Alteration of adenosine [alpha]₁ and [beta]₁ receptor-mediated responses in the aging rat hearts and the effect of dietary restriction
Authors:   , Gao Erhe , Roberts Jay 
Published: July 2024

Adenosine (Ado) is a naturally-occurring myocardial metabolite and its concentration is increased during hypoxia, ischemia and aging processes. Adenosine is an endogenous cardioprotectant and its effects are mediated largely by interaction with adenosine [alpha]₁ receptors ([alpha]₁-AdoR). Stimulation of [alpha]₁-AdoR inhibits [beta]-adrenoceptor mediated activation of adenylyl cyclase (AC) and decreases production of cAMP, thus decreasing myocardial contraction and reducing oxygen demand which subsequently exerts a cardiovascular protective effect. In order to investigate the effect of age and dietary restriction (DR) on [beta]-adrenoceptor and [alpha]₁-AdoR signal transduction in the heart, the effects of isoproterenol (ISO), forskolin and specific [alpha]₁-AdoR agonists on AC activity were measured in crude cardiac membrane from 6-, 12- and 24-month-old male Fisher 344 Ad libitum (AL) and DR (fed 60% of AL) rats. There were no differences in basal AC activity with age. The [beta]-adrenoceptor dependent stimulation of AC activity decreased with age. In AL rats, the ISO-stimulated cAMP production reduced 36% in 12- and 53% in 24-month-old rats when compared to 6-month-old rats. There was also an age-related decline in receptor independent activation of AC activity. The forskolin-stimulated cAMP production was reduced 33% in 12- and 55% in 24-month-old rats compared with 6-month-old rats. In contrast, the decline in ISO- and forskolin-stimulated AC activity was attenuated in DR animals. There was an age-related decline in the capacity of [alpha]₁-AdoR agonists, such as N⁶-sulfophenyladenosine (SPA), to inhibit ISO- and forskolin-stimulated AC activity in AL rats. SPA produced 22% and 20% inhibition of ISO- and forskolin-stimulated AC activity in 6-month-old rats respectively but only inhibited 5% and 6% in 12-month-old rats and 3% and 4% in 24-month-old rats, respectively. Compared with AL rats, this age-related decline in [alpha]₁-AdoR function was attenuated in DR rats. Studies with radioligand ([³H]-DPCPX) showed that there were no differences in [alpha]₁-AdoR density with age and diet. Analysis of the dose-dependent displacement of [³H]-DPCPX binding by SPA, yielded two affinity binding sites in both 6- and 24-month-old AL and DR rats. However, the proportion of high-affinity [alpha]₁-AdoRs was significantly lower in 24-month-old AL rats (23.5%) compared with 24-month-old DR (42%), 6-month-old AL (54.9%) and 6-month-old DR rats (57%) (P < 0.05). Co-immunoprecipitation of [alpha]₁-AdoR with associated G proteins showed that [alpha]₁-AdoR coupled with G_([alpha]i3) and G_([alpha]o) proteins. Both basal and SPA-enhanced [alpha]₁-AdoR coprecepitation with G_([alpha]i3) and G_([alpha]o) proteins were reduced with age. These results indicate that in the absence of changes in [alpha]₁-AdoR density and G protein level, the age-related decline in [alpha]₁-AdoR response is partially due to the reduction in the coupling between [alpha]₁-AdoRs and their G proteins. DR maintains [alpha]₁-AdoR function by preventing the loss of high affinity [alpha]₁-AdoR sites. The present study has generated a significant finding which indicates that cardiac [alpha]₁-AdoR function decrease with age. This may partly explain the increased incidence of morbidity and mortality from heart disease in the elderly. This study also demonstrated that dietary restriction maintains [beta]-adrenoceptor, [alpha]₁-AdoR and receptor independent-modulation of AC activity. These findings provide important information that should prompt future hypotheses and experimentation to explore ways attenuating the aging process in the heart.