Syncytiotrophoblast membrane vesicles: A model for examining the human placental cholinergic system

Abstract

A membrane vesicle preparation was used to examine characteristics of the human placental cholinergic system. Plasma membrane vesicles were prepared from the microvillous surface of the human placental Syncytiotrophoblast. Membranes were purified 18‐ to 20‐fold as indicated by 5′‐nucleotidase activity. Vesicle cholinesterase activity was enriched and had a substrate preference consistent with that of acetylcholinesterase (acetylcholine > acetyl‐β‐methylcholine > butyryl‐choline). Choline acetyltransferase specific activity was reduced 80%. The synthetic muscarinic ligand, [³H]‐quinuclidinyl benzilate (QNB), was used to identify two classes of muscarinic cholinergic binding sites. The dissociation constant of QNB binding was 80 pM and 30 nM for the two sites. The sites were saturable and bound 9 fmoles and 910 fmoles per mg protein for the high and low affinity sites, respectively. Specific binding was inhibited by scopolamine, atropine, carbamylcholine (CCh), and diphenhydramine, but not by non‐muscarinic ligands‐i.e. GABA, glycine, d‐amphetamine, α‐bungarotoxin and nicotine. The cholinergic agonist CCh had no effect on active AIB transport, although pharmacologic doses (1 mM) of atropine, scopolamine and lidocaine reduced Na‐gradient active transport of α‐aminoisobutyric acid (AIB). No effect on Na‐independent AIB transport was observed. Thus, these drugs apparently reduced AIB uptake through their shared local anesthetic activity and not through a central cholinergic mechanism. In contrast, CCh was able to stimulate Ca²⁺ uptake by the vesicles in a dose‐dependent manner paralleling its ability to inhibit QNB binding. The CCh‐stimulated Ca²⁺ uptake was inhibited by scopolamine, implying its mediation via cholinergic‐type binding sites. The membrane vesicle preparation therefore provides a useful model for examination of the role of the human placental cholinergic system.