1. We investigated the mechanism of action of two novel nonsulphonylurea ATP-sensitive potassium channel (K(ATP)) inhibitors, PNU-99963 and PNU-37883A, on four types of cloned K(ATP) channels. 2. Whole-cell currents were recorded in a symmetrical potassium (140 mM) gradient in HEK-293 cells stably expressing Kir6.2/SUR1, Kir6.2/SUR2A, Kir6.2/SUR2B or Kir6.1/SUR2B. 3. PNU-99963 potently inhibited the four K(ATP) channel clones. The concentration at which half-maximum current was inhibited (IC(50)) was 66, 41, 43 and 11 nM for Kir6.2/SUR1, Kir6.2/SUR2A, Kir6.2/SUR2B and Kir6.1/SUR2B, respectively. In contrast, PNU-99963 up to a concentration of 3 microM had no significant effect on current generated in HEK-293 cells by transiently expressing Kir6.2Delta26, a C-terminal truncated pore-forming subunit of Kir6.2. 4. PNU-37883A inhibited four types of K(ATP) channels, but to different extents. Inhibition of the putative smooth muscle K(ATP) channel types, Kir6.2/SUR2B (IC(50); 15 microM) and Kir6.1/SUR2B (IC(50); 6 microM), was significantly greater than inhibition of either the pancreatic beta cell or cardiac K(ATP) channel clones. Moreover, PNU-37883A significantly inhibited currents generated by expressing Kir6.2Delta26 alone, with an IC(50) of 5 microM, which was significantly increased to 38 microM when Kir6.2Delta26 was expressed with SUR2B. 5. In conclusion, two structurally different nonsulphonylurea compounds, PNU-99963 and PNU-37883A, inhibit K(ATP) channels via different mechanisms, namely through the sulphonylurea receptor (SUR) and the pore-forming subunits, respectively, although SUR2B reduced the inhibitory effect of PNU-37883A. While PNU-99963 potently inhibits all the four cloned K(ATP) channels, PNU-37883A has a degree of selectivity towards both smooth muscle K(ATP) channels, but could not discriminate between them.