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The enzyme 11β-hydroxysteroid dehydrogenase (11β-HSD) catalyses the interconversion of corticosterone, the major glucocorticoid of the rat, and the biologically-inactive 11-dehydrocorticosterone. In the placenta, 11β-HSD is thought to regulate glucocorticoid transport between maternal and fetal compartments, and may also affect the local action of glucocorticoids. The present study assessed whether 11β-dehydrogenase (corticosterone to 11-dehydrocorticosterone) and 11-oxoreductase (11-dehydrocorticosterone to corticosterone) activities are both present in rat placenta, and whether these activities change with advancing pregnancy. Enzyme activity was estimated on days 16, 19 and 22 of pregnancy (term=day 23) in placental fragments incubated for 6 h with either [3H]corticosterone or [3H] 11-dehydrocorticosterone. The percentage conversion of these substrates to [3H] 11-dehydrocorticosterone and [3H] corticosterone, respectively, were determined at the end of the incubation. Both 11-oxoreductase and 11β-dehydrogenase activities were clearly evident in placental tissue fragments, and while 11-oxoreductase activity declined with advancing pregnancy (P<0·01), 11β-dehydrogenase activity increased (P<0·01). Thus, 11-oxoreductase exceeded (P<0·05) 11β-dehydrogenase at day 16, but thereafter activities were similar. These changes do not appear to be glucocorticoid-induced, since pretreatment of rats with either metyrapone or dexamethasone acetate from day 15 of pregnancy did not affect placental 11β-HSD on day 22. To allow comparison with earlier studies, estimates of 11β-HSD were also made in placental homogenates at each stage of pregnancy. In contrast to observations in placental fragments, 11β-dehydrogenase was always the dominant reaction in homogenates, presumably due to the loss of 11-oxoreductase activity following tissue homogenisation.
These data demonstrate that net 11β-dehydrogenase activity in the rat placenta increases towards term, thereby increasing the capacity for placental inactivation of active glucocorticoid. This pattern of 11β-HSD is consistent with reduced transfer of active glucocorticoid between the mother and fetus near term, and thus should promote independence of their hypothalamic-pituitary-adrenal axes.
Journal of Endocrinology (1994) 143, 505–513
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