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Regulation of the β-Adrenergic Receptor Signaling Pathway in Sustained Ligand-Activated Preconditioning

Sustained ligand-activated preconditioning (SLP), induced with chronic opioid receptor (OR) agonism, enhances tolerance to ischemia/reperfusion injury in young and aged hearts. Underlying mechanisms remain ill-defined, although early data implicate phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) during the induction phase, and β2-adrenoceptor (β2-AR), Gs alpha subunit (Gαs), and protein kinase A (PKA) involvement in subsequent cardioprotection. Here, we tested for induction of a protective β2-AR/Gαs/PKA signaling axis with SLP to ascertain whether signaling changes were PI3K-dependent (by sustained cotreatment with wortmannin), and whether the downstream PKA target Rho kinase (ROCK) participates in subsequent cardioprotection (by acute treatment with fasudil). A protected phenotype was evident after 5 days of OR agonism (using morphine) in association with increased membrane versus reduced cytosolic levels of total and phosphorylated β2-ARs; increased membrane and cytosolic expression of 52 and 46 kDa Gαs isoforms, respectively; and increased phosphorylation of PKA and Akt. Nonetheless, functional sensitivities of β2-ARs and adenylyl cyclase were unchanged based on concentration-response analyses for formoterol, fenoterol, and 6-[3-(dimethylamino)propionyl]-forskolin. Protection with SLP was not modified by ROCK inhibition, and changes in β2-AR, Gαs, and PKA expression appeared insensitive to PI3K inhibition, although 5 days of wortmannin alone exerted unexpected effects on signaling (also increasing membrane β2-AR and PKA expression/phosphorylation and Gαs levels). In summary, sustained OR agonism upregulates cardiac membrane β2-AR expression and phosphorylation in association with increased Gαs subtype levels and PKA phosphorylation. While Akt phosphorylation was evident, PI3K activity appears nonessential to OR upregulation of the β2-AR signal axis. This opioidergic remodeling of β2-AR signaling may explain β2-AR, Gαs, and PKA dependence of SLP protection.