Characterization and ontogeny of PGE2 and PGF2α receptors on the retinal vasculature of the pig

The vasoconstrictor effects of PGE₂ and PGF_2α are less pronounced on retinal vessels of the newborn than of the adult pig. We tested the hypothesis that the decreased vasomotor response to these prostaglandins might be due to relatively fewer receptors and/or different receptor subtypes (in the case of PGE₂) on retinal vessels of the newborn animal. Binding studies using [³H]PGE₂ and [³H]PGF_2α revealed that PGE₂ (EP) and PGF_2α (FP) receptor densities in retinal microvessel membrane preparations from newborn animals were approximately 25% of those found in vessels from the adult. The K_d for PGF_2α did not differ; however, the K_d for PGE₂ was less in newborn than in adult vessels. Competition binding studies using AH 6809 (EP₁ antagonist), butaprost (EP₂ agonist), M&B 28,767 (EP₃ agonist), and AH 23848B (EP₄ antagonist) suggested that the retinal vessels of the newborn contained approximately equal number of EP₁ and EP₂ receptor subtypes whereas the main receptor subtype in the adult vessels was EP₁. In addition, PGE₂ and butaprost produced comparable increases in adenosine 3′,5′-cyclic monophosphate synthesis in newborn and adult vessels. PGE₂, 17-phenyl trinor PGE₂ (EP₁agonist) and PGF_2α caused a 2.5 to 3-fold greater increase in inositol1,4,5-triphosphate (IP₃) formation in adult than in newborn preparations. It is concluded that fewer PGF_2α receptors and an associated decrease in receptor-coupled IP₃ formation in the retinal vessels of the newborn could lead to weaker vasoconstrictor effects of PGF_2α on retinal vessels of the newborn than of adult pigs; fewer EP₁ receptors (associated with vasoconstriction) and a relatively greater proportion of EP₂ receptors (associated with vasodilation) might be responsible for the reduced retinal vasoconstrictor effects of PGE₂ in the newborn.     

Characterisation of the prostanoid receptors mediating contraction of guinea-pig isolated trachea

The receptors mediating prostanoid-induced contraction of guinea-pig isolated trachea have been characterised in terms of a recently proposed general classification of prostanoid receptors. Results obtained on the trachea were compared with those obtained on guinea-pig fundus, which contains a sub-type of PGE₂-sensitive (EP-) receptor termed the EP₁-receptor, and guinea-pig lung strip, which contains a thromboxane-sensitive or TP-receptor. The following agonists were studied, PGE₂, PGF₂α and the thromboxane-like agonists U-46619 and Wy17186. The antagonists studied were SC-19220 which selectivity blocks EP₁-receptors, and AH19437 which selectively blocks TP-receptors. On guinea-pig fundus the rank order of agonist potency was PGE₂ > PGF₂α > Wy17186 U-46619, and responses to all agonists were antagonised by SC-19220 but not by AH19437. On guinea-pig lung strip the rank order of potency was U-46619 > Wy17186 PGF₂α > PGE₂ and responses to all agonists tested were blocked by AH19437 but not by SC-19220. On the trachea, the rank order was PGE₂ = U-46619 > Wy17186 = PGF₂α. SC-19220 antagonised responses to PGE₂ and PGF₂α, but not those to U-46619 or Wy17186. Conversely, AH19437 antagonised responses to U-46619 and Wy17186 but not those to PGE₂ or PGF₂α. It is concluded that prostanoid-induced contractions of guinea-pig trachea can be mediated by both EP₁- and TP-receptors.     

Effects of selective PGE2 receptor antagonists in esophageal adenocarcinoma cells derived from Barrett's esophagus

Accumulating evidence suggests that COX-2-derived prostaglandin E₂ (PGE₂) plays an important role in esophageal adenocarcinogenesis. Recently, PGE₂ receptors (EP) have been shown to be involved in colon cancer development. Since it is not known which receptors regulate PGE₂ signals in esophageal adenocarcinoma, we investigated the role of EP receptors using a human Barrett's-derived esophageal adenocarcinoma cell line (OE33). OE33 cells expressed COX-1, COX-2, EP₁, EP₂ and EP₄ but not EP₃ receptors as determined by real time RT-PCR and Western-blot. Treatment with 5-aza-dC restored expression, suggesting that hypermethylation is involved in EP₃ downregulation. Endogenous PGE₂ production was mainly due to COX-2, since this was significantly suppressed with COX-2 inhibitors (NS-398 and SC-58125), but not COX-1 inhibitors (SC-560). Cell proliferation (³H-thymidine uptake) was significantly inhibited by NS-398 and SC-58125, the EP₁ antagonist SC-51322, AH6809 (EP₁/EP₂ antagonist), and the EP₄ antagonist AH23848B, but was not affected by exogenous PGE₂. However, treatment with the selective EP₂ agonist Butaprost or 16,16-dimethylPGE₂ significantly inhibited butyrate-induced apoptosis and stimulated OE33 cell migration. The effect of exogenous PGE₂ on migration was attenuated when cells were first treated with EP₁ and EP₄ antagonists. These findings suggest a potential role for EP selective antagonists in the treatment of esophageal adenocarcinoma.     

Prostanoids receptors signaling in different diseases/cancers progression

Abstract

Prostanoids, that is, prostaglandins (PGs) PGE₂, PGF_2α, PGI₂, PGD₂ and thromboxane A₂(TXA₂), are the oldest members of the eicosanoid family. The PGs are a family of lipid mediators formed in response to various stimuli. They are transported into the extracellular microenvironment by specific multidrug resistance-associated proteins (MRPs) after synthesis. Once exported to the microenvironment, prostanoids bind to G-protein coupled receptors that contain seven transmembrane spanning domains. There are eight types of the prostanoid receptors conserved in mammals from mouse to human. They are the PGD receptor (DP), four subtypes of the PGE receptor (EP₁, EP₂, EP₃ and EP₄), the PGF receptor (FP), PGI receptor (IP) and TXA receptor (TP). Recently, several studies have revealed the roles of PG receptor signaling in various pathological conditions, and suggest that selective manipulation of the prostanoid receptors may be beneficial in treatment of the pathological conditions. Here we review these recent findings of roles of prostanoid receptor signaling and their therapeutic implications.     

Molecular and pharmacological characterization of zebrafish ‘contractile’ and ‘inhibitory’ prostanoid receptors

Prostanoids comprising prostaglandins (PGs) and thromboxanes (TXs) have been shown to play physiological and pathological roles in zebrafish. However, the molecular basis of zebrafish prostanoid receptors has not been established. Here, we demonstrate that there exist at least five ‘contractile’ (Ca²⁺-mobilizing) and one ‘inhibitory’ (G_i-coupled) prostanoid receptors in zebrafish; five ‘contractile’ receptors consisting of two PGE₂ receptors (EP1a and EP1b), two PGF_2α receptors (FP1 and FP2), and one TXA₂ receptor TP, and one ‘inhibitory’ receptor, the PGE₂ receptor EP3. [³H]PGE₂ specifically bound to the membranes of cells expressing zebrafish EP1a, EP1b and EP3 with a Kd of 4.8, 1.8 and 13.6 nM, respectively, and [³H]PGF_2α specifically bound to the membranes of cells expressing zebrafish FP1 and FP2, with a Kd of 6.5 and 1.6 nM, respectively. U-46619, a stable agonist for human and mouse TP receptors, significantly increased the specific binding of [³⁵S]GTPγS to membranes expressing the zebrafish TP receptor. Upon agonist stimulation, all six receptors showed an increase in intracellular Ca²⁺ levels, although the increase was very weak in EP1b, and pertussis toxin abolished only the EP3-mediated response. Zebrafish EP3 receptor also suppressed forskolin-induced cAMP formation in a pertussis toxin-sensitive manner. In association with the low structural conservation with mammalian receptors, most agonists and antagonists specific for mammalian EP1, EP3 and TP failed to work on each corresponding zebrafish receptor. This work provides further insights into the diverse prostanoid actions mediated by their receptors in zebrafish.     

Prostanoid Receptors Involved in Regulation of the Beating Rate of Neonatal Rat Cardiomyocytes

Although prostanoids are known to be involved in regulation of the spontaneous beating rate of cultured neonatal rat cardiomyocytes, the various subtypes of prostanoid receptors have not been investigated in detail. In our experiments, prostaglandin (PG)F_2α and prostanoid FP receptor agonists (fluprostenol, latanoprost and cloprostenol) produced a decrease in the beating rate. Two prostanoid IP receptor agonists (iloprost and beraprost) induced first a marked drop in the beating rate and then definitive abrogation of beating. In contrast, the prostanoid DP receptor agonists (PGD₂ and BW245C) and TP receptor agonists (U-46619) produced increases in the beating rate. Sulprostone (a prostanoid EP₁ and EP₃ receptor agonist) induced marked increases in the beating rate, which were suppressed by SC-19220 (a selective prostanoid EP₁ antagonist). Butaprost (a selective prostanoid EP₂ receptor agonist), misoprostol (a prostanoid EP₂ and EP₃ receptor agonist), 11-deoxy-PGE₁ (a prostanoid EP₂, EP₃ and EP₄ receptor agonist) did not alter the beating rate. Our results strongly suggest that prostanoid EP₁ receptors are involved in positive regulation of the beating rate. Prostanoid EP₁ receptor expression was confirmed by western blotting with a selective antibody. Hence, neonatal rat cardiomyocytes express both prostanoid IP and FP receptors (which negatively regulate the spontaneous beating rate) and prostanoid TP, DP₁ and EP₁ receptors (which positively regulate the spontaneous beating rate).     

Studies of the characterisation of prostanoid receptors: A proposed classification

Comparison of rank orders of agonist potency of the anturally occurring prostanoids. PGD₂, PGE₂, PGF₂α and PGI₂ as well as the stable TxA₂ mimetic, U-46619, on a range of smooth muscle preparations provides evidences evidence for the existence of distinct receptors for PGE₂. PGF₂α and TxA₂. Since others have provided evidence for the existence of distinct receptors for PGD₂ and PGI₂, we suggest that receptors exist for each of these naturally occurring 2-series prostanoids. Results obtained with two specific prostanoid receptor blocking drugs, SC-19220 and AH 19437, supported and extend these conclusions. SC-19220 selectively block some but not all PGE-sensitive receptors. While AH 19437 selectively blocks all U-46619/TxA₂-sensitive receptors. A nomenclature for prostanoid receptors is proposed,; in which each receptor is designated the letter P preceded by a letter signifying the most potent natural prostanoid agonist at than receptor, such that receptors sensitivity to PGs D₂, E₂, F₂α, I₂ and TxA₂ become DP-, EP-, FP- and TP- receptors respectively. Where some sub-division is required within a receptor group, e.g. EP-receptors (SC-19220-sensitive and SC-19220-insensitive), subcript numbers may be used such that these are EP₁ and EP₂ subtypes. The resulting scheme is a working hypothesis and its confirmation requires the development of potent selective prostanoid receptor blocking drugs for each postulated type.     

Prostaglandins and congeners XIII (1). The synthesis of dℓ-erythro-16-methoxyprostaglandins

$\text{d?-Erythro}$-16-methoxy-PGE₂, PGA₂, PGF_2α, 11-deoxy PGE₁, and 11-deoxy PGF_1α have been prepared via the cuprate conjugate addition procedure. These congeners are less potent than the parent prostaglandins as stimulators of isolated gerbil colon contractions and as bronchodilators in the guinea pig Konzett assay.     

Prostaglandin E2 promotes proliferation of skeletal muscle myoblasts via EP4 receptor activation

We recently demonstrated that conditioned media (CM) from osteocytes enhances myogenic differentiation of myoblasts, suggesting that signaling from bone may be important for skeletal muscle myogenesis. The effect of CM was closely mimicked by prostaglandin E₂ (PGE₂), a bioactive lipid mediator in various physiological or pathological conditions. PGE₂ is secreted at high levels by osteocytes and such secretion is further enhanced under loading conditions. Although four types of receptors, EP1 to EP4, mediate PGE₂ signaling, it is unknown whether these receptors play a role in myogenesis. Therefore, in this study, the expression of EPs in mouse primary myoblasts was characterized, followed by examination of their roles in myoblast proliferation by treating myoblasts with PGE₂ or specific agonists. All four PGE₂ receptor mRNAs were detectable by quantitative real-time PCR (qPCR), but only PGE₂ and EP4 agonist CAY 10598 significantly enhance myoblast proliferation. EP1/EP3 agonist 17-phenyl trinor PGE₂ (17-PT PGE₂) and EP2 agonist butaprost did not have any significant effects. Moreover, treatment with EP4 antagonist L161,982 dose-dependently inhibited myoblast proliferation. These results were confirmed by cell cycle analysis and the gene expression of cell cycle regulators. Concomitant with the inhibition of myoblast proliferation, treatment with L161,982 significantly increased intracellular reactive oxygen species (ROS) levels. Cotreatment with antioxidant N-acetyl cysteine (NAC) or sodium ascorbate (SA) successfully reversed the inhibition of myoblast proliferation and ROS overproduction caused by L161,982. Therefore, PGE₂ signaling via the EP4 receptor regulates myogenesis by promoting myoblast proliferation and blocking this receptor results in increased ROS production in myoblasts.     

The effects of synthetic prostaglandin analogs on canine hepatic bile flow

The synthetic prostaglandin analogs 16, 16-dimethyl PGF_2α and 16, 16-dimethyl PGE₂ were administered to dogs with chronic biliary and gastric fistulas. The effects of 16, 16 diMePGF_2α and 16, 16 diMePGE₂ were evaluated on bile flow and composition and bile adenosine 3′, 5′ monophosphate (cyclic AMP) secretion. 16, 16 diMePGF_2α in doses of 0.125 and 0.25 μg-kg-min significantly increased hepatic bile flow. The choleresis was characterized by increased cloride and bicarbonate secretion. Measurement by radioimmunoassay of bile cyclic AMP concentration demonstrated no evident increase in bile cyclic AMP secretion associated with the choleresis produced by 16, 16 diMePGF_2α. The administration of 16, 16 diMePGE₂ in a dose range 0.01 to 1.0 μg-kg-min did not significantly alter bile flow rates or composition. Bile erythritol-¹⁴C clearance, a measure of canalicular bile flow, was significantly increased by PGF_2α but not by 16, 16-dimethyl PGF_2α, suggesting that the mechanism of action of PGF_2α in stimulating hepatic bile flow may be different from that involved in 16, 16-dimethyl PGF_2α choleresis. The results of this study indicate that the synthetic PGF_2α analog produces a choleretic response not mediated by adenylate cyclase and associated with increased chloride and bicarbonate secretion.     

Prostaglandin IX - synthesis of (±)-15-methyl-11-deoxy PGE1 (doxaprost) - a potent bronchodilator - and its C-15-epimer

The communication describes the total synthesis of (±)-15-methyl-11-deoxy PGE₁ and its C-15-epimer. The synthesis of (±)-15-methyl-11-deoxy PGF₁ and (±)-15-methyl-11-deoxy PGF_1β is also reported. Preliminary data for the bronchodilator activity is presented.     

Heterocyclic 1,7-disubstituted indole sulfonamides are potent and selective human EP3 receptor antagonists

We have developed a pharmacophore model for the EP₃ receptor antagonists based on its endogenous ligand PGE₂. This ligand-based design yielded a series of novel peri-substituted [4.3.0] bicyclic aromatics featuring 1-alklyaryl 7-heterocyclic sulfonamide substituents. The synthesized molecules are potent antagonists of human EP₃ receptor in vitro and show inhibition of rat platelets aggregation. Optimized derivatives display high selectivity over IP, FP, and other EP receptor panels.     

Prostaglandin E1 (PGE1), but not prostaglandin E2 (PGE2), alters luteal and endometrial luteinizing hormone (LH) occupied and unoccupied LH receptors and mRNA for LH receptors in ovine luteal tissue to prevent luteolysis

Loss of luteal progesterone secretion at the end of the ovine estrous cycle is via uterine PGF₂α secretion. However, uterine PGF₂α secretion is not decreased during early pregnancy in ewes. Instead, the embryo imparts a resistance to PGF₂α. Prostaglandins E (PGE; PGE₁ + PGE₂) are increased in endometrium and uterine venous blood during early pregnancy in ewes to prevent luteolysis. Chronic intrauterine infusion of PGE₁ or PGE₂ prevents spontaneous or IUD, estradiol-17β, or PGF₂α-induced premature luteolysis in nonbred ewes. The objective was to determine whether chronic intrauterine infusion of PGE₁ or PGE₂ affected mRNA for LH receptors, occupied and unoccupied receptors for LH in luteal and caruncular endometrium, and luteal function. Ewes received Vehicle, PGE₁, or PGE₂ every 4 h from days 10 to 16 of the estrous cycle via a cathether installed in the uterine lumen ipsilateral to the luteal-containing ovary.Jugular venous blood was collected daily for analysis of progesterone and uterine venous blood was collected on day-16 for analysis of PGF₂α and PGE. Corpora lutea and caruncular endometrium were collected from day-10 preluteolytic control ewes and day-16 ewes treated with Vehicle, PGE₁ or PGE₂ for analysis of the mRNA for LH receptors and occupied and unoccupied receptors for LH. Luteal weights on day-16 in ewes treated with PGE₁ or PGE₂ and day-10 control ewes were similar (P ≥ 0.05), but were greater (P ≤ 0.05) than in day-16 Vehicle-treated ewes. Progesterone profiles on days 10–16 differed (P ≤ 0.05) among treatment groups: PGE₁ > PGE₂ > Vehicle-treated ewes. Concentrations of PGF₂α and PGE in uterine venous plasma on day-16 were similar (P ≥ 0.05) in the three treatment groups. Luteal mRNA for LH receptors and unoccupied and occupied LH receptors were similar (P ≥ 0.05) in day-10 control ewes and day-16 ewes treated with PGE₂ and were lower (P ≤ 0.05) in day-16 Vehicle-treated ewes. PGE₂ prevented loss (P ≤ 0.05) of day-16 luteal mRNA for LH receptors and occupied and unoccupied LH receptors. Luteal and caruncular tissue mRNA for LH receptors and occupied and unoccupied LH receptors were greater (P ≤ 0.05) on day-16 of PGE₁-treated ewes than any treatment group. mRNA for LH receptors and occupied and unoccupied receptors for LH in caruncules were greater (P ≤ 0.05) in day-16 Vehicle or PGE₂-treated ewes than in day-10 control ewes. It is concluded that PGE₁ and PGE₂ share some common mechanisms to prevent luteolysis; however, only PGE₁ increased luteal and endometrial mRNA for LH receptors and occupied and unoccupied LH receptors. PGE₂ prevents a decrease in luteal mRNA for LH receptors and occupied and unoccupied receptors for LH without altering endometrial mRNA for LH receptors or occupied and unoccupied receptors for LH.     

EP2 receptor activation by prostaglandin E2 leads to induction of HO-1 via PKA and PI3K pathways in C6 cells

Recently we proposed that COX-2 induction precedes expression of HO-1 in ischemic preconditioned rat brain. In the current study, we investigated the molecular mechanism by which prostaglandin E₂, one of COX-2 metabolites, induces HO-1 in rat C6 brain cells. We demonstrated that concentration of PGE₂ increased HO-1 expression in C6 cells in vitro. The effects of PGE₂ were mimicked by PGE₂ receptor EP₂ agonists, 11-deoxy PGE₂, and cAMP analog, dibutyl-cAMP. HO-1 expression by PGE₂ was inhibited by LY294002, PI3K inhibitor and H89, PKA inhibitor. The EP₂-specific antagonist, AH8006 also inhibited PGE₂-mediated HO-1 expression in a concentration-dependent manner. Finally, PGE₂ inhibited GOX-induced apoptosis as assayed by FACS analysis or DNA strand breaks assay, and this cell death was reversed by ZnPPIX, HO-1 inhibitor. In addition to HO-1 induction, PGE₂ also increased phosphorylation of Bad by PKA- and PI3K-depednent manner. Taken together, we conclude that PGE₂ induces HO-1 protein expression through PKA and PI3K signaling pathways via EP₂ receptor in C6 cells. The induction of HO-1 along with increase of p-Bad by PGE₂ is responsible for anti-apoptosis against oxidant stress.     

Prostaglandin E2 suppresses β1-integrin expression via E-prostanoid receptor in human monocytes/macrophages

β₁-Integrins mediate cell attachment to different extracellular matrix proteins, intracellular proteins, and intercellular adhesions. Recently, it has been reported that prostaglandin E₂ (PGE₂) has anti-inflammatory properties such as inhibition of the expression of adhesion molecules or production of chemokines. However, the effect of PGE₂ on the expression of β₁-integrin remains unknown. In this study, we investigated the effects of PGE₂ on the expression of β₁-integrin in the human monocytic cell line THP-1 and in CD14⁺ monocytes/macrophages in human peripheral blood. For this, we examined the role of four subtypes of PGE₂ receptors and E-prostanoid (EP) receptors on PGE₂-mediated inhibition. We found that PGE₂ significantly inhibited the expression of β₁-integrin, mainly through EP₄ receptors in THP-1 cells and CD14⁺ monocytes/macrophages in human peripheral blood. We suggest that PGE₂ has anti-inflammatory effects, leading to the inhibited expression of β₁-integrin in human monocytes/macrophages, and that the EP₄ receptor may play an important role in PGE₂-mediated inhibition.     

Investigation of the pronounced synergism between prostaglandin E2 and other constrictor agents on rat femoral artery

This study investigates the pronounced synergism between the weak contractile action of prostaglandin E₂ (PGE₂) and strong actions of phenylephrine, U-46619 and K⁺ on rat isolated femoral artery. The potency ranking for synergism was SC-46275 (prostanoid receptor agonist selectivity: EP₃EP₁)=sulprostone (EP₃>EP₁)>17-phenyl PGE₂ (EP₁>EP₃). The novel EP₃ antagonist L-798106 (0.2–1 μM) blocked the enhanced action of sulprostone (pA₂=7.35–8.10), while the EP₁ antagonist SC-51322 (1 μM) did not (pA₂<6.0). Matching responses to priming agent and priming agent/sulprostone were similarly suppressed by nifedipine (300 nM) and the selective Rho-kinase inhibitors H-1152 (0.1–1 μM) and Y-27632 (1–10 μM). Our findings implicate an EP₃ receptor in the prostanoid component of contractile synergism. While the synergism predominantly operates through a Ca²⁺ influx–Rho-kinase pathway, the EP₃ receptor does not necessarily transduce via Rho-kinase.     

EP2 Receptor Signaling Pathways Regulate Classical Activation of Microglia

Activation of EP2 receptors by prostaglandin E2 (PGE₂) promotes brain inflammation in neurodegenerative diseases, but the pathways responsible are unclear. EP2 receptors couple to Gα_s and increase cAMP, which associates with protein kinase A (PKA) and cAMP-regulated guanine nucleotide exchange factors (Epacs). Here, we studied EP2 function and its signaling pathways in rat microglia in their resting state or undergoing classical activation in vitro following treatment with low concentrations of lipopolysaccharide and interferon-γ. Real time PCR showed that PGE₂ had no effect on expression of CXCL10, TGF-β1, and IL-11 and exacerbated the rapid up-regulation of mRNAs encoding cyclooxygenase-2, inducible NOS, IL-6, and IL-1β but blunted the production of mRNAs encoding TNF-α, IL-10, CCL3, and CCL4. These effects were mimicked fully by the EP2 agonist butaprost but only weakly by the EP1/EP3 agonist 17-phenyl trinor PGE₂ or the EP4 agonist CAY10598 and not at all by the EP3/EP1 agonist sulprostone and confirmed by protein measurements of cyclooxygenase-2, IL-6, IL-10, and TNF-α. In resting microglia, butaprost induced cAMP formation and altered the mRNA expression of inflammatory mediators, but protein expression was unchanged. The PKA inhibitor H89 had little or no effect on inflammatory mediators modulated by EP2, whereas the Epac activator 8-(4-chlorophenylthio)-2′-O-methyladenosine 3′,5′-cyclic monophosphate acetoxymethyl ester mimicked all butaprost effects. These results indicate that EP2 activation plays a complex immune regulatory role during classical activation of microglia and that Epac pathways are prominent in this role.     

3H] prostaglandins binding to dispersed bovine luteal cells: evidence for discrete prostaglandin receptors.

Suspensions of dispersed bovine luteal cells prepared by collagenase digestion of luteal tissue specifically bound [³H]Prostaglandin (PG) E₁ and [³H]PGF_2α. While the number of sites per cell (～ 1.8 × 10⁵) were about the same for both [³H]PGs, the apparent Kds were different: [³H]PGE₁ ? 2.4 nM; [³H]PGF_2α ? 11 nM. The [³H]PGs binding was inhibited in a dose-dependent manner in the presence of increasing concentrations of unlabeled PGs. Potency order for inhibition of [³H]PGE₁ binding was: PGE₂ > PGE₁ > PGF_2α > PGF_1α. The corresponding data for [³H]PGF_2α was: PGF_2α > PGF_1α > PGE₂ > PGE₁. While [³H]PGE₁ and [³H]PGF_2α bind to their own receptors with high affinity, their affinities for each other's binding were extremely low. Thus, these results demonstrate that luteal cells, like plasma membranes isolated from luteal tissue, contain receptors for PGEs and PGF_2α which are discrete with respect to specificity and affinity.     

Paradoxical endogenous synthesis of a coronary dilating substance from arachidonate

Isolated bovine, canine, and human coronary arteries exhibited dose dependent contractions to prostaglandin (PG) E₂ and F_2α (50 ng/ml to 10 μg/ml). The ED₅₀ value for both PGE₂ and PGF_2α was 500 ng/ml in the bovine and human coronary arteries. Paradoxically, although PGE₂ and PGF_2α are vasoconstrictors, administration of their precursor, arachidonate (100 ng/ml to 10 μg/ml) caused relaxation of the bovine, canine and human coronary arteries. This observation suggests that arachidonate is not being converted by the coronary PG synthetase to PGE₂ or PGF_2α. However, the arachidonate induced coronary relaxation was inhibited by pretreatment with PG synthetase inhibitors, indomethacin, meclofenemate and aspirin. Indomethacin addition to the strips previously relaxed by arachidonate caused contraction. In contrast to other PGs (E₂ and F_2α), PGE₁ (10 ng/ml to 10 μg/ml) caused dose dependent relaxation of the bovine coronary arteries (ED₅₀ = 100 ng/ml). Indomethacin induced further relaxation of the blood vessels previously relaxed by PGE₁. Since PGE₁ cannot arise from arachidonate, the arachidonate coronary dilation and reversal by indomethacin must be independent of PGE₁ formation. Linolenate (100 ng/ml to 10 μg/ml) and oleate (100 ng/ml to 10 μg/ml) also caused relaxation of the bovine coronary blood vessels both before and after indomethacin, thereby eliminating a direct non-specific fatty acid effect as the cause of the arachidonate relaxation. These results suggest that in isolated coronaries, arachidonate undergoes a novel conversion, possibly by PG synthetase, to a dilating substance which exerts different contractile effects than exogenously administered PGE₂, PGF_2α and PGE₁.This work was supported by (USPHS) training grants NS 05221, RCDA (P.N.) HL-19586, HL-11771A, HL-14397 and SCOR grant HL-17646, HL-17646-0.