The identification and characterization of β-adrenergic receptors in human amnion tissue

With the use of [¹²⁵I]iodocyanopindolol as a β-receptor ligand, β-receptors were identified and characterized in human amnion tissue. [¹²⁵I]Iodocyanopindolol was found to bind to a total particulate fraction prepared from amnion tissue obtained at term. At low concentrations of [¹²⁵I]iodocyanopindolol, more than 80% of total [¹²⁵I]iodocyanopindolol bound was at specific high-affinity sites and could be displaced by an excess of (±)-propranolol. The K_d and B_max for binding of [¹²⁵I]iodocyanopindolol to amnion β-receptors were 10.1 ± 1.1 pM and 46.8 ± 3.2 fmol/mg protein, respectively. Analysis of the competition for binding to amnion β-receptors between [¹²⁵I]iodocyanopindolol and ligands that discriminated between β₁- and β₂-receptors revealed that the β-receptors of human amnion were almost entirely of the β₂-subtype. The density of β-receptors found in the amnion at term was approximately three times that found early in the second trimester of gestation. The β-receptors in human amnion appear to be functional since the in vitro exposure of amnion tissue pieces to isoproterenol (10^-5M), resulted in a fivefold increase in the intracellular concentration of cyclic adenosine monophosphate. The presence of β-receptors in the amnion is in keeping with the proposed importance of the catecholamines found in amniotic fluid in the regulation of prostaglandin production by the amnion.