Radioligands with high specific activity and high affinity have recently become available to study the beta-adrenergic receptor (BAR). BAR may be important in the placenta for mediating a variety of metabolic and hemodynamic effects of catecholamines including placental hormone synthesis and secretion, placental glycogenolysis, and placental blood flow. Little is known regarding the development of the BAR. We have used the tritiated radioligand dihydroalprenolol to study the ontogenesis of BAR in the fetal and maternal portions of the ovine placenta. The receptor number decreased from 415 fmoles . mg-1 of protein at 120 days' gestation to 226 fmoles . mg-1 at 145 days; this trend was highly significant (r = -0.822, p less than 0.01). Similar changes were noted in the dissociation constant. Downward regulation as a result of fetal neurosympathetic maturation is proposed as an explanation for this observation.
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