| Abstract: | - 1 The human placenta is a rich source of β-adrenoreceptors. However, previous work has provided conflicting evidence as to the predominant receptor subtype present. This study was designed to clarify this situation.
- 2 The radioligand 3H-dihydroalprenolol (3H-DHA) was used to identify β-adrenoreceptor binding sites in membranes prepared from human placentae obtained immediately post partum from normal full term pregnancies.
- 3 3H-DHA binding to human placental membranes was saturable, of high affinity and exhibited the pharmacological characteristics of a β-adrenoreceptor. Displacement by β-adrenoreceptor agonists suggested a predominant β1-subtype with a hierarchy of potency isoprenaline > adrenaline > noradrenaline. Analysis of the displacement of the specific binding of 3H-DHA by β-adrenoreceptor antagonists yielded Hill plots with an apparent slope (nH) of one for the non selective antagonist (?)propranolol, and of less than one for the highly selective antagonists practolol (β1), and ICI 118,551 (β2) indicating a possible heterogeneity of binding sites.
- 4 A direct comparison of the displacement of 3H-DHA binding by selective antagonists in the placenta with that occurring in rat and rabbit lung, further established that a mixture of β1- and β2-adrenoreceptor binding sites was present.
- 5 Graphical analysis by Hofstee plot and computer assisted iterative curve fitting was used to further evaluate the displacement by the selective agents and established the presence of an heterogeneous population of β-adrenoreceptor subtypes in the human placenta with approximately 65%β1 and 35%β2.
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