Direct Pituitary Effects of Estrogen and Progesterone on Gonadotropin Secretion in the Ovariectomized Ewe

The direct pituitary effects of estrogen and progesterone on the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were studied in ovariectomized (OVX) ewes in which the pituitary had been disconnected surgically from the hypothalamus (hypothalamo-pituitary disconnection, HPD). Gonadotropin secretion was restored with hourly pulses of 500 ng gonadotropin-releasing hormone (GnRH) via intra-atrial cannulae. Intramuscular injections of 50 micrograms estradiol benzoate (EB) to 5 sheep initially caused reductions (approximately 50%) in plasma LH baseline, peak values and LH pulse amplitude. Thereafter all parameters of plasma LH concentration increased 2- to 3-fold above starting values. After these 5 sheep had received 2 subcutaneous progesterone implants (mean +/- SEM plasma levels 5.3 +/- 1.5 nmol/l), the biphasic LH response to EB was still apparent and increases in LH peak values (267 +/- 19%) and LH pulse amplitudes (262 +/- 23%) were greater (p less than 0.05) than those seen with EB alone (195 +/- 11 and 172 +/- 14%, respectively). The presence of 2 progesterone implants alone did not change plasma LH baseline, peak values or pulse amplitude, or plasma FSH values. In the second experiment, where 4 OVX-HPD ewes were given 4 progesterone implants (plasma progesterone 27.7 +/- 3.4 nmol/l), there were no effects on basal plasma LH or plasma FSH values. The LH responses to EB were more marked in 4 OVX-HPD ewes given 4 progesterone implants than in the animals given EB alone. Also, the estrogen-induced LH surge occurred earlier in the ewes given 4 progesterone implants than in those given estrogen alone.(ABSTRACT TRUNCATED AT 250 WORDS)