The role of ATP-sensitive K+ channels (KATP) in ischemia and reperfusion (I/R) was studied in isolated rat lungs. I/R produced a sixfold increase in endothelial permeability as measured by the capillary filtration coefficient. Cromakalim (10 microM) given at 46 min after reperfusion reversed the filtration coefficient increase. This effect was not blocked by either a protein kinase A inhibitor (adenosine-3',5'-cyclic monophosphothioate; 100 microM) or an adenosine antagonist [8-(p-sulfophenyl)-theophylline; 20 microM]. Cromakalim given before ischemia or at the beginning of reperfusion protected the endothelial barrier from injury. Glibenclamide (500 microM) given before the ischemic period, at the beginning of reperfusion, or 46 min after reperfusion did not alter the changes in microvascular permeability produced by I/R. Glibenclamide blocked the ability of cromakalim to reverse endothelial damage but not the ability of either isoproterenol (10 microM) or an adenosine A2-receptor agonist, CGS-21680 (300 nM). We conclude that opening of KATP channels does not produce endothelial injury in I/R. The activation of KATP channels can both protect against and reverse the endothelial damage associated with I/R. This novel mechanism(s) is independent from known pathways that employ cAMP-protein kinase system and adenosine.