177 PARADOXICAL EFFECT OF PROTEIN-BINDING ON PLACENTAL TRANSFER OF STEROIDS

Abstract

The relation of protein-binding to the transfer rates of a series of polar and non-polar steroids has been studied in an in vitro perfusion system of human placenta. The clearance indices (C.I. = clearance substrate:clearance antipyrine) for the polar steroids (dexamethasone, betamethasone, prednisolone, cortisol, estriol) from buffer solutions ranged from 0.36 to 0.74. The addition of serum albumin, lg/dl, to maternal and fetal perfusates had relatively little effect (C.I. 0.25 - 0.51). The C.I. of the non-polar steroids (ethynilestradiol, progesterone, estrone, diethylstilbesterol) from buffer solutions ranged from 0.11 to 0.26. The addition of lg/dl of serum albumin to the perfusates caused a sharp increase in C.I. (0.52 - 0.83).

Percent binding to serum albumin measured by equilibrium dialysis for the polar steroids were 35 to 60; for the non-polar steroids, 89 to 94. Binding to 20% placental homogenate were 12 to 31 and 60 to 96, respectively.

The transfer of lipid-soluble materials between maternal and etal plasma is best understood as a series of equilibria in which binding to placenta plays a pivotal role.

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