Progesterone directly inhibits pituitary luteinizing hormone secretion in an estradiol-dependent manner.
We recently demonstrated that progesterone and estradiol inhibit pituitary LH secretion in a synergistic fashion. This study examines the direct feedback of progesterone on the estradiol-primed pituitary. Nine ovariectomized (OVX) ewes underwent hypothalamic-pituitary disconnection (HPD) and were infused with 400 ng GnRH every 2 h throughout the experiment. After 7 days of infusion, estradiol was implanted s.c. Four days later, estradiol implants were exchanged for blank implants in 4 ewes and for progesterone implants in 5 ewes. These implants remained in place for another 4 days. Blood samples were collected around exogenous GnRH pulses before and 0.5 to 96 h after implant insertion and exchange. Serum LH and progesterone concentrations were determined through RIA. One month later, 4 of the HPD-OVX ewes previously implanted with steroids were reinfused with GnRH and the implantation protocol was repeated using blank implants only. In estradiol-primed ewes, progesterone significantly lowered LH secretion after 12 h of implantation and LH secretion remained inhibited while progesterone implants were in place (p less than 0.05). Removing estradiol transiently lowered LH secretion, and this effect was significant only 24 h after estradiol withdrawal (p less than 0.05). These data suggest that progesterone has a direct, estradiol-dependent inhibitory effect on pituitary LH release and that estradiol may sustain pituitary gonadotrope response to GnRH.
Girmus
R L
RL
Department of Animal Science, University of Arizona, Tucson 85721.
Wise
M E
ME
D000818:Animals; D004343:Drug Implants; D004958:Estradiol / Q000008:administration & dosage / Q000494:pharmacology*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000008:administration & dosage / Q000494:pharmacology; D007262:Infusions, Intravenous; D007279:Injections, Subcutaneous; D007986:Luteinizing Hormone / Q000097:blood*; D010052:Ovariectomy; D010902:Pituitary Gland / Q000378:metabolism* / Q000502:physiology; D011374:Progesterone / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*; D011863:Radioimmunoassay; D012756:Sheep
D004343:Drug Implants; D007987:Gonadotropin-Releasing Hormone; D011374:Progesterone; D004958:Estradiol; D007986:Luteinizing Hormone