The interaction between the cardioprotective effect of endothelin (ET) receptor blockade and nitric oxide (NO) during ischemia-reperfusion injury was investigated. Anesthetized pigs were subjected to 45 (protocol 1) or 30 min (protocol 2) coronary artery ligation and 4 h reperfusion. In protocol 1, five groups were given vehicle, the ET(A) receptor antagonist LU-135252 (LU), the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine (L-NNA), L-NNA in combination with LU, or L-NNA in combination with the NO precursor L-arginine (L-Arg) and LU intravenously before ischemia. In protocol 2, two groups were given vehicle or L-NNA. In protocol 1, the infarct size (IS) was 79 +/- 5% of the area at risk in the vehicle group and 93 +/- 2% in the L-NNA group. LU reduced the IS to 43 +/- 7% (P < 0.001). The cardioprotective effect of LU was abolished in the presence of L-NNA (IS 76 +/- 6%), whereas addition of L-Arg restored its cardioprotective effect (IS 56 +/- 2%; P < 0.05 vs. vehicle and L-NNA + LU groups). In protocol 2, the IS was 49 +/- 6% in the vehicle group and 32 +/- 4% in the L-NNA group (P = not significant). Myocardial ET-like immunoreactivity (ET-LI) increased in the vehicle group of protocol 1. ET-LI in the ischemic-reperfused myocardium was lower in the groups given LU (P < 0.01) and L-NNA + L-Arg + LU (P < 0.05) but not in the group given L-NNA + LU compared with the vehicle group. These results suggest that the cardioprotective effect of the ET(A) receptor antagonist is mediated via a mechanism related to NO.