Diminished alpha1-adrenergic-mediated contraction and translocation of PKC in senescent rat heart

Myocardial reserve function declines with aging due in part to reduced alpha- and beta-adrenergic receptor (AR)-mediated contractile augmentation. Whereas specific age-associated deficits in beta-AR signaling have been identified, it is not known which components of the alpha1-AR signaling cascade, e.g., protein kinase C (PKC) and associated anchoring proteins (receptors for activated C kinase; RACKs), underlie deficits in alpha1-AR contractile function with aging. We therefore assessed cardiac contraction (dP/dt) in Langendorff perfused hearts isolated from adult (5 mo) and senescent (24 mo) Wistar rats following maximal alpha1-AR stimulation with phenylephrine (PE), and we measured the subcellular distribution of PKCalpha and PKCepsilon, and their respective anchoring proteins RACK1 and RACK2 by Western blotting. The maximum dP/dt response to PE (10(-5) M) was significantly reduced by 41% in 24-mo-old vs. 5-mo-old (P < 0.01). Inhibitory effects of PKC blockade (chelerythrine; 10 microM) on dP/dt following alpha1-AR stimulation with PE observed in adult hearts were absent in 24-mo-old hearts (P < 0.01). In 5-mo-old hearts, PE elicited reductions in soluble PKCalpha and PKCepsilon levels, while increasing particulate PKCalpha and PKCepsilon levels to a similar extent. In contrast, soluble PKCalpha and PKCepsilon levels in 24-mo-old hearts were increased in response to PE; particulate PKCepsilon and PKCalpha were unchanged or reduced and associated with significant reductions in particulate RACK1 and RACK2. The results indicate, for the first time, that selective translocation of PKCalpha and PKCepsilon in response to alpha1-AR stimulation is disrupted in the senescent myocardium. That age-related reductions in particulate RACK1 and RACK2 levels were also observed provide evidence that alterations in PKC-anchoring proteins may contribute to impaired PKC translocation and defective alpha1-AR contraction in the aged rat heart.