Effect of aging on A1-adenosine receptor-mediated inhibition of norepinephrine release in the rat heart.

Adenosine inhibits norepinephrine (NE) release from cardiac adrenergic nerves and reduces the postsynaptic β-adrenergic mediated actions of NE, leading to decreased myocardial force of contraction. The actions of adenosine are mediated by pre- and postsynaptic adenosine A 1 receptors (A 1 -AdoR). We reported that adenosine inhibition of postsynaptic β-adrenergic receptor-mediated cyclic adenosine monophosphate (cAMP) production declines with age in male F344 rat hearts. In this study, cardiac synaptosomes, isolated intact adrenergic nerve terminals, were used to examine the effect of age on adenosine inhibition of NE release. Cardiac synaptosomes were prepared from the hearts of 6- and 24-month-old male F344 rats, loaded with [ 3 H]NE, and placed in a superfusion system. [ 3 H]NE release was induced by high [K + ] exposure in the presence of varying concentrations of adenosine or the specific A 1 -AdoR agonist, N 6 -p-sulfophenyladenosine (SPA). [ 3 H]NE release was significantly reduced in old rats compared with young rats. Inhibition of [ 3 H]NE release by adenosine and SPA was significantly greater in young rats compared with old rats. The A 1 -AdoR antagonist, 8-(p-sulfophenyl)-theophylline, blocked the actions of adenosine on [ 3 H]NE release, and the specific adenosine A 2 -receptor agonist, cyclopropylcarboxamidoadenosine, had no effect on [ 3 H]NE release. Our data suggest that presynaptic A 1 -AdoR-mediated inhibition of NE release in the rat heart declines with age.