Adrenergic Receptors in Inner and Outer Layers of Human Myometrium Near Term: Characterization of Beta-Adrenergic Receptor Sites by [125I]-Iodocyanopindolol Binding

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Abstract
The Β-adrenergic receptors present in inner and outer layers of human myometrium near term were characterized using the radioiodinated antagonist iodocyanopindolol (ICYP). In both layers ICYP binding is saturable, rapidly reversible, stereoselective, and appears to occur in a single class of sites with a KD of 30 pmol/l. Adrenergic agonists compete for ICYP binding sites with an order of potency consistent with Β2-adrenergic potencies: isoproterenol > epinephrine > > norepinephrine. Studies in which CGP 20712 A, a Β1 adrenergic antagonist, competes for ICYP binding sites in human myometrium reveal that at least 65% of the Β-receptors present are Β2-subtype, whatever the layer considered. At the 35th week of pregnancy, the density of Β-adrenergic receptors in the inner layer (15.2 fmol/mg of protein) is about 50% higher than in the outer layer. At term, the densities of Β-adrenergic receptors are reduced and exhibit the same values for both layers (5 fmol/mg of protein). These results indicate that the Β-adrenergic receptors in the two layers diminish during pregnancy and reach, at term, an equal and low level of density.