The advent of transgenic techniques provides a unique opportunity to study adenosine receptor-mediated cardioprotection. Transgenic A 1 receptor overexpression increases myocardial protection without impairing intrinsic function, but at the expense of resting bradycardia and a blunted response to catecholamine stimulation. Thus, we investigated whether an optimal degree of A 1 receptor overexpression exists whereby hearts would be protected from ischemic injury with minimal sequelae. Hearts from two transgenic lines were evaluated (high-level expressors, HL, and low-level expressors, LL) and compared to controls. Both transgenic lines showed enhanced myocardial protection as measured by postischemic recovery of developed tension (HL 49 ยฑ 3% of baseline, LL 45 ยฑ 3%, and control 31 ยฑ 3%; P < 0.05). Resting heart rates in conscious restrained mice were lowest in HL (620 ยฑ 14 bpm), intermediate in LL (681 ยฑ 11 bpm), and highest in control (713 ยฑ 8 bpm), P < 0.05, while systolic blood pressure was similar in all groups. The inotropic response to isoproterenol was comparably blunted in HL and LL hearts (+dP/dt increased 33 ยฑ 4% and 39 ยฑ 3%, respectively) compared to control (58 ยฑ 7%), P < 0.05. b-adrenergic receptor density was upregulated only in the HL hearts (90 ยฑ 12 fmol/mg) compared to LL and control (41 ยฑ 9 and 32 ยฑ 5, respectively, P < 0.05). In summary, both HL and LL hearts afforded cardioprotection from ischemia, with LL hearts exhibiting less bradycardia and less b-adrenergic cross-regulation. While the data suggest that genetic manipulation of receptor systems may provide a powerful tool to improve myocardial tolerance to injury, they also highlight the need for careful screening for effects on the integrated physiology of the system.