Endothelin-1 (ET)-1 within the corpus luteum (CL) is rapidly up-regulated during natural or PGF(2 alpha)-induced luteolysis; however, such an increase was not observed at early luteal stage, when the CL is refractory to PGF(2 alpha). The mature and active form of ET-1 is derived from the inactive intermediate peptide, big ET-1, by ET-converting enzyme (ECE)-1. This study therefore examined the developmental and cell-specific expression of ECE-1 in bovine CL. A significant, 4-fold, elevation in ECE-1 expression (mRNA and protein levels) occurred during the transition of the CL from early to midluteal phase. Analysis using in-situ hybridization and enriched luteal cell subpopulations showed that both steroidogenic and endothelial cells of the CL expressed high levels of ECE-1 mRNA; prepro ET-1 mRNA, on the other hand, was only expressed by resident endothelial cells. These data suggest that luteal parenchymal and endothelial cells may cooperate in the biosynthesis of mature bioactive ET-1. In the mature CL, ECE-1 mRNA increase occurred both in steroidogenic and endothelial cells and was accompanied by a significant rise in ET-1 peptide. However, in contrast to ECE-1, prepro ET-1 mRNA levels were similar in early and midluteal-phase CL. Low ECE-1 levels during the early luteal phase, restricting the production of active ET-1, may explain why the immature CL is able to withstand PGF(2 alpha)-induced luteolysis.

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A study was conducted to examine the effects of gonadotropins on ovarian follicular development and differentiation in GnRH agonist (GnRHa)-treated cattle. Holstein cows were allotted into two pre-treatment groups: controls (n 5 5) and GnRHa-treated (n 5 9). Ovaries were removed from control cows on day 5 following a synchronized estrus. Treatment with GnRHa resulted in follicular arrest at < 5 mm. Following follicular arrest, GnRHa-treated cows received a constant infusion of FSH for 96 h (GnRHa/FSH), with a randomly selected subset receiving hourly pulses of LH in addition to FSH during the last 48 h of infusion (GnRHa/FSH 1 LH). At the end of infusion, ovaries were removed, follicles were counted and measured, and follicular fluid samples were collected from large follicles (>10 mm). Differences in expression of mRNA for LH receptor, FSH receptor, cytochrome P450 side-chain cleavage, 3b-hydroxysteroid dehydrogenase, cytochrome P450 17a-hydroxylase (P450c17) and cytochrome P450 aromatase were determined in large follicles using in situ hybridization. The number of large follicles did not differ between GnRHa/FSH-treated and GnRHa/FSH 1 LH-treated cows (P 5 0.64), but was greater than control animals (P # 0.004). Follicular fluid concentrations of estradiol-17b and androstenedione were highest in GnRHa/FSH 1 LH-treated cows (P # 0.04), intermediate in control cows, and lowest in GnRHa/FSH-treated cows. Hybridization intensity of P450c17 was greater in GnRHa/FSH 1 LH-treated versus control or GnRHa/FSH-treated cows (P # 0.03). These results indicate that while FSH can support bovine follicular growth >10 mm, LH increases androgen production and expression of P450c17.

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Two experiments in lactating dairy cows examined ovarian follicular responses to high, frequent doses of exogenous LH pulses at levels associated with follicular cysts. In Experiment 1, estrus was synchronized in 12 cyclic lactating cows >40 d postpartum. Emergence of the second follicular wave (d 0) was determined by ultrasonography. Starting on d 1, cows received LH (40 microg/h; n = 7) or saline (2 mL/h; n = 5) in hourly pulses for up to 5 (n = 5) or 7 (n = 7) d. On d 2, all cows received two injections of PGF2alpha, 12 h apart. In experiment 2, 14 lactating cows (7 to 12 d postpartum) received LH (40 microg/h; n = 7) or saline (1 mL/h; n = 7) in hourly pulses for 7 d, beginning 24 h after start of the first follicular wave. Daily samples were used to determine serum concentrations of progesterone (P4), estradiol-17beta (E2), LH, and FSH. Profiles of LH were determined from blood samples collected at 12-min intervals for 8 h on d 3. During infusion of LH, serum P4 and FSH were similar across treatments in both experiments. Serum E2 concentrations were similar in experiment 1, but serum E2 was greater on d 2, 3, and 5 in LH-treated cows in experiment 2. Infusion increased LH pulse frequency and amplitude in both experiments. Formation of cysts did not differ between LH- and saline-treated cows in either experiment (1 of 7 vs. 0 of 5 and 1 of 6 vs. 0 of 7, respectively). Cows that ovulated had similar intervals to ovulation in experiment 1 [6.0 +/- 0.1 d (LH) vs. 6.4 +/- 0.2 d (saline)], but in experiment 2, ovulation was 14 d earlier in LH-treated cows (5.6 +/- 1.8 d vs 19.9 +/- 1.5 d). In conclusion, high concentrations of LH are not solely responsible for formation of cysts in lactating dairy cows. Pulsatile infusion of LH stimulated follicular growth and steroidogenesis and decreased time to first ovulation in anestrous postpartum cows.

This study was conducted to examine the effects of metestrus administration of SyncroMate-B (SMB) on PGF2alpha secretion and corpus luteum (CL) development. In a study replicated over 2 yr, cows were observed for spontaneous estrus in yr 1, and cows received an injection of 25 mg of PGF2alpha and were observed for subsequent estrus in yr 2. At standing estrus (estrus = d 1), cows were randomly allotted to receive either the standard SMB regimen (n = 40) on d 3 of the estrous cycle or no treatment (n = 8). Fifty percent (n = 20) of SMB-treated cows were administered PGF2alpha on d 10 of the estrous cycle 48 h prior to implant removal. Twice-daily blood samples were collected in the morning (AM) and evening (PM) from d 2 AM through d 14 AM of the treated estrous cycle and subsequently analyzed for progesterone (P4) and PGF2alpha metabolite (PGFM). Prior to statistical analysis, SMB- and SMB/PGF2alpha-treated cows were sorted according to P4 concentration at d 10 of the treated estrous cycle to either a CL functional group (P4 > or = 1 ng/mL; n = 20) or a CL nonfunctional group (P4 < 1 ng/mL; n = 17). Following d 10 AM administration of PGF2alpha, functional and nonfunctional groups were further subdivided based on treatment. The groups were as follows: untreated control cows (n = 8); SMB-treated cows retaining a functional CL (SMB-F; n = 8); SMB-treated cows with a nonfunctional CL (SMB-N; n = 11); SMB/PGF2alpha-treated cows retaining a functional CL (SMB/PG-F; n = 12); and SMB/PGF2alpha-treated cows with a nonfunctional CL (SMB/PG-N; n = 6). Of all SMB-treated cows, 54% retained a functional CL through d 10 AM of the treated estrous cycle. Mean serum P4 concentrations increased for cows in all groups until d 7, after which P4 concentrations increased for cows in SMB/PG-F, SMB-F, and control groups and decreased for cows in SMB/PG-N and SMB-N groups. Following PGF2alpha administration on d 10, mean serum P4 concentrations remained < 1 ng/mL for cows in SMB/PG-N and SMB-N groups, decreased to < 1 ng/mL for cows in the SMB/ PG-F group, and remained > 1 ng/mL for cows in SMB-F and control groups. Mean serum PGFM concentrations tended (P = .06) to increase in cows with nonfunctional CL compared with control cows on d 8 AM and were greater (P < .05) in cows with functional CL on d 8 PM through d 9 PM. These results indicate that retention of a functional rather than a nonfunctional CL following metestrus administration of SMB is dependent on a premature release of uterine PGF2alpha.

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