Patterns of prostaglandin production in the bovine fetal and maternal vasculature

Prostaglandin (PG) synthesis by the blood vessel wall may contribute to the regulation of vascular tone and prevent deposition of platelets. The pattern of prostaglandin synthesis was investigated in vascular tissues from fetal, maternal and non-pregnant cows. Thin slices of blood vessels were incubated in Krebs' solution to which was added [1-¹⁴C] arachidonic acid. The radioactive products were extracted from the incubation medium, separated by thin-layer chromatography, and quantitated by scintillation counting. In all blood vessels studied the principal prostaglandin recovered from the incubation medium was 6 keto-PGF_1α, the stable hydrolysis product of prostacyclin; the highest yield of radioactive prostaglandins was obtained with fetal tissues. Only the mesenteric arteries obtained from pregnant and non-pregnant cows demonstrated a biosynthetic capacity comparable to those of the fetal vasculature; pulmonary arteries and aortas of adult animals had a much lower capacity to synthesize prostaglandins. Release of prostaglandins by the ductus arteriosus was also measured by mass fragmentography. Again 6 keto-PGF_1α was the predominant product: 6 keto-PGF_1α = 223 versusPGE₂ = 10 and PGF_2α = 15 ng/g of wet weight of tissue. The identity of 6 keto-PGF_1α was confirmed by mass spectroscopy. The high capacity of fetal blood vessels to synthesize PGI₂ may reflect an important role for this substance in the regulation of the fetal circulation.     

Synthesis of prostaglandins by the ductus arteriosus of the bovine fetus

Previous studies demonstrated that prostaglandins are local or tissue hormones which can be released from blood vessel walls. In the present study, we investigated the capacity of bovine ductus arteriosus to synthetize prostaglandins in vitro. After incubation of slices of ductus arteriosus in Krebs' solution with (1-14C) arachidonic acid for 3 hours, more than 40% of the radiolabeled material recovered from the incubating medium were metabolites of arachidonic acid. The major product was indistinguishable from 6 keto-PGF1alpha as determined by its chromatographic motility and resistance to alkaline conversion to PGB. The PGI2 synthetic capacity of the ductus arteriosus, as revealed by the predominance of its major metabolite 6 keto-PGF1alpha, suggests that this metabolic pathway of arachidonic acid may contribute to the hemodynamic changes occurring during fetal life and at birth.     

PGE2 is a more potent vasodilator of the lamb ductus arteriosus than is either PGI2 or 6 keto PGFα

It has been shown in vitro that the lamb ductus arteriosus forms prostaglandins PGE₂, PGF_2α, 6 keto PGF_1α (and its unstable precursor PGI₂). In this study the relative potencies of these endogenous prostaglandins were investigated on isolated lamb ductus arteriosus preparations contracted by exposure to elevated PO₂ and indomethacin. All the prostaglandins (except PGF_2α) relaxed the vessel. This is consistent with the hypothesis that endogenous prostaglandins inhibit the tendency of the vessel to contract in response to oxygen. Only PGE₂, however, relaxed the vessel at concentrations below 10⁻⁸M. PGI₂ and 6 keto PGF_1α had approximately 0.001 and 0.0001 times the activity of PGE₂. Although PGE₂ has been observed to be a minor product of prostaglandin production in the lamb ductus arteriosus, the tissue's marked sensitivity to PGE₂ might make it the most significant prostaglandin in regulating the patency of the vessel.     

Prostaglandin I2 is less relaxant than prostaglandin E2 on the lamb ductus arteriosus

Prostaglandin(PG) I₂ and its stable metabolite, 6-keto-PGF_1α, were tested on the isolated ductus arteriosus from mature fetal lambs. PGI₂ relaxed the ductus in high doses (threshold 10⁻⁶M) and its activity disappeared on standing at room temperature for 30 minutes. 6-keto-PGF_1α was inactive at all doses. By contrast, PGE₂ produced a dose-dependent relaxation over a range between 10⁻¹⁰ and 10⁻⁶ M. These findings confirm that PGE₂ is the most potent ductal relaxant among the known derivatives of arachidonic acid. PGE₂ probably maintains ductus patency in the fetus and, together with PGE₁, remains the compound of choice in the management of newborns requiring a viable ductus for survival.     

Responsiveness of the lamb ductus arteriosus to prostaglandins and their metabolites

The relative potencies of the prostaglandins A₁, A₂, E₁, E₂, F_2α and their 15-keto-, 15-keto-13,14-dihydro-, and 13,14-dihydro-metabolites were investigated on isolated lamb ductus arteriosus preparations contracted by exposure to elevated PO₂. All the prostaglandins (except PGF_2α and its 15-keto-metabolites) relaxed the tissue. However, only PGE₁, E₂, and their 13,14-dihydro-metabolites, were effective at concentrations below 10⁻⁸ M. Therefore, events that alter metabolism of circulating PGs in the perinatal period may have significant effects on the relative patency or closure of the ductus arteriosus.     

The effects of prostaglandins, prostaglandin inhibitors, and oxygen on the closure of the ductus arteriosus, pulmonary arteries and umbilical vessels in vitro

Three prostaglandins (PGF₂α and PGE₁, PGE₂) have been found in maternal and fetal circulation during labour. Two of these prostaglandins (PGF₂α and PGE₂) are present in elevated levels in maternal circulation during labour and their presence in fetal vessels has been shown.These three prostaglandins have been tested for their effects on fetal vessels in vitro (umbilical artery and vein, ductus arteriosus, and smaller pulmonary artery). These vessels were selected as being crucial in the conversion from fetal to extra-uterine circulation in mammalian species. Responses of these vessels to the prostaglandins under varying oxygen regimes have been examined as well as their responses to prostaglandin inhibitors. Activity of vessels of varying gestational ages exposed to PGF₂α was also examined. The following results were obtained:

1. All vessels, with the exception of pulmonary arteries, contracted in the presence of oxygen over the range 20–100mmHg pO₂. At a pO₂ of < 20mmHg the ductus arteriosus remained inactive or dilated. Pulmonary arteries dilated at high pO₂.

2. All vessels contracted in response to exogenous PGF₂α with the exception of the pulmonary arteries which dilated. In the presence of PGF₂α, the umbilical veins dilated under low (< 20mmHg) pO₂ and contracted at higher levels. Contraction also occurred at lower levels after a period of time.

3. Although PGF₂α was capable of causing contraction in the ductus arteriosus at near zero pO₂, oxygen, (or possibly the products of oxygenation), appear to be required for continued contraction in the presence of PGF₂α. A synergistic relationship between oxygen and PGF₂α responses was found as oxygen tensions increased. A synergistic response between PGF₂α and oxygen with umbilical arteries which did not increase with increased pO₂ was also found. Oxygen tension appeared to have little effect on the response of other vessels to PGF₂α.

4. PGE₁ caused dilations in all vessels examined. Such dilations appearing to be independent of the oxygen regime prevailing. However, an increase in oxygen during experiments reversed any dilation caused by the prostaglandins.

5. PGE₂ caused contractions in umbilical vessels which were independent of oxygen. PGE₂ caused contraction of pulmonary arteries. However, in the ductus arteriosus, PGE₂ caused an initial contraction followed by a strong dilation. This dilation became weaker as pO₂ increased.

6. Additions of prostaglandin inhibitors (Naproxen and Indomethacin) to the bathing solution in which the ductus arteriosus and umbilical arteries were contracting (in response to PGF₂α, or oxygen alone) caused a decrease in contractions, and sometimes a slight decrease when the vessel had been pretreated with PGF₂α suggesting a possible need for endogenously synthesised prostaglandins for the maintenance of oxygen mediated contractions (in vivo).

7. Vessels responsed to PGF₂α at an early gestational age. A role for prostaglandins and oxygen in the closure of fetal vessels is discussed.

     

In vitro prostaglandin synthesis by human glomeruli and papillae

We have investigated in vitro prostaglandin synthesis by human isolated glomeruli and papillary homogenates and compared the results with those obtained in parallel studies using rat material. Prostaglandins were measured by two methods, namely radiometric high performance liquid chromatography after incubation with ¹⁴C arachidonic acid and radioimmunoassay. The relative abundance of various prostaglandins synthesized by glomeruli was different in man (6 keto PGF_1α > TXB₂ > PGF_2α > PGE₂) and in the rat (PGE₂ TXB₂ > 6 keto PGF_1α). Unidentified peaks eluting between 6 keto PGF_1α and TXB₂ were observed only in rat glomeruli. These peaks were suppressed by indomethacin. Direct radioimmunoassay of prostaglandins in the incubation medium of human glomeruli confirmed the predominance of 6 keto PGF_1α synthesis and showed its stimulation by arachidonic acid, its progressive decrease with time and its linear relationship with glomerular protein at low concentrations. On the contrary, the profile of prostaglandin synthesis by the papilla was similar in man and in the rat, PGE₂ and PGF_2α being the major products in both species. However, related to one mg of protein, papillary synthesis of these two prostaglandins was greater in the rat. These results show that PGI₂ is the major prostaglandin synthesized in human glomeruli and suggest a role for this prostaglandin in glomerular physiology in man.     

Prostanoid synthesis in whole blood cells from fish of the Arabian Gulf

The ability to synthesise prostaglandins and thromboxane from ¹⁴C-labelled arachidonic acid was investigated in 11 species of fish from the Arabian Gulf. Cyclooxygenase activity was assessed in washed whole blood cells. Arachidonic acid and its metabolites were extracted and separated on silicic acid columns and thin layer chromatography (silica gel G). Total capacity to convert [¹⁴C]arachidonic acid to prostanoids varied from 1 to 35% among the 11 fish species studied. Gray shark (Chiloscyllium griseum) blood cells had the highest capacity (37±0.4%) to convert arachidonate into prostanoids and two species of catfish (Arius bilineatus and A. thalassinus) exhibited greater than 10% capacity to convert [¹⁴C]arachidonate into prostanoids. The major prostanoid synthesised by the two catfish (A. bilineatus and A thalassinus) was 6-keto PGF_1α, a stable metabolite of prostacyclin, PGI₂. In contrast, A. teunispinis synthesised thromboxane B₂, a stable metabolite of thromboxane A₂. Thromboxane B₂ (TXB₂) was the major product synthesised by all three species of shark studied (Chil. griseum, Carcharhinus plumbeus, Carch. melanopterus), with 6-keto PGF_1α a minor product. Other fish studied showed a varied pattern of prostanoid synthesis. The synthesis of these prostanoids was almost completely blocked by preincubation of the whole blood cells from catfish and shark with indomethacin (0.5 μM) suggesting the involvement of cyclooxygenase-mediated prostanoid synthesis.     

Effects of superovulation, arachidonic acid or endometrium on release of prostaglandins and synthesis of proteins by bovine trophoblast

This study was conducted in vitro to examine factors that may regulate prostaglandin release by bovine trophoblast and endometrial slices. Trophoblastic tissues and endometrial slices were recovered from superovulating and normally-ovulating cattle on day 16 or 20 of pregnancy and incubated for 24 h. Release of PGF_2^α and 13,14-dihydro-15-keto-PGF_2^α (PGMF), and incorporation of [¹⁴C]-leucine into proteins were quantified and expressed per μg DNA, which gives a measure of cellular activity. Activity of trophoblastic tissue for synthesizing protein was decreased (P<.05) and for releasing PGMF was increased (P<.05) on day 20 compared to day 16 of pregnancy. Neither supercovulation nor day of pregnancy altered trophoblastic activity for releasing PGF_2^α. Supercovulation increased (P<.05) endometrial release of PGF_2^α. Endometrial release of PGF_2^α was less (P<.05) on day 20 than on day 16 of pregnancy. When arachidonic acid (0, 100, 200 or 400 μg) was added at the start of incubation, trophoblastic release of PGF_2^α changed (P<.05) quadratically with dose of arachidonic acid. When arachidonic acid was added 8 h after the start of incubation, triphoblastic release of PGF_2^α increased linearly (P<.01) with dose of arachidonic acid. Adding arachidonic acid to incubation medium did not affect trophoblastic or endometrial protein synthesis. Endometrial slices suppressed (P<.05) trophoblastic protein synthesis and release of PGF_2^α. Apparently, endometrium can modulate trophoblastic release of prostaglandins and synthesis of proteins in vitro, and trophoblastic tissue from supercovulated cattle 16 or 20 days pregnant can be used to study trophoblastic synthesis of prostaglandins and proteins.     

The influence of mono- and divalent cations on the cardiac metabolism of arachidonic acid

Our previous study indicated that, in the isolated rabbit heart, perfusion with Ca²⁺ free Krebs Henseleit buffer (KHB) results in increased conversion of exogenous arachidonic acid to PGE₂ and 6-keto-PGF_1α, probably as the result of increased availability of substrate to cuclooxygenase. Since perfusion with Ca²⁺ free buffer is known to cause alterations in the cardiac content of various mono- and divalent cations, the present study was performed to determine: a) The relationship between the conversion of exogenous arachidonic acid to prostaglandins and cardiac content of Na⁺, K⁺, Ca²⁺ and Mg²⁺; and b) Whether enhanced arachidonic acid conversion to prostaglandins during Caa²⁺ free perfusion is due to reduced incorporation of this fatty acid into tissue lipids. Perfusion of the rabbit heart with Ca²⁺ free buffer produced a significant reduction in the tissue content of Na⁺, K⁺, Ca²⁺ and Mg²⁺. However, the production of 6-keto-PGF_1α from exogenous arachidonic acid was linearly correlated with tissue Mg²⁺. These observations, together with our finding that perfusion with Ca²⁺ free KHB reduced the incorporation of [³H] arachidonic acid into tissue lipids, suggests that Ca²⁺ free perfusion may, by reducing the activity of arachidonyl CoA synthetase (a Mg²⁺ dependent enzyme), decrease the acylation of arachidonic acid into lipids, thus increasing the availability of arachidonic acid to cyclooxygenase.     

Arachidonate metabolism in bovine gallbladder muscle

Incubation of [1-¹⁴C]arachidonic acid (AA) with homogenates of bovine gallbladder muscle generated a large amount of radioactive material having the chromatographic mobility of 6-keto-PGF_1α (stable product of PGI₂) and smaller amounts of products that comigrated with PGF_2α and PGE₂. Formation of these products was inhibited by the cyclooxygenase inhibitor indomethacin. The major radioactive product identified by thin-layer chromatographic mobility and by gas chromatography - mass spectrometric analysis was found to be 6-keto-PGF_1α. The quantitative metabolic pattern of [1-¹⁴C]PGH₂ was virtually identical to that of [1-¹⁴C]AA. Incubation of arachidonic acid with slices of bovine gallbladder muscle released labile anti-aggregatory material in the medium, which was inhibited by aspirin or 15-hydroperoxy-AA.These results indicate that bovine gallbladder muscle has a considerable enzymatic capacity to produce PGI₂ from arachidonic acid.     

7beta-hydroxy-epiandrosterone modulation of 15-deoxy-delta12,14-prostaglandin J2, prostaglandin D2 and prostaglandin E2 production from human mononuclear cells

7β-hydroxy-epiandrosterone (7β-OH-EPIA) has been shown to be cytoprotective in various organs including the brain. It has also been shown that prostaglandin D₂ (PGD₂) and its spontaneous metabolite 15-deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂) are also cytoprotective. It is possible that these prostaglandins derived from circulating mononuclear cells may mediate the actions of 7β-OH-EPIA. The aim of this study, therefore, was to ascertain the effect of 7β-OH-EPIA (in the absence or presence of tumour necrosis factor-α (TNF-α)), a pro-inflammatory stimulus, on the biosynthesis of PGD₂, PGE₂ and 15d-PGJ₂ from human mononuclear cells. Prostaglandins were measured by enzyme immunoassay (EIA). 7β-OH-EPIA alone induced a concentration-dependant increase in the production of PGD₂. TNF-α increased PGD₂ levels which were enhanced by 7β-OH-EPIA. 7β-OH-EPIA increased 15d-PGJ₂ levels both in the absence and presence of TNF-α. 7β-OH-EPIA alone had no effect on PGE₂ biosynthesis but suppressed TNF-α-induced PGE₂ circa 50%. 7β-OH-EPIA also increased the level of free arachidonic acid and radiolabelled prostaglandins in cells pre-incubated with radiolabelled arachidonic acid, indicating that the increase may occur via the enhanced release of substrate arachidonic acid. 7β-OH-EPIA did not affect levels of the anti-inflammatory cytokine IL-10 indicating that this is an unlikely mechanism by which 7β-OH-EPIA induces its actions but more likely exerts its effects via the production of cytoprotective prostaglandins.     

Synthesis of thromboxane B2 and 6-keto-prostaglandin F1α by bovine gastric mucosal and muscle microsomes

Bovine gastric mucosal and muscle microsomes synthesize prostaglandins and thromboxane B₂ (TXB₂) from arachidonic acid (AA). TXB₂ and 6-keto-prostaglandin F₁α (6-keto-PGF₁α) were the major products synthesized by pylorus, body, and cardiac region of the gastric mucosa. Gastric muscle mainly synthesized 6-keto-PGF₁α. TXB₂ and 6-keto-PGF₁α synthesis occurs at an appreciable rate from endogenous precursors but more rapidly with added arachidonate. Prostaglandins E₂, F₂α and D₂ were synthesized in smaller amounts under the conditions studied.     

Effect of prostaglandins E1, E2 and F2α on neutrophil aggregation

Recently we have found that chemotactic factors stimulate neutrophils in suspension to aggregate. Because of an obvious analogy to platelet aggregation, we examined the influence of three prostaglandins on this process. Prostaglandins E₁, E₂ and F_2α alone did not cause aggregation of the neutrophils but were able to partially inhibit the aggregation response induced by the synthetic chemotactic tripeptide, formly-methionyl-leucyl-phenylalanine. The minimal inhibitory concentrations for prostaglandins E₁, E₂ and F_2α were 10⁻⁷, 10⁻⁶ and 10⁻⁵M, respectively. These results are similar to those found for the prostaglandin-induced inhibition of platelet aggregation. It may be, therefore, that neutrophil aggregation, like platelet aggregation, is modulated by intracellular prostaglandins and other products of arachidonic acid metabolism.     

Biosynthesis of prostaglandins and thromboxane B2 by fetal lung homogenates

The conversion of arachidonic acid to prostaglandins (PG's) and thromboxane B₂ (TXB₂) was investigated in homogenates from fetal and adult bovine and rabbit lungs. Adult bovine lungs were very active in converting arachidonic acid (100 μg/g tissue) to both PGE₂ (10.7 μg/g tissue) and TXB₂ (6.2 μ/g tissue). Smaller amounts of PGF_2α (0.9 μ/g) and 6-oxoPGF_1α were formed. Homogenates from fetal calf lungs during the third trimester of pregnancy were quite active in converting arachidonic acid to PGE₂, but formed very little TXB₂, PGF_2α or 6-oxoPGF_1α. Homogenates from rabbit lungs converted arachidonic acid (100 μg/g) mainly to PGE₂, both before and after birth. The amount of PGE₂ formed increased during gestation to a maximum of about 6 μg/g tissue at 28 days of gestation. It then decreased to a minimum (1.5 μg/g) which was observed 8 days after birth, followed by an increase to about 4 μg/g in older rabbits.     

The effects of alpha1-adrenergic and muscarinic cholinergic stimulation on prostaglandin release by rabbit iris

We have investigated the effects of norepinephrine (NE) and acetylcholine (ACh) on prostaglandin (PGE₂ and 6 keto-PGF_1α) production by rabbit iris, measured by radioimmunoassay (RIA), and the type of phospholipase activated by NE in irides in which phosphatidylinositol (PI) was doubly prelabeled with [³H] myo-inositol and [1-¹⁴C] arachidonic acid (¹⁴C-AA), quantitated by radiometric and chromatographic methods. PGE₂ output in 60 min (3.6 μg/g tissue) was 2.6 times greater than 6 keto-PGF_1α. PG production is time-dependent and it is stimulated by NE and ACh in a dose-dependent manner. The Ne- and ACh-induced release of PGE₂, measured by RIA, is mediated through α₁-adrenergic and muscarinic cholinergic receptors, respectively, and it requeires Ca²⁺ for maximal stimulation. Studies on the mechanism of AA release PI in irides doubly prelabeled with ¹⁴C-AA and [³H] myo-inositol revelased the following: (a) Both Ne and ACh increased the breakdown of PI, and this was accompanied by a significant increase in the release of AA and consequently PGE₂. The stimulatory effects of NE and ACh are mediated through α₁-adrenergic and muscainic cholinergic receptors respectively. (b) The NE-induced formation of ³H-lyso PI and the NE-induced metabolism of ¹⁴C-1,2-diacyl-glycerol (DG) are time-dependent. Two pathways for AA release from PI are probably operaitve in the iris: (a) An indirect release by PI-specific phospholipase C which producers DG, followed by the actions of DG- and monoacylglycerol lipases on DG to release AA. (b) A direct release by phospholipase A₂. Whether lyso PI is a product of the polypholipids such as prosphatidylcholine and phosphatidylethanolamine could also serve as a source for AA in PG synthesis. In conclusion, the data presented provide evidence that in the iris the neurotransmitter-stimulated release of PG and AA, from phosphoinositides, for PG synthesis is coupled to the activation of α₁-adrenergic and muscarinic cholinergic receptors.     

Studies on closure of the ductus arteriosus. X. effect of prostaglandin

The effect of prostaglandins F_2∝, E₁ and of 7-oxa-13-prostynoic acid on the newborn rat and rabbit ductus can be studied using the whole-body freezing technique. PGF_2∝ and PGE₁ were able to re-open the closing ductus arteriosus in adequately oxygenated animals. PGF_2∝ administration was accompanied by a strong physical reaction in the rat but less in the rabbit. PGF₁ had sedative effects in both animals. A prostaglandin antagonist, 7-oxa-13-prostynoic acid had no effect on normal ductal closure nor did it counteract the effects of PGF_2∝ and PGE₁. The role of prostaglandins in homeostasis during the fetal and newborn period may be to modify ductal tone.     

Anti-inflammatory drugs, prostanoid and proteoglycan production by cultured bovine articular chondrocytes

The effect of various anti-inflammatory drugs on the production of prostaglandins E₂ and F₂α, 6 keto PGF₁α and thromboxane B₂ by bovine articular chondrocytes was measured by radioimmunoassay. While indomethacin and meclofenamic acid caused a dose-dependent inhibition of all prostanoids measured, the effects of hydrocortisone and colchicine varied with respect to different prostanoids. Hydrocortisone (10⁻⁷M – 10⁻³M) both in the presence and absence of added arachidonic acid, resulted in an inhibition of prostaglandins E₂ and F₂, and to a lesser extent, 6 keto PGF₁α, but T×B₂ production was only slightly inhibited by the drug in the absenced of arachidonic acid and markedly increased in its presence. Colchicine (10⁻⁷M – 10⁻³M) had the opposite effect, causing an inhibition of T×B₂ and stimulating PGE₂ and 6 keto PGF₁α production. These findings suggest that certain anti-inflammatory drugs may, in addition to their action on phospholipase A₂ and cyclo-oxygenase, exert potent effects at the level of the different synthetases. In order to see whether these alterations in relative prostanoid levels affected proteoglycan metabolism, the effect of anti-inflammatory drugs on proteoglycan synthesis by cultured chondrocytes was tested using ³⁵SO₄ labeling methodology. The results showed that the concentrations tested (10⁻⁵M to 10⁻⁷M), indomethacin, dexamethasone, hydrocortisone and colchicine inhibited ³⁵SO₄ incorporation into newly synthesized proteoglycan molecules both in the presence (10⁻⁶M) and absence of exogenous arachidonic acid. In the same concentration range choroquine had no effect.These results do not support the hypothesis of direct prostanoid involvement in the modulation of proteoglycan synthesis in articular cartilage.     

Potent constriction of cat coronary arteries by hydroperoxides of arachidonic acid and its blockade by anti-inflammatory agents

The ω-6 and ω-9 hydroperoxides of arachidonic acid caused dose-dependent constriction of cat coronary arteries in concentrations of 10⁻⁸ to 10⁻⁵M. Their potency was comparable to that of prostaglandin (PG) E₂, and PGF_2α and 100 times greater than that of arachidonic acid. The cyclooxygenase inhibitor, meclofenamate markedly reduced constriction caused by the hydroperoxides but potentiated constriction caused by the prostaglandins. The effects of the hydroperoxides were also reduced by indomethacin and dexamethasone but were unaffected by the thromboxane synthetase inhibitor imidazole. Since the hydroperoxides are not substrates for cyclooxygenase, it is suggested that they have a direct effect on the arteries which can be antagonized by anti-inflammatory drugs.     

Prostaglandin production from homogenates of separated glandular epithelium and stroma from human endometrium

The biosynthesis of prostaglandins by isolated epithelial glandular and stromal cells was studied after collagenase digestion of endometrium collected from women at various stages of the menstrual cycle. Homogenates of the separate cell types were incubated for 60 minutes with 2.08 μg 1¹⁴C arachidonic acid and the products separated by thin layer chromatography. Both glandular and stromal homogenates synthesised PGF_2α. More PGF_2α was synthesised by glandular epithelium separated from both proliferative and secretory endometrium than by stroma. The ratio of PGF_2α/PGE₂ was greater in glands and stroma isolated from secretory than proliferative endometrium. Small but significant amounts of 6-keto-PGF₁α were produced by all cell types. These results suggest that the increased synthesis of PGF₂α from secretory endometrium is due, at least in part, to increased activity of cyclo-oxygenase enzyme in the glandular epithelium.