Luteolysis Induced by Prostaglandin F2α Before and After Hysterectomy in Heifers

In a Latin Square design, each of 6 heifers was given (im) 5, 10 and 15 mg PGF20 during diestrus during three consecutive estrous cycles. Then the uterus and cervix were removed. Thirty to 45 days after surgery, each hysterectomized heifer was given 30 mg PGF3 � to induce estrus before the same experimental doses of PGF20 were resumed at 15-day intervals beginning 10 to 12 days after this induced estrus. Ten and 15 mg PGF20 were luteolytic in 10 of 12 heifers before and after hysterectomy. While 5 mg PGF3 � was not luteolyticin intactheifers,it caused luteolysis in three of four heifers after hysterectomy. Thus, the uterus is not required for luteolysis induced by exogenous PGF20 in heifers. To the contrary, the uterus may interfere with luteolysis induced by PGF20. Serum (jugular) progesterone changes were similarforthe first 6 h after 5, 10 or 15 mg PGF20 in heifers before and after hysterectomy, whether or not PGF2 � was luteolytic. After the first 6 h, progesterone remained elevated at about 60 percent of pretreatment values in heifers which did not exhibit estrous behavior, while progesterone declined to less than 1 ng/ml within 36 h in heifers which exhibited estrus. Following the initial rapid decline in progesterone, serum LH doubled within 3 h after administration of PGF20 and LH remained elevated until 6 h in all heifers; it returned to pretreatment values within 12 to 24 h when heifers exhibited no estrus, but it remained elevated when PGF20 was luteolytic. Serum estradiol increased within 18 h when PGF20 was luteolytic, and estrus began at 78 ± 5 h in these heifers. There was no rise in serum estradiol when heifers failed to exhibit estrus after PGF2 �. Thus, beginning within 18 h, elevated estrogen secretion may participate in the Iuteolysis begun by exogenous PGF2 �. Serum testosterone was unaffected by PGF20 injections before or after hysterectomy.