Prostaglandin F2α antagonizes thromboxane A2-induced human platelet aggregation

The effects of prostaglandin F_2α on human blood platelet function were investigated. PGF_2α at 15 μM completely blocked platelet aggregation induced by 500 μM arachidonic acid or 3 μM U46619 but had no effect on aggregatin induced by 7.5 μM ADP. A similar specificity of action was not obtained with either PGI₂ or PGE₂. Thus concentrations of PGI₂ (3 nM) or PGE₂ (20 μ M) which inhibited U46619-induced aggregation by 100% also blocked ADP-stimulated aggregation.The inhibitory properties of PGF_2α were not related to increases in platelet cAMP, since direct measurement of intracellular cAMP revealed that 15 μ M PGF_2α produced no substantial change in cAMP levels. This finding was in direct contrast to results obtained using either PGI₂ or PGE₂. Both PGI₂ (3 nM) and PGE₂ (20 μ M) induced significant increases in platelet cAMP levels.The possibility that PGF_2α directly interacts at the platelet TXA₂/PGH₂ receptor was investigated by measuring [³H]PGF_2α binding to isolated platelet membranes. It was found that [³H] PGF_2α binding reached equilibrium within 30 min at room temperature and could be 90% displaced by addition of 1000 fold excess of unlabelled PGF_2α. Furthermore, when 1000 fold excess of either the TXA₂/PGH₂ “mimetic” U46619 or the TXA₂/PGH₂ antagonist 13-azaprostanoic acid was added, specific [³H] PGF_2α binding was displaced by 95% and 85% respectively. In contrast, the same molar excess of 6-keto-PGF_1α, azo analog 1, or TXB₂, caused displacement of only 15%, 20% or 25% of the [³H] PGF_2α binding. Scatchard analysis indicated that [³H] PGF_2α has two binding sites; i.e., a high affinity binding site with an apparent Kd of 50 nM and a low affinity binding site with apparent Kd of 320 nM. These results suggest that the selective inhibition by PGF_2α of AA or U46619-induced aggregation may be mediated through interaction at the platelet TXA₂/PGH₂ receptor.     

Cerebral endothelial cell culture II. Adenylate cyclase response to prostaglandins and their interaction with the adrenergic system

The response of endothelial adenylate cyclase (AC) to prostaglandins (PGE₁, PGE₂, PGF_1α, PGF_2α, PGD₂ and PGI₂) and the relationship of PGE₂ to adrenergic systems were investigated in cerebrovascular endothelial cultures. E-type prostaglandins and PGI₂ were more effective in stimulating endothelial AC (EC₅₀ = 3 × 10^?7M, and 3 × 10^?7M, respectively) than prostaglandins of the F-series and PGD₂ which activated AC at high doses only. A modulation of endothelial AC response to either PGE₂ or norepinephrine (NE) was observed in the presence of both agents in the system. It was manifested by a dose-dependent NE inhibition of the PGE₂-stimulated formation of cAMP, which was partially restored by phentolamine. Alpha and β-adrenergic agonists (α, clonidine and 6-fluoronorepinephrine; β, isoproterenol) also partly blocked while forskolin and PGE₂ synergistically stimulated the production of cAMP in the endothelial cultures. These findings strongly suggest that the interaction of prostaglandins and α- and β-adrenergic agonists with the AC system in cerebrovascular endothelium may play a role in the regulation of the cerebral microcirculation and/or blood pressure.     

Involvement of prostaglandins in the spawning of the scallop,Patinopecten yessoensis

1. A high performance liquid chromatography (HPLC) using 9-anthryldiazomethane (ADAM) as a fluorescent reagent was employed to detemine the levels of endogenous prostaglandins (PGs) in the central nervous system, gonad, gill and hemolymph of the scallop, and the authors have also verified the involvement of PGs during spawning induced by u.v. ray-irradiated seawater.2. PGF_2α, PGE₂, PGD₂, 6-keto-PGF_1α. and TXB₂ were identified in all tissue and hemolymph, while no PGD₂ was found in the hemolymph by HPLC.3. PGF_2α, PGE₂ and PGD₂ levels in the ovary were about four times as much as those in the testis during the spawning season.4. PGF_2α, PGE₂ and PGD₂ levels in the ovary decreased during spawning, while, on the contrary, those in the testis increased during spawning. No changes of PGs levels were observed in the central nervous system.5. These results suggest the possibility that PGF_2α and PGE₂ are, especially, implicated in the spawning of the scallop; however, they also indicate that a difference between the functional mechanism of PGs in the ovary and that in the testis exists during spawning.     

Thromboxane A2 is the major arachidonic acid metabolite of human cortical hydronephrotic tissue

Human cortical hydronephrotic microsomes converted [¹⁴C] arachidonic acid to [¹⁴C] thromboxane B₂ as the major metabolic product. Using [¹⁴C] PGH₂ as substrate, similar enzymatic conversions were noted with HHT>TXB₂6KPGF_1αPGE₂PGF_2α as the major products. Inhibition of thromboxane synthetase with imidazole 5 mM reduced thromboxane B₂ production by 60% and the major product then was 6 keto PGF_1α. After addition of imidazole, the metabolic profile showed 6KPGF_1αPGE₂HHT>PGF_2α. Control experiments were carried out using normal cortical tissue obtained from kidneys removed surgically for carcinoma of kidney and rejected for transplantation secondary to fracture as a consequence of blunt trauma. These control kidneys, while they demonstrated an ability to generate thromboxane B₂$\text{in vitro}$, had much less activity than hydronephrotic kidneys and with PGH₂ as substrate PGE₂TxB₂. In addition, inhibition with imidazole produced mainly PGE₂. Thus, like the rabbit and rat, there is enhanced thromboxane and prostacyclin synthesis in human ureteral obstruction and are, therefore, potential vasoactive compounds which may in part be responsible for the hemodynamic alterations occurring in human obstructive uropathy.     

Accumulation of Cyclooxygenase Products of Arachidonic Acid Metabolism in Gerbil Brain During Reperfusion After Bilateral Common Carotid Artery Occlusion

: Several of the cyclooxygenase products of arachidonic acid were measured in the cerebral hemispheres of gerbils subjected to transient interruption of the cerebral circulation. The levels of PGD₂, PGF₂α, PGE₂, TXB₂, 13,14-H₂-15-keto-PGE₂, and the stable nonenzymic product of prostacyclin, 6-keto-PGF₁α, were not altered at the end of a 5-min period of ischemia. However, the onset of reperfusion was accompanied by a rapid accumulation of these products. Levels were highest during the initial period of reperfusion, then decreased to approach control levels after 120 min. PGD₂, PGF₂α, and PGE₂ were the predominant metabolites detected. This postischemic accumulation of arachidonic acid metabolites could be blocked by prior administration of inhibitors of cyclooxygenase activity.     

Prostaglandin levels in isolated brain microvessels and in normal and norepinephrine-stimulated cat brain homogenates

The levels of PGD₂, PGE₂, PGF_2α and 6-keto-PGF_1α (6KF_1α) produced from endogenous archidonic acid (AA) were quantitated in cat cerebral cortical homogenates and microvessels isolated from cat cerebral cortex using gas chromatography/mass spectrometry (GC/MS). There was a six-fold enrichment of 6KF_1α levels in isolated microvessels, compared to homogenates, suggesting that 6KF_1α is of vascular, rather than neuronal origin. In order to further understand any possible role that norepinephrine (NE)_might have on modulation of PG synthesis, we studied the effects of 0.5 mM NE on PG synthesis from endogenous AA and from ³H-PGG₂, the endoperoxide precursor of PGs. In cat cortical homogenates NE induced a 74% increase in PGD₂ and PGF_2α, a 62% increase in PGE₂, and a 36% increase in 6KF_1α, as measured by GC/MS. NE caused a twofold increase in the conversion of ³H-PGG₂ to ³h-PGG_2α, with a concomitant decrease in ³H-PGE₂ and ³H-6KF_1α formation, and no change in ³H-PGD₂ synthesis. NE had no effect on the total conversiob of ³H-PGG₂ to ³H-PGs, nor on the breakdown of ³H-PGG₂ in the absence of brain tissue. We conclude that NE stimulates extravascular synthesis of PGD₂, PGE₂ and PGF_2α by stimulation of the prostaglandin synthetase complex, in addition to NE's stimulatory effect on the conversion of PGG₂ to PGF_2α, and that the lack of effect of NE on 6KF_1α synthesis reflects either a failure to achieve an adequate concentration at the vascular tissue, or an absence of the mechanism whereby NE stimulates PG synthetase.     

Metabolism and effect of prostaglandin H2 in adipose tissue

Prostaglandin H₂ (PGH₂) inhibited noradrenaline induced cyclic AMP accumulation in isolated rat fat cells in a dose-dependent manner. IC₅₀ was 10 – 25 ng/ml both in the absence and in the presence of theophylline. The degree of inhibition produced by PGH₂ increased with time of incubation. A stable PGH₂ analog did not inhibit cyclic AMP accumulation. PGH₂ was rapidly converted by isolated fat cells to PGD₂, PGE₂ and PGH_2α, but no formation of thromboxane B₂ was found either or . PGE₂ was a more potent inhibitor than PGH₂ of noradrenaline induced cyclic AMP accumulation. PGD₂ enhanced cyclic AMP accumulation in a limited concentration interval, while PGF_2α was essentially uneffective.Our results suggest that PGH₂ is an inhibitor of cyclic AMP formation in isolated rat fat cells only after conversion to PGE₂. A physiological role for PGH₂ as a modulator of lipolysis is considered unlikely.     

Inhibition of GSH-dependent PGH2 isomerase in mammary adenocarcinoma cells by allicin.

We have reported tha allicin, a constituent of garlic oil, has no effect on the activities of platelet cyclooxygenase or thromboxane synthase, or vascular PGI₂ synthase. The effect of allicin on glutathione (GSH) dependent PGH₂ to PGE₂ isomerase is unknown. We therefore studied the effect of allicin on PGE₂ biosynthesis in a murine mammary adenocarcinoma cell line (No 4526). Intact or sonicated cells were incubated with either ¹⁴C-arachidonic acid (AA) or ¹⁴C-PHG₂, respectively. Following metabolism, products were extracted, separated by TLC and analyzed by radiochromatographic scan. PGE₂ was predominantly formed with minimal amounts of PGF_2α and PGD₂. Formation of 6-keto-PGF_1α or TXB₂ was not detected indicating the absence of TXA₂ and PGI₂ synthase activity. Indomethacin and ibuprofen inhibited the PGE₂ formation (p < 0.05). The enzymatic PGE₂ formation in sonicates was blocked by depletion of the cellular non-protein thiols by buthionine sulfoximine and was shown to be dependent on GSH. Allicin, over the range of 10–1000 μM, inhibited the formation of PGE₂ in cells exposed to 2.0 μM ¹⁴C-AA for 20 min. and in sonicated cells incubated with 20.0 μM ¹⁴C-PGH₂ for 2 min (p < 0.05). Allicin did not alter cyclooexygenase-mediated oxygen utilization in ram seminal vessicle microsomes, suggesting that allicin selectively inhibits the GSH-dependent PGH₂ to PGE₂ isomerase in this adenocarcinoma cell line.     

Prostaglandin synthesis by the cochlea

The exogenous and endogenous syntheses of prostaglandins (PG's) by the cochlea of adult mongolian gerbils were studied . After incubation of the whole membraneous cochlea with [³H]-arachidonic acid (AA), syntheses of PGF_2α, 6-keto PGF_1α, PGE₂, thromboxane (TX) B₂ and PGD₂ were evidenced in this order. The synthesis of radioactive PG's was almost completely inhibited by incubation with 10⁻⁵ M indomethacin. No significant amounts of those PG's were detected by radioimmunoassay (RIA) in the cochlea obtained from animals killed by microwave irradiation at 5.0 kw for 0.8 sec. However, when the homogenate of the whole membraneous cochlea obtained from animals without microwave irradiation was incubated at 37°C for 0–15 min, PGD₂, PGE₂, PGF₂α and 6-keto PGF₁α were found to be formed from endogenous AA in the cochlea by RIA. PG's were formed already at 0 time to considerable level (PGD₂, PGF₂α and 6-keto PGF₁α, 90–120 pg/cochlea; PGE₂, 370 pg/cochlea), reached to the maximum level (PGD₂, PGF₂α and 6-keto PGF₁α, 170–200 pg/cochlea; PGE₂, 500 pg/cochlea) at a 5-min incubation, and then gradually decreased. On the other hand, the amount of TXB₂ was lower than the detection limit by RIA (<50 pg/cochlea) even after the incubation. The cochlea was dissected into three parts: organ of Corti + modiolus (OC + M), lateral wall (LW), and cochlear nerve (CN), and then PG's formed by these tissues were determined after a 5-min incubation of the homogenates. In the CN and OC + M, PGE₂ was the major PG (100 and 160 pg/tissue, respectively), and the amounts of PGD₂, PGF₂α and 6-keto PGF₁α were about of those of PGE₂. In the LW, the amounts of PGD₂, PGE₂, PGF₂α and 6-keto PGF₁α were about the same level (70–100 pg/LW).     

Desensitization of prostaglandin-activated platelet adenylate cyclase

Prostaglandin D₂ (PGD₂) is one of several prostaglandins that can inhibit platelet aggregation and activate adenylate cyclase. Platelets were exposed to varying concentrations of PGD₂, washed, and the adenylate cyclase response to prostaglandins, epinephrine, and sodium fluoride determined. Incubating platelets with 5 × 10^?5 M PGD₂ for 2 hr resulted in a 45% decrease in PGD₂ activation of adenylate cyclase and a 25% decrease in stimulation by PGE₁. Fluoride activation (7-fold) epinephrine inhibition (30%) and basal enzyme activity were unchanged by exposure of the platelets to PGD₂. Desensitization was concentration dependent, with loss of enzyme activity first noted when platelets were incubated with 10^?7 M PGD₂. Enzyme sensitivity could be partially restored when desensitized platelets were washed free of PGD₂ and incubated in buffer for 2 hr; complete resensitization required incubation for 24 hr in plasma. Regulation of prostaglandin sensitive platelet adenylate cyclase could be of importance in mediating the response of platelets to aggregating agents.     

Effects of all CIS-5, 8,11,14,17 eicosapentaenoic acid and PGH3 on platelet aggregation

In human platelet-rich plasma (PRP) eicosapentaenoic acid (EPA) inhibited platelet aggregation induced by a stable analogue of PGH₂ (U46619), arachidonic acid, collagen or ADP. EPA was more potent than oleic, linoleic, α-linolenic or γ-linolenic acids. In aspirin-treated platelets, aggregation induced by U46619 was inhibited to a similar extent by arachidonic acid or by EPA over a range of concentrations of 0.05–0.3 mM. EPA incubated with PRP did not induce the generation of a thromboxane (TXA)-like activity; indeed it prevented the formation of TXA₂ induced by arachidonic acid or by collagen. The anti-aggregatory activity of EPA was not influenced by inhibitors of cyclo-oxygenase and lipoxygenase. The anti-aggregatory action of EPA may be caused by a rapid occupancy by EPA of TXA₂/PGH₂ “receptors” on platelet membrane as well as by a slower displacement of arachidonic acid from platelet phospholipids by chemically unchanged molecules of EPA.Not all samples of PRP were irreversibly aggregated by PGH₂, but in those that were, PGH₃ also induced an immediate dose-dependent but reversible aggregation. After a 4 min incubation of non-aggregating doses of PGH₂ or PGH₃ (100–300 nM) with PRP a stable anti-aggregatory compound was detected. The inhibitory activity produced from PGH₃ was apparently more potent (ca 10 times) than that obtained from PGH₂. The anti-aggregating compounds were identified by TLC and GLC-MS as PGD₂ and PGD₃. The apparent difference of potency between PGD₂ and PGD₃ was attributed to the concurrent production of PGE₂ and PGE₃. PGE₂ prevented the inhibitory effect of PGD₂ whereas PGE₃ did not affect the activity of PGD₃.It is concluded that one of the reasons for the low incidence of myocardial infarction in Eskimos could be that the pro-aggregatory arachidonic acid is replaced in their phospholipids by the anti-aggregatory EPA.     

Modulation of human platelet adenylate cyclase by prostacyclin (PGX)

Prostacyclin (PGX) ($\text{5Z}\text{)-9-}\text{deoxy}\text{-6,9α-}\text{epoxy}\text{-Δ}{}^5\text{-}\text{PGF}{}_{\mathrm{1\alpha }}$ has been found to be a potent stimulator of cAMP accumulation in human platelet rich plasma (PRP), and a direct stimulator of platelet microsome adenylate cyclase. Prostacyclin is, on a molar basis, at least 10 times more potent a stimulator of cAMP accumulation in platelets than PGE₁. The prostacyclin stimulation of platelet cAMP accumulation can be antagonized by the prostaglandin endoperoxide PGH₂, and a PGH₂-induced platelet aggregation is antagonized by prostacyclin. A model of platelet homeostasis is proposed that suggests platelet aggregation is controlled by a balance between the adenylate cyclase stimulating activity of prostacyclin, and the cAMP lowering activity of PGH₂.     

Hyperglycemia promotes elevated generation of TXA2 in isolated rat uteri

The relationship between high glucose concentrations and arachidonic acid metabolism in uterine tissue from control and diabetic ovariectomized rats was evaluated. Uterine tissue from diabetic rats produced amounts of PGE₂ and PGF_2α similar to controls, while a lower production of 6-keto-PGF_1α (indicating the production of prostacyclin) and a higher production of TXB₂ (indicating the generation of TXA₂) was found in the diabetic group. A group of diabetic rats was treated with phlorizin to diminish plasma glucose levels. Phlorizin treatment did not alter production of PGE₂, PGF_2α, and 6-keto-PGF_1α in the diabetic group. A diminished production of TXB₂ was found in the treated diabetic uteri when compared to the non-treated diabetic group. Moreover, a positive correlation between plasma glucose levels and uterine TXB₂ generation was observed. When control uterine tissue was exposed in vitro to high concentrations of glucose (22 mM) and compared to control tissue incubated in the presence of glucose 11 mM alterations in the generation of PGE₂, PGF_2α, and 6-keto-PGF_1α were not found, but a higher production of TXB₂ was observed and values were similar to those obtained in the diabetic tissue. Alteration in the production of the prostanoids evaluated were not observed when diabetic tissue was incubated in the presence of high concentrations of glucose. These results provide evidence of a direct relationship between plasma glucose levels and uterine production of TXA₂.     

Prostaglandin endoperoxides IV. Effects on smooth muscle

The effect on smooth muscle of the endoperoxides PGG₂ and PGH₂, which are intermediates in prostaglandin biosynthesis, was studied in different systems $\text{in vitro}$ and $\text{in vivo}$. On gastrointestinal smooth muscle (gerbil colon, rat stomach) PGG₂ and PGH₂ produced contractions comparable to those of PGE₂ and PGF_2a whereas contractions elicited on vascular (rabbit aorta) and airway (guinea-pig trachea) smooth muscle were considerably greater than those of PGE₂ and PGF_2a respectively. On intravenous injection into guinea-pigs PGG₂ and PGH₂ caused a triphasic change in blood pressure and were 8–10 times more effective than PGF_2a in producing an increase in tracheal insufflation pressure. When given as aerosols the unstable endoperoxides were less effective than PGF_2a. It is concluded that the endoperoxides are potent smooth muscle stimulants and that they are more effective than their degradation products (PGD₂, PGE₂, PGF_2a) in some systems.     

Patterns of prostaglandin synthesis and degradation in isolated superficial and proliferative colonic epithelial cells compared to residual colon

Prostaglandin (PG) synthesis and degradation were examined in different regions (epithelial versus non-epithelial structures) of the rat distal colon by both HPLC analysis of [¹⁴C] arachidonate (AA) metabolites and by specific radioimmunoassays. Intact isolated colonic epithelial cells synthesized mainly PGF₂α and TXA₂, as monitored from the formation of its stable degradation product TXB₂ (PGF_2α > TXB₂ > 6-keto-PGF_1α, the stable degradation product of PGI₂=PGD₂=PGE₂=13,14-dihydro-15-keto-PGF_2α). The profile of PG products of isolated surface epithelial cells was identical to that of proliferative epithelial cells. However, generation of PGs by surface epithelium was 2 to 3-fold higher than by proliferative cells both basally and in the presence of a maximal stimulating concentration (0.1 mM) of AA. The latter implied a greater synthetic capacity of surface epithelium, rather than differences due to endogenous AA availability. The major sites of PG synthesis in colon clearly resided in submucosal structures; the residual colon devoid of epithelial cells accounted for at least 99% of the total PGs produced by intact distal colon. The profile of AA metabolites formed by submucosal structures also differed markedly from that of the epithelium. The dominant submucosal product was PGE₂. PGE₂ and its degradation product 13,14-dihydro-15-keto-PGE₂ accounted for 63% of the PG products formed by submucosal structures (PGE₂ PGD₂ > 13,14-dihydro-15-keto-PGE₂ > PGF_2α=TXB₂=6-keto-PGF_1α > 13,14-dihydro- 15-keto-PGF_2α). By contrast, epithelial cells, and particularly surface epithelium, contributed disproportionately to the PG degradative capacity of colon, as assessed from the metabolism of either PGE₂ or PGF_2α. When expressed as a percentage, epithelial cells accounted for 71% of total colonic PGE₂ degradative capacity but only 23% of total colonic protein. Approximately 15% of [³H] PGE₂ added to the serosal side of everted colonic loops crossed to the mucosal side intact. Thus, at least a portion of the PGE₂ formed in the submucosa reaches, and thereby can potentially influence functions of the epithelium.     

Some new prospects in the mechanism of control of arachidonate metabolism in human placenta and amnion

The conversion of (1-¹⁴C) PGH₂ was studied in human placental and fetal membrane cellular preparations (tissue fragments, homogenate, cytosol, microsomes). Placental and amnion homogenates convert labelled PGH₂ into PGE₂ through a very active PGE₂ isomerase. However isolated placental microsomes do not metabolise PGH₂ into PGE₂ but into T×A₂ (identified as T×B₂ by GC-MS) and presumably 12-HHT. This microsomal T×A₂ synthetase is not active in the whole tissue nor in the homogenate. Placental cytosol gives mainly PGD₂. No conversion into PGI₂ (identofied as 6 keto PGF_1α) nor PGF_2α was observed in any fraction.Some aspects of PG synthesis regulation by the placental cytosol were studied: the cytosol contains a heat-stable factor that inhibits T×A₂ synthesis and shifts PGH₂ placental microsome metabolism towards PGE₂. In addition the placental cytosol inhibits human platelet-aggregation through a heat-labile factor which is not PGI₂ nor PGD₂. A multiple step regulation of the various PG metabolites synthetised from arachidonic acid in the placenta can be outlined and its physiological implications are discussed.     

Influence of angiotensins (I,II & III) on prostaglandin production by minced rat renal medulla

Minced rat renal medulla was incubated for 30 min at 37 °C in the presence of angiotensin, I, II or III (100 ng/ml) to determine the existence of a direct stimulating effect on prostaglandin (PG) production. PGE₂, PGF_2α, 6-keto PGF_1α and Thromboxane B₂ (TXB₂)_were determined by radioimmunoassay.For analysis of data variance, the results were separated according to whether the net output of PGE₂ was above or below 1.5 ng PGE₂ equivalent/mg tissue/30 min. Under low-output conditions, angiotensin I, II or III stimulated PGE₂ production significantly (p<0.02) and tended to augment PGF_2α production, while under high-output conditions no effect on PGE₂ or PGF_2α production was observed.Under either output condition, angiotensin I, II or III had no effect on 6-keto PGF_1α and TXB₂.     

Thromboxane synthase inhibition: Implications for prostaglandin endoperoxide metabolism: I Characterisation of an acute intravenous challenge model to measure prostaglandin endoperoxide metabolism

It has been proposed that thromboxane synthase inhibition (TXSI) may be a useful form of anti-thrombotic therapy and that this is due, in part, to redirection of PGH₂ metabolism in favour of PGI₂, a potent vasodilator and anti-platelet agent. While redirection has been observed there are conflicting reports of its occurrence . We now describe the characterisation of an acute intravenous challenge model using thrombin, collagen, arachidonic acid (AA) and PGH₂ for the study of PGH₂ metabolism. Following challenge, plasma concentrations of TXB₂, 6-oxo-PGF_1α, alleged metabolites of PGI₂ (PGI₂m) and PGE₂ were measured by radioimmunoassay (RIA). Thrombin and collagen challenge resulted in a dose-related increase in plasma TXB₂ while AA and PGH₂, in addition, elevated 6-oxo-PGF_1α and PGI₂m. Injection of PGH₂ elevated 6-oxo-PGF_1α, PGI₂m, TXB₂ and PGE₂ levels. Experimental conditions were defined such that challenge with thrombin (40 NIH units kg⁻¹), collagen (100 kg⁻¹), AA (1mg kg⁻¹) and PGH₂ (5μg kg⁻¹) and measurement of eicosanoids 0.5min following challenge (5μg kg⁻¹) and measurement of eicosanoids 0.5min following challenge were optimal for detection of redirection of PGH₂ metabolism . The identity of immunoreactive TXB₂ and 6-oxo-PGF_1α was further supported by experiments in which the extracted immunoreactive eicosanoids co-eluted with authentic [³H]standards when subject to reverse phase high performance liquid chromatography (RPHPLC). Evidence is also presented that the levels of plasma eicosanoids measured in this model reflect biosynthesis.     

Comparison of the effects of prostaglandins D2, F2α and E1 on spontaneous contractions of rabbit oviduct

The effects of PGD₂, PGF_2α and PGE₁ were studied on the circular muscle of post-ovulatory rabbit oviducts $\text{in vitro}$. PGE₁ inhibited spontaneous contractile activity. Lower concentrations of PGD₂ and PGF_2α were stimulatory and higher concentrations were inhibitory. Since PGD₂ may be produced in the oviduct, any hypothesis concerning the role of prostaglandins in the control of oviductal motility and ovum transport should include PGD₂ as well as PGFs and PGEs.