RELATIONSHIP BETWEEN FOETAL CORTICOSTEROIDS, MATERNAL PROGESTERONE AND PARTURITION IN THE RAT

Abstract

The concentration of total corticosteroids, corticosteroid binding globulin (CBG) and progesterone were determined in maternal and fetal/neonatal plasma from rats on days 5, 4, 3, 2, 1 before birth and days 0, 1 and 4 after birth. In addition free corticosteroids and adrenal weight/unit body weight were measured on the fetuses/neonates and the fetuses, respectively. Although the concentration of maternal total corticosteroids and CBG ranged from 53.0 .+-. 12.5 to 31.0 .+-. 13.1 .mu.g/100 ml (.hivin.x .+-. SEM [standard error of the mean]) and 26.6 .+-. 2.2 to 45.1 .+-. 0.9 .mu.g corticosteroid bound/100 ml plasma, respectively, the changes in the concentration of these constituents were not related to the initiation of either parturition or lactation. The concentration of total corticosteroids in fetal plasma increased significantly (P < 0.05) from day 5 (14.6 .mu.g/100 ml) to reach peak concentrations (44.9 .mu.g/100 ml) on day 3 before birth and then decreased to low levels (7.7 .mu.g/100 ml) at birth. The pattern of change in fetal adrenal weight/unit body weight closely followed the pattern of change in the concentration of total corticosteroids in fetal plasma during the last 5 days of gestation. There was a significant (P < 0.05) daily decrease in the concentration of CBG in the fetuses from 4 days before until 1 day before birth, which resulted in a significant (P < 0.01) increase from 3.08 to 5.94 .mu.g/100 ml of free corticosteroids in the fetal plasma between day 2 and day 1 before birth, respectively. This peak corresponded with a significant fall (P < 0.02) in the maternal progesterone (measured by protein binding assay) from 2.57 .mu.g/100 ml to 0.62 .mu.g/100 ml between day 2 and day 1 before birth. Fetal progesterone (measured by radioimmunoassay) showed the same changes as maternal progesterone but was between 25-50% of that in maternal plasma. The changes in fetal free corticosteroids and maternal progesterone may be important in the initiation of parturition in the rat.