β-glucuronidase is not required for transfer of [ 3H]-estrone-[ 14C]glucuronide across guinea pig fetal membranes

Abstract

To understand the means whereby a charged, estrogen conjugate may be transferred across guinea pig amnion and chorion, the permeability to [ 3H]estrone-[ 14C]glucuronide was examined at 45 days and near term. No evidence of deconjugation was obtained in either early or late amnion, despite significantly greater transfer near term. Early amnion was virtually impermeable, regardless of ATP depletion. In contrast, early chorion transferred estrone-glucuronide without any requirement for deconjugation or ATP. No effect of tissue orientation was observed in amnion; whereas, incubations from maternal to fetal side of late chorion exhibited β-glucuronidase activity. Inhibition of the latter demonstrated that hydrolysis was concomitant with but not required for transport. [ 3H]Estrone produced by deconjugation was enzymatically reduced after pubic symphysis relaxation, although β-glucuronidase activity began prior to this stage. Transport across late fetal membranes was not saturable and chorion incubations from maternal to fetal side demonstrated a lower transport capacity. In either tissue orientation, late chorion displayed a lower rate of transfer than amnion. These results indicate that fetal membranes possess distinct abilities for transferring intact estrone-glucuronide, depending on stage of development and tissue orientation. The passive nature of transport and its dependence on structural characteristics is consistent with possible regulation of tight junctions.