[H-3]dofetilide binding to cardiac myocytes: Modulation by extracellular potassium

The radioligand [H-3]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 [H-3]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 [H-3]dofetilide binding. Results: under physiological conditions (Na+ 135 mM, K+ 5 mM), [H-3]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 mu 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 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, [H-3]dofetilide binding to its high-affinity site is modulated by extracellular potassium. (C) 1997 Academic Press Limited.