Transgenic A(1 )adenosine receptor overexpression increases myocardial resistance to ischemia

Activation of myocardial A(1) adenosine receptors (A(1)AR) protects the heart from ischemic injury. In this study transgenic mice were created using the cardiac-specific α-myosin heavy chain promoter and rat A(1)AR cDNA. Heart membranes from two transgene positive lines displayed ≈1,000-fold overexpression of A(1)AR (6,574 ± 965 and 10,691 ± 1,002 fmol per mg of protein vs. 8 ± 5 fmol per mg of protein in control hearts). Compared with control hearts, transgenic Langendorff-perfused hearts had a significantly lower intrinsic heart rate (248 beats per min vs. 318 beats per min, P < 0.05), lower developed tension (1.2 g vs. 1.6 g, P < 0.05), and similar coronary resistance. The difference in developed tension was eliminated by pacing. Injury of control hearts during global ischemia, indexed by time-to-ischemic contracture, was accelerated by blocking adenosine receptors with 50 μM 8-(p-sulfophenyl) theophylline but was unaffected by addition of 20 nM N(6)-cyclopentyladenosine, an A(1)AR agonist. Thus A(1)ARs in ischemic myocardium are presumably saturated by endogenous adenosine. Overexpressing myocardial A(1)ARs increased time-to-ischemic contracture and improved functional recovery during reperfusion. The data indicate that A(1)AR activation by endogenous adenosine affords protection during ischemia, but that the response is limited by A(1)AR number in murine myocardium. Overexpression of A(1)AR affords additional protection. These data support the concept that genetic manipulation of A(1)AR expression may improve myocardial tolerance to ischemia.