Cardiac β(2)-Adrenergic Receptor Phosphorylation at Ser(355/356) Regulates Receptor Internalization and Functional Resensitization

Previous studies have demonstrated that β(2)-adrenergic receptors (β(2)ARs) can be phosphorylated by G protein-coupled receptor kinases (GRKs) and protein kinase A (PKA), affecting β(2)AR internalization and desensitization. However, the exact physiological function of β(2)ARs in cardiomyocytes is unknown. In this study, we showed that neonatal mouse cardiomyocytes had different contraction and internalization responses to sustained or repeated, transient agonist stimulation. Specifically, short-time stimulation (10 min) with epinephrine or norepinephrine increased the cardiomyocyte contraction rate, reaching a maximum at 5 min, followed by a slow decline. When the agonist was re-added after a 60-min wash-out period, the increase in the cardiomyocyte contraction rate was similar to the initial response. In contrast, when cardiomyocytes were exposed continuously to epinephrine or norepinephrine for 60 min, the second agonist stimulation did not increase the contraction response. These results indicated that continuous β(2)AR stimulation caused functional desensitization. Phosphorylation of β(2)ARs at serine (Ser)(355/356) GRK phosphorylation sites, but not at Ser(345/346) PKA phosphorylation sites increased with continuous epinephrine stimulation for 60 min. Accordingly, β(2)AR internalization increased. Interestingly, β(2)AR internalization was blocked by mutations at the GRK phosphorylation sites, but not by mutations at the PKA phosphorylation sites. Furthermore, inhibition of β(2)AR dephosphorylation by okadaic acid, a phosphatase 2A inhibitor, impaired the recovery of internalized β(2)ARs and reduced the cardiomyocyte contraction rate in response to epinephrine. Finally, epinephrine treatment induced the physical interaction of β-arrestin with internalized β(2)ARs in cardiomyocytes. Together, these data revealed the essential role of the Ser(355/356) phosphorylation status of β(2)ARs in regulating receptor internalization and physiological resensitization in neonatal cardiomyocytes to contraction functions.