ATP-dependent bile acid transport across microvillous membrane of human term trophoblast

Abstract

The main fate for fetal bile acids is to be transferred to the mother by the trophoblast. In this study, ATP-dependent bile acid transport across the maternal- and the fetal-facing plasma membranes (mTPM and fTPM, respectively) of the human trophoblast was investigated. With the use of [14C]glycocholate (GC) and a rapid-filtration technique, GC transport by mTPM and fTPM was measured in the absence or the presence of 3 mM ATP plus an ATP-regenerating system. GC efflux from preloaded mTPM or fTPM vesicles was found to be insensitive to ATP. By contrast, GC uptake by mTPM, but not by fTPM, was significantly increased (approximately threefold) by ATP. This was temperature sensitive and occurred into an osmotically reactive space. Kinetic analysis revealed that GC uptake by mTPM was saturable and fit the Michaelis-Menten equation both in the absence and in the presence of ATP. ATP-dependent transport was not abolished by a protonophore (carbonyl cyanide p-trifluormethoxyphenyl hydrazone) together with 100 mM K+ (in = out) plus a K+ ionophore (valinomycin). It specifically required hydrolyzable ATP, although CTP had a slight stimulatory effect. Neither Na+ nor Cl- (100 mM, in = out) was mandatory. Moreover, 100 mM gradients of either Na+ (in << out) or Cl- (in >> out) had no effect on ATP-dependent GC uptake. This was inhibited by vanadate and bile acid analogues but not by several cholephilic organic anions and a variety of adenosine triphosphatase inhibitors. These results provide strong evidence for the existence of an ATP-dependent transport system for bile acids across the apical membrane of human trophoblast, which may play an important role in the control of the overall fetal-maternal bile acid traffic.