Control of oxytocin secretion by ovine corpora lutea: effects of arachidonic acid, phospholipases, and prostaglandins

A New Perspective on Thyroid Hormones: Crosstalk with Reproductive Hormones in Females

Accumulating evidence has shown that thyroid hormones (THs) are vital for female reproductive system homeostasis. THs regulate the reproductive functions through thyroid hormone receptors (THRs)-mediated genomic- and integrin-receptor-associated nongenomic mechanisms, depending on TH ligand status and DNA level, as well as transcription and extra-nuclear signaling transduction activities. These processes involve the binding of THs to intracellular THRs and steroid hormone receptors or membrane receptors and the recruitment of hormone-response elements. In addition, THs and other reproductive hormones can activate common signaling pathways due to their structural similarity and shared DNA consensus sequences among thyroid, peptide, and protein hormones and their receptors, thus constituting a complex and reciprocal interaction network. Moreover, THs not only indirectly affect the synthesis, secretion, and action of reproductive hormones, but are also regulated by these hormones at the same time. This crosstalk may be one of the pivotal factors regulating female reproductive behavior and hormone-related diseases, including tumors. Elucidating the interaction mechanism among the aforementioned hormones will contribute to apprehending the etiology of female reproductive diseases, shedding new light on the treatment of gynecological disorders.

Delayed luteolysis after intra-uterine infusions of nordihydroguaiaretic acid in the ewe

Intrauterine administration of nordihydroguaiaretic acid (5 mg, bid. NDGA), an inhibitor of the enzyme 5-lipoxygenase, on days 10-14 of the oestrous cycle, maintained luteal function and delayed oestrus in the ewe. The duration (mean +/- SD) of the oestrous cycle in the treatment group (n = 4) was 24 +/- 1 days, which was significantly (P < 0.001) longer than that of 16 +/- 1 days in vehicle-treated controls (n = 4); plasma progesterone concentrations were also significantly (P < 0.01) higher in the treatment group. On days 13 and 14 of the cycle (oestrus = Day 0) in the control group large pulses of 13,14-dihydro-15-keto prostaglandin F2alpha (PGFM) were evident, with mean (+/- SD) maximum concentrations of 232.5 +/- 66 and 415 +/- 309 pg ml(-1), respectively. In the treatment group, however, concentrations of PGFM were below detection level (< 50 pg ml(-1)). Similarly, in the control group, oxytocin release was highly pulsatile, with mean (+/- SD) peak concentrations of 21.8 +/- 5 and 18.5 +/- 6 pg ml(-1) on days 13 and 14, respectively; these were significantly (P < 0.01) higher than values of 7.6 +/- 3 and 6.1 +/- 3 pg ml(-1) in NDGA-treated ewes, where pulses were of relatively low amplitude. These results suggest that 5-lipoxygenase products of arachidonic acid metabolism may be involved in the positive feedback mechanism between luteal oxytocin and uterine PGF2alpha during luteolysis in the ewe.

Suppression of ovarian oxytocin secretion after intra-luteal administration of the arachidonate 5-lipoxygenase inhibitor BWA4C in the ewe

The effect of an intra-luteal injection of the 5-lipoxygenase (5-LO) inhibitor BWA4C (2 mg in 50 microl DMSO) on the secretion of oxytocin (OT) from the corpus luteum in response to a close-arterial infusion of prostaglandin (PG)-F2alpha (5 ng min(-1)) was examined in anaesthetised sheep. Within 30 minutes of administration both basal (pre-infusion) and PGF2alpha-stimulated OT release into the posterior vena cava were significantly (P<0.01) reduced. These results are consistent with the proposition that 5-LO products of arachidonic acid may modulate OT secretion from the ovine corpus luteum.

Prostaglandin F2 alpha-induced release of oxytocin from ovine corpora lutea in vitro

Prostaglandin (PG)F2 alpha, in the dose range 1-10(4) nM, failed to elicit oxytocin secretion in vitro from ovine luteal tissue on day 12 of the estrous cycle (estrus = Day 0), during a 60 minute period. Preincubation of luteal slices for 6 hours prior to treatment suggested that tissue desensitization due to the release of endogenous prostaglandins by tissue preparation is not responsible for this lack of response. However, in luteal tissue collected on day 6 of the ovine estrous cycle, PGF2 alpha stimulated oxytocin release in a dose-dependent manner. This apparent change in sensitivity of the ovine corpus luteum in vitro may be due to a combination of a reduction in the luteal oxytocin available for release and saturation of PGF2 alpha receptors in the more mature tissue.

Oxytocin synthesis and secretion from bovine corpora lutea exposed in vitro to cycloheximide and colchicine

Two experiments were conducted to study the effects of cycloheximide and colchicine on prostaglandin F2 alpha (PGF2 alpha)-induced secretion and synthesis of oxytocin in bovine luteal tissue in vitro. Corpora lutea were collected from beef heifers on Day 8 of the estrous cycle. In Experiment 1, incorporation of [14C]-leucine into oxytocin synthesized and secreted by luteal slices after exposure to PGF2 alpha, cycloheximide and cycloheximide plus PGF2 alpha was examined. In Experiment 2, synthesis and secretion of oxytocin were evaluated in luteal slices incubated with colchicine and PGF2 alpha alone and in combination. Cycloheximide inhibited incorporation of labeled leucine into luteal proteins by more than 90% and no labeled oxytocin was detected in the media or tissue. Prostaglandin F2 alpha induced significant secretion of oxytocin that was not inhibited by cycloheximide. Tissue levels of oxytocin after incubation with cycloheximide and/or PGF2 alpha did not differ and were similar to those of the incubated control. Colchicine alone did not suppress oxytocin secretion and did not alter the ability of PGF2 alpha to induce significant secretion of this nonapeptide. Tissue concentrations of oxytocin after incubation with colchicine and/or PGF2 alpha did not differ. These studies indicate that secretion and replenishment of luteal oxytocin in vitro is not contingent upon de novo protein synthesis. Inability of colchicine to suppress oxytocin secretion and synthesis may have been due to the short duration of exposure of luteal tissue to the drug.

Pulsatile release of oxytocin after suppression of prostaglandin synthesis in hysterectomised ewes

On days 13 and 14 after oestrus (day 0) oxytocin and 13,14-dihydro-15-keto-prostaglandin (PG)F2 alpha (PGFM) concentrations were measured in jugular plasma of hysterectomised sheep with or without systemic treatment with the PG cyclo-oxygenase inhibitor indomethacin (4 mg kg-1 three times a day on days 12, 13 and 14 subcutaneously. Pulsatile increases of oxytocin were observed in both untreated and treated sheep with mean (+/- SD) peak heights of 18.4 +/- 9.6 pg ml-1 (n = 11) and 23.5 +/- 9.4 pg ml-1 (n = 8), respectively; these means were not significantly different. Plasma concentrations of PGFM remained consistently low in both groups (under 100 pg ml-1) with no significant peaks observed. The data suggest that PGF2 alpha may not be the only stimulus for the release of luteal oxytocin, or that there may be a contribution by the posterior pituitary to oxytocin secretion during the luteal phase of the ovine oestrous cycle.

Arachidonic acid and cell signalling in the ovary and placenta

Arachidonic acid (AA) and its metabolites make up a diverse group of signalling molecules important to mediation of metabolic and endocrine function of ovarian and placental cell membranes. This paper reviews recent literature examining AA and eicosanoid involvement in the functional dynamics of follicular development, ovulation and corpus luteum function. The putative roles of AA metabolites in establishment and maintenance of pregnancy are reviewed with reference to decidualization, trophoblast invasion and implantation, maintenance of perfusion of the feto-placental unit and lipid transfer. Finally, recent evidence implicating AA metabolism in mediation of enzyme activity following hormone-receptor coupling within various cells types comprising the placental membranes is reviewed.

Unique metabolites of eicosapentaenoic acid interfere with corpus luteum function in the ewe

Experiments were conducted to determine the in vivo and in vitro effects of metabolites of eicosapentaenoic acid on ovine luteal function. Injection of 750 micrograms methyl eicosapentaenoic acid (EPA) or methyl 12(R),13(S)-dihydroxyeicosapentaenoic acid (12,13-diHEPE) into the ovarian artery of ewes on day 10 of the estrous cycle caused a reduction in serum concentrations of progesterone by 48 h posttreatment compared with levels of this steroid in arachidic acid-treated controls (p < 0.005). Although mean serum concentrations of progesterone in methyl EPA-treated ewes during the remainder of the cycle did not differ from those in control ewes, levels in methyl 12,13-diHEPE-treated ewes remained significantly suppressed. Duration of the estrous cycle did not differ among treatment groups (p > 0.05), but more of the methyl 12,13-diHEPE-treated animals (3/5) had exhibited estrus within 3 days after injection than methyl EPA-treated (1/5) or control ewes (0/5). Slices of corpus luteum removed from ewes on day 10 of the estrous cycle were incubated with arachidic acid (controls), 12,13-diHEPE or docosatetraenoic acid (DTA). Regardless of fatty acid treatment, all tissues retained the ability to produce basal levels of progesterone during subsequent incubation. Luteal slices previously exposed to arachidic acid or DTA exhibited an increase in progesterone production in response to subsequent treatment with LH (p < 0.05). In contrast, luteal slices incubated with 12,13-diHEPE did not respond to LH with a significant increase in production of this steroid above that observed in controls. All tissues displayed a marked increase in progesterone synthesis upon treatment with 8-Br-cAMP relative to incubation of tissue alone (p < 0.001). Subcellular distribution of [14C]-12,13-diHEPE in luteal cells after incubation revealed that the majority of the fatty acid was associated with the plasma membrane. These data suggest that metabolites of eicosapentaenoic acid with hydroxyl groups on adjacent carbon atoms interfere with luteal function in the ewe, perhaps in part by altering luteal response to LH.

In vivo desensitization of a high affinity PGF2 alpha receptor in the ovine corpus luteum

The corpus luteum (CL) of the sheep exhibits a differential sensitivity to PGF2 alpha in vivo in terms of an increase in oxytocin (OT) secretion and a decrease in progesterone secretion, pointing to the presence in vivo of both high and low affinity receptors for PGF2 alpha. The presence of the high affinity PGF2 alpha receptor was assessed by monitoring the secretion rate of OT from the ovine CL in response to subluteolytic infusions of PGF2 alpha. Rapid desensitization to PGF2 alpha occurred after only one hour of infusion, while a minimum rest period of six hours was required to restore sensitivity. The possibility that these findings could be explained by the depletion and resynthesis of OT was excluded by demonstrating an increase in OT secretion rate with supra-physiological levels of PGF2 alpha two hours after desensitization. Collectively, these results indicate the presence of a high affinity receptor for PGF2 alpha in the ovine CL which exhibits desensitization and recovery in vivo. The temporal nature of the desensitization and recovery of the high affinity PGF2 alpha receptor controlling luteal OT secretion may contribute to the pulsatile nature of PGF2 alpha release from the ovine uterus.

Crotoxin, a phospholipase A2 neurotoxin from snake venom, interacts with epithelial mammary cells, is internalized and induces secretion

Prolactin (PRL) induces liberation of arachidonic acid (AA) from phospholipids of lactating mammary epithelial cells and stimulates casein secretion. In order to investigate the possible involvement of phospholipase A2 (PLA2) activity in the hormonal control of casein secretion by PRL, we examined the effects of crotoxin, a PLA2 neurotoxin from snake venom, on mammary epithelial cells. Crotoxin is made of two subunits: a basic PLA2 with low toxicity (component B, CB) and an acidic, non-toxic and enzymatically inactive component A (CA) which enhances the pharmacological action of CB. While CA is inactive, the PLA2 subunit (CB) induces an accumulation of secretory products in the lumen of mammary acini, an extensive development of the Golgi apparatus. The secretion of newly synthesized casein is increased in the presence of CB and this effect is inhibited by nordihydroguaiaretic acid (NDGA) and caffeic acid, two inhibitors of the lipoxygenase pathway which also prevent stimulation of secretion by PRL. Further, CB transiently induces the release of radiolabelled AA from mammary tissues previously labelled with [14C]AA, the highest release being observed between 15 s and 5 min of contact with CB and CA. Immunofluorescence labelling by anti-CB antibodies of epithelial mammary tissues previously incubated with CA, CB or a combination of CA and CB indicates that CB binds to epithelial cells and is internalized, at least in part, and that CA enhances both CB binding and its internalization. These observations emphasize the involvement of PLA2 in the control of casein secretion and suggest that PLA2 acts intracellularly.

Arachidonic acid and its metabolites increase cytosolic free calcium in bovine luteal cells

We studied the effects of arachidonic acid and its metabolites on intracellular free calcium concentrations ([Ca2+]i) in highly purified bovine luteal cell preparations. Corpora lutea were collected from Holstein heifers between days 10 and 12 of the estrous cycle. The cells were dispersed and small and large cells were separated by unit gravity sedimentation and flow cytometry. The [Ca2+]i was determined by spectrofluorometry in luteal cells loaded with the fluorescent Ca2+ probe, Fura-2. Arachidonic acid elicited a dose-dependent increase in [Ca2+]i in both small and large luteal cells, having an effect at concentrations as low as 5 microM; and was maximally effective at 50 microM. Several other fatty acids failed to exert a similar response. Addition of nordihydroguaiaretic acid (NDGA) or indomethacin failed to suppress the effects of arachidonic acid. In fact, the presence of both inhibitors resulted in increases of [Ca2+]i, with NDGA exerting a greater stimulation of [Ca2+]i than indomethacin. Prostaglandin F2 alpha (PGF2 alpha) as well as prostaglandin E2 (PGE2) increased [Ca2+]i in the small luteal cells. These results support the idea that arachidonic acid exerts a direct action in mobilizing [Ca2+]i, in the luteal cells. Furthermore, they demonstrate that the cyclooxygenase (PGF2 alpha and PGE2) and lipoxygenase products of arachidonic acid metabolism also play a role in increasing [Ca2+]i in bovine luteal cells. Since the bovine corpus luteum contains large quantities of arachidonic acid, these findings suggest that this compound may regulate calcium-dependent functions of the corpus luteum, including steroid and peptide hormone production and secretion.

Prostaglandin F2 alpha, oxytocin and progesterone secretion by bovine luteal cells at several stages of luteal development: effects of oxytocin, luteinizing hormone, prostaglandin F2 alpha and estradiol-17 beta

Bovine luteal cells from Days 4, 8, 14 and 18 of the estrous cycle were incubated for 2 h (1 x 10(5) cells/ml) in serum-free media with one or a combination of treatments [control (no hormone), prostaglandin F2 alpha (PGF), oxytocin (OT), estradiol-17 beta (E) or luteinizing hormone (LH)]. Luteal cell conditioned media were then assayed by RIA for progesterone (P), PGF, and OT. Basal secretion of PGF on Days 4, 8, 14 and 18 was 173.8 +/- 66.2, 111.1 +/- 37.8, 57.7 +/- 15.4 and 124.3 +/- 29.9 pg/ml, respectively. Basal release of OT and P was greater on Day 4 (P less than 0.01) than on Day 8, 14 and 18 (OT: 17.5 +/- 2.6 versus 5.6 +/- 0.7, 6.0 +/- 1.4 and 3.1 +/- 0.4 pg/ml; P: 138.9 +/- 19.5 versus 23.2 +/- 7.5, 35.4 +/- 6.5 and 43.6 +/- 8.1 ng/ml, respectively). Oxytocin increased (P less than 0.01) PGF release by luteal cells compared with control cultures irrespective of day of estrous cycle. Estradiol-17 beta stimulated (P less than 0.05) PGF secretion on Days 8, 14 and 18, and LH increased (P less than 0.01) PGF production only on Day 14. Prostaglandin F2 alpha, E and LH had no effect on OT release by luteal cells from any day. Luteinizing hormone alone or in combination with PGF, OT or E increased (P less than 0.01) P secretion by cells from Days 8, 14 and 18. However on Day 8, a combination of PGF + OT and PGF + E decreased (P less than 0.05) LH-stimulated P secretion. These data demonstrate that OT stimulates PGF secretion by bovine luteal cells in vitro. In addition, LH and E also stimulate PGF release but effects may vary with stage of estrous cycle.