Amiloride derivatives are potent blockers of K-ATP channels

In cardiomyocytes sarcolemmal K-ATP channels open massively when the cytoso lic [ATP] drops into the range of tens of micromolar, as during acute ische mia. The diuretic drug amiloride and related derivatives are well establish ed as drugs blocking the Na+/H+- and the Na+/Ca2+-exchange, protecting the ischemic heart. Herein, the blocking action of amiloride and its derivative s 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 myocy tes, amiloride, DCB, and EIPA reversibly blocked the K-ATP channels with th e 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(I R)6.2 subunit with one of the sulfonylurea receptors SUR1 and SUR2A. All th ree 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 current s more effectively than inward currents whereas the block by DCB and EIPA w as voltage independent. In cardiomyocytes, also whole-cell I-KATP was block ed by the three drugs. In conclusion, amiloride, EIPA, and DCB block the po re-forming K(IR)6.2 subunit of cardiac K-ATP channels with higher potency t han the Na+/H+- and the Na+/Ca2+-exchange, precluding a specific block of t he exchanges under ischemic conditions.