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3β-Hydroxysteroid dehydrogenase (3β-HSD) activity in bovine cotyledons was much higher than in caruncles throughout the gestation period. The activity of this enzyme in cotyledons increased greatly to 150.6 ± 5.8 pmol min−1 mg−1 protein during the seventh month of gestation, reached a peak of 221.0 ± 34.9 pmol min−1 mg−1 protein during the eighth month, and decreased at parturition. Progesterone and 20α-hydroxyprogesterone concentrations in cotyledons also increased sharply to 2.69 ± 0.30 and 2.15 ± 0.42 ng mg−1 protein, respectively, during the seventh month of gestation, reaching peaks of 2.86 ± 0.47 and 2.51 ± 0.36 ng mg−1 protein, respectively, during the eighth month and decreasing at parturition, in a manner similar to the activity of 3β-hydroxysteroid dehydrogenase. The fluctuation of 17α-hydroxyprogesterone concentration in cotyledons was different from that of progesterone and 20α-hydroxyprogesterone. These findings indicate that the activity of 3β-hydroxysteroid dehydrogenase in the placenta is enhanced during the third trimester, and progesterone synthesized in the cotyledons is converted concurrently to 20α-hydroxyprogesterone before progesterone is transferred to the fetal blood.
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Two peaks of aromatase activities were detected in bovine cotyledons, 2.76 ± 0.96 pmol min−1 mg−1 protein at month five of gestation and 3.62 ± 0.78 pmol min−1 mg−1 protein immediately after parturition, but aromatase activity in caruncles was very low throughout gestation. Oestrone concentrations in the cotyledons and caruncles were 0.93 ± 0.50 and 1.51 ± 0.69 ng mg−1 protein at month five of gestation, but 4.31 ± 1.66 and 3.36 ± 0.98 ng mg−1 protein immediately after parturition, indicating a biphasic pattern. Our findings suggest that oestrogen synthesis in the bovine placenta increased to a maximum at least twice during the period from gestation to parturition; at month five of gestation and at parturition the changes in aromatase activity may only be partially explained by changes in oestrogen production.
