Nitric Oxide as a Local Mediator of Prostaglandin F2α-Induced Regression in Bovine Corpus Luteum: An In Vivo Study

Prostaglandins as local regulators of ovarian physiology in ruminants

Prostaglandins are synthesized from arachidonic acid through the catalytic activities of cyclooxygenase, while the production of different prostaglandin types, prostaglandin F2 alpha (PGF) and prostaglandin E2 (PGE), are regulated by specific prostaglandin synthases (PGFS and PGES). Prostaglandin ligands (PGF and PGE) bind to specific high-affinity receptors and initiate biologically distinct signalling pathways. In the ovaries, prostaglandins are known to be important endocrine regulators of female reproduction, in addition to maintaining local function through autocrine and/or paracrine effect. Many research groups in different animal species have already identified a variety of factors and molecular mechanisms that are responsible for the regulation of prostaglandin functions. In addition, prostaglandins stimulate their intrafollicular and intraluteal production via the pathway of prostaglandin self-regulation in the ovary. Therefore, the objective of the review article is to discuss recent findings about local regulation patterns of prostaglandin ligands PGF and PGE during different physiological stages of ovarian function in domestic ruminants, especially in bovine. In conclusion, the discussed local regulation mechanisms of prostaglandins in the ovary may stimulate further research activities in different methodological approaches, especially during final follicle maturation and ovulation, as well as corpus luteum formation and function.

Cloprostenol, a synthetic analog of prostaglandin F2α induces functional regression in cultured luteal cells of felids†

In the present study, we investigated the effect of the synthetic analog of prostaglandin F2α (PGF2α)-cloprostenol-on cultured steroidogenic luteal cells of selected felid species over a 2-day culture period. The changes induced by cloprostenol were measured based on progesterone concentration and mRNA expression analysis of selected genes. Cloprostenol significantly reduced concentration of progesterone in cell culture medium of small luteal cells isolated from domestic cat corpora lutea (CL) at the development/maintenance stage (P < 0.05), but did not influence progesterone production in cultured cells from the regression stage. A decrease or complete silencing of progesterone production was also measured in cultured luteal cells of African lion (formation stage) and Javan leopard (development/maintenance stage). Gene-expression analysis by real-time PCR revealed that treatment with cloprostenol did not have an influence on expression of selected genes coding for enzymes of steroidogenesis (StAR, HSD3B, CYP11A1) or prostaglandin synthesis (PTGS2, PGES), nor did it effect hormone receptors (AR, ESR1, PGR, PTGER2), an anti-oxidative enzyme (SOD1) or factors of cell apoptosis (FAS, CASP3, TNFRSF1B, BCL2) over the studied period. Significant changes were measured only for expressions of luteinizing hormone (P < 0.05), prolactin (P < 0.05) and PGF2α receptors (P < 0.005) (LHCGR, PRLR, and PTGFR). The obtained results confirm that PGF2α/cloprostenol is a luteolytic agent in CL of felids and its impact on progesterone production depends on the developmental stage of the CL. Cloprostenol short-term treatment on luteal cells was associated only with functional but not structural changes related to luteal regression.

Current Knowledge on the Multifactorial Regulation of Corpora Lutea Lifespan: The Rabbit Model

Simple Summary

Corpora lutea (CL) are temporary endocrine structures that secrete progesterone, which is essential for maintaining a healthy pregnancy. A variety of regulatory factors come into play in modulating the functional lifespan of CL, with luteotropic and luteolytic effects. Many aspects of luteal phase physiology have been clarified, yet many others have not yet been determined, including the molecular and/or cellular mechanisms that maintain the CL from the beginning of luteolysis during early CL development. This paper summarizes our current knowledge of the endocrine and cellular mechanisms involved in multifactorial CL lifespan regulation, using the pseudopregnant rabbit model.

Abstract

Our research group studied the biological regulatory mechanisms of the corpora lutea (CL), paying particular attention to the pseudopregnant rabbit model, which has the advantage that the relative luteal age following ovulation is induced by the gonadotrophin-releasing hormone (GnRH). CL are temporary endocrine structures that secrete progesterone, which is essential for maintaining a healthy pregnancy. It is now clear that, besides the classical regulatory mechanism exerted by prostaglandin E2 (luteotropic) and prostaglandin F2α (luteolytic), a considerable number of other effectors assist in the regulation of CL. The aim of this paper is to summarize our current knowledge of the multifactorial mechanisms regulating CL lifespan in rabbits. Given the essential role of CL in reproductive success, a deeper understanding of the regulatory mechanisms will provide us with valuable insights on various reproductive issues that hinder fertility in this and other mammalian species, allowing to overcome the challenges for new and more efficient breeding strategies.

Corpora lutea in superovulated ewes fed different planes of nutrition

The corpus luteum (CL) is an ovarian structure which is critical for the maintenance of reproductive cyclicity and pregnancy support. Diet and/or diet components may affect some luteal functions. FSH is widely used to induce multiple follicle development and superovulation. We hypothesized that FSH would affect luteal function in ewes fed different nutritional planes. Therefore, the aim of this study was to determine if FSH-treatment affects (1) ovulation rate; (2) CL weight; (3) cell proliferation; (4) vascularity; (5) expression of endothelial nitric oxide (eNOS) and soluble guanylate cyclase (sGC) proteins; and (6) luteal and serum progesterone (P4) concentration in control (C), overfed (O), and underfed (U) ewes at the early- and mid-luteal phases. In addition, data generated from this study were compared to data obtained from nonsuperovulated sheep and described by Bass et al. Ewes were categorized by weight and randomly assigned into nutrition groups: C (2.14 Mcal/kg; n = 11), O (2xC; n = 12), and U (0.6xC; n = 11). Nutritional treatment was initiated 60 d prior to day 0 of the estrous cycle. Ewes were injected with FSH on day 13-15 of the first estrous cycle, and blood samples and ovaries were collected at early- and mid-luteal phases of the second estrous cycle. The number of CL/ewe was determined, and CL was dissected and weighed. CL was fixed for evaluation of expression of Ki67 (a proliferating cell marker), CD31 (an endothelial cell marker), and eNOS and sGC proteins using immunohistochemistry and image analysis. From day 0 until tissue collection, C maintained, O gained, and U lost body weight. The CL number was greater (P < 0.03) in C and O than U. Weights of CL, cell proliferation, vascularity, and eNOS but not sGC expression were greater (P < 0.001), and serum, but not luteal tissue, P4 concentrations tended to be greater (P = 0.09) at the early- than mid-luteal phase. Comparisons of CL measurements demonstrated greater (P < 0.01) cell proliferation and serum P4 concentration, but less vascularity at the early and mid-luteal phases, and less CL weight at the mid-luteal phase in superovulated than nonsuperovulated ewes; however, concentration of P4 in luteal tissues was similar in both groups. Thus, in superovulated ewes, luteal cell proliferation and vascularity, expression of eNOS, and serum P4 concentration depend on the stage of luteal development, but not diet. Comparison to control ewes demonstrated several differences and some similarities in luteal functions after FSH-induced superovulation.

Relationships among nitric oxide metabolites and pulses of a PGF2α metabolite during and after luteolysis in mares

Hourly circulating concentrations of a PGF2α metabolite (PGFM), progesterone (P4), and LH were obtained from a reported project, and concentrations of nitric oxide (NO) metabolites (NOMs; nitrates and nitrites) were determined in eight mares. Unlike the reported project, hormone concentrations were normalized to the peak of the first PGFM pulse of luteolysis (early luteolysis), second PGFM pulse (late luteolysis), and a pulse after luteolysis. The duration of luteolysis was 23.1 ± 1.0 hours, and the peak of the first and second PGFM pulses occurred 6.5 ± 0.9 and 14.8 ± 0.8 hours after the beginning of luteolysis. Concentration of P4 decreased progressively within and between the PGFM pulses Changes were not detected in LH concentration in association with the PGFM pulses. Concentration of NOMs was greater (P < 0.05) at the peak of the PGFM pulse during early luteolysis (88.8 ± 15.0 μg/mL) than during late luteolysis (58.8 ± 9.0 μg/mL). Concentration of NOMs began to decrease (P < 0.05) 4 hours before the peak of the PGFM pulse of early luteolysis. Concentration began to increase (P < 0.05) an hour after the peak of the PGFM pulse of late luteolysis. An NOM decrease and increase was not detected during the PGFM pulse after luteolysis. On a temporal basis, results indicated that NO either is not required for luteolysis in mares or has a role in or responds only during late luteolysis. A caveat is that the relative contribution of the CL versus other body tissues to circulating concentrations of NOMs in mares has not been determined.

Circulating nitric oxide metabolites during luteolysis and the effect of luteinizing hormone on circulating nitric oxide metabolites in heifers

Temporal relationships among circulating concentrations of nitric oxide metabolites (NOM), progesterone (P4), and luteinizing hormone (LH) within the hours of a PGFM pulse were studied during luteolysis in heifers. The peak of a PGFM pulse was designated Hour 0. All of the following increases and decreases were significant. Within a spontaneous PGFM pulse (experiment 1; n = 7), concentrations of P4 and LH decreased between Hours -1 and 0 and increased between Hours 0 and 1; NOM increased between Hours -1 and 2. In experiment 2, PGFM pulses were simulated by intrauterine infusion of PGF2α (PGF group, n = 6), and another group was also treated with acyline to inhibit LH secretion (acyline-PGF group, n = 6). Averaged over the two groups, concentration of P4 decreased between Hours -2 and 0, increased (rebounded) between Hours 0 and 1, and decreased after Hour 2. In the PGF group, concentration of LH decreased between Hours -2 and -0.5 and increased between Hour 0 and Hour 1.5 to a maximum at Hour 1.5; NOM decreased between Hours -2 and -1.5 and increased between Hours 0 and 1.5. In the acyline-PGF group, the effect of hour was not significant for concentrations of LH and NOM. The absence of an increase in NOM concentration when LH was inhibited is a novel finding. The hypotheses were supported that concentrations of LH and NOM are temporally related, and LH has a role in the increase in NOM within the hours of a PGFM pulse.

Role of the cell cycle in regression of the corpus luteum

The corpus luteum contains differentiated steroidogenic cells that have exited the cell cycle of proliferation. In some tissues, deletion of quiescent, differentiated cells by apoptosis in response to injury or pathology is preceded by reentry into the cell cycle. We tested whether luteal cells reenter the cell cycle during the physiological process of luteolysis. Ovaries were obtained after injection of cows with a luteolytic dose of prostaglandin F(2)(α) (PGF). In luteal sections, cells co-staining for markers of cell proliferation (MKI67) and apoptosis (cPARP1) increased 24  h after PGF, indicating that cells that reenter the cell cycle undergo apoptosis. The percent of steroidogenic cells (CYP11A1-positive) co-staining for MKI67 increased after PGF, while co-staining of non-steroidogenic cells did not change. Dispersed luteal cells were stained with Nile Red to distinguish lipid-rich steroidogenic cells from nonsteroidogenic cells and co-stained for DNA. Flow cytometry showed that the percent of steroidogenic cells progressing through the cell cycle and undergoing apoptosis increased after PGF. Culturing luteal cells induced reentry of steroidogenic cells into the cell cycle, providing a model to test the influence of the cell cycle on susceptibility to apoptosis. Blocking cells early in the cell cycle using inhibitors reduced cell death in response to treatment with the apoptosis-inducing protein, Fas ligand (FASL). Progesterone treatment reduced progression through the cell cycle and decreased FASL-induced apoptosis. In summary, steroidogenic cells reenter the cell cycle upon induction of luteal regression. While quiescent cells are resistant to apoptosis, entry into the cell cycle promotes susceptibility to apoptosis.

After 65 years, research is still fun

In 1946, at the end of World War II, I entered graduate school at Cornell University, where I remained for 44 years. During that time, my laboratory produced more than 300 publications in the field of reproductive biology, including studies on nutrition and reproduction, the role of the hypothalamus in pituitary gonadotropin release, corpus luteum formation and function, hormone assays, and estrous cycle synchronization. At age seventy, I retired from Cornell and accepted the Gordon Cain Endowed Professorship at Louisiana State University, where I continued my work on the bovine corpus luteum and added research on the collection, maturation, in vitro fertilization, and culture of bovine oocytes. In 1994, I moved to the Pennington Biomedical Research Center and soon thereafter started the research that led to development of the lytic peptide-gonadotropin conjugates, which target and destroy cancer cell membranes. I am continuing my work on the development of targeted cancer cell drugs and, yes, research is still fun!

Ovarian steroids modulate tumor necrosis factor-α and nitric oxide-regulated prostaglandin secretion by cultured bovine oviductal epithelial cells

Ovarian steroids assure an optimum environment for the final maturation of oocytes, gamete transport, fertilization, and early embryonic development. The aim of experiment 1 was to examine the influence of ovarian steroids on tumor necrosis factor-α (TNF-α)- or nitric oxide (NO)-regulated prostaglandin (PG), and nitrite/nitrate (NO₂/NO₃) secretion by cultured bovine oviductal epithelial cells (BOECs). BOECs were pretreated with 17β-estradiol (E₂; 10⁻⁹ M) and/or progesterone (P₄; 10⁻⁷ M) for 24 h. For the next 24 h, BOECs were treated with TNF-α (10 ng/mL) or spermine nitric oxide complex (NONOate; 10⁻⁵ M). Prostaglandin F(2α) and PGE₂ secretion was measured in medium by ELISA. The pretreatment of cells with P₄ (progesterone), E₂ (17 β-estradiol), or E₂/P₄ augmented TNF-α-induced PGF(2α) and PGE₂ secretion (P < 0.01). The pretreatment of cells with E₂ or E₂/P₄ increased NONOate-induced PGF(2α) and PGE₂ secretion (P < 0.01). TNF-α induced NO₂/NO₃ production by BOECs. The pretreatment of cells with E₂ augmented only TNF-α-induced NO₂/NO₃ production (P < 0.05). The aim of experiment 2 was to examine the influence of TNF-α, NO, and ovarian steroids on the protein content of enzymes specifically involved in PG and NO production, PG synthases, and NO synthases (NOSs). BOECs were treated with TNF-α (10 ng/mL) or NONOate (10⁻⁵ M). TNF-α increased the protein content of PGG/H synthase, PGF synthase, and PGE synthase (P < 0.05) and endothelial and inducible NOSs (P < 0.05). Nitric oxide increased the protein content of PGF synthase, PGE synthase, endothelial NOS, and inducible NOS (P < 0.05). These results show possible linkage between TNF-α and NO, modulated by ovarian steroids, in the regulation of PG synthesis by BOECs that may be important for triggering the process of oviductal contractions.

Leukotrienes are auto-/paracrine factors in the bovine corpus luteum: an in vitro study

The aim of this study was to determine leukotrienes (LTs) functions in the bovine corpus luteum (BCL) during the oestrous cycle. In steroidogenic CL cells we examined the effect of luteotropic [LH, prostaglandin E(2) (PGE(2))] and luteolytic (PGF(2α), cytokines) factors on: the levels of LTB(4) and C(4), the expression of 5-lipoxygenase (LO), LT receptors type I (LTR-I) and LTR-II, and the effects of LTB(4) and C(4) stimulations on the levels of progesterone (P4), PGE(2), F(2α) and nitric oxide (NO) metabolites. Both luteolytic and luteotropic factors stimulated 5-LO expression on days 2-4 and 17-19 of the cycle. Leukotriene receptors type I expression increased after PGE(2) and tumour necrosis factor α with interferon γ (TNF/IFN) stimulation on days 2-4 of the cycle. Leukotriene receptor type II expression increased after PGE(2α) and TNF/IFN stimulation on days 2-4 and 17-19 of the cycle, and LTR-II expression on days 8-10 of the cycle was unchanged after cell stimulation with any factor. Leukotriene B(4) level increased after BSC incubation with luteotropic factors during all examined days of the cycle and after cytokine stimulation at early- and mid-luteal stages, whereas luteolytic factors stimulated LTC(4) secretion over the entire cycle. Leukotriene B(4) stimulated P4 secretion at the mid-luteal stage and stimulated NO secretion during all examined phases. Leukotriene B(4) stimulated PGE(2) secretion at the early- and mid-luteal stage. Leukotriene C(4) inhibited P4 secretion at the mid- and regressing-luteal stage, stimulated NO (entire cycle) and PGF(2α) at mid- and regressing-luteal phases. Leukotrienes modulate steroidogenic cells functions, depending on the stage of the cycle. Leukotriene B(4) plays a luteotropic role stimulating P4 and PGE(2) secretions; LTC(4) stimulates the secretion of luteolytic factors and enhances the luteolytic cascade within BCL.

Tumor necrosis factor-α inhibits the stimulatory effect of luteinizing hormone and prostaglandin E(2) on progesterone secretion by the bovine corpus luteum

Tumor necrosis factor-α (TNF-α) is involved in the tissue remodeling that occurs in the corpus luteum (CL) during its development and regression. This cytokine is also implicated in the regulation of reproduction by its actions on ovarian steroidogenic cells. The aim of this study was to examine the influence of TNF-α on (1) progesterone (P(4)) output by the bovine CL and on (2) the responsiveness of the CL to LH or prostaglandin E(2) (PGE(2)) in vitro. In experiment 1, CL (days 8 to 10 of the estrous cycle) were perfused by using an in vitro microdialysis system with TNF-α (0.1, 0.5, or 1 μg/mL) alone or with TNF-α (1 μg/mL) followed by LH (1000 ng/mL) or PGE(2) (2 × 10(-5) M). Basal P(4) release (P < 0.05) was increased by TNF-α (0.5 or 1 μg/mL). Moreover, TNF-α (1 μg/mL) inhibited the stimulatory effect of LH or PGE(2) on P(4) output (P < 0.05). In experiment 2, 4 h after intrauterine infusion of TNF-α (0.01 μg/mL or 1 μg/mL), CL (days 8 to 10 of the estrous cycle) were collected by colpotomy, cultured, and stimulated with LH (10 ng/mL) or PGE(2) (10(-6) M). Intrauterine infusion of TNF-α at a concentration of 1 μg/mL increased basal P(4) output by CL (P < 0.05). Moreover, the intrauterine infusion of TNF-α at a concentration of 0.01 μg/mL inhibited the stimulatory effect of LH or PGE(2) on P(4) output (P < 0.05). These results indicate that TNF-α (1) does not have an effect on the autonomous, pulsatile release of P(4); (2) increases P(4) secretion by bovine CL with increasing doses, and (3) reduces in a dose-dependent manner the responsiveness of CL to luteotropic factors both directly (after infusion to CL) and indirectly (after intrauterine infusion).

Nitric oxide stimulates progesterone and prostaglandin E2 secretion as well as angiogenic activity in the equine corpus luteum

Cytokines and nitric oxide (NO) are potential mediators of luteal development and maintenance, angiogenesis, and blood flow. The aim of this study was to evaluate (i) the localization and protein expression of endothelial and inducible nitric oxide synthases (eNOS and iNOS) in equine corpora lutea (CL) throughout the luteal phase and (ii) the effect of a nitric oxide donor (spermine NONOate, NONOate) on the production of progesterone (P4) and prostaglandin (PG) E(2) and factor(s) that stimulate endothelial cell proliferation using equine luteal explants. Luteal tissue was classified as corpora hemorrhagica (CH; n = 5), midluteal phase CL (mid-CL; n = 5) or late luteal phase CL (late CL; n = 5). Both eNOS and iNOS were localized in large luteal cells and endothelial cells throughout the luteal phase. The expression of eNOS was the lowest in mid-CL (P < 0.05) and the highest in late CL (P < 0.05). However, no change was found for iNOS expression. Luteal explants were cultured with no hormone added or with NONOate (10(-5) M), tumor necrosis factor-α (TNFα; 10 ng/mL; positive control), or equine LH (100 ng/mL; positive control). Conditioned media by luteal tissues were assayed for P4 and PGE(2) and for their ability to stimulate proliferation of bovine aortic endothelial cells (BAEC). All treatments stimulated release of P4 in CH, but not in mid-CL. TNFα and NONOate treatments also increased PGE(2) levels and BAEC proliferation in CH (P < 0.05). However, in mid-CL, no changes were observed, regardless of the treatments used. These data suggest that NO and TNFα stimulate equine CH secretory functions and the production of angiogenic factor(s). Furthermore, in mares, NO may play a role in CL growth during early luteal development, when vascular development is more intense.

Local regulation of corpus luteum development and regression in the cow: Impact of angiogenic and vasoactive factors

The corpus luteum (CL) of the estrous cycle in the cow is a dynamic organ which has a life time of approximately 17-18 days. The main function of the CL is to secrete a large amount of progesterone (P) thereby supporting the achievement of pregnancy. As the CL matures, the steroidogenic cells establish contact with many capillaries and the matured CL is composed of many vascular endothelial cells that account for up to 50% of all CL cells. The bovine CL produces several major angiogenic and vasoactive foctors such as vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), angiopoietin-1 and -2 (ANPT-1 and -2), prostaglandin F(2alpha) (PGF(2alpha)), endothelin-1 (EDN1), angiotensin II (Ang II) and nitric oxide (NO). These factors regulate P secretion directly and/or indirectly within the CL. Moreover, different actions of PGF(2alpha) in the early cycle CL (non-luteolytic) and the mid cycle CL (luteolytic) may provide insight into the luteolysis cascade in the cow. The aim of the present review is to describe the current concepts of the local mechanisms for the cascade of development and regression of the bovine CL as regulated by luteal angiogenic and vasoactive factors.

Regulation of luteal function and corpus luteum regression in cows: hormonal control, immune mechanisms and intercellular communication

The main function of the corpus luteum (CL) is production of progesterone (P4). Adequate luteal function to secrete P4 is crucial for determining the physiological duration of the oestrous cycle and for achieving a successful pregnancy. The bovine CL grows very fast and regresses within a few days at luteolysis. Mechanisms controlling development and secretory function of the bovine CL may involve many factors that are produced both within and outside the CL. Some of these regulators seem to be prostaglandins (PGs), oxytocin, growth and adrenergic factors. Moreover, there is evidence that P4 acts within the CL as an autocrine or paracrine regulator. Each of these factors may act on the CL independently or may modify the actions of others. Although uterine PGF(2 alpha) is known to be a principal luteolytic factor, its direct action on the CL is mediated by local factors: cytokines, endothelin-1, nitric oxide. The changes in ovarian blood flow have also been suggested to have some role in regulation of CL development, maintenance and regression.

Effects of exogenous tumour necrosis factor-alpha on the secretory function of the bovine reproductive tract depend on tumour necrosis factor-alpha concentrations

The aim of study was to correlate tumour necrosis factor-alpha (TNF) infused doses used with the TNF concentrations achieved and with the secretory function of both the ovary and the uterus in cows. We evaluated the concentrations of progesterone (P4), prostaglandin (PG)F(2alpha), PGE(2) nitric oxide (NO) and TNF in the jugular vein and vena cava caudalis as parameters of exogenous TNF action on the female reproductive tract. Aortae abdominalis of cows (n = 18) were infused with saline or two doses of TNF (luteolytic--1 microg or luteotrophic--10 microg). In the peripheral blood, 1 microg TNF concentrations achieved within the range of 30-45 pg/ml, and 10 microg TNF provoked a sharp increase in achieved concentrations at a range of 250-450 pg/mL). The TNF concentrations achieved in vena cava caudalis were five to six times higher than that in peripheral blood (p < 0.001). One microgram TNF increased PGF(2alpha) and NO (p < 0.001) and decreased P4 (p < 0.05). The higher TNF dose stimulated P4 and PGE(2) (p < 0.01). TNF infusion at luteolytic dose achieved its concentrations at the physiological range previously observed in cows. Luteotrophic TNF dose achieved the concentrations in vena cava caudalis that are much higher than physiological level and were previously noted in pathological circumstances (i.e. mastitis, metritis).

Intrauterine infusion of BQ-610, an endothelin type A receptor antagonist, delays luteolysis in dairy heifers

Three separate in vivo experiments were conducted to evaluate the putative role of endothelin-1 (ET-1) during luteal regression in heifers. In Experiment 1, a single intraluteal injection of 500 microg BQ-610 [(N,N-hexamethylene) carbamoyl-Leu-D-Trp (CHO)-D-Trp], a highly specific endothelin A (ETA) receptor antagonist, did not diminish the decline in plasma progesterone following a single exogenous injection of 25 mg prostaglandin F2 alpha (PGF2alpha) administered at midcycle of the estrous cycle. In Experiment 2, six intrauterine infusions of 500 microg BQ-610 given every 12 h on days 16-18 delayed spontaneous luteolysis, as evidenced by an extended elevation (P=0.054) of plasma progesterone concentration. In Experiment 3, heifers were administered six intrauterine infusions of BQ-610 or saline on days 16-19, and peripheral blood samples were collected from day 11 to 16 (before infusion), hourly on days 16-19 (during infusion), and on days 20-25 (after infusion). BQ-610 treated heifers had markedly higher (P<0.0001) levels of plasma progesterone compared with saline controls, and this effect was most notable during the infusion period (treatment by period interaction; P<or=0.05). Heifers infused with BQ-610 also had higher progesterone levels on day 21 (treatment by time interaction; P<or=0.05). Mean plasma concentrations of 13,14-dihydro-15-keto-PGF2alpha (PGFM), the primary metabolite of PGF2alpha, were measured in the samples collected hourly and were not different (P>or=0.05) between treatments. These results indicate that the in vivo antagonism of the ETA receptor can delay functional luteolysis, and supports the theory that ET-1 regulates luteal function in ruminants.

Luteolytic effect of prostaglandin F 2 alpha on bovine corpus luteum depends on cell composition and contact

Prostaglandin F(2 alpha) (PGF(2 alpha)) is a main luteolytic factor in vivo; however, its direct luteolytic influence on steroidogenic cells of bovine corpus luteum (CL) is controversial and not fully understood. The aim of the study was to clarify PGF(2 alpha) action on bovine CL in different in vivo and in vitro conditions and to examine whether the contact among all main types of CL cells is necessary for luteolytic PGF(2 alpha) action. In experiment 1, the bovine CL (day 15 of the oestrous cycle) was perfused using in vivo microdialysis system with dinoprost (an analogue of PGF(2 alpha)) for 0.5 h. Dinoprost caused a short-time increase in progesterone (P4), whose concentration decreased thereafter (at 6-, 10-, 12- and 24-h after treatment). In experiment 2, the direct effect of PGF(2 alpha) on P4 accumulation in CL steroidogenic cells cultured in monolayer (day 15 of the cycle) was determined. PGF(2 alpha) after 24 h of incubation increased P4 accumulation in steroidogenic CL cells. In experiment 3 steroidogenic, endothelial CL and immune cells (day 15 of the cycle) were incubated with PGF(2 alpha) in cocultures for 24 h in glass tubes and the levels of P4, stable metabolites of nitric oxide (NO) and leukotriene (LT) C(4) were determined. Although PGF(2 alpha) treatment increased P4 secretion in homogeneous steroidogenic CL cell culture, the decrease in P4 secretion in cocultures of all types of CL cells was observed. The secretion of NO and LTC(4) increased after the treatment of PGF(2 alpha) both in pure cultures of CL cells and in cocultures. The interactions between endothelial and immune cells with steroidogenic CL cells are needed for luteolytic PGF(2 alpha) action within the bovine CL. Our results indicate that the cell coculture model, including the main types of CL cells, is the most approximate to study PGF(2 alpha) role in vitro.

Endometrial nitric oxide production and nitric oxide synthases in the equine endometrium: Relationship with microvascular density during the estrous cycle

Nitric oxide (NO) plays an important role in angiogenesis and in the regulation of the blood flow. This study was carried out to investigate (i) the effects of endogenous estrogens and progestins and exogenous progesterone (P(4)) (5 ng/ml or 1 microg/ml) or estradiol 17beta (E(2)beta) (50 pg/ml or 1 microg/ml) on in vitro endometrial NO synthesis; (ii) the presence of different isoforms of NO synthase; (iii) and their relationship to microvascular density in the equine endometrium during the estrous cycle. NOS expression was also evaluated in the myometrium. Expression of endothelial and inducible forms of NOS in the uterus was assessed by Western blot and immunocytochemistry. Vascular density in endometrial tissue was determined on histologic sections. In the luteal phase, compared to the follicular phase, endometrial NO production increased without exogenous hormones and with exogenous E(2)beta (1 microg/ml). Although immunocytochemistry revealed iNOS and eNOS expression in the endometrium, no positive signal for iNOS was detected by Western blot. Endothelial NOS was observed in endometrial glands, endothelial cells, fibroblasts, blood and lymphatic vessels. Endometrial eNOS expression was the highest in the follicular and mid-luteal phases while it was found to be the lowest in the early luteal phase. In the follicular phase, hyperplasia of endometrial tissue with respect to myometrium was detected. No difference in vascular density was present between phases. All together, NO may play some roles in both proliferative and secretory phases of endometrial development in the mare.

Infusion of exogenous tumor necrosis factor dose dependently alters the length of the luteal phase in cattle: differential responses to treatment with indomethacin and L-NAME, a nitric oxide synthase inhibitor

We examined whether prostaglandins (PGs) and nitric oxide (NO) mediate tumor necrosis factor (TNF) actions in the estrus cycle. On Day 14 of the cycle, the following solutions were infused into the aorta abdominalis of a total of 51 heifers (Experiments 1 and 2): saline; 1 or 10 microg of TNF; 480 mg indomethacin (INDO), an inhibitor of prostaglandin H synthase; 800 mg L-NAME, an inhibitor of NO synthase; and TNF (1 or 10 microg) in combination with INDO or L-NAME. TNF at 1 microg infused directly into aorta abdominalis increased the level of PGF(2alpha) and decreased the level of progesterone (P4) in the peripheral blood and shortened the estrus cycle. The high TNF dose stimulated P4 and PGE(2) and prolonged the corpus luteum (CL) lifespan. INDO blocked the effects of both TNF doses on the CL lifespan and hormone output. L-NAME completely blocked the effects of the luteolytic TNF dose, whereas the effects of the luteotropic TNF dose were not inhibited. In Experiment 3 (Day 14), saline or different TNF doses were infused into the jugular vein (n = 9) or into the uterine lumen (n = 18). The CL lifespans of the different groups were not different when TNF was infused into the jugular vein. Although high TNF doses (1 and 10 microg) infused into the uterine lumen prolonged the CL lifespan, low doses (0.01 and 0.1 microg) induced premature luteolysis. We suggest that the actions of exogenous TNF on the CL lifespan depend on PG synthesis stimulated by TNF in the uterus. TNF at low concentrations initiates a positive cascade between uterine PGF(2alpha) and various luteolytic factors, including NO, to complete premature luteolysis. PGE(2) is a good candidate mediator of the luteotropic actions of exogenous TNF action.

Inverse relationship between nitric oxide synthases and endothelin-1 synthesis in bovine corpus luteum: interactions at the level of luteal endothelial cell

Endothelin-1 (ET-1) and nitric oxide (NO) play pivotal roles in corpus luteum (CL) function. The present study examined the interplay between NO and ET-1 synthesis in the bovine CL. We found similar inducible and endothelial NO synthase (iNOS and eNOS, respectively) activities in the young CL (d 1-5) expressing the highest levels of both eNOS and iNOS mRNA. These values later declined at mid-cycle (d 8-15) and remained low at later stages (d 16-18). Luteolysis, initiated by prostaglandin F2alpha analog administration, further reduced NOS mRNA and by 24 h, NOS values dropped to approximately 15% of those at mid-cycle. eNOS protein levels followed a similar pattern to its mRNA. Because endothelial cells (ECs) are the main site for ET-1 and NO production in the CL, we examined the direct effects of the NO donor, NONOate on luteal ECs (LECs). Elevated NO levels markedly decreased ET-1 mRNA, and peptide concentrations in cultured and freshly isolated LECs in a dose-dependent manner. In agreement, NOS inhibitor, NG-nitro-l-arginine methyl ester, stimulated ET-1 mRNA expression in these cells. Interestingly, NO also up-regulated prostaglandin F2alpha receptors in LECs. These data show that there is an inverse relationship between NOS and ET-1 throughout the CL life span, and imply that this pattern may be the result of their interaction within the resident LECs. NOS are expressed in a physiologically relevant manner: elevated NO at an early luteal stage is likely to play an important role in angiogenesis, whereas reduced levels of NO during luteal regression may facilitate the sustained up-regulation of ET-1 levels during luteolysis.

Vascular composition, apoptosis, and expression of angiogenic factors in the corpus luteum during prostaglandin F2alpha-induced regression in sheep

Corpora lutea and blood samples were collected from superovulated ewes 0, 4, 8, 12 and 24 h after prostaglandin F(2alpha) (PGF) analog injection on day 10 of the estrous cycle. Changes in vascular cell and fibroblast composition, apoptosis and mRNA expression for several angiogenic factors in the corpus luteum (CL) were determined. While peripheral progesterone concentration decreased at 24 h after PGF injection, CL weight did not change. The area of positive BS-1 lectin staining (endothelial cell marker), smooth muscle cell actin (SMCA; pericyte and SMC marker), collagen type 1 (fibroblast marker), and the rate of cell death changed in luteal tissues after PGF treatment. In association with these cellular changes, mRNA for several angiogenic factors including vascular endothelial growth factor (VEGF) and receptors (Flt and KDR), basic fibroblast growth factor (FGF2) and receptor, angiopoietin (ANGPT) 1 and receptor Tie-2, endothelial nitric oxide synthase (NOS3), and angiotensin II receptor 1 (AT1) were altered. Changes in endothelial cell marker expression were positively correlated with changes in VEGF and NO systems. In addition, changes in mRNA expression for VEGF, Flt and KDR were positively correlated with changes in ANGPT2, Tie-2, and NOS3, indicating a functional relationship. This data demonstrates that after an initial increase, the endothelial component of the vascular bed decreases during PGF-induced luteal regression. However, SMCA expression remained high during luteal regression, potentially indicating a role of pericytes and vascular SMC in luteolysis, likely to regulate tissue remodeling and to maintain the integrity of larger blood vessels. Further, it appears that early regression may increase collagen type 1 production and/or expression by fibroblasts. Expression of angiogenic factors is influenced by PGF-induced luteolysis and may serve to maintain vascular structure in order to aid luteal regression.

Role of intraluteal prostaglandin F2α, progesterone and oxytocin in basal and pulsatile progesterone release from developing bovine corpus luteum

The present study examined the role of intra-luteal prostaglandin (PG) F(2alpha), progesterone (P4) and oxytocin (OT) on the corpus luteum function by using specific hormone antagonists. Luteal cells from the developing CL (days 5-7 of the estrous cycle) were exposed to P4 antagonist (onapristone, OP, 10(-4)M), OT antagonist (atosiban, AT; 10(-6)M) or indomethacin (INDO; 10(-4)M), for 12h and then stimulated with PGF(2alpha) (10(-8)M) for 4h. Pre-treatment of the cells with OP, AT or INDO resulted in an increase in P4 secretion in response to PGF(2alpha). To examine the temporal effects of P4, OT and PGs on P4 secretion, dispersed luteal cells were pre-exposed to OP, AT or INDO for 1, 2, 4, 6 or 12h. Prostaglandin F(2alpha) stimulated P4 secretion (P<0.05) after 2h of pre-exposition. In the microdyalisis study, the spontaneous release of P4 from developing CL tissue was of pulsatile nature with irregular peaks at 1-2h intervals. Treatment with OP increased the number of P4 peaks (P<0.05), whereas AT and INDO significantly reduced the number of P4 peaks detected (P<0.05). Interestingly, INDO completely blocked the pulsatile nature in the release of P4, but it secretion remained stable throughout the experimental period. These results demonstrate that luteal PGF(2alpha), OT, and P4 are components of an autocrine/paracrine intra-ovarian regulatory system responsible for the episodic (pulsatile) release of P4 from the bovine CL during the early luteal phase.

Ovarian function in ruminants

The purpose of this overview is to highlight important steps of ovarian regulation during follicle development, ovulation and the life span of corpus luteum (CL) in ruminants. The ovarian cycle is central to reproductive function. It is characterized by repeating patterns of cellular proliferation, differentiation and transformation that encompass follicular development and ovulation as well as the formation, function and regression of the CL. In the first part, the importance and regulation of final follicle growth and especially of angiogenesis and blood flow during folliculogenesis, dominant follicle development and CL formation are described. Our results underline the importance of growth factors especially of insulin-like growth factor (IGF), vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) for development and completion of a dense network of capillaries (angiogenesis) during follicle growth and CL formation. In the second part, the regulation of CL function by endocrine/paracrine and autocrine acting regulators is discussed. There is evidence that besides the main endocrine hormones luteinizing hormone (LH) and growth hormone (GH) local regulators as growth factors, peptides, steroids and prostaglandins are important modulators of luteal function. During early CL development until midluteal stage oxytocin (OT), prostaglandins and progesterone (P) itself stimulate luteal cell proliferation and function supported by the luteotropic action of a number of growth factors. The still high mRNA expression, protein concentration and localization of VEGF, FGF and IGF family members in the cytoplasm of luteal cells during midluteal stage suggest that they play pivotal role in the maintenance (survival) of this endocrine tissue. The major function of the CL is to secrete P. Progesterone itself regulates the length of the estrous cycle via influencing the timing of the luteolytic PGF2alpha signal from the endometrium. At the end of a nonfertile cycle, the regression of CL commences, steroidogenic capacity is lost (functional luteolysis), cell death is initiated, and tissue involution as well as resorption occurs within a few days (structural luteolysis). The cascade of mediators during luteolysis is very complex and still awaits elucidation. Evidence is given for participation of blood flow, inflammatory cytokines, vasoactive peptides (angiotensin II and endothelin-1), and decrease of the classical luteotropic mediators.

Role of tumor necrosis factor-alpha and nitric oxide in luteolysis in cattle

Although prostaglandin (PG) F2alpha is known to be a principal luteolytic factor, its action on the bovine corpus luteum (CL) is mediated by other intra-ovarian factors. Tumor necrosis factor-alpha (TNFalpha) and its specific receptors are present in the bovine CL with the highest expressions at luteolysis. TNFalpha in combination with interferon-gamma reduced progesterone (P4) secretion, increased PGF2alpha and leukotriene C4 (LTC4) production, and induced apoptosis of the luteal cells in vitro. Low concentrations of TNFalpha caused luteolysis, which resulted in a decreased level of P4, and increased levels of PGF2alpha, LTC4 and nitrite/nitrate (stable metabolites of nitric oxide-NO) in the blood. Inhibition of local NO production counteracts spontaneous and PGF2alpha-induced luteolysis. Therefore, NO is a likely candidate for the molecule that mediates PGF2alpha and TNFalpha actions during luteolysis. Both PGF2alpha and TNFalpha increase NO concentrations in blood, and stimulate NO synthase expression on protein level in the bovine CL cells. NO stimulates PGF2alpha and LTC4 secretion, inhibits P4 production and reduces the number of viable luteal cells. TNFalpha and NO induce apoptotic death of the CL by modulating expression of bcl-2 family genes and by stimulating expression and activity of caspase-3. The above findings indicate that TNFalpha and NO play crucial roles in functional and structural luteolysis in cattle.

Real-Time Relationships in Intraluteal Release among Prostaglandin F2α, Endothelin-1, and Angiotensin II During Spontaneous Luteolysis in the Cow1

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.

Effects of Prostaglandin F2.ALPHA. and Nitric Oxide on the Secretory Function of Bovine Luteal Cells

The objective of the present study was to investigate the influence of prostaglandin F(2alpha) (PGF (2alpha)) and nitric oxide (NO) on production of steroids and PGs by culturing bovine luteal cells obtained from ovaries on days 8-12 of the estrous cycle with a nitric oxide (NO) donor (Spermine NONOate), and a NO synthase inhibitor (N(G)-nitro-L-arginine methyl ester dihydrochloride: L-NAME). When the cells were exposed for 24 h to PGF(2alpha) (10(-7)-10(-5) M), production of progesterone (P(4)) increased significantly at all doses used (P<0.05). Moreover, PGF(2alpha) stimulated PGF(2alpha) production (P<0.01), depressed testosterone (T) production (P<0.05), but did not affect synthesis of prostaglandin E(2) (PGE(2)). Spermine NONOate decreased P(4) production to 66%, 47% and 34% of the control concentration after treatment with 10(-5) M, 10(-4) M and 10(-3) M, respectively, but did not affect T production, and increased PGF(2alpha) synthesis (P<0.05) and PGE(2) (P<0.01) at all doses used. L-NAME increased production of P(4) (P<0.01) but did not affect (P>0.05) secretion of T, PGF(2alpha) and PGE(2). Estradiol-17beta (E(2)) was detectable on the level of sensitivity of assay and was not significantly altered by any treatments. The overall results suggest that PGF(2alpha) and NO produced locally in bovine CL play roles in the regulation of the secretory function of the bovine CL as auto/paracrine factors.