Em Kudlacz

Control of Adenylate Cyclase Activity in Developing Rat Heart and Liver: Effects of Prenatal Exposure to Terbutaline or Dexamethasone

Adrenergic stimulation and glucocorticoids have been hypothesized to control the development of beta-adrenergic receptors and responsiveness. In the current study, rats were exposed to a beta-agonist (terbutaline) or a glucocorticoid (dexamethasone) during late gestation, and the development of adenylate cyclase activity and beta-receptor binding was evaluated in membranes prepared from the heart and liver. In control heart, basal, isoproterenol-stimulated and forskolin-stimulated adenylate cyclase showed distinct developmental spikes of activity that were unrelated to changes in receptor binding sites, but that instead corresponded temporally to periods of sympathetic neuronal activation. Prenatal exposure to terbutaline initially enhanced all three enzymatic measures in the immediate postpartum period but delayed the appearance and exaggerated the magnitude of the subsequent peaks; again, these changes occurred without specific relation to effects on receptor binding. Dexamethasone produced a similar shift in the peaks of cardiac enzyme activity. In the liver, where beta-adrenergic receptors and responsiveness decline after birth, terbutaline and dexamethasone had much smaller effects on adenylate cyclase. These results suggest that beta-adrenergic stimulation serves as a trophic factor controlling the ontogenetic rise of adenylate cyclase activity; regulation involves the level of the enzyme itself rather than changes in receptor binding capabilities or receptor-specific linkages. Consequently, prenatal administration of beta-agonists or glucocorticoids can cause long-term alterations in enzyme activity and responsiveness.

Fetal terbutaline exposure causes selective postnatal increases in cerebellar α-adrenergic receptor binding

beta-Adrenergic agonists used in therapy of premature labor and asthma cross the placenta and can affect development of the fetal nervous system. In the current study, pregnant rats were given 10 mg/kg of terbutaline on gestational days 17, 18 and 19 and adrenergic receptor binding capabilities examined in brain regions of the offspring. Despite the absence of body or brain growth impairment, selective increases were seen postnatally in cerebellar alpha 1- and alpha 2-receptor subtypes, whereas the same receptor populations were decreased by small amounts in cerebral cortex and midbrain + brainstem. beta-Adrenergic receptors showed little or no change in any region. The regional and subtype selectivity are compatible with primary deficits in the development of noradrenergic projections to the cerebellum identified in previous studies and provide further evidence that therapeutic use of beta-adrenergic agonists may produce neurobehavioral teratology.

Prenatal terbutaline treatment: Tissue-selective dissociation of perinatal changes in ß-adrenergic receptor binding from regulation of adenylate cyclase activity

The role of prenatal beta-receptor stimulation in development of adrenergic reactivity was examined by administering the beta-agonist, terbutaline, to pregnant rats on gestational days 17, 18 and 19. On gestational day 20, liver membrane beta-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 unchanged by terbutaline exposure and enzymatic reactivity to beta-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 beta-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 beta-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.

Effects of prenatal terbutaline exposure on cellular development in lung and liver of neonatal rat: ornithine decarboxylase activity and macromolecules

ABSTRACT: Endogenous ß-adrenergic input has been hypothesized to control patterns of cellular maturation in sympathetic target tissues by providing a timing signal for patterns of cell replication, differentiation, and responsiveness. In the current study, we examined the consequences of fetal exposure to a ß-agonist, terbutaline (2 or 10 mg/kg on gestational d 17, 18, and 19), in developing rat lung and liver, as assessed with ornithine decarboxylase activity and measurements of tissue macromolecules. Drug treatment caused an acute stimulation of ornithine decarboxylase in fetal and neonatal lung and blunted the peak of enzyme activity seen at 20-30 d postnatally without affecting acute responsiveness to ß-adrenergic stimulation (isoproterenol). Consistent with these results, patterns of cell replication and differentiation were altered, characterized by a deficit in cell acquisition (DNA content); in the tissue displaying fetal ODC stimulation (lung), there was also evidence for developmental abnormalities in RNA and protein. Thus, by prematurely stimulating ß-adrenergic receptors, prenatal administration of adrenergic agonists may have long-lasting, adverse effects on tissue development and responsiveness.

Maturation of the Sympathetic Nervous System: Role in Neonatal Physiological Adaptations and in Cellular Development of Peripheral Tissues

It obvious that the development of the nervous system is marked by the progression from immaturity of function to the acquisition of neurotransmission and integrated control of synaptic activity. However, there is an increasing realization that neural function in the fetus and neonate also serve specialized needs which are particular to development, and that the onset of maturity equally represents the loss of these unique patterns of neural activity. Nowhere is this more evident than in the sympathetic nervous system and its endocrine counterpart, the adrenal medulla. This review will detail recent work which demonstrates how catecholamines released first by the adrenal, and later by neurons, mediate the transition from fetal to neonatal physiological function as well as the subsequent programming of postsynaptic reactivity and cellular differentiation in target tissues. We have chosen the rat for study as this species is altricial, and therefore develops neural function relatively late. Thus, in the rat many fetal neural characteristics persist into the postnatal period.