Effects of Ovarian Steroids on Luteal Function: Prevention of Luteolysis following LH-Neutralization in the Pseudopregnant Rat 1

Abstract

The ability of estradiol (E) and progesterone (P) to act as luteotrophins in place of LH was investigated in pseudopregnant (PSP) rats. The length of PSP was extended by either hysterectomy (hyst.) or by the experimental induction of decidual tissue (DT) on Day 5 (Day 1 = ovulation). On Day 9, LH was removed from circulation by a sc injection of a specific LH antiserum (LHAS) and each rat was simultaneously treated with various dosages of E and/or P. Control rats were treated with normal horse serum (NHS) and/or oil vehicle. Blood samples were collected at 0, 24, and 72 hr post-treatment for the estimation of serum P levels. Day 5 hyst. rats treated with any dose (0.1 − 10.0 μg) of E or P (2 mg), alone or in combination, and simultaneously injected with NHS or LHAS exhibited elevated P levels through Day 12 and an extended diestrous cycle. In contrast, oil-treated, LHAS-injected rats underwent immediate luteolysis. DT-bearing PSP rats treated with 10 μg of E failed to undergo luteolysis following LHAS treatment; however, neither the lower E dosages nor P were able to prevent corpus luteum regression in these rats. The placement of estradiol (5 μg) pellets under the ovarian bursa maintained luteal function in both hyst. and DT-bearing PSP rats after the Day 9 LHAS injection. To determine if E prevented or merely delayed LH-dependent P secretion, 10.0 μg of E was administered on Day 9 and LHAS injected on Day 12. The E treatment effectively overrode the requirement for LH in the hyst.-PSP rat. In contrast, the DT-bearing PSP animals underwent luteolysis following the Day 12 LHAS treatment. NHS injections had no effect on luteal function. The results of these studies suggest that both E and P can serve as direct luteotrophins, replacing the requirement for LH by the rat corpus luteum. Higher doses of E are necessary in DT-bearing PSP rats than in hyst. animals to maintain luteal function. Also, while E eliminates the need for LH in the hyst.-PSP rat, it merely delays the luteal dependency on LH in the DT-bearing rat.