Sulfonylurea derivatives glibenclamide and tolbutamide are selective blockers of ATP-sensitive K+ (KATP) channels. However, their ability to prevent cellular K+ loss and shortening of action potential duration during ischemia or hypoxia in the intact heart is modest compared with their efficacy at blocking KATP channels in excised membrane patches. In the isolated arterially perfused rabbit interventricular septum, the increase in unidirectional K+ efflux and shortening of action potential duration during substrate-free hypoxia were effectively blocked by glibenclamide, but only by very high concentrations (100 microM); during hypoxia with glucose present, glibenclamide was only partially effective at reducing K+ loss. During total global ischemia (10 minutes), up to 100 microM glibenclamide or 1 mM tolbutamide attenuated shortening of action potential duration but only reduced [K+]0 accumulation by a maximum of 32 +/- 6%. In isolated patch-clamped guinea pig ventricular myocytes in which the whole-cell ATP-sensitive K+ current was activated by exposure to the metabolic inhibitors, glibenclamide (up to 100 microM) and tolbutamide (10 mM) were only partially effective at blocking the whole-cell ATP-sensitive K+ current (maximum block, 51 +/- 10% and 50 +/- 9%, respectively), especially when ADP was included in the patch electrode solution. In inside-out membrane patches excised from these myocytes, glibenclamide blocked unitary currents through KATP channels with a Kd of 0.5 microM and a Hill coefficient of 0.5 in the absence of ADP at the cytosolic membrane surface, but block was incomplete when 100 microM ADP (+2 mM free Mg2+) was present. ADP had a similar effect on block of KATP channels by tolbutamide. These findings suggest that free cytosolic [ADP], which rises rapidly to the 100 microM range during early myocardial ischemia and hypoxia, may account for the limited efficacy of sulfonylureas at blocking ischemic and hypoxic cellular K+ loss under these conditions.