Progesterone, Estradiol and Testosterone Levels in Ovarian Venous Blood of Pregnant Rabbits

Secretion rates of progesterone, estradiol and testosterone were measured by radioimmunoassay in the ovarian venous plasma of pregnant rabbits between the time of implantation and parturition. In the 72 cases studied, progesterone output rose rapidly after implantation on Day 7–8 post coitwn to reach peak values (78.3 ± 9.3 μg/ovary/hr) between Days 14 and 18, and thereafter remained high until the final week of gestation. Just prior to parturition (Days 30–32) progesterone outputs dropped to low levels (11.6 ± 2.8 μ-g/ovary/hr). Testosterone levels in ovarian venous plasma were about 10-fold higher than those of estradiol, and fluctuations in these 2 steroids tended to parallel each other. There was no evidence of an increased release of either estrogen or testosterone as parturition approached. Our results suggest that progesterone withdrawal, and the resultant decrease in the progesterone/ estrogen ratio, constitute the important stimuli for the onset of parturition in the rabbit. (Endocrinology93: 1235, 1973)