β₂-Adrenergic Stimulation Compartmentalizes β₁ Signaling Into Nanoscale Local Domains by Targeting the C-Terminus of β₁-Adrenoceptors.

RATIONALE:βARs (β-adrenergic receptors) are prototypical GPCRs (G protein-coupled receptors) that play a pivotal role in sympathetic regulation. In heart cells, β₁AR signaling mediates a global response, including both l-type Ca²⁺ channels in the sarcolemma/T tubules and RyRs (ryanodine receptors) in the SR (sarcoplasmic reticulum). In contrast, β₂AR mediates local signaling with little effect on the function of SR proteins. OBJECTIVE:To investigate the signaling relationship between β₁ARs and β₂ARs. METHOD AND RESULTS:Using whole-cell patch-clamp analyses combined with confocal Ca²⁺ imaging, we found that the activation of compartmentalized β₂AR signaling was able to convert the β₁AR signaling from global to local mode, preventing β₁ARs from phosphorylating RyRs that were only nanometers away from sarcolemma/T tubules. This offside compartmentalization was eliminated by selective inhibition of β₂AR, 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 β₁AR C terminus, a component of the putative βarr1 binding site and GRK2 phosphorylation site, eliminated the offside compartmentalization conferred by β₂AR activation. CONCLUSIONS:β₂AR stimulation compartmentalizes β₁AR signaling into nanoscale local domains in a phosphodiesterase-4-dependent manner by targeting the C terminus of β₁ARs. This finding reveals a fundamental negative feed-forward mechanism that serves to avoid the cytotoxicity of circulating catecholamine and to sharpen the transient β₁AR response of sympathetic excitation.

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