Interaction of progesterone, GnRH and estradiol in the control of LH release in castrate heifers.

Thirteen ovariectomized heifers were randomly divided into two groups for each of three experiments. In the first experiment, 10 mg estradiol-17~ (E) was injected intramuscularly into all heifers. Seven heifers also received intramuscularly (im) 25 mg progesterone (P) at 1 2 , O, +12 and +24 hr relative to E injection. Estradiol had a biphasic effect on serum luteinizing hormone (LH) measured at various intervals. Serum LH was significantly (P<.01) depressed at 12 hr but preovulatory-like peaks of LH were noted between 20 and 32 hr after E. Neither the inhibitory feedback of E at 12 hr nor the stimulatory feedback at about 24 hr was significantly affected by P treatment. In the second experiment, 40 /.tg gonadotropin-releasing hormone (GnRH) was injected im into all 13 heifers. Seven heifers also received 25 mg P at 2 8 , 1 6 and 4 hr relative to GnRH. Peaks of serum LH were observed 1 to 3 hr post-injection in five of six heifers receiving only GnRH. Only three of seven heifers presented with P exhibited LH peaks after GnRH and their peaks tended to be smaller. Average serum LH for all heifers was decreased for at least 3 hr post-GnRH by progesterone pretreatment. In the third experiment, the conditions of experiment 2 were duplicated except that all