title: KATP channel openers of the benzopyran type reach their binding site via the cytosol.
abstract: OBJECTIVE
  ATP-sensitive K+ (KATP) channels are composed of pore-forming subunits (Kir6.x) and of sulphonylurea receptors (SUR). Both sulphonylureas and K(ATP) channel openers act by binding to SUR. Sulphonylureas reach their binding site from the cytosol but it remains unknown whether this holds for openers too.
  
  
  METHODS
  A poorly membrane-permeant sulphonic acid derivative of the benzopyran-type opener, bimakalim, was synthesized, descyano-bimakalim-6-sulphonic acid (BMSA). Binding of BMSA and bimakalim was compared in membranes and intact cells expressing the Kir6.2/SUR2B channel and channel opening was compared in inside-out patches and whole cells.
  
  
  RESULTS
  In membranes, bimakalim and BMSA bound to Kir6.2/SUR2B with Ki values of 61 nM and 4.3 microM, showing that the negative charge decreased affinity 69-fold. In intact cells, however, binding of BMSA was much weaker than in membranes (75-fold) whereas that of bimakalim was unchanged. The Ki value of BMSA decreased with increasing incubation time. In inside-out patches, bimakalim (1 microM) and BMSA (100 microM) opened the Kir6.2/SUR2B channel closed by MgATP to a similar degree whereas in whole-cell experiments, only bimakalim was effective.
  
  
  CONCLUSIONS
  Despite its negative charge, BMSA is an effective channel opener. The fact that BMSA binds and acts more effectively when applied to the inner side of the cell membrane shows that benzopyran openers reach their binding site at SUR from the cytosol. This suggests that the binding pocket of SUR is only open on the cytoplasmic side.
authors.affiliation: Department of Pharmacology and Toxicology, Medical Faculty, University of Tübingen, Tübingen, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Stephan
authors.forename: E
authors.initials: E
authors.lastname: Salamon
authors.forename: H
authors.initials: H
authors.lastname: Weber
authors.forename: U
authors.initials: U
authors.lastname: Russ
authors.forename: H
authors.initials: H
authors.lastname: Lemoine
authors.forename: U
authors.initials: U
authors.lastname: Quast
mesh_terms: D001578:Benzopyrans / Q000494:pharmacology*; D002460:Cell Line; D003600:Cytosol / Q000378:metabolism*; D004095:Dihydropyridines / Q000494:pharmacology*; D006801:Humans; D015221:Potassium Channels / Q000819:agonists*
chemical_list: D001578:Benzopyrans; D004095:Dihydropyridines; D015221:Potassium Channels; C117780:mitochondrial K(ATP) channel; C061050:bimakalim