Expression of two α2-adrenergic receptor subtypes in human placenta: Evidence from direct binding studies

Summary Adrenergic receptors may play an important role for mediating a variety of metabolic and haemodynamic effects of catecholamines including placental blood flow. The α-adrenergic receptors of the human placenta were characterized in vitro by the use of [ 3 H]rauwolscine and [ 3 H]prazosin as radioligands. Saturation experiments would suggest that the α-adrenoceptors in the human placenta are α 2 . Comparative binding studies were performed, using recently synthesized compounds (Beecham Pharmaceuticals, UK) selective for α 2A (BRL-44408) and α 2B (BRL-41992) subtypes. The results indicate that human placenta contains at least two pharmacologically distinct α 2 -adrenoceptor subtypes with approximately 60 per cent α 2A and 40 per cent α 2B receptors. In contrast with the pattern of increasing β-adrenoceptor density, the concentration of α 2 -adrenoceptors in term placentae is significantly lower than in placentae from the first trimester.