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 3 H-dihydroalprenolol /DHA/. Steady-state binding was reached by 15 min at 25°C, and the association and dissociation rate constants were 0.0422 nM −1 min −1 and 0.0874 min −1 , respectively. From saturation experiments, B max of 82.0 ± 38 fmol/mg protein and K D =1.85 ± 0.92 nM were calculated. Inhibition of 3 H-DNA 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 3 H-DHA binding sites in early human lung may have a developmental importance which is not yet understood.