Placental transfer of steroids: effect of binding to serum albumin and to placenta

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Abstract
The transfer rates and placental retention of a series of steroids were measured using an in vitro perfusion system of an isolated cotyledon of human placenta. The steroids were added to the maternal inflow and rates of appearance in maternal and fetal outflows were measured, from which data were calculated the transfer rate and placental retention. With a low concentration of albumin (0.01 g/dl) in the maternal and fetal perfusates, transfer rates of diethylstilbestrol and ethynylestradiol were initially low, with considerable retention of the steroids within the placenta. Transfer rates increased with duration of perfusion. With high concentrations of albumin (1 g/dl), placental retention was greatly reduced and transfer rates very rapidly reached high levels. Albumin in the fetal circulation was the effective factor in increasing transfer rate; maternal albumin reduced it. The results with estrone and progesterone were qualitatively similar but not as striking, posssibly because of the large endogenous concentrations of these two hormones. Placental retention of dexamethasone, a more polar steroid that does not bind to placenta and binds poorly to albumin, was low and there was little difference between transfer from low- and high-protein perfusates.