Metabolism of PGI2 by 15-hydroxyprostaglandin-dehydrogenase and Δ-reductase in different vascular beds of the cat in vivo

The metabolism of endogenous PGI₂ (released by angiotensin II or bradykinin) and exogenous PGI₂ by 15-hydroxy-PG-dehydrogenase and Δ¹³-reductase was studied in five different vascular beds of the anaesthetized cat. Plasma concentrations of 6-keto-PGF_1α (the product of spontaneous hydrolysis of PGI₂) and 6,15-diketo-13,14-dihydro-PGF_1α (the metabolite formed from PGI₂ by 15-hydroxy-PG-dehydrogenase and Δ¹³-reductase) were determined in the efferent vessels of the respective vascular beds by specific radioimmunoassays.No major metabolism of PGI₂ by 15-hydroxy-PG-dehydrogenase and Δ¹³-reductase was detected in the head and the hindlimbs of the cat. In the lung exogenous (circulating) PGI₂ was not metabolized, whereas PGI₂ synthetized in the lung itself was converted to 6,15-diketo-13,14-dihydor-PGF_1α. No significant amounts of 6,15-diketo-13,14-dihydro-PGF_1α-immunoreactivity were detected in hepatic venous blood after infusion of PGI₂ into the portal vein. However as also no 6-keto-PGF_1α was found, the liver seems to efficiently extract PGI₂ from the circulation. The cat kidney had the highest capacity of all vascular beds investigated to release endogenous and exogenous PGI₂ as 6-15-diketo-13,14-dihydro-PGF_1α. In other organs (vascular beds) investigated PGI₂ is either metabolized less efficiently by the 15-hydroxy-PG-dehydrogenase or further transformed to other metabolites.     

Actions of prostacyclin (PGI2) and its product, 6-oxo-PGF1α on the rat gastric mucosa in vivo and in vitro

The effects of prostacyclin (PGI₂) and its breakdown product 6-oxo-PGF_1α on various aspects of gastric function were investigated in the rat. PGI₂ increased mucosal blood flow when infused intravenously. PGI₂ was a more potent inhibitor of gastric acid secretion in vivo than PGE₂. Like PGE₂, PGI₂ inhibited acid secretion from the rat stomach in vitro. PGI₂ had comparable activity to PGE₂ in inhibiting indomethacin-induced gastric erosions. Thus prostacyclin shares several of the activities of PGE₂, and may be involved in the regulation of gastric mucosal function.     

Effect of bromocriptine on prostacyclin release and cyclic nucleotides on rat aortic and uterine tissues

The effect of bromocriptine mesylate on cyclic nucleotides and PGI₂ release by rat aortic and uterine tissues was investigated. Treatment of rats with bromocriptine (10 mg kg⁻¹ I.P. daily for 14 days) increased PGI₂ release by the thoracic aorta from 0.67 ± 0.02 to 1.4 ± 0.03 ng/mg wet tissue (P < 0.001; n = 6). This increase was antagonized by treatment with sulpiride (15 mg kg⁻¹). Incubation of the arterial tissue with bromocriptive (50 ug ml⁻) in vitro also stimulated PGI₂ release. Mepacrine (160 μg ml⁻) significantly decreased both basal and stimulated PGI₂ release. Incubation of myometrial tissue from pregnant rats with bromocriptine (50 μg ml⁻¹) in vitro significantly decreased PGI₂ release from 1.25 ± 0.07 to 0.60 ± 0.08 ng/mg wet tissue (P < 0.05, n = 6).It also elevated uterine cAMP from 40 ± 2 to 64 ± 3 pmoles/100 mg wet tissue. Both effects were antagonized by sulpiride. Bromocriptine did not affect uterine cGMP or the cyclic nucleotides in the aorta. It is concluded that the increase in aortic PGI₂ was mediated via activation of dopamine D-2 receptors that stimulate phospholipase A₂ enzyme. The decrease in myometrial PGI₂ release may be related to the increase in uterine cAMP resulting from activation of dopamine D-1 receptors. Previous studies suggested a role for PGI₂ in implantation in the rat. The results suggest that the inhibitory effèct on uterine PGI₂ may underlie the reported inhibition of bromocriptine on implantation. On broad basis, the decrease in uterine PGI₂ together with the reported luteolytic effect of bromocriptine point to a potential role for the compound in postcoital contraception.     

Prostacyclin as neuromodulator in the sympathetically stimulated rabbit heart

Prostaglandin E₂ (PGE₂) has previously been shown to inhibit sympathetic neurotransmission in different organs and species. Based on this inhibitory effect and on its reversal by cyclo-oxygenase inhibitors, PGE₂ has been claimed to be a physiological modulator of in vivo release of norepinephrine (NE) from sympathetic nerves. It is now recognized that prostacyclin (PGI₂) is the main cyclo-oxygenase product in the heart. We therefore addressed the question whether PGI₂, within the same preparation, is formed in increased amounts during sympathetic nerve stimulation and has neuromodulatory activity.The effluent from isolated rabbit hearts subjected to sympathetic nerve stimulation or to infusion of NE or adenosine (ADO) was collected, and its content of PGE₂ and 6-keto-PGF_1α (dehydration product of PGI₂) was analyzed using gas chromatography/mass spectrometry, operated in the negative ion/chemical ionization mode. Other hearts were infused with PGI₂ and nerve stimulation induced outflow of endogenous NE into the effluent was analyzed using HPLC with electrochemical detection. Nerve stimulation at 5 or 10 Hz (before but not after adrenergic receptor blockade), as well as infusion of NE (10⁻⁶–10⁻⁵M) or ADO (10⁻⁴M) increased the cardiac outflow of 6-keto-PGF_1α. Basal and nerve stimulation induced efflux of 6-keto-PGF_1α was approximately 5 times higher than the corresponding efflux of PGE₂. PGI₂ dose-dependently inhibited the outflow of NE from sympathetically stimulated hearts, the inhibition at 10⁻⁶M being approximately 40%.On the basis of these observations we propose that PGI₂ is a more likely candidate than PGE₂ as a potential modulator of neurotransmission in cardiac tissue in vivo.     

Effect of bisenoic prostaglandins on the uterine vasculature of the nonpregnant sheep

The effects of the bisenoic prostaglandins on the uterine vasculature and uterine contractile activity have been evaluated in an unanesthetized chronically catheterized nonpregnant sheep preparation. Changes in uterine blood flow were monitored with electromagnetic flow probes while uterine contractile activity and tone were determined via an intra-uterine balloon connected to a pressure transducer. Prostaglandins A₂, D₂, E₂, and prostacyclin (PGI₂) were all found to be vasodilators. PGD₂ and PGI₂ were much more potent than PGA₂ and PGE₂ in dilating the uterine vasculature. The prostacyclin breakdown product 6-keto PGF_1α, PGF_2α, thromboxane B₂, and the endoperoxide analogues U44069 and U46619 produced vasoconstriction of the uterine vasculature. Prostaglandins A₂, D₂ and F_2α increased while PGI₂ decreased uterine contractile activity. PGF_2α also increased uterine tone suggesting that a portion of its vasoconstrictor activity may be due to mechanical compression of the uterine vasculature.     

Effect of taurine on arterial, uterine and cardiac PGI2 and TXA2 synthesis in the rat

The influence of taurine (in drinking water for 6 weeks) on PGI₂ and TXA₂ synthesis by some female rat organs was investigated using radioimmunoassay and platelet antiaggregatory bioassay. Taurine 100 and 200 mg/kg/day increased aortic PGI₂ release from 0.59 ± 0.04 (control) to 0.85 ± 0.05 and 1.01 ± 0.06 ng/mg, respectively and that by the myometrium from 0.24 ± 0.02 (control) to 0.38 ± 0.01 and 0.50 ± 0.04 ng/mg wet tissue, respectively (P < 0.05, n = 6). It did not affect PGI₂ and TXA₂ production in the heart or TXA₂ in the aorta. Taurine 200 mg/kg depressed uterine TXA₂ synthesis from 148.6 ± 9.8 (control) to 85.4 ± 6.8 pg/mg (P < 0.05, n = 6). Furthermore taurine 0.4 and 0.8 mM in vitro stimulated PGI₂ released by the myometrial and aortic tissues from pregnant rats. The stimulant effect of taurine on PGI₂ may be related to its antioxidant effect whereas its inhibitory effect on uterine TXA₂ may result from direction of synthesis towards PGI₂. It is concluded that endogenous taurine may participate in regulation of PGs synthesis and that prostanoids may contribute to its known actions. On broad basis, taurine-induced release of PGI₂ may prove of potential value in those ailments characterised by deficiency in PGI₂ release.     

Differential effects of prazosin and rauwolsin on the release of prostaglandins I2 and E2 from the perfused mesenteric arterial bed of the rabbit following nerve stimulation

Sympathetic nerve stimulation of the perfused mesenteric arterial bed of the rabbit, , increase the secretion of prostaglandin (PG)I₂ and PGE₂. Prazosin (4.8 × 10⁻⁶), and α₁ adrenergic receptor antagonist, inhibited this inrease in release of PGI₂ but not of PGE₂ whereas rauwolsin (10⁻⁷ M), an α₂ adrenergic receptor antagonist, inhibited the increase in release of PGE₂ but not of PGI₂. Prazosin (10⁻⁶ M) completely blocked the vasoconstrictor response to nerve stimulation, and to norepinephrine and phenylephrine administration, suggesting there to be little of an α₂ adrenergic receptor component in this response. It is concluded that the increase in PGI₂ release follows the activation of α₁ adrenergic receptors and is therefore post-junctional in origin, whereas the increase in PGE₂ release follows the activation of α₂ adrenergic receptors and may be pre- and/or post-junctional in origin.Indomethacin (2.8 × 10⁻⁷, 5.6 × 10⁻⁷ and 1.12 × 10^{−6 M} did not affect the vasoconstrictor responses to nerve stimulation at 10 Hz, whereas rauwolsin (10⁻⁷ M) in the presence of indomethacin substantially increased them. These results indicate that PGE₂ does not regulate norepinephrine release following nerve stimulation at 10 Hz to rabbit mesenteric arteries, and that the inhibition of norepinephrine release following stimulation of α₂ pre-junctional receptors is independent of PG involvement.     

Effect of PGI2, PGE2 and 6-keto PGF1α on canine gastric blood flow and acid secretion

Prostaglandins (PG)I₂, PGE₂ and 6-keto PGF₁α were infused directly into the gastric arterial supply at 10⁻⁹, 10⁻⁸ and 10⁻⁷ g/kg/min during an intra-gastric artery pentagastrin infusion in anesthetized dogs. 6-keto PGF₁α was also infused at 10⁻⁶ g/kg/min. Gastric arterial blood flow was measured continuously with a non-cannulating electromagnetic flow probe and gastric acid collected directly from the stomach. PGI₂ and PGE₂ produced similar dose-dependent increases in blood flow with an increase of more than four-fold at the highest dose. Both PGs inhibited acid output over this dose range with PGE₂ having 10 times the potency of PGI₂. 6-keto PGF₁α was at least 1000 times less active than PGI₂ or PGE₂ at increasing blood flow and failed to inhibit acid output even at 10⁻⁶ g/kg/min.     

Increased production of prostacyclin (PGI2) by vascular intimal smooth muscle cells from atherosclerotic rabbit aorta

PGI₂ synthesis by aortic strips obtained from thoracic aorta of rabbits fed a high cholesterol diet was examined and compared with that of control rabbits fed a normal diet. In this report, the amounts of PGI₂ produced were shown as 6-keto-PGF_1α per μg of aortic tissue DNA instead of per mg wet weight. We also investigated PGI₂ synthesis by cultured smooth muscle cells (SMC) obtained from atherosclerotic intima.Basal PGI₂ production by aortic strips from atherosclerotic rabbit aorta was significantly augmented compared with that of controls. Arachidonic acid (AA)-induced PGI₂ production by atherosclerotic aorta was also significantly higher than that of controls. PGI₂ producing capacities of intimal and medial layers, separated from atherosclerotic aorta, were examined and the intimal layer was found to elicit a significantly greater PGI₂ production than the medial layer.Furthermore, cultured intimal SMC obtained from atherosclerotic rabbit aorta produced a greater amount of PGI₂ than medial SMC from normal rabbit aorta at various cultured conditions. These results suggest that the possibility of enhanced PGI₂ production by atherosclerotic aorta may well be considered as a defence mechanism of the vessel wall against damaging stimuli.     

Role of prostacyclin on steroidogenesis by frog interrenal gland

The role of prostacyclin (PGI₂) on amphibian adrenal steroidogenesis was studied in perifused interrenal fragments from adult male frogs. Exogenous PGI₂ (3×10⁻⁸ M to 3×10⁻⁵ M) and, in a lesser extent, 6-keto-PGF_1α increased both corticosterone and aldosterone production in a dose-related manner. Short pulses (20 min) of 0.88 μM PGI₂ administered at 90 min intervals within the same experiment did not induce any desensitization phenomenon. A prolonged administration (6 h) of PGI₂ gave rise to an important increase in steroid production followed by a decline of corticosteroidogenesis. Indomethacin (IDM, 5 μM) induced a marked reduction of the spontaneous secretion of corticosteroid which confirmed the involvement of endogenous PGs in the process of corticosteroid biosynthesis. The IDM-induced blockade of corticosterone and aldosterone secretion was totally reversed by administration of exogenous PGI₂ in our model. Angiotensin II (AII) induced a massive release of 6-keto-PGF_1α, the stable metabolite of PGI₂. The increase of 6-keto-PGF_1α preceded the stimulation of corticosterone and aldosterone secretions. In contrast, the administration of ACTH did not modify the release of 6-keto-PGF_1α. These results indicate that PGI₂ might be an important mediator of adrenal steroidogenesis in frog. They confirm that the corticosteroidogenic actions of ACTH and AII are mediated by different mechanisms.     

Actions of prostacyclin (PGI2) on adrenergic neuroeffector transmission in the rabbit kidney

In the Tyrode's perfused rabbit kidney PGI₂ (1.3 × 10⁻⁸-3.3 × 10^−7M) dose-dependently inhibited vasoconstrictor responses to sympathetic nerve stimulation, as did PGE₂. The dose-effect curve of the two compounds differed, making PGI₂ the less potent in the low concentration and the more potent in the high. PGI₂ also inhibited the vasoconstrictor response to exogenous noradrenaline, but it had no effect on transmitter release. The main metabolite of PGI₂, 6-keto-PGF_1α, was ineffective both on noradrenaline release and on vascular responses to nerve stimulation or exogenous noradrenaline. It is suggested that PGI₂,if a significant renal prostaglandin, may modulate renal neuroeffector transmission post-junctionally, thereby forming a complement to the prejunctional action of PGE₂.     

Depressant effects of prostacyclin in human atrial fibers and cardiomyocytes

The aim of this study was to characterize the electropharmacological effects of prostacyclin (PGI₂) in human atrial fibers and cardiomyocytes. Atrial tissues obtained from the hearts of 28 patients undergoing corrective cardiac surgery were used. Transmembrane action potentials were recorded using a conventional microelectrode technique, and twitch force by a transducer. Effects of PGI₂ (1 nM–10 µM) on action potential characteristics and contraction of atrial fibers were evaluated in normal [K]_o (4 mM) and high [K]_o (27 mM) in the absence and presence of cardiotonic agents. In addition, atrial and ventricular myocytes were isolated enzymatically from atrial tissues and hearts of 4 patients undergoing cardiac transplant. The effects of PGI₂ on Na- and Ca-dependent inward currents (I_Na and I_Ca) of cardiomyocytes were tested. In 9 human atrial fibers showing fast-response action potentials (mean dV/dt_max = 101 ± 15 Vs^–1) in 4 mM [K]_o, PGI₂ did not influence dV/dt_max of phase 0 depolarization even at 1 µM. However, at a concentration as low as 10 nM, PGI₂ depressed spontaneous rhythms or slow-response action potentials in high-K-depolarized fibers. PGI₂ also depressed delayed afterdepolarizations and aftercontractions induced by cardiotonic agents. In isolated cardiomyocytes, PGI₂ reduced I_Ca but not I_Na. The present findings show that, in human atrial fibers and cardiomyocytes, PGI₂ induces greater depressant effects on the slow-response action potential, I_Ca and triggered activity than on the fast-response action potential. It is suggested that PGI₂ may act through a selective reduction of transmembrane Ca influx.     

Release of prostaglandins I2 and E2 from the perfused mesenteric arterial bed of normotensive and hypertensive rats. Effects of sympathetic nerve stimulation and norepinephrine administration

The spontaneous output of prostaglandin (PG) I₂ from the perfused mesenteric arterial bed in vitro was significantly higher in hypertensive rats than in normotensive rats. Sympathetic nerve stimulation (at 10Hz) of the mesenteric arterial bed from normotensive rats caused a rapid and short-lived (< 4 min) two-fold increase in PGI₂ output and a smaller increase in PGE₂ output. Sympathetic nerve stimulation (at 10Hz) of the mesenteric arterial bed from hypertensive rats failed to increase PGI₂ and PGE₂ output. It is not possible to conclude whether this lack of response is a cause or a result of hypertension. Surprisingly, norepinephrine administration stimulated PGI₂ and PGE₂ release from the mesenteric arterial bed of both normotensive and hypertensive rats. Obviously, differences exist in the responsiveness of rat mesenteric arteries to endogenous and exogenous norepinephrine concerning PG release between the normotensive and hypertensive states.     

Prostacyclin (PGI2) Synthase Is a Constitutively Expressed Enzyme in Human Endothelial Cells

Biogenesis of prostanoids is under the control of some polypeptide growth factors. Cytosolic phospholipase A₂, a form specific for arachidonic acid containing phospholipids, is activated by a translocation mechanism regulated by growth factors, while prostaglandin H synthase isoforms are induced de novo in several cell types. No information is available as far as PGI₂ synthase is concerned. Human umbilical vein endothelial cells were cultured under conditions favoring proliferation or differentiation or capillary-like network formation in the presence of collagen gels. Basic fibroblast growth factor (bFGF 0.5-4 ng/ml) was used as a mitogen, interleukin-1α (IL-1α 10-60 UI/ml) as a differentiating agent, and prostacyclin (PGI₂) biosynthesis was evaluated. Under the first condition, basal PGI₂ production was unaffected while, in the presence of IL-1α, a marked stimulation of PGI₂ synthesis was observed. It is known that IL-1α is a potent inducer of PGH synthase, while it is not known whether PGI₂ synthase is also induced. Two lines of evidence indicate that PGI₂ synthase is a constitutively expressed not inducible enzyme: (a) proliferating nonproducing cells when added with PGH₂ produce an amount of PGI₂ not different from the amount produced by cells stimulated with IL-1α; (b) under this condition PGI₂ synthase was immunodetectable either by immunofluorescence detected by confocal microscopy or by ELISA and, on microsomes isolated from endothelial cells, by Western blotting. It is concluded that the limiting step in the conversion arachidonate-PGI₂ is represented solely by the level of PGH synthase. These results strongly suggest, but do not prove, the constitutive nature of the enzyme. The final demonstration requires the availability of a probe to detect mRNA level, a trial we are carrying out at the moment.     

Synthesis and chiroptical characterization of prostacyclin diastereomers

In the mercuri- and halo-cyclizations of PGF₂α methyl ester and its 11,15-bis(α-ethoxyethyl)-ether (or other protected forms) the exo-PGI₁ derivative predominates independent of reagent and degree of protective of the PGF₂α sample used. Diastereomerically pure samples of exo- and endo-PGI₁ and prostacyclin (PGI₂) were prepared. PGI₀ epimers were prepared: catalytic hydrogenation of PGI₂ Me ester provides exclusively the endo isomer. PGI₂ methyl ester was found to be stable to extensive chromatography on silica, and to storage for at least a year in anhydrous ethanol at −20°C. At pH 7.4 in 2:1 H₂O:EtOH, the ester has a half-life in excess of 5 hr at 25°C. A reproducible small scale (0.4–3 mg) synthesis of prostacyclin uses a modification of Whittaker's iodocyclization followed by DBN treatment. This procedure, developed with 15-³H-PGF₂α, proved widely applicable to PGF₂α analogs and diastereomers. The following prostacyclins (in the Me ester and Na salt forms) bearing the 5-en-6-yl ether unit were prepared in this way: ent-PGI₂, rac-PGI₂, 15-epi-PGI₂, ent-15-epi-PGI₂, 11-epi-PGI₂, 8,9,12-epi-PGI₂, -PGI₂, 13,14-dihydro-PGI₂, and 13,14-dihydro-15-epi-PGI₂. NMR comparisons for the methyl esters reveal that of the resonances (H-5,9,11,15) that appear at δ4.0±0.6 ppm, the most deshielded is H-9 so long as the 5,6-olefin is . The 8 ,9 -6,9-oxido- -5,6-ene unit is most readily characterized by its strong positive dichroic absorption at 210–230 nm. CD spectroscopy not only serves to confirm the presence of this unit in analogs, but also can be used for quantitative analysis of PGI₂ solutions and for monitoring the rate of hydrolytic cleavage of these enol ethers.     

Prostacyclin and thromboxane formation in human umbilical arteries following stimulation with vasoactive autacoids

The formation of prostacyclin (PGI₂) and thromboxane A₂ (TXA₂) (measured as the stable metabolites 6-keto-PGF_1α and TXB₂) during stimulation with vasoactive autocoids was registered in human umbilical arteries perfused . Responses were registered within 3–4 minutes after addition of the subtances. Both angiostensin I and II were found to increase the formation of PGI₂ while depressing that of TXA₂. Serotonin increased the formation of TXA₂ but not that of PGI₂. Both PGE₂ and PGF_2α stimulated the PGI₂ formation. The TXA₂ mimetic U46619, increased PGI₂ production, whereas PGI₂ slighlty increased the formation of TXA₂. All responses were found to be completely inhibited by indomethacin.     

Prostaglandin synthesis by fetal rat bone in vitro: Evidence for a role of prostacyclin

Prostaglandin synthesis by fetal rat bones was examined by thin-layer chromatography of culture media after preincubation with labeled arachidonic acid. Cultures in rabbit complement (non-heat inactivated serum) were compared with cultures in heat-inactivated serum or cultures treated with indomethacin. The major complement-dependent products were PGE₂, PGF_2α and 6-keto-PGF_1α, the metabolite of prostacyclin (PGI₂). Since PGI₂ had not been previously identified in bone its ability to stimulate bone resorption was tested. Repeated addition of PGI₂ stimulated release of previously incorporated ⁴⁵Ca from fetal rat long bones in both short-term and long-term cultures at concentrations of 10⁻⁵ to 10⁻⁹M. Because of the short half life of PGI₂ in solution at neutral pH, we tested a sulfur analog, thiaprostacyclin (S-PGI₂) which was found to be a stimulator of bone resorption at concentrations of 10⁻⁵ to 10⁻⁶M. These studies suggest that endogenous PGI₂ production may play a role in bone metabolism. Since vessels produce PGI₂ it is possible that PGI₂ release may be responsible for the frequent association between vascular invasion and resorption of bone or calcified cartilage in physiologic remodeling and pathologic osteolysis.     

Testing the “redirection hypothesis” of prostaglandin metabolism in the kidney

Furosemide increases the synthesis of two major renal eicosanoids, prostacylin (PGI₂) and thromboxane A₂ (TXA₂), by stimulating the release of arachidonic acid which in turn is metabolized to PGG₂/PGH₂, then to PGI₂ and TXA₂. PGI₂ may mediate, in part, the early increment in plasma renin activity (PRA) after furosemide. We hypothesized that thromboxane synthetase inhibition should direct prostaglandin endoperoxide metabolism toward PGI₂, thereby enhancing the effects of furosemide on renin release. Furosemide (2.0 mg.kg⁻¹ i.v.) was injected into Sprague-Dawley rats pretreated either with vehicle or with U-63, 557A (a thromboxane synthetase inhibitor, 2 mg/kg⁻¹ followed by 2 mg/kg⁻¹.hr⁻¹). Urinary 6ketoPGF₁ α and thromboxane B₂ (TXB₂), reflecting renal synthesis of PGI₂ and TXA₂, as well as PRA and serum TXB₂, were measured. Serum TXB₂ was reduced by 96% after U-63, 557A. U-63, 557A did not affect the basal PRA. Furosemide increased PRA in both vehicle and U63, 557A treated rats. However, the PRA-increment at 10, 20 and 40 min following furosemide administration was greater in U-63, 557A-treated rats than in vehicle-treated rats and urine 6ketoPGF₁ α excretion rates were increased. These effects of thromboxane synthesis inhibition are consistent with a redirection of renal PG synthesis toward PGI₂ and further suggest that such redirection can be physiologically relevant.     

Arachidonic acid stimulates interleukin-6 release from rat peritoneal macrophages in vitro: Evidence for a prostacyclin-dependent mechanism

Interleukin-6 (IL-6) is a cytokine involved in the differentiation of B-cells to antibody secreting plasma cells, the activation of T-cells, and the stimulation of hepatocyte production of acute phase proteins. Because of the pro-inflammatory effects of this cytokine, we investigated the ability of the fatty acid arachidonic acid (AA) to regulate the release of IL-6 from rat resident peritoneal macrophages (Mø) in vitro. AA (0.5–16 μM) stimulated IL-6 release during a 4 h incubation period in a biphasic manner, with 4 μM AA generating a peak of IL-6 release (3-5-fold). AA (0.5–16 μM) also induced an increasing release of the AA metabolite thromboxane B₂ (TXB₂). The AA-induced release of IL-6 occurred within 1–2 h of incubation, whereas TXB₂ concentrations were elevated within 5 min of AA treatment. The TX synthetase inhibitor CGS 12970 (4.0 μM and 40.0 μM) effectively blocked the generation of TXB₂, but increased prostacyclin (PGI₂) generation and potentiated the release of IL-6. In addition, PGI₂, as well as the PGI₂ agonists iloprost and cicaprost, stimulated IL-6 release from Mø by greater than 5-fold over vehicle-treated basal levels. These data suggest that PGI₂ (but not TXA₂) is involved in AA-induced IL-6 release from peritoneal Mø.     

On the multiplicity of platelet prostaglandin receptors I. Evaluation of competitive antagonism by aggregometry

Methods for the evaluation of competitive interactions at receptors associated with platelet activation and inhibition using aggregometry of human PRP have been developed. The evidence supports the suggestion that PGE₁ and PGI₂ share a common receptor for inhibition of platelet reactivity, but only a portion (if any) of the aggregation stimulation associated with PGE₂ is the result of PGE₂ binding (without efficacy) to this receptor. PGE₂ (@.3–20 μ ) is an effective antagonist of PGE₁, PGI₂, producing a shift of about one order of magnitude in the IC₅₀-values obtained from complete aggregation inhibition dose response curves. The antagonism of PGD₂ inhibition is particularly notable, 80 n PGE₂ levels are detectable. This and other actions of PGE₂ indicate another platelet receptor for PGE₂. PGE₁ acts at both the PGE₂ and PGI₂ receptor. Other substances showing PGI₂-like actions only at high doses (1–30 μ ), display additive responses with PGI₂ indicative of decreased affinity for the I₂/E₁ receptor and the absence of PGE₂-like aggregation stimulation activity.PGI₂ methyl ester has intrinsic inhibitory action not associated with in situ ester hydrolysis. The methyl ester is dissaggregatory showing particular specificity for inhibition of release and second wave aggregation.