Are developing β-adrenoceptors able to desensitize? Acute and chronic effects of β-agonists in neonatal heart and liver
During fetal and neonatal development, β-adrenergic receptors (β-ARs) appear to be resistant to desensitization by β-agonist drugs. To determine the mechanisms underlying the regulatory differences between adults and neonates, we administered isoproterenol, a mixed β1/β2-AR agonist, and terbutaline, a β2-selective agonist. Effects were examined in the ensuing 4 h after a single injection, or after the last of four daily injections. We prepared cell membranes from heart (predominantly β1-ARs) and liver (predominantly β2-ARs) and assessed signal transduction in the adenylyl cyclase (AC) pathway. In the first few hours after a single administration of isoproterenol to adult rats, cardiac β-ARs showed activation of G proteins (elevated AC response to forskolin) and desensitization of β-AR-mediated responses; after the fourth injection, heterologous desensitization emerged, characterized by a loss of signaling mediated either through β-ARs or glucagon receptors. Terbutaline evoked an increase in the forskolin response but no desensitization of receptor-mediated responses. When we gave the same treatments to neonatal rats, we observed cardiac G protein activation, but there was neither homologous nor heterologous desensitization of β-ARs or glucagon receptors. In the adult liver, isoproterenol and terbutaline both failed to evoke desensitization, regardless of whether the drugs were given once or for 4 days. In neonates, however, acute or chronic treatment elicited homologous desensitization of β-AR-mediated AC signaling, while sensitizing the response to glucagon. These results show that neonatal β-ARs are inherently capable of desensitization in some, but not all, cell types; cellular responses can be maintained through heterologous sensitization of signaling proteins downstream from the receptor. Differences from adult patterns of response are highly tissue selective and are likely to depend on ontogenetic differences in subtypes of β-ARs and AC.
Auman, JT; Seidler, FJ; Tate, CA; Slotkin, TA
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