PHARMACOLOGICAL CHARACTERIZATION AND DISTRIBUTION OF MUSCARINIC RECEPTORS IN HUMAN PLACENTAL SYNCYTIOTROPHOBLAST BRUSH-BORDER AND BASAL PLASMA-MEMBRANES

Based on the existence of choline acetyltransferase and acetylcholine in human placenta, we have investigated the presence of muscarinic ace tylcholine receptors in brush-border and basal plasma membranes from h uman term placenta. Radioligand binding assay, using [H-3]N-methyl-sco polamine as tracer, showed the existence of acetylcholine muscarinic r eceptors in brush-border(K-d 0.28+/-0.04 nM; B-max 9.4+/-1.6 fmol/mg p rotein) and basal plasma membranes(K-d 0.24+/-0.05 nM; B-max 34.3+/-6. 3 fmol/mg protein). In order to perform a pharmacological characteriza tion of these receptors, competition binding experiments were carried out using the muscarinic receptor antagonists pirenzepine, nylacetyl-5 -11-dihydro-6H-pyrido(14)benzodiazepine (AF-DX 116), himbacine, 4-diph enylacetoxy-N-methylpiperidine methiodide (4-DAMP), dicyclomine and he xahydro-sila-difenidol (HHSD). The results obtained showed that the mu scarinic receptors in brush-border and basal plasma membranes belong t o different subtypes. In brush-border membranes, the receptor found ma tch in terms of affinity for the antagonists with the muscarinic M(1) receptor subtype (K-i pirenzepine, 13.6+/-8.2 nM; K-i AF-DX 116, 1680/-271 nM; K-i himbacine, 212+/-6.5 nM; K-i 4-DAMP, 1.5+/-0.4 nM; K-i d icyclomine, 5.1+/-0.8 nM; K-i HHSD, 34.3+/-7.3 nM), whereas the recept or in basal plasma membrane seems to be of the muscarinic M(2) recepto r subtype (K-i pirenzepine, 202+/-48 nM; K-i AF-DX 116, 124+/-60 nM; K -i himbacine, 20.6+/-4.8 nM; K-i 4-DAMP, 4.5+/-1.2 nM; K-i dicyclomine , 54.6+/-22 nM; K-i HHSD, 89.2+/-15.8 nM). The results obtained show t he existence of muscarinic acetylcholine receptors in brush-border and basal plasma membranes from human term placenta with a different dist ribution pattern in terms of number of receptors and distribution of d ifferent subtypes. The functional significance of these findings is as yet unknown, but these receptors probably mediate different functions as they belong to different subtypes and are coupled to different sec ond messengers.