Prostaglandin hyperalgesia, IV: a metabolic process

Prostaglandin E2, prostacyclin and Db-cAMP injected into the rat paw induce hyperalgesia. This hyperalgesic effect of the prostaglandins but not of Db-CAMP was blocked by pre-treatment of the animals with cycloheximide. Prostaglandin hyperalgesia thus seems to be dependent on the triggering of some metabolic process which enhances the effects of physical or chemical stimuli.     

Prostaglandin hyperalgesia, V: A peripheral analgesic receptor for opiates

Prostaglandin E₂ injected in the rat paw causes hyperalgesia which is antagonized by local injections of opiate and opiate antagonists. In the present investigation in rats it i shown at naloxone has an analgesic effect at doses as low as 2 μg/site, injected into the rat hind paw. At a dose that has no analgesic effect (1 μg/site) naloxone antagonized the analgesia produced by either local or systematic administration of morphine. Local administration of levorphanol (50 μg/site) caused a 50% reduction in the intensity of the hyperalgesia induced by prostaglandin E₂. A dose four times greater of its isomer, dextrorphan, had little analgesic effect. The present results support the suggestion that this peripheral analgesia is the result of an action of opiates in receptors located at the nociceptors.     

Role of the sensory neuron cytoskeleton in second messenger signaling for inflammatory pain

Prostaglandin E(2) (PGE(2)) and epinephrine act directly on nociceptors to produce mechanical hyperalgesia through protein kinase A (PKA) alone or through a combination of PKA, protein kinase C epsilon (PKCepsilon), and extracellular signal-regulated kinase (ERK), respectively. Disruptors of the cytoskeleton (microfilaments, microtubules, and intermediate filaments) markedly attenuated the hyperalgesia in rat paws caused by injection of epinephrine or its downstream mediators. In contrast, the hyperalgesia induced by PGE(2) or its mediators was not affected by any of the cytoskeletal disruptors. These effects were mimicked in vitro, as measured by enhancement of the tetrodotoxin-resistant sodium current. When PGE(2) hyperalgesia was shifted to dependence on PKCepsilon and ERK as well as PKA, as when the tissue is "primed" by prior treatment with carrageenan, it too became dependent on an intact cytoskeleton. Thus, inflammatory mediator-induced mechanical hyperalgesia was differentially dependent on the cytoskeleton such that cytoskeletal dependence correlated with mediation by PKCepsilon and ERK.     

Activation of ERK signalling by Src family kinases (SFKs) in DRG neurons contributes to hydrogen peroxide (H2O2)-induced thermal hyperalgesia

Concomitant generation of reactive oxygen species during tissue inflammation has been recognised as a major factor for the development and the maintenance of hyperalgesia, out of which H₂O₂ is the major player. However, molecular mechanism of H₂O₂ induced hyperalgesia is still obscure. The aim of present study is to analyse the mechanism of H₂O₂-induced hyperalgesia in rats. Intraplantar injection of H₂O₂ (5, 10 and 20 µmoles/paw) induced a significant thermal hyperalgesia in the hind paw, confirmed by increased c-Fos activity in dorsal horn of spinal cord. Onset of hyperalgesia was prior to development of oxidative stress and inflammation. Rapid increase in phosphorylation of extracellular signal regulated kinase (ERK) was observed in neurons of dorsal root ganglia after 20?min of H₂O₂ (10 µmoles/paw) administration, which gradually returned towards normal level within 24?h, following the pattern of thermal hyperalgesia. The expression of TNFR1 followed the same pattern and colocalised with pERK. ERK phosphorylation was observed in NF-200-positive and -negative neurons, indicating the involvement of ERK in C-fibres as well as in A-fibres. Intrathecal preadministration of Src family kinases (SFKs) inhibitor (PP1) and MEK inhibitor (PD98059) prevented H₂O₂ induced augmentation of ERK phosphorylation and thermal hyperalgesia. Pretreatment of protein tyrosine phosphatases (PTPs) inhibitor (sodium orthovanadate) also diminished hyperalgesia, although it further increased ERK phosphorylation. Combination of orthovanadate with PP1 or PD98059 did not exhibit synergistic antihyperalgesic effect. The results demonstrate SFKs-mediated ERK activation and increased TNFR1 expression in nociceptive neurons during H₂O₂ induced hyperalgesia. However, the role of PTPs in hyperalgesic behaviour needs further molecular analysis.     

Effects of Prostaglandin E2 and Capsaicin on Behavior and Cerebrospinal Fluid Amino Acid Concentrations of Unanesthetized Rats: A Microdialysis Study

Abstract: Prostaglandin E₂ (PGE₂) delivered to the spinal cord produces an increased sensitivity to noxious (hyperalgesia) and innocuous (allodynia) stimuli. The mechanisms that underlie this effect remain unknown, but a PGE₂-evoked enhancement of spinal neurotransmitter release may be involved. To address this hypothesis, we examined the effect of PGE₂ on CSF concentrations of amino acids and also the modulatory effect of PGE₂ on capsaicin-evoked changes of spinal amino acid concentrations using a microdialysis probe placed in the lumbar subarachnoid space. Amino acids were quantified using HPLC with fluorescence detection. Addition of 1 mM, but not 10 or 100 µM, PGE₂ to the perfusate for a 10-min period (flow rate, 5 µl/min) evoked an immediate increase (80–100%) in glutamate (Glu), aspartate (Asp), taurine (Tau), glycine (Gly), and γ-aminobutyric acid (GABA) concentrations. Similarly, capsaicin infusion (0.1–10 µM) induced a dose-dependent increase in Glu, Asp, Tau, Gly, GABA, and ethanolamine levels. Significant increases in amino acid levels evoked by PGE₂ or capsaicin were associated with a touch-evoked allodynia. The combination of PGE₂ (10 µM) and capsaicin (0.1 or 1.0 µM) at concentrations that individually had no effect together evoked a significant increase (60–100%) in Glu, Asp, Tau, Gly, and GABA concentrations and produced tactile allodynia. These data demonstrate that spinally delivered PGE₂ or capsaicin substantially elevates CSF concentrations of both excitatory and inhibitory amino acids. The capacity of PGE₂ to enhance and prolong capsaicin-evoked amino acid concentrations may be one of the mechanisms by which spinal PGE₂ produces hyperalgesia and allodynia.     

Acidosis Mediates the Switching of Gs-PKA and Gi-PKCε Dependence in Prolonged Hyperalgesia Induced by Inflammation

Chronic inflammatory pain, when not effectively treated, is a costly health problem and has a harmful effect on all aspects of health-related quality of life. Previous studies suggested that in male Sprague Dawley rats, prostaglandin E₂ (PGE₂)-induced short-term hyperalgesia depends on protein kinase A (PKA) activity, whereas long-lasting hyperalgesia induced by PGE₂ with carrageenan pre-injection, requires protein kinase C_ε (PKC_ε). However, the mechanism underlying the kinase switch with short- to long-term hyperalgesia remains unclear. In this study, we used the inflammatory agents carrageenan or complete Freund’s adjuvant (CFA) to induce long-term hyperalgesia, and examined PKA and PKC_ε dependence and switching time. Hyperalgesia induced by both agents depended on PKA/PKC_ε and G_s/G_i-proteins, and the switching time from PKA to PKC_ε and from G_s to G_i was about 3 to 4 h after inflammation induction. Among the single inflammatory mediators tested, PGE₂ and 5-HT induced transient hyperalgesia, which depended on PKA and PKC_ε, respectively. Only acidic solution-induced hyperalgesia required G_s-PKA and G_i-PKC_ε, and the switch time for kinase dependency matched inflammatory hyperalgesia, in approximately 2 to 4 h. Thus, acidosis in inflamed tissues may be a decisive factor to regulate switching of PKA and PKC_ε dependence via proton-sensing G-protein–coupled receptors.     

Prostaglandin hyperalgesia, V: a peripheral analgesic receptor for opiates

Prostaglandin E2 injected in the rat paw causes hyperalgesia which is antagonized by local injections of opiate and opiate antagonists. In the present investigation in rats it is shown that naloxone has an analgesic effect at doses as low as 2 micrograms/site, injected into the rat hind paw. At a dose that has no analgesic effect (1 microgram/site) naloxone antagonized the analgesia produced by either local or systemic administration of morphine. Local administration of levorphanol (50 micrograms/site) caused a 50% reduction in the intensity of the hyperalgesia induced by prostaglandin E2. A dose four times greater of its isomer, dextrorphan, had little analgesic effect. The present results support the suggestion that this peripheral analgesia is the result of an action of opiates in receptors located at the nociceptors.     

Red blood cell deformability and the prostaglandins

The effects of Prostaglandin E₁ and Prostaglandin E₂ on the deformability of the human red blood cell have been studied using the glass micropipette method. In the range of concentrations 10^?13 to 10^?5 M, neither PGE₁ nor PGE₂ alters the red cell deformability.     

Comparison of the effects of prostaglandin I2 and prostaglandin E2 stimulation of the rat kidney adenylate cyclase-cyclic AMP systems

Prostacyclin (Prostaglandin I₂) effects on the rat kidney adenylate cyclase-cyclic AMP system were examined. Prostaglandin I₂ and prostaglandin E₂, from 8 · 10^?4 to 8 · ^?7 M stimulated adenylate cyclase to a similar extent in cortex and outer medulla. In inner medulla, prostaglandin I₂ was more effective than prostaglandin E₂ at all concentrations tested. Both prostaglandin I₂ and prostaglandin E₂ were additive with antidiuretic hormone in outer and inner medulla. Prostaglandin I₂ and prostaglandin E₂ were not additive in any area of the kidney, indicating both were working by similar mechanisms. Prostaglandin I₂ stimulation of adenylate cyclase correlated with its ability to increase renal slice cyclic AMP content. Prostaglandin I₂ and prostaglandin E₂ (1.5 · 10^?4 M) elevated cyclic AMP content in cortex and outer medulla slices. In inner medulla, with Santoquin® (0.1 mM) present to suppress endogenous prostaglandin synthesis, prostaglandin I₂ and prostaglandin E₂ increased cyclic AMP content. 6-Ketoprostaglandin F_1α, the stable metabolite of prostaglandin I₂, did not increase adenylate cyclase activity or tissue cyclic AMP content. Thus, prostaglandin I₂ activates renal adenylate cyclase. This suggests that the physiological actions of prostaglandin I₂ may be mediated through the adenylate cyclase-cyclic AMP system.     

Involvement of Spinal Somatostatin Receptor SST2A in Inflammation-Induced Thermal Hyperalgesia: Ultrastructural and Behavioral Studies in Rats

Our previous results have shown that somatostatin receptor subtype SST_2A is responsible for thermal, but not mechanical nociceptive transmission in the rat spinal cord. The present study was undertaken to further examine the ultrastructural localization of SST_2A receptor in lamina II of the spinal dorsal horn and the role of SST_2A receptor in thermal hyperalgesia following Complete Freund’s Adjuvant (CFA)-induced inflammation. We found that SST_2A receptors in lamina II are located primarily in postsynaptic dendrites and soma, but not in axons or synaptic terminals. CFA-induced inflammation markedly increased SST_2A receptor-like immunoreactivity in lamina II. Paw withdrawal latency (PWL) evoked by noxious heating was obviously shortened 1 h after intraplantar injection of CFA, exhibiting thermal hyperalgesia. Pre-blocking SST_2A activity by intrathecal pre-administration of CYN154806, a broad-spectrum antagonist of SST₂ receptor, or specific antiserum against SST_2A receptor (anti-SST_2A) significantly attenuated thermal hyperalgesia in a dose-dependent fashion in CFA-treated rats. But, administration of anti-SST_2A or CYN154806 after CFA treatment had no effect upon thermal hyperalgesia. Intrathecal application of SST_2A agonist SOM-14 at different doses prior to CFA treatment did not influence thermal hyperalgesia in inflamed rats, but at a low dose shortened PWL evoked by noxious heating in normal rats. These results suggest that spinal SST_2A receptors play a key role in triggering the generation, but not maintenance, of thermal hyperalgesia evoked by CFA-induced inflammation. The up-regulation of SST_2A receptors in the spinal cord may be one of the mechanisms underlying inflammation-induced thermal hyperalgesia. Special issue article in honor of Dr. Ji-Sheng Han. Jun Zhao and Jiang-Yuan Hu—contributed equally in this paper.     

Inflammatory effects of prostacyclin (PGI2) and 6-oxo-PGF1α in the rat paw

In the rat paw prostacyclin was 5–10 times less potent than PGE₂ in causing oedema, and 5 times less potent in potentiating carrageenin-induced oedema, which it did in a dose-related manner. Prostacyclin was 5 times more potent than PGE₂ in producing hyperalgesia and as potent as PGE₂ in restoring carrageenin-induced hyperalgesia. The effects on oedema were longer lasting than those on hyperalgesia.6-oxo-PGF_1α was 500 times less potent than PGE₂ in causing oedema by itself and in potentiating carrageenininduced oedema. It had no hyperalgesic activity in this test.     

P2X3 receptors contribute to transition from acute to chronic muscle pain

This study aimed to evaluate whether the development and/or maintenance of chronic-latent muscle hyperalgesia is modulated by P2X3 receptors. We also evaluate the expression of P2X3 receptors and PKCε of dorsal root ganglions during these processes. A mouse model of chronic-latent muscle hyperalgesia, induced by carrageenan and evidenced by PGE₂, was used. Mechanical muscle hyperalgesia was measured by Randall-Selitto analgesimeter. The involvement of P2X3 receptors was analyzed by using the selective P2X3 receptors antagonist A-317491 by intramuscular or intrathecal injections. Expression of P2X3 and PKCε in dorsal root ganglion (L4-S1) were evaluated by Western blotting. Intrathecal blockade of P2X3 receptors previously to carrageenan prevented the development and maintenance of acute and chronic-latent muscle hyperalgesia, while intramuscular blockade of P2X3 receptors previously to carrageenan only reduced the acute muscle hyperalgesia and had no effect on chronic-latent muscle hyperalgesia. Intrathecal, but not intramuscular, blockade of P2X3 receptors immediately before PGE₂, in animals previously sensitized by carrageenan, reversed the chronic-latent muscle hyperalgesia. There was an increase in total and phosphorylated PKCε 48 h after the beginning of acute muscle hyperalgesia, and in P2X3 receptors at the period of chronic muscle hyperalgesia. P2X3 receptors expressed on spinal cord dorsal horn contribute to transition from acute to chronic muscle pain. We also suggest an interaction of PKCε and P2X3 receptors in this process. Therefore, we point out P2X3 receptors of the spinal cord dorsal horn as a pharmacological target to prevent the development or reverse the chronic muscle pain conditions.

     

Prostaglandin E in peptic ulcer disease

Prostaglandin E₁ and E₂ inhibit gastric secretion in vivo and in vitro under a variety of conditions. It is not known whether these compounds may play a role in normal gastric secretory physiology or in the pathophysiology of peptic ulcer disease. Six normal adults and six patients with documented duodenal ulcer disease were studied under basal conditions and during gastric secretory stimulation with betazole. Prostaglandin E in plasma and gastric juice was measured by radioimmunoassay. Prostaglandin E was significantly higher in the plasma of normal volunteers both in the basal state and during stimulation. Gastric juice prostaglandin E was also significantly higher in normal volunteers during the basal state but the difference disappeared during stimulation. The relative deficiency of prostaglandin E in the ulcer group may indicate a role for prostaglandins in the pathophysiology of gastric hypersecretion.     

Prostaglandin D2 lowers nuclear DNA polymerase activity in cultured mastocytoma cells

Prostaglandin D₂ strongly inhibited growth of cultured mastocytoma P-815, 2-E-6 cells, which were established and cloned from mouse mast tumor cells. The inhibition was dose-dependent (IC₅₀ = 2.09 × 10⁻⁵ M). Prostaglandin D₂ also inhibited the DNA synthesizing activity of the cells dose-dependently. We next measured the activities of endogenous DNA polymerases extracted from untreated and prostaglandin D₂-treated cells. Prostaglandin D₂ pretreatment reduced DNA polymerase α activity by 52%. The sedimentation coefficients of the enzymes from untreated and prostaglandin D₂-treated cells were the same suggesting there was no gross change in the size of the enzyme. Prostaglandin D₂ pretreatment of the cells reduced endogenous DNA polymerase β activity to 68% of the control value; the sedimentation coefficients of the enzymes from treated and untreated cells were both 3.5 S. Interestingly, prostaglandin D₂ had no direct inhibitory effect on the activity of either DNA polymerase α or β. Our results indicate that the activities of DNA polymerase α and β are lower in prostaglandin D₂-treated mastocytoma cells. This finding account for the lower level of DNA synthesis in these cells.     

Sphingosine 1-Phosphate Mediates Hyperalgesia via a Neutrophil-Dependent Mechanism

Novel classes of pain-relieving molecules are needed to fill the void between non-steroidal anti-inflammatory agents and narcotics. We have recently shown that intraplantar administration of sphingosine 1-phosphate (S1P) in rats causes peripheral sensitization and hyperalgesia through the S1P₁ receptor subtype (S1PR₁): the mechanism(s) involved are largely unknown and were thus explored in the present study. Intraplantar injection of carrageenan in rats led to a time-dependent development of thermal hyperalgesia that was associated with pronounced edema and infiltration of neutrophils in paw tissues. Inhibition of 1) S1P formation with SK-I, a sphingosine kinase inhibitor, 2) S1P bioavailability with the S1P blocking antibody Sphingomab, LT1002 (but not its negative control, LT1017) or 3) S1P actions through S1PR₁ with the selective S1PR₁ antagonist, W146 (but not its inactive enantiomer, W140) blocked thermal hyperalgesia and infiltration of neutrophils. Taken together, these findings identify S1P as an important contributor to inflammatory pain acting through S1PR₁ to elicit hyperalgesia in a neutrophil-dependant manner. In addition and in further support, we demonstrate that the development of thermal hyperalgesia following intraplantar injection of S1P or SEW2871 (an S1PR₁ agonist) was also associated with neutrophilic infiltration in paw tissues as these events were attenuated by fucoidan, an inhibitor of neutrophilic infiltration. Importantly, FTY720, an FDA-approved S1P receptor modulator known to block S1P-S1PR₁ signaling, attenuated carrageenan-induced thermal hyperalgesia and associated neutrophil infiltration. Targeting the S1P/S1PR₁ axis opens a therapeutic strategy for the development of novel non-narcotic anti-hyperalgesic agents.     

The effect of orally administered prostaglandins A1, A2 and 15 Epi-A2 on human gastric acid secretion

Prostaglandins A₁, A₂ and 15 Epi-A₂ were administered orally to human male volunteers. Prostaglandin A₁ and 15 Epi-A₂ did not consistently affect gastric acid secretion either in terms of the pH values within individual tests or in respect of comparative control test data. Prostaglandin A₂ administration resulted in a transient inhibition of secretion in all 6 subjects tested, with the pH rising above 6 in every case.It is concluded that none of these compounds is likely to have therapeutic application to the peptic ulceration problem.     

The effect of prostaglandin E2 on the cardiovascular response to angiotensin II in pregnant rabbits

The rise in arterial blood pressure in response to angiotensin II was studied in the last third of pregnancy in rabbits. The response was compared with that of pregnant rabbits during infusion of prostaglandin E₂ and F_2α. Prostaglandin E₂ significantly diminished the rise in diastolic pressure in response to angiotensin II. Prostaglandin F_2α did not alter the response. Intravenous indomethacin elevated the blood pressure and caused an absolute increase in the pressor response. It did not mediate a change in the percentage rise in blood pressure in response to angiotensin II.     

Prostaglandin biosynthesis and catabolism in the developing fetal sheep lung

The prostaglandin biosynthetic and catabolic capacity of homogenates of lungs fetal sheep of various gestational ages was measured. Prostaglandin biosynthesis was assayed by the deuterium-isotope dilution technique making us e of mas fragmentography whereas prostaglandin catabolism was measured by the radioisotope-dilution method described previously (Pace-Asciak, C.R. and Rangaraj, G. (1976) J. Biol. Chem. 251, 3381–3385).Homogenates of lung sform fetuses of all ages tested (40 days to term) formed both prostaglandins E₂ and F_2α; although prostaglandin F_2α was formed to a greater extent than prostaglandin E₂ by the 40 day lung, prostaglandin E₂ increased with increasing age until at term the ratio of both prostaglandins approached unity. Total prostaglandin biosynthesis (E₂ + F_2α) rose gradually with age (approx. 3 fold increase between 40 days and term). Prostaglandin F_2α catabolism occurred mainly by the prostaglandin 15-hydroxy dehydrogenase pathway; this activity was detectable even at 40 days and remained unchanged up to 80 days. Prostaglandin catabolic activity rose sharply at 90 days (approx. 3 fold) with a maximum around 110 days (approx. 4 fold) decreasing back to 40 day levels by term (143 days).The increasing prostaglandin catabolic activity around 90–100 days in this species is discussed in relation to the hemodynamic changes in the lungs starting around this age and the appearance of surfactant. Prostaglandin catabolism might play an important role in the developing organ controlling steady state concentrations of prostaglandins during certain periods of organogenesis.     

Comparison of the peripheral mediator background of heat injury- and plantar incision-induced drop of the noxious heat threshold in the rat

AimsPreviously we described the drop of the noxious heat threshold in response to mild heat injury or plantar incision. While mild heat injury elicits an immediate and short-lasting thermal hyperalgesia, surgical incision leads to a delayed and sustained heat hyperalgesia. Only very few peripheral mediators of these phenomena have been identified. Therefore the present study aimed at comparing the peripheral mediator background of heat hyperalgesia evoked by mild heat injury or surgical incision.Main methodsHeat hyperalgesia was assessed by measuring the behavioural noxious heat threshold in conscious rats employing an increasing-temperature water bath.Key findingsThe heat threshold drop evoked by a mild heat injury and measured 10 min afterwards was reduced by intraplantarly applied HOE 140, a bradykinin B₂ receptor antagonist, NDGA, a non-selective lipoxygenase inhibitor, L-NOARG, a non-selective nitric oxide synthase inhibitor, TNP-ATP, a P2X purinoceptor antagonist and AMG9810, an antagonist of the transient receptor potential vanilloid type 1 (TRPV1) receptor. The heat threshold drop evoked by plantar incision and measured 18 h later was reduced by intraplantarly applied HOE 140, [des-Arg¹⁰]-HOE 140, a bradykinin B₁ receptor antagonist, L-NOARG, TNP-ATP and the TRPV1 receptor antagonist SB-366791.SignificanceOnly small differences have been revealed between the examined peripheral mediators of the acute heat hyperalgesia evoked by mild heat injury and the sustained increase in heat responsiveness induced by surgical incision. The B₂ and B₁ bradykinin receptor, P2X purinoceptors, TRPV1 receptor, nitric oxide synthase and lipoxygenase(s) are involved in at least one of these hyperalgesia models.     

Specific prostaglandine E1 and A1 binding sites in rat a dipocyte plasma membranes

Rat adipocyte plasma membranes sacs have been shown to be a sensitive and specific system for studying prostaglandin binding. The binding of prostaglandin E₁ and prostaglandin A₁ increases linearly with increasing protein concentration, and is a temperature-sensitive process. Prostaglandin E₁ binding is not ion dependent, but is enhanced by GTP. Prostaglandin A₁ binding is stimulated by ions, but is not affected by GTP.Discrete binding sites for prostaglandin E₁ and A₁ were found. Scatchard plot analysis showed that the binding of both prostaglandins was biphasic, indicating two types of binding sites. Prostaglandin E₁ had association constants of 4.9 · 10⁹ 1/mole and 4 · 10⁸ 1/mole, while the prostaglandin A₁ association constants and binding capacities varied according to the ionic composition of the buffer. In Tris-HCl buffer, the prostaglandin A₁ association constants were 8.3 · 10⁸ 1/mole and 5.7 · 10⁷ 1/mole, while in the Krebs—Ringer Tris buffer, the results were 1.2 · 10⁹ 1/mole and 8.6 · 10⁶ 1/mole.Some cross-reactivity between prostaglandin E₁ and A₁ was found for their respective binding sites. Using Scatchard plot analysis, it was found that a 10-fold excess of prostaglandin E₁ inhibited prostaglandin A₁ binding by 1–20% depending upon the concentration of prostaglandin A₁ used. Prostaglandin E₁ competes primarily for the A prostaglandin high-affinity binding site. Similar Scatchard analysis using a 20-fold excess of prostaglandin A₁ inhibited prostaglandin E₁ binding by 10–40%. Prostaglandin A₁ was found to compete primarily for the E prostaglandin low-affinity receptor.All of the bound [³H]prostaglandin E₁, but only 64% of the bound [³H]-prostaglandin A₁ can be recovered unmetabolized from the fat cell membrane. There is no non-specific binding of prostaglandin E₁, but 10–15% of prostaglandin A₁ binding to adipocyte membranes is non-specific. Using a parallel line assay to measure relative affinities for the E binding site, prostaglandin E₁ > prostaglandin A₂ > prostaglandin F_2α. Prostaglandin E₂ and 16,16-dimethyl prostaglandin E₂ were equipotent with prostaglandin E₁, while other prostaglandins had lower relative affinities. 7-Oxa-13-prostynoic acid does not appear to antagonize prostaglandin activity in adipocytes at the level of the receptor.