Amiloride derivatives are potent blockers of KATP channels

In cardiomyocytes sarcolemmal K-ATP channels open massively when the cytosolic [ATP] drops into the range of tens of micromolar, as during acute ischemia. The diuretic drug amiloride and related derivatives are well established as drugs blocking the Na+/H+- and the Na+/Ca2+-exchange, protecting the ischemic heart. Herein, the blocking action of amiloride and its derivatives 2', 4'-dichlorobenzamil (DCB) and 5-(N-ethyl-N-isopropyl)amiloride (EIPA) on K-ATP channels was tested. In inside-out patches of mouse cardiac myocytes, amiloride, DCB, and EIPA reversibly blocked the K-ATP channels with the IC50 values 102, 1.80, and 2.14 mu mol/l (-80 mV), respectively. Similar IC50 values were obtained in recombinant channels when coexpressing the K(IR)6.2 subunit with one of the sulfonylurea receptors SUR1 and SUR2A. All three drugs also blocked currents generated by the C-terminus deletion mutant K(IR)6.2 Delta 26 in the absence of SUR. Amiloride blocked outward currents more effectively than inward currents whereas the block by DCB and EIPA was voltage independent. In cardiomyocytes, also whole-cell I-KATP was blocked by the three drugs. In conclusion, amiloride, EIPA, and DCB block the pore-forming K(IR)6.2 subunit of cardiac K-ATP channels with higher potency than the Na+/H+- and the Na+/Ca2+-exchange, precluding a specific block of the exchanges under ischemic conditions.