The effects of continuous infusions of prostacyclin-Na (epoprostenol-sodium) on platelet counts, ADP-induced aggregation, and cyclic AMP levels in normal volunteers

The effect of a prostaglandin DP-receptor partial agonist (192C86) on platelet aggregation and the cardiovascular system in healthy volunteers

1. PGD2 (DP)-receptors mediate inhibition of platelet aggregation and vasodilatation. If receptor reserve were greater on platelets it might be possible to separate these effects. To determine whether such a difference in receptor reserve exists, we have examined the effects of a highly selective DP-receptor partial agonist 192C86 on platelet aggregation and the cardiovascular system in healthy volunteers. 2. Using an open, dose-escalating study design, four male volunteers received constant rate intravenous infusions of 192C86 for up to 60 min. Ex vivo platelet aggregation to ADP and collagen in platelet-rich plasma (PRP) and whole blood (WB) was studied at baseline, after 15, 30 and 60 min of each infusion and at 180 min post-infusion. Heart rate (HR), systolic and diastolic (DBP) blood pressure were measured at frequent intervals. Adverse experiences were monitored by checklist. Facial flushing was assessed by the volunteer using a visual analogue scale, by an observer using a numerical scale and by full-face colour photographs. Blood was taken for assay of plasma 192C86 concentrations by radio-immunoassay (r.i.a.). 3. 192C86 (0.007-0.058 micrograms kg-1 min-1) inhibited platelet aggregation to ADP and collagen both in PRP and WB in a dose-dependent manner. However, this was always accompanied by a decrease in DBP, increase in HR and facial flushing. Plasma concentrations of 192C86 were at or below the limits of sensitivity of the r.i.a. (0.5 ng ml-1). 4. The highest infusion rate was stopped after 20 min due to symptomatic hypotension on standing.(ABSTRACT TRUNCATED AT 250 WORDS)

Secondary signals mediated by GPIIb/IIIa in thrombin-activated platelets

We have previously found that stimulation of aequorin-loaded platelets by thrombin produced a two-peaked increase in intracellular free calcium concentration ([Ca2+]i), and the development of the second peak of [Ca2+]i was closely related with the aggregation. In this report, we studied the interrelationship between the GPIIb/IIIa complex, aggregation, cytoskeletons and [Ca2+]i of platelets. The pretreatment of the platelets with dihydrocytochalasin B (4 microM), an actin polymerization inhibitor, did not inhibit aggregation and TXB2 production, but did inhibit both actin polymerization and the second peak of [Ca2+]i increase induced by thrombin, suggesting that actin polymerization and the second peak of [Ca2+]i are interrelated. GRGDSP (100 microM), a synthetic anti-adhesive peptide, has already been reported to inhibit platelet aggregation and the second peak of [Ca2+]i induced by thrombin. It also inhibited actin polymerization and TXB2 production, suggesting that aggregation was important for not only the generation of the second peak of [Ca2+]i but also for actin polymerization and TXB2 production. PGI2 (5 nM) did not abolish but only delayed aggregation, TXB2 production, actin polymerization and the second peak of [Ca2+]i increase. These findings suggest that the secondary signals are caused by aggregation (fibrinogen-binding to the GPIIb/IIIa) in thrombin-aggregated platelets, which results in the TXA2 production and the secondary peak of [Ca2+]i increase, and the latter was dependent on actin polymerization.

Prostacyclin and vascular function: implications for hypertension and atherosclerosis

Prostacyclin and endothelium-derived relaxing factor (or nitric oxide) are unstable mediators produced by the vascular endothelium, that are important for local regulation of platelet behavior and blood flow. This review focuses on the basic biochemistry and pharmacology of prostacyclin, its interactions with nitric oxide and nitrovasodilator drugs, and the implications of disturbances in this system for vascular disease, particularly hypertension and atherosclerosis. Prostacyclin and its stable analogs are also finding limited therapeutic applications in preservation of platelet function, pulmonary hypertension, and investigation into the cytoprotective and antiatherosclerotic properties is continuing.

Epoprostenol (prostacyclin) in unstable angina

The purpose of this randomized, double-blind multicenter trial was to investigate the potential therapeutic effect of epoprostenol (prostacyclin, PGI2) in patients with unstable angina, as compared with placebo, and to investigate the safety of this agent. Of the 184 patients enrolled, 28 did not fit the study criteria; of the remaining 156 patients, 30 received prostacyclin in an open-label fashion. In the double-blind portion of the study, 63 patients each received prostacyclin or placebo. The drug or its vehicle was infused intravenously up to 5 ng/kg/min dose for 72 hours with a tapering off period for the last 12 hours. Both treatment groups from the double-blind portion were comparable in regard to the demographic data, length of infusion, and total dose received. There were no significant differences between the placebo and prostacyclin group in the following clinical endpoints: levels of cardiac enzymes throughout hospitalization period (with the exception of lower SGOT level in the prostacyclin group at day 2), and severity of angina (throughout the study), and at the end of the study (day 30). The number of patients who had congestive heart failure, new myocardial infarction, balloon pump insertion, coronary artery bypass grafting, or percutaneous coronary angioplasty was similar in both groups. Similar results in regard to the efficacy endpoints were also apparent in the prostacyclin group that was treated under open-label fashion. There was also no difference in the New York Heart Association (NYHA) functional status at the end of the double-blind study.(ABSTRACT TRUNCATED AT 250 WORDS)

Desensitization of platelets to iloprost. Loss of specific binding sites and heterologous desensitization of adenylate cyclase

The binding characteristics of [3H]prostacyclin and [3H]iloprost ([3H]5-[(E)-(1S,5S,6R,7R)-7-hydroxy-6-[(E)-(3S,4RS) -3-hydroxy-4-methyl-1-octen-6-inyl]-bicyclo[3.3.0]octan-3-yl idene] -pentanoic acid) and platelet adenylate cyclase activities were investigated in platelet-rich plasma preincubated with iloprost. The exposure of platelets to 0.1 microM iloprost (12 h, 20 degrees C) caused a significant loss of iloprost binding sites (P less than 0.01) without causing changes in binding affinity. This loss of specific [3H]iloprost binding was time- and dose-dependent. The reduction of iloprost receptor density was accompanied by an impaired responsiveness of platelet adenylate cyclase to iloprost, prostaglandin D2 and forskolin. In contrast, basal adenylate cyclase activity was not affected by iloprost pretreatment. The diminished response of the enzyme to GTP and NaF pointed to an involvement of the stimulatory guanyl nucleotide-binding protein (Gs) in iloprost-induced heterologous desensitization. Consequently, [32P]NAD+ and cholera toxin were used for the direct labelling of Gs. Platelet membranes desensitized to iloprost incorporated less label into the 45 kD subunit of Gs. These data suggest that the site of action of iloprost for heterologous desensitization of human platelet adenylate cyclase is located on Gs.

Platelet and vessel associated prostacyclin and thromboxane A2/prostaglandin endoperoxide receptors

Synthetic stable analogues of thromboxane A2 (TXA2), cyclic endoperoxides (PGH2) and prostacyclin (PGI2) opened up new opportunities for investigating the mechanisms of action of these compounds. They proved to be useful pharmacological probes for characterizing PGI2 and TXA2/PGH2 receptors. Over the past few years, new synthetic antagonists with high specificity allowed the modulation of biological responses to endogenous eicosanoids. These compounds will, therefore, considerably promote our understanding of the biological function and significance of arachidonate metabolites. The present review summarizes current concepts that have arisen concerning platelet and vascular PGI2 and TXA2/PGH2 receptors, their transmembrane signal transduction, as well as their possible implications in the pathophysiology of cardiovascular disease.

Effects of iloprost (ZK 36374), a prostacyclin derivative, on platelet function after ischaemic exercise in patients with stable angina pectoris

The effect of a chemically stable prostacyclin analogue (Iloprost) on platelet function was investigated in a controlled study in patients with angiographically confirmed stable angina pectoris after ischaemic exercise. In placebo experiments, ADP platelet aggregation was increased after exercise only when measured in whole blood and not in PRP. While plasma thromboxane B2 levels were unchanged, those of 6-keto PGF1 alpha were significantly although transiently increased after exercise. Iloprost displayed a potent antiaggregating activity in PRP and also reversed platelet hyperaggregation occurring in whole blood determinations after exercise. Plasma thromboxane B2 levels were significantly reduced but occasionally a rebound increase occurred 30 min. after end of the infusion. In contrast plasma level of 6-keto PGF1 alpha did not change after Iloprost and its recorded post-exercise increase was counteracted, thus suggesting a negative feed-back mechanism between Iloprost and natural prostacyclin. The data also suggest that degradation of the analogue is probably accomplished through pathways different from those of PGI2.

Per-operative topical administration of ZK 36 374 (Iloprost) acts favorably on patency of small artery anastomoses in rats

Aspirin (group I, 20 mg/kg, 0.5 hour preoperatively, ip, xylocaine (group II, topical administration of 1 ml of a 2% solution), and ZK 36,374 (Iloprost) either pre- or per- or postoperatively (group III, 10 micrograms/kg 0.5 hour peroperatively iv; group IV, topical administration of 1 ml of a solution containing 25 micrograms/ml peroperatively; and group V, 10 micrograms/kg 0.5 hour postoperatively iv) were given to groups of BN female rats in order to improve the patency rate of small artery (less than 0.5 mm) anastomoses. The rats in group VI received saline peroperatively by topical application, this group served as a control. The patency was established by means of arteriography and macroscopical examination. Neither aspirin nor xylocaine improved the number of successful anastomoses. However, Iloprost administered topically, while performing the anastomosis, substantially improved the patency rate when compared with group VI, the control group. In the control group only five out of 21 animals showed patent anastomoses, whereas in group IV (Iloprost, topical administration) seven out of ten operations were successful. The results suggest that Iloprost applied locally could be helpful in clinical microsurgery for elective operations as well as in replantation surgery.

Platelet prostacyclin binding in coronary artery disease

Reduced responsiveness of platelets to prostacyclin, reported in vitro in patients with coronary artery disease, has been thought to be a factor predisposing toward coronary thrombosis and vasospasm as a result of enhanced in vivo release of cyclic endoperoxides and thromboxane A2 by the platelets. In this study, specific binding of prostacyclin to intact platelets was determined in patients with coronary artery disease by direct binding studies using 9-3H-prostacyclin sodium salt. In addition, the inhibitory effect of prostacyclin on primary aggregation induced by adenosine diphosphate and cyclic adenosine monophosphate (cyclic AMP) accumulation stimulated by prostacyclin was examined. Twenty patients with angiographically documented coronary artery disease and stable angina, 8 patients with acute myocardial infarction, 14 healthy volunteers and 10 patients with normal angiograms were studied. In patients with stable angina, binding capacity and affinity of platelet prostacyclin binding sites and prostacyclin-induced cyclic AMP accumulation were not different from those of control subjects. In patients with acute myocardial infarction, however, binding capacity of platelet prostacyclin receptors was significantly reduced (0.69 +/- 0.45 versus 1.35 +/- 0.37 pmol/10(9) platelets, p = 0.001) and the postreceptor response, represented by platelet responsiveness to prostacyclin and prostacyclin-induced cyclic AMP synthesis, was impaired. Because all patients with myocardial infarction were receiving intravenous heparin and nitroglycerin, which might interfere with platelet prostacyclin binding, competition experiments were performed in vitro. Neither heparin (3 to 250 IU/ml) nor nitroglycerin (0.8 to 22 microM) displaced specifically bound 9-3H-prostacyclin. L-Epinephrine in concentrations up to 10 microM also exhibited no competition with specific platelet prostacyclin binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition and subsequent enhancement of platelet responsiveness by prostacyclin in the rabbit. Relationship to platelet adenosine 3',5'-cyclic monophosphate

Methods were developed for measuring changes in platelet sensitivity to a release-inducing stimulus and in platelet cyclic AMP in fresh whole blood samples from rabbits. These techniques permitted detection of the effects of exogenous and endogenous prostacyclin on circulating platelets. In these methods, rabbit platelets were labeled in vitro by incubation with [14C]serotonin and [3H]adenine and then transfused into other rabbits. Release of platelet [14C]serotonin by a standard dose of synthetic platelet-activating factor (40 pmol/ml) and the platelet cyclic [3H]AMP levels were then measured in citrated blood from the conscious animals within 2 min of arterial puncture. Bolus intravenous injections of prostacyclin (1-10 nmol/kg) caused concentration-dependent increases in platelet cyclic AMP after 2 min, which decreased approximately 75% by 5 min, and disappeared after 30 min. Significant inhibition of the platelet release reaction was detected 2 min but not 5 min after injection of 10 nmol of prostacyclin per kilogram. With lower doses, significant enhancement of the release of [14C]serotonin was observed after 5 min. Similar changes in platelet responsiveness and cyclic [3H]AMP were observed after release of endogenous prostacyclin by intravenous injection of angiotensin II (5 nmol/kg); inhibition of the release of [14C]serotonin after 2 min was followed by potentiation after 5 min, though platelet cyclic [3H]AMP remained above control values. In these experiments, the time course of the changes in platelet cyclic [3H]AMP correlated closely with values for blood prostacyclin obtained previously (Haslam, R.J., and M.D. McClenaghan, 1981, Nature [Lond.]., 292:364-366). Prostacyclin also had a biphasic effect on the release of [14C]serotonin when added to citrated blood in vitro, though both the increase in sensitivity to platelet-activating factor and the return of platelet cyclic [3H]AMP towards control values took place more slowly. At all times, addition of platelet-activating factor decreased platelet cyclic [3H]AMP towards but not below the control level observed in the absence of prostacyclin. Our results indicate that although transient increases in platelet cyclic AMP cause an immediate decrease in platelet responsiveness in vivo or in vitro, a period of enhanced platelet sensitivity follows as platelet cyclic AMP falls.

Thrombocytopenia and lupus-like anticoagulant in a patient with peripheral vascular disease: response to infusion of prostacyclin

A 46 year old man with intermittent claudication due to severe peripheral vascular disease had a circulating lupus like anticoagulant (LLAC), thrombocytopenia (79 X 109/1), markedly reduced platelet survival and a normal bone marrow. He was treated with intravenous prostacyclin (PGI2) infusions which resulted in improvement of the patient's exercise tolerance and normalisation of his platelet count (300 X 109/1) and platelet aggregation could then be assessed. The platelets were markedly hyperaggregable and generated supranormal quantities of thromboxane A2. A diagnosis of consumptive thrombocytopenia secondary to peripheral vascular disease and platelet hyperaggregability was made. Despite therapy with aspirin and dipyridamole, gradual and progressive reduction in platelet count followed and his exercise tolerance declined over the next three months. Immunoglobulin prepared from the patient's serum did not inhibit vascular PGI2 synthesis in vitro. To our knowledge this is the first reported case of consumptive thrombocytopenia due to severe peripheral vascular disease and platelet hyperaggregability. PGI2 administration caused a transient resolution of these features which was not sustained by aspirin and dipyridamole.