Prenatal terbutaline treatment: Tissue-selective dissociation of perinatal changes in ß-adrenergic receptor binding from regulation of adenylate cyclase activity
The role of prenatal ß-receptor stimulation in development of adrenergic reactivity was examined by administering the ß-agonist, terbutaline, to pregnant rats on gestational days 17, 18 and 19. On gestational day 20, liver membrane ß-receptor binding capabilities showed the depression characteristic of down-regulation, but heart and kidney receptor binding were essentially normal. Basal adenylate cyclase activity in the fetal liver membrane preparation was uncharged by terbutaline exposure and enzymatic reactivity to ß-adrenergic stimulation showed only a slight lowering; forskolin stimulation, however, was markedly increased in the terbutaline group. By postnatal day 2, receptor binding had returned to normal in the liver and remained at control levels in the other two tissues. Responsivity of adenylate cyclase to ß-receptor stimulation was markedly elevated in heart and kidney membranes; the effect represented an alteration at the level of the cyclase itself, rather than the receptor, since both basal activity and forskolin stimulation of the enzyme showed equivalent enhancement. These data thus suggest that early ß-adrenergic stimulation promotes cellular reactivity by fostering the development of membrane transduction mechanisms, rather than through effects on the receptor ligand binding site per se.