Age-associated reductions in cardiac beta1- and beta2-adrenergic responses without changes in inhibitory G proteins or receptor kinases.

Abstract

While an age-associated diminution in myocardial contractile response to beta-adrenergic receptor (beta-AR) stimulation has been widely demonstrated to occur in the context of increased levels of plasma catecholamines, some critical mechanisms that govern beta-AR signaling must still be examined in aged hearts. Specifically, the contribution of beta-AR subtypes (beta1 versus beta2) 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 (Gi) are increased with aging has not been examined. Both these inhibitory mechanisms are upregulated in chronic heart failure, a condition also associated with diminished beta-AR responsiveness and increased circulatory catecholamines. In this study, the contractile responses to both beta1-AR and beta2-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 beta-AR subtype stimulation was observed. This was associated with a nonselective reduction in the density of both beta-AR subtypes and a reduction in membrane adenylyl cyclase response to both beta-AR subtype agonists, NaF or forskolin. However, the age-associated diminutions in contractile responses to either beta1-AR or beta2-AR stimulation were not rescued by inhibiting Gi with pertussis toxin treatment. Further, the abundance or activity of beta-adrenergic receptor kinase, GRK5, or Gi did not significantly change with aging. Thus, we conclude that the positive inotropic effects of both beta1- and beta2-AR stimulation are markedly decreased with aging in rat ventricular myocytes and this is accompanied by decreases in both beta-AR subtype densities and a reduction in membrane adenylate cyclase activity. Neither GRKs nor Gi proteins appear to contribute to the age-associated reduction in cardiac beta-AR responsiveness.

Increased expression of iNOS is associated with endothelial dysfunction and impaired pressor responsiveness in streptozotocin-induced diabetes

“…It has been suggested that the increase in MABP with age is a normal age-dependent process that may be a compensatory mechanism for loss of arterial compliance (4). Similarly, the decrease in HR is usually attributed to a diminution in the efficacy of ␤-adrenergic stimulation of the heart, which again is normal and an age-dependent process (41,43) The depressed MABP and HR in diabetic rats observed in the present study is in agreement with many other studies in STZ diabetic rats (9,14,18,20,23,31,39,44). Previous studies from our own laboratory have shown lower HR and reduced systolic blood pressure in conscious STZ-induced diabetic rats (44).…”

Age Increases Cardiac Gαi2 Expression, Resulting in Enhanced Coupling to G Protein-coupled Receptors

“…These results are similar to other reports on ␤AR density in Fisher 344 rat hearts (21, 38 -42), though Mader et al (43) did report a decrease in affinity. In other rat strains there may be moderate decreases in receptor number (6,19,25,44).…”

“…PTX ADP-ribosylates G␣ i /G␣ o and prevents their interaction with receptors, thereby removing the inhibition of AC by these G protein subtypes (46,47). Our previous studies have shown that PTX treatment of human atrial membranes results in an increase in AC stimulation through ␤ 2 AR and other G␣ s -coupled receptors (4), and others have shown that PTX treatment of myocytes increases ␤ 2 ARmediated contractility (2,6). As shown in Table II, the inactivation of G␣ i /G␣ o by PTX increases both basal and GPCRstimulated AC activity, and this effect is greater in the old than in the young left ventricular membranes for the G␣ i -coupled ␤ 2 ARs and glucagon receptors.…”

“…In Sprague-Dawley rats, Bohm et al (19) found that age increases cardiac G␣ i , and the increase in G␣ i is reduced by exercise. In Wistar rats, Bazan et al (25) reported an increase in cardiac G␣ i with age, but Xiao et al (6) and Miyamoto et al (26) found no change. Some of the reported differences on the effect of age may be attributable to experimental design.…”

“…These results are similar to other reports on ␤AR density in Fisher 344 rat hearts (21, 38 -42), though Mader et al (43) did report a decrease in affinity. In other rat strains there may be moderate decreases in receptor number (6,19,25,44).We also examined whether there are age-dependent changes in the ability of muscarinic receptors to inhibit ␤ 1 AR-and ␤ 2 AR-stimulated AC in rat ventricle. As shown in Table I, stimulation of muscarinic receptors strongly inhibits ␤AR-stimulated AC in both young and old rat hearts.…”