Biosynthesis and catabolism of prostaglandin F2alpha (PGF2alpha) are controlled by progesterone in the rat uterus during pregnancy

Myometrial quiescence is a key factor in all species to accomplish a successful gestation. PGs play a crucial role in mediating parturition events, and their synthesis and metabolism are regulated by cyclooxygenases (COXs) and NAD(+)-dependent 15-hydroxy-PG dehydrogenase (PGDH), respectively. Progesterone (P(4)) is the hormone responsible for maintaining uterine smooth muscle quiescence during pregnancy. In this work, we have studied the effect of P(4) on the activity of COXs and PGDH, the uterine enzymes involved in the biosynthesis and metabolism of prostanoids in the rat. We found that during pregnancy PGF(2alpha) production and also protein levels of COX-1 and COX-2 were decreased. The exogenous administration of P(4) significantly inhibited the uterine production of PGF(2alpha) and also the protein level of COX-2. PGF(2alpha), metabolism was assessed by PGDH activity, which resulted high during pregnancy and increased as a result of P(4) administration. These results indicate that PGs levels were negatively modulated by P(4), which could be exerting its effect by increasing PGs metabolism through stimulation on PGDH activity and an inhibition on COX and that is a major mechanism for maintain uterine quiescence in pregnancy.     

Localization of prostaglandin F in the ovine uterus during early pregnancy

As part of a study on the anti-luteolytic action of the sheep conceptus, the distribution of prostaglandin F (PGF) within the uteri of 19 pregnant and 14 non-pregnant ewes was studied using three antisera to PGF_2α and fluorescent antibody tracing. In the uteri of seven non-pregnant ewes up to Day 11 after estrus (Day 0) and in uteri of $\text{18}\text{19}$ pregnant ewes up to Day 50, PGF was localized mainly in the lamina propria, with very little on epithelial cells lining the uterine lumen. After Day 11, in the uteri of seven non-pregnant ewes, PGF was localized on the surface of luminal epithelial cells and throughout their cytoplasm, and to a lesser extent in the lamina propria. The distribution of PGF on Days 14 and 17 in the gravid and non-gravid horns of the uteri of 13 ewes made unilaterally pregnant (conceptus confined to one uterine horn) was similar to that described above for normal pregnant and non-pregnant ewes after Day 11 respectively.These results are interpreted to indicate a change in the distribution of PGF in sheep uteri due to the presence of a conceptus.     

Expression of PIK3IP1 in the murine uterus during early pregnancy

The ovarian steroid hormones, estrogen (E2) and progesterone (P4), are essential regulators of uterine functions necessary for development, embryo implantation, and normal pregnancy. ARID1A plays an important role in steroid hormone signaling in endometrial function and pregnancy. In previous studies, using high density DNA microarray analysis, we identified phosphatidylinositol-3-kinase interacting protein 1 (Pik3ip1) as one of the genes up-regulated by ARID1A. In the present study, we performed real-time qPCR and immunohistochemistry analysis to investigate the regulation of PIK3IP1 by ARID1A and determine expression patterns of PIK3IP1 in the uterus during early pregnancy. The expression of PIK3IP1 was strong at the uterine epithelial and stromal cells of the control mice. However, expression of PIK3IP1 was remarkably reduced in the Pgr^cre/+Arid1a^f/f mice and progesterone receptor knock-out (PRKO) mice. During early pregnancy, PIK3IP1 expression was strong at day 2.5 of gestation (GD 2.5) and then slightly decreased at GD 3.5?at the epithelium and stroma. After implantation, PIK3IP1 expression was detected at the secondary decidualization zone. To determine the ovarian steroid hormone regulation of PIK3IP1, we examined the expression of PIK3IP1 in ovariectomized control, Pgr^cre/+Arid1a^f/f, and PRKO mice treated with P4 or E2. P4 treatment increased the PIK3IP1 expression at the luminal and glandular epithelium of control mice. However, the PIK3IP1 induction was decreased in both the Pgr^cre/+Arid1a^f/f and PRKO mice, compared to controls. Our results identified PIK3IP1 as a novel target of ARID1A and PGR in the murine uterus.     

Expression of enzymes of cyclooxygenase pathway and secretion of prostaglandin E2 and F2alpha by porcine myometrium during luteolysis and early pregnancy

Past studies of uterine prostaglandin (PGs) and pig reproduction have focused on endometrial rather than myometrial PGs. This study documents the synthesis and secretion of myometrial prostaglandins (PGs) in pigs and the involvement of oxytocin (OT) in these processes. Cyclooxygenase-2 (COX-2) expression was similar in myometrial explants from cyclic and pregnant pigs (days 14-16) and OT (10(-7) M) in vitro significantly increased COX-2 protein regardless of reproductive state. Basal expression of prostaglandin E2 synthase (PGES) was higher during pregnancy than during luteolysis. Conversely, prostaglandin F synthase (PGFS) was highest during luteolysis and lower in myometrium from gravid animals. OT had no influence on the expression of PGES and PGFS. In another tissue culture experiment, myometrial slices produced more PGE2 than PGF2alpha regardless of reproductive state of the female. OT stimulated PGE2 production in myometrium harvested during luteolysis and increased PGF2alpha production in all tissues examined. Progesterone (P4; 10(-5) M) blocked stimulatory effect of OT on myometrial PG release. Myometrial OTr mRNA was higher (P=0.03) during luteolysis than during pregnancy. In conclusion: (1) oxytocin increases myometrial COX-2 expression, but does not influence the expression of terminal enzymes of PGs synthesis (PGES and PGFS); (2) porcine myometrium preferentially produces PGs during early pregnancy and secretes more PGE2 than PGF2alpha; (3) myometrial OT and OTr support secretion of PGs from myometrium during luteolysis.     

Effects of indomethacin, luteinizing hormone (LH), prostaglandin E2 (PGE2), trilostane, mifepristone, ethamoxytriphetol (MER-25) on secretion of prostaglandin E (PGE), prostaglandin F2alpha (PGF2alpha) and progesterone by ovine corpora lutea of pregnancy or the estrous cycle

Two experiments were conducted to determine the luteotropin of pregnancy in sheep and to examine autocrine and paracrine roles of progesterone and estradiol-17 beta on progesterone secretion by the ovine corpus luteum (CL). Secretion of progesterone per unit mass by day-8 or day-11 CL of the estrous cycle was similar to day-90 CL of pregnancy (P > or = 0.05). In experiment 1, secretion of progesterone in vitro by slices of CL from ewes on day-8 of the estrous cycle was increased (P < or = 0.05) by LH or PGE2. Secretion of progesterone in vitro by CL slices from day-90 pregnant ewes was not affected by LH (P > or = 0.05) while PGE2 increased (P < or = 0.05) secretion of progesterone. Day 8 ovine CL of the estrous cycle did not secrete (P > or = 0.05) detectable quantities of PGF2alpha or PGE while day-90 ovine CL of pregnancy secreted PGE (P < or = 0.05) but not PGF2alpha. Secretion of progesterone and PGE in vitro by day-90 CL of pregnancy was decreased (P < or = 0.05) by indomethacin. The addition of PGE2, but not LH, in combination with indomethacin overcame the decreases in progesterone by indomethacin (P < or = 0.05). In experiment 2, secretion of progesterone in vitro by day-11 CL of the estrous cycle was increased at 4-h (P < or = 0.05) in the absence of treatments. Both day-11 CL of the estrous cycle and day-90 CL of pregnancy secreted detectable quantities of PGE and PGF2alpha (P < or = 0.05). In experiment 1, PGF2alpha secretion by day-8 CL of the estrous cycle and day-90 ovine CL of pregnancy was undetectable, but was detectable in experiment 2 by day-90 CL. Day 90 ovine CL of pregnancy also secreted more PGE than day-11 CL of the estrous cycle (P < or = 0.05), whereas day-8 CL of the estrous cycle did not secrete detectable quantities of PGE (P > or = 0.05). Trilostane, mifepristone, or MER-25 did not affect secretion of progesterone, PGE, or PGF2alpha by day- 11 CL of the estrous cycle or day-90 CL of pregnancy (P > or = 0.05). It is concluded that PGE2, not LH, is the luteotropin at day-90 of pregnancy in sheep and that progesterone does not modify the response to luteotropins. Thus, we found no evidence for an autocrine or paracrine role for progesterone or estradiol-17 36 on luteal secretion of progesterone, PGE or PGF2alpha.     

Suppression of prostaglandin E(2)-mediated c-fos mRNA induction by interleukin-4 in murine macrophages

When murine peritoneal macrophages were stimulated for 30 min with arachidonic acid, the growth-associated immediate early gene c-fos was induced in a concentration-dependent manner as assessed by Northern blot analysis. The arachidonic acid-induced c-fos mRNA expression was inhibited by a cyclooxygenase inhibitor, indomethacin, but not by a lipoxygenase inhibitor, nordihydroguaiaretic acid. Macrophages produced prostaglandin (PG) E(2) from arachidonic acid as determined by an enzyme immunoassay. Northern blot analysis revealed the expression of PGE receptor EP2 and EP4 subtypes, but not EP1 and EP3 in murine macrophages. PGE(2) brought about a marked elevation of cAMP, and c-fos mRNA expression was increased by PGE(2) and dibutyryl cAMP in these cells. These results suggest that arachidonic acid is transformed to PGE(2), which then binds to EP2 and EP4 receptors to increase intracellular cAMP and c-fos mRNA expression. Furthermore, the induction of c-fos by arachidonic acid, PGE(2), and cAMP was suppressed by pretreatment with interleukin (IL)-4. We also showed that the tyrosine phosphorylation of a Janus kinase, JAK3, is enhanced by IL-4 treatment, suggesting that the PGE(2)-mediated c-fos mRNA induction is inhibited by IL-4 through the tyrosine phosphorylation of JAK3.     

Differential expression of prostaglandin E receptor subtype EP2 in rat uterus during early pregnancy

PGE2 is essential for mammalian female reproduction. This study was to examine the expression of EP2 gene in the rat uterus during early pregnancy, delayed implantation and artificial decidualization by in situ hybridization and immunohistochemistry. There was no detectable EP2 mRNA expression in the uterus from days 1 to 4 of pregnancy (day 1 = day of vaginal sperm). A low level of EP2 immunostaining was observed in the luminal and glandular epithelium from days 1 to 4 of pregnancy. Both EP2 mRNA and protein expression were highly detected in the luminal epithelium at implantation sites on day 6 of pregnancy. EP2 expression decreased from day 7 of pregnancy and was undetectable on days 8 and 9 of pregnancy. After delayed implantation was terminated by estrogen treatment and the embryo implanted, both EP2 mRNA and protein expression were strongly observed in the luminal epithelium at the implantation site. There was no detectable EP2 expression in both control and decidualized uteri. In conclusion, these data suggest that EP2 expression at implantation site may play an important role during embryo implantation in rats.     

Effect of mifepristone on pregnancy,pregnancy-specific protein B (PSPB), progesterone,estradiol-17beta,prostaglandin F2alpha (PGF2alpha) and prostaglandin E (PGE) in ovariectomized 90-day pregnant ewes

The objective of this experiment was to determine the effect of mifepristone, a progesterone receptor antagonist, on pregnancy and secretion of steroids, pregnancy-specific protein B (PSPB) and prostaglandins at mid-pregnancy in ewes. Ninety-day pregnant ewes were ovariectomized (OVX) and treatments were initiated 72 h post-OVX. Ewes received (1) vehicle, (2) prostaglandin F2alpha (PGF2alpha, 8 mg/58 kg/bw, i.m.) 84 h post-OVX, (3) mifepristone (50 mg intrajugular at 72, 84, 96, and 108 h post-OVX), (4) mifepristone (50mg) + PGF2alpha, (5) mifepristone (100 mg intrajugular at 72, 84, 96, and 108 h), and (6) mifepristone (100 mg) + PGF2alpha. Ewes treated with vehicle or PGF2alpha alone did not abort (P > or = 0.05). But, 60, 80, 60, and 100% of ewes treated with mifepristone (50 mg), mifepristone (50 mg) + PGF2alpha, mifepristone (100 mg), and mifepristone (100 mg) + PGF2alpha, respectively, aborted (P < or = 0.05). Profiles of progesterone, estradiol-17beta, prostaglandin E (PGE), or PSPB did not differ (P > or = 0.05) among treatment groups. Profiles of PGF2alpha of treatment groups receiving mifepristone with or without PGF2alpha differed (P < 0.05) from vehicle or PGF2alpha alone-treated ewes. It is concluded that progesterone actions are necessary to suppress uterine/placental secretion of PGF2alpha and that maintenance of critical progesterone: estradiol-17beta and PGE:PGF2alpha ratios are necessary for maintenance of pregnancy.     

Effects of indomethacin, prostaglandin E2, prostaglandin F2 alpha and 6-keto-prostaglandin F1 alpha on hatching of mouse blastocysts

The present study has been performed to investigate how PGs would participate the hatching process. Effects of indomethacin, an antagonist to PGs biosynthesis, on the hatching of mouse blastocysts were examined in vitro. Furthermore, it was studied that prostaglandin E2 (PGE2), prostaglandin F2 alpha (PGF2 alpha) or 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) were added to the culture media with indomethacin. The hatching was inhibited by indomethacin yet the inhibition was reversible. In the groups with indomethacin and PGE2, no improvement was seen in the inhibition of hatching and the inhibition was irreversible. In the groups with indomethacin and PGF2 alpha, inhibition of hatching was improved in comparison with the group with indomethacin. In the groups with indomethacin and 6-keto-PGF1 alpha, no improvement was seen. The above results indicated that PGF2 alpha possibly had an accelerating effect on hatching and a high concentration of PGE2 would exert cytotoxic effect on blastocysts.     

Induction of interleukin-1alpha production in murine Sertoli cells by interleukin-1

In the present study we examined the involvement of interleukin (IL)-1alpha, -1beta, FSH, and lipopolysaccharide (LPS) in the regulation of IL-1alpha and -1beta production by Sertoli cells under in vitro conditions. Sertoli cell cultures from immature mice produced constitutively basal levels of intracellular IL-1alpha. Stimulation of Sertoli cell cultures with LPS (5 microgram/ml) resulted in a maximal production of intracellular IL-1alpha 2 h after the stimulation. Thereafter, these levels decreased but remained significantly higher within 24 h after stimulation than those in control cultures. The effect of LPS on IL-1alpha production was dose dependent. FSH did not show any effect on intracellular IL-1alpha production by Sertoli cells. IL-1alpha could not be detected in supernatants of unstimulated or stimulated Sertoli cell cultures. Sertoli cell cultures stimulated with recombinant IL-1alpha induced optimal intracellular levels of IL-1alpha within 2 h of stimulation. These levels remained high 24 h after stimulation. However, stimulation of Sertoli cell cultures with IL-1beta induced a peak of IL-1alpha production 8 h after stimulation. These levels decreased 24 h after the stimulation but were still found to be significantly higher than those in control cultures. The addition of IL-1 receptor antagonist (IL-1ra) to Sertoli cell cultures did not significantly alter their capacity to produce IL-1alpha. However, the stimulatory effects of recombinant IL-1alpha on IL-1alpha production by Sertoli cell cultures were reversed by the concomitant addition of recombinant IL-1ra. No immunoreactive IL-1beta could be detected in lysates or conditioned media of immature murine Sertoli cells under any of the stimulatory conditions outlined. Our results may suggest the involvement of physiological (IL-1) and pathophysiological factors (LPS) in the regulation of spermatogenesis and spermiogenesis processes and male fertility.     

Prostaglandin F(2alpha) regulates cytokine responses of mast cells through the receptors for prostaglandin E

There is an increasing body of evidence that prostanoids modulate mast cell functions and contribute to the development of allergic inflammation. The present study aimed to identify an undetermined function of prostaglandin (PG) F_2α in mast cell activation and the signaling mechanism involved in it. Simultaneous quantification of prostanoids by liquid chromatography/tandem mass spectrometry revealed the constitutive release of PGF_2α, thromboxane B₂, and 6-keto-PGF_1α from bone marrow-derived mast cells (BMMCs). Upon activation of BMMCs by lipopolysaccharide, the cytokine production in BMMCs was enhanced when the culture was supplemented with PGF_2α. However, F prostanoid receptor—a selective receptor for PGF_2α—was not detected in BMMCs. Further investigations performed using prostanoid receptor antagonists revealed an alternative mechanism wherein the receptors for PGE species—E prostanoid receptors—mediated the PGF_2α signal in BMMCs. The present study provides an insight into a novel function of PGF_2α, i.e., an autocrine accelerator for mast cell activation.     

Cardiovascular effects of prostaglandin F(2 alpha) and prostaglandin E(2) in Atlantic cod (Gadus morhua)

Little is known of the cardiovascular functions of prostaglandins in non-mammalian vertebrates. There are indications that prostaglandins may have a function in haemostasis by constricting blood vessels in filament arteries in the fish gill after injury. Our aim was to examine the cardiovascular effect of the prostaglandins F(2 alpha) (PGF(2 alpha)) and E(2) (PGE(2)) with emphasis on branchial circulation. Intra-arterial injections of PGF(2 alpha) (10, 40, 160, 400 nmol kg(-1)) in cod caused a dose-dependent increase in ventral aortic blood pressure, a reduction in cardiac output, and an increase in gill vascular resistance. A contraction of filament arteries was observed with in vivo microscopy only seconds after injection. PGF(2 alpha) may therefore possibly be involved in a haemostatic vasoconstriction. In contrast, the most significant effects of PGE(2) appeared to be on the heart. PGE(2) also reduced dorsal aortic blood pressure.     

The induction of labour by the intravenous infusion of prostaglandin F2alpha

Labour was induced by the intravenous infusion of prostaglandin F2alpha in 106 patients at 36-44 weeks of pregnancy. The induction was successful in 80% of the women. The total dose needed ranged from 0.1 to 14.2 mg of PGF2alpha. The uterine activity and fetal heart rate were recorded by cardiotocography. The contraction pattern and induction-delivery time were the same as reported for induction with oxytocin. In one case uterine hyperactivity occurred after rupture of the membranes. No serious adverse effects were seen, but in a few cases local irritation was noted at the site of infusion. The condition of the infants was generally good. It might be concluded that PGF2alpha seems valuable for the induction of labour, but due to the risk for over-stimulation careful supervision is needed.     

Farrowing induction with a combination of prostaglandin F(2alpha) and a peripherally acting alpha(2)--adrenergic agonist AGN 190851 and a combination of prostaglandin F(2alpha) and oxytocin

We studied the effect of prostaglandin (PG) F(2alpha)-AGN 190851 on farrowing induction and compared it with that of PGF(2alpha)-oxytocin. Eighty crossbred, multiparous sows were randomly assigned to the following 4 treatment groups of 20 sows each: 1) control, saline-saline; 2) PGF(2alpha) (10 mg/sow)-oxytocin (30 IU/sow); 3) PGF(2alpha) (10 mg/sow)-AGN 190851 (0.06 mg/kg); and 4) PGF(2alpha) (10 mg/sow)-AGN 190851 (0.1 mg/kg). Either PGF(2alpha) or saline was administered intramuscularly on Day 111 of gestation at 11:30 h; AGN 190851, oxytocin or saline was administered intramuscularly 20 h after the first injection. The PGF(2alpha)-AGN 190851 (0.1 mg/kg) treated sows had the shortest mean farrowing interval (2.1 +/- 1.6 h, mean +/- SD) compared with the remaining treatment groups (control: 67.1 +/- 26.2 h; PGF(2alpha)-oxytocin: 5.6 +/- 6.7 h; PGF(2alpha)-AGN 190851 [0.06 mg/kg]: 3.0 +/- 2.8 h). Duration of farrowing, litter size, litter weight and interval from weaning to first estrus in sows were not significantly changed by these treatments. The PGF(2alpha)-oxytocin group had a significantly higher stillbirth rate than the control group, whereas the PGF(2alpha)-AGN 190851 (0.1 mg/kg) group had the lowest number of pigs born dead and stillbirth rate among the 4 treatment groups. These results suggested that the PGF(2alpha)-AGN 190851 combination can be used as an alternative method to PGF(2alpha)-oxytocin for synchronizing farrowing.     

Doses of prostaglandin F(2)alpha effective for induction of estrus in goats

A total of 11 cycling does weighing between 24 and 50 kg were injected with varying dosages of prostaglandin F(2)alpha (PGF(2)alpha) between 7 and 10 days into each estrous cycle. Five injections each of 1.25, 2.5, 5.0, or 7.5 mg PGF(2)alpha were alternated with five injections of 1.0 ml saline. Saline treated does served as controls. All does were teased twice daily with a buck and observed for signs of estrus for 5 days post-injection. Daily systemic concentrations of progesterone (P(4)) were determined by radioimmunoassay. The mean (+/- S.E.) hours from injection to estrus was 47 +/- 3.3, 42 +/- 4.3, 44 +/- 8.5, and 43 +/- 5.5 for does receiving 1.25, 2.5, 5.0, and 7.5 mg PGF(2)alpha, respectively. None of the does receiving saline exhibited estrus in the 5-day post-injection observation period. Mean (+/- S.E.) concentrations of systemic P(4) in all does on the day of injection was 4.22 +/- 0.45 ng/ml. Concentrations 24 hours post-injection were 0.21 +/- 0.02, 0.15 +/- 0.05, 0.17 +/- 0.04, 0.16 +/- 0.04, and 4.5 +/- 1.36 ng/ml for does receiving 1.25, 2.5, 5.0, and 7.5 mg PGF(2)alpha, and 1.0 ml saline, respectively. The results suggested that 1.25 mg PGF(2)alpha was effective for induction of estrus in the cycling goat.     

Prostacyclin, prostaglandin F2 alpha and progesterone production by bovine luteal cells during the estrous cycle

Corpora lutea (CL) were collected from Holstein heifers on Days 5, 10, 15 and 18 (5/day) of the estrous cycle. Dispersed luteal cell preparations were made and 10(6) viable luteal cells were incubated with bovine luteinizing hormone (LH) and different amounts of arachidonic acid in the presence and absence of the prostaglandin (PG) synthetase inhibitor indomethacin. The concentrations of progesterone, PGF2 alpha and 6-keto-PGF1 alpha, the stable inactive metabolite of prostacyclin (PGI2), were measured. Day 5 CL had the greatest initial content of 6-keto-PGF1 alpha (1.01 +/- 0.16 ng/10(6) cells), and synthesized more 6-keto-PGF1 alpha (2.55 +/- 0.43) than CL collected on Days 10 (0.57 +/- 0.11), 15 (0.08 +/- 0.05) and 18 (0.19 +/- 0.03) during a 2-h incubation period. Arachidonic acid stimulated the production of 6-keto-PGF1 alpha by Days 10, 15 and 18 luteal tissue. PGF2 alpha was produced at a greater rate on Day 5 (0.69 +/- 0.17 ng/10(6) cells) than on Days 10 (0.06 +/- 0.01), 15 (0.04 +/- 0.02) and 18 (0.08 +/- 0.01). Arachidonic acid stimulated and indomethacin inhibited the production of PGF2 alpha, in most cases. The initial content of 6-keto-PGF1 alpha was higher than that of PGF2 alpha on all days of the cycle and more 6-keto-PGF1 alpha was synthesized in response to arachidonic acid addition. The ratio of 6-keto-PGF1 alpha content to PGF2 alpha content was 4.39, 2.30, 1.25 and 1.13 on Days 5, 10, 15 and 18, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Morphine diminishes the constancy of spontaneous uterine contractions, antagonizes the positive inotropic effects of prostaglandin E2, but not of prostaglandin F2 alpha and inhibits prostaglandin E and F outputs from the uterus of ovariectomized rats

The effects of morphine on the constancy of spontaneous contractions (isometric developed tension = IDT and contractile frequency = CF), in uterine strips isolated from ovariectomized rats and the influence of naloxone, were explored. The inotropic responses to added prostaglandins (PGs) E2 and F2 alpha and the influences of morphine and of morphine in the presence of naloxone on PG actions, were also determined. Moreover, the synthesis and outputs of PGs E and F from uteri and the effects of morphine alone and of morphine plus naloxone, were studied. Morphine (10(-6) M) significantly depressed uterine constancy of IDT during the first hours following delivery, but its action on CF did not differ from controls. Naloxone, neither at 10(-8) M nor at 10(-6) M, altered the negative inotropic influence of morphine on IDT. Exogenous PGs E2 and F2 alpha, stimulated uterine inotropism in a concentration-dependent fashion. Morphine altered dose-response curves for exogenous PGE2, evoking a parallel surmountable shift to the right, but did not affect the inotropic action of added PGF2 alpha. This antagonistic effect of the opioid was not altered by preincubation with naloxone. Basal synthesis and outputs of PGs E and F in uteri from ovariectomized rats were significantly depressed by morphine (10(-6) M) but not altered by incubating tissues with morphine in presence of naloxone. Results are discussed in terms of a presumptive dual action of morphine on uterine motility, i.e., antagonizing PGE2 receptors and inhibiting the synthesis of some PGs by the uterus. These influences of morphine do not appear to be subserved by the activation of mu opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)