We tested the hypothesis that ATP-sensitive K+ channels are involved in the mechanism mediating coronary autoregulation in open-chest dogs. We perfused the left anterior descending coronary artery with arterial blood from an extracorporeal circuit and measured steady-state coronary blood flow (CBF) with stepwise changes in coronary perfusion pressure (CPP) between 50 and 150 mm Hg during an intracoronary infusion of vehicle or glibenclamide (a putative blocker of ATP-sensitive K+ channels). CBF was relatively stable over CPP between 50 and 110 mm Hg during vehicle infusion, indicating the presence of autoregulation at the CPP range. During glibenclamide infusion (10 micrograms.min-1 x kg-1), CBF progressively decreased with reduction in CPP below 110 mm Hg, whereas the CPP-CBF relation at CPP above 110 mm Hg was not altered by glibenclamide. The autoregulation index [1-(delta F/F)/(delta P/P), where F indicates CBF and P indicates CPP] was greater than 0 over the CPP range between 50 and 100 mm Hg during vehicle infusion and was less than 0 during glibenclamide infusion. Glibenclamide did not alter systemic arterial pressure, heart rate, left ventricular pressure, and changes in regional myocardial oxygen consumption associated with changes in CPP. In the absence of glibenclamide, the CPP-CBF relation was reproducible in the repeated studies for time control. These results suggest that ATP-sensitive K+ channels play an important role in mediating coronary autoregulation at the lower range of CPP in the blood-perfused dog heart.