Atypical beta-adrenergic receptors in rat liver: evidence for transient expression during aging

This study was designed to determine whether functional beta 3-adrenergic receptors exist in the rat liver and whether there are alterations in the beta 3-adrenergic response with age in a manner similar to that which occurs for beta 2-adrenergic receptors. The beta 3 stimulation of adenylyl cyclase activity was assessed using the novel beta 3-specific agonist, CGP-12177A. Adenylyl cyclase activation by CGP-12177A was seen only in the adults. Isoproterenol-stimulated adenylyl cyclase activity was high in the neonates, declined by 45% in the adults, and was high again in the senescent rats. ICI 118551, a beta 2-selective antagonist, attenuated the isoproterenol-stimulated activity by two-thirds but had no effect on the CGP-12177A-stimulated activity. These data demonstrated the presence of the beta 3-adrenergic receptor in the rat liver, although only at the adult stage of development. In addition, these data confirm earlier findings that beta 2-adrenergic activation of adenylyl cyclase is biphasic with age and indicate that the emergence of the beta 3-stimulated activity coincides with the attenuation of beta 2-stimulated activity.