Real-time relationships in intraluteal release among Prostaglandin F2alpha, endothelin-1, and angiotensin II during spontaneous luteolysis in the cow

It is well known that prostaglandin F(2alpha) (PGF(2alpha)) is a physiological luteolysine, and that its pulsatile release from the endometrium is a luteolytic signal in many species. There is now clear evidence that the vasoactive peptides endothelin-1 (ET-1) and angiotensin II (Ang II) interact with PGF(2alpha) in the luteolytic cascade during PGF(2alpha)-induced luteolysis in the cow. Thus, we investigated the local secretion of PGF(2alpha), ET-1, and Ang II in the corpus luteum (CL) and their real-time relationships during spontaneous luteolysis in the cow. For this purpose, an in vivo microdialysis system (MDS) implanted in the CL was utilized to observe local secretion changes within the CL microenvironment. Each CL of cyclic Holstein cows (n = 6) was surgically implanted with MDS capillary membranes (18 lines/6 cows) on Day 15 (estrus = Day 0) of the estrous cycle. The concentrations of PGF(2alpha), ET-1, Ang II, and progesterone (P) in the MDS samples were determined by enzyme immunoassays. The intraluteal PGF(2alpha) secretion slightly increased from 12 h after the onset of luteolysis (0 h) and drastically increased (by about 300%) from 24 h. Intraluteal ET-1 secretion increased from 12 h. Intraluteal Ang II secretion was elevated from 0 h and was maintained at high levels (about 180%) toward estrus. In each MDS lines (in the same microenvironment) within the regressing CL, the local releasing profiles of PGF(2alpha), ET-1, and Ang II CL positively correlated with each other (P < 0.05) at high proportions in 18 MDS lines (PGF(2alpha) vs. ET-1, 44.4%; PGF(2alpha) vs. Ang II, 55.6%; ET-1 vs. Ang II, 38.9%). In contrast, there was no clear relationship among these substances released into different MDS lines implanted in the same CL (with different microenvironments). In conclusion, we propose that the increase of PGF(2alpha), ET-1, and Ang II within the CL during luteolysis is a common phenomenon for both PGF(2alpha)-induced and spontaneous luteolysis. Moreover, this study illustrated the in vivo relationships in intraluteal release among PGF(2alpha), ET-1, and Ang II during spontaneous luteolysis in the cow. The data suggest that these vasoactive substances may interact with each other in a local positive feedback manner to activate their secretion in the regressing CL, thus accelerating and completing luteolysis.     

Administration of prostaglandin f(2 alpha) during the early bovine luteal phase does not alter the expression of ET-1 and of its type A receptor: a possible cause for corpus luteum refractoriness

Luteal regression is initiated by prostaglandin F(2 alpha) (PGF(2 alpha)). In domestic species and primates, demise of the corpus luteum (CL) enables development of a new preovulatory follicle. However, during early stages of the cycle, which are characterized by massive neovascularization, the CL is refractory to PGF(2 alpha). Our previous studies showed that endothelin-1 (ET-1), which is produced by the endothelial cells lining these blood vessels, plays a crucial role during PGF(2 alpha)-induced luteolysis. Therefore, in this study, we compared the effects of PGF(2 alpha) administered at the early and mid luteal phases on ET-1 and its type A receptors (ETA-R) along with plasma ET-1 and progesterone concentrations, and the mRNA levels of PGF(2 alpha) receptors (PGF(2 alpha)-R) and steroidogenic genes. As expected, ET-1 and ETA-R mRNA levels were markedly induced in midcycle CL exposed to luteolytic dose of PGF(2 alpha) analogue (Cloprostenol). In contrast, neither ET-1 mRNA nor its receptors were elevated when the same dose of PGF(2 alpha) analogue was administered on Day 4 of the cycle. In accordance with ET-1 expression within the CL, plasma ET-1 concentrations were significantly elevated 24 h after PGF(2 alpha) injection only on Day 10 of the cycle. The steroidogenic capacity of the CL (plasma progesterone as well as the mRNA levels of steroidogenic acute regulatory protein and cytochrome P450(scc)) was only affected when PGF(2 alpha) was administered during midcycle. Nevertheless, PGF(2 alpha) elicited certain responses in the early CL: progesterone and oxytocin secretion were elevated, and PGF(2 alpha)-R was transiently affected. Such effects probably result from PGF(2 alpha) acting on luteal steroidogenic cells. These findings may suggest, however, that the cell type mediating the luteolytic actions of PGF(2 alpha), possibly the endothelium, could yet be nonresponsive during the early luteal phase.     

Changes in steady-state concentrations of messenger ribonucleic acids in luteal tissue during prostaglandin F2alpha induced luteolysis in mares

Transvaginal ultrasound-guided luteal biopsy was used to evaluate the effects of prostaglandin (PG)F2alpha on steady-state concentrations of mRNA for specific genes that may be involved in regression of the corpus luteum (CL). Eight days after ovulation (Hour 0), mares (n=8/group) were randomized into three groups: control (no treatment or biopsy), saline+biopsy (saline treatment at Hour 0 and luteal biopsy at Hour 12), or PGF2alpha+biopsy (5mg PGF2alpha at Hour 0 and luteal biopsy at Hour 12). The effects of biopsy on CL were compared between the controls (no biopsy) and saline+biopsy group. At Hour 24 (12h after biopsy) there was a decrease in circulating progesterone in saline group to 56% of pre-biopsy values, indicating an effect of biopsy on luteal function. Mean plasma progesterone concentrations were lower (P<0.001) at Hour 12 in the PG group compared to the other two groups. The relative concentrations of mRNA for different genes in luteal tissue at Hour 12 was quantified by real time PCR. Compared to saline-treated mares, treatment with PGF2alpha increased mRNA for cyclooxygenase-2 (Cox-2, 310%, P<0.006), but decreased mRNA for LH receptor to 44% (P<0.05), steroidogenic acute regulatory protein to 22% (P<0.001), and aromatase to 43% (P<0.1) of controls. There was no difference in mRNA levels for PGF2alpha receptor between PG and saline-treated groups. Results indicated that luteal biopsy alters subsequent luteal function. However, the biopsy approach was effective for collecting CL tissue for demonstrating dynamic changes in steady-state levels of mRNAs during PGF2alpha-induced luteolysis. Increased Cox-2 mRNA concentrations suggested that exogenous PGF2alpha induced the synthesis of intraluteal PGF2alpha. Thus, the findings are consistent with the concept that an intraluteal autocrine loop augments the luteolytic effect of uterine PGF2alpha in mares.     

Prostaglandin f(2alpha) induces distinct physiological responses in porcine corpora lutea after acquisition of luteolytic capacity

This study examines differences in intracellular responses to cloprostenol, a prostaglandin (PG)F(2alpha) analog, in porcine corpora lutea (CL) before (Day 9 of estrous cycle) and after (Day 17 of pseudopregnancy) acquisition of luteolytic capacity. Pigs on Day 9 or Day 17 were treated with saline or 500 microgram cloprostenol, and CL were collected 10 h (experiment I) or 0.5 h (experiment III) after treatment. Some CL were cut into small pieces and cultured to measure progesterone and PGF(2alpha) secretion. In experiment I, progesterone remained high and PGF(2alpha) low in luteal incubations from either Day 9 or Day 17 saline-treated pigs. Cloprostenol increased PGF(2alpha) production 465% and decreased progesterone production 87% only from Day 17 luteal tissue. Cloprostenol induced prostaglandin G/H synthase (PGHS)-2 mRNA (0.5 h) and protein (10 h) in both groups. In cell culture, cloprostenol or phorbol 12, 13-didecanoate (PDD) (protein kinase C activator), induced PGHS-2 mRNA in luteal cells from both groups. However, acute cloprostenol treatment (10 min) decreased progesterone production and increased PGF(2alpha) production only from Day 17 luteal cells. Thus, PGF(2alpha) production is induced by cloprostenol in porcine CL with luteolytic capacity (Day 17) but not in CL without luteolytic capacity (Day 9). However, this change in PGF(2alpha) production is not explained by a difference in induction of PGHS-2 mRNA or protein.     

Macrophage migration inhibitory factor in the bovine corpus luteum: characterization of steady-state messenger ribonucleic acid and immunohistochemical localization

Macrophage migration inhibitory factor (MIF) is a pro-inflammatory cytokine produced by T cells and macrophages. A number of tissues also produce MIF during states of active differentiation and/or proliferation. The purpose of this study was to determine whether MIF is present in the corpus luteum (CL). The steady-state mRNA for MIF was examined in CL by Northern analysis on Day 5, Days 9-12, and Day 18 of the estrous cycle and at 0.5, 1, 4, 12, 24, and 36 h after a luteolytic injection of prostaglandin F(2alpha) (PGF(2alpha)) (n = 4 CL per time point). The greatest amount of MIF mRNA was observed in Day 5 CL compared with midcycle and Day 18 CL. Messenger RNA for MIF in CL collected 0.5 h post-PGF(2alpha) was greater than in midcycle and all other regressing CL. Immunohistochemical analysis (n = 4) revealed that MIF was present in the bovine CL throughout the estrous cycle and appeared to be localized to large luteal cells. It was concluded that MIF is produced within the bovine CL, mRNA expression is maximal in the early CL, and the protein is primarily localized to large luteal cells. The functional significance of MIF remains to be determined.     

Positive association, in local release, of luteal oxytocin with endothelin 1 and prostaglandin F2alpha during spontaneous luteolysis in the cow: a possible intermediatory role for luteolytic cascade within the corpus luteum

Luteolysis is caused by a pulsatile release of prostaglandin F(2alpha) (PGF(2alpha)) from the uterus in ruminants, and a positive feedback between endometrial PGF(2alpha) and luteal oxytocin (OXT) has a physiologic role in the promotion of luteolysis. The bovine corpus luteum (CL) produces vasoactive substances, such as endothelin 1 (EDN1) and angiotensin II (Ang II), that mediate and progress luteolysis. We hypothesized that luteal OXT has an additive function to ensure the CL regression with EDN1 and Ang II, and that it has an active role in the luteolytic cascade in the cow. Thus, the aim of the present study was to observe real-time changes in the local secretion of luteal OXT and to determine its relationship with other local mediators of luteolysis. Microdialysis system (MDS) capillary membranes were implanted surgically into each CL of six cyclic Holstein cows (18 lines total among the six cows) on Day 15 (estrus == Day 0) of the estrous cycle. Simultaneously, catheters were implanted to collect ovarian venous plasma ipsilateral to the CL. Although the basal secretion of OXT by luteal tissue was maintained during the experimental period, the intraluteal PGF(2alpha) secretion gradually increased up to 300% from 24 h after the onset of luteolysis (0 h; time in which progesterone started to decrease). In each MDS line (microenvironment) within the CL, the local releasing profiles of OXT were positively associated with PGF(2alpha) and EDN1 within the CL in all 18 MDS lines implanted in the six CLs (OXT vs. PGF(2alpha), 50.0%; OXT vs. EDN1, 72.2%; P < 0.05). On the other hand, the intraluteal OXT was weakly related to Ang II (OXT vs. Ang II, 27.7%). In the ovarian vein, the peak concentration of PGF(2alpha) increased significantly when the peak of PGF(2alpha) coincided with the peak of OXT after the onset of spontaneous luteolysis (P < 0.05). In conclusion, intraluteal OXT may locally modulate secretion of vasoactive substances, particularly EDN1 and PGF(2alpha) within the CL, and thus might be one of the luteal mediators of spontaneous luteolysis in the cow.     

Analysis of luteal tissue inhibitor of metalloproteinase-1, -2, and -3 during prostaglandin F(2alpha)-induced luteolysis

Increased matrix metalloproteinase (MMP) expression and activities help to mediate tissue involution through increasing extracellular matrix remodeling and promoting dedifferentiation and, ultimately, apoptosis. Therefore, we hypothesized that prostaglandin (PG) F(2alpha) administration would decrease expression of the tissue inhibitor of metalloproteinase (TIMP)-1, -2, and -3 and effectively increase the MMP:TIMP ratio, leading to glandular involution. In experiment 1, we tested the effects of PGF(2alpha) administration (Day 10 postestrus; Day 0 = estrus) on luteal TIMP-1, -2, and -3 mRNA and protein expression. Corpora lutea were collected at 0, 15, or 30 min or at 1, 2, 4, 6, 12, 24, and 48 h following PGF(2alpha) administration (n = 3-9 animals/time point). Following PGF(2alpha) administration, TIMP-1 mRNA levels decreased (P < 0.05) at 1 and 2 h relative to 0 h (controls), then increased to levels greater than controls at 4 and 6 h. In contrast, TIMP-2 and -3 mRNA levels did not decrease following PGF(2alpha) administration. The TIMP-1, -2, and -3 proteins were localized to large luteal cells (LLCs) within control (untreated) tissues. However, histodepletion of TIMP-1 within LLCs was evident within 30 min (earliest time point collected) following PGF(2alpha) injection and continued through 48 h. Luteal concentration of TIMP-1, as determined by RIA, was decreased (P < 0.05) by 15 min (earliest time point collected) following PGF(2alpha) administration and remained low through 48 h. In contrast, TIMP-2 and -3 immunolocalization was not altered by PGF(2alpha) administration. Experiment 2 was conducted to determine if PGF(2alpha) could initiate the preceding changes in TIMP-1 in early (Day 3) corpora lutea that can bind PGF(2alpha) but are refractory to its luteolytic effects. Serum concentrations of progesterone and luteal concentrations of TIMP-1 mRNA and protein were similar at 0 and 6 h after PGF(2alpha) injection on Day 3 postestrus. These data suggest that an early and sustained effect of PGF(2alpha) is the specific depletion of TIMP-1 within LLCs that are capable of responding to the luteolytic action of PGF(2alpha). This action may increase the MMP:TIMP-1 ratio, creating an environment that favors extracellular matrix degradation and, thereby, facilitates both functional and structural regression.     

Changes in prostaglandin secretion by the regressing bovine corpus luteum

Secretion of prostaglandins (PGs) by the regressing corpus luteum (CL) was investigated in the cow. Six cows were implanted with microcapillary dialysis membranes of a microdialysis system (MDS) into the CL during Days 8-9 (Day 0 = estrus), and a prostaglandin (PG) F2alpha analogue (Estrumate) was injected intramuscularly (i.m.) to induce luteolysis. Acute increases in intraluteal release of PGF2alpha and PGE2 were observed during the first 4 h, followed by decreases over the next 8 h. Intraluteal release of both PGs gradually increased again during the period 48-72 h. Concentrations of PGF2alpha in ovarian venous plasma (OVP) were 4-13 times higher than those of jugular venous plasma (JVP) (P < 0.001) during the period of the experiment, and increased from 24 h after treatment with Estrumate (P < 0.05). Cyclooxygenase (COX)-2 mRNA expression increased (P < 0.05) at 2 and 24 h after treatment with Estrumate. The results indicated that local release of PGF2alpha and PGE2, and COX-2 mRNA expression were increased by Estrumate in the regressing CL at the later stages of luteolysis. Thus, luteal secretion of PGs may be involved in the local mechanism for structural rather than functional luteolysis.     

Effect of local interaction of reactive oxygen species with prostaglandin F(2alpha) on the release of progesterone in ovine corpora lutea in vivo

Prostaglandin (PG) F(2alpha) is implicated in the process of luteal regression in many species, and has been shown to increase the generation of reactive oxygen species. In this study, the role of reactive oxygen species in the local regulatory mechanisms of functional luteolysis in the ewe was examined. In Experiment 1, we studied local effects of hydrogen peroxide (H(2)O(2)) and its interaction with PGF(2alpha) on P secretion in ovine corpus luteum (CL) in vivo. For this purpose, a microdialysis system (MDS) was used, where only the cells surrounding the capillary membrane in the microenvironment of the CL are exposed to these factors, and the P secretory ability of the CL is maintained as if intact. The study used a multiple CL model to implant the MDS, enabling us to examine in parallel several experimental infusions into the MDS implanted in different CLs (one MDS line per CL) developed after superovulation in one ewe. On Day 8 after GnRH treatment, the MDS were implanted into multiple CL in both ovaries of six ewes. A 4-h infusion with PGF(2alpha) (10(-6)M) at 8-12 h slightly increased P release during infusion, while a 4-h infusion with H(2)O(2) (10(-3)M) at 20-24 h decreased P release at 27-38 h. A pre-infusion with PGF(2alpha) for 4h at 8-12h, followed by infusion of H(2)O(2) at 20-24 h rapidly decreased the P release at 20-40 h (P<0.05); this decrease occurred 7h earlier than in the CL treated with H(2)O(2) alone. In Experiment 2, by utilizing the MDS we also applied free radical scavengers to examine their possible weakening effect on the inhibition of P secretion in the microenvironment within the regressing CL induced by PGF(2alpha) treatment. On Day 8 after estrus, the MDS were implanted into the CL (single CL model, two MDS lines per CL). Infusion of free radical scavengers, superoxide dismutase (SOD;50mg/ml)+catalase (CAT; 10mg/ml), at 0-28 h first increased P release until 12 h (P<0.05), and consequently delayed the decrease in P release until 30 h after administration of PGF(2alpha) i.m. (P<0.05). The present results support the concept that the leading pathway from PGF(2alpha) induces an increase of reactive oxygen species in luteolysis in the ewe.     

Apoptosis during spontaneous and prostaglandin F(2alpha)-induced luteal regression in the buffalo cow (Bubalus bubalis): involvement of mitogen-activated protein kinases

The present study was conducted to evaluate whether the corpus luteum (CL) of the water buffalo (Bubalus bubalis) cow undergoes luteal regression by the process of apoptosis and to examine the involvement of mitogen-activated protein (MAP) kinases during prostaglandin (PG) F(2alpha)-induced luteolysis. Sections of CL from late in the estrous cycle, i.e., during spontaneous luteolysis, stained for 4',6'-diamidino-2-phenylindole revealed increased numbers of condensed nuclei, indicating cell death by apoptosis, which was confirmed further by the occurrence of pronounced oligonucleosome formation. For morphological and biochemical characterization during PGF(2alpha)-induced apoptosis, CL were collected at 0, 4, 12, and 18 h after injection of 750 micro g of Tiaprost, a synthetic analogue of PGF(2alpha), to midestrous buffalo cows. Serum progesterone concentrations fell within 4 h and decreased (P < 0.05) maximally by 18 h. Concomitant decreases (P < 0.05) in the levels of steroidogenic acute regulatory mRNA and protein were observed in CL during 12-18 h, with the more profound effect on mRNA levels. Quantitative analysis of the genomic DNA showed a >5-fold increase (P < 0.05) in the low molecular weight DNA fragments by 18 h postinjection. Immunoblot analysis of CL tissue lysates showed increased (P < 0.05) levels of phospho-Jun N-terminal kinase (JNK) 1 (4- to 14-fold during 4-18 h) and phospho-p38 (2- to 4-fold at 18 h). Immunohistochemical evaluation of CL sections revealed an increased nuclear localization of phospho-JNK after treatment. These findings demonstrate that the CL of the buffalo cow undergoes cell death by the process of apoptosis both during spontaneous and PGF(2alpha)-induced luteolysis and that MAP kinases are involved during PGF(2alpha)-mediated apoptosis in the CL.     

Endothelin-1 mediates prostaglandin F(2alpha)-induced luteal regression in the ewe

A diversified series of experiments was conducted to determine the potential role of endothelin-1 (ET-1) in ovine luteal function. Endothelin-1 inhibited basal and LH-stimulated progesterone production by dispersed ovine luteal cells during a 2-h incubation. This inhibition was removed when cells were preincubated with cyclo-D-Asp-Pro-D-Val-Leu-D-Trp (BQ123), a highly specific endothelin ET(A) receptor antagonist. Administration of a luteolytic dose of prostaglandin F(2alpha) (PGF(2alpha)) rapidly stimulated gene expression for ET-1 in ovine corpora lutea (CL) collected at midcycle. Intraluteal administration of a single dose of BQ123 to ewes on Day 8 or 9 of the estrous cycle mitigated the luteolytic effect of PGF(2alpha). Intramuscular administration of 100 microg ET-1 to ewes at midcycle reduced plasma progesterone concentrations for the remainder of the estrous cycle. Following pretreatment with a subluteolytic dose of PGF(2alpha), i.m. administration of 100 microg ET-1 caused a rapid decline in plasma progesterone and shortened the length of the estrous cycle. These data complement and extend previously published reports in the bovine CL and are the strongest evidence presented to date in support of a role for ET-1 in PGF(2alpha)-mediated luteal function in domestic ruminants.     

Role of the DLL4-NOTCH system in PGF2alpha-induced luteolysis in the pregnant rat

We investigated the expression and cell localization of NOTCH1, NOTCH4, and the delta-like ligand DLL4 in corpus luteum (CL) from pregnant rats during prostaglandin F2alpha (PGF2alpha)-induced luteolysis. We also examined serum progesterone (P(4)) and CL proteins related to apoptosis after local administration of the notch inhibitor N-[N-(3,5-difluorophenacetyl-l-alanyl)]-S-phenylglycine t-butyl ester (DAPT). Specific staining for NOTCH1 and NOTCH4 receptors was detected predominantly in large and small luteal cells. Furthermore, in line with the fact that the notch intracellular domain is translocated to the nucleus, where it regulates gene expression, staining was evident in the nuclei of luteal cells. In addition, we detected diffuse cytoplasmic immunostaining for DLL4 in small and large luteal cells, in accordance with the fact that DLL4 undergoes proteolytic degradation after receptor binding. The mRNA expression of Notch1, Notch4, and Dll4 in CL isolated on Day 19 of pregnancy decreased significantly after administration of PGF2alpha. Consistent with the mRNA results, administration of PGF2alpha to pregnant rats on Day 19 of pregnancy decreased the protein fragment corresponding to the cleaved forms of NOTCH1/4 CL receptors. In contrast, no significant changes were detected in protein levels for the ligand DLL4. The local intrabursal administration of DAPT decreased serum P(4) levels and increased luteal levels of active caspase 3 and the BAX:BCL2 ratio 24 h after the treatment. These results support a luteotropic role for notch signaling to promote luteal cell viability and steroidogenesis, and they suggest that the luteolytic hormone PGF2alpha may act in part by reducing the expression of some notch system members.     

Insulin-like growth factor (IGF)-I, IGF-I receptor, and IGF binding protein-3 messenger ribonucleic acids and protein in corpora lutea from prostaglandin F(2alpha)-treated gilts

Insulin-like growth factor-I (IGF-I) is produced within the porcine corpus luteum (CL) and is thought to play an autocrine/paracrine role in CL development/function during the early luteal phase. This study examines the hypotheses that the luteolytic actions of prostaglandin F(2alpha) (PGF(2alpha)) during the early luteal phase may involve either a decrease in IGF-I or IGF receptor (IGF-IR), or an increase in IGF binding protein (IGFBP)-3, expression, any of which could interfere with the luteotropic actions of IGF-I in this tissue. Cycling gilts were treated twice daily with PGF(2alpha) (or saline) on Days 5-9 of the cycle to induce premature luteolysis. CL were collected on Days 6-9, and RNA, protein, or progesterone was extracted. By slot blot analysis, steady-state levels of IGF-I and IGFBP-3 mRNA were not different in PGF(2alpha)-treated vs. control animals; however, IGF-IR mRNA was increased in treated animals on Day 9. No changes in IGF-I content (ng/CL measured by RIA) were observed with respect to treatment. According to ligand blot analysis, the levels of IGFBP-3 increased on Day 6 and decreased on Days 8-9, while IGFBP-2 was higher on Days 6-7 and decreased on Day 9 in treated animals. IGF-IR levels, determined from Western blots, were higher on Day 7 (P < 0.05) and lower on Day 9 in PGF(2alpha)-treated animals vs. control animals (P < 0.05). In conclusion, PGF(2alpha)-induced premature luteolysis was associated with an increase in steady-state levels of IGF-IR mRNA, but it did not appear to be linked to changes in mRNA levels for IGF-I or IGFBP-3. However, since IGFBP-2 and -3 protein levels increased early in the treatment period (Days 6-7), it is possible that they may mediate the luteolytic actions of PGF(2alpha) by sequestering IGF-I and preventing its interaction with the IGF-IR.     

Intraluteal administration of a nitric oxide synthase blocker stimulates progesterone and oxytocin secretion and prolongs the life span of the bovine corpus luteum

To test the role of nitric oxide (NO) in secretory functions of bovine corpora lutea (CL), two groups of four Holstein heifers each were treated as follows: Group 1, Nomega-Nitro-L-Arginine Methyl Ester (L-NAME), an inhibitor of nitric oxide synthase (NOS), on Day 11 or 12 of the cycle and Group 2, L-NAME on Days 17 and 18 of the cycle. All treatments were administered by an intraluteal microdialysis system (MDS). Drugs were infused for 4-hr periods on the designated days, and the treatment periods were preceded and followed by 4-hr control periods. Perfusate and jugular blood samples were collected at half-hour intervals. Perfusate samples were analyzed for progesterone (P4), oxytocin (OT), prostaglandin F2alpha (PGF2alpha), and leukotriene C4 (LTC4); jugular plasma samples were analyzed for P4, OT, and LH. Perfusion of L-NAME on Day 11 or 12 consistently increased P4 concentration in the perfusate, but had no effect on the life span of the CL. Perfusion of L-NAME on Days 17-18 also elevated P4 levels in the perfusate, and in addition, maintained P4 levels in the plasma of three of the four treated animals through Day 25 of the cycle. L-NAME perfusion also increased OT release concomitant with P4 into the perfusate at both the mid- and late-luteal phase treatments. For the most part, concentrations of LH, OT, and P4 in the jugular plasma samples collected during the perfusions were unaffected by treatments. L-NAME perfusion caused small, but significant (P < 0.05) increases in perfusate PGF2alpha and LTC4 at Days 17 and 18 and in LTC4 on Day 11 or 12. These data indicate that NO plays a direct luteolytic role in regression of the bovine CL.     

An intra-corpus luteum site for the luteolytic action of prostaglandin F2 alpha in the rhesus monkey

It has not been possible to demonstrate prostaglandin F2 alpha (PGF2 alpha) participation in primate luteolysis under conditions of systemic administration or of acute intraluteal injection. These study designs were hampered by the short biological half-life in the first instance and brevity of administration in the latter. In this study, luteolysis has resulted from chronic, intraluteal delivery of PGF2 alpha. Using the Alzet osmotic pump-cannula system, normally cycling rhesus monkeys were continuously infused, until menses occurred, with PGF2 alpha (10 ng/1/hr) directly into the corpus luteum (CL, n = 6), into the stroma of the ovary not bearing the corpus luteum (NCL, n = 3), or subcutaneously (SC, n = 5). An additional 5 monkeys received vehicle (V) into the corpus luteum. All experiments commenced 5-7 days after the preovulatory estradiol surge. Luteal function was assessed by the daily measurements of plasma progesterone, estradiol, and LH. Intraluteal PGF2 alpha caused premature functional luteolysis in all monkeys, as reflected by a highly significant decline in circulating progesterone and estradiol and the early onset of menstruation, when compared to the other groups. V, NCL, and SC infusions had no effect on either circulating steroid levels or luteal phase lengths. None of the experimental groups showed any change in plasma LH concentrations. These are the first data to indicate that PGF2 alpha can induce functional luteolysis in the primate, and the site of action appears to be the corpus luteum.     

Regulation of intraluteal production of prostaglandins

There is clear evidence for intraluteal production of prostaglandins (PGs) in numerous species and under a variety of experimental conditions. In general, secretion of PGs appears to be elevated in the early corpus luteum (CL) and during the period of luteolysis. Regulation of intraluteal PG production is regulated by a variety of factors. An autoamplification pathway in which PGF-2alpha stimulates intraluteal production of PGF-2alpha has been identified in a number of species. The mechanisms underlying this autoamplification pathway appear to differ by species with expression of Cyclooxygenase-2 (Cox-2) and activity of phospholipase A2 acting as important physiological control points. In addition, a number of other responses that are induced by PGF-2alpha (decreased luteal progesterone, increased endothelin-1, increased cytokines) also have been found to increase intraluteal PGF-2alpha production. Thus, regulation of intraluteal PG production may serve to initiate or amplify physiological signals to the CL and may be important in specific aspects of luteal physiology particularly during luteal regression.