Sulfonylureas stimulate insulin secretion from pancreatic beta-cells by closing ATP-sensitive K+ (K(ATP)). The beta-cell and cardiac muscle K(ATP) channels have recently been cloned and shown to possess a common pore-forming subunit (Kir6.2) but different sulfonylurea receptor subunits (SUR1 and SUR2A, respectively). We examined the mechanism underlying the tissue specificity of the sulfonylureas tolbutamide and glibenclamide, and the benzamido-derivative meglitinide, using cloned beta-cell (Kir6.2/SUR1) and cardiac (Kir6.2/SUR2A) K(ATP) channels expressed in Xenopus oocytes. Tolbutamide inhibited Kir6.2/SUR1 (Ki approximately 5 micromol/l), but not Kir6.2/SUR2A, currents with high affinity. Meglitinide produced high-affinity inhibition of both Kir6.2/SUR1 and Kir6.2/SUR2A currents (Kis approximately 0.3 micromol/l and approximately 0.5 micromol/l, respectively). Glibenclamide also blocked Kir6.2/SUR1 and Kir6.2/SUR2A currents with high affinity (Kis approximately 4 nmol/l and approximately 27 nmol/l, respectively); however, only for cardiac-type K(ATP) channels was this block reversible. Physiological concentrations of MgADP (100 micromol/l) enhanced glibenclamide inhibition of Kir6.2/SUR1 currents but reduced that of Kir6.2/SUR2A currents. The results suggest that SUR1 may possess separate high-affinity binding sites for sulfonylurea and benzamido groups. SUR2A, however, either does not possess a binding site for the sulfonylurea group or is unable to translate the binding at this site into channel inhibition. Although MgADP reduces the inhibitory effect of glibenclamide on cardiac-type K(ATP) channels, drugs that bind to the common benzamido site have the potential to cause side effects on the heart.