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

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 m . Because previous electrophysiological studies have shown that hyperkalemia could alter the pharmacological responses to IKrchannel 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 m , K+5 m ), [3H]dofetilide bound to two sites on guinea-pig ventricular myocytes (a high-affinity site, Kd26±8 n , Bmax81±12 fmol/106cells; and a low-affinity site, Kd1.6±0.8μm , Bmax1003±173 fmol/106cells,n=11). Binding properties were not altered by changes in osmolarity or extracellular sodium. However, when extracellular K+was increased to 20 m , a single binding site was observed with an affinity Kdof 120±12 n and a Bmaxof 303±57 fmol/106cells (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 Kdvalues for binding to fetal mouse cardiac myocytes at an extracellular K+of 5 m and 20 m were also significantly different, 29±10 and 230±46 n , respectively. In conclusion, [3H]dofetilide binding to its high-affinity site is modulated by extracellular potassium.