Activation of the nuclear enzyme poly(ADP-ribose) synthetase (PARS) is important in the cellular response to oxidative stress. During ischemia and reperfusion (I/R) increased free radical production leads to DNA breakage that stimulates PARS which in turn results in an energy-consuming metabolic cycle and initiation of the apoptotic process. Previous studies have reported that PARS inhibition confers protection in various models of I/R-induced cardiovascular damage. The purpose of this study was to determine the role of PARS inhibition in I/R-induced injury of smooth muscle cells and endothelium in the coronary circulation of the isolated guinea-pig heart. Control hearts and those treated with a PARS inhibitor--benzamide (100 micromol L(-1)), were subjected to 30 min of subglobal ischemia and subsequent reperfusion (90 min). To analyze the functional integrity of smooth muscle cells and endothelium, one-minute intracoronary infusions of endothelium-independent (sodium nitroprusside, NaNP; 3 micromol L(-1)) and endothelium-dependent (substance P, SP; 10 nmol L(-1)) vasodilators were used before ischemia and at the reperfusion time. The degree of the injury of coronary smooth muscle and endothelial cells induced by I/R was estimated in terms of diminished vasodilator responses to NaNP (at 55 min and 85 min of reperfusion) and to SP (at 70 min of reperfusion), respectively, and expressed as the percentage of preischemic response. I/R reduced vasorelaxant responses to both vasodilators by half (to 54.1 +/- 5.1% and to 53.6 +/- 4.9% of preischemic value for NaNP at 55 min and 85 min of reperfusion, respectively and to 45.9 +/- 6.5% for SP at 70 min of reperfusion). PARS inhibition provided complete restoration of vasorelaxation induced by NaNP (107.6 +/- 13.3% and 104 +/- 14.4% of preischemic response at the two time points of reperfusion, respectively). However, there was no effect on the SP-induced response (48+12.1% of preischemic response). We conclude that pharmacological PARS inhibition with benzamide protects coronary smooth muscle cells but not endothelium against I/R-induced reperfusion injury in the coronary circulation of the guinea-pig heart.