Developmental changes in β2-adrenergic receptor signaling in ventricular myocytes:: the role of Gi proteins and caveolae microdomains

Cardiomyocyte beta(2)-adrenergic receptors (beta-ARs) provide a source of inotropic support and influence the evolution of heart failure. Recent studies identify distinct mechanisms for beta(2)-AR actions in neonatal and adult rat cardiomyocytes. This study examines whether ontogenic changes in cardiac beta(2)-AR actions can be attributed to altered Gi expression or changes in the spatial organization of the beta(2)-AR complex in membrane subdomains (caveolae). We show that beta(2)-ARs increase cAMP, calcium, and contractile amplitude in a pertussis toxin (PTX)insensitive manner in neonatal cardiomyocytes. This is not caused by lack of Gi; Galphai expression is higher in neonatal cardiomyocytes than in those of adult rats. beta(2)-ARs provide inotropic support without detectably increasing cAMP, in adult cardiomyocytes. This cannot be attributed to dual coupling of beta(2)-ARs to Gs and Gi, because beta(2)-ARs do not promote cAMP accumulation in PTX-pretreated adult cardiomyocytes. Spatial segregation of beta(2)-ARs, Galphas/Galphai, and adenylyl cyclase to distinct membrane subdomains also is not a factor, because all of these proteins copurify in caveolin-3-enriched vesicles isolated from adult cardiomyocytes. However, these studies demonstrate that enzyme-based protocols routinely used to isolate ventricular cardiomyocytes lead to proteolysis of beta-ARs. The functional consequences of this limited beta-AR proteolysis is uncertain, because truncated beta(1)-ARs promote cAMP accumulation and truncated beta(2)-ARs provide inotropic support in adult cardiomyocytes. Collectively, these studies indicate that components of the beta(2)-AR signaling complex compartmentalize to restricted membrane subdomains in adult rat cardiomyocytes. Neither compartmentalization nor changes in Gi expression fully explain the ontogenic changes in beta(2)-AR responsiveness in the rat ventricle.