[3H]Dofetilide Binding to Cardiac Myocytes: Modulation by Extracellular Potassium☆

Abstract The radioligand [ 3 H]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 [ 3 H]dofetilide to cardiac myocytes only at extracellular potassium of 135 m m . 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 [ 3 H]dofetilide binding. Results: under physiological conditions (Na + 135 m m , K + 5 m m ), [ 3 H]dofetilide bound to two sites on guinea-pig ventricular myocytes (a high-affinity site, K d 26±8 n m , B max 81±12 fmol/10 6 cells; and a low-affinity site, K d 1.6±0.8 μ m , 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 m m , a single binding site was observed with an affinity K d of 120±12 n m and a B max of 303±57 fmol/10 6 cells ( P 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 m m and 20 m m were also significantly different, 29±10 and 230±46 n m , respectively. In conclusion, [ 3 H]dofetilide binding to its high-affinity site is modulated by extracellular potassium.