title: [3H]dofetilide binding to cardiac myocytes: modulation by extracellular potassium.
abstract: The radioligand [3H]dofetilide binds specifically to the delayed rectifier potassium channel and provides a biochemical approach to study interactions of Class III drugs with this channel. However, previous studies have examined the binding of [3H]dofetilide to cardiac myocytes only at extracellular potassium of 135 mM. Because previous electrophysiological studies have shown that hyperkalemia could alter the pharmacological responses to I(Kr) channel blockers, the hypothesis tested in this study was that changing ionic conditions would alter characteristics of [3H]dofetilide binding.
  
  
  RESULTS
  under physiological conditions (Na+ 135 mM, K+ 5 mM), [3H]dofetilide bound to two sites on guinea-pig ventricular myocytes (a high-affinity site, K(d) 26+/-8 nM, B(max) 81+/-12 fmol/10(6) cells: and a low-affinity site, K(d) 1.6+/-0.8 microM, B(max) 1003+/-173 fmol/10(6) cells, n=11). Binding properties were not altered by changes in osmolarity or extracellular sodium. However, when extracellular K+ was increased to 20 mM, a single binding site was observed with an affinity K(d) of 120+12 nM and a B(max) of 303+/-57 fmol/10(6) cells (P<0.05; n=6). To establish whether this effect was mediated at the high-affinity site we assessed the effects of elevated extracellular potassium on a biological model, neonatal mouse myocytes, that expressed solely the high-affinity binding sites. The K(d) values for binding to fetal mouse cardiac myocytes at an extracellular K+ of 5 mM and 20 mM were also significantly different, 29+/-10 and 230+/-46 nM, respectively. In conclusion, [3H]dofetilide binding to its high-affinity site is modulated by extracellular potassium.
authors.affiliation: Department of Medicine, University of Calgary, Alberta, Canada.
authors.forename: H J
authors.initials: HJ
authors.lastname: Duff
authors.forename: Z P
authors.initials: ZP
authors.lastname: Feng
authors.forename: C
authors.initials: C
authors.lastname: Fiset
authors.forename: L
authors.initials: L
authors.lastname: Wang
authors.forename: J
authors.initials: J
authors.lastname: Lees-Miller
authors.forename: R S
authors.initials: RS
authors.lastname: Sheldon
mesh_terms: D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000378:metabolism*; D005318:Fetal Heart / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D006168:Guinea Pigs; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D051379:Mice; D009994:Osmolar Concentration; D010627:Phenethylamines / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology*; D011869:Radioligand Assay; D012964:Sodium / Q000494:pharmacology; D013449:Sulfonamides / Q000378:metabolism*; D014316:Tritium
chemical_list: D000889:Anti-Arrhythmia Agents; D010627:Phenethylamines; D013449:Sulfonamides; D014316:Tritium; D012964:Sodium; C063533:dofetilide; D011188:Potassium