G(1) protein-mediated functional compartmentalization of cardiac beta(2)-adrenergic signaling

In contrast to beta(1)-adrenoreceptor (beta(1)-AR) signaling, beta(2)-AR st imulation in cardiomyocytes augments L-type Ca2+ current in a cAMP-dependen t protein kinase (PKA)dependent manner but fails to phosphorylate phosphola mban, indicating that the beta(2)-AR-induced cAMP/PKA signaling is highly l ocalized. Here we show that inhibition of G(i) proteins with pertussis toxi n (PTX) permits a full phospholamban phosphorylation and a de novo relaxant effect following beta(2)-AR stimulation, converting the localized beta(2)- AR signaling to a global signaling mode similar to that of beta(1)-AR. Thus , beta(2)-AR-mediated G(i) activation constricts the cAMP signaling to the sarcolemma. PTX treatment did not significantly affect the beta(2)-AR-stimu lated PKA activation. Similar to G(i) inhibition, a protein phosphatase inh ibitor, calyculin A (3 x 10(-8) M), selectively enhanced the beta(2)-AR but not beta(1)-AR-mediated contractile response. Furthermore, PTX and calycul in A treatment had a non-additive potentiating effect on the beta(2)-AR-med iated positive inotropic response. These results suggest that the interacti on of the beta(2)-AR-coupled G(i), and G(s) signaling affects the local bal ance of protein kinase and phosphatase activities. Thus, the additional cou pling of beta(2)-AR to G(i) proteins is a key factor causing the compartmen talization of beta(2)-AR-induced cAMP signaling.