Ischaemic tolerance in aged mouse myocardium: The role of adenosine and effect of A₁ adenosine receptor overexpression

The genesis of the ischaemia intolerant phenotype in aged myocardium is poorly understood. We tested the hypothesis that impaired adenosine-mediated protection contributes to ischaemic intolerance, and examined whether this is countered by A₁ adenosine receptor (A₁AR) overexpression. Responses to 20 min ischaemia and 45 min reperfusion were assessed in perfused hearts from young (2-4 months) and moderately aged (16-18 months) mice. Post-ischaemic contractility was impaired by ageing with elevated ventricular diastolic (32 ± 2 vs. 18 ± 2 mmHg in young) and reduced developed (37 ± 3 vs. 83 ± 6 mmHg in young) pressures. Lactate dehydrogenase (LDH) loss was exaggerated (27 ± 2 vs. 16 ± 2 IU g^-1 in young) whereas the incidence of tachyarrhythmias was similar in young (15 ± 1%) and aged hearts (16 ≠ 1%). Functional analysis confirmed equipotent effects of 50 μM adenosine at A₁ and A₂ receptors in young and aged hearts. Nonetheless, while 50 μM adenosine improved diastolic (5 ± 1 mmHg) and developed pressures (134 ± 7 mmHg) and LDH loss (6 ± 2 IU g^-1) in young hearts, it did not alter these variables in the aged group. Adenosine did attenuate arrhythmogenesis for both ages (to ∼10%). In contrast to adenosine, 50 μM diazoxide reduced ischaemic damage and arrhythmogenesis for both ages. Contractile and anti-necrotic effects of adenosine were limited by 100 μM 5-hydroxydecanoate (5-HD) and 3 μM chelerythrine. Anti-arrhythmic effects were limited by 5-HD but not chelerythrine. Non-selective (100 μM 8-sulfophenyltheophylline) and A₁-selective (150 nM 8-cyclopentyl-1,3-dipropylxanthine) adenosine receptor antagonism impaired ischaemic tolerance in young but not aged hearts. Quantitative real-time PCR and radioligand analysis indicated that impaired protection is unrelated to changes in A₁AR mRNA transcription, or receptor density (∼8 fmol mg^-1 protein in both age groups). However, A₁AR overexpression improved tolerance for both ages, restoring adenosine-mediated protection. These data reveal impaired protection via exogenous and endogenous adenosine contributes to ischaemic intolerance with ageing. This is independent of A₁AR expression, and involves ineffective activation of a 5-HD-/diazoxide-sensitive process. The effects of A₁AR overexpression indicate that the age-related failure in signalling can be overcome.