Effects of estradiol on concentrations of gonadotropin-releasing hormone receptor messenger ribonucleic acid following removal of progesterone

Effect of progesterone on magnitude of the luteinizing hormone surge induced by two different doses of gonadotropin-releasing hormone in lactating dairy cows

Ovulation to the first GnRH injection of Ovsynch-type protocols is lower in cows with high progesterone (P4) concentrations compared with cows with low P4 concentrations, suggesting that P4 may suppress the release of LH from the anterior pituitary after GnRH treatment. The objectives of this study were to determine the effect of 1) circulating P4 concentrations at the time of GnRH treatment on GnRH-induced LH secretion in lactating dairy cows and 2) increasing the dose of GnRH from 100 to 200 μg on LH secretion in a high- and low-P4 environment. A Double-Ovsynch (Pre-Ovsynch: GnRH, PGF(2α) 7d later, GnRH 3d later, and Breeding-Ovsynch 7d later: GnRH, PGF(2α) 7d later, and GnRH 48 h later) synchronization protocol was used to create the high- and low-P4 environments. At the first GnRH injection of Breeding-Ovsynch (high P4), all cows with a corpus luteum ≥ 20 mm were randomly assigned to receive 100 or 200 μg of GnRH. At the second GnRH injection of Breeding-Ovsynch (low P4) cows were again randomized to receive 100 or 200 μg of GnRH. Blood samples were collected every 15 min from -15 to 180 min after GnRH treatment, and then hourly until 6h after GnRH treatment. As expected, mean P4 concentrations were greater for cows in the high- than the low-P4 environment. For cows receiving 100 μg of GnRH, the LH peak and area under the curve (AUC) were greater in the low- than in the high-P4 environment. Similarly, for cows receiving 200 μg of GnRH, the LH peak and AUC were greater in the low- than the high-P4 environment. Cows receiving 100 or 200 μg of GnRH had greater mean LH concentration in the low- than the high-P4 environment from 1 to 6h after GnRH treatment. On the other hand, when comparing the effect of the 2 GnRH doses in the high- and low-P4 environments, cows receiving 200 μg of GnRH had a greater LH peak and AUC than cows treated with 100 μg of GnRH both in the high- and low-P4 environments. For the high-P4 environment, mean LH was greater from 1.5 to 5h after GnRH treatment for cows receiving 200 μg of GnRH than for those receiving 100 μg of GnRH. In the low-P4 environment, mean LH was greater for cows receiving 200 μg of GnRH than for those receiving 100 μg of GnRH from 1 to 2.5h after GnRH treatment. We conclude that the P4 environment at GnRH treatment dramatically affects GnRH-induced LH secretion, and that a 200-μg dose of GnRH can increase LH secretion in either a high- or a low-P4 environment.

Estrogen replacement inhibits intimal hyperplasia and the accumulation and effects of transforming growth factor beta1

BACKGROUND

The role of estrogens in providing atheroprotection has been well documented in both epidemiologic and experimental studies. This phenomenon has traditionally been attributed to the beneficial lipid-modifying effects of estrogens. Previous studies have used models of either diet- or injury-induced atherosclerosis. As such, the interrelationship between estrogens, lipids, and atherosclerosis remains unclear. We hypothesized that estrogens are atheroprotective independent of changes in serum lipids by directly influencing the accumulation and effects of the peptide growth factor transforming growth factor beta1 (TGF-beta1).

MATERIAL AND METHODS

Thirteen female sheep (8 years old) were randomized to sham, ovariectomy, or ovariectomy with 17beta-estradiol replacement. Serum lipid levels were serially measured. At 9 months, necropsy was performed with histologic morphometric analysis of the aortoiliac bifurcation. Levels of TGF-beta1 were determined in serum and aortic tissue. Human aortic smooth muscle cells were isolated and cultured.

RESULTS

Serum triglyceride, lipoprotein a, and total, low-density lipoprotein, and high-density lipoprotein cholesterol levels were similar and normal between groups. Ovariectomy resulted in aortoiliac intimal hyperplasia compared with sham (P < 0.001) and hormone replacement (P < 0.001) groups. Compared with ovariectomy, estrogen replacement attenuated aortic accumulation of TGF-beta1 (P < 0.02). In vitro, estradiol potentiated TGF-beta1 inhibition of human vascular smooth muscle cell (VSMC) proliferation and increased TGF-beta1 release in stimulated VSMCs (P < 0.001).

CONCLUSIONS

Without dietary manipulation, ovarian ablation induces aortic intimal hyperplasia in the ewe. Estradiol abrogates this response independently of its effects on serum lipids. Hormone replacement decreases the accumulation of TGF-beta1, suggesting that estrogens may provide atheroprotection both by modifying local production and by modulating the influence of TGF-beta1 on VSMC growth.

Ovarian ablation alone promotes aortic intimal hyperplasia and accumulation of fibroblast growth factor

BACKGROUND

Estrogen-mediated cardiovascular protection is incompletely explained by its beneficial lipid-modifying effects. Previous studies interrogating direct vascular effects of estrogens have used models of either diet- or injury-induced atherosclerosis. The purpose of this study was to determine the influence of ovarian ablation alone on vascular remodeling. We hypothesized that estrogens are atheroprotective, independent of their influence on lipid metabolism, by directly influencing the production and effects of a prototypical atherogenic mediator, basic fibroblast growth factor (bFGF).

METHODS AND RESULTS

Twenty-five female sheep were randomized to sham operation, ovariectomy, or ovariectomy plus 17beta-estradiol replacement. Serum cholesterol and triglyceride levels were serially measured for 1 year and were similar among groups and in the normal range (30 to 60 mg/dL). At 6, 9, and 12 months, ovariectomy resulted in aortoiliac intimal hyperplasia compared with sham (P<0.01) and hormone replacement (P<0.01) groups. The neointima of ovariectomized animals was characterized immunohistochemically by increased vascular smooth muscle cells (VSMCs). Levels of bFGF protein were determined in adjacent aortic segments. Ovariectomized sheep had 2-fold more FGF than sham or ovariectomized sheep that received hormone replacement. In vitro, estradiol inhibited the mitogenic effect of bFGF on human aortic VSMCs.

CONCLUSIONS

Without dietary manipulation, ovarian ablation alone induces aortic intimal hyperplasia in the ewe. Estradiol abrogates this response independently of its effects on serum lipids. Hormone replacement decreases the accumulation of the atherogenic peptide bFGF in vivo and inhibits the mitogenic response of VSMCs to bFGF in vitro. These results suggest that estrogens may provide atheroprotection both by modulating local production and by attenuating the influence of bFGF on VSMC growth.

Effects of ovariectomy and hypothalamic-pituitary disconnection on amounts of steroidogenic factor-1 mRNA in the ovine anterior pituitary gland

Steroidogenic factor-1 (SF-1) is a transcription factor involved in regulation of steroidogenic enzymes. Recent evidence indicates that SF-1 is also important in the anterior pituitary gland, where it may influence gene expression in gonadotropes. We isolated a cDNA encoding ovine SF-1 and demonstrated that the SF-1 gene is expressed in the anterior pituitary gland of sheep. SF-1 transcripts and luteinizing hormone (LH) were colocalized in gonadotropes by in situ hybridization and immunohistochemistry, respectively. To test the hypothesis that GnRH stimulates pituitary expression of ovine SF-1 mRNA, ewes were ovariectomized to increase endogenous secretion of GnRH. Compared to ovary-intact ewes, ovariectomy resulted in three- and fourfold increases in steady-state amounts of mRNA encoding SF-1 and LH beta subunit, respectively. In ovariectomized ewes in which delivery of GnRH to the anterior pituitary gland was prevented by hypothalamic-pituitary disconnection (HPD), steady-state amounts of mRNA encoding SF-1 and LH beta-subunit were decreased. These results provide evidence that pituitary SF-1 gene expression in sheep is regulated by GnRH. Coordinate regulation of mRNAs encoding SF-1 and LH beta-subunit raises the possibility that SF-1 may be an important transcriptional regulator of LH beta-subunit gene expression in ovine gonadotropes.