The effect of prostaglandin F2 alpha during various stages of the oestrous cycle of beef heifers

Review: Luteal prostaglandins: mechanisms regulating luteal survival and demise in ruminants

The corpus luteum (CL) is critical for establishment and maintenance of pregnancy in all mammals. However, the fate of the CL in ruminants is dependent on the presence of a functional uterus or signals from a developing embryo to modify uterine function to ensure its own survival. The key molecule secreted by the uterus that must be modified is prostaglandin F2alpha (PGF2A). At the same time, there is evidence that mechanisms within the CL may influence the ability of PGF2A to cause luteolysis. This review focuses on prostaglandins and steroidogenic capacity as endogenous modulators of the sensitivity of the CL to exogenous PGF2A. Early luteal development and early pregnancy are two different luteal stages in which sensitivity of the CL to PGF2A renders it incapable, or less capable, respectively, of undergoing luteolysis in response to PGF2A compared to a midcycle CL. An analysis of molecular changes that occur during these two stages provides some novel insight into molecules and pathways worth exploring to explain the regulation of luteolytic capacity in corpora lutea of ruminants.

Comparison of the initial ovarian response, the synchrony of oestrus and ovulation and chronic stress response after administration of 100 or 250 μg of GnRH to randomly cycling Bos indicus cattle

OBJECTIVE

This study investigated the effects of administering saline, 100 or 250 μg of gonadotrophin releasing hormone (GnRH) on ovarian response, synchrony of oestrus and ovulation and chronic stress response in Bos indicus cattle.

DESIGN

Randomised control.

METHODS

Animals were either left untreated (n = 20) or on day 0 treated with an intravaginal progesterone releasing device and either saline (n = 24), 100 μg (n = 35), or 250 (n = 35) μg of GnRH, intramuscular (IM). Blood was sampled 1.4 h after administration of treatment to monitor concentrations of luteinising hormone (LH) and P4 in serum and again 5 days later. On day 5 intravaginal P4 releasing device were removed, cloprostenol was administered IM and again 8 h later. Oestrus and ovulation were then monitored with ultrasonography for 6.5 days. Hair was clipped on day 55 for analysis of hair cortisol concentrations (HCC).

RESULTS

No significant differences were found between Saline and GnRH treatments in the odds of inducing a new corpus luteum (CL) and the synchrony of oestrus or ovulation. HCC did not differ significantly between treatments. Mean concentrations of LH in serum on day 0 were less in the Saline compared to 100 and 250 μg GnRH treatments but did not differ between different doses of GnRH.

CONCLUSION

Mean concentrations of LH and the odds of inducing a new CL were not increased after administering 250 μg compared to 100 μg of GnRH. Animal handling events in the study did not influence HCC. Further research is needed to better optimise responses to GnRH in B. indicus cattle.

Effect of PGF2α treatments during early corpus luteum development on circulating progesterone concentrations and ovulation in breeding-age Holstein heifers

The objective of this study was to test the effect of low circulating concentrations of progesterone (P4) on pre-ovulatory follicle development in heifers as part of an overarching objective to develop a model to understand this phenomenon in dairy cattle without the confounding factors of lactation. Holstein heifers between 12 and 13 mo of age were pre-synchronized to ensure all heifers were on d 6 of the estrous cycle at the start of the Ovsynch program. Only heifers with CL regression and ovulation to the following pre-treatment strategy were used in the study: 0.5 mg cloprostenol (PGF_2α), 2 d later, 0.1 mg GnRH, 6 d later, GnRH (G1; 1st GnRH of Ovsynch). Heifers (n = 159) responding to pre-treatment were randomly assigned to 4 groups and completed the Ovsynch program: high P4 control (HPC), low P4 control (LPC; PGF_2α 24 h after G1), PG2 (PGF_2α 24 and 48 h after G1) and PG3 (PGF_2α 24, 48, and 96 h after G1). Only heifers that had ovulation to G1 remained in the study. Blood samples were collected in all heifers on d 7 (n = 157) and in a subset of heifers on d 1, 2, 3, 4 (n = 82) after G1 to measure serum P4. Pre-ovulatory follicle size at G1 (13.0 ± 0.1 mm; P = 0.53) and mean serum P4 24 h after G1 (d 1; 3.62 ± 0.11 ng/mL; P = 0.46) did not differ among treatments. HPC heifers had greater (P < 0.001) mean serum P4 compared to LPC, PG2 and PG3 on d 2, 3, 4, and 7. On d 2, 3 and 4, mean serum P4 of LPC, PG2 and PG3 heifers did not differ (P > 0.10). On d 7, LPC heifers had greater (P < 0.001) serum P4 compared to PG2 and PG3 heifers. Mean ± SEM serum P4 on d 7 after G1 was 8.43 ± 0.39, 2.55 ± 0.36, 1.58 ± 0.20, and 1.21 ± 0.15 ng/mL for HPC, LPC, PG2, and PG3, respectively. Percentage of heifers with P4 < 0.50 ng/mL on d 7 was greater (P < 0.05) for LPC, PG2 and PG3 (27, 32 and 26%, respectively) compared to HPC (0%). A greater (P < 0.05) proportion of heifers ovulated before G2 in the LPC, PG2 and PG3 than in the HPC. For heifers that ovulated after G2, low serum concentrations of P4 in LPC, PG2 and PG3 induced double ovulations in 6/97 heifers after the final GnRH of Ovsynch compared to 0/33 in HPC. In summary, PGF_2α treatments during early CL development reduced circulating P4 concentrations 7 d after G1 compared with both HPC and LPC. However, it did not effectively control CL and follicle function to be utilized as a model to test high vs. low serum P4 on fertility parameters in Holstein heifers.

Treatment with prostaglandin F2α and an intravaginal progesterone insert promotes follicular maturity in advance of gonadotropin-releasing hormone among postpartum beef cows

An experiment was designed to evaluate treatments to promote ovarian follicular maturity in advance of administration of exogenous gonadotropin-releasing hormone (GnRH; 100 μg gonadorelin) for control of the bovine estrous cycle. We hypothesized prostaglandin F_2α (PGF_2α; 500 μg cloprostenol) followed by an intravaginal progesterone-releasing insert (CIDR; 1.38 g progesterone) would induce greater follicle size and serum estradiol at the time of GnRH administration. Postpartum cows (n = 194) in two locations were assigned to one of five treatments based on age, days postpartum, and body condition score. Cows in Treatment 1 were treated with the standard 7-d CO-Synch + CIDR protocol: administration of GnRH and CIDR insertion on Day -10, and administration of PGF_2α and CIDR removal on Day -3. Treatments 2-5 were designed in a 2 × 2 factorial arrangement, with Treatment 1 included as an additional reference. On Day -17, cows in Treatments 2-5 received a CIDR insert, either with (Treatments 2 and 3) or without (Treatments 4 and 5) administration of PGF_2α at CIDR insertion. On Day -10, all cows were administered GnRH, and CIDR inserts were either removed (Treatments 2 and 4) or remained in place until Day -3 (Treatments 3 and 5). Treatment with PGF_2α and CIDR in advance of GnRH (Treatments 2 and 3) resulted in increased diameter of the largest ovarian follicle (P < 0.001) and increased serum concentrations of estradiol (P < 0.0005) on Day -10. In addition, variation among cows in CL status (no CL vs. a single CL vs. multiple CL) on Day -3 tended to be decreased (P = 0.08), with cows more likely to have a single CL rather than no CL or multiple CL. Lastly, the proportion of cows expressing estrus prior to fixed-time artificial insemination tended (P = 0.08) to be improved. Results support the hypothesis that administration of PGF_2α and treatment with a CIDR for 7 days prior to GnRH promotes follicular maturity in advance of GnRH administration and may provide an approach by which to enhance response of postpartum beef cows to GnRH-based estrus synchronization programs.

Luteolysis and the Auto-, Paracrine Role of Cytokines From Tumor Necrosis Factor α and Transforming Growth Factor β Superfamilies

Successful pregnancy establishment demands optimal luteal function in mammals. Nonetheless, regression of the corpus luteum (CL) is absolutely necessary for normal female cyclicity. This dichotomy relies on intricate molecular signals and rapidly activated biological responses, such as angiogenesis, extracellular matrix (ECM) remodeling, or programmed cell death. The CL establishment and growth after ovulation depend not only on the luteinizing hormone-mediated endocrine signal but also on a number of auto-, paracrine interactions promoted by cytokines and growth factors like fibroblast growth factor 2, vascular endothelial growth factor A, and tumor necrosis factor α (TNF), which coordinate vascularigenesis and ECM reorganization as well as steroidogenesis. With the organ fully developed, the release of the uterine prostaglandin F2α activates luteolysis, an intricate process supported by intraluteal interactions that ensure the loss of steroidogenic function (functional luteolysis) and the involution of the organ (structural luteolysis). This chapter provides an overview of the local action of cytokines during luteal function, with particular emphasis on the role of TNF and transforming growth factor β superfamilies during luteolysis.

A resynchronization of ovulation program based on ovarian structures present at nonpregnancy diagnosis reduced time to pregnancy in lactating dairy cows

Our objective was to evaluate time to pregnancy after the first service postpartum and pregnancy per artificial insemination (P/AI) in dairy cows managed with 2 resynchronization of ovulation programs. After first service, lactating Holstein cows were blocked by parity (primiparous vs. multiparous) and randomly assigned to the d 32 Resynch (R32; n = 1,010) or short Resynch (SR; n = 1,000) treatments. Nonpregnancy diagnosis (NPD) was conducted 32 ± 3 d after AI by transrectal ultrasonography. Nonpregnant cows in R32 received the Ovsynch protocol: GnRH, PGF_2α 7 d later, GnRH 56 h later, and timed AI (TAI) 16 to 18 h later. Cows in SR with a corpus luteum (CL) ≥15 mm and a follicle ≥10 mm at NPD received PGF_2α, PGF_2α 24 h later, GnRH 32 h later, and TAI 16 to 18 h later. Cows in SR without a CL ≥15 mm or a follicle ≥10 mm at NPD received a modified Ovsynch protocol with 2 PGF_2α treatments and progesterone (P4) supplementation (GnRH plus CIDR, PGF_2α and CIDR removal 7 d later, PGF_2α 24 h later, GnRH 32 h later, and TAI 16 to 18 h later). Blood samples were collected from a subgroup of cows at the GnRH before TAI (R32 = 114; SR = 121) to measure P4 concentration. Binomial outcomes were analyzed with logistic regression and hazard of pregnancy (R32 = 485; SR = 462) with Cox's proportional regression in SAS (SAS Institute, Cary, NC). For P/AI analysis, the TAI service was the experimental unit (R32 = 720; SR = 819). Models included treatment and parity as fixed effects and farm as random effect. The hazard of pregnancy was greater for the SR treatment (hazard ratio = 1.18; 95% confidence interval: 1.01-1.37). Median time to pregnancy was 95 and 79 d for the R32 and SR treatments, respectively. At NPD, 71.3 and 71.2% of cows had a CL for the R32 and SR treatments, respectively. Treatment did not affect overall P/AI 32 ± 3 d after AI (R32 = 31.0% vs. SR = 33.9%) or for cows with a CL at NPD (R32 = 32.7% vs. SR = 32.8%). For cows with no CL at NPD, P/AI was greater for the SR treatment (36.9%) than for the R32 treatment (28.6%). Pregnancy loss from 32 to 63 d after AI was similar for all services combined (R32 = 8.3% vs. SR = 10.4%) and for cows with no CL at NPD (R32 = 13.2% vs. SR = 7.2%) but tended to be affected by treatment for cows with a CL at NPD (R32 = 6.8% vs. SR = 11.9%). Treatment affected the proportion of cows with P4 ≤0.5 ng/mL at the GnRH before TAI for all cows (R32 = 68.4% vs. SR = 81.8%), tended to have an effect among cows with a CL (R32 = 70.0% vs. SR = 81.8%), and had no effect for cows with no CL (R32 = 64.7% vs. SR = 81.8%). We concluded that the SR program reduced time to pregnancy because of a reduction of the interbreeding interval for cows with a CL at NPD and greater P/AI in cows with no CL at NPD.

Onset of luteolytic action of exogenous prostaglandinF-2α during estrous cycle in goats

The objectives of these two experiments were to determine the day of onset of luteolysis after exogenous PGF-2α administration during the estrous cycle and the fertility of this synchronized estrus in goats. In the first experiment, during the breeding season, 48 Nubian does were estrous synchronized, using intravaginal sponges impregnated with a progestin, and estrus was detected by vasectomized bucks. The does were divided at random into three groups of 16 does each to be treated at days 2, 3, and 4 of the estrous cycle (estrus = day 0). Then, at each day of injection, the does were again randomly divided to receive a single dose of natural prostaglandin F-2α im (PGF-2α; 5 mg/doe; treatment [TRE] group) or sterile saline solution (control [CON] group; 1 mL/doe). Finally, the following groups were originated: TRE-2, CON-2, TRE-3, CON-3, TRE-4, and CON-4. The overall estrus response after treatment with PGF-2α (TRE group, 70.8%) was higher than saline (CON group, 12.5%, P ≤ 0.001). Estrus response for TRE-2, CON-2, TRE-3, CON-3, TRE-4, and CON-4 was 25% (2 of 8), 12.5% (1 of 8), 87.5% (7 of 8), 12.5% (1 of 8), 100% (8 of 8), and 0% (0 of 8) for the same groups, respectively. Estrus response was different between day 2 and days 3 and 4 (P ≤ 0.04) and not between day 3 and day 4 (P ≥ 0.05). In the second experiment, 15 multiparous Boer does were estrous synchronized with control internal drug release (CIDR, 300 mg progesterone = P₄) and PGF-2_α and randomly divided to receive one single dose of PGF-2α im at days 2, 3 or 4, after synchronized estrus (n = 5 at each day). The does were detected twice a day for estrus, and blood was collected daily for P₄ determination for 11 days after the synchronized estrus. Each doe in estrus was bred by hand mating to a proven male. All the does with a functional corpus/corpora luteum/lutea (CL; ≥1.0 ng/mL of P₄) responded to PGF-2α with a drop in P₄ levels that either lasted only 24 h for the does that did not show estrus (0.27 ± 0.10 ng/mL; n = 4) or persisted longer in all the does that showed estrus (0.22 ± 0.18 ng/mL; n = 10; P = 0.47). Estrus response for days 2, 3, and 4 was 20% (1 of 5), 80% (4 of 5), and 100% (5 of 5), respectively (P = 0.05). The conception rate was 100%, 100%, and 80% for the same days of administration, respectively (P = 0.64). It was concluded that luteolytic action of PGF-2α begins at day 3 of the estrous cycle by inducing an ovulatory and fertile estrus in goats.

Lack of complete regression of the Day 5 corpus luteum after one or two doses of PGF2α in nonlactating Holstein cows

The early corpus luteum (CL) (before Day 6) does not regress after a single PGF2α treatment. We hypothesized that increasing PGF2α dose or number of treatments would allow regression of the early CL (Day 5). Nonlactating Holstein cows (N = 22) were synchronized using the Ovsynch protocol. On Day 5 (Day 0 = second GnRH treatment), cows were assigned to: (1) control (N = 5): no further treatment; (2) 1PGF (N = 6): one dose of 25 mg PGF2α; (3) 2PGF (N = 5): two doses of 25 mg PGF2α (50 mg) given 8 hours apart (second PGF2α on Day 5 at the same time as the other PGF2α treatments); (4) DPGF (N = 6): double dose of 25 mg PGF2α (50 mg) given on Day 5. Blood samples were collected to monitor progesterone (P4) profiles in two periods. In the first period (0 to 24 hours), there were effects of treatment (P = 0.01), time (P < 0.01), and an interaction of treatment and time (P = 0.02). Group 1PGF versus control was different only at 12 hours (P = 0.02). Cows treated with DPGF were different than control at 4 hours (P = 0.04), 12 hours (P < 0.01), and 24 hours (P < 0.01). Only cows treated with 2PGF had lower P4 than control during the entire period and low P4 (0.37 ± 0.17 ng/mL) at 24 hours, usually indicative of luteolysis. In the second period (Day 5 to 15 of the cycle), there were effects of treatment (P < 0.01), time (P < 0.01), and interaction of treatment and time (P = 0.002). Group 1PGF was not different than control from Day 5 to 13 and P4 was greater than control on Day 14 (P = 0.01) and 15 (P < 0.01). Circulating P4 in DPGF cows was lower than control from Day 7 (P = 0.05) through 12 (P < 0.01). Likewise, there were differences between control and 2PGF from Day 7 to 13, but not on Day 14 and 15. On Day 15, all PGF2α-treated groups had circulating P4 consistent with an active CL. Ultrasound evaluation confirmed that no CL from any group completely regressed during the experiment and no new ovulations occurred to account for functional CL later in cycle. In summary, a double dose of PGF2α (twice on Day 5 or 8 hours apart) can dramatically decrease P4, consistent with classical definitions of luteolysis; however, these CL recover and become fully functional. Thus, the Day 5 CL of mature Holstein cows do not regress even to two doses of PGF2α.

Hormonal manipulations in the 5-day timed artificial insemination protocol to optimize estrous cycle synchrony and fertility in dairy heifers

Objectives were to determine the effects of GnRH at the initiation of the 5-d timed artificial insemination (AI) program combined with 2 injections of PGF2α on ovarian responses and pregnancy per AI (P/AI) in dairy heifers, and the role of progesterone concentrations on LH release and ovulation in response to GnRH. In study 1, heifers received a controlled internal drug release (CIDR) insert containing 1.38 g of progesterone on d 0, an injection of 25 mg of PGF2α and CIDR removal on d 5, and an injection of 100 μg GnRH concurrently with AI on d 8. Heifers were assigned to receive no additional treatment (control; n=559) or an injection of GnRH on d 0 and a second injection of PGF2α on d 6 (G2P; n=547). In study 2, all heifers were treated as described for the control in study 1, and were allocated to receive no additional treatment (control; n=723), an injection of PGF2α on d 6 (NG2P; n=703), or an injection of GnRH on d 0 and an injection of PGF2α on d 6 (G2P; n=718). In study 3, heifers received a CIDR on d 7 after ovulation and were assigned randomly to a low-progesterone (LP; n=6) treatment in which 2 injections of 25 mg of PGF2α each were administered 12h apart, on d 7 and 7.5 after ovulation, or to a high-progesterone (HP; n=12) treatment in which no PGF2α was administered. On d 8, heifers received 100 μg of GnRH and blood was sampled at every 15 min from -30 to 180 min relative to the GnRH for assessment of LH concentrations. Additionally, 94 heifers were assigned to LP or HP and ovulation in response to GnRH was evaluated. In study 1, P/AI was greater for G2P than for the control on d 32 (59.4 vs. 53.5%) and 60 after AI (56.6 vs. 51.3%). In study 2, administration of GnRH on d 0 increased the proportion of heifers with a new corpus luteum on d 5 (control=21.9 vs. NG2P=20.1 vs. G2P=34.4%). Administration of a second PGF2α increased the proportion of heifers with progesterone <0.5 ng/mL at AI (control=83.1 vs. NG2P=93.0 and G2P=87.2%). Pregnancy per AI was greater for G2P than for control and NG2P on d 32 (control=52.9 vs. NG2P=55.0 vs. G2P=61.7%) and 60 (control=49.0 vs. NG2P=51.6 vs. G2P=59.1%). In study 3, HP attenuated LH release and reduced ovulation (19.0 vs. 48.4%) in response to GnRH compared with LP. Combining GnRH and 2 doses of PGF2α in the 5-d timed AI protocol improved follicle turnover, luteolysis, and P/AI in heifers. Elevated concentrations of progesterone suppressed LH release and are linked with the low ovulatory response to the initial GnRH treatment of the protocol.

The effect of dose and type of cloprostenol on the luteolytic response of dairy cattle during the Ovsynch protocol under different oestrous cycle and physiological characteristics

The objective of this study was to characterize the effect of dose and type of cloprostenol (CLO) on the luteolytic response of dairy cattle during the Ovsynch protocol under different oestrus cycle and physiological characteristics. Twelve non-lactating dairy cows and 111 lactating dairy cows were used in three experiments. In Experiment I, cows were synchronized so that they had only a 5.5- to 6-day-old corpus luteum (CL) at the time of the prostaglandin F2α (PGF2α ) treatment of Ovsynch. In Experiment II, cows were synchronized so that they had at least a CL of approximately 14 days old at the time of PGF2α treatment and an accessory CL if they had responded to the first GnRH of Ovsynch. Furthermore, in each experiment, cows received either a standard or a double dose of d-CLO as the luteolytic treatment. In Experiment III, lactating cows were blocked by parity and assigned to one of three luteolytic treatments during Ovsynch: 500 μg d,l-CLO, 150 or 300 μg of d-CLO. In Experiment I, the dose of d-CLO had an effect (p = 0.08) on the percentage of cows with full luteolysis, but not in Experiment II (p > 0.1). More cows in Experiment II had full luteolysis than did cows of Experiment I (87% vs 58%, respectively; p = 0.007). In Experiment III, 87.1%, 84.4% and 86.2% lactating dairy cows had full luteolysis and 37.8%, 36.8% and 36.1% of cows became pregnant after treatment with 500 μg d,l-CLO, 150 or 300 μg of d-CLO, respectively (p > 0.05).

Treatments to Optimize the Use of Artificial Insemination and Reproductive Efficiency in Beef Cattle under Tropical Environments

Bos indicus cattle, the preferred genetic group in tropical climates, are characterized by having a lower reproductive efficiency than Bos taurus. The reasons for the poorer reproductive efficiency of the Bos indicus cows include longer lengths of gestation and postpartum anestrus, a short length of estrous behavior with a high incidence of estrus occurring during the dark hours, and puberty at older age and at a higher percentage of body weight relative to mature body weight. Moreover, geography, environment, economics, and social traditions are factors contributing for a lower use of reproductive biotechnologies in tropical environments. Hormonal protocols have been developed to resolve some of the reproductive challenges of the Bos indicus cattle and allow artificial insemination, which is the main strategy to hasten genetic improvement in commercial beef ranches. Most of these treatments use exogenous sources of progesterone associated with strategies to improve the final maturation of the dominant follicle, such as temporary weaning and exogenous gonadotropins. These treatments have caused large impacts on reproductive performance of beef cattle reared under tropical areas.

Prostaglandin F2alpha differentially affects mRNA expression relating to angiogenesis, vasoactivation and prostaglandins in the early and mid corpus luteum in the cow

Administration of prostaglandin (PG) F(2alpha) in cattle during the mid-luteal phase (Days 8-12 of the estrous cycle) drastically reduces the plasma progesterone concentrations and the volume of the corpus luteum (CL). However, PGF(2alpha) does not induce luteolysis during the early luteal phase (up to Day 5 of the estrous cycle). To characterize the possible distinct difference in acute response to a luteolytic dose of PGF(2alpha) administration, we determined various mRNA expressions in the early and mid CL relating to angiogenesis, vasoactivation and PG-related factors at 30 min after PGF(2alpha) injection in cyclic cows. The experiments were conducted on Day 4 (early CL) and Days 10-12 (mid CL). Cows were either injected with 500 microg PGF(2alpha) analogue or saline as the control (early CL control, n=5; early CL PGF(2alpha) treated, n=5; mid CL control, n=5; mid CL PGF(2alpha) treated, n=7). Thirty min after injection of PGF(2alpha) or saline, the cows were ovariectomized transvaginally, and the CL tissues were collected from regions designated as the periphery and center of the CL. Administration of PGF(2alpha) up-regulated the mRNA expressions of angiogenic-related factors such as vascular endothelial growth factors, vasohibin, fibroblast growth factor 2 and insulin-like growth factor-II in the early CL, whereas PGF(2alpha) down-regulated these mRNA expressions in the mid CL. In the vasoactive factors, PGF(2alpha) stimulated the mRNA expressions of endothelin-1, angiotensin converting enzyme, endothelial nitric oxide synthase (NOS) and inducible NOS in the periphery area of the mid CL, but not in the early CL. However, PGF(2alpha) drastically down-regulated PGF(2alpha) receptor mRNA expression in both regions of the early and mid CL. The results indicated a clear difference in the acute action of PGF(2alpha) depending not only on the luteal phase (immature vs. mature) but also the region (periphery vs. center) within the CL at 30 min after PGF(2alpha) injection in the cow.

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.

The roles of PGF(2alpha) and PGE(2) in regression of the corpus luteum after intrauterine infusion of Arcanobacterium pyogenes in cows

To study the effect of bacteria in the uterus on the fate of the corpus luteum (CL), Arcanobacterium pyogenes was inoculated into the uteri of cows on Day 3 (Day 0=day of spontaneous ovulation). Plasma concentrations of 13,14-dihydro-15-keto-PGF(2alpha) (PGFM), 13,14-dihydro-15-keto-PGE(2) (PGEM) and progesterone (P(4)) were determined. In five cows, the developing CL regressed and first-wave dominant follicles, which normally become atretic, ovulated (Group OV) after bacterial inoculation. In another five cows (Group NOV) and five control cows, the developing CL did not regress and first-wave dominant follicles did not ovulate. In Group OV, PGFM concentrations increased by 126.2pg/mL (from 36.8+/-7.8pg/mL on Day 3 to 163+/-37.2pg/mL on Day 6), with an increase ratio of 5.8-fold. Conversely, in Group NOV, PGFM had a greater increase of 198.4pg/mL (from 128.2+/-27.8pg/mL on Day 3 to 326.6+/-115.1pg/mL on Day 5), but the increase ratio was only 2.3-fold. Although PGEM tended to increase in both groups, raw increases and increase ratios were small. Bacterial inoculation into the uterus stimulated the release of prostaglandins and affected the fate of the CL; in that regard, the CL was affected more by PGF(2alpha) than by PGE(2), and the increase ratio of PGF(2alpha) was more important than the raw increase.

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.

Prostaglandin F 2 alpha stimulates endothelial nitric oxide synthase depending on the existence of bovine granulosa cells: analysis by co-culture system of endothelial cells, smooth muscle cells and granulosa cells

Prostaglandin F(2 alpha) (PGF(2 alpha)) induces luteolysis in the mid but not in the early luteal phase; despite this, both the early and the mid corpus luteum (CL) have PGF(2 alpha) receptor (FPr). We previously indicated that the luteal blood flow surrounding the CL drastically increases prior to a decrease of progesterone (P) in the cows, suggesting that an acute increase of luteal blood flow may be an early sign of luteolysis in response to PGF(2 alpha) and that this may be induced by a vasorelaxant nitric oxide (NO). The aim of this study was to investigate the luteal stage-dependent and the site-restricted effect of PGF(2 alpha) and NO on the mRNA expressions and P secretion. To mimic the local luteal region both of peripheral and central areas of the CL, we utilized co-cultures using bovine aorta endothelial cells (EC), smooth muscle cells (SMC) and luteinizing granulosa cells (GC) or fully-luteinized GC. PGF(2 alpha) stimulated the expression of endothelial NO synthase (eNOS) mRNA at 0.5 h in mix-cultures of EC and SMC with fully-luteinized GC but not with luteinizing GC. The expression of eNOS mRNA in EC was increased by PGF(2 alpha) at 1 h only when EC was cultured together with fully-luteinized GC but not with luteinizing GC. In all co-cultures, PGF(2 alpha) did not affect the mRNA expression of FPr. Treatment of NO donor inhibited P secretion at 0.5 h. In conclusion, the present study suggests that the coexistence of the mature luteal cells (fully-luteinized GC) with EC/SMC may be crucial for acquiring functional NO synthesis induced by PGF(2 alpha).

Prostaglandin F2alpha increases endothelial nitric oxide synthase in the periphery of the bovine corpus luteum: the possible regulation of blood flow at an early stage of luteolysis

Prostaglandin F(2)(alpha) (PGF(2)(alpha)) released from the uterus causes alterations in luteal blood flow, reduces progesterone secretion, and induces luteolysis in the bovine corpus luteum (CL). We have recently discovered that luteal blood flow in the periphery of the mature CL acutely increases coincidently with pulsatile increases in a metabolite of PGF(2)(alpha) (PGFM). In this study, we characterized changes in regional luteal blood flow together with regional alterations in endothelial nitric oxide synthase (eNOS) expression during spontaneous luteolysis and in response to PGF(2)(alpha). Smooth muscle actin-positive blood vessels larger than 20 microm were observed mainly in the periphery of mature CL. PGF(2)(alpha) receptor was localized to luteal cells and large blood vessels in the periphery of mid-CL. PGF(2)(alpha) acutely stimulated eNOS expression in the periphery but not in the center of mature CL. Injection of the NO donor S-nitroso-N-acetylpenicillamine into CL induced an acute increase in luteal blood flow and shortened the estrous cycle. In contrast, injection of the NOS inhibitor l-NAME into CL completely suppressed the acute increase in luteal blood flow induced by PGF(2)(alpha) and delayed the onset of luteolysis. In conclusion, PGF(2)(alpha) has a site-restricted action depending on not only luteal phase but also the region in the CL. PGF(2)(alpha) stimulates eNOS expression, vasodilation of blood vessels, and increased luteal blood flow in periphery of mature CL. Furthermore, the increased blood flow is mediated by NO, suggesting that the acute increase in peripheral blood flow to CL is one of the first physiological indicators of NO action in response to PGF(2)(alpha).

Bovine endothelial cells interact with fully-luteinized, but not luteinizing, granulosa cells in the mRNA expression of endothelin-1 system in response to prostaglandin F(2alpha)

The corpus luteum (CL) undergoes regression by prostaglandin (PG)F(2alpha) from uterus and endothelin-1 (ET-1) plays an important role during luteolysis as a local mediator of PGF(2alpha) in the cow. Endothelial cells (EC) and luteal cells are main cell types making up the CL and their interactions are vital for CL function. We aimed to examine the relevance of interactions between EC and luteal cells on stimulation of genes which involved ET-1 synthesis by PGF(2alpha). We further focused the impact of maturity of luteal cells on the stimulation of the genes. To make a microenvironment which resembles the CL, we used bovine aortic endothelial cells (BAEC) and luteinizing or fully-luteinized granulosa cells (GC) and evaluated the effect of PGF(2alpha) on the expression for mRNA of ET-1 system by using real-time RT-PCR. PGF(2alpha) stimulated the expression of preproET-1 and endothelin converting enzyme-1 mRNA only in the co-cultures of BAEC with fully-luteinized GC, but not with luteinizing GC. The data suggest that interactions between BAEC and fully-luteinized GC enhance the capability of BAEC to produce ET-1 in response to PGF(2alpha). This mechanism may contribute to the local induction of luteolytic action of PGF(2alpha) which is dependent on the age/maturation of the CL.

A cooperative action of endothelin-1 with prostaglandin F(2alpha) on luteal function in the cow

Prostaglandin F(2alpha) (PGF(2alpha)) is the primary luteolysin in the cow, and luteal endothelin-1 (ET-1) interacts with PGF(2alpha) during the process of luteolysis. In contrast, a developing corpus luteum (CL) is refractory to exogenous administration of PGF(2alpha). Thus, the present study was aimed to investigate the functional relationship between ET-1 and PGF(2alpha) in the mid-CL (PGF(2alpha)-sensitive) and early-CL (PGF(2alpha)-refractory). In the mid-CL model, cows (n = 6/treatment) were assigned to receive one of five types of treatments on day 10 of the estrous cycle: (1) an injection of saline; control, (2) a 500 microg of PGF(2alpha) analogue (sufficient dose to induce luteolytis); full-PG, (3) an intraluteal injection of 0.25 mg ET-1; ET-1, (4) a 125 micro g of PGF(2alpha) (insufficient dose to induce luteolytis); 1/4PG or (5) an intraluteal injection of 0.25 mg ET-1 after administration of a insufficient dose of PGF(2alpha) analogue; 1/4PG/ET. In the early-CL model, cows were assigned to receive one of two types of treatments on day 5 of the estrous cycle: (1) a sufficient dose of PGF(2alpha) analogue; PG (n = 5) or (2) an intraluteal injection ET-1 after a sufficient dose of PGF(2alpha); PG/ET (n = 7). In the mid-CL model, 1/4PG/ET resulted in a rapid reduction of progesterone (P) concentrations similar to that in full-PG from the next day. However, the levels of P in 1/4PG/ET (1.5-2.5 ng/ml) kept significantly higher than that in full-PG (< 0.5 ng/ml). ET-1 or 1/4PG did not decrease plasma P concentrations (4-6 ng/ml). The plasma ET-1 levels increased with the full-PG administration. In the early-CL model, both treatments had no effect on plasma P increase and ET-1 levels. The overall results indicate that the intraluteal ET-1 injection after administration of insufficient dose of PGF(2alpha) induces the depression of P secretion in vivo during the mid luteal phase in the cow, supporting the concept that ET-1 is one of a local mediator of functional luteolysis in the cow. The result further indicates that the early-CL is not only PG-refractory but also ET-1-refractory.

Efficacy of either a single or split treatment of PGF2α after a 14 day melengestrol acetate treatment to synchronize estrus and induce luteolysis in Bos indicus×Bos taurus heifers

Two experiments evaluated a modified delivery of prostaglandin F2alpha (PGF2alpha) after a melengestrol acetate (MGA) treatment in Angus and Bos indicus x Bos taurus (BI) heifers. Experiment 1 was replicated three times with yearling BI heifers (n = 695). Heifers received MGA (0.5 mg head(-1) day(-1)) for 14 days. In Replications 1 and 2, heifers received either 25 mg of PGF2alpha im 19 days after MGA (single) or 12.5 mg of PGF2alpha im 19 and 20 days after MGA (split). In Replication 3, heifers received the same treatments, with PGF2alpha initiated either 18 or 19 days after MGA. Estrus was detected for 72 h after PGF2alpha, with AI commencing 8-12 h after a detected estrus. Heifers not observed in estrus by 72 h were timed-AI concomitant with GnRH (100 microg im). Heifers from Replication 2 (n = 146) had blood samples collected at the initial PGF2alpha and at timed-AI to determine corpus luteum (CL) regression by evaluating plasma progesterone concentrations. The interval from MGA withdrawal to PGF2alpha did not have a significant effect on any variable in Replication 3 and there were no treatment by replication effects for any variables, therefore data were pooled. Modifying the PGF2alpha treatment from a single treatment to two treatments on consecutive days increased (P < 0.05) 72 h estrous response (43.2% versus 50.1%), timed-AI (23.9% versus 33.5%) and total-AI pregnancy rates (34.5% versus 42.5%), and CL regression (79.1% versus 92.5%), respectively. In Experiment 2, yearling Angus (n = 66) and 2-year-old BI (n = 68) heifers were synchronized as per Experiment 1 (with the initial PGF2alpha 19 days after MGA). Neither breed nor PGF2alpha treatment effected (P > 0.05) 72 h estrous response, total-AI pregnancy rate, or CL regression rate. In conclusion, treating yearling BI heifers with split treatments of PGF2alpha (given on two consecutive days) improved estrous response and pregnancy rates by increasing PGF2alpha-induced luteolysis.

Expression of prostaglandin G/H synthase (PGHS) and heat shock protein-70 (HSP-70) in the corpus luteum (CL) of prostaglandin F2 alpha-treated immature superovulated rats

In this study we examined the mechanism of corpus luteum (CL) regression by measuring changes in expression of prostaglandin G/H synthase-1 (PGHS-1) and -2 (PGHS-2) in day 4 CL and inducible heat shock protein 70 (HSP-70) in day 4 and day 9 CL of immature superovulated rats. The rats were superovulated and treated with 500 microg of prostaglandin F2alpha (PGF2alpha) on day 4 or day 9 after CL formation. Ovaries and serial blood samples were removed during the 24-hour period following treatment. Plasma progesterone was determined by radioimmunoassay while mRNA abundance and protein expression were assessed by semiquantitative RT-PCR and immunoblot analysis, respectively. One hour after PGF2alpha, both day 4 and day 9 rats exhibited a significant decrease in progesterone secretion; however, there was a greater decrease in day 9 rats. In ovarian samples removed on day 4, there was a significant increase in mRNA for PGHS-2 at 1 hour after PGF2alpha. PGHS-1 mRNA content remained unchanged. Immunoblot analyses showed an increase in PGHS-2 protein expression only at 8 h. There were no changes in PGHS-1 protein expression. In day 9 rats, ovarian HSP-70 protein levels increased by 50% after PGF2alpha injection; however, on day 4 there was no change in expression of this protein over the sampling period. These results suggest that expression of PGHS-2 may be involved in inhibiting progesterone production and that expression of HSP-70 may be required for complete CL regression in the rat.

Fertility of Bunaji (zebu) cows after treatment with PRID with or without PGF2alpha

A study was undertaken to determine the oestrus response and fertility rates of zebu cows treated with PRID alone or in combination with PGF2alpha. A total of 184 non-suckled cycling Bunaji cows were allotted randomly to four treatment groups of 46 animals per group as follows: group 1 (PRID-12), PRID was inserted for 12 days; group 2 (PRID+7+PGF2alpha-6, PRID was inserted for 7 days and PGF2alpha was administered intramuscularly 1 day prior to PRID withdrawal; group 3 (PRID-7+PGF2alpha-7, PRID was inserted for 7 days and PGF2alpha was administered intramuscularly on the day of PRID withdrawal; group 4 (2 x PGF2alpha-13, two intramuscular injections of PGF2alpha 13 days apart. At the end of each treatment period, the cows were observed for 7 days for behavioural oestrus and were inseminated 12 h following detection of oestrus. Pregnancy was diagnosed by rectal palpation 30-40 days post-insemination. The respective oestrus response rates were 78.3%, 76.1%, 87.0% and 89.1% for groups 1-4. While the corresponding pregnancy rates were 39.1%, 41.3%, 52.2% and 52.2%, the conception rates were 50.0%, 54.3%, 60.3% and 58.6% for groups 1-4. Although individual variations in progesterone levels were observed, the progesterone profiles were generally typical and normal. The results of the study have confirmed the effectiveness of the four regimes in synchronizing and controlling oestrus and ovulation in Bunaji cows. However, groups 3 and 4 showed some superiority over the other treatments. The results of this study provide feasible options from which clinicians involved in intensive breeding programmes and herd health fertility programmes can choose.

Prostaglandin F(2alpha) regulation of the bovine corpus luteum endothelin system during the early and midluteal phase

Recent evidence in the cow suggests that endothelin-1 (ET-1) plays a role during prostaglandin (PG) F(2alpha)-induced luteal regression. We have examined the effects of treatment with PGF(2alpha) during the early and midluteal phases on three components of the endothelin system: endothelin-converting enzyme-1 (ECE-1), ET type A receptor (ET(A)), and ET-1 in the bovine corpus luteum (CL). Cyclic beef cows were injected (0 h) on Day 4 or 10 with either saline or the PGF(2alpha) analogue Lutalyse (15 mg). The CL were collected at 2 (n = 11), 10 (n = 23), 24 (n = 15), or 48 h (n = 12) after treatment. The cows in which CL were removed after 10 h comprised of two experimental groups. The first group (n = 11) received one injection; the second group (n = 12) received two injections, one at 0 h and one at 8 h. The cows in which CL were collected after 24 and 48 h received one injection every 8 h. Semiquantitative reverse transcriptase-polymerase chain reaction was used to evaluate the mRNA encoding ECE-1, ET(A), and ET-1. The ECE-1 and ET(A) proteins were evaluated by semiquantitative Western blot analysis. The ET-1 was the most likely component of the endothelin system target for PGF(2alpha) regulation during the midluteal phase. The ET(A) and ECE-1 genes were constitutively expressed in the Day 4 and Day 10 CL. A practical application of this observation is that it may be possible to target the ET-1 gene as a way to manipulate the luteolytic action of PGF(2alpha).

Sensitivity of bovine corpora lutea to prostaglandin F2alpha is dependent on progesterone, oxytocin, and prostaglandins

Prostaglandin (PG) F2alpha that is released from the uterus is essential for spontaneous luteolysis in cattle. Although PGF2alpha and its analogues are extensively used to synchronize the estrous cycle by inducing luteolysis, corpora lutea (CL) at the early stage of the estrous cycle are resistant to the luteolytic effect of PGF2alpha. We examined the sensitivity of bovine CL to PGF2alpha treatment in vitro and determined whether the changes in the response of CL to PGF2alpha are dependent on progesterone (P4), oxytocin (OT), and PGs produced locally. Bovine luteal cells from early (Days 4-5 of the estrous cycle) and mid-cycle CL (Days 8-12 of the estrous cycle) were preexposed for 12 h to a P4 antagonist (onapristone: OP; 10(-4) M), an OT antagonist (atosiban: AT; 10(-6) M), or indomethacin (INDO; 10(-4) M) before stimulation with PGF2alpha. Although OP reduced P4 secretion (p < 0.001) only in early CL, it reduced OT secretion in the cells of both phases examined (p < 0.001). OP also reduced PGF2alpha and PGE2 secretion (p < 0.01) from early CL. However, it stimulated PGF2alpha secretion in mid-cycle luteal cells (p < 0.001). AT reduced P4 secretion in early and mid-cycle CL (p < 0.05). Moreover, PGF2alpha secretion was inhibited (p < 0.05) by AT in early CL. The OT secretion and the intracellular level of free Ca2+ ([Ca2+]i) were measured as indicators of CL sensitivity to PGF2alpha. PGF2alpha had no influence on OT secretion, although [Ca2+]i increased (p < 0.05) in the early CL. However, the effect of PGF2alpha was augmented (p < 0.01) in cells after pretreatment with OP, AT, and INDO in comparison with the controls. In mid-cycle luteal cells, PGF2alpha induced 2-fold increases in OT secretion and [Ca2+]i. However, in contrast to results in early CL, these increases were magnified only by preexposure of the cells to AT (p < 0.05). These results indicate that luteal P4, OT, and PGs are components of an autocrine/paracrine positive feedback cascade in bovine early to mid-cycle CL and may be responsible for the resistance of the early bovine CL to the exogenous PGF2alpha action.

Does increased PGF2 alpha receptor concentration mediate PGF2 alpha-induced luteolysis during early diestrus in the pig?

The mechanism by which multiple injections of PGF2 alpha result in premature luteolysis in pigs is unknown. In the present study we evaluated whether PGF2 alpha receptor concentrations on large luteal cells changed when gilts were injected IM with 12.5 mg of PGF2 alpha every 12 hours from the morning of day 5 of an estrous cycle (estrus = day 0) until ovariectomy on day 6, 7, 8, or 9. Luteal PGF2 alpha receptor concentrations remained constant from day 6 through 9 in the PGF2 alpha-treated group, but increased linearly (P > 0.05) in control gilts from day 6 to 9. Receptor affinity for PGF2 alpha did not change throughout the study in either PGF2 alpha-treated or control gilts. Luteal progesterone concentrations were significantly lower in PGF2 alpha-treated gilts than in control gilts only on day 9. Histological examination of luteal tissue obtained from PGF2 alpha-treated gilts revealed definite evidence of luteolysis by day 8. We conclude that PGF2 alpha-induced premature luteolysis is not mediated by an increase in luteal PGF2 alpha receptor concentrations and, based on luteal progesterone concentrations and histological features, that the PGF2 alpha-based protocol used to shorten the estrous cycle is accompanied by premature functional and structural luteal regression.

Progesterone regulation of luteinizing hormone receptors on cultured bovine luteal cells

During development of the corpus luteum (CL), the numbers of luteinizing hormone (LH) receptors increase. Cultured bovine luteal cells from developing and mature CL were used to examine the influence of progesterone (P4) on this receptor. CL were obtained from dairy cows during the early or middle phase of the estrous cycle. In early CL, the number of receptors per cell was increased by exogenous progesterone treatment but there was no effect on receptor numbers in cells from midcycle CL. Binding affinities did not change with respect to age or treatment. Forskolin elevated endogenous progesterone and also enlarged the receptor population. The action did not appear to be an unmasking of cryptic receptors since the effect was not seen in luteal particulates. Elevation of LH receptor numbers by progesterone in immature CL may be a form of intraluteal regulation contributing to the functional maturation of these steroidogenic cells.

Synchronization of estrus in early diestral dairy heifers with Prostaglandin F2α and estradiol benzoate

Following observation of estrus, 134 Holstein heifers were given injections of Prostaglandin F(2)alpha (PGF(2)alpha) between Days 5 and 10 of their cycle (estrus = Day 0). They were then randomly assigned to either a group receiving 400 microg of estradiol benzoate (E(2)B) 40 h or maintained as controls. Heifers observed in estrus within 120 h of PGF(2)alpha administration were inseminated (approximately 12 h after initial observation of estrus). Blood samples for progesterone determination were drawn from the coccygeal vein on Days 15 and 21 after insemination. Pregnancy was confirmed by palpation per rectum between Days 5.0 and 60 post insemination. When control and treated heifers were compared it was found that a higher percentage of heifers treated with E(2)B exhibited estrus after PGF(2)alpha, but there had been no effect on subsequent progesterone concentrations or pregnancy rates.

Regulation of prostaglandin synthesis by progesterone in the bovine corpus luteum

The objective of the present study was to investigate the influence of progesterone on prostaglandin synthesis by the corpus luteum (CL). Corpora lutea were obtained from dairy cows on days 4, 6, 10, and 18 of the estrous cycle, dissociated, and placed in serum-free culture. The addition of luteinizing hormone (LH) resulted in a slight, but non-significant (p greater than 0.05), increase in levels of 6-keto-PGF1 alpha, and had no effect on PGF2 alpha. Progesterone treatment caused a significant, dose-dependent decrease in both PGF2 alpha and 6-keto-PGF1 alpha in 6-day and 10-day corpora lutea, but not in 4-day or 18-day corpora lutea. In the 6- and 10-day corpora lutea, progesterone treatment resulted in a greater inhibition of PGF2 alpha than 6-keto-PGF1 alpha production. Therefore, progesterone treatment brought about an increase in the 6-keto-PGF1 alpha to PGF2 alpha ratio in these cells (12.9 vs. 21.3). It is concluded from these studies that progesterone can modulate luteal prostacyclin and PGF2 alpha synthesis, suggesting an interaction of progesterone and prostaglandin production within the corpus luteum.

Prostaglandin E2 counteracts the effects of PGF2 alpha in indomethacin treated cycling gilts

Twenty crossbred gilts with at least 2 consecutive estrous cycles of 18 to 21 days in length were used to study the effects of prostaglandins E2 and F2 alpha (PGE2 and PGF2 alpha) on luteal function in indomethacin (INDO) treated cycling gilts. Intrauterine and jugular vein catheters were surgically placed before day 7 of the treatment estrous cycle and gilts were randomly assigned to 1 of 5 treatment groups (4/group). With exception of the controls (Group I) all gilts received 3.3 mg/kg INDO every 8 h, Groups III, IV and V received 2.5 mg PGF2; 2.5 mg PGF2 alpha + 400 micrograms PGE2 every 4 hr, or 400 micrograms PGE2 every 4 h, respectively. All treatments were initiated on day 7 and continued until estrus or day 23. Jugular blood for progesterone analysis was collected twice daily from day 7 to 30. Estradiol-17 beta (E2-17 beta) concentrations were determined in samples collected twice daily, from 2 d before until 2 d following the day of estrus onset. When compared to pretreatment values, estrous cycle length was unaffected (P greater than 0.05) in Group I, prolonged (P less than 0.05) in Groups II, IV and V; and shortened (P less than 0.05) in Group III. The decline in plasma progesterone concentration that normally occurs around day 15 was unaffected (P greater than .05) in Group I; delayed (P less than 0.05) in Groups II, IV and V; and occurred early (P less than 0.05) in Group III. Mean E2-17 beta remained high (31.2 +/- 4.9 to 49.3 +/- 3.1 pg/ml) in Groups III and IV, while the mean concentrations in Groups III and V varied considerably (17.0 +/- 2.0 to 52.2 +/- 3.5 pg/ml). The results of this study have shown that PGE2 will counteract the effects of PGF2 alpha in INDO treated cycling gilts. The inclusion of PGF2 alpha appeared to either stimulate E2-17 beta secretion or maintain it at a higher level than other treatments.

Influence of luteinizing hormone and prostaglandin F2α 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.