Platelet-activating factor stimulates eicosanoid production in cultured feline tracheal explants.

Eicosanoids have been shown to be major mediators of airway inflammation. Platelet-activating factor (PAF), a potent bronchoconstrictor and stimulator of respiratory mucous secretion, may mediate some of its effects via eicosanoid production. To explore eicosanoid generation by cultured feline tracheal explants, eicosanoids were measured following PAF stimulation. After labeling the explants with [3H]arachidonic acid, supernatant from control and PAF treated explants was fractionated by reverse phase high-performance liquid chromatography (HPLC). The resulting elution pattern suggested the release of arachidonic acid (AA), 15-hydroxyeicosatetraenoic acid (HETE), leukotriene (LT)B4, C4, prostaglandin (PG) D2/E2/F2 alpha, and 6-keto-PGF1 alpha. Radioimmunoassay (RIA) following HPLC resolution confirmed that PAF induced a significantly increased release of peptido-leukotrienes, PGD2, PGE2, PGF2 alpha, LTB4, and 5-HETE, as well as thromboxane (TX) B2. The most remarkable increase was LTC4/D4/E4 (15 x control) and PGD2 (4 x control). The PAF antagonist Ro 19-3704 had an inhibitory effect on the PAF-stimulated release of peptido-leukotrienes. We conclude that PAF stimulates the production of a variety of lipoxygenase and cyclooxygenase pathway metabolites in feline airways.     

Roles of prostaglandin production and mitogen-activated protein kinase activation in hepatocyte growth factor—mediated rat hepatocyte proliferation

Hepatocyte growth factor (HGF) and epidermal growth factor (EGF)-stimulated DNA synthesis in primary cultured rat hepatocytes. HGF-induced DNA synthesis was concentration-dependently inhibited by a cyclooxygenase inhibitor, indomethacin. BW755C, a dual inhibitor for cyclooxygenase and lipoxygenase activities, also inhibited hepatocyte growth. Prostaglandin E₁ (PGE₁), PGE₂, and PGF_2α induced DNA synthesis even at such a low concentration as 5 nmol/L and potentiated [³H] thymidine incorporation induced by HGF in an additive manner. HGF caused arachidonic acid (AA) release and eicosanoid production. These events were detectable within 10 minutes after stimulation and lasted for at least 60 minutes. Furthermore, two proteins with approximately 40 kd were tyrosine phosphorylated by HGF. These proteins were identified as p42/p44 mitogenactivated protein (MAP) kinases by anti-MAP kinase immunoblots, which were known to activate cytosolic phospholipase A₂ (cPLA₂), a key enzyme in AA release. Activation of MAP kinases was detectable within 5 minutes after stimulation with HGF and lasted for at least 60 minutes. EGF-mediated DNA synthesis was also inhibited by the above cyclooxygenase inhibitors. EGF caused AA release and tyrosine phosphorylation of MAP kinases. These results suggest that HGF as well as EGF causes AA release, probably through activation of cPLA₂ mediated by MAP kinases, and that PGs, metabolites of AA, might play a pivotal role in hepatocyte proliferation in an autocrine mechanism.     

Prostaglandins,other eicosanoids and endothelial cells

Summary Endothelial cells are an important source of eicosanoid formation in the cardiovascular systems. All major pathways of eicosanoid production have been demonstrated in endothelial cells, yielding significant amounts of prostacyclin (PGI₂), PGE₂, PGF₂, thromboxane A₂, leukotrienes and a number of hydroxy fatty acids. The regulation of eicosanoid formation by endothelial cells is poorly understood. There is evidence that precursors, such as arachidonic acid or prostaglandin endoperoxides, may also be provided by other cell types. Endothelial cell-derived eicosanoids are involved in the regulation of local vessel tone, intravascular platelet activation, cell locomotion and, eventually, cell proliferation. Most of the available information considers PGI₂. This compound is the quantitatively dominating eicosanoid in endothelial cells. Major actions of PGI₂ include inhibition of platelet activation and aggregation, relaxation of arterial vessels and inhibition of growth-factor release. There is probably a tight interaction with other biologically active mediators which needs further evaluation. This also applies to the clinical significance of eicosanoid-related pathways for the mechanism of action of cardiovascular drugs, such as organic nitrates or acetylsalicylic acid. The unique property of the cicosanoid system to become activated only in response to stimulation, the local nature of this reaction, the multiplicity of products formed and the short half-time of most of them are currently the most significant obstacles to define the role of endothelial cell-derived eicosanoids in clinical practice.     

Effects of third ventricle injection of prostaglandins on gonadotropin secretion in goldfish,Carassius auratus

The effects of injection of prostaglandin (PG) E₁, PGE₂, and PGF_2α into the third ventricle on serum gonadotropin (GTH) concentrations in the goldfish were tested. Blood samples were taken at 30 min postinjection for radioimmunoassay of serum GTH. PG dosages of 0.5 and 1.0 μg were ineffective. However, PGE₂ and PGF_2α at the 2.0-μg dosage significantly suppressed serum GTH. PGE₁ at a 2.0-μg dosage had no effect. There were no effects on serum GTH when 2.0 μg of PGE₁, PGE₂, or PGF_2α were injected intraperitoneally. The results indicate that PGE₂ and PGF_2α suppress gonadotropin secretion by some action, presumably on the hypothalamus. However, action of PGE₂ and PGF_2α by diffusion from the site of injection to some other brain site or the pituitary cannot be eliminated in the present study.     

Effects of oestrogen on progesterone synthesis and arachidonic acid metabolism in human luteal cells

OBJECTIVE Locally produced oestrogens and prostaglandins (PGs) are implicated in the regulation of luteal lifespan in the human ovary. This study (1) assesses direct effects of these factors on progesterone synthesis in isolated luteal cells, and (2) explores interactions between luteal age and treatment with gonadotrophin or oestrogen on the metabolism of arachidonic acid (prostaglandin precursor) by steroidogenic luteal cells in vitro. DESIGN Primary monolayer cultures of human luteal cells obtained at different stages of the luteal phase were used to investigate the effect of oestradiol, catechol oestrogens (2- and 4-hydroxyoestradiol), diethylstilboestrol, PGE₂ and PGF_2x on basal and human chorionic gonadotrophin (hCG) stimulated progesterone production in vitro. The role of PGs as modulators of luteal cell function was further investigated by studying the metabolic fate of radioactively labelled arachidonic acid in hormone treated (oestradiol and hCG) and control cultures, assessed by high performance liquid chromatography. ATIENTS Corpora lutea were enucleated from nine women with regular ovulatory cycles undergoing microsurgical reversal of tubal sterilization. Granulosa cell aspirates were obtained from three patients undergoing in-vitro fertilization treatment. RESULTS PGE₂ and PGF_2α, at various concentrations did not have a consistent effect, whereas oestradiol, diethylstilboestrol (and 2-hydroxyoestradiol in early luteal cell cultures) significantly inhibited basal and hCG stimulated progesterone biosynthesis. Evidence for direct inhibition of 3β-hydroxysteroid dehydrogenase enzymic activity by oestradiol was obtained. Both major metabolic pathways of arachidonic acid (lipoxygenase and cyclo-oxygenase) were operative in steroidogenic luteal cells recovered throughout the luteal phase. The ratio of PGE₂ to PGF₂ synthesis in vitro by human luteal cells from endogenously incorporated arachidonic acid did not change significantly with corpus luteum age, with PGE₂ tending to predominate. Oestradiol treatment shifted arachidonic acid metabolism from the lipoxygenase towards the cyclo-oxygenase pathway in cells isolated from ageing corpora lutea. CONCLUSIONS Oestradiol, at relatively high concentrations, is a potent inhibitor of basal and hCG induced luteal cell steroidogenesis in vitro. No support is provided for the concept that luteolysis is mediated by local production of PGF_2α. The putative luteolytic effect of oestradiol may entail reduced metabolism of arachidonic acid to lipoxygenase derived products by luteal cells rather than direct stimulation of prostaglandin production by itself.     

Comparisons of prostaglandin vasoactive effects and interactions in the in vivo microcirculation of the rat urinary bladder

A combination of techniques for in vivo transillumination, topical application of vasoactive agents, and direct microscopic observation of microcirculatory responses was utilized to evaluate the vasomotor actions of prostaglandins (PGs) E₁, E₂, F_1α, F_2α, A₁, and A₂ on rat urinary bladder arterioles and venules. The effects of PGE₁ and histamine (HIS) on arteriolar responsiveness to norepinephrine (NE), serotonin (5-HT), and PGF_2α were measured. Histochemical studies were completed to determine the primary site of prostaglandin (PG) metabolic deactivation in the urinary bladder. Arteriolar dilatation occurred with HIS, PGE₁, PGE₂, PGA₁, PGA₂, and PGF_1α, all of which (with the exception of HIS) demonstrated significant dose-related responses. Overall, PGE₁ and PGE₂ were of greatest potency. Significant dose-related arteriolar constriction occurred with NE > PGF_2α > 5-HT (in order of decreasing potency). HIS, PGE₁, PGE₂, and PGA₁ produced significant venular dilatation; PGE₁ and HIS were dose related. Only NE resulted in significant venoconstriction. Arteriolar responsiveness to NE and PGF_2α decreased after pretreatment with PGE₁ but was unchanged by HIS pretreatment, whereas application of 5-HT following pretreatment with PGE₁ or HIS produced equivalent levels of arteriolar constriction. The primary site of deactivation of PGE₁ was histochemically localized to bundles of smooth muscle fibers in the muscular coat of the rat urinary bladder wall.     

Age-related changes in arachidonic acid metabolism of normal,leukemic and autoimmune strain mice

Spleen synthesis of prostaglandins (PGs) and thromboxane (Tx) was examined as a function of age for autoimmune, leukemic and normal mouse strains. Prostaglandin and thromboxane production from ¹⁴C-arachidonic acid was assessed from 20 minute synthesis by tissue homogenates. Values for young animals, six to 10 weeks of age, were compared to those for eight-to 12-month-old mice, except in the cases of the early autoimmune strain BXSB male and MRL/Ipr mice which were examined at five to seven months of age. Total eicosanoid synthesis, as well as PGF_2α , PGE₂, and PGD₂ production showed, in general, a pattern of increase with age in nonautoimmune strain BALB/c, C57BI/6 and female BXSB mice. In contrast, PGE₂ and PGF_2α synthesis decreased in the autoimmune MRL/Ipr and male BXSB models and in the nonautoimmune, but lymphoma-developing AKR strain. The late autoimmune MRL/++ strain showed decreased PGE₂ but no change in PGF_2α with age. Age-related PGE₂ and PGF_2α changes were minimal in the nonautoimmune DBA/2 and in the late autoimmune PN strains. Thromboxane B₂ appeared to be conserved in all strains studied. The results suggest that diminished spleen PG production with age is linked to development of autoantibodies and lymphoproliferative disorders. Further studies are necessary to establish whether decreased synthesis of immunoregulatory PGs plays a causal role in disease development or arises as a consequence of disease.     

Shell gland responsiveness to prostaglandins F2α and E1 and to arginine vasotocin during the laying cycle of the domestic hen (Gallus domesticus)

Contractile responses of isolated shell gland (SG) strips from laying hens displayed no significant differences 6 hrs before oviposition, at oviposition, and 6 hrs after oviposition when stimulated with arginine vasotocin (AVT), prostaglandin E₁ (PGE₁), or prostaglandin F_2α (PGF_2α). Dose-response curves show that the sensitivity of the SG to these agents, in vitro, is: AVT > PGF₂ > PGE₁. PGF_2α, however, produces the largest contractile response, while PGE₁ appears to be a poor agonist of contractile activity in vitro. These results are discussed in relation to the known hormonal patterns during the ovulatory cycle of the hen and the physiological roles attributed to these oxytocics in the control of oviposition.     

Does of the local myocardial release of prostaglandin E2 or F2α contribute to the early consequences of acute myocardial ischaemia?

The possible role of prostaglandins in acute myocardial ischaemia was investigated in anaesthetized greyhounds which were subjected to either short (3 min) occlusions or permanent ligation of the left anterior descending coronary artery. Plasma PGE₂ and PGF_2α concentrations were measured by radioimmunoassay in blood from the aorta, the coronary sinus (draining the essentially normal myocardium) and from a local coronaryvein (draining the area rendered ischaemic by coronary artery ligation). In the permanent ligation studies PGE₂ and PGF_2α were measured before, and 2, 10 and 30 min post-ligation. At 2 and 10 min post ligation there were no significant changes in the concentrations of either prostaglandin in blood from the essentially normal myocardium or from the acutely ischaemic myocardium. After 30 min of coronary artery ligation there was a significant increase in PGE₂ in the local coronary vein. This release of PGE₂ from the ischaemic myocardium was not related to the occurrence of cardiac dysrhythmias but may reflect the onset of changes in cellular integrity in the developing area of infarction. Despite electrocardiographic and metabolic evidence of acute myocardial ischaemia there was no increase in PGE₂ or PGF_2α values in either the coronary sinus or the local coronary vein following the release of a 3-min coronary artery occlusion. Since neither PGE₂ nor PGF_2α is released from the acutely ischaemic myocardium during the first 10 min post-ligation or during reperfusion following 3-min occlusions, it is unlikely that either of these prostaglandins is involved in the early consequence of coronary artery ligation.     

Prostaglandin effects upon cyclic AMP levels, corticosterone output, and aldosterone output in the adrenal of the frog, Rana berlandieri forreri.

The mechanism of prostaglandin action in the adrenal of the frog (Rana berlandieri forreri) was evaluated in vitro. The prostaglandins evoked transient (PGA₂, PGB₁, PGE₂), continued (PGA₁, PGB₂, PGF_1α, PGF_2α), or no effects (PGE₁) upon cyclic AMP (cAMP) levels. Further, the cAMP levels were depressed (PGA₁, PGB₁, PGB₂), elevated (PGA₂, PGE₂, PGF_1α, PGF_2α), or unchanged (PGE₁) compared to the controls. Prostaglandins regulate cAMP levels in the frog adrenal. In addition, specific prostaglandins evoke specific effects in this regard. The frog adrenal corticosterone and aldosterone outputs are modulated by the prostaglandins. The modulations produced differ with the prostaglandins tested. The greatest adrenocortical sensitivity is to PGB₂ which evoked about a 13-fold increase in corticosterone output and a 5-fold increase in aldosterone output at 16 min. Only PGF_1α inhibited steroid output; the other prostaglandins stimulated steroid outputs to varying degrees. A close correlation of the prostaglandin-evoked corticosterone and aldosterone output responses with the cAMP changes was not present in the frog adrenal. The prostaglandins differentially affected cAMP levels and when compared to adrenocorticoid outputs, different prostaglandins produce different effects. The responses of the frog adrenal indicate that the mechanisms controlling adrenocortical function are more complex than originally visualized. In this regard, the site(s) of action (plasma membrane, mitochondrion, etc.) of a given factor must also be considered. Thus, the current concepts must be broadened to include a number of interacting factors among which are the cyclic nucleotides and the prostaglandins.     

Eicosanoid production in isolated perfused lungs stimulated by calcium ionophore A23187

The patterns of lung eicosanoid production were investigated in five different species by stimulating isolated lungs with calcium ionophore A23187 (10(-5) M). The species studied were the rat, ferret, hamster, guinea pig, and rabbit. The eicosanoids measured included the cyclooxygenase metabolites 6-keto-PGF1 alpha, TxB2, and PGE2 and the lipoxygenase metabolites LTC4, LTD4, and LTB4. Since these metabolites are either retained in the lungs or released into the perfusate, eicosanoids were measured in lung tissue and in the perfusate. In all animal species, calcium ionophore stimulated the production of all the eicosanoid metabolites measured, although in different relative proportions and quantities. The sulfidopeptide LTC4 was predominantly retained in the lung tissue while other eicosanoids were distributed more evenly between tissue and perfusate, with tissue levels generally being higher. The ferret lung produced large quantities of eicosanoids, more than 90% being lipoxygenase products. In contrast, the guinea pig lung produced predominantly cyclooxygenase products, especially TxB2. The rat, hamster, and rabbit lungs showed a more even distribution of cyclooxygenase and lipoxygenase products. Species differences in the ability to produce lipid mediators in the lung may be important in determining the vasoconstrictive, bronchoconstrictive, or inflammatory response to physiologic stimuli.     

Eicosanoid synthesis in children with cholestatic disease

Summary The possibility of malabsorption of triglycerides contained in the diets of children with cholestasis suggests a deficiency of essential fatty acids and, therefore, probable effects on eicosanoid metabolism. Children with either biliary atresia (BA) or syndromatic paucity of interlobular bile ducts (PILBD) were evaluated as to plasma and platelet total lipid fatty acid composition and synthesis of prostaglandins (PG) E₁, PGE₂, PGI₂, PGF₂, and thromboxane (TXB₂) by whole blood incubated at 37°C for 10 min. In both diseases linoleate deficiency was present as shown by low 18:2 fatty acids in plasma lipids. The children with BA had lower plasma arachidonate than controls but normal eicosanoid synthesis except for excess PGI₂. Those with PILBD had low platelet arachidonate and were severely deficient in TXB₂ synthesis (<10% of controls). Three children with PILBD were fed a supplement of structured triglyceride (Captex 810) intended to provide as much as 10% of energy as linoleate for 2–3 months. Results for these three cases suggested that insufficient linoleate was absorbed to increase plasma linoleate and differences in eicosanoids could not be attributed to linoleate supplementation.Journal Paper No. J-12248 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Project Number 2728.     

In vitro release of prostaglandins and leukotrienes from synovial tissue,cartilage, and bone in degenerative joint diseases

Objective. To determine the major source of eicosanoid release in arthritic joint tissues and to examine the modulation of this release by indomethacin and diclofenac. Methods. Release of prostaglandin E₂ (PGE₂), 6-keto-PGF_1α, leukotriene B₄ (LTB₄), and LTC₄ was measured in supernatants of synovial tissue, cartilage, and bone incubates from patients with osteoarthritis, active rheumatoid arthritis (RA), inactive RA, and pseudogout. Radioimmunoassay (RIA) was used to determine the levels of the eicosanoids. Results. Addition of the divalent cation ionophore A23187 resulted in significant release of all eicosanoids measured from synovial tissue, but not from cartilage, cortical bone, or cancellous bone. PG release was significantly inhibited by the addition of indomethacin or diclofenac at either 10_–5 moles/liter or 10_–7 moles/liter. The amount of LTC₄ released from cartilage and bone was only slightly above the detection limit of the RIA, whereas large amounts were released from synovial tissue. Neither indomethacin nor diclofenac had an effect on LTC₄ release. LTC₄ release from synovial tissue of patients with inactive RA was significantly decreased in comparison with the levels from synovial tissue of patients with the other joint diseases. There was no significant difference in PG release among patients in the various disease groups. Conclusion. Synovial tissue appears to be the major source of eicosanoids in synovial fluid. Indomethacin and diclofenac inhibit the release of PG, but not LT, from various joint tissues.     

Arachidonic acid metabolites and glucocorticoid regulatory mechanism in cultured porcine tracheal smooth muscle cells

To elucidate the signal transduction system in the production of prostaglandin E₂ (PGE₂) by porcine tracheal smooth muscle cells in culture (PTSMC), we examined the pattern of arachidonic acid metabolites released from PTSMC and the relationship between bradykinin-stimulated rises in intracellular calcium concentration ([Ca²⁺]_i) and PGE₂ production by PTSMC. We next examined the effect of dexamethasone on these parameters. Bradykinin induced a dose-dependent increase in both the rise in [Ca²⁺]_i and PGE₂ production by PTSMC. The increase in [Ca²⁺]_i paralleled an increase in PGE₂ production. High-performance liquid chromatography (HPLC) revealed that dexamethasone-treated PTSMC were suppressed to release arachidonic acid metabolites such as PGE₂ and prostaglandin F₂ (PGF₂). Incubation of PTSMC with 10^–6M dexamethasone for 24 h significantly suppressed both the rise in [Ca²⁺]_i and PGE₂ production by PTSMC in response to bradykinin, and also significantly suppressed bradykinin-stimulated release of radioactivity from PTSMC prelabeled with ³H-labeled arachidonic acid (³H-AA). When PTSMC pretreated with dexamethasone were incubated with 170 nM prostaglandin H₂ (PGH₂) or 20 M arachidonic acid; PTSMC synthesized less PGE₂ than control PTSMC. Results suggest that bradykinin stimulates PTSMC to produce PGE₂ via the signal transduction system including Ca²⁺, and dexamethasone appeared to suppress PGE₂ production by reducing the activity of cytosolic phospholipase A₂ (cPLA₂) and PGE₂ synthase. However, we failed to demonstrate the suppression of the activity of cyclooxygenase in PTSMC by dexamethasone. Since the elevation of [Ca²⁺]_i is necessary for the contraction of airway smooth muscles, dexamethasone seems to reduce the contraction of airway.Offprint requests to: Hideki Tanaka     

An investigation into the role of prostaglandins in zebrafish oocyte maturation and ovulation

This study explored the potential for ovarian-derived prostaglandins (PGs) to be involved in the regulation of oocyte maturation and ovulation in zebrafish. It was demonstrated that cultured vitellogenic follicles have the capacity to produce prostaglandin E₂ (PGE₂) and PGF_2α in response to arachidonic acid (AA) in a concentration-dependent manner, and that AA stimulates the in vitro production of 17β-estradiol (E₂). The production of AA-stimulated PGF_2α was significantly reduced by treatment with the non-selective cyclooxygenase (COX) inhibitor, indomethacin (INDO). Treatment of full-grown follicles with AA did not induce oocyte maturation as assessed by germinal vesicle breakdown, but INDO significantly decreased the rate of spontaneous maturation. Using Real-Time PCR, it was shown that follicles of different developmental size classes (primary growth and pre-vitellogenic, early-vitellogenic, and mid- to full-grown vitellogenic) express enzymes that release (cytosolic phospholipase A₂ (cPLA₂); phospholipase Cγ1) or metabolize (COX-1, COX-2, and prostaglandin synthase-2) AA to PG metabolites. The expression of cPLA₂ was found to be significantly greater in full-grown follicles compared to follicles of the pre- and early-vitellogenic stages. In vivo studies demonstrated that breeding groups of zebrafish exposed to 100 μg/L INDO exhibited reduced spawning rates and clutch sizes compared with control and 1 μg/L INDO exposed fish. In other studies, it was shown that naturally spawning groups of females exhibit increased ovarian levels of PGF_2α, E₂, and 17α,20β-dihydroxy-4-pregnen-3-one (a maturation-inducing hormone in zebrafish) near the time of ovulation compared with non-breeding females. Collectively, these experiments indicate that the AA pathway in zebrafish ovaries is involved in the regulation of oocyte maturation and ovulation and a non-selective inhibitor of COX disrupts these processes.     

Synthesis of prostaglandins by cultured rat hearts myocytes and cardiac mesenchymal cells

Ischemia, trauma and hormonal stimulation elicit the release of prostaglandins (PGs) from the heart. Although PGI₂ is synthesized by coronary arteries, the capacities for PG synthesis of individual types of cells comprising the heart have not been elucidated. Accordingly, synthesis of prostaglandins by cultured rat cardiac myocytes and mesenchymal cells was evaluated by radiochromatography of products obtained by incubating cells with [1-¹⁴C]arachidonate, and verified by assessing the effects of cell incubation medium on platelet aggregation. Cultured mesenchymal cells synthesized PGs E₂, F_2α and 6-keto-F_1α, a metabolite of PGI₂ (2076 ± 183, 1284 ± 158 and 1194 ± 152 dpm/mg protein/30 min, respectively). Medium from mesenchymal cells inhibited platelet aggregation, an effect abolished by preincubating the cells with indomethacin, further indicating that these cells synthesized PGI₂. Cardiac myocytes synthesized PGE₂ and PGF_2α (952 ± 227 and 287 ± 104 dpm/mg protein/30 mins, respectively), but no PGI₂. Medium from myocytes did not inhibit platelet aggregation. Prostaglandins D₂, A₂ and thromboxane were not synthesized by either type of cell. Thus, PGI₂ is synthesized by cardiac mesenchymal cells and the hitherto uncharacterized sources of PGE₂ and PGF_2α found in coronary sinus effluent may include cardiac myocytes as well as mesenchymal cells.     

Effects of PAF on cardiac function and eicosanoid release in the isolated perfused rat heart: comparison between normotensive and spontaneously hypertensive rats

The aim of this study was (a) in isolated perfused rat heart to characterize the effects of platelet-activating factor (PAF) on coronary flow, ventricular contractility, and eicosanoid release and (b) to determine whether PAF effects are altered in hearts from spontaneously hypertensive rats (SHR). PAF (10^–10–10^–7 mol) dose-dependently decreased coronary flow and ventricular contractility; concomitantly, coronary effluent concentrations of thromboxane (TX)B₂ and prostaglandin F₂ (PGF₂) were elevated but not those of prostacyclin. The PAF receptor antagonist WEB 2086 (10^–7–10^–5 mol/l) concentration-dependently antagonized these PAF effects. In addition, the cyclo-oxygenase inhibitor indomethacin (5×10^–5 mol/l) prevented PAF (10^–9–10^–7 mol) induced eicosanoid release; in the presence of indomethacin PAF caused coronary constriction and ventricular depression only at the highest dose (10^–7 mol) but had no effect at 10^–9 or 10^–8 mol. Moreover, the TXA₂ antagonist SQ 29,548 (10^–6 mol/l) completely inhibited 10^–8 mol PAF induced ventricular depression but did not effect coronary constriction. In SHR PAF (10^–9–10^–7 mol) evoked decreases in coronary flow and ventricular contractility did not differ from those in normotensive Wistar-Kyoto rats while PAF induced TXA₂ and PGF₂ release was markedly enhanced. In addition, decreases in coronary flow and ventricular contractility induced by the TXA₂ agonist U 46619 (10^–7 mol/l) were markedly depressed in SHR. We conclude that in isolated perfused rat heart PAF causes coronary constriction and depression of ventricular function mainly indirectly through released TXA₂ and/or PGF₂. Moreover, the fact that in SHR the PAF effects on coronary flow and ventricular function are not altered despite markedly enhanced TXA₂ and PGF₂ release supports the view that in the SHR the receptors mediating TXA₂ and/or PGF₂ effects are desensitized.     

Prostaglandins, Slow-reacting Substances (Leukotrienes) and the Lung

Abstract: Prostaglandins (PGs) are synthesised from arachidonic acid in human lung tissue and their effects on bronchial smooth muscle have suggested a possible role in the pathogenesis of airflow obstruction in asthmatic subjects, who are exquisitely sensitive to inhalation of PGF_2α. More recently it has become evident that the endoperoxides, which are unstable intermediates in the biosynthesis of PGs, may be converted to thromboxane A₂ which is more potent than PGF_2α in animal experimental models of airflow obstruction. Prostacyclin which is released by the lung into the circulation inhibits platelet aggregation but appears to have minimal effect on broncho-motor tone. Non-steroidal anti-inflammatory drugs such as aspirin and indomethacin prevent the formation of PGs and throm-boxanes by inhibiting the cyclooxygenase enzyme which converts arachidonic acid to the endoperoxides. The failure of indomethacin to prevent allergen-induced and exercise-induced asthma mitigates against a direct involvement of cyclooxygenase products. However experiments with human lung tissue in vitro have shown increased antigen-induced release of slow-reacting substance of anapbylaxis (SRS-A) in the presence of indomethacin. Since SRS-A is a potent broncho-constrictor it is possible that an enhanced release of SRS-A, in subjects pre-treated with indornethacin, was contributing to their continued bronchoconstriction following antigen or exercise challenge. A similar mechanism may be involved in aspirin-sensitive asthmatics. The recent discovery that SRS-A is a lipoxygenase product of arachidonic acid has emphasised the importance of this alternative pathway for the metabolism of arachidonic acid. The structure of SRS-A has been identified as leukotriene D and chemically pure leukotrienes have been synthesised to enable detailed studies of their role in the patho-pbysiology of asthma and other lung diseases. It should now be possible to develop leukotriene synthesis inhibitors and receptor antagonists for clinical use.     

Prostanoids and the Cough Reflex

Prostanoids such as prostaglandin (PG) D₂, PGE₂, PGF_2α, prostacyclin (PGI₂), and thromboxane (Tx) A₂ act via five classes of receptors named DP, EP, FP, IP, and TP, respectively, and mediate a diverse range of physiological effects. Prostanoids are commonly associated with many diseases as a proinflammatory mediator; however, in the lung, prostanoids, particularly PGE₂, seem to have a protective role. Inhaled PGE₂ has been shown to be anti-inflammatory and a bronchodilator but causes cough. This has hindered the development of prostanoids for the treatment of airway inflammatory diseases. We discuss here the extensive research into the role of prostanoids in the airways and their modulation of the cough reflex.