Identification of beta-adrenergic binding sites in rabbit myometrium

Rabbit uterine muscle may contract or relax with adrenergic stimulation depending on the hormonal milieu. This difference in contractile activity has been shown to be due to alteration of adrenergic response between alpha-adrenergic (contraction) and beta-adrenergic (relaxation). When rabbits are treated with estrogen followed by progesterone, norepinephrine produces myometrial relaxation. This effect is blocked by propranolol, indicating that it is mediated by beta receptor activation. A subcellular preparation of this myometrium has adenylate cyclase activity that can be stimulated by isoproterenol + guanyl-5'-yl-imidodi-phosphate (Gpp(NH)p). The radioligand [125I]iodohydroxybenzylpindolol binds to the same preparation. The binding is rapid, 80% maximal in 10 min, and readily reversible (t1/2 = 5 min). The binding is high affinity (Kd = 0.12 nM), low capacity (15 fmol/mg protein), and is to a single class of binding sites. Binding is competed for stereoselectively by beta adrenergic agonists and antagonists. The competition of beta adrenergic agonists for binding, isoproterenol = ritodrine greater than epinephrine greater than norepinephrine, is consistent with interactions at a beta2-adrenergic receptor.