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.
Keywords: calcium signaling; phosphorylation; sarcolemma; sarcoplasmic reticulum; serine.