Amiloride derivatives are potent blockers of K_ATP channels

Abstract.

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 µmol/l (–80 mV), respectively. Similar IC₅₀ values were obtained in recombinant channels when coexpressing the K_IR6.2 subunit with one of the sulfonylurea receptors SUR1 and SUR2A. All three drugs also blocked currents generated by the C-terminus deletion mutant K_IR6.2Δ26 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_IR6.2 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.