Choline transport in human placental brush-border membrane vesicles

Abstract

Pathways for transport of choline by human placental epithelia were investigated using brush border membrane vesicles isolated by divalent cation precipitation. The presence of choline transport mechanisms mediating Na +-choline cotransport, choline/H + exchange and facilitated diffusion were assessed from [ 3H]choline tracer flux measurements. The rate and magnitude of intravesicular choline accumulation was unaffected by the imposition of an inwardly directed Na + gradient suggesting an absence of a mechanism mediating brush border membrane Na +-choline cotransport. The imposition of inside-acid or inside-alkaline pH gradients was observed to have no significant effect on choline uptake suggesting choline is not a substrate for placental epithelial organic cation/H + exchange. Conditions favoring the development of an inside-negative K + diffusion potential was observed to induce a concentrative accumulation of choline to levels exceeding equilibrium suggesting the presence of a conductive uptake pathway for choline in placental brush border membrane. Evidence to suggest conductive choline uptake resulted from a mediated transport process includes a demonstration of the counterflow phenomena, the concentration-dependent inhibition by hemicholinium-3 ( I C50 ≅ 100 μ M ) and the saturable rate of conductive choline uptake ( K m ≅ 300 μ M , V max ≅ 30 nmol/mg per min ). Substrate specificity studies of the mechanism mediating conductive choline uptake suggest the interaction of choline with the transport protein occurs at a minimum of two sites: a site of negativity with the positively charged nitrogen group and a site of hydrogen bonding to the primary alcohol. Several commonly prescribed pharmaceuticals known to cross the placental barrier including imipramine, verapamil, propranolol, quinine, flurazepam, amiloride and ritodrin were observed to inhibit conductive choline uptake suggesting an interaction with the mechanism mediating conductive choline transport. Conductive choline uptake was unaffected by the presence of the basic amino acids lysine, arginine and histidine; the neurotransmitters serotonin, dopamine and histamine and the vitamins thiamine and carnitine which suggests the mechanism mediating conductive choline transport is not a pathway for placental uptake of these compounds.