Induction of Ovulation in Hypophysectomized Rats by Progesterone*

Abstract

Hypophysectomy performed at 1550 hr on proestrous day blocked ovulation in 4-day cyclic rats kept under 14 L, 10 D (light on 0500 hr), although a sharp rise of peripheral LH level had already been initiated by 1500 hr in intact animals. Three successive injections of 5 or 10 mg progesterone to animals hypophysectomized at 1550 hr beginning at time of operation resulted in complete restoration of ovulation. This restoration failed when the first injection was omitted and was only partially effective when the last two injections were omitted. Progesterone treatment of animals hypophysectomized earlier than 1550 hr was much less effective, suggesting that the release of an adequate amount of LH is also obligatory in inducing follicular rupture. Hypophysectomy at 1550 hr caused a marked reduction of progesterone level at 2400 hr, while in sham-operated animals the level increased between 1600 and 2400 hr. This period covers the ovulatory process. In these experiments progesterone induced ovulation directly and not via the hypophysis, suggesting that the continual and accelerated secretion of progesterone observed during the preovulatory stage is essential for the final step of follicular rupture. (Endocrinology95: 1322, 1974