Gi-dependent localization of β2-adrenergic receptor signaling to L-type Ca2+channels

A plausible determinant of the specificity of receptor signaling is the cellular compartment over which the signal is broadcast. In rat heart, stimulation of beta (1)-adrenergic receptor (beta (1)-AR), coupled to G(s)-protein, or beta (2)-AR, coupled to G(s)- and G(i)-proteins, both increase L-type Ca2+ current, causing enhanced contractile strength. But only beta (1)-AR stimulation increases the phosphorylation of phospholamban, troponin-l, and C-protein, causing accelerated muscle relaxation and reduced myofilament sensitivity to Ca2+. beta (2)-AR stimulation does not affect any of these intracellular proteins. We hypothesized that beta (2)-AR signaling might be localized to the cell membrane. Thus we examined the spatial range and characteristics of beta (1)-AR and beta (2)-AR signaling on their common effector, L-type Ca2+ channels. Using the cell-attached patch-clamp technique, we show that stimulation of beta (1)-AR or beta (2)-AR in the patch membrane, by adding agonist into patch pipette, both activated the channels in the patch. But when the agonist was applied to the membrane outside the patch pipette, only beta (1)-AR stimulation activated the channels. Thus, beta (1)-AR signaling to the channels is diffusive through cytosol, whereas beta (2)-AR signaling is localized to the cell membrane. Furthermore, activation of Gi is essential to the localization of beta (2)-AR signaling because in pertussis toxin-treated cells, beta (2)-AR signaling becomes diffusive. Our results suggest that the dual coupling of beta (2)-AR to both G(s)- and G(i)-proteins leads to a highly localized beta (2)-AR signaling pathway to modulate sarcolemmal L-type Ca2+ channels in rat ventricular myocytes.