Reperfusion of the ischemic myocardium results in the loss of endothelium-dependent relaxation. We have shown recently that the alternate complement pathway is activated immediately on reperfusion of the ischemic porcine myocardium. We hypothesized that complement activation directly attenuates endothelium-dependent relaxation of porcine coronary arteries. Bradykinin (BK) or substance P concentration-dependently relaxed precontracted (U46619, 50 nmol/L) left anterior descending coronary artery (LAD) rings in vitro. Addition of zymosan to human (10%) or porcine (10%) serum for 30 minutes significantly (P < 0.05) increased the EC50 of BK-induced LAD relaxation from 4 +/- 1 to 418 +/- 159 nmol/L (n = 8) and from 9 +/- 3 to 281 +/- 132 nmol/L (n = 7), respectively. Similarly, addition of zymosan to 10% human serum (HS) for 30 minutes increased the EC50 of substance P-induced LAD relaxation from 0.4 +/- 0.1 to 30 +/- 14 nmol/L (n = 9, P < .05). Basal release of nitric oxide was reduced significantly in LAD rings exposed to zymosan-activated HS compared with HS alone. Addition of soluble CR1 (sCR1, 10 nmol/L) to zymosan-activated HS preserved BK-induced relaxation (EC50) of the LAD rings (control, 4 +/- 1 nmol/L; sCR1 + zymosan+serum, 2 +/- 1 nmol/L; n = 6). Zymosan-activated C8-depleted HS (10%) did not attenuate the EC50 of BK-induced coronary artery relaxation (3 +/- 1 to 3 +/- 1 nmol/L, n = 7, P = NS). Zymosan-activated C8-depleted HS plus C8 (6 micrograms/mL) increased the EC50 of BK-induced coronary artery relaxation from 4 +/- 1 to 423 +/- 141 nmol/L (n = 12, P < .05). We have further demonstrated that C5b-9 complexes can be found on the luminal surface of LAD endothelial cells after 5 minutes of exposure to zymosan-activated HS by using C5b-9 reactive monoclonal antibody fluorescent immunohistochemistry and confocal microscopy.(ABSTRACT TRUNCATED AT 250 WORDS)