Effect of Progestin and Progesterone on Ovulation in the Rabbit

Although the post partum rabbit (oestrus) uniformly ovulates after coitus, the pregnant or pseudopregnant rabbit does not ovulate after coitus. This failure of post-coital ovulation in the pregnant or pseudopregnant rabbit, as well as the absence of spontaneous ovulation during pregnancy in other species, has been attributed to some inhibitory influence of the corpus luteum. It is obvious that this inhibition could be effected either by rendering the ovarian follicles refractory to the normal concentrations of the gonadotropic hormone, or by interfering with the normal supply of this hormone to the ovarian follicles.

To determine the mechanism of this inhibition, post partum rabbits were injected daily with varying doses of progestin∗ or progesterone† for 5 days. At the end of the fifth day an attempt was made to mate the treated animals. Those animals which refused the male were immediately injected with one minimal ovulating dose of pregnancy urine extract‡ which had just been closely assayed by the rabbit method. 1 , 2 Laparotomy was performed 18–24 hours later to determine whether or not ovulation had occurred.

Of the 24 females which had been injected with corpus luteum preparations, 9 accepted the male. In not a single instance did coitus provoke ovulation. The remaining 15 females were injected with the P. U. extract, and this was followed by ovulation in 10 animals. From these results it is clear that post-coital ovulation in the oestrous rabbit is prevented by the daily injection of corpus luteum preparations in amounts equal to, or greater than, the quantity necessary to sustain pregnancy in the castrate rabbit. 3 It is equally clear that this inhibition of ovulation is not effected by altering the sensitivity of the ovarian follicle as measured by the response to P.U.