Stability of prostaglandin I2 in human blood

Evaluation of the stability, bioavailability, and hypersensitivity of the omega-3 derived anti-leukemic prostaglandin: Δ(12)-prostaglandin J3

Previous studies have demonstrated the ability of an eicosapentaenoic acid (EPA)-derived endogenous cyclopentenone prostaglandin (CyPG) metabolite, Δ¹²-PGJ₃, to selectively target leukemic stem cells, but not the normal hematopoietic stems cells, in in vitro and in vivo models of chronic myelogenous leukemia (CML). Here we evaluated the stability, bioavailability, and hypersensitivity of Δ¹²-PGJ₃. The stability of Δ¹²-PGJ₃ was evaluated under simulated conditions using artificial gastric and intestinal juice. The bioavailability of Δ¹²-PGJ₃ in systemic circulation was demonstrated upon intraperitoneal injection into mice by LC-MS/MS. Δ¹²-PGJ₃ being a downstream metabolite of PGD₃ was tested in vitro using primary mouse bone marrow-derived mast cells (BMMCs) and in vivo mouse models for airway hypersensitivity. ZK118182, a synthetic PG analog with potent PGD₂ receptor (DP)-agonist activity and a drug candidate in current clinical trials, was used for toxicological comparison. Δ¹²-PGJ₃ was relatively more stable in simulated gastric juice than in simulated intestinal juice that followed first-order kinetics of degradation. Intraperitoneal injection into mice revealed that Δ¹²-PGJ₃ was bioavailable and well absorbed into systemic circulation with a C_max of 263 µg/L at 12 h. Treatment of BMMCs with ZK118182 for 12 h resulted in increased production of histamine, while Δ¹²-PGJ₃ did not induce degranulation in BMMCs nor increase histamine. In addition, in vivo testing for hypersensitivity in mice showed that ZK118182 induces higher airways hyperresponsiveness when compared Δ¹²-PGJ₃ and/or PBS control. Based on the stability studies, our data indicates that intraperitoneal route of administration of Δ¹²-PGJ₃ was favorable than oral administration to achieve effective pharmacological levels in the plasma against leukemia. Δ¹²-PGJ₃ failed to increase histamine and IL-4 in BMMCs, which is in agreement with reduced airway hyperresponsiveness in mice. In summary, our studies suggest Δ¹²-PGJ₃ to be a promising bioactive metabolite for further evaluation as a potential drug candidate for treating CML.

The influence of platelets on the promotion of invasion by tumor cells and inhibition by antiplatelet agents

OBJECTIVE

Using a chemoinvasion assay, we show that platelets promote invasiveness of 5 pancreatic adenocarcinoma cell lines.

METHODS

Gelatin zymography and Western blot analysis were performed to detect metalloproteinase-9 (MMP-9) secreted from tumor cells in the presence or absence of platelets. The effects of antiplatelet agents on the invasiveness of tumor cells and the secretion level of MMP-9 were evaluated.

RESULTS

The number of traversed tumor cells significantly increased when incubated with platelets compared without platelets in all cell lines. The MMP-9 band was detected in all tumor cell lines, and the intensity was obviously greater in conditions of incubation with platelets than without. In the experiment of antiplatelet agents effects, it was confirmed that invasiveness of tumor cells significantly decreased following incubation with cilostazol depending on the concentration in spite of the presence of platelets. The level of MMP-9 also significantly decreased in the ELISA analysis.

CONCLUSIONS

These data mean platelets activate invasiveness of tumor cells because of enhanced MMP-9 secretion. Furthermore, anti-platelet drugs may inhibit invasiveness of tumor cells due to decreased MMP-9 secretion, and this inhibition may lead to the suppression of tumor cell invasion. We propose that antiplatelet agents are applicable in clinical treatment to inhibit metastasis of malignant tumor cells.

Pathogenesis of acute myocardial infarction. Novel regulatory systems of bioactive substances in the vessel wall

Rupture of the lipid-rich atheromatous plaque, intraplaque hemorrhage, and intraluminal thrombus are three pathological hallmarks most commonly recognized in the infarct-related coronary artery at the site of acute myocardial infarction. Rupture of the atheromatous plaque is closely related to but does not fully explain the genesis of occlusive intracoronary thrombus formation and thus the development of acute myocardial infarction. Besides a variety of hematologic disorders, one should emphasize the role of the platelet-derived mediators that promote an environment where thrombosis and vasoconstriction occur, including TXA2, serotonin, ADP, platelet-derived growth factor, tissue factor, and the diminished availability of those natural endogenous substances that inhibit platelet aggregation, such as EDRF, tissue plasminogen activator, and PGI2. PGI2 released from vascular endothelial cells is extremely unstable. Our group provided the first evidence that HDL stabilizes PGI2 through the newly discovered function of Apo A-I, which is associated with the surface of HDL particles and identified as PGI2 stabilizing factor. Decrease in HDL-associated Apo A-I in patients with unstable angina and during the acute phase of myocardial infarction indicates that HDL plays an important role in preventing coronary atherosclerosis and intracoronary thrombus formation by stabilizing PGI2 in addition to the generally accepted biochemical property of HDL to prevent the accumulation of cholesterol by mobilizing free cholesterol from tissues or macrophages. There is also a PGI2 synthesis-stimulating factor in serum that has not yet been identified chemically. EDRF or nitric oxide provides another important regulating system in the vessel wall. Lipoproteins are inhibitors of endothelium-dependent relaxation of rabbit aorta.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical and pharmacological activity of arene-fused prostacyclin analogues on human platelets

Human platelets have been employed as an assay system to evaluate the pharmacological activity of a group of stable, arene-fused prostacyclin analogs. Prostacyclin (PGI2) is a highly active member of the eicosanoid family and is relatively unstable under physiological conditions. Prostacyclin's best characterized activities are those of inhibition of platelet aggregation and relaxation of vascular smooth muscle. These activities are mediated in large part via elevation of intracellular levels of cyclic AMP subsequent to receptor occupation and activation of adenylate cyclase. We previously described the synthesis of a series of arene-fused prostacyclin analogs with stability in aqueous media at physiological pH. Several of these compounds have prostacyclin-like activities, i.e., competitive binding at the platelet prostacyclin receptor, elevation of intraplatelet cyclic AMP levels and inhibition of human platelet aggregation. One compound in particular (11a) demonstrated these activities with potency similar to PGI2, i.e., Kd at platelet receptor of 3.7 nM and IC50 for inhibition of collagen-induced human platelet aggregation in plasma of 2.9 nM.

Regulation of PGI2 activity by serum proteins: serum albumin but not high density lipoprotein is the PGI2 binding and stabilizing protein in human blood

Although previous studies have shown that serum albumin binds PGI2 and protects it from rapid degradation, it remains debatable whether it is physiologically important due to its low binding affinity for PGI2. We were intrigued by the observations of Yui et al. (J. Clin. Invest. 82 (1988) 803-807) which suggested that apo A-I of the high density lipoprotein (HDL) is the "serum PGI2 stabilizing factor". To clarify this, we carried out experiments to determine the binding kinetics and parameters of HDL and albumin purified from normal pooled human serum. Despite the use of multiple binding assays, we could not detect any binding activity in HDL2, HDL3 or nascent HDL preparations, nor could we demonstrate any PGI2 protecting activity by these molecules. By contrast, purified albumin exhibited essentially identical binding parameters as the native serum from which the albumin was purified. The binding activity of various albumin preparations was not due to the contamination of apo A-I. Computer simulation analysis also failed to provide evidence to support the notion that HDL bound and prolonged PGI2 activity. To determine whether physiological concentrations of albumin influence PGI2 binding to platelet receptors, we measured PGI2 binding to platelet membrane in the absence and presence of albumin. Albumin at 40 mg/ml increased the KD of PGI2 binding to the receptors by 2-3 fold. These findings indicate that albumin plays a major role in protecting PGI2 activity and regulating its availability for platelet PGI2 receptors.

Decreased prostaglandin-I2 stability in acute myocardial infarction

It has been demonstrated that under certain conditions the in-vitro half-life of biologically active PGI2 in plasma is extremely shortened, which may result in-vivo in a local haemostatic imbalance. In 36 patients suffering from acute myocardial infarction a sequential change in in-vitro half-life of synthetic PGI2 was therefore studied during 3 weeks. 21 patients admitted turning out not to develop myocardial infarction served as follow-up controls. During and shortly after the acute episode the plasmatic half-life of PGI2 in-vitro was shortened by about 40%, improving continuously thereafter. No certain influence of either risk factors, sex or age could be discovered. A possible influence of various drugs administered in the hospital period has been excluded in 43 patients with proven coronary artery disease. No such changes occurred during acute angina pectoris attack in 12 patients. It remains to be established, whether the short-lasting destabilisation of PGI2 may be an acute disease-associated finding, or an important pathogenetic factor.

Characterization of the interaction between prostacyclin and human serum albumin using a fluorescent analogue, 2,6-dichloro-4-aminophenol iloprost

We synthesized a fluorescent probe, 2,6-dichloro-4-aminophenol iloprost or dichlorohydroxyphenylamide of iloprost (DCHPA-iloprost) by reacting the stable prostacyclin analog, iloprost (ZK 35 374), with 2,6-dichloro-4-aminophenol with a yield of 60%. This probe exhibited an optical spectrum which overlapped with the emission spectrum of the sole tryptophan of human serum albumin (HSA). Energy transfer from the tryptophan residue to the phenol moiety of DCHPA-iloprost was observed. We utilized this donor-quenching phenomenon to quantitate the binding stoichiometry and affinity as well as the association rate of DCHPA-iloprost binding to HSA. As DCHPA-iloprost showed similar binding characteristics similar to those of iloprost and prostacyclin and competed with iloprost for HSA binding sites, we used DCHPA-iloprost as a probe to locate the binding domain of prostacyclin (PGI2) in HSA. The distance between the tryptophan indole and the phenol group of DCHPA-iloprost was estimated to be 15-18 A. Because iloprost binding to HSA was competitive with warfarin and not with free fatty acid, we propose that PGI2 binds to the 'domain 2' of HSA was competitive with warfarin and not with free fatty acid, we propose that PGI2 binds to the 'domain 2' of HSA molecules. A possible molecular mechanism by which HSA reduces the chemical degradation of PGI2 and stabilizes its activity could be derived from this model.

The association between apolipoprotein A-I and prostacyclin binding in human serum

The stability of Prostacyclin in human blood is dependent upon binding to circulating proteins. Although this binding has been considered to be primarily due to albumin, a recent report has suggested that apolipoprotein A-I is responsible. We compared prostacyclin binding to the concentration of albumin and apolipoprotein A-I in several groups of sera with known differences in the ability to bind prostacyclin. Our results indicate a strong correlation with albumin concentration but no correlation between binding and the concentration of apolipoprotein A-I. It appears that in the human circulation albumin concentration is the most important determinant of prostacyclin binding.

Influence of albumin and non-esterified fatty acids on serum prostacyclin binding in pregnancy

We investigated the etiology of the previously documented decrease in serum prostocyclin binding during pregnancy. Addition of albumin to the serum of pregnant women failed to raise binding to non-pregnant levels. Pregnancy serum bound significantly more prostacyclin following the removal of non-esterified fatty acids and the addition of fatty acid free albumin resulted in a rise in binding to non-pregnant levels. We conclude that serum protein prostacyclin binding is affected by both albumin concentration and non-esterified fatty acids.

Serum prostacyclin stabilizing factor is identical to apolipoprotein A-I (Apo A-I). A novel function of Apo A-I

Serum PGI2 stabilizing factor (PSF) was purified from human serum to a single protein with a molecular weight of 28,000 D by SDS-PAGE. Analyses of NH2-terminal sequence (32 residues), COOH-terminal sequence (3 residues) and the composition of amino acids disclosed its homology with human apolipoprotein A-I (Apo A-I), a major apolipoprotein of HDL. Apolipoprotein A-II, C-I, C-II, C-III, D and E, as well as LDL, and VLDL did not possess this activity. The alpha-helix structure of Apo A-I is necessary for the binding of PGI2. HDL and nascent HDL reconstituted from Apo A-I and phospholipid significantly prolonged the half-life of PGI2. PGI2 stabilization by HDL and Apo A-I may be an important protective action against the accumulation of platelet thrombi at sites of vascular damage. The beneficial effect of HDL in the prevention of coronary artery disease may be partly due to this action.

Prostacyclin degradation in patients with quantitative platelet disorders

Plasma prostacyclin (PGI2) degradation rates were measured at 1, 5, 15 and 30 min in a group of patients with platelet quantitative disorders of various pathogeneses, including 13 with thrombocytosis, 16 with thrombocytopenia from impaired production in the bone marrow, 11 with thrombocytopenia from peripheral destruction, and 28 normal, healthy persons. Patients with thrombocytosis had a low PGI2 degradation rate, whereas patients with thrombocytopenia due to impaired production had a high PGI2 degradation rate. Of the patients with thrombocytopenia caused by peripheral destruction, six with idiopathic thrombocytopenia purpura (ITP) had a slow PGI2 degradation in contrast to five with systemic lupus erythematosus (SLE) - four concurrently had cryoglobulinemia - who had a rapid PGI2 degradation. The findings suggest that: (1) a platelet-derived substance in the human plasma may have a PGI2 stabilising activity; (2) presence of cryoglobulin or immune complex in plasma may interfere with PGI2 stability.

Stabilization mechanism of prostacyclin by human serum: an approach by binding kinetics using a stable prostaglandin I2 analogue, iloprost

We used a gel filtration method and a stable prostaglandin I2 (prostacyclin) analogue, iloprost, to study the kinetics of prostaglandin I2 binding by human serum proteins. Binding equilibrium experiments conducted at physiological prostaglandin I2 concentration (nM) yielded a KD of 10(-9) and a capacity of approx. 50 nM for the serum binding protein(s). Kinetic measurements gave a dissociation rate constant of 10(-3) s-1. When binding equilibrium was established at various ligand concentrations ranging from nM to microM, a result indicating an unsaturable binding was obtained utilizing this method. On the other hand, saturation was achieved with a ligand concentration as high as 50-100 microM by another binding method. A KD of 7 X 10(-5) and a capacity of approx. 600 microM was obtained. This apparent discrepancy was resolved by performing parallel experiments using purified human serum albumin samples and serum. It is concluded that the large quantity of serum albumin, approx. 600 microM, in serum may compensate for its low KD (approx. 10(-5] for prostaglandin I2, thus simulating a binding protein with a KD of 10(-9) and a limited capacity. These data offer direct information regarding how prostaglandin I2 is stabilized by serum and is transported to the platelet prostaglandin I2 receptors. There is a strong implication that serum albumin is the major if not the only protein responsible for binding of prostaglandin I2.

Effects of plasma on the platelet antiaggregatory action of prostacyclin in pregnancy

Platelet-poor plasma from 13 nonpregnant, 33 normally pregnant, and eight preeclamptic women was incubated with prostacyclin. The ability of platelet-poor plasma to diminish the antiaggregatory effect of prostacyclin was assessed by measuring this effect on arachidonic acid-induced platelet aggregation. Platelet aggregation greater than 95% in response to arachidonic acid was observed when incubated plasma without exogenous prostacyclin was assayed. When prostacyclin and platelet-poor plasma were preincubated together, subsequent arachidonic acid-induced platelet aggregation measured 9.6% in samples from nonpregnant patients, 40.3% in samples from normally pregnant patients, and 78.2% in samples from patients with preeclampsia. The ability of exogenous prostacyclin to inhibit arachidonic acid-induced platelet aggregation was significantly different in the presence of plasma from nonpregnant, normally pregnant, and preeclamptic women. These observations suggest that a plasma factor may affect the action of prostacyclin during pregnancy.

The single prostacyclin receptor of gel-filtered platelets provides a correlation with antiaggregatory potency of PGI2 mimics

Gel-filtered human platelets (GFP) display only a single binding site for [3H]-PGI2: KD = 61nM, 234 fmol/10(8) platelets (1410 sites/platelet). Platelet-rich plasma (PRP) displays the same receptor density but the KD value increases to 123 nM due to protein binding of PGI2 which lowers its effective concentration. The [3H]-PGI2/GFP binding assay has been used to evaluate the molecular basis of aggregation inhibition for prostacyclin analogs and mimics, three PGE type structures, and PGD2. Antiaggregatory IC50s and radioligand binding IC50s correlate for PGE2, E1, and six PGI2 analogs. PGD2, and to a lesser extent 6-oxo-PGE1, display greater antiaggregatory potency than expected based on PGI2-binding site affinity data.

Stability of prostacyclin in human and rabbit whole blood and plasma

The stability of prostacyclin (PGI2) in whole blood and plasma was studied in vitro by measuring the disappearance rate of labeled prostacyclin during a 37 degrees C incubation. Prostacyclin was assayed using a quantitative chromatographic method. The half-life of PGI2 was 6.3 +/- 0.8 minutes (mean +/- s.d., n = 6) in citrated human whole blood, significantly shorter (p less than 0.001) than the 10.7 +/- 2.3 minute half-life in citrated human plasma (n = 7). Prior freezing and thawing of plasma did not affect the rate of PGI2 hydrolysis. These values, including the prolonged half-life in plasma, were similar in the blood (5.4 +/- 1.8 min, n = 7) and plasma (9.0 +/- 1.9 min, n = 14) of diabetic patients. In plasma samples from patients with thrombotic thrombocytopenic purpura, the half-life of prostacyclin (4.9 +/- 1.0 min, n = 4) was significantly shortened (p less than 0.001) compared to that in plasma from normal volunteers. The stability of prostacyclin in rabbit blood and plasma was also quantified. The PGI2 half-life in citrated rabbit plasma (10.8 +/- 1.1 min, n = 3) was similar to that in citrated human plasma from control subjects. In contrast to the findings in human blood, the half-life of PGI2 in citrated rabbit whole blood (11.7 +/- 3.3 min, n = 4) was not different from the rabbit plasma value. Substitution of EDTA for citrate did not affect the half-life in rabbit blood or plasma.

Binding of prostacyclin by plasma glycoproteins

The binding of prostacyclin (PGI2) to plasma proteins and the resulting increase in PGI2 stability was investigated. Using gel filtration to separate bound and free PGI2, we have found that Cohn Fraction VI can bind PGI2, and retard its hydrolysis to 6-keto-PGF1 alpha (6KPGF1 alpha). The biological activity of the bound PGI2 correlated well with the quantity of bound PGI2, measured as 6KPGF1 alpha by RIA. Fraction VI bound a greater percentage of PGI2 than the other eicosanoids tested (i.e., PGI2 greater than TXB2 greater than LTB4 greater than PGE1 greater than PGF2 alpha). The PGI2 binding activity of Fraction VI was lost after neuraminidase treatment. Our data suggest that Fraction VI glycoproteins may play an important role in the binding and stabilization of PGI2 by plasma proteins.

Prostacyclin and thromboxane in gynecology and obstetrics

The gynecologic and obstetric implications of the smooth muscle-relaxing, antiaggregatory prostacyclin and its endogenous antagonist, thromboxane A2, are reviewed. In addition to the vascular wall and circulating platelets, which are primary sources for prostacyclin and thromboxane A2, respectively, reproductive tissues produce great amounts of these prostanoids, evidently for the regulation of the vascular tone and/or vascular platelet interaction. Several gynecologic and obstetric disorders are characterized by abnormalities in prostacyclin and/or thromboxane A2. In primary menorrhagia the uterine release of prostacyclin is increased, and consequently menstrual blood loss can be reduced with various prostaglandin synthesis inhibitors. Prostacyclin relaxes the nonpregnant myometrium in vitro and may also do so in vivo, although intravenous infusion of prostacyclin has no effect upon the uterine contractility in nonpregnant or pregnant subjects. Patients with pelvic endometriosis may have increased levels of prostacyclin and thromboxane A2 metabolites in the peritoneal fluid. The prostacyclin/thromboxane A2 balance shifts to thromboxane A2 dominance in patients with gynecologic cancer. During pregnancy the production of prostacyclin and thromboxane A2 increases in the mother and fetoplacental tissue. Preeclampsia and other chronic placental insufficiency syndromes are accompanied by prostacyclin deficiency in the mother and in fetomaternal tissues and by an overproduction of thromboxane A2, at least in the placenta. These changes may account for the vasoconstriction and platelet hyperactivity, which are pathognomonic for hypertensive pregnancies. By directing the prostacyclin/thromboxane A2 balance to prostacyclin dominance (by dietary manipulation, administration of prostacyclin and/or its analogues, drugs with prostacyclin-stimulating and/or thromboxane A2-inhibiting action), it may be possible to prevent and/or treat hypertensive pregnancy complications in the future.

Serum prostacyclin binding defects in thrombotic thrombocytopenic purpura

To understand the pathophysiologic significance of abnormal serum prostacyclin (PGI2) binding activities in thrombotic thrombocytopenic purpura (TTP), we evaluated the PGI2 binding characteristics in three chronic TTP sera and 19 normal sera. PGI2 binding by serum was rapid and reversible. The binding activity in TTP sera (22.1 +/- SD, 4.4%) was significantly lower than that of normal sera (42.2 +/- 6.2%). Moreover, the antiaggregating activity and 6-keto-prostaglandin F1 alpha (6KPGF1 alpha) content in the gel filtrates representing the binding peak was proportionally lower in a TTP serum than normal serum. Although normal and TTP sera bound [14C]arachidonate with similar activity, and neither bound [3H]6KPGF1 alpha, there was a difference in prostaglandin E1 (PGE1) binding. Binding of [3H]PGE1 was subnormal in two TTP sera (W.J. and T.G.) and normal in the third (H.S.). Normal serum corrected the binding defects of TTP serum. Interestingly, the mixture of two TTP sera (W.J. and H.S.) mutually corrected their PGI2 binding defects. In addition, although in vivo plasma transfusions improved the PGI2 binding activity of W.J. and H.S., there existed a striking difference in the nature of their response. These observations indicate that there is at least two types of PGI2 binding defects in TTP. Our data indicate that TTP is associated with diminished serum binding of PGI2. This defect may reduce the availability of PGI2 to damaged vascular sites and decrease an important modulator of platelet thrombus formation at times of severe vascular insult.

Does human placenta produce prostacyclin?

We have previously demonstrated that the human placenta possesses potent platelet anti-aggregatory activity. This activity was exhibited only when aggregation was induced by adenosine diphosphate (ADP), but not when induced by adrenaline, ristocetin or collagen. We have also shown that placental extracts degrade ADP. We therefore concluded that the placenta's anti-aggregatory activity, in vitro, was not due to prostacyclin (PGI2) but to an 'ADPase'. In view of some reports claiming that the human placenta produces PGI2, we carried out a series of experiments to establish whether human placental tissue can convert [14C]-arachidonic acid [( 14C]-AA) to 6-oxo-PGF1 alpha, the stable metabolite of PGI2. Tissue from placenta and the membranes did not show any appreciable conversion of [14C]-AA into 6-oxo-PGF1 alpha. This finding was confirmed by radioimmunoassay techniques where the placenta was shown to produce spontaneously only minimal amounts of 6-oxo-PGF1 alpha. We conclude that placental tissue and the fetal membranes do not synthesize a significant amount of PGI2, certainly not enough to account for the potent platelet anti-aggregatory activity of the placenta in vitro. Placental platelet anti-aggregatory activity in vitro, is probably due entirely to ADPase activity.

Reduction of serum prostacyclin stability in ischemic stroke

Prostacyclin is a powerful vasodilator and inhibitor of platelet aggregation that has been implicated to play a role in cerebrovascular disease. Prostacyclin is unstable in aqueous solution and stabilized in serum by binding to an unidentified serum protein as measured by gel filtration. In 15 patients with ischemic stroke we measured the serum prostacyclin binding capacity and the rate of degradation of exogenously added prostacyclin. There was a significant reduction in serum prostacyclin binding capacity and a significant increase in rate of degradation in the patients with ischemic stroke as a whole compared to controls, and in patients with persistent deficits. Decreased serum prostacyclin binding capacity and accelerated rate of prostacyclin degradation in vitro, may reflect an accelerated rate in vivo of prostacyclin degradation, thereby increasing susceptibility to stroke. Since only a small number of patients were investigated, the findings are of a preliminary nature and must be confirmed by further studies with large numbers of patients and appropriate patient controls.

Systemic and cerebral effects of prostacyclin-induced arterial hypotension in the dog

Prostacyclin has strong vasodilating and antiplatelet properties. This study was performed to investigate its potential for producing profound intraoperative hypotension. Five dogs were anesthetized with morphine, nitrous oxide, and oxygen, paralyzed with pancuronium, and ventilated to a PaCO2 of 40 torr. Mean arterial blood pressure (MABP) was lowered to 40 mm Hg with an intravenous infusion of prostacyclin in 0.05 M Tris buffer (average rate of infusion 3 +/- 1 micrograms/kg/min). Blood flow was determined using the radioactive microsphere technique. Measurements were made before and after 20, 40, and 60 minutes of hypotension; and after a 40-minute recovery period. Infusion of prostacyclin reduced MABP 63% while increasing heart rate 51%. Tachyarrhythmias occurred in all dogs, and cardiac index decreased 18%. Myocardial blood flow decreased an average of 29%, cerebral blood flow decreased 30%, cerebellar blood flow decreased 18%, and blood flow in the brain stem and spinal cord was unchanged. Cerebral metabolic rate of oxygen, determined by measuring the oxygen content of the sagittal sinus, was unchanged. Hypotension was easily induced and maintained using prostacyclin, without apparent tachyphylaxis. However, the cardiac changes caused by this drug are more severe than those accompanying hypotension induced by most other agents, and may represent a serious contraindication to its clinical use.

Comparison of the effect of a conventional heparin and a low molecular weight heparinoid on platelet function

In a study comparing the in vitro effects of heparin and a low molecular weight heparinoid (Organon 10172) on aggregation of platelets from normal subjects, we have demonstrated that whereas heparin markedly enhances platelet aggregation induced by other aggregators and inhibits the anti-aggregatory effect of epoprostenol (prostacyclin, PGI2), heparinoid does not produce such effects. The use of heparinoid may thus have a significant advantage over that of heparin in situations where enhanced platelet aggregation is the main factor leading to thrombosis or where heparin treatment is followed by thrombocytopaenia.

Inhibition of indium-111 platelet accretion onto venous thrombi in dogs by prostacyclin

The known platelet anti-aggregant effects of prostacyclin (epoprostenol) suggest that it may have therapeutic potential in conditions in which the platelet plays a pathophysiological role. The growth of venous thrombi is one such condition. We have attempted to determine, in a canine model of fresh venous thrombosis, whether prostacyclin infusion inhibits platelet accretion in vivo and how this in vivo event related to hemodynamic and in vitro platelet anti-aggregant effects. Gamma camera imaging over thrombi for accretion of indium-111-labeled platelets disclosed that prostacyclin, at an infusion rate of 50 ng/kg per min, inhibited platelet accretion in vivo and resulted in a 95 +/- 4% decrease in in vitro adenosine diphosphate-induced platelet aggregation, and a decrease in mean arterial pressure to 86 +/- 4% of pre-infusion values. Step-wise decrements of prostacyclin infusion demonstrated that platelet accretion occurred in vivo at infusion rates of approximately 10-20 ng/kg per min and correlated with an in vitro adenosine diphosphate-induced aggregation of 54 +/- 13% of control values. Thus, prostacyclin, in a dose that causes only a mild decrease in systemic pressure, can completely inhibit platelet uptake onto fresh venous thrombi in the dog, and this inhibition correlates closely with in vitro adenosine diphosphate-induced platelet aggregation. The potential therapeutic implications of these findings are discussed.

Measurement of human venous plasma prostacyclin and metabolites by radioimmunoassay: a reappraisal

Two radioimmunoassays, the first specific for 6-oxo-PGF1 alpha and the second for 6-oxo-PGF1 alpha, 13, 14-dihydro-6-oxo-PGF1 alpha, 13,14-dihydro-6, 15-dioxo-PGF1 alpha and 6-oxo-PGE1 are described. These radioimmunoassays were used to measure levels of immunoreactive 6-oxo-PGF1 alpha and alleged metabolites of prostacyclin in human venous plasma. Procedures for the direct measurement and extraction of plasma 6-oxo-PGF1 alpha are described and the limitations to which radioimmunoassay of plasma 6-oxo-PGF1 alpha is exposed are discussed. Direct measurement of plasma immunoreactive 6-oxo-PGF1 alpha gave maximal levels of 6.1 pg ml-1 and 4 pg ml-1 in the first and second radioimmunoassays respectively. The latter value reflected combined levels of less than 2 pg ml-1 of 6-oxo-PGF1 alpha and cross-reacting metabolites in venous blood. Extraction of human plasma gave 88.7 +/- 2.2% recovery (mean +/- S.E.M. n = 5 experiments) of [3H]-6-oxo-PGF1 alpha and 80 - 86% recovery of exogenous 6-oxo-PGF1 alpha as immunoreactive 6-oxo-PGF1 alpha. Basal plasma levels of extractable immunoreactive 6-oxo-PGF1 alpha were less than 2.5 pg ml-1. Prostacyclin incubated in vitro with blood was recovered in plasma as immunoreactive 6-oxo-PGF1 alpha and confirmed that conversion to metabolites that cross-reacted with the second antibody, in particular 6-oxo-PGE1, did not occur under experimental conditions. Extraction of [3H]-6-oxo-PGF1 alpha from acidified plasma with methanol resulted in formation of a prostanoid that had properties consistent with the methylated hemiketal isomer of 6-oxo-PGF1 alpha. Under radioimmunoassay conditions this prostanoid was less immunogenic than native [3H]-6-oxo-PGF1 alpha and [3H]-6-oxo-PGF1 alpha extracted from plasma using methyl formate. The low levels of plasma 6-oxo-PGF1 alpha which we report questions the validity of clinical studies that previously described altered levels of plasma 6-oxo-PGF1 alpha in pathophysiological conditions. These studies which were based upon measurement of plasma 6-oxo-PGF1 alpha by radioimmunoassay and GC/MS should now be re-evaluated. Preliminary results from this study have been reported elsewhere (1, 2).

Prostacyclin induces a surprising long-lasting motility in quiescent uterine strips (indomethacin-treated) isolated from ovariectomized rats

Dose-response curves for several prostaglandins (PGI2; PGD2; PGF2 and PGE2); BaCl2 or prostaglandin metabolites (15-keto-PGF2 alpha; 13,14-diOH-15-keto-PGF2 alpha; 6-keto-PGF1 alpha and 6-keto PGE1 in quiescent (indomethacin-treated) uterine strips from ovariectomized rats, were constructed. All PGs tested as well as BaCl2, triggered at different concentrations, evident phasic contractions. Within the range of concentrations tested the portion of the curves for the metabolites of PGF2 alpha was shifted to the right of that for PGF2 alpha itself; the curve for 6-keto-PGF1 alpha was displaced to the right of the curve for PGI2 and that for 6-keto-PGE1 to the left. It was also demonstrated that the uterine motility elicited by 10(-5) M PGF2 alpha and its metabolites was long lasting (more than 3 hours) and so it was the activity evoked by PGI2;6-keto-PGF1 alpha and BaCl2, but not the contractions following 6-keto-PGE1, which disappeared much earlier. The contractile tension after PGF2 alpha; 15-keto-PGF2 alpha; 13,14-diOH-15-keto-PGF2 alpha and PGI2, increased as time progressed whilst that evoked by 6-keto-PGF1 alpha or BaCl2 fluctuated during the same period around more constant levels. The surprising sustained and gradually increasing contractile activity after a single dose of an unstable prostaglandin such as PGI2, on the isolated rat uterus rendered quiescent by indomethacin, is discussed in terms of an effect associated to its transformation into more stable metabolites (6-keto-PGF1 alpha, or another not tested) or as a consequence of a factor which might protects prostacyclin from inactivation.

Effect of nonesterified fatty acids on the stability of prostacyclin activity

The half-life of the platelet antiaggregatory activity of prostacyclin (PGI2) is prolonged when PGI2 is incubated in human plasma or in fatty-acid-free human albumin solutions. This "PGI2-protective" effect is diminished when fatty acids are added to the incubate, and the diminution is proportional to the concentration of the nonesterified fatty acids added. Unsaturated fatty acids were more potent in causing this reduction than saturated acids. It is possible that nonesterified fatty acids displace PGI2 from binding sites on the albumin molecule and that they may thus alter the biologic half-life of any circulating PGI2. This observation may partly account for the well-documented association between raised serum nonesterified fatty acid levels and thrombosis/platelet activation.

Platelet-mediated collagen and fibrin retraction: effect of prostaglandins, cyclic AMP, calcium antagonists and N-ethylmaleimide

Collagen gel retraction (CGR) and thrombin-induced fibrin clot retraction (FCR) could be abolished by EGTA and EDTA, trifluoperazine - a calmodulin inhibitor - and by NEM. Both processes appeared insensitive to hydrocortisone (up to 100 microM), indomethacin (up to 60 microM), PGI2 and its stable analogue 6-keto-PGE1 alpha (10 microM). A slight inhibitory effect on both CGR and FCR was provoked by dibutyryl-cAMP and PGD2. PGE1 1 microM strongly inhibited CGR and FCR. Possible interference of the inhibitors with anchorage of collagen and fibrin on platelet surface and with platelet contractile system is discussed.

Effect of vacuum curettage on the concentrations of plasma 6-keto-prostaglandin F1 alpha and serum thromboxane B2

Serial plasma samples collected before and after vacuum curettage followed by methylergometrine injection in 10 women were assayed for 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha). The mean 6-keto-PGF1 alpha concentration was 97.2 (SE 8.8) pg/ml before cervical dilatation. The concentration rose to 128.2 (SE 13.5) pg/ml (P less than 0.10) immediately and to 133.3 (SE 17.8) pg/ml (P less than 0.05) 1 h after curettage and returned to the initial value within 5 h. Neither methylergometrine nor anaesthesia, nor non-gynaecological surgery, caused changes in the level of plasma 6-keto-PGF1 alpha. The capacity of the platelets to produce thromboxane A2 during spontaneous clotting of blood did not change during vacuum curettage, anaesthesia and non-gynaecological surgery, nor after methylergometrine. The evidence suggests that the pregnant myometrium and/or intrauterine tissues capable of generating prostacyclin (PGI2) in vitro may release PGI2 also in vivo.

Increased plasma concentrations of prostacyclin metabolite 6-keto-PGF1 alpha in essential hypertension. Influence of therapy with labetalol

To evaluate the role of the vasoactive prostaglandins prostacyclin and thromboxane A2 in essential hypertension, the stable metabolites 6-keto-PGF1 alpha and thromboxane B2, respectively, were measured in plasma before and after therapy in 7 patients. During the placebo phase, plasma 6-keto-PGF1 alpha levels were significantly greater than normal. Plasma thromboxane B2 levels were not statistically different from those in normal subjects. After intravenous administration of labetalol to the point of blood pressure reduction, neither plasma 6-keto-PGF1 alpha nor thromboxane B2 values changed. With prolonged oral labetalol therapy and concurrent regulation of blood pressure, a significant decrease in plasma 6-keto-PGF1 alpha levels occurred while thromboxane B2 values remained unaltered. Elevation of plasma 6-keto-PGF1 alpha in untreated hypertensive subjects suggests that enhanced vessel wall prostacyclin synthesis may be a protective mechanism to prevent organ damage. As blood pressure is controlled this increase is no longer needed, and prostacyclin generation returns to normal.

Effect of human plasma proteins on stabilisation of platelet anti-aggregatory activity of prostacyclin

Following the demonstration that biological activity of prostacyclin is more stable in plasma than in buffer solutions at physiological pH, we investigated the possibility that a plasma protein may be responsible for this effect. The duration of platelet anti-aggregatory activity of prostacyclin in fatty acid-free human albumin solution was significantly longer than in buffer solutions. The duration of this 'protection' was proportional to the concentration of albumin; alpha, beta, and gamma-globulin preparations had no 'protective' effect. The degree of 'protection' by albumin solutions was lower than that by plasma despite identical albumin concentrations. This discrepancy may in part be explained by the tendency of plasma to become alkaline on standing, a change that would tend to stabilise prostacyclin. The clinical relevance of our findings is discussed.

Isolation of prostacyclin from whole blood

A method was development for the isolation of prostacyclin (PGI2) from whole blood in a fraction suitable for high pressure liquid chromatography (HPLC) separation of PGI2 and 6-keto-prostaglandin F1 alpha (6-K-PGF1 alpha). Prostacyclin was stabilized in whole blood by rapidly raising the pH to 10 with Na2CO3 and cooling the samples to 0 degree C. Under these conditions, 2.9% hydrolysis was observed after 20 min. Reverse phase extraction columns were used to directly extract both PGI2 and 6-K-PGF1 alpha from the alkaline plasma with recoveries of greater than 95% using an acetonitrile/2mM Na2B4O7, pH 10, 40/60 elution solvent mixture. An additional 1.7% hydrolysis was found during the column extraction procedure. Final separation of PGI2 and 6-K-PGF1 alpha was performed with HPLC using an alkaline solvent system. This method is capable of rapidly and efficiently extracting and separating PGI2 and 6-K-PGF1 alpha from whole blood or plasma. It introduces less than 5% hydrolysis of PGI2, thus providing a means of applying highly sensitive 6-K-PGF1 alpha assays to the determination of PGI2 levels in physiological samples.

Accelerated prostacyclin degradation in the thrombotic thrombocytopenic purpura

Plasma prostacyclin (PGI2) degradation rate was measured in a 39-year-old man with chronic thrombotic thrombocytopenic purpura (TTP). His disease responded to plasma exchange, or plasma infusion alone, given at 3-4 week intervals. Plasmapheresis with albumin replacement had an adverse effect. PGI2 degradation rate was measured by incubation of exogenous PGI2 with plasma at 37 degrees C and recording of PGI2 activity after one, five, and fifteen min incubation by measurement of inhibition of platelet aggregation. The PGI2 degradation rate of the patient was significantly higher than that of normal subjects. The degradation rate improved after each plasma treatment and correlated well with clinical improvement. Moreover, the degradation rate of PGI2 could be corrected in vitro by the addition of normal plasma. When the patients plasma was incubated with aortic rings, PGI2 activity was reduced but the level of its inactive end product, 6-keto-PGF1 alpha, was normal. These findings indicate that our patient had normal PGI2 stimulating activity but had an abnormal rate of PGI2 degradation. Accelerated PGI2 degradation which leads to PGI2 deficiency may be important in the pathogenesis of microvascular thrombosis.

Subcellular localization and secretion of factor V from human platelets

Factor V, a plasma protein cofactor necessary for optimal conversion of prothrombin to thrombin, is also present in considerable concentration in blood platelets (9.9 units per 10(9) platelets). Subcellular fractionation by two methods has localized factor V in the alpha granules of unstimulated platelets. ADP and epinephrine cause release of 4.6% and 6.4%, respectively, of the total factor V, a process completely inhibited by cyclooxygenase alkylation by aspirin. In contrast, collagen causes release of 25% of platelet factor V, a process only partially suppressed by aspirin. Secretion of factor V depends on the availability of metabolic energy, because antimycin A, an inhibitor of aerobic metabolism, and 2-deoxyglucose, an inhibitor of anaerobic glycolysis, together almost totally inhibited the secretion of factor V induced by collagen. The data establish that factor V is not normally available on unstimulated platelets but can be secreted from alpha granules upon stimulation with physiological agents such as ADP, epinephrine, and collagen. Because factor V is known to serve as a receptor for factor Xa, the exposure of factor V on platelets consequent to release would accelerate the process of blood coagulation.