β-Adrenergic Receptors and Catecholamine-Sensitive Adenylate Cyclase of the Human Placenta*

Abstract

β-Adrenergic receptor and β-adrenergic sensitive adenylate cyclase were demonstrated in membrane fractions of human placenta. Placental membranes from normal term pregnancies bound the β-adrenergic antagonist (−)[³H]dihydroalprenolol to a single saturable class of sites (Kd = 2.31 ± 0.23 nM; n = maximal capacity, 112±9 fmol/mg). Competition for binding was stereoselective for (−)isomers of propranolol, and β-adrenergic agonists displayed competition for the placental receptor in the order (−)isoproterenol > (−)epinephrine > (−)norepinephrine, typical of a β₂ type receptor. β-Adrenergic receptor was present in placental tissue as early as 10 weeks gestational age, and binding capacity decreased slightly with advancing gestation. [³H]Dihydroalprenolol binding was coupled to epinephrine-stimulated adenylate cyclase activity throughout gestation. The subcellular distribution of both β-adrenergic receptors and epinephrine-stimulated adenylate cyclase suggest their localization primarily in nonbrush border membrane fractions, presumably from plasma membranes more closely related to the fetal rather than to the maternal circulation. Epinephrinesensitive adenylate cyclase was not present in purified brush border preparations which were directly exposed to maternal blood in the intervillous space.