Potassium channel openers (KCOs; e.g., P1075, pinacidil) exert their effects on excitable cells by opening ATP-sensitive potassium channels. These channels are heteromultimers composed with a 4:4 stoichiometry of an inwardly rectifying K(+) channel subunit plus a regulatory subunit comprising the receptor sites for hypoglycemic sulfonylureas and KCOs (a sulfonylurea receptor). To elucidate stoichiometry of KCO action, we analyzed P1075 sensitivity of channels coassembled from sulfonylurea receptor isoforms with high or low P1075 affinity. Concentration activation curves for cDNA ratios of 1:1 or 1:10 resembled those for channel opening resulting from interaction with a single site, whereas models for activation requiring occupation of two, three, or four sites were incongruous. We conclude KCO-induced channel activation to be mediated by interaction with a single binding site per tetradimeric complex.