Binding properties of beta-adrenergic receptors in early human fetal lung

The characteristics of beta-adrenergic receptors in human fetal lung were examined using the beta-adrenoceptor antagonist 3H-dihydroalprenolol /DHA/. Steady-state binding was reached by 15 min at 25 degrees C, and the association and dissociation rate constants were 0.0422 nM-1 min-1 and 0.0874 min-1, respectively. From saturation experiments, Bmax of 82.0 +/- 38 fmol/mg protein and KD = 1.85 +/- 0.92 nM were calculated. Inhibition of 3H-DHA binding by beta-1 /metoprolol/ and beta-2 /zinterol, IPS-339, fenoterol/ selective drugs resulted in biphasic displacement curves with slope factors less than 1.0. Analysis of these curves revealed a beta-1: beta-2 ratio of 40:60 in human fetal lung. The presence of 3H-DHA binding sites in early human lung may have a developmental importance which is not yet understood.