Effects of Placental Luteotropin and Estrogen on the Growth of Artificially Formed Secondary Corpora Lutea of Pregnancy in Rats1

The tropic effects of placental luteotropin and estrogen on the rapid increase in weight and function of corpora lutes (CL) during midpregnancy were studied in PMSG-treated prepubertal pregnant rats. An injection of 20 IU of human chorionic gonadotropin (hCG) on Day 5 or 7 of pregnancy (sperm present = Day 0) produced secondary CL in addition to the primary CL of pregnancy (designated as Day-5-hCG rats and Day-7-hCG rats, respectively). The primary CL were dis-distinguished from the secondary by intraluteally injected India ink. Sixty-seven % of the hCG-treated rats continued pregnancy until term and 77% of them showed prolonged gestation. Ovaries bearing 13-day-old secondary CL secreted 127 μg/100 ml or more of progesterone on Day 21 of pregnancy, while control pregnant rats secreted insignificant amounts of progesterone; 32% of the ovaries bearing 14–15-day-old secondary CL secreted progesterone at levels comparable to the second half of pregnancy. The primary CL showed rapid growth after Day 10 of pregnancy when the secretion of placental luteotropin was expected. While the weight of the secondary CL of both Day-5- and Day-7-hCG rats exhibited no significant increase at the time of placental luteotropin secretion, it increased significantly on Days 16–18 of pregnancy or at Day 10 of CL age in both groups. Estradiol receptor contents in nuclei of the secondary CL were at low levels at Day 7 of CL age and increased 2-fold at Day 10. Daily injections of clomiphene citrate to normal pregnant and Day-5-hCG-treated rats during Days 9–11 and Days 15–17 of pregnancy, respectively, inhibited both the rapid growth of CL on Days 10–13 of age and the increment of progesterone secretion. These findings suggest that the placental luteotropin is not indispensable for the rapid increase in weight and function of rat CL during midpregnancy and that estrogen is the principal factor of the luteotropic complex at this stage.