Caveolar localization dictates physiologic signaling of beta 2-adrenoceptors in neonatal cardiac myocytes

There is a growing body of evidence that G protein-coupled receptors function in the context of plasma membrane signaling compartments. These compartments may facilitate interaction between receptors and specific downstream signaling components while restricting access to other signaling molecules. We recently reported that beta(1)- and beta(2)-adrenergic receptors (AR) regulate the intrinsic contraction rate in neonatal mouse myocytes through distinct signaling pathways. By studying neonatal myocytes isolated from beta(1)AR and beta(2)AR knockout mice, we found that stimulation of the beta(1)AR leads to a protein kinase A-dependent increase in the contraction rate. In contrast, stimulation of the beta(2)AR has a biphasic effect on the contraction rate. The biphasic effect includes an initial protein kinase A-independent increase in the contraction rate followed by a sustained decrease in the contraction rate that can be blocked by pertussis toxin. Here we present evidence that caveolar localization is required for physiologic signaling by the beta(2)AR but not the beta(1)AR in neonatal cardiac myocytes. Evidence for beta(2)AR localization to caveolae includes co-localization by confocal imaging, co-immunoprecipitation of the beta(2)AR and caveolin 3, and co-migration of the beta(2)AR with a caveolin-3-enriched membrane fraction. The beta(2)AR-stimulated increase in the myocyte contraction rate is increased by approximately 2-fold and markedly prolonged by filipin, an agent that disrupts lipid rafts such as caveolae and significantly reduces co-immunoprecipitation of beta(2)AR and caveolin 3 and co-migration of beta(2)AR and caveolin-3 enriched membranes. In contrast, filipin has no effect on beta(1)AR signaling. These observations suggest that beta(2)ARs are normally restricted to caveolae in myocyte membranes and that this localization is essential for physiologic signaling of this receptor subtype.