β-Adrenergic Receptors and Cyclic Adenosine Monophosphate Generation in Human Fetal Lung

Abstract

ABSTRACT. We designed experiments to determine whether P-adrenergic receptors are present and functional in human fetal lung during the 2nd trimester of gestation. To determine the presence of p receptors, characterize their binding sites, and assess changes in receptor with gestational age, we performed radioligand binding assays with the specific, high-affinity P antagonist, '251-iodocyanopindolol, in membrane particulates from the lungs of 2nd trimester abortuses (15-23 wk). Binding of '251-iodocyanopindolol was saturable and of high affinity (dissociation constant = 40 pM). Binding was stereoselective as determined by competition studies with (-) and (+) stereoisomers of propranolol. Agonist affinities (isoproterenol > epinephrine >> norepinephrine) were consistent with a predominance of 8-2 receptors; this predominance was confirmed by competition studies with the specific P-2 receptor antagonist ICI 118-551 (75% 8-2,25% P-1). The concentration of B-adrenergic receptors increased with gestational age. To assess the functional coupling of the fi receptors, we tested the ability of receptor occupancy to activate adenylate cyclase. For this assay, we incubated minced human fetal lung with P agonists and determined the amount of cAMP generated. P Agonists stimulated cAMP generation more than 2-fold. We conclude that Padrenergic receptors are present and functional in human fetal lung as early as the 2nd trimester. (Pediatr Res 21:142-147,1986) Abbreviations ICYP, '2~-iodocyanopindolol ATP, adenosine triphosphate CAMP, cyclic adenosine monophosphate Gpp(NH)p, 5'-guanylylimidodiphosphate GTP, guanosine triphosphate K,,, dissociation constant There is increasing evidence for an integral role of adrenergic responses in the complex transition which the fetal lungs undergo in preparation for extrauterine life. Indirect evidence comes from clinical situations in which infants are not exposed to high levels of the adrenergic catecholamines, epinephrine and norepinephrine. For example, infants born by cesarean section without labor Received March 26, 1986: accepted September 18, 1986 have lower concentrations of catecholamines in amniotic fluid and cord blood than do those delivered vaginally or by cesarean section after labor (I). In association with these lower catecholamine levels, infants born by cesarean section without labor have a higher incidence of breathing difficulties (2), less mature biochemical indices of lung maturity (3,4), and lower tidal volumes and lung compliance (1) than do other infants. In contrast, infants born to mothers who received P-adrenergic agonists immediately before delivery are reported to have a lower incidence of hyaline membrane disease (5).In other species, there is substantial experimental evidence that P-adrenergic responses are involved in fetal lung maturation (6). Administration of P-adrenergic agonists to fetal sheep and rabbits significantly decreases lung water content (7,8), increases surfactant release (9, lo), and increases lung stability (1 1, 12). In fetal rabbits,...

β-Adrenergic Receptors and cAMP Response Increase during Explant Culture of Human Fetal Lung: Partial Inhibition by Dexamethasone

“…The difference between our studies and those in animals may simply reflect a difference between species in the effect of glucocorticoids on the P-adrenergic response cascade. The @,-receptor subtype, which predominates in rabbit fetal lung (I), may respond differently to dexamethasone treatment than the &receptor subtype, which predominates in human fetal lung (2,3). This could not explain the difference between rat and human fetal lung, both of which contain P2-receptors.…”

“…Samples for determination of receptor concentration were frozen and stored at -70°C until membrane preparation. We have previously shown that under these conditions P-adrenergic receptor concentration was not different from that in fresh specimens (3). Samples for determination of cAMP generation were processed immediately after harvesting.…”

“…Partially purified membrane particulates were prepared by differential centrifugation as previously described (3). Protein concentration was determined using the Bradford method (14) with BSA as the standard.…”

“…1 clase, as evidenced by the ability of isoproterenol to stimulate cAMP generation in human fetal lung as early as 20 wk gestation (3).…”

Intrapulmonary Terbutaline and Aminophylline Decrease Lung Liquid in Fetal Lambs

“…epinephrine and dibutyryl-CAMP have a greater effect in causing lung liquid absorption toward term than they do earlier in gestation (1,4,5,14). Previous studies of the ontogeny of 0-adrenergic receptors in the lungs of several species, including sheep, indicate that the concentration of P-adrenergic receptors in the lung increases with advancing gestation (7,9,(16)(17)(18). In fetal sheep, the concentration of pulmonary P-adrenergic receptors doubles between 125 and 135 d gestation (7), which might account for the increased effect of terbutaline that we observed over this range of gestation.…”