Purpose: In a mouse model of oxygen-induced retinopathy (OIR), a well-established model of retinopathy of prematurity (ROP), blocking beta-adrenoreceptors (β-ARs), and, in particular, β2-ARs, counteracts retinal responses to hypoxia. In the present work, we determined the effects of the β-AR agonist isoproterenol on retinal angiogenesis and β-AR signaling to better clarify the role of sympathetic transmission in ROP.
Methods: Isoproterenol was administered subcutaneously. Protein kinase A activity was determined by a colorimetric assay to assess drug effectiveness. Blood pressure and heart-to-body weight ratio were measured. Vascular endothelial growth factor (VEGF) and norepinephrine were measured with ELISA. Retinal neovascularization was assessed by CD31 immunohistochemistry. β-AR-coupled adenylyl cyclase (AC) activity was measured with a competition assay. β-ARs, G-protein-coupled receptor kinase (GRK)2, and β-arrestins were determined by Western blot. Association of β-arrestins with β2-ARs was assessed by immunoprecipitation.
Results: Isoproterenol-induced modulation of protein kinase A activity suggests that the drug was effective at the receptor level. Isoproterenol did not affect cardiovascular parameters, but decreased retinal levels of VEGF and reduced pathogenic neovascularization, likely through an influence on sympathetic transmission. In fact, isoproterenol downregulated β2-AR expression, recovered the hypoxia-induced increase in β-AR-coupled AC activity, and increased GRK2 and β-arrestins, which promote β-AR desensitization through the uncoupling of G-protein-coupled receptors from G proteins. Immunoprecipitation studies demonstrated that β-AR desensitization involved β2-ARs.
Conclusions: Our findings suggest that hypoxia-induced retinal neovascularization depends at least in part on increased sympathetic transmission, as reduction of sympathetic drive by agonist-induced β2-AR desensitization inhibits some of the hallmarks of OIR.