PROGESTERONE AND 20-ALPHA-HYDROXYPREGN-4-EN-3-ONE (20-ALPHA-OHP) IN THE PREGNANT BABOON - SELECTIVE PLACENTAL SECRETION OF 20-ALPHA-OHP INTO THE FETAL COMPARTMENT

The serum concentrations of progesterone and 20 alpha-hydroxy-pregn-4- en-3-one (20 alpha-OHP) were measured by RIA in blood draining the mat ernal and fetal sides of the placenta on Day 100 and Day 170 of gestat ion (term Day 184) in the baboon to determine the qualitative and quan titative patterns of progestins within the maternal-placental-fetal co mpartment and to ascertain whether production of placental progestins is increased with advancing gestation. The mean (+/- SEM) concentratio n of progesterone in maternal serum was similar at mid- (10 +/- 2 ng/m l) and late (11 +/- 3 ng/ml) gestation and not different than that of 20 alpha-OHP (6 +/- 1 ng/ml). In the uterine vein, progesterone levels were greater (p < 0.05) at Day 100 (82 +/- 13 ng/ml) than at Day 170 (30 +/- 7 ng/ml) and exceeded (p < 0.05) those of 20 alpha-OHP at both mid- (8 +/- 2 ng/ml) and late (4 +/- 1 ng/ml) gestation. Progesterone concentrations in the umbilical vein (128 +/- 24 ng/ml) and artery (7 9 +/- 12 ng/ml) at midgestation exceeded respective values at term (20 +/- 1 and 14 +/- 1 ng/ml). In contrast, 20 alpha-OHP concentrations i n the umbilical vein (17 +/- 4 ng/ml) and artery (12 +/- 3 ng/ml) at m idgestation increased more than 2-fold by Day 170. The estimated secre tion rate of placental progesterone into the fetus was similar at mid- (752 +/- 154 ng/min) and late (681 +/- 171 ng/min) gestation, whereas that for 20 alpha-OHP was negligible at midgestation (57 +/- 71 ng/mi n) and increased 15-fold (p < 0.05) by term (892 +/- 241 ng/min). Beca use 20 alpha-OHP is a metabolite of progesterone, total placental prog esterone production was greater (p < 0.05) at term (1595 +/- 400 ng/mi n) than at midgestation (809 +/- 171 ng/min). This study demonstrates that placental secretion of progesterone is bidirectional whereas that of 20 alpha-OHP occurs selectively into the fetus. Although progester one and 20 alpha-OHP levels in the fetus were lower at term than at mi dgestation, because of the developmental increase in umbilical blood f low as determined by others, placental progesterone production was act ually increased during this interval. Therefore, we suggest that the e strogen-dependent developmental increase in key components of the prog esterone biosynthetic pathway, recently demonstrated by us in the babo on placenta, is associated with a corresponding increase in progestero ne production.