Age-associated reductions in cardiac β₁- and β₂-adrenergic responses without changes in inhibitory G proteins or receptor kinases

While an age-associated diminution in myocardial contractile response to β-adrenergic receptor (β-AR) stimulation has been widely demonstrated to occur in the context of increased levels of plasma catecholamines, some critical mechanisms that govern β-AR signaling must still be examined in aged hearts. Specifically, the contribution of β-AR subtypes (β₁ versus β₂) to the overall reduction in contractile response with aging is unknown. Additionally, whether G protein-coupled receptor kinases (GRKs), which mediate receptor desensitization, or adenylyl cyclase inhibitory G proteins (G(i)) are increased with aging has not been examined. Both these inhibitory mechanisms are upregulated in chronic heart failure, a condition also associated with diminished β-AR responsiveness and increased circulatory catecholamines. In this study, the contractile responses to both β₁-AR and β₂-AR stimulation were examined in rat ventricular myocytes of a broad age range (2, 8, and 24 mo). A marked age-associated depression in contractile response to both β-AR subtype stimulation was observed. This was associated with a nonselective reduction in the density of both β-AR subtypes and a reduction in membrane adenylyl cyclase response to both β-AR subtype agonists, NaF or forskolin. However, the age-associated diminutions in contractile responses to either β₁-AR or β₂-AR stimulation were not rescued by inhibiting G(i) with pertussis toxin treatment. Further, the abundance or activity of β-adrenergic receptor kinase, GRK5, or G(i) did not significantly change with aging. Thus, we conclude that the positive inotropic effects of both β₁- and β₂-AR stimulation are markedly decreased with aging in rat ventricular myocytes and this is accompanied by decreases in both β-AR subtype densities and a reduction in membrane adenylate cyclase activity. Neither GRKs nor G, proteins appear to contribute to the age-associated reduction in cardiac β-AR responsiveness.