A comparison of the inhibitory effects of prostacyclin and carbacyclin on platelet adhesion to collagen

The effect of carbacyclin, a chemically stable analogue of prostacyclin (PGI2), on the adhesion of platelets to collagen has been examined. The compound was compared to PGI2 which is unstable and rapidly hydrolysed to the inactive derivative, 6-oxo-PGF 1 alpha. The adhesion of 111Indium-labelled human platelets to collagen in the absence of platelet aggregation and secretion was measured. The cAMP level in the platelets was also monitored. Both PGI2 and carbacyclin inhibited platelet-collagen adhesion and caused a rise in the platelet cAMP level. Carbacyclin was approximately 15-fold less effective than PGI2, however, its effect was longer lasting, remaining constant for at least 30 minutes.     

Effects of prostacyclin analogue (carbacyclin) on the interaction of platelets with different collagen substrates. Inhibition of cAMP increase by collagens type I, III, and IV

We investigated the effects of a stable prostacyclin analogue, carbacyclin, on the interaction of platelets with collagen substrates differing in their ability to activate platelets: human collagens type I, III, IV and V (CI, CIII, CIV and CV), and commercial calf skin collagen type I (CSC). The total adhesion was measured using 51Cr-labelled platelets, and quantitative morphometry of adherent platelets was performed by scanning electron microscopy (SEM). Carbacyclin in the concentrations inducing a 10-fold rise in platelet cAMP did not affect the adhesion of platelets to weak substrates, CV and CSC, but reduced the adhesion to strong substrates, CIV (by 49%) and CI/CIII (by 78%), which stimulated massive spreading and formation of surface-bound aggregates respectively. Carbacyclin inhibited all morphological manifestations of platelet activation associated with adhesion: conversion of native discoid platelets to spherical ones on CSC; massive spreading on CIV; and aggregate formation on CI/CIII. Massive spreading and aggregation on a weak substrate (CSC) stimulated by arachidonic acid and thrombin was also inhibited by carbacyclin. Under the same concentration of agonists aggregation of platelets was more sensitive to the action of carbacyclin, than spreading. Strong collagen substrates CI, CIII and CIV, but not CV and gelatin, inhibited the carbacyclin-induced rise in platelet cAMP.     

Effect of prostacyclin analogue (carbacyclin) on the interaction of platelets with different collagen substrates. Inhibition of camp inccrease by collagens type I, III and IV

We investigated the effects of a stable prostacyclin analogue, carbacylcin, on the interaction of platelets with collagen substrates differing in thier ability to activate platelets: human collagens type I. III, IV and V (CI, CIII, CIV and CV), and commercial calf skin collagen type I (CSC). The total adhension was measured using ⁵¹Cr-labelled platelets, and quantitative morphometry of adherent platelets was performed by scanning electron microscopy (SEM). Carbacyclin in the concentrations inducing a 10-fold rise in platelets cAMP did not effect the adhension of platelets to weak substrates, CV and CSV, but reduced the adhesion to strong substrates, CIV and *by 49%) and CI/CIII (by 78%), which stimulated massive spreading and formation of surface-boud aggregates respectively. Carbacyclin inhibited all morphological manifestations of platelet activation associated with adhension: conversion of native discoid platelets to spherical ones on CSC; massive spreading on CIV; and aggregate formation on CI/CIII. Massive spreading and aggregation on a weak substrate (CSC) stimulated by arachidonic acid and thrombin was also inhibited by carbacyclin. Under the same concentration of angonists aggregation of platelets was more sensitive to the action of carbacyclin, than spreading. Strong collagen substrates CI, CIII and CIV, but not CV and gelatin, inhibited the carbacyclin-induced rise in platelet cAMP.     

Binding to protaglandin (PG) receptors and activation of adenyl cyclase by 20-isopropylidene-PGS in rabbit platelet membranes

20-Isopropylidene-PGE₁ (Isop-PGE₁) was about 10 times more potent than PGE₁ in inhibition of thrombin-induced aggregation of rabbit washed platelets. Likewise, 20-isopropylidene-17(R)-methyl-carbacyclin (CS-570), a stable PGI₂ analogue, was more potent than carbacyclin in the anti-aggregatory activity. In order to define the platelet-prostaglandin interactions, a binding assay was done using platelet membranes with [³H]-PGE₁ as a radioligand. Isop-PGE₁ (IC₅₀μM) bound to the PG receptors more potently than PGE₁ (IC₅₀ = 2.1 μM). CS-570 (IC₅₀=0.39 μM) was more potent than carbacyclin (C₅₀=1.9 μM). These indicate that introduction of an isopropylidene group to the carbon 20 of PGs increases the binding ability to the receptors. These PGE₁ and PGE₂ analogues activated platelet membrane adenyl cyclase and increased intracellular cAMP levels with the same potency series obtained in the binding experiments. All these results suggest that the binding to the receptors by these PGs is coupled to the activation of adenyl cyclase, followed by the increase in cAMP levels in platelets and the inhibition of platelet aggregation. Thus, the increased anti-aggregatory activity of 20-isop-PGs may be explained by their increased affinity for the PG receptors and stimulation of adenyl cyclase.15-Epimeric-20-isopropylidene-PGE₁ (15-Epi-isop-PGE₁), which has an unnatural configuration of the 15-hydroxyl group, was much less potent than isop-PGE₁ in the binding experiment and the other three investigations. This indicates that the configuration of the 15-hydroxyl group is important for the binding to the PG receptors and the consequent activities in platelets.     

The influence of epinephrine on prostacyclin (PGI2) induced dissociation of adp aggregated platelets

Prostacyclin (PGI₂) is a powerful inhibitor of platelet function and recent studies have indicated that it can dissociate aggregated platelets in vivo. The present investigation has evaluated the mechanism involved in the dissociation of ADP aggregated platelets by PGI2, the influence of epinephrine on the process of PGI₂ mediated disaggregation, and effects of the catecholamine on the refractory state of PGI2 dispersed cells. Dissociation of ADP aggregates by PGI₂ was concentration and time dependent, caused complete disappearance of sensitive platelet clumps and restored platelet discoid form. The dispersed platelets were refractory to further stimulation by ADP, thrombin, arachidonate or the calcium ionophore, A23187. Measurement of cAMP revealed no change in control levels after aggregation by ADP, but a sharp increase during and after dissociation by PGI₂. Epinephrine added to ADP aggregates before or after PGI2 blocked disaggregation, or stopped dissociation if already in progress, and caused a rapid fall in the high levels of cAMP stimulated by PGI₂. Agents to which PGI2 dissociated platelets were refractory, including ADP, arachidonate and A23187 caused no lowering of elevated cAMP levels. Reduction of high levels of CAMP in PGI₂ dispersed platelets by epinephrine did not result in reaggregation. However, epinephrine addition to refractory platelets restored in large measure their sensitivity to aggregation by arachidonate, thrombin, ADP and A23187.     

Effect of 7-fluoro prostacyclin,a stable prostacyclin analogue,on cAMP accumulation and prostaglandin binding in mastocytoma P-815 cells

The effect of 7-fluoro proscyclilin (PGI₂-F), a chemically stable analogue of prostacyclin, on cAMP accumulation in and [³H]PGE binding to mastocytoma P-815 cells was compared with those of the Na salt and methyl ester of prostacyclin (PGI₂Na or PGI₂Me), which are rapidly inactivated in aqueous solution or metabolized in the tissue.PGIF was as effective as PGI₂Me, and slightly less effective than PGI₂Na in stimulating cAMP accumulation in mastocytoma cells and rabbit platelets. PGI₂F was also more stable than PGI₂Me or PGI₂Na, and retained its original cAMP elevating activity even after incubation with or without cells for 4 h at 37°C. Cells which had been exposed to PGI₂F and then washed free of unbound reagent continued to produced cAMP for more than 3 h. PGI₂F was also as effective as PGE₁ or PGE₂ in displacing [³H]PGE₂ bound to the cells. Non-competitive inhibition by PGI₂F or PGI₂Me of [³H]PGE₂ binding to the cells, with apparent K_is of 1.29 μM and 1.13 μM, respectively, indicates the presence of different receptors for PGE₂ and for PGI₂F or PGI₂Me in mastocytoma P-815 cells.     

Antithrombotic effect of topically applied 3-Oxa-Methano-prostaglandin I1 in the microcirculation and antiplatelet functions of its active form

The antithrombotic effect of topical application of the 3-oxamethano-prostaglandin (PG) I₁ analog, SM-10902 in the microcirculation and in vitro antiplatelet functions of its active form SM-10906 were estimated in comparison with PGI₂ and PGE₁. In rat platelets, SM-10906 evoked accumulation of intracellular cyclic adenosine 3′,5′-monophosphate, and exhibited antiaggregatory and disaggregatory activities, which were all enhanced by the phosphodiesterase inhibitor theophylline. Additionally, SM-10906 was shown to inhibit platelet adhesion to collagen in human platelet-rich plasma. PGI₂ and PGE₁ also showed in vitro antiplatelet effects in the order of PGI₂ > SM-10906 ≥ PGE₁. SM-10902 exhibited a dose-dependent antithrombotic effect in the guinea pig mesenteric arteriole by a topical application, and this activity might be exerted by the antiplatelet functions of SM-10906. Although SM-10906, PGI₂ and PGE₁ also showed the antithrombotic effects, SM-10902 was the most potent. In conclusion, the present studies indicate that an external topical preparation of SM-10902 may be useful for the therapy of peripheral circulatory insufficiency.     

Influence of pH on the prostacyclin (PGI2) mediated inhibition of platelet function

Prostacyclin (PGI₂) stimulates platelet adenylate cyclase, elevates intracellular levels of cyclic adenosine monophosphate and blocks the response to aggregating agents. It is rapidly hydrolyzed (T 1–5 min) to 6-keto prostaglandin F_1α at acid or neutral pH.As a result, platelets incubated with PGI₂ will recover spontaneously and respond to aggregating agents within 15–60 min, depending on the initial PGI₂ concentration. In the present study we have evaluated the influence of temperature and pH on the stability of PGI₂ and its effects on platelet function. PGI₂ in Tris buffer was stabilized at several pH levels and stored at 37°C, 23°C, and 4°C. Inhibitory influence on platelet function was lost rapidly at pH 7.2–7.4, lasted several hours at pH 7.8 and was retained indefinitely at pH 8 or above. PGI₂ (2.8 hM) completely inhibited the response to arachidonic acid for 15 min. at pH 7.4, for at least 1 hour at pH 7.8 and showed no reversal of inhibition after 48 hours at pH 8. However, PGI₂ inhibited samples at pH 8 completely recovered their sensitivity to arachidonic acid when the pH was reduced to 7.4. These findings indicate that the biological activity of PGI₂, though labile at neutral pH, is stable at pH 8 and can inhibit cAMP mediated platelet functions for at least 48 hours. Because of its pH dependence, PGI₂ may be a useful agent for prolonging the sensitivity of stored platelets.     

Prostacyclin inhibits 5-hydroxytryptamine release but stimulates thromboxane synthesis during cardiopulmonary bypass

The antiaggregating agent prostacyclin (PGI₂) was infused into ten dogs during cardiopulmonary bypass (CPB) to minimize thrombocytopenia and platelet dysfunction. The animals were anesthetized, placed on mechanical ventilation and underwent thoracotomy. After heparinization with 300 u/kg, animals were assigned to control (n=5) or PGI₂ treated groups (n=5). Thoracotomy and then CPB decreased platelet numbers to below 30, 000/mm³ (p < 0.05) and fibrinogen to less than 150 mg/dl (p < 0.05). PGI₂ at 100 ng/kg·min was infused for the 2 h period of CPB. PGI₂ infusion did not prevent these changes, but did prevent platelet serotonin release. In the control group after CPB, platelet serotonin fell from the baseline value of 1.11 μg/10⁹ to 0.35 μg/10⁹ platelets (p < 0.05). In contrast, PGI₂ treatment resulted in a serotonin increase to 2.27 μg/10⁹ platelets (p < 0.05). Thromboxane B₂ concentrations of platelets and plasma rose during CPB (p < 0.05). Surprisingly, PGI₂ infusion accentuated this rise in platelet and plasma thromboxane B₂ (p < 0.05). These data indicate that during CPB, an infusion of PGI₂: 1) does not prevent thrombocytopenia; 2) increases platelet serotonin uptake despite, 3) an associated rise in platelet and plasma thromboxane B₂.     

Potentiation of anti-aggregating activity of PGI2 by 7-ethoxycarbonyl-6,8-dimethyl-4-hydroxymethyl-1(2H)-phthalazinone (EG-626) in rabbit platelets in vitro

Anti-aggregating activity of 7-ethoxycarbonyl-6,8-dimethyl-4-hydroxymethyl-1(2H)-phthalazinone (EG-626) was tested using rabbit platelets in vitro. EG-626 alone, when added before, prevented platelet aggregation induced by ADP, as did PGI₂, papaverine and dipyridamole. Spontaneous disaggregation was also accelerated when EG-626 was added after the maximal aggregation induced by ADP. EG-626 alone also inhibited platelet aggregation induced by collagen and arachidonic acid. ID₅₀s of these agents in ADP-induced aggregation were 7–9 nM for PGI₂, 223 μM for EG-626, 266 μM for papaverine and 957 μM for dipyridamole. When EG-626 was used in combination with PGI₂, a threshold dose (50 μM) of EG-626 potentiated the anti-aggregating effect of subthreshold dose (3 nM) of PGI₂ upto 100% inhibition in collagen-induced platelet aggregation. The marked potentiating effect of EG-626 was accompanied by an accumulation of cyclic AMP in the platelets. These effects might be due to inhibition of phosphodiesterase. Papaverine and dipyridamole, other phosphodiesterase inhibitors, also potentiated the anti-aggregating activity of PGI₂. The activity of papaverine, however, was one eighth of EG-626 and that of dipyridamole was much less. The most effective combination of PGI₂ and EG-626 to induce 50% inhibition was obtained with 20% of ID₅₀ of each agent, whereas that of PGI₂ and papaverine or dipyridamole was 39 or 41%, respectively.     

Aggregation and thromboxane B2 formation in platelets and vascular prostacyclin production from genetically obese rats

Obesity, diabetes, hyperlipidaemia and age are conditions predisposing to atheroscleorosis and arterial occlusion. Recently it has been claimed that increased synthesis of thromboxane A₂ by platelets and decreased synthesis of prostacyclin (PGI₂) by blood vessels play an important role. The “Zucker” rat, a genetically obese animal with hyperlipidaemia, hyperinsulinaemia and normoglycaemia was used to study platelet aggregation, thromboxane (TXB₂) production and aortic PGI₂ synthesis. Two age groups (6–8 months and 14–16 months old) and their homozygote lean controls were used. In the obese rats no increased aggregation was found with ADP, arachidonic acid and collagen. On the contrary platelets from young fatty rats were less sensitive to ADP than platelets from lean young animals. An increase in platelet sensitivity to aggregating agents with age was observed, especially in the obese rats. TXB₂ measured in platelet rich plasma after exposure to ADP, arachidonic acid, arachidonic acid plus ADP and collagen was similar in the fatty and lean animals.Production of PGI₂ from incubated aortic rings was lowest in young lean animals. No differences existed between the other groups of rats studied. Insulin added to aortic rings had no influence on PGI₂ production. It is concluded that age rather than obesity, hyperlipidaemia or hyperinsulinaemia may cause platelet hyperresponsiveness to aggregating agents. Thromboxane and plateletaggregation do not closely correlate. PGI₂ production is not reduced by metabolic alterations, thought to predispose to atherosclerosis.     

Prostaglandins: specific inhibition of platelet adhesion to collagen and relationship with cAMP level

The effect of 3 prostaglandins (PG's) (I2, D2 and E1) on the adhesion of platelets to purified type III collagen has been investigated. A quantitative method for a specific evaluation of the adhesion has been applied and has revealed an inhibition of adhesion by low concentrations (10(-10)M) of PGs added before collagen; the effect varied as a function of the dose of PGs (maximum at 10(-6)M) which also induced an increase in the level of platelet cAMP. The inhibition of adhesion and the elevation of platelet cAMP followed the same time course and were either of short duration (rapid decrease in the induced effects after 15 and 45 seconds in the case of PGE1) or longer lasting (maximum effect maintained for 5 minutes in the case of PGI2 and D2). These effects were potentiated by a phosphodiesterase inhibitor such as theophylline (10(-3)M). The addition of PGs after collagen resulted in a reduction of the enhancement of cAMP, associated with a decrease in the inhibition of adhesion. Moreover, the addition of exogenous cAMP (dibutyryl N6-02' cAMP) induced a comparable inhibition. A correlation between the adhesion of platelets to collagen and the level of either endogenous or exogenous cAMP has been established. The PGs also inhibited the platelet release reaction from the alpha granules (beta TG) and the dense bodies. (5-HT and ADP). A greater inhibition of release than of adhesion was observed for the same doses of PGs added.     

Characterization of prostacyclin synthesis in cultured human arterial smooth muscle cells, venous endothelial cells and skin fibroblasts.

The interaction of human platelets with one another and with the blood vessel wall is thought to be regulated in part by a balance between two arachidonic acid metabolites: thromboxane A₂, synthesized by platelets, and prostacyclin (PGI₂), synthesized by the vessel wall. We have studied the ability of cultured human vascular cells to synthesize PGI₂ from arachidonic acid. Four strains of human arterial smooth muscle cells synthesized a mean of 1.36 ng PGI₂ per 10⁵ cells, with a range of 0.2–5.3 ng PGI₂ per 10⁵ cells among the different strains. Human umbilical vein endothelial cells synthesized a mean of 7.16 ng PGI₂ per 10⁵ cells with a range of 2.3–14.0 ng per 10⁵ cells. In contrast, cultured human diploid skin fibroblasts synthesized only 0.27 ng PGI₂ per 10⁵ cells with a range of 0.05–0.6 ng per 10⁵ cells. When cultured cells were mixed with platelets, PGI₂ synthesis from added arachidonate was reduced rather than stimulated. Thus the major precursor cyclic endoperoxides utilized for PGI₂ synthesis are formed within the cells and not from endoperoxides synthesized by platelet cyclooxygenase. Aspirin has been proposed as an anti-thrombotic agent. Aspirin could be ineffective, however, if it inhibited not only platelet cyclooxygenase but that of vessel wall cells as well. Measurement of the rate constant or potency for aspirin inhibition of PGI₂ synthesis in cultured cells indicates that the cyclooxygenase in both cell types of the blood vessel wall is 14–44 fold less sensitive to aspirin inactivation than that in platelets, and appropriate levels of aspirin can selectively block human platelet thromboxane A₂ synthesis without compromising the capacity of the vasculature to produce PGI₂.     

Role of lipoxygenase metabolism in platelet function: Effect of aspirin and salicylate

Aspirin inhibits thromboxane A₂ (TxA₂) production whereas its salicylate moiety inhibits 12-hydroxy-eiosatetraenoic acid (12-HETE) production in the platelet. The significance of the latter effect on platelet function is unclear. We examined the effects of aspirin and salicylate on (i) platelet/ collagen adhesion using ³H-adenine-labelled human platelets and collagen- coated discs, (ii) platelet aggregation induced by thrombin, collagen, ADP and arachidonic acid, and (iii) platelet TxA₂ and 12-HETE synthesis as measured by radioimmunoassay and high pressure liquid chromatography respectively. Aspirin (50 μM) decreased platelet aggregation and increased platelet adhesion. The decrease in aggregation was associated with inhibition of TxA₂ production and the increase in adhesion was associated with enhanced 12-HETE production. Salicylate had the opposite effects. Platelet aggregation was increased and platelet adhesion decreased. The increased aggregation was associated with enhanced TxA₂ production and the decrease in aggregation was associated with inhibition of 12-HETE production. These observations suggest that 12-HETE facilitates platelet adhesion which can be altered by salicylate treatment.     

Effects of nitrogen dioxide exposure on prostacyclin synthesis in lung and thromboxane A2 synthesis in platelets in rats

Effects of nitrogen dioxide (NO₂) exposure on prostacyclin (PGIP₂) synthesis in the rat lung and thromboxane A₂ (TXA₂) synthesis in the platelets were studied. Male Wistar rats were exposed to 10 ppm NO₂ for 1, 3, 5, 7 and 14 days. PGI₂ synthesizing activity of homogenized lung decreased. The damage of PGI₂ synthesizing activity reaches its maximum at 3 days. At 14 days, PGI₂ synthesizing activity returned to the normal level. The activity of PGI₂ synthetase decreased significantly. The formation of lipid peroxides due to NO₂ exposure may cause the depression of PGI₂ synthesizing activity of lung. On the other hand, platelet TXA₂ synthesizing activity increased. This increased TXA₂ synthesizing activity lasted at least till 3 days. Then, it returned to the normal level. The counts of platelet were decreased significantly by 1, 3, 5 and 7 days NO₂ exposure. Then the decreased counts of platelet returned to the normal level at 14 days NO₂ exposure. These results indicate that the depression of PGI₂ synthesizing activity lung by NO₂ exposure cause an increase in TXA₂ synthesizing activity of platelets. It may contribute to induce platelet aggregation and to the observed decrease in the number of platelets during NO₂ exposure.     

Sex-related difference in the effect of aspirin on prostaglandin metabolism in rat platelet and aorta

The effect of aspirin on the production of the arterial prostacyclin (PGI₂)-like substance and platelet malondialdehyde (MDA) was investigated in rats of both sexes. No significant sex difference observed with the arterial PGI₂-like substance. But, following the aspirin treatment, the production of the PGI₂-like substance was significantly decreased in male rats. There was significant sex difference in the production of platelet MDA before the aspirin treatment. And after the aspirin treatment, platelets of both sexes produced significantly less MDA. It is possible that sex difference in the effect of aspirin is related to the quantitative difference of cyclooxygenase activity between platelets and vasal wall.     

Prostacyclin stimulates the adenylate cyclase system of human thyroid tissue

Since Prostacyclin (PGI₂) is a major product of arachidonic acid metabolism in the human thyroid, we have studied the effects of PGI₂ on cAMP accumulation in human thyroid slices and cultured thyrocytes. In both systems, PGI₂ caused a dose- and time-dependent increase of cAMP accumulation with higher potency and efficacy than PGE₂. Two optically active isomers of 5,6-dihydro-PGI₂, i.e. stable synthetic analogs of PGI₂, had qualitatively similar effects to PGI₂. The relative potency ratio between the α- and β- isomer as well as their potency compared to PGI₂ were substantially similar to their potency in inhibiting human platelet aggregation. In thyroid slices, PGI₂ and its stable analogs had a greater than TSH in causing cAMP accumulation; however, in contrast to TSH, this effect was not associated with increased iodothyronine release except at maximal PGI₂ concentrations. TSH had no detectable effect on thyroidal PGI₂ synthesis and release. In cultured thyrocytes the effects of PGI₂ and its stable analogs were considerably less than those obtained with TSH and required higher concentrations. Such a discrepancy was not found in the case of PGE₂. These findings suggest the existence of a specific PGI₂-responsive adenylate cyclase system in human significance.     

Interrelationships among prostaglandins, vasopressin and cAMP in renal papillary collecting tubile cells in culture

To determine the influence of prostaglandins on cAMP metabolism in renal papillary collecting tubule (RPCT) cells, intracellular cAMP levels were measured after incubating cells with prostaglandins (PGs) alone or in combination with arginine vasopressin (AVP). PGE₁, PGE₂ and PGI₂, but not PGD₂ or PGF_2α, increased intracellular cAMP concentrations. At maximal concentrations (10⁻⁵ tthe effects of PGE₂ plus PGI₂ (or PGE₁), but not of PGI₂ plus PGE₁, were additive suggesting that at least two different PG receptors may be present in RPCT cell populations. Bradykinin treatment of RPCT cells caused an accumulation of intracellular cAMP which was blocked by aspirin and was quantitatively similar to that observed with 10⁻⁵ PGE₂. PGs, when tested at concentrations (e.g. 10⁻⁹ ) which had no independent effect on intracellular cAMP levels, did not inhibit the AVP-induced accumulation of intracellular cAMP in RPCT cells. These results indicate that PGs do not block AVP-induced accumulation of intracellular cAMP in RPCT cells at concentrations of PGs which have been shown to inhibit the hydroosmatic effect of AVP on perfused collecting tubule segments. However, at higher concentrations of PGs (e.g. 10⁻⁵ ), the effects of AVP plus PGE₁, PGE₂, PGI₂ or bradykinin on intracellular cAMP levels were not additive. Thus, under certain conditions, there is an interaction between PGs and AVP at the level of cAMP metabolism in RPCT cells.     

Differential Effects of Prostaglandin E1 and Prostaglandin E2 on Growth and Differentiation of Murine Myeloid Leukemic Cell Line,M1

Effects of prostaglandins (PGs) of the E series on growth and differentiation of murine myeloid leukemic cell line M1 were studied. PGE₁, but not PGE₂, inhibited the growth of M1 cells. PGE₂ neither inhibited nor augmented the antiproliferative effect of PGE₁. PGE₁ augmented the differentiation of M1 cells into macrophage-like cells induced by interleukin 6. PGE₂, however, did not exhibit any effect on the differentiation. PGE₁ caused a marked increase in intracellular cAMP level in M1 cells, whereas PGE₂ had no effect. These results indicate that M1 cells are able to respond only to PGE₁. Radiolabeled PGE₁ binding experiments, however, revealed that there was no specific binding in M1 cells, suggesting that the cells express low numbers of receptors or very low affinity receptors specific for PGE₁. Stable agonists of PGI₂, iloprost, cicaprost or carbacyclin, also potently inhibited the growth of M1 cells. These findings suggest that PGE₁ as well as PGI₂ may play a role in the differentiation of monocyte-macrophage lineage cells.     

Modulation of platelet thromboxane A2 and arterial prostacyclin by dietary vitamin E

Platelets from vitamin E-deficient and vitamin E-supplemented rats generate the same amount fo thromboxane A₂ (TxA₂) when they are incubated with unesterified arachidonic acid. Platelets from vitamin E-deficient rats produce more TxA₂ than platelets from vitamin E-supplemented rats when the platelets are challenged with collagen. Arterial tissue from vitamin E-deficient rats generates less prostacyclin (PGI₂) than arterial tissue from vitamin E-supplemented rats. The vitamin E effect with arterial tissue is observed when the tissue is incubated with and without added unesterified arachidonic acid. These data show that arterial prostacyclin synthesis is diminished in vitamin E-deficient rats. Vitamin E, $\text{in}$$\text{vivo}$, inhibits platelet aggregation both by lowering platelet TxA₂ and by raising arterial PGI₂.