title: Mechanism of inhibition of mouse Slo3 (KCa 5.1) potassium channels by quinine, quinidine and barium.
abstract: OBJECTIVE
  The Slo3 (KCa 5.1) channel is a major component of mammalian KSper (sperm potassium conductance) channels and inhibition of these channels by quinine and barium alters sperm motility. The aim of this investigation was to determine the mechanism by which these drugs inhibit Slo3 channels.
  
  
  METHODS
  Mouse (m) Slo3 (KCa 5.1) channels or mutant forms were expressed in Xenopus oocytes and currents recorded with 2-electrode voltage-clamp. Gain-of-function mSlo3 mutations were used to explore the state-dependence of the inhibition. The interaction between quinidine and mSlo3 channels was modelled by in silico docking.
  
  
  RESULTS
  Several drugs known to block KSper also affected mSlo3 channels with similar levels of inhibition. The inhibition induced by extracellular barium was prevented by increasing the extracellular potassium concentration. R196Q and F304Y mutations in the mSlo3 voltage sensor and pore, respectively, both increased channel activity. The F304Y mutation did not alter the effects of barium, but increased the potency of inhibition by both quinine and quinidine approximately 10-fold; this effect was not observed with the R196Q mutation.
  
  
  CONCLUSIONS
  Block of mSlo3 channels by quinine, quinidine and barium is not state-dependent. Barium inhibits mSlo3 outside the cell by interacting with the selectivity filter, whereas quinine and quinidine act from the inside, by binding in a hydrophobic pocket formed by the S6 segment of each subunit. Furthermore, we propose that the Slo3 channel activation gate lies deep within the pore between F304 in the S6 segment and the selectivity filter.
authors.affiliation: School of Biomedical Sciences, Faculty of Biological Sciences, University of Leeds, Leeds, UK.
authors.forename: David C
authors.initials: DC
authors.lastname: Wrighton
authors.affiliation: School of Biomedical Sciences, Faculty of Biological Sciences, University of Leeds, Leeds, UK.
authors.forename: Stephen P
authors.identifier: http://orcid.org/0000-0001-6869-4414
authors.initials: SP
authors.lastname: Muench
authors.affiliation: School of Biomedical Sciences, Faculty of Biological Sciences, University of Leeds, Leeds, UK.
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Lippiat
mesh_terms: D000818:Animals; D001464:Barium / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D005260:Female; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000378:metabolism*; D051379:Mice; D026902:Potassium Channel Blockers / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D017433:Protein Structure, Secondary; D011802:Quinidine / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D011803:Quinine / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D014982:Xenopus laevis
chemical_list: C110627:Kcnu1 protein, mouse; D051036:Large-Conductance Calcium-Activated Potassium Channels; D026902:Potassium Channel Blockers; D001464:Barium; D011803:Quinine; D011802:Quinidine