β-Adrenergic Receptor Overexpression in the Fetal Rat: Distribution, Receptor Subtypes, and Coupling to Adenylate Cyclase Activity via G-Proteins

Abstract

Biogenic amines are hypothesized to play a role in the control of cell differentiation. We assessed the development of β-adrenergic receptors and their linkage to adenylate cyclase activity in order to determine whether catecholaminergic cell signaling can take place early in development. On Gestational Day 12, β-receptors were present in rat embryo in concentrations comparable to those in mature adrenergic target tissues; the concentrations climbed fivefold by Gestational Day 18. β-Receptor expression was higher in liver than in heart and brain, as identified both by binding to isolated membrane preparations and by receptor autoradiography; nevertheless, receptor distribution was quite widespread, with labeling visible throughout the fetus. Receptor subtype selectivities (β₂ in liver, β₁ in heart, predominantly β₂ in whole fetus) were already in place in early development, but receptor coupling to adenylate cyclase via G-proteins showed substantial developmental changes. Agonist-induced displacement of radioligand binding showed little or no GTP sensitivity on Gestational Day 12, suggesting relatively poor receptor linkage to G(s). In contrast, by Day 18, GTP produced a large shift in the agonist displacement curve. Receptor stimulation of adenylate cyclase by isoproterenol also showed a developmental spike by Gestational Day 18; the pattern for isoproterenol stimulation was distinct from the ontogeny of adenylate cyclase itself and from stimulation by forskolin-Mn²⁺ (which bypasses the need for receptors or G-proteins) or by fluoride (which activates G-proteins nonselectively). Thus, β-receptors are highly expressed during fetal development and the receptors are readily capable of modulating intracellular cAMP production. Fetal catecholamines, which are produced and released by the adrenal medulla, extraadrenal chromaffin tissue, and cells that transiently express adrenergic phenotype, can thus have a direct impact on the differentiation of a wide variety of cells.