OESTRADIOL POTENTIATES A PROLONGED PROGESTERONE-INDUCED SUPPRESSION OF LH RELEASE IN OVARIECTOMISED COWS

摘要

Our working hypothesis was that the negative feedback effect of progesterone on LH secretion in ovariectomised cows is greatly enhanced by oestradiol when both hormones are administered intravaginally. Each of eight ovariectomised cows had a progesterone releasing device ('CIDR-1'; Eazi-breed CIDR-B-TM, InterAg, Hamilton, New Zealand) inserted into the vagina for 3 days, followed by an injection of 1 mg oestradiol benzoate (ODB) in peanut oil 24 h after device removal. The timing of this injection was designated as Day 0. The CIDR-1 was reinserted concurrently with an injection of 0.4 mg ODB on Day 5. An additional device ('CIDR-2') was inserted 4 days later (Day 9). In four of the cows, an ODB capsule (CIDIROL(TM), Douglas Pharmaceuticals, Auckland, New Zealand) was placed into a groove on the surface of CIDR-2 preceding insertion. CIDR-2 was withdrawn after 5 days (Day 14) and CIDR-1 was withdrawn 2 days later (Day 16). The treatment regimen was repeated in a cross-over design where each cow received CIDR-2 with or without an ODB capsule. Blood samples were collected at intervals of 15 min for a period of 16 h beginning 8 h before insertion of CIDR-2 to determine changes in LH secretion pattern, and then at intervals of 8 h until 48 h after removal of CIDR-1 to determine mean LH and steroid concentrations.