Low peripheral progesterone and late embryonic/early fetal loss in suckled beef and lactating dairy cows.

Pregnancy failure during placentation in lactating dairy cows was associated with low concentrations of serum progesterone. Beef cows have greater serum progesterone and less pregnancy failure. Experiment 1 determined that reduction of serum progesterone affected late embryonic/early fetal loss in suckled beef cows. Cows (n=40) received progesterone from two new or used controlled internal drug releasing devices, replaced every 5d, beginning on Day 28 of gestation (mating=Day 0); CL were enucleated on Day 29. Retention of pregnancy was 77% in treated cows and 97% in 78 control cows (P<0.05). Experiment 2 determined how pregnant, lactating dairy cows with high or low progesterone concentrations during Days 28-34 differed in luteal function or in serum progesterone during replacement therapy. Luteal tissue from such cows was assayed for progesterone and expression of mRNA for genes of endothelin and prostaglandin (PG) systems. Secretion of progesterone and prostaglandins by dispersed luteal cells was determined during incubation with LH, endothelin-1, or arachidonic acid. Neither luteal progesterone nor mRNAs for endothelin or prostaglandin systems differed. Endothelin-1 inhibited secretion of progesterone more (P<0.05) in luteal cells from cows with low versus high serum progesterone, when incubated with arachidonic acid. Secretion of prostaglandin F(2)alpha was increased and that of 6-keto-PGF(1)alpha decreased by endothelin-1 in vitro. Serum progesterone during replacement was lower (P<0.05) for cows with low than high serum progesterone at lutectomy. Thus, clearance, more than luteal production, determined peripheral progesterone in pregnant, lactating dairy cows.

Relationship of fertility to ovarian follicular waves before breeding in dairy cows.

Cows with two waves of follicular growth during the estrous cycle yield follicles that are older and larger at ovulation compared with cows having three waves. The objectives of the current research were 1) to compare fertility in cows with two or three follicular waves and 2) to examine associations between luteal function, follicular development, and fertility after breeding. Follicular waves were monitored by ultrasonography during the estrous cycle before insemination in 106 dairy cows. Fewer cows had three follicular waves before next estrus and ovulation than two waves (P < 0.01; 30% vs 68%, respectively), but pregnancy rate was higher (P = 0.058; 81 vs 63%, respectively). Cows with two waves had shorter estrous cycles (P < 0.01), with the ovulatory follicle being both larger (P < 0.05) and older (P < 0.01). In cows with three waves, luteal function was extended (P < 0.05) and the peak in plasma progesterone occurred later (P < 0.05) in the estrous cycle compared to two wave cows. Considering cows that became pregnant, luteal phase length was shorter (P < 0.05) during the estrous cycle preceding insemination than for nonpregnant cows. In conclusion, fertility was greater in lactating cows inseminated after ovulation of the third-wave follicle that had developed for fewer days of the estrous cycle as compared with two-wave cows.

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.

Inter-relationships between endothelin and prostaglandin F2alpha in corpus luteum function.

In cattle and other species, the corpus luteum plays a central role in the regulation of cyclicity and maintenance of pregnancy. In the absence of fertilization and implantation, the corpus luteum undergoes functional and morphological regression or luteolysis. Luteal regression is initiated in domestic ruminants by surges of prostaglandin F2alpha (PGF2alpha) from the uterus. Despite intensive investigation, the mechanisms by which PGF2alpha causes luteal regression remain undetermined. Recent studies from several laboratories have demonstrated that endothelial cells and their product, endothelin 1, are required for the manifestation of the luteolytic effects of PGF2alpha. Experimental evidence strongly supports the concept that luteal endothelin 1 inhibits luteal steroidogenesis and mediates the effects of PGF2alpha. Endothelin 1 caused a dose-dependent reduction in both basal and luteinizing hormone-stimulated biosynthesis of progesterone and prostacyclin, and an increase in PGF2alpha by ovine and bovine luteal cells. Specific receptors for endothelin 1 were identified on large and small bovine luteal cells, and the addition of specific endothelin receptor antagonists abolished the inhibitory effects of endothelin 1. Luteal endothelin 1 content increased as the cyclic corpus luteum aged, and the highest concentrations were observed during luteolysis. The amount of mRNA encoding endothelin 1 was greatly increased during the period of luteolysis. Gene expression for endothelin 1 was increased, in a time-dependent manner, in corpora lutea collected from heifers and ewes after exogenous administration of PGF2alpha. In heifers, exogenous PGF2alpha resulted in increased luteal output of endothelin 1. In ewes, the luteolytic effects of PGF2alpha were mitigated by pretreatment with a specific endothelin receptor antagonist. Administration of endothelin 1 or a sub-luteolytic dose of PGF2alpha to ewes reduced concentrations of jugular venous progesterone but did not shorten luteal lifespan. However, a combination of endothelin 1 and PGF2alpha acted synergistically to bring about complete luteolysis and reduced lifespan of the corpus luteum. In summary, endothelin 1 appears to have a direct effect on luteal cells in cattle and sheep, and it plays an essential role in mediating the luteolytic effects of PGF2alpha.

Intraovarian regulation of luteolysis.

The corpus luteum is a transient gland, which is only functional for 17-18 days in the cyclic cow or for up to 200 days in the pregnant cow. Regression of the corpus luteum is essential for normal cyclicity as it allows the development of a new ovulatory follicle, whereas prevention of luteolysis is necessary for the maintenance of pregnancy. Evidence acquired over the past three decades indicated that PGF2 alpha is the luteolytic hormone in ruminants. Nevertheless, the detailed mechanisms of PGF2 alpha action are just beginning to be clarified. A pivotal role for an endothelial cell product endothelin 1 (ET-1) has been documented in PGF2 alpha-induced luteal regression. ET-1 inhibited progesterone production by luteal cells in a dose-dependent manner via selective ET-1 binding sites (ETA). The inhibitory action of PGF2 alpha on progesterone secretion (in vivo and in vitro) was blocked by a selective ETA receptor antagonist. This implied that ET-1 (through ETA receptors present on steroidogenic cells) may have mediated the inhibitory effect of PGF2 alpha. The involvement of ET-1 in luteal regression was also suggested by the observation that the highest concentrations of ET-1 coincide with uterine PGF2 alpha surges. Furthermore, PGF2 alpha administration upregulated ET-1 expression within the corpus luteum. Later stages of luteal regression, which involve programmed cell death (PCD), are presumably mediated by immune cells. ET-1 may also be involved in this process by promoting leukocyte migration and stimulating macrophages to release tumour necrosis factor alpha (TNF alpha). The TNF alpha receptor type 1 (p55) is present on luteal cells (endothelial and steroidogenic cells) and could initiate PCD and the structural demise of the corpus luteum.

Elevated lipoprotein lipids and gestational hormones in women with diet-treated gestational diabetes mellitus compared to healthy pregnant controls.

The objective of this study was to describe plasma and lipoprotein perturbations in gestational diabetes mellitus (GDM) compared to controls, and determine if alterations in lipids are related to gestational hormones and/or glucose control. Maternal HbA1c, free fatty acids (FFA), beta-estradiol, progesterone, prolactin, and plasma, very-low-density lipoprotein (VLDL), low-density lipoprotein (LDL), high-density lipoprotein (HDL), HDL2 and HDL3 triglyceride (TG), cholesterol, and dietary intake were determined for women with diet-treated GDM and controls in a longitudinal design. Subjects (N = 25/group) were matched for age, race, and body-mass index (BMI). Women with GDM had significantly higher HbA1c than controls, although both groups were within the normal range (4%-6%). However, body weight gain was less for women with GDM. There was a trend for higher plasma FFAs at 37-38 weeks in GDM versus controls. Plasma and lipoprotein TG among the groups increased over the third trimester, and mean concentrations were greater for women with GDM. In GDM versus controls, VLDL and HDL3 TGs were higher at all times, HDL and HDL2 TGs at 33-34 and 37-38 weeks, and LDL TGs at 37-38 weeks. In VLDL, core lipids (TG + cholesterol) increased over gestation and were greater in GDM. In HDL, the TG/cholesterol ratio was greater in GDM. In GDM versus controls, plasma progesterone and prolactin were higher at all times; beta-estradiol was elevated at 37-38 weeks. HbA1c, progesterone, and prolactin correlated with all lipoprotein TG fractions. Exaggerated hypertriglyceridemia, particularly in the VLDL and HDL fractions, is a feature of GDM. The increase in VLDL TG is likely due to an increase in VLDL synthesis, whereas particle enrichment in TG is a plausible explanation for changes in HDL TG. Slight perturbations in glucose control and gestational hormones in diet-treated GDM may contribute to the observed increase in plasma and lipoprotein TG.

Regulation of endothelin-1 expression in the bovine corpus luteum: elevation by prostaglandin F 2 alpha.

Prostaglandin F2alpha (PGF2alpha) has been recognized as the physiological luteolysin in ruminants and other species for more than three decades; however, the mechanisms involved in its action are poorly understood. We previously have shown that endothelin-1 (ET-1) mediates, at least in part, the action of PGF2alpha, and the current study examines the effect of PGF2alpha on the expression of ET-1 in bovine corpus luteum (CL). Endothelins (ETs) were extracted from CL, collected at various times of the estrous cycle, and highest levels were found during luteolysis. The expression of prepro-ET-1 was also highest in regressing CL, suggesting that PGF2alpha may have elevated ET-1 expression. This was confirmed by demonstrating that administration of PGF2alpha to heifers at midcycle elevated luteal ET-1 expression. Levels were induced as soon as 2 h after PGF2alpha treatment and 24 h later were 7-fold higher than preinjection levels. Endothelial cells isolated from bovine CL produced ET-1, and addition of PGF2alpha, oxytocin (OT), and vasopressin-augmented ET biosynthesis. Induction of ET-1 expression by PGF2alpha in these cells was evident after a short incubation time (15-90 min). Taken together, these data suggest that stimulation of luteal ET-1 expression by PGF2alpha may be achieved by several nonmutually exclusive mechanisms: 1) by acting directly on luteal endothelial cells; 2) indirectly, via OT release from large luteal cells; and 3) by causing hypoxia in the CL (as a result of ET-1-induced vasoconstriction). The latter mechanism may serve to augment ET-1 secretion in a positive-feedback process.

Long chain polyunsaturated fatty acids and bovine luteal cell function.

Experiments were conducted to determine the effects of exogenous polyunsaturated fatty acids (PUFA) on the production of progesterone and prostanoids by dispersed bovine luteal cells and to characterize endogenous luteal fatty acids throughout the estrous cycle. The addition of eicosapentaenoic acid (20:5, n3) resulted in a dose-dependent reduction of progesterone production and an increase in production of prostaglandin (PG) F2 alpha (PGF2 alpha) (p < 0.05). Nordihydroguaiaretic acid abolished the inhibitory effects of 20:5, n3 on progesterone production, while indomethacin did not alter these effects. The addition of 10 micrograms docosahexaenoic acid (22:6, n3) resulted in a suppression of progesterone synthesis (p < 0.05) and reduced PGF2 alpha synthesis. The addition of 0.1, 1, and 10 micrograms docosatetraenoic acid (22:4, n6) inhibited basal progesterone production, whereas only the highest dose decreased LH-stimulated synthesis of progesterone. The addition of 22:4, n6 resulted in increased PGF2 alpha synthesis (p < 0.05) and in lowered synthesis of prostacyclin (p < 0.05). Variations in luteal fatty acids were confirmed by an experiment in which endogenous fatty acids were characterized throughout the estrous cycle. The percentages and ratios of PUFA were altered throughout the estrous cycle. In summary, PUFA have potent inhibitory effects on the production of progesterone and PGI2 in vitro and may play a role in bovine luteal cell function by mechanisms yet to be determined.

Luteotropic and luteolytic mechanisms in the bovine corpus luteum.

The function of the corpus luteum (CL) is a key element in many reproductive processes including ovulation, length of the estrous cycle, recognition of pregnancy and embryo survival in all mammalian species. The main function of the CL is to produce progesterone which acts on its tissues to prepare them for successful pregnancy. The CL is controlled by numerous biological compounds which provide luteotropic support during the estrous cycle and pregnancy and for inducing luteolysis at the end of the cycle The purpose of this paper is to review the mechansims responsible for controlling the endocrine function of this tissue in the bovine ovary.

Effect of endothelin-1 on bovine luteal cell function: role in prostaglandin F2alpha-induced antisteroidogenic action.

Endothelin-1 (ET-1) a vasoactive peptide, is synthesized and secreted by endothelial cells. In the bovine corpus luteum (CL), endothelial cells constitute a major proportion (53.5%) of total CL cells. This study was designed to examine the effects of ET-1 on bovine luteal cell functions and its involvement in the action of PGF2alpha. To better define the cells implicated in this process, we used CL slices, whole CL-derived cells, and steroidogenic large (LLC) and small (SLC) luteal-like cells. High affinity binding sites for ET-1 (K(d), approximately 0.3 x 10(-9)) were present in both steroidogenic luteal cells. The binding affinity of ET-1 was 3 orders of magnitude higher than that of ET-3, and a selective ETA receptor antagonist (BQ123) competed similarly to ET-1, suggesting the presence of ETA receptors. The lack of effect of ET-3 on CL-derived cells further supported this conclusion. Both basal progesterone secretion and bovine LH (5 ng/ml)-stimulated progesterone secretion from CL-derived cells were significantly inhibited by ET-1 in a dose-dependent manner, whereas preincubation of these cells with ETA receptor antagonist prevented the inhibitory effect of added ET-1. Incubation of LLC with 10(-8) M ET-1 inhibited their progesterone secretion (114.8 vs. 176.7 ng/10(5) cells-20 h; P < 0.05). On the other hand, ET-1 did not affect progesterone production from SLC despite the presence of ET-binding sites. PGF2alpha only inhibited LH-stimulated progesterone secretion by luteal slices. This antisteroidogenic effect of PGF2alpha could be prevented by the addition of a selective ETA receptor antagonist. Luteal tissue and microvascular endothelial cells isolated from bovine CL produced ET-1; in contrast, the peptide was undetectable in the culture medium or in cell extracts of either LLC or SLC. These data support the concept that ET-1 may play a paracrine regulatory role in bovine luteal function and propose a novel role for this peptide in mediating PGF2alpha-induced luteal regression.

Bovine corpus luteum function after removal of granulosa cells from the preovulatory follicle.

Experiments were conducted to determine the effects of removing granulosa cells from bovine preovulatory follicles on subsequent corpus luteum (CL) function. Holstein heifers were assigned to three groups: untreated controls (n = 6), removal of granulosa cells (n = 9) and removal and return of granulosa cells (n = 7). Surgery was performed 18-24 hr after the onset of estrus and in all cases after the preovulatory luteinizing hormone (LH) surge. Jugular venous blood was collected and estrous activity monitored twice daily. Corpora lutea were formed in six heifers in each group. Concentrations of plasma progesterone were reduced (P less than 0.05) on Days 7-17 in heifers from which granulosa cells were removed when compared to the other two groups. There were no differences in the lengths of the estrous cycles nor concentrations of LH in the three groups. In summary, these experiments support the concept that granulosa cells make a substantial contribution to the output of progesterone by the cyclic CL but may have a limited role in determining the functional lifespan of the CL. These experiments also establish the fact that granulosa cells develop into functional luteal cells after their removal and return to the preovulatory follicle.

Function and lifespan of corpora lutea in ewes treated with exogenous oxytocin.

Oxytocin was administered to Dorset and Shropshire ewes in one experiment and to Dorset ewes in a further 4 experiments. In Exp. 1, concentrations of plasma progesterone and lengths of the oestrous cycle in ewes given oxytocin subcutaneously twice a day on Days 0-3, 2-5, 4-7, 6-9, 8-11, 10-13, 12-15 or 14-17 were similar to those of control ewes. In Exp. 2, intraluteal infusions of oxytocin from Day 2 to Day 9 after oestrus had no effect on concentration of progesterone, weight of CL collected on Day 9 or length of the oestrous cycle. In Exp. 3, intraluteal infusions of oxytocin on Days 10-15 after oestrus had no effect on weight of CL collected on Day 15. In Exp. 4, s.c. injections of oxytocin on Days 3-6 after oestrus had no effect on weight of CL collected on Day 9, concentrations of progesterone or length of the oestrous cycle. In Exp. 5, s.c. injections of oxytocin twice a day did not affect the maintenance and outcome of pregnancy in lactating and nonlactating ewes. Exogenous oxytocin, therefore, does not appear to affect luteal function at any stage of the ovine oestrous cycle although oxytocin has been reported by others to alter ovine CL function.

Differential origin and control mechanisms in small and large bovine luteal cells.

Studies of the calcium requirement and the relationship of intracellular calcium to progesterone synthesis in highly purified preparations of bovine luteal cells reveal a remarkably close relationship between intracellular calcium levels and steroidogenesis. The differential responses of the two cell types, summarized in Table 2, are beginning to reveal how the two cell types may co-operate to produce both luteotrophic and luteolytic responses at different stages of the oestrous cycle and early pregnancy. The luteotrophic mechanisms in the small cells are fairly clear; in addition to the luteotrophic effects of LH and cAMP, activation of protein kinase C leads to increased progesterone synthesis. Accordingly, PGF-2 alpha and several other prostanoids are luteotrophic in these cells. PGF-2 alpha stimulates phospholipase C activity in the small cells but does not reduce LH-stimulated cAMP or progesterone accumulation (Davis et al., 1989). This acute stimulus of protein kinase C activation to progesterone production in bovine small luteal cells is rapidly desensitized, although its stimulus to prostanoid production continues for at least 24 h. Large cells respond to LH, but only at relatively high levels. In addition, we have no good evidence for a role for protein kinase C in the control of progesterone synthesis in the large bovine luteal cells from mid-cycle corpora lutea. Phorbol esters have no effect on steroidogenesis and it is not yet established that protein kinase C provides the same high affinity receptor for phorbol esters that is found in the small cells. Experiments with inhibitors of protein kinase C, such as staurosporine, in large cells have been inconclusive. Evidence for several species suggests that both cell types co-operate, in ways not yet fully understood, to bring about maximal progesterone production at mid-cycle. Some evidence suggests that they may also co-operate to bring about luteolysis. The concept that PGF-2 alpha initiates luteolysis by inhibiting LH stimulated progesterone production in the large cells must be revised in light of the relative insensitivity of these cells to LH and the fact that they probably constitutively express the cholesterol side-chain cleavage enzymes (P-450scc) that represent the rate-limiting step in progesterone production. Oonk et al. (1989) have reported that, once P-450scc mRNA is induced in rat granulosa cells by the LH surge, it is constitutively maintained by the luteinized cells in the absence of gonadotrophins and is no longer regulated by cAMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of histone H2A on progesterone production by bovine luteal cells.

Histone H2A competitively inhibits binding of GnRH to high affinity rat ovarian receptor sites and blocks gonadotropin-stimulated steroid and cAMP accumulation during culture of rat granulosal or luteal cells. The objective of our study was to examine the progesterone suppressive effects of histone H2A on bovine luteal cells. In the first study, luteal cells were treated at Time = 0 h with a partially purified preparation of bovine ovarian histone H2A (3 ng GnRH equivalents, 800 micrograms protein), equivalent amounts of GnRH (3 ng), or BSA (800 micrograms) and incubated for a total of 4 h. At Time = 2 h, cells were treated with 5 ng bovine LH (bLH) or with medium. Histone H2A completely blocked both basal and LH-induced accumulation of progesterone compared with untreated cultures or cultures treated with bLH. Neither BSA nor GnRH suppressed LH-induced progesterone accumulation. In the second study, histone H2A was added to cultures at Time = 0 h and bovine luteal cells were cultured for 8 h. After 2 h of treatment, histone H2A (3 ng GnRH equivalents) was removed from selected cultures and replaced with fresh medium. Four hours later cultures were treated with 5 ng bLH or medium. LH treatment of cultures from which histone H2A had been removed resulted in an increase in accumulation of progesterone compared with control cultures treated throughout the treatment period with histone H2A. The third study examined the effect of 9-181 pg GnRH equivalents (1.7-34 micrograms protein) of a highly purified preparation of bovine ovarian histone H2A on basal and LH-induced progesterone production during 2 or 3 h of culture.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of an antigonadotrophic GnRH-like protein in the ovaries of diverse mammals.

A GnRH-like protein, detected with a receptor assay for GnRH in rat ovarian membranes, is present in ovarian extracts of the rat, human, cow and ewe at levels much higher than immunoreactive GnRH. This protein has been partly purified and characterized from bovine ovaries. Highest levels of this protein are found in granulosa cells, but substantial activity is present in luteal, thecal and stromal tissue. No detectable GnRH-like activity is found in follicular fluid, or in ovarian venous or jugular plasma. The GnRH-like protein is structurally and immunologically different from GnRH, but it competitively and reversibly inhibits binding of GnRH to high-affinity sites in rat ovarian membranes. The GnRH-like protein markedly inhibits the action of both LH and FSH in cultures of rat luteal and granulosa cells, respectively, to a much greater extent than GnRH. Co-elution of receptor-binding activity and antigonadotrophic activity occurs within and between several chromatography procedures, but the protein has not been completely purified. The presence of this antigonadotrophic GnRH-like protein in the ovaries of diverse species raises the possibility that it may represent a novel paracrine regulator of ovarian function.

Involvement of lipoxygenase products of arachidonic acid metabolism in bovine luteal function.

A series of experiments was conducted to determine the effects of lipoxygenase products of arachidonic acid (AA) metabolism on the function of the bovine corpus luteum (CL). In the first experiment, reaction products of soybean lipoxidase-AA were added to dispersed bovine luteal cells in increasing concentrations. These lipoxygenase products resulted in a dose-related reduction in the biosynthesis of progesterone and 6-keto-prostaglandin (PG)F1 alpha, while the synthesis of PGF2 alpha was unaffected. In a second experiment, the addition of 5-hydroxyeicosatetraenoic acid (5-HETE), a specific lipoxygenase product, again resulted in a reduction in progesterone and 6-keto-PGF1 alpha, with no change in PGF2 alpha synthesis. Extremely high endogenous concentrations of 5-HETE were measured in luteal tissues (36 +/- 17 to 46 +/- 13 ng/10(6) cells) in a third experiment. In the fourth experiment, an inhibitor of the lipoxygenase pathways, nordihydroguaiaretic acid (NDGA) infused into the uterine lumen twice daily on Days 14-18 of the estrous cycle delayed luteolysis and resulted in lengthened estrous cycles (27.2 +/- 0.3 vs 21.5 +/- 1.0 days for controls, p less than 0.05). Thus, an inhibitor of the lipoxygenase pathway of arachidonic acid metabolism delays luteolysis, possibly by removing the preferential inhibition of PGF1 alpha biosynthesis caused by 5-HETE and other products of the lipoxygenase system. Collectively, these results suggest that products of the lipoxygenase pathway are involved in luteolysis in normal heifers.

Role of luteal prostaglandins in the control of bovine corpus luteum functions.

Current and emerging concepts on regulation of bovine corpus luteum function by various metabolites of arachidonic acid are reviewed. A series of experiments are presented which support the concept that prostacyclin (PGI2), a metabolite of arachidonic acid via the cyclooxygenase pathway, plays a luteotropic role, and that products of the lipoxygenase pathway of arachidonic acid metabolism, particularly 5-hydroxyeicosatetraenoic acid (5-HETE), play a luteolytic role in the function of the bovine corpus luteum (CL). These ideas are supported by the following findings: injection of PGI2 directly into CL at mid-cycle produced a prolonged increase in peripheral plasma concentrations of progesterone; PGI2 stimulated synthesis of progesterone by dispersed luteal cells; synthesis of PGI2 by luteal cells was greatest during the period of early CL development (d 5 and 10), and diminished as the CL aged unless pregnancy ensued, causing a maintenance of the CL and synthesis of PGI2; administration of indomethacin, a blocker of synthesis of prostaglandin by the cyclooxygenase pathway, twice daily on d 4 to 6 of the estrous cycle inhibited CL development and caused a reduction in cycle length, suggesting the presence of a luteotropic prostaglandin; oxytocin administration twice daily on d 4 through 6 inhibited CL development and was accompanied by a 50% reduction in luteal synthesis of PGI2 by CL collected on d 7; large quantities of 5-HETE were found in luteal tissue; the addition of 5-HETE to dispersed luteal cells inhibited synthesis of progesterone and PGI2, while production of PGF2 alpha was unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of bovine luteal function by indomethacin.

Experiments were conducted to determine the effects of infusing indomethacin, a prostaglandin synthetase inhibitor, into the uterine lumen on the development and function of the bovine corpus luteum in the presence and absence of concurrently administered oxytocin. Each treatment was given twice daily on d 4, 5 and 6 of the estrous cycle. Treatments (six heifers/group) and resulting estrous cycle lengths were as follows: (1) untreated controls, 20.6 +/- .4 d; (2) .2 M phosphate buffer vehicle infused into the uterine lumen, 21.0 +/- .6 d; (3) 40 mg indomethacin infused into the body of the uterus, 16.5 +/- 1.0 d; (4) 150 USP units oxytocin injected sc, 10.0 +/- 1.2 d and (5) a combination of oxytocin and indomethacin as in treatments 3 and 4, 14.1 +/- 1.3 d. Plasma concentrations of progesterone were lower (P less than .05) in each treatment group from d 7 onward, when compared with untreated and vehicle-treated controls. Indomethacin alone effectively inhibited the development and function of the corpus luteum, and was without effect on oxytocin-induced inhibition of luteal function. In summary, it appears that a prostaglandin of either uterine or ovarian origin, or both, is required for the normal development and function of the bovine corpus luteum.

Prolongation of the bovine estrous cycle with a gonadotropin-releasing hormone analog.

A series of in vitro and in vivo experiments was conducted to determine the effects of gonadotropin-releasing hormone (GnRH) on bovine luteal function. Biosynthesis of progesterone by bovine luteal cells during a 2-h incubation was determined following addition of 0, 10, 20, 40, 80 and 100 ng GnRH. Synthesis of progesterone was significantly depressed only by the 100-ng dose of GnRH. Luteal cells were incubated with 0.1 and 1.0 microgram GnRH in the presence and absence of 0, 1, 2 and 5 ng bovine luteinizing hormone (LH). Again, only the highest dose of GnRH significantly depressed LH-stimulated production of progesterone. Three experiments were conducted to assess the effects of repetitively administered GnRH. In the first, twice daily intrauterine infusions of 100 micrograms GnRH on Days 12, 13 and 14 of the bovine estrous cycle was without effect on plasma concentrations of progesterone and the functional life span of the corpus luteum (CL). In the second experiment, 10 micrograms of a highly potent GnRH analog (GnRH-A) was injected subcutaneously four times daily on Days 9-12 of the estrous cycle. GnRH-A-treated heifers had longer (P less than 0.05) mean estrous cycle lengths (26.2 +/- 0.72 days) when compared to control heifers (20.25 +/- 0.25 days). Plasma concentrations of progesterone were higher in the GnRH-A-treated group on Days 9-13 and Days 15-22 of the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of flunixin meglumine on prostacyclin accumulation in the equine eye.

A study was performed to identify prostacyclin (PGI2) in equine aqueous humor, demonstrate an increase in PGI2 following anterior chamber paracentesis, and determine the effects of subconjunctival injection of flunixin meglumine on PGI2 accumulation. Twenty ponies were found to be normal upon ocular examination and were placed under general anesthesia. Anterior chamber paracentesis was performed in both eyes (right and left); immediately afterward, 25 mg of flunixin meglumine was injected (subconjunctivally) in the left eye. Aqueous paracentesis was performed 1.5 hours later (both eyes). Aqueous humor samples were assayed for 6-keto PGF1alpha, the stable, inactive metabolite of PGI2. Eyes treated with flunixin meglumine contained less 6-keto PGF1alpha than did control eyes.