β 2 -Adrenergic Stimulation Compartmentalizes β 1 Signaling Into Nanoscale Local Domains by Targeting the C-Terminus of β 1 -Adrenoceptors

Rationale: βARs (β-adrenergic receptors) are prototypical GPCRs (G protein–coupled receptors) that play a pivotal role in sympathetic regulation. In heart cells, β 1 AR signaling mediates a global response, including both l -type Ca 2+ channels in the sarcolemma/T tubules and RyRs (ryanodine receptors) in the SR (sarcoplasmic reticulum). In contrast, β 2 AR mediates local signaling with little effect on the function of SR proteins. Objective: To investigate the signaling relationship between β 1 ARs and β 2 ARs. Method and Results: Using whole-cell patch-clamp analyses combined with confocal Ca 2+ imaging, we found that the activation of compartmentalized β 2 AR signaling was able to convert the β 1 AR signaling from global to local mode, preventing β 1 ARs from phosphorylating RyRs that were only nanometers away from sarcolemma/T tubules. This offside compartmentalization was eliminated by selective inhibition of β 2 AR, GRK2 (GPCR kinase-2), βarr1 (β-arrestin-1), and phosphodiesterase-4. A knockin rat model harboring mutations of the last 3 serine residues of the β 1 AR C terminus, a component of the putative βarr1 binding site and GRK2 phosphorylation site, eliminated the offside compartmentalization conferred by β 2 AR activation. Conclusions: β 2 AR stimulation compartmentalizes β 1 AR signaling into nanoscale local domains in a phosphodiesterase-4–dependent manner by targeting the C terminus of β 1 ARs. This finding reveals a fundamental negative feed-forward mechanism that serves to avoid the cytotoxicity of circulating catecholamine and to sharpen the transient β 1 AR response of sympathetic excitation.