Gene dosage-dependent effects of cardiac-specific overexpression of the A₃ adenosine receptor

We used a genetic approach to determine whether increasing the level of A₃ adenosine receptors (A₃ARs) expressed in the heart confers protection against ischemia without causing cardiac pathology. We generated mice carrying one (A₃tg.1) or six (A₃tg.6) copies of a transgene consisting of the cardiomyocyte-specific α-myosin heavy chain gene promoter and the A₃AR cDNA. A₃tg.1 and A₃tg.6 mice expressed 12.7±3.15 and 66.3±9.4 fmol/mg of the high-affinity G protein-coupled form of the A₃AR in the myocardium, respectively. Extensive morphological, histological, and functional analyses demonstrated that there were no apparent abnormalities in A₃tg.1 transgenic mice compared with nontransgenic mice. In contrast, A₃tg.6 mice exhibited dilated hearts, expression of markers of hypertrophy, bradycardia, hypotension, and systolic dysfunction. When A₃tg mice were subjected to 30 minutes of coronary occlusion and 24 hours of reperfusion, infarct size was reduced ≈30% in A₃tg.1 mice and ≈40% in A₃tg.6 mice compared with nontransgenic littermates. The reduction in infarct size in the transgenic mice was not related to differences in risk region size, systemic hemodynamics, or body temperature, indicating that the cardioprotection was a result of increased A₃AR signaling in the ischemic myocardium. The results demonstrate that low-level expression of A₃ARs in the heart provides effective protection against ischemic injury without detectable adverse effects, whereas higher levels of A₃AR expression lead to the development of a dilated cardiomyopathy.