Ovine Luteinizing Hormone–Induced Steroid and Luteinizing Hormone Secretion, and Ovulation in Intact and Pregnant Mare Serum Gonadotropin–Primed Hens

Abstract

Studies were conducted to investigate the effect of ovine luteinizing hormone (LH) treatment on steroid and LH secretion and ovulation in intact, saline-primed hens and in pregnant mare serum gonadotropin (PMSG)-primed hens. Intact, saline-primed hens, injected 12 hr prior to the first (C1) ovulation of the sequence, responded with fully potentiated preovulatory surges of progesterone (P4) (peak value was 371% above preinjection values), estradiol-17β (E2) (117% above baseline), LH (220% above baseline), and premature ovulation. By comparison, those hens injected at the same time prior to the second (C2) ovulation responded with a lesser increase in plasma P4 (peak value was 305% above baseline) and E2 (72% above baseline), and there was no significant increase in plasma LH. Within this group, 4 of 5 hens failed to ovulate prematurely.All groups of PMSG-primed hens had significantly higher preinjection concentrations of P4 and E2, and lower basal concentrations of LH, compared to saline-primed hens. Subsequent to treatment of PMSG-primed hens with 25 or 100 μg ovine LH, there was a significant increase in plasma P4 to approximately 200% above preinjection concentrations. By contrast, there was a nonsignificant increase in plasma LH (50% above baseline values) and no significant increase in plasma E2. The ovulatory response following PMSG-priming was greatest in hens injected with 100 μg ovine LH (5 of 5 hens ovulated), while 3 of 5 hens injected with 25 μg ovine LH and 6 of 6 saline-challenged hens failed to ovulate. It was concluded: 1) that the inability to induce premature ovulation when ovine LH is injected 12 hr prior to a C2 ovulation is due to the inability of the follicle(s) to release sufficient concentrations of steroid to initiate and maintain a positive feedback interaction with LH; and 2) that PMSG-priming alters the steroid-producing capabilities of the preovulatory follicles and the ability to secrete LH.