleotides induced by prostacyclin in isolated platelets

Isolated platelets are strongly sensitized by the presence of low-density lipoproteins (LDL) so that they aggregate with very low concentrations of other agonists or exhibit spontaneous aggregation. Prostacyclin (PGI2) is a potent inhibitor of aggregation, but its action was reversed by LDL. This effect of LDL was accompanied by a decrease in the synthesis of cyclic AMP induced by PGI2, but its efficacy depended on the relative concentrations of LDL and PGI2. PGI2 also enhanced the synthesis of cyclic GMP, but this was completely reversed by the presence of LDL. LDL did not remove inhibitory prostaglandins, e.g. E1, from their receptor sites. The lipoproteins also decreased cyclic AMP synthesis induced by forskolin, which has its effect on the GTP-sensitive protein or the catalytic unit of the adenylate cyclase enzyme complex. It is proposed that LDL may act on the enzyme catalytic unit via an inhibitory GTP-sensitive protein or by a separate mechanism which indirectly impedes the production of cyclic AMP.

Human platelets are defective in processing of cholera toxin

Cholera toxin is unable to elevate cyclic AMP levels in intact human platelets despite being very efficacious in this respect in other mammalian cells; in the presence of 0.5 mM-isobutylmethylxanthine, we found that 3-6nM-cholera toxin over 3h at 37 degrees C elevated platelet cyclic AMP from 33 +/- 13 to 39 +/- 12pmol/mg of protein (means +/- S.D.; n = 12). We have investigated the basis for this lack of response. 125I-labelled cholera toxin bound to platelets both saturably and with high affinity (Kd congruent to 60pM; Bmax. congruent to 50fmol/mg of protein). Incubation of platelets with the putative cholera toxin receptor monosialoganglioside GM1 enhanced 125I-labelled cholera toxin binding at least 40-fold but facilitated only a minimal (less than or equal to 3-fold) elevation of platelet cyclic AMP levels. In contrast, dithiothreitol-activated cholera toxin markedly stimulated adenylate cyclase activity in platelet membranes. Platelet cytosol both enhanced stimulation of adenylate cyclase activity by activated cholera toxin (A1 subunit) and supported stimulation by the A1-A2 subunit of cholera toxin. Neither GTP nor NAD+, both necessary for response to cholera toxin, was lacking in intact platelets. However, we found that platelets were unable to cleave cholera toxin to the active A1 subunit (as assessed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis). By contrast, murine S49 lymphoma cells were able to generate the A1 subunit with a time course that closely resembled the kinetics of toxin-mediated cyclic AMP accumulation in these cells. Thus we conclude that human platelets are defective in their ability to process surface-bound cholera toxin. These results indicate that binding of cholera toxin to surface receptors is necessary, but not sufficient, for expression of the toxin effect and the generation of the A1 subunit of the toxin may be rate-limiting for expression of cholera toxin response.

Decreased [20-3H]phorbol-12,13-dibutyrate binding to phospholipase C pretreated epidermal cells

Incubation of mouse epidermal cells (HEL-37) with phospholipase C (Clostridium perfringens) caused about a 50% decrease in the specific binding of [20-3H]phorbol-12,13-dibutyrate. Phospholipase C caused a decrease in the apparent number of binding sites from 2.86 X 10(5) to 1.21 X 10(5) per cell with little effect on ligand affinity. The decrease was observed with purified phospholipase containing no detectable protease activity, and when cells were fixed with glutaraldehyde. The phorbol-12,13-dibutyrate binding capacity of treated cells was recovered within 4 h incubation in complete medium. The results suggest that not all phorbol ester binding sites are equivalent, with differences arising either from varying phospholipid requirements or from membrane localisation.

Elevated membrane cholesterol concentrations inhibit glucagon-stimulated adenylate cyclase

A method was devised which increases the cholesterol concentration of rat liver plasma membranes by exchange from cholesterol-rich liposomes at low temperature (4 degrees C). When the cholesterol concentration of liver plasma membranes is increased, there is an increase in lipid order as detected by a decrease in mobility of an incorporated fatty acid spin probe. This is accompanied by an inhibition of adenylate cyclase activity. The various ligand-stimulated adenylate cyclase activities exhibit different sensitivities to inhibition by cholesterol, with inhibition of glucagon-stimulated greater than fluoride-stimulated greater than basal activity. The bilayer-fluidizing agent benzyl alcohol is able to reverse the inhibitory effect of cholesterol on adenylate cyclase activity in full. The thermostability of fluoride-stimulated cyclase is increased in the cholesterol-rich membranes. Elevated cholesterol concentrations abolish the lipid-phase separation occurring at 28 degrees C in native membranes as detected by an incorporated fatty acid spin probe. This causes Arrhenius plots of glucagon-stimulated adenylate cyclase activity to become linear, rather than exhibiting a break at 28 degrees C. It is suggested that the cholesterol contents of both halves of the bilayer are increased by the method used and that inhibition of adenylate cyclase ensues, owing to the increase in lipid order and promotion of protein-protein and specific cholesterol-phospholipid interactions.

Protein-stimulated enrichment of human platelet membranes in linoleylphosphatidylcholines. Effect upon adenylate cyclase and fluidity

In order to study the effect of linoleyl enrichment of platelet membranes upon adenylate cyclase activity and membrane fluidity, manipulations of platelet phospholipids are carried out with phosphatidylcholine-loaded high-density lipoproteins (HDL) or phospholipid-exchange protein and phospholipid-cholesterol mixed vesicles. Incubation with HDL does not appear to be valuable for this purpose. On the other hand, phospholipid-exchange protein and mixed vesicles can be used successfully. Phospholipid-exchange protein stimulated 3-fold the spontaneous exchange of 2-linoleylphosphatidylcholine between the vesicles and the platelets. Linoleyl enrichment of platelets by dilinoleylphosphatidylcholine is about 25% and by 2-linoleylphosphatidylcholine is about 45-50%. The unsaturation index remains constant when the enrichment is performed using dilinoleylphosphatidylcholine but it increases with 2-linoleylphosphatidylcholine. Basal and prostaglandin E1-stimulated adenylate cyclase activities are not modified by dilinoleylphosphatidylcholine, while they increase significantly in the case of 2-linoleylphosphatidylcholine. There is no significant variation in diphenyl hexatriene fluorescence polarization parameters, either with dilinoleylphosphatidylcholine or with 2-linoleylphosphatidylcholine.

Biochemical pharmacology of paradoxical sleep

The role of noradrenergic cells in the regulation of paradoxical sleep is still controversial, and experimental data have given rise to contradictory interpretations. Early investigations focused primarily on chemical neurotransmissions. However, the process of information transmission between cells involves many other factors, and the cell surface is an important site for transduction of messages into modifications of the activity of postsynaptic cells. alpha-adrenoceptors are believed to play an important role in the control of wakefulness and paradoxical sleep. Experimental evidence suggests that physiological modulation of receptor sensitivity, possibly by specific neuro-modulators, may be a key mechanism in synaptic transmission. In the investigation of the mechanisms involved in paradoxical sleep regulation, lesions of the locus coeruleus have given equivocal results. Collateral inhibition, probably mediated by alpha 2-adrenoceptors, appears to be a powerful mechanism. The exact temporal relationship between noradrenergic cell activation and paradoxical sleep production is not established, but 5-HT appears to be involved. Differences between paradoxical sleep and waking may be related to a physiological modulation of alpha 2-adrenoceptor sensitivity.

[3H]Dihydroergocryptine binding to alpha-adrenergic receptors of human platelets. A reassessment using the selective radioligands [3H]prazosin, [3H]yohimbine, and [3H]rauwolscine

Which subtype(s) of the alpha-adrenergic receptor occurs on human platelets? Studies of platelet responsiveness to adrenergic compounds and indirect radioligand binding studies addressing this question have yielded contradictory conclusions. These bindings studies employed the ligand [3H]dihydroergocryptine ( [3H]DHE), an alpha-adrenergic antagonist that does not select between alpha 1- and alpha 2-adrenergic receptors and that also binds to other receptor types in some tissues. To determine the subtype of the platelet alpha-adrenergic receptor, we have examined the binding to intact human platelets of [3H]prazosin (alpha 1-selective), [3H]yohimbine (alpha 2-selective), and [3H]rauwolscine (alpha 2-selective), and we have compared the binding of these selective radioligands with that of [3H]DHE. [3H]Yohimbine and [3H]rauwolscine both bound with high affinity (Kd = 2.7 and 4.6 nM, respectively) to an equal number and a single class (Hill coefficient approximately 1.0) of sites ( approximately 300 per platelet), but [3H]yohimbine yielded lower nonspecific binding than did [3H]rauwolscine. In paired experiments, [3H]DHE bound to 1.5 times as many (phentolamine-displaceable) sites as did [3H]yohimbine or [3H]rauwolscine. Unlabeled yohimbine and epinephrine competed for fewer [3H]DHE binding sites than did phentolamine. Thus, in addition to binding to the alpha 2-adrenergic receptors identified by [3H]yohimbine and [3H]rauwolscine, [3H]DHE seems to bind to other sites on human platelets. The nature of these sites is not clear. We found that [3H]prazosin did not identify alpha 1-adrenergic receptors on platelets, and that phenoxybenzamine only inhibited [3H]yohimbine and [3H]DHE binding at higher concentrations than usually observed for alpha 1-adrenergic receptors. We conclude that (1) all alpha-adrenergic sites on human platelets are of the alpha 2 subtype, (2) [3H]DHE may bind to additional, as yet ill-defined, sites in addition to those sites identified by [3H]yohimbine and [3H]rauwolscine, and (3) [3H]yohimbine is the preferred antagonist radioligand for studying the alpha 2-adrenergic receptors on human platelets.