In vivo and in vitro Ovarian Steroidogenesis in the Pregnant Rat

The interrelationship between follicular and luteal function during pregnancy in the rat was examined by in vivo and in vitro methods. Corpora lutea (CL) and nonluteal ovarian tissues (NLO) were removed on Days 2 to 22 (at 2 day intervals) (day sperm-positive = Day 1 of pregnancy), and incubated separately for 2 h to determine the production rate of steroids. Changes in progesterone (P), 20α-dihydroprogesterone (20α-OHP), testosterone (T), and estradiol-17β (E2) in peripheral blood, CL, and NLO, and levels of LH and FSH in serum and pituitary were measured by radioimmunoassay. A marked decline in the in vitro production rate of E2 and T in NLO occurred between Days 14 and 18 followed by an abrupt increase on Days 20 and 22. These changes correlated with the levels of serum LH between Days 14 and 22. Serum FSH also declined on Day 16 but returned to basal levels by Day 18. Despite the fall in NLO production of E2, serum E2 started to increase on Day 14 with a progressive rise continuing until term. A marked increase in luteal content of E2 occurred on Day 16 and continued until Day 22, whereas in vitro production of E2 by CL declined from Day 14 onward. Serum T also began to increase on Day 14 with the peak attained between Days 18 and 20, paralleling the rising E2 levels. In vitro production of luteal T increased on Day 12 with peak values present between Days 16 and 20, paralleling the pattern of serum T. Maximal values of serum P occurred between Days 14 and 16 and decreased after Day 18. In vitro production of luteal P increased abruptly on Day 4, with peak values on Day 10 and a decline by Day 16. There was a second peak in in vitro production of P on Day 22, although serum and luteal levels of P were already low. Changes in serum levels of 20α-OHP and its in vitro production rate were inversely related to changes in P throughout pregnancy. These findings indicate that at the end of pregnancy the CL produce large amounts of P in vitro after functional luteolysis has occurred in vivo. The high levels of P during midpregnancy may be involved in the suppression of follicular maturation, probably by lowering basal levels of serum LH. Furthermore, between Days 14 and 18, secretion of T and E2 represents proportionally more luteal than follicular sources of the hormones in the pregnant rat.