Delta-opioid receptors are known to participate in the protection found following ischemic preconditioning (IPC), but the role of kappa-receptors in IPC is currently controversial. Langendorff-perfused rat hearts received 35 min regional ischemia and 2 h reperfusion. PC (2 cycles 5 min global ischemia) substantially reduced infarct size. Pharmacological PC with the delta-agonist DADLE (10 nmol/L) had similar protective effects. However, higher dose DADLE (1 micromol/L) had a less beneficial effect, and in conjunction with the delta-antagonist naltrindole unexpectedly increased infarct size (61.5 +/- 2.0%, p<0.05 v 45.9 +/- 2.4% in controls) suggesting a non-delta effect. The universal kappa-opioid agonist bremazocine (30 nmol/L) increased infarct size (61.3 +/- 1.6%, p<0.05 v controls), an effect abrogated by the selective kappa1-antagonist nor-binaltorphimine (BNI). Since opiates are known to have anti-adrenergic effects, which hypothetically may help to mediate IPC, cyclic AMP levels were measured in DADLE and in bremazocine-treated hearts. Decreased levels of cyclic AMP at the start of the regional ischemic period were found in low dose DADLE hearts (0.485 +/- 0/020, n = 8, vs controls, 0.654 +/- 0.025 nmol/g wet weight, p<0.001), but not in high dose DADLE nor in bremazocine treated hearts. Thus, in the isolated rat heart kappa1-opioid receptor activation exacerbates infarct size through an as yet unknown mechanism, suggesting that there could be an "antipreconditioned state". In contrast, delta-activity mediates protection which may be associated with a reduction of tissue cyclic AMP levels.