Phosphatidylinositol 3-Kinase Functionally Compartmentalizes the Concurrent Gs Signaling During &bgr;2-Adrenergic Stimulation

Compartmentation of intracellular signaling pathways serves as an important mechanism conferring the specificity of G protein-coupled receptor (GPCR) signaling. In the heart, stimulation of &bgr;2-adrenoceptor (&bgr;2-AR), a prototypical GPCR, activates a tightly localized protein kinase A (PKA) signaling, which regulates substrates at cell surface membranes, bypassing cytosolic target proteins (eg, phospholamban). Although a concurrent activation of &bgr;2-AR-coupled Gi proteins has been implicated in the functional compartmentation of PKA signaling, the exact mechanism underlying the restriction of the &bgr;2-AR-PKA pathway remains unclear. In the present study, we demonstrate that phosphatidylinositol 3-kinase (PI3K) plays an essential role in confining the &bgr;2-AR-PKA signaling. Inhibition of PI3K with LY294002 or wortmannin enables &bgr;2-AR-PKA signaling to reach intracellular substrates, as manifested by a robust increase in phosphorylation of phospholamban, and markedly enhances the receptor-mediated positive contractile and relaxant responses in cardiac myocytes. These potentiating effects of PI3K inhibitors are not accompanied by an increase in &bgr;2-AR-induced cAMP formation. Blocking Gi or G&bgr;&ggr; signaling with pertussis toxin or &bgr;ARK-ct, a peptide inhibitor of G&bgr;&ggr;, completely prevents the potentiating effects induced by PI3K inhibition, indicating that the pathway responsible for the functional compartmentation of &bgr;2-AR-PKA signaling sequentially involves Gi, G&bgr;&ggr;, and PI3K. Thus, PI3K constitutes a key downstream event of &bgr;2-AR-Gi signaling, which confines and negates the concurrent &bgr;2-AR/Gs-mediated PKA signaling.