Gi-Dependent Localization of b2-Adrenergic Receptor Signaling to L-Type Ca 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 b1-adrenergic receptor (b1-AR), coupled to Gs-protein, or b2-AR, coupled to Gsand Gi-proteins, both increase L-type Ca 21 current, causing enhanced contractile strength. But only b1-AR stimulation increases the phosphorylation of phospholamban, troponin-I, and C-protein, causing accelerated muscle relaxation and reduced myofilament sensitivity to Ca. b2-AR stimulation does not affect any of these intracellular proteins. We hypothesized that b2-AR signaling might be localized to the cell membrane. Thus we examined the spatial range and characteristics of b1-AR and b2-AR signaling on their common effector, L-type Ca 21 channels. Using the cell-attached patch-clamp technique, we show that stimulation of b1-AR or b2-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 b1-AR stimulation activated the channels. Thus, b1-AR signaling to the channels is diffusive through cytosol, whereas b2-AR signaling is localized to the cell membrane. Furthermore, activation of Gi is essential to the localization of b2-AR signaling because in pertussis toxin-treated cells, b2-AR signaling becomes diffusive. Our results suggest that the dual coupling of b2-AR to both Gsand Gi-proteins leads to a highly localized b2-AR signaling pathway to modulate sarcolemmal L-type Ca 21 channels in rat ventricular myocytes.