A High Affinity Binding Site for [3H]-Dofetilide on Human Leukocytes

Certain Class III anti-arrhythmic agents have been shown to interact with human leukocytes and after antigenic and mitogenic activation. We hypothesized that a binding site for the Class III anti-arrhythmic agent, dofetilide, would exist on human leukocytes. Analysis of binding isotherms defined the presence of a single high affinity binding site on mononuclear cells and neutrophils: Kd26±4 n , Bmax61±14 fmol/106cells and Kd33±14 n , Bmax163±45 fmol/106cells, respectively. Other Class III drugs inhibited [3H]-dofetilide binding at physiologically relevant concentrations, but the IC50values of E4031 and quinidine were significantly higher for leukocytes than for cardiac myocytes. Interestingly, verapamil inhibited [3H]-dofetilide binding to leukocytes, but not to cardiac myocytes at physiologic concentrations (10μm). Charybdotoxin and tetraethlyammonium inhibited [3H]-dofetilide binding to leukocytes atμmand m concentrations, respectively, however, apamin did not inhibit binding even at 1μmconcentrations. These data suggest that a Ca2+-activated K+channel, like K(Ca) mini (apamin-insensitive isoform), is a candidate for the leukocyte [3H]-dofetilide binding site. To assess the functional significance of defetilide binding to leukocyte biology, we evaluated fMLP-stimulated superoxide production in the presence or absence of dofetilide. Dofetilide, at 30 n suppressed of superoxide production. In conclusion, dofetilide binds to human leukocytes at physiologic concentrations and this binding alters leukocyte function possibly through interaction with a Ca2+-activated K+channel.