The hour of transition into luteolysis in horses and cattle: a species comparison

Hourly blood sampling in both horses and cattle indicate that the transition between the end of preluteolysis and the beginning of luteolysis occurs within 1 h, as manifested by a change in progesterone concentrations. Each species presents a separate temporality enigma on the relationship between pulses of a prostaglandin (PG) F2α metabolite (PGFM) and the hour of the progesterone transition. In horses, relatively small pulses of PGFM occur during preluteolysis (before transition) and at transition. Oxytocin, but not estradiol, increases and decreases concomitantly with the small PGFM pulse at transition but not with previous pulses and may account for the initiation of luteolysis during the small PGFM pulse. In cattle, the last PGFM pulse of preluteolysis occurs hours before transition (e.g., 4 h), and the next pulse occurs well after transition (e.g., 9 h); unlike in horses, a PGFM pulse does not occur at transition. During the last PGFM pulse before transition, progesterone concentration decreases during the ascending portion of the PGFM pulse. Concentration then rebounds in synchrony with an LH pulse. The rebound returns progesterone to the concentration before the PGFM pulse. During luteolysis, an LH-stimulated progesterone rebound may occur after the peak of a PGFM pulse, but progesterone does not return to the concentration before the PGFM pulse. A similar LH-stimulated progesterone rebound does not occur in horses, and therefore progesterone fluctuations are more shallow in horses than in cattle.     

Loss of luteal sensitivity to luteinizing hormone underlies luteolysis in cattle: A hypothesis

By virtue of the secretion of progesterone (P4), corpus luteum (CL) is important not only for normal cyclicity but also for conception and continuation of pregnancy in female mammals. Luteolysis (also called luteal regression) is defined as loss of the capacity to synthesize and secrete P4 followed by the demise of the CL. There is strong evidence that sequential pulses of prostaglandin F2α (PGF) secreted from the uterus near the end of luteal phase induces luteolysis in farm animals. Loss of luteal sensitivity to luteinizing hormone (LH) at the end of menstrual cycle has been reported to be critical for initiation of luteolysis in primates, however this has not been investigated in farm animals. A closer observation of the published real-time profiles of circulating hormones (P4, LH, and PGF) and their inter-relationships around the time of the beginning of spontaneous luteolysis in cattle revealed- 1) A natural pulse of PGF causes a transient P4 suppression lasting a couple of hours followed by a rebound in P4 concentration, 2) The P4 secretions that occur in response to LH pulses before the beginning of luteolysis (i.e., preluteolysis) either fail or do so to a lesser extent during luteolysis indicating a loss of sensitivity to LH, and 3) The loss of sensitivity coincides with the beginning of luteolysis (i.e., transition), and apparently luteolysis does not initiate until there is loss of sensitivity to LH. The CL is sensitive to LH during preluteolysis, and the LH-stimulated P4-dependent and/or independent local survival mechanisms maintain the steroidogenic capability and viability of the CL until the very end of preluteolysis. Luteolysis does not appear to initiate with the PGF pulse(s) that occur during this period. With the loss of sensitivity to LH at the transition, however, a progressive decline in P4 begins initiating luteolysis. Also, the survival mechanisms become compromised making the CL less viable. The uterine PGF pulses that occur after the beginning of luteolysis induces increase in the local luteolytic factors, which contribute to further luteolysis, more importantly, structural luteolysis with ultimate demise of the CL. Therefore, I hypothesize that the loss of luteal sensitivity to LH underlies luteolysis in cattle. The hypothesis not only unifies the basic mechanism of luteolysis in a farm animal and primates but also provides a perspective to view luteolysis as a process rather than a factor-mediated event. A novel unified working model for luteolysis in a farm animal and primates is described. A better understanding of the luteal physiology including how responsiveness to LH diminishes in aging CL would help in the development of novel strategies in modulating CL structure-function to improve and/or control fertility in humans as well as in animals.     

Role of luteinizing hormone in changes in concentrations of progesterone and luteal blood flow during the hours of a simulated pulse of 13,14-dihydro-15-keto-prostaglandin F(2alpha) (PGFM) in heifers

A bolus treatment (e.g., 25 mg) of prostaglandin F(2alpha) (PGF) in the study of luteolysis in cattle results in dubious interpretations. Therefore, in experiment 1 of the present study, a 13,14-dihydro-15-keto-PGF (PGFM) pulse was simulated by incremental intrauterine (IU) infusion of PGF for 2.7 h on Day 14 postovulation. Concentrations of PGFM during the first hour of infusion and at the maximum were not different between simulated (n = 7) and spontaneous (n = 7) pulses. In experiment 2, four groups (n = 6 per group) were treated at Minute 0 (beginning of infusion) as follows: saline (infused IU), PGF (infused IU), acyline/saline, and acyline/PGF. Two hours before Minute 0, each heifer was given flunixin meglumine to inhibit endogenous PGF secretion, and heifers in the acyline/saline and acyline/PGF groups were given acyline to inhibit luteinizing hormone (LH). Plasma progesterone concentrations were similar among groups during Minutes 0 to 60, with no indication of an initial transient progesterone increase in the two PGF groups. Progesterone began to decrease in the PGF groups at Minute 60 and to rebound at Minute 135 after the PGFM peak at Minute 120. The rebound was complete in association with an increase in LH in the PGF group, but it was not complete when LH was inhibited in the acyline/PGF group. Luteal blood flow increased during PGF infusion in the two PGF groups and remained elevated for approximately 2 h after the PGFM peak in the PGF group but not in the acyline/PGF group. Novel findings were that an initial transient increase in progesterone did not occur with the simulated PGFM pulse and that LH stimulated a progesterone rebound and maintained the elevated luteal blood flow after the PGFM peak.     

The transition between preluteolysis and luteolysis in cattle

Novel characterization of the transition between preluteolysis and luteolysis was done in seven heifers. Blood samples were collected hourly and assayed for progesterone (P4), 13-14-dihydro-15-keto-PGF2α (PGFM), and estradiol (E2). The peaks of P4 oscillations were used to designate the transitional hour in each heifer. The interval from the peak of the last PGFM pulse of preluteolysis to the peak of the first pulse during luteolysis (transitional period) was longer (P < 0.0001) than the interval between the first and second pulses during luteolysis (13.4 ± 1.3 h vs. 7.0 ± 0.9 h). The long intervals from the last PGFM pulse of preluteolysis to the transitional hour (4.0 ± 0.9 h) and from the transitional hour to the first PGFM pulse of luteolysis (9.4 ± 1.3 h) resulted in the illusion that the beginning of luteolysis was not associated temporally with a PGFM pulse. The E2 and PGFM concentrations were less (P < 0.05) during the last PGFM pulse of preluteolysis than during the first pulse of luteolysis. Concentration of P4 was suppressed at the peak of the last PGFM pulse of preluteolysis and consistently rebounded at the transitional hour to the concentrations before the PGFM pulse. In four of seven heifers, one or two P4 rebounds occurred between the peak of the PGFM pulse and the rebound at the transitional hour. Results indicated that the prolonged transitional period may be related, at least in part, to increasing concentration of E2, intervening P4 rebounds between the peak of the last PGFM pulse of preluteolysis and the transitional hour, and the complete P4 rebound at the transitional hour.     

Pulsatile output of prostaglandin F(2alpha) does not increase around the time of luteolysis in the pregnant goat.

Prostaglandin (PG) F(2alpha) secreted from the uterus is the luteolysin of the estrous cycle and is also believed to be responsible for luteolysis in the pregnant doe at term. We have reported that basal progesterone concentrations decrease before basal PGF(2alpha) concentrations increase, which is inconsistent with this view. In this study we investigated whether luteolysis is associated with increased frequency or amplitude of pulsatile PGF(2alpha) secretion in does over the last 2 wk of gestation. Progesterone concentrations decreased approximately 1 wk before parturition. There was no accompanying increase in PGF(2alpha) concentrations or pulse frequency, and those pulses that were observed were of lesser amplitude and duration than those that have been associated with luteolysis in cycling ewes. A small increase in PGF(2alpha) pulse frequency was identified during the 3 days before parturition, but this was not associated with any change in progesterone concentrations. The biological significance of these small changes in PGF(2alpha) pulse frequency is obscure, although the high concentration of this eicosanoid at labor may have been related to the final, precipitous decline in plasma progesterone concentrations. These findings do not support the notion that PGF(2alpha) is the principal luteolysin in the pregnant doe at term.     

Luteinizing hormone receptors and progesterone content in porcine corpora lutea after prostaglandin F2 alpha

The effect of prostaglandin F2 alpha (PGF2 alpha) on luteinizing hormone (LH) receptors, weight and progesterone content of corpora lutea (CL), and serum progesterone concentrations was studied in gilts. Fifteen gilts were hysterectomized between Days 9 to 11 of the estrous cycle. Twelve gilts were injected i.m. with 10 mg of PGF2 alpha and 3 with saline on Day 20. Ovaries were surgically removed from each of 3 gilts at 4, 8, 12 and 24 h following PGF2 alpha treatment and from the 3 control gilts 12 h following saline injection. Jugular blood samples for progesterone analysis were collected from all gilts at 0, 2 and 4 h following treatment and at 8, 12 and 24 h for gilts from which ovaries were removed at 8, 12 and 24 h, respectively. Mean serum progesterone and CL progesterone concentrations decreased within 4 h after PGF2 alpha treatment (P less than 0.05) and remained low through 24 h after treatment. The number of unoccupied LH receptors decreased by 4 h (P less than 0.05) and this trend continued through 24 h. There were no differences in luteal weight or affinity of unoccupied LH receptors of luteal tissue at 4, 8 12 and 24 h after PGF2 alpha when compared to luteal tissue from controls. These data indicate that during PGF2 alpha-induced luteolysis in the pig, luteal progesterone, serum progesterone concentrations and the number of LH receptors decrease simultaneously.     

Failure of exogenous LH to prevent PGF2alpha-induced luteolysis in beef cows

Henderson and McNatty (Prostaglandins 9:779, 1975) proposed that LH from the preovulatory LH surge attached to receptors on luteal cells and that this attachment might protect the early corpus luteum from PGF2alpha induced luteolysis. To test this hypothesis, experiments were performed on heifers at day 10-12 of the cycle. Both jugular veins were catheterized and infusions of either saline (0.64 ml/min) or LH-NIH-B9 (10 microgram/min; 0.64 ml/min) were given. Saline infusions were from 0-12 h; LH infusions were for 10 h and were preceded by a 2 h saline infusion. All animals were given 25 mg PGF2alpha im at 6 h (6 h into the saline infusion and 4 h into the LH infusion). Blood samples were taken at 0.5 h, 1 h and 4 h intervals from 0-12h, 13-18 h and 12-42 h respectively. Serum was assayed for LH and progesterone by radioimmunoassay methods. Two animals received saline and two received LH in each experiment. Each treatment was replicated 6 times. LH infusion resulted in a mean serum LH of 75 ng/ml compared to 0.90 ng/ml in saline infused animals. This elevation of LH did not alter PGF2alpha induced luteolysis as indicated by decline in serum progesterone. This experiment does not support the hypothesis that the newly formed corpus luteum is resistant to PGF2alpha because of protection afforded by the proestrus LH surge.     

Luteolysis in the hamster: abrogation by gonadotropin and prolactin pretreatment

The luteolysis which terminated pseudopregnancy (PSP) in superovulated hamsters was studied. Spontaneous luteolysis occurred before 1100 on Day 7 of PSP and was characterized by a rapid decline in circulating progesterone levels. Luteolysis induced by prostaglandin F2 alpha (PGF2 alpha) on Day 5 of PSP displayed a similar rapid reduction in progesterone over 24 hours. In both cases levels of the progesterone metabolite 20 alpha hydroxypregn-4-ene-3-one (20 alpha-OHP) were less than 2 percent of progesterone levels and declined in a manner similar to progesterone. This suggests that conversion of progesterone or its precursors to 20 alpha-OHP was not a functional aspect of luteolysis in the hamster. Pretreatment with either prolactin (PRL), luteinizing hormone (LH) or follicle stimulating hormone (FSH) failed to prevent PGF2 alpha-induced luteolysis on Day 5 in the superovulated PSP hamster. Combinations of PRL and LH, LH and FSH or PRL and FSH were also unsuccessful in abrogating luteolysis. However, pretreatment with a combination of PRL, FSH and LH prevented luteolysis in 11/14 animals. These results suggest that luteotropic agents can reverse the luteolytic effects of PGF2 alpha in the hamster.     

Prostaglandin E1 and F2alpha specific binding in bovine corpora lutea: comparison with luteolytic effects.

Preliminary characterization indicated the presence of separate prostaglandin (PG)E1 and (PG)F2alpha binding sites in membrane fractions prepared from bovine corpora lutea. These differ in the rate and temperature dependence of the specific binding. Equilibrium binding data indicate the apparent dissociation constants as 1.32 x 10(-9)M and 1.1 x 10(-8)M for PGE1 and PGF2alpha, respectively. Competition of several natural prostaglandins for the PGE1 and PGF2alpha bovine luteal specific binding sites indicates specificity for the 9-keto or 9alpha-hydroxyl moiety, respectively. Differences in relative ability to inhibit 3H-PG binding were found due to sensitivity to the absence or presence of the 5, 6-cis-double bond as well. Bovine luteal function was affected following treatment of heifers with 25 mg PGF2alpha as measured by reduced estrous cycle length, decreased corpus luteum size and significantly decreased plasma progesterone levels. In contract, treatment with 25 mg PGE1 resulted in cycle lengths comparable to those of non-treated herdmates with no apparent modification in corpus luteum size. However, plasma progesterone levels were increased significantly following PGE1 treatment compared to pretreatment values. In so far as data obtained in vitro on PGF2alpha relative binding affinity to the bovine CL can be compared to data obtained independently in vitro on PGF2alpha induced luteolysis in the bovine, PGF2alpha relative binding to the CL and luteolysis appeared to be associated. By similar reasoning, there was no apparent relationship between PGE1 relative binding affinity in the luteal fractions and luteolysis in estrous cyclic cattle.     

Involvement of endothelin-1 and its receptors in PGF2alpha-induced luteolysis in the rat

The possible mediatory role of endothelin-1 (ET-1) in prostaglandin F(2alpha) (PGF(2alpha))-induced luteolysis in the rat was examined. The effect of PGF(2alpha) was tested on day 9 of pregnancy either in vivo, by injecting cloprostenol, an analog of PGF(2alpha) or in vitro, in isolated intact corpora lutea incubated with PGF(2alpha). Luteolysis was confirmed by progesterone determination in the peripheral blood serum or in the culture medium, respectively. Administration of cloprostenol (.0025 mg/rat) induced within 1 hr, a significant fall (from 56.8 to 27.6 ng/ml, P < 0.0001) in serum progesterone concentrations that was associated with an increased expression of the mRNA to ET-1 and its protein product in rat luteal tissue. Elevated level of ET-1 were also determined at the spontaneous regression of the CL, upon parturition. Expression of the ET receptors, ETA and ETB was not affected by cloprostenol. On the other hand, this PGF(2alpha) analog induced expression of luteal VEGF mRNA. In vitro experiments demonstrate that the LH (100 ng/ml)-induced increase in luteal progesterone secretion was reduced by PGF(2alpha) (1 microg/ml). The inhibitory effect of PGF(2alpha) was reversed by BQ123 (10(- 7) M), that is a selective ETA receptor antagonist. We conclude that the PGF(2alpha)-induced elevation in luteal expression of ET-1 combined with the reversal of its luteolytic effect by an ETA receptor antagonist suggest that ET-1 may take part in the PGF(2alpha)-induced luteolysis in the rat.     

Influence of luteinizing hormone and prostaglandin F(2alpha) on progesterone secretion in superfused minced bovine luteal tissue in the early stage of the estrous cycle

Minced luteal tissue of bovine corpora lutea from Day 4, 5, and 6 of the estrous cycle (n = 4 corpora lutea each) was superfused for 9 h, and the progesterone secretion under the influence of 100 ng luteinizing hormone (LH)/ml and/or 1,000 ng prostaglandin F(2alpha) (PGF(2alpha))/ml was determined. In vivo, this period of the estrous cycle is characterized by a transition from PGF(2alpha) refractoriness to PGF(2alpha) sensitivity. The investigations were carried out in order to examine whether this transition is reflected by a change in the hormone secretion pattern in vitro. The basal secretion was higher on Day 6 than on Day 4 and 5 (P < 0.01). PGF(2alpha) slightly increased the progesterone secretion, but there was no statistically significant difference (P > 0.05). LH, however, stimulated the progesterone secretion by about 30% in luteal tissue collected from Day 4 and 5 (P < 0.01). In luteal tissue collected from Day 6, the LH-induced increase in hormone secretion was not statistically significant due to two corpora lutea that showed no response at all to LH. The progesterone secretion of the two other corpora lutea, however, was increased by 30% (P < 0.01). When PGF(2alpha) and LH were simultaneously added, the LH-induced progesterone secretion was not inhibited; PGF(2alpha) even seemed to intensify the action of LH. The difference between the hormone secretion under the influence of LH alone and that under the influence of a combination of LH and PGF(2alpha), however, was not statistically significant. It is concluded that in cattle the end of the refractoriness to PGF(2alpha) in vivo is not reflected by a corresponding change of the hormone secretion pattern in vitro.     

In vivo oxytocin release from microdialyzed bovine corpora lutea during spontaneous and prostaglandin-induced regression

The release of luteal oxytocin during spontaneous and prostaglandin-induced luteolysis was investigated in cows. A continuous-flow microdialysis system was used in 11 cows to collect dialysates of the luteal extracellular space between Days 12 and 24 postestrus. Seven cows were untreated and were expected to exhibit spontaneous luteolysis during sampling, whereas 4 cows received prostaglandin F(2alpha) (PGF(2alpha)) systemically between Days 13 and 15 to induce luteolysis during sampling. Oxytocin was detectable in the dialysate of all cows before Day 16 postestrus and occurred as 2 or 3 discrete pulses per 12-h sampling period. For non-PGF(2alpha)-treated cows, dialysate oxytocin content began to decline spontaneously on Day 15 postestrus and was undetectable by Day 17 postestrus. Oxytocin decay curves preceded onset of serum progesterone decline by at least 72 h and were not related temporally with onset of progesterone decline within cow. Exogenous PGF(2alpha) (25 mg, i.m.) produced a 10-fold increase in dialysate oxytocin within 1 h (1.9 +/- 0.3 pg/ml to 20.8 +/- 3.0 pg/ml; P < 0. 01). Dialysate oxytocin then declined to pretreatment concentrations within 2 h and was undetectable within 8 h posttreatment. A second PGF(2alpha) injection given 20 h after the first did not result in a measurable increase in dialysate oxytocin, probably because luteolysis was underway. Although robust luteal oxytocin release was observed after treatment with a pharmacological dose of PGF(2alpha), the lack of detectable oxytocin secretion during spontaneous luteolysis suggests that the contribution of luteal oxytocin in the cow may be less than that proposed for the ewe.     

Destruction of bovine ovarian follicles: effects on the pulsatile release of luteinizing hormone and prostaglandin F2 alpha-induced luteal regression

Destruction of ovarian follicles during diestrus prolongs the lifespan of corpora lutea in cows, but the site(s) of action is unclear. Thus, ovarian follicles were destroyed in 10 beifers (X-IRRAD) on Day 9 postestrus, while 10 additional beifers (SHAM) served as a control group. To investigate changes in luteotropic support resulting from destruction of ovarian follicles, pulses of luteinizing hormone (LH) were characterized on Days 8, 13, and 15 postestrus. To study the interaction between products from ovarian follicles and prostaglandin F2 alpha (PGF2 alpha) in luteolysis, changes in serum concentrations of progesterone were monitored after an injection of saline or PGF2 alpha on Day 14 postestrus. Frequency and amplitude of pulses of LH increased by Day 13 in X-IRRAD beifers. An increase of similar magnitude in amplitude but not frequency of pulses of LH occurred between Day 13 and Day 15 postestrus in SHAM beifers. Exogenous PGF2 alpha was significantly less efficacious in causing luteolysis in X-IRRAD animals. We suggest that increased luteotropic support may be involved in but is not the only cause for lengthening the lifespan of corpora lutea following destruction of ovarian follicles. Additionally, we suggest that regression of bovine corpora lutea involves a synergistic action between products from ovarian follicles and PGF2 alpha.     

Effect of prostaglandin E2 alpha on the hCG-stimulated progesterone production by human corpora lutea

The effect of prostaglandin PGF2 alpha on the hCG stimulated and basal progesterone production by human corpora lutea was examined in vitro. hCG (40 i.u./ml) stimulated progesterone formation in corpora lutea of early (days 16-19 of a normal 28 day cycle), mid (days 20-22) and late (days 23-27) luteal phases. This stimulation was inhibited by PGF2 alpha (10 micrograms/ml) in corpora lutea of mid and late luteal phases. PGF2 alpha alone did not show a consistent effect on basal progesterone production. The inhibition of hCG stimulated progesterone production by PGF2 alpha at times corresponding to luteolysis indicates a role for that prostaglandin in the process of luteolysis in the human corpus luteum.     

Growth hormone, but not luteinizing hormone, acts with luteal peptides on prostaglandin F2alpha and progesterone secretion by bovine corpora lutea in vitro*

Prostaglandin F2alpha (PGF2alpha) is a major physiological luteolysin in the cow. However, injection of PGF2alpha before day 5 (day 0 = estrus) of the estrous cycle dose not induce luteolysis. On the other hand, the early corpus luteum (CL) actively produces PGF2alpha. This indicates that luteal PGF2alpha may play a key role in the refractoriness to PGF2alpha injected during the early luteal phase when angiogenesis is active in the CL. Thus, this study aimed to investigate the possible interaction between pituitary hormones and local factors (luteal peptides) on secretion of PGF2alpha and progesterone (P) by the early bovine CL, and to evaluate the effect of growth hormone (GH) as well as its interactions on production of PGF2alpha in the developing CL. A RT-PCR analysis revealed that mRNA for GH receptor in CL was fully expressed from early in the luteal phase throughout the estrous cycle, while luteinizing hormone (LH) receptor mRNA was expressed less by the early and regressing CL than those at mid or late luteal phases (P < 0.05). For the stimulation test, an in vitro microdialysis system (MDS) was used as a model. Each bovine early CL (days 3-4) was implanted with the MDS, and maintained in an organ culture chamber. The infusion of GH, insulin-like growth factor-1 (IGF-1) and oxytocin (OT) increased (P < 0.05) PGF2alpha and P release. In contrast, LH had no effect (P > 0.05) on PGF2alpha secretion and little effect on P release. Unexpectedly, there was no distinct interaction between pituitary hormones and luteal peptides on secretion of PGF2alpha and P. These results indicate that GH is a more powerful stimulator of PGF2alpha and P production in the early bovine CL than LH and suggest that GH and luteal peptides, IGF-1 and OT, contribute to maintenance of elevated PGF2alpha production in the developing bovine CL.     

Growth hormone, but not luteinizing hormone, acts with luteal peptides on prostaglandin F2alpha and progesterone secretion by bovine corpora lutea in vitro

Prostaglandin F2alpha (PGF2alpha) is a major physiological luteolysin in the cow. However, injection of PGF2alpha before day 5 (day 0 = estrus) of the estrous cycle dose not induce luteolysis. On the other hand, the early corpus luteum (CL) actively produces PGF2alpha. This indicates that luteal PGF2alpha may play a key role in the refractoriness to PGF2alpha injected during the early luteal phase when angiogenesis is active in the CL. Thus, this study aimed to investigate the possible interaction between pituitary hormones and local factors (luteal peptides) on secretion of PGF2alpha and progesterone (P) by the early bovine CL, and to evaluate the effect of growth hormone (GH) as well as its interactions on production of PGF2alpha in the developing CL. A RT-PCR analysis revealed that mRNA for GH receptor in CL was fully expressed from early in the luteal phase throughout the estrous cycle, while luteinizing hormone (LH) receptor mRNA was expressed less by the early and regressing CL than those at mid or late luteal phases (P < 0.05). For the stimulation test, an in vitro microdialysis system (MDS) was used as a model. Each bovine early CL (days 3-4) was implanted with the MDS, and maintained in an organ culture chamber. The infusion of GH, insulin-like growth factor-I (IGF-I) and oxytocin (OT) increased (P < 0.05) PGF2alpha and P release. In contrast, LH had no effect (P > 0.05) on PGF2alpha secretion and little effect on P release. Unexpectedly, there was no distinct interaction between pituitary hormones and luteal peptides on secretion of PGF2alpha and P. These results indicate that GH is a more powerful stimulator of PGF2alpha and P production in the early bovine CL than LH and suggest that GH and luteal peptides, IGF-1 and OT, contribute to maintenance of elevated PGF2alpha production in the developing 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.     

Effects of progesterone administered to dairy heifers on sensitivity of corpora lutea to PGF(2)alpha and on plasma LH concentration

To determine whether progesterone facilitates PGF(2)alpha-induced luteolysis prior to day 5 of the estrous cycle, 48 Holstein-Friestian heifers were assigned at random to four treatments: 1) 4 ml corn oil/day + 5 ml Tris-HCl buffer (control); 2) 25 mg prostaglandin F(2)alpha (PGF(2)alpha); 3) 100 mg progesterone/day (progesterone); 4) 100 mg progesterone/day + 25 mg PGF(2)alpha (combined treatment). Progesterone was injected subcutaneously daily from estrus (day 0) through day 3. The PGF(2)alpha was injected intramuscularly on day 3. Estrous cycle lengths were decreased by progesterone: 20.2 +/- 0.56, 19.2 +/- 0.31 (control and PGF(2)alpha); 13.2 +/- 1.40, and 11.7 +/- 1.27 (progesterone and combined). The combination of progesterone and PGF(2)alpha did not shorten the cycle any more than did progesterone alone (interaction, P>0.05). PGF(2)alpha treatment reduced progesterone concentrations on day 6 (P<0.05) and both progesterone and PGF(2)alpha reduced plasma progesterone on day 8 (P<0.01 and P<0.05, respectively). LH was measured in blood samples collected at 10- min intervals for 4 hr on day 4 from three heifers selected at random from each of the four treatment groups. Mean LH concentration for control heifers ranged from 0.35 to 0.63 ng/ml (overall mean, 0.49 ng/ml) and for progesterone-treated heifers ranged from 0.12 to 0.30 ng/ml (overall mean, 0.23 ng/ml). LH concentrations were greater in control heifers (P<0.01). The mean LH pulse rate for control heifers was 2.7 pulses/heifers/4 hr, while that for the progesterone-treated heifers was 1.7 pulses/heifer/4 hr. The mean pulse amplitude for control and progesterone treatments was 0.47 ng/ml and 0.36 ng/ml, respectively. Neither pulse amplitude nor frequency were different between treatment groups.     

Effects of in vivo and in vitro administration of prostaglandin F2 alpha on lipoprotein utilization in cultured bovine luteal cells

Two experiments were conducted to determine if a loss in the ability to utilize lipoprotein-cholesterol is one mechanism whereby prostaglandin F2 alpha (PGF2 alpha) decreases steroidogenesis in bovine luteal cells. In the first experiment, serum-free cultures of bovine luteal cells were treated with PGF2 alpha (100 ng/ml) for 5 days prior to addition of lipoproteins. Exposure to PGF2 alpha completely suppressed low-density lipoprotein (LDL)- and high-density lipoprotein (HDL)-stimulated progesterone production (p less than 0.01) compared to control (no PGF2 alpha) cultures. Luteal cells cultured in the presence of LDL + luteinizing hormone (LH, 10 ng/ml) + PGF2 alpha produced significantly less progesterone than luteal cells cultured with LDL + LH (p less than 0.05). Treatment with PGF2 alpha had no significant effect on HDL + LH-stimulated progesterone synthesis. In the second experiment, cows were injected with a luteolytic dose of PGF2 alpha (25 mg), and the corpora lutea were removed at 0 (no PG), 1, 4, or 12 h post-injection. Dissociated luteal cells were placed in culture for 7 days, either with or without LH (10 ng/ml), and lipoproteins were added on Days 5-7. LH stimulation of progesterone production was apparent in cultures obtained at 0 and 12 (p less than 0.05) but not 1 and 4 h post-PGF2 alpha. Addition of either LDL or HDL increased progesterone synthesis in all cultures, regardless of time following in vivo administration of PGF2 alpha. It is concluded that PGF2 alpha can inhibit bovine luteal cell utilization of either LDL or HDL in vitro. However, luteal cell utilization of lipoproteins in vitro is not adversely affected by in vivo exposure to PGF2 alpha, if collected within 12 h post-PGF2 alpha.     

Temporal interrelationships at 15-min intervals among oxytocin, LH, and progesterone during a pulse of a prostaglandin F2α metabolite in heifers

A pulse of a PGF2α metabolite (PGFM) was induced by treatment with 0.1 mg of estradiol-17β on Day 15 (Day 0=ovulation; n=9 heifers). Blood samples were taken every 15 min for 9h beginning at treatment (Hour 0). For PGFM and LH, an intraassay-CV method was used to detect fluctuations in the 15-min samples and pulses in the hourly samples. A mean of 6.9 ± 0.4 PGFM fluctuations/9 h were superimposed on the hourly PGFM concentrations, compared to 2.1 ± 0.5 LH fluctuations/9 h (P<0.02). An increase (P<0.02) in oxytocin began 15 min before the beginning nadir of the PGFM pulse. A transient increase in progesterone did not occur at the beginning nadir of the PGFM pulse. Progesterone decreased (P<0.02) during the ascending portion and increased (P<0.03) as a rebound during the descending portion of the PGFM pulse. The peak of an LH pulse occurred 1.5 ± 0.4 h (range, 0.25-2.75 h) after the peak of the PGFM pulse. The wide range in the interval from a PGFM peak to an LH peak obscured the contribution of increasing LH to the rebound. The results did not support the hypothesis that oxytocin and PGFM increase concurrently. Results supported the hypothesis that the immediate transient progesterone increase that has been demonstrated with exogenous PGF2α does not occur during the ascending portion of an endogenous PGFM pulse. The hypothesis that the progesterone rebound after the peak of a PGFM pulse is temporally related to an LH pulse was supported.