Effect of prostaglandin E1 on platelet behaviour in vitro and in vivo

Anti-aggregatory effect of prostacyclin (PGI2) in vivo

Prostacyclin (PGI2) when infused intravenously reduced the mortality of rabbits given high intravenous doses of arachidonic acid (AA). Prostaglandins E1 and D2 were ineffective. Indomethacin pretreatment abolished the toxic AA effect. Since the lethal effect of AA is partly due to the formation of platelet aggregates it is concluded that PGI2 is a most potent anti-aggregatory prostaglandin in vivo.

Effect of prostaglandins E1, E2 and F2alpha on neutrophil aggregation

Recently we have found that chemotactic factors stimulate neutrophils in suspension to aggregate. Because of an obvious analogy to platelet aggregation, we examined the influence of three prostaglandins on this process. Prostaglandins E1, E2 and F2alpha alone did not cause aggregation of the neutrophils but were able to partially inhibit the aggregation response induced by the synthetic chemotactic tripeptide, formyl-methionyl-leucyl-phenylalanine. The minimal inhibitory concentrations for prostaglandins E1, E2 and F2alpha were 10(-7), 10(-6) and 10(-5)M, respectively. These results are similar to those found for the prostaglandin-induced inhibition of platelet aggregation. It may be, therefore, that neutrophil aggregation, like platelet aggregation, is modulated by intracellular prostaglandins and other products of arachidonic acid metabolism.

Prostacyclin (PGI2) inhibits the formation of platelet thrombi in arterioles and venules of the hamster cheek pouch

1 Isolated rings of hamster aorta produced an unstable substance which inhibited platelet aggregation in vitro and had the same characteristics as prostacyclin. 2 Prostacyclin inhibited adenosine diphosphate (ADP)-induced aggregation of hamster platelets in vitro. 3 The effects of prostacyclin on ADP-induced platelet thrombi in the microcirculation of the hamster cheek pouch were studied with a television microscope. 4 Prostacyclin caused a dose-dependent increase in the time of iontophoretic application of ADP which was required to induce platelet thrombi formation and embolization in venules (30 to 40 micron diameter). 5 Prostacyclin caused a dose-dependent reduction in the total time during which ADP-induced thrombi were observed following local electrical damage to arterioles (40 to 80 micron diameter). 6 Thrombus formation in venules and arterioles was abolished by 500 ng/ml prostacyclin in the Krebs solution superfusing the hamster cheek pouch. 7 Prostacyclin was approximately twenty times more potent than prostaglandin E1 in preventing thrombus formation in the microcirculation.

Platelet and fibrin(ogen) deposition in the artificial kidney. The influence of haematocrit, fibrin monomer and platelet inhibitors. An in vitro study

The effect of variations in haematocrit (hct) on the retention of 51Cr-labelled platelets and 125I-labelled fibrinogen from normal, heparinized, human blood, circulated in an experimental "mini-Kiil" dialyzing system, was studied. Platelet retention (PR) on dialysis membranes and blood lines was found to increase with increasing hct. When only platelet rich plasma was circulated, a significantly lower PR was noted on the blood lines. Fibrin(ogen) deposition on the same surfaces showed a similar tendency. When 125I-labelled fibrin monomer was added to normal blood, prior to dialysis, fibrin deposition on the membranes and blood lines was of a much higher degree than in the control. However, PR was not significantly influenced by the presence or deposition of the fibrin monomer. Reduced platelet function, caused by incubation with acetylsalicylic acid prior to dialysis, led to less PR on the blood lines, but not on the membranes. The observed difference was, however, not statistically significant (p = 0.06). Pretreatment of platelets with prostaglandin E1, however, was able to reduce PR significantly both on dialysis membranes and blood lines.

Prostaglandins and model aspects of thrombosis

The possible role of prostaglandins (PGs) in thrombosis was determined by examining their effects in models of thrombosis which included platelet aggregation (in vitro), blood flow (in vivo) and thrombus formation (in vivo). It was found that blood flow and thrombosis can be effectively modulated by the various types of prostaglandins produced by the blood and the vascular tissue.

On the interrelationship of prostaglandin endoperoxide G2 and cyclic nucleotides in platelet function

The prostaglandin endoperoxide G2 caused rapid aggregation and relase of ADP and [14C]serotonin in human platelets. Since the presence of the ADP phosphorylating system creatine phosphate/creatine phosphokinase markedly inhibited the aggregation caused by the endoperoxide, this effect seemed to be mediated mainly by ADP, which is instantaneously released by the endoperoxide. The prostaglandin G2 counteracted the increasing effect of prostaglandin E1 on the adenosine 3':5'-monophosphate (cAMP) levels in platelet-rich plasma. This effect of prostaglandin G2 was only observed when ADP was released by the endoperoxide. This finding indicates that the effect of prostaglandin G2 on the cAMP levels in platelet-rich plasma is principally mediated by ADP. The rapid release of ADP by prostaglandin G2 and the time courses for the effects of the endoperoxide and ADP on the level of cAMP give further evidence for this hypothesis. ADP also caused primary aggregation in the presence of indomethacin, and prostaglandin synthesis inhibitors did not influence the decreasing effect of ADP on the cAMP levels. N2,O2-Dibutyrylguanosine 3':5'-monophosphate did not influence the aggregation and release-reaction caused by ADP and no changes of the cGMP levels were observed after addition of prostaglandin G2.

Metabolism of 8,11,14-eicosatrienoic acid in human platelets

The following labeled compounds were isolated and identified after incubation of 8,11,14-eicosatrien [1-14C] oic acid with human platelets: 12-L-hydroxy-8,10,14-eicosatrienoic acid, 8,11,12-trihydroxy-9,14-eicosadienoic acid, 8,9,12-trihydroxy-10,14-eicosadienoic acid, 12-L-hydroxy-8,10-heptadecadienoic acid, prostaglandin E1, prostaglandin D1, and 8-(1-hydroxy-3-oxopropyl)-9,12-dihydroxy-10-heptadecenoic acid (thromboxane B1).

Effect of anticoagulants and inhibitors of platelet aggregation on thrombotic occlusion of endarterectomized cat carotid arteries

Cat carotid arteries measuring 1.8 to 2 mm in diameter were endarterectomized under the operating microscope over a 1-cm segment and the arteriotomy was closed with a 9-0 monofilament nylon suture. Vessels exhibiting significant narrowing of the lumen due to faulty closure of the arteriotomy were excluded from the study. The vessels were divided into six groups according to the method of treatment of the animals: control, aspirin, Coumadin, Coumadin plus aspirin, heparin for less than four hours, and heparin for four to eight hours. All vessels in the untreated group subjected to simple arteriotomy and closure remained patent. Only heparin demonstrated an apparent beneficial effect after endarterectomy with 100% of the vessels treated more than four hours and 30% of those treated less than four hours remaining patent. This is contrasted to a 0% patency in other endarterectomized vessels.

Electron microscopic studies on the blood-aqueous barrier of prostaglandin-treated rabbit eyes. I. Iridial and ciliary processes

Prostaglandins E1 and E2 were applied topically to rabbit eyes. Structures related to the blood-aqueous barrier in the iridial and ciliary processes, as well as the permeability of the ciliary epithelium to the protein tracer horseradish peroxidase, were studied with the electron microscope. Marked morphological changes, including dilations of the intercellular spaces and separation of the two epithelial cell layers, were found in the epithelium of the iridial processes. Only minor structural changes were found in the epithelium of the ciliary processes. Leakage of peroxidase through the intercellular spaces of the epithelium was demonstrated in the iridial processes and in the anterior parts of the ciliary processes. In the ciliary vessels of the same regions, opening of interendothelial gaps, platelet aggregations, microthrombi, and haemorrhages were found. In a previous in vitro study on the effects of prostaglandins on the movement of peroxidase in the ciliary epithelium, no structural changes of the epithelium were found, and the epithelial diffusion barrier to peroxidase was found to be intact. It is assumed that the breakdown of this barrier in vivo is secondary to vascular changes.

Angiotensin II stimulation of prostaglandin production in cultured human vascular endothelium

Immunoreactive material resembling prostaglandin E accumulates in the medium of cultured human umbilical vein endothelial cells. Prodcution is inhibited by indomethacin and stimulated by angiotensin II. Prostaglandin secretion by endothelium may be important in platelet-dependent thrombotic phenomena, and in local control of vascular permeability and tone in vivo.

Platelet aggregation: effects of cardiopulmonary bypass

This study was designed to determine whether reduction in platelet aggregate microembolization during the first 30 minutes of cardiopulmonary bypass is due to thrombocytopenia or to decreased ability of platelets to aggregate. The total volume of platelet aggregates induced in blood by adenosine diphosphate (ADP) was measured with a Coulter counter. The volume of platelets in blood was calculated by multiplying hemocytometry platelet counts by the mean platelet volume. Immediately before cardiopulmonary bypass, the total volume of aggregates induced in blood by ADP (2muM) was reduced when compared to normal donors because of (1) a slight fall in the volume of platelets, and (2) reduction in the percentage by volume of platelets which aggregated. After 30 minutes on bypass, the volume of both platelets and aggregates fell, but a greater percentage of platelets aggregated. This indicates that reduction of platelet aggregate formation during cardiopulmonary bypass is due to thrombocytopenia. It also suggests that anesthesia, surgical trauma and heparinization alter platelet reactivity more than cardiopulmonary bypass.

The coagulation mechanism in labor at term induced with prostaglandin F2 alpha

The prostaglandins, especially the PGE series, produce profound effects on platelet function in vitro. It has been reported that PGF2alpha and the PGE series do not affect the clotting mechanism when used to induce midterm abortion, in contrast to that induced with hypertonic saline but the effect of these drugs on the clotting mechanism when used to induce term labor has not been reported. Labor was induced with intravenous PGF2alpha in eight patients, at 32 to 41 weeks' gestation, with premature rupture of the membranes. Three samples were obtained: (1) anteceding the administration of the drug, (2) during the peak drug effect during active labor, and (3) approximately 12 hours post partum. No significant changes were seen in the prothrombin or partial thromboplastin times, platelet numbers or aggregation with ADP, fibrinogen levels, euglobulin lysis times, and circulating fibrin split products. The circulation of thrombin as shown by a specific test for fibrin monomer was not demonstrated.

Platelets, thrombosis and drugs

The development of thrombosis involves 4 main factors: the vessel wall, the formed elements of the blood, blood coagulation, and blood flow. In venous thrombosis, however, the major part in both the initiation and growth of thrombi is played by the platelets. In selecting drugs which inhibit platelet function it is helful to know which of the platelet reactions that contribute to thrombus formation can be inhibited by various agents. Platelets adhere to the damaged vessel wall, collagen being probably the most important constituent involved. They are then stimulated to release the contents of their storage granules. Release-inducing agents promote the discharge of adenosine diphosphate (ADP) which causes platelets in the vicinity to swell to a more spherical shape, extend pseudopods and adhere to each other. Platelet aggregation is reversible, and a number of drugs have been shown to be capable of inhibiting platelet function at various stages, both in vitro and in vivo. Adrenaline, noradrenaline, oestrogens and nicotine enhance aggregation. Drugs which inhibit platelet function include the non-steroidal anti-inflammatory drugs, the pyrimido-pyrimidines (e.g. dipyridamole), hydroxychloroquine, clofibrate, and dextran. In this review the effects of drugs which inhibit platelet function are outlined and the extent to which they can be used to influence the course of thromboembolic disease in man is discussed. It is suggested that combination of anti-platelet drugs with anticoagulants could prove clinically useful.

Effect of intravenous prostaglandin E 2 on platelet function, coagulation, and fibrinolysis

Prostaglandin E(2) (PGE(2)) was infused intravenously to eight women for the termination of pregnancy and tests of platelet function: coagulation and fibrinolysis were studied before and during the infusion.Platelet adhesiveness, as measured by a cellophane membrane test-cell system, was significantly diminished by PGE(2), a change which was not noted by the glass-bead column technique. The administration of PGE(2) caused more rapid platelet disaggregation following ADP-induced aggregation but had no effect on the platelet count, collagen-induced aggregation, or platelet factor 3 activity. An increase in plasma antithrombin concentration and euglobulin lysis activity was also noted. These results support the concept that prostaglandin E(2) might have a role in the prevention of thrombosis.

The effects of thrombin on adenyl cyclase activity and a membrane protein from human platelets

Washed human platelets were incubated with 0.1-1.0 U/ml human thrombin and the effects on adenyl cyclase activity and on a platelet membrane protein (designated thrombin-sensitive protein) were studied. Adenyl cyclase activity was decreased 70-90% when intact platelets were incubated with thrombin. The T(1/2) for loss of adenyl cyclase activity was less than 15 sec at 1 U/ml thrombin. There was no decrease of adenyl cyclase activity when sonicated platelets or isolated membranes were incubated with these concentrations of thrombin. Loss of adenyl cyclase activity was relatively specific since the activities of other platelet membrane enzymes were unaffected by thrombin. Prior incubation of platelets with dibutyryl cyclic adenosine monophosphate (AMP), prostaglandin E(1), or theophylline protected adenyl cyclase from inhibition by thrombin. Incubation of intact but not disrupted platelets with thrombin resulted in the release of thrombin-sensitive protein from the platelet membrane. The rapid release of this protein (T(1/2) < 15 sec) at low concentrations of thrombin suggested that removal of thrombin-sensitive protein from the platelet membrane is an integral part of the platelet release reaction. This hypothesis is supported by the parallel effects of thrombin on adenyl cyclase activity and thrombin-sensitive protein release in the presence of dibutyryl cyclic AMP, prostaglandin E(1), and theophylline at varying concentrations of thrombin.

The influence on platelet aggregation of drugs that affect the accumulation of adenosine 3':5'-cyclic monophosphate in platelets

1. The involvement of intracellular 3':5'-cyclic AMP in the inhibition of platelet aggregation by prostaglandin E(1), isoprenaline and adenosine has been examined by a radiochemical technique. Platelet-rich plasma was incubated with radioactive adenine to incorporate (14)C radioactivity into platelet nucleotides. Pairs of identically treated samples were taken, one for the photometric measurement of platelet aggregation induced by ADP, the other for estimation of the radioactivity of 3':5'-cyclic AMP. 2. Theophylline, papaverine, dipyridamole and 2,6-bis-(diethanolamino)-4-piperidinopyrimido[5,4d]pyrimidine (compound RA233) were found to inhibit 3':5'-cyclic AMP phosphodiesterase from platelets. At concentrations of 3':5'-cyclic AMP greater than 50mum the most active inhibitor was dipyridamole; at 3':5'-cyclic AMP concentrations less than 19mum, papaverine and compound RA233 were more active than dipyridamole. 3. In the presence of compound RA233 (50mum), the effectiveness of prostaglandin E(1) as an inhibitor of platelet aggregation was increased tenfold. Compound RA233 also increased the stimulation by prostaglandin E(1) of the incorporation of radioactivity into 3':5'-cyclic AMP. 4. Compound RA233 (50mum) increased the effectiveness of both adenosine and 2-chloroadenosine as inhibitors of aggregation by 70-100-fold, and in the presence of compound RA233 both adenosine and 2-chloroadenosine stimulated the incorporation of radioactivity into 3':5'-cyclic AMP; the extent of the stimulation was proportional to the logarithm of the nucleoside concentration. 5. Compound RA233 (100-500mum) inhibited platelet aggregation by itself and caused small increases in the radioactivity of 3':5'-cyclic AMP. Partial positive correlations were found between the radioactivity of 3':5'-cyclic AMP in platelets measured at the time of addition of the aggregating agent (ADP) and the extent to which the aggregation was inhibited. 6. The results are interpreted as indicating that adenosine, 2-chloroadenosine, isoprenaline, prostaglandin E(1) and drugs that inhibit platelet 3':5'-cyclic AMP phosphodiesterase all inhibit aggregation by a common mechanism involving intracellular 3':5'-cyclic AMP.