The amiodarone derivative 2-methyl-3-(3,5-diiodo-4-carboxymethoxybenzyl) benzofuran (KB130015) opens large-conductance Ca2+-activated K+ channels and relaxes vascular smooth muscle

2-Methyl-3-(3,5-diiodo-4-carboxymethoxybenzyl)benzofuran (KB130015) has been developed to retain the antiarrhythmic properties of the parent molecule amiodarone but to eliminate its undesired side effects. In patch-clamp experiments, KB130015 activated large-conductance, Ca2+- activated BKCa channels formed by hSlol (alpha) subunits in HEK 293 cells. Channels were reversibly activated by shifting the open-probability/voltage (P-o/V) relationship by about -60 mV in 3 mu M intracellular free Ca2+ ([Ca2+](in)). No effect on the single-channel conductance was observed. KB130015-mediated activation of BKCa channels was half-maximal at 20 mu M with a Hill coefficient of 2.8. BKCa activation by KB130015 did not require the presence of Ca2+ and still occurred with saturating (100 mu M) [Ca2+](in). Effects of the prototypic BKCa activator NS1619 (1,3-dihydro-l-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one) and those of KB130015 were not additive suggesting that both activators may at least partially share a common mechanism of action. KB130015-mediated activation was observed also for BKCa channels from insects and for human BKCa channels with already profoundly left-shifted voltage-dependence. In contrast, human intermediate conductance Ca2+-activated channels were inhibited by KB130015. Using segments of porcine pulmonary arteries, KB130015 induced endothelium-independent vasorelaxation, half-maximal at 43 mu M KB130015. Relaxation was inhibited by 1 mM tetraethylammonium, suggesting that KB130015 can activate vascular smooth muscle type BKCa channels under physiological conditions. Interestingly, the shift in the P-o/V relationship was considerably stronger (-90 mV in 3 mu M [Ca2+](in)) for BKCa channels containing Slo-beta 1 subunits. Thus, KB130015 belongs to a novel class of BKCa channel openers that exert an effect depending on the subunit composition of the channel complex. (c) 2006 Elsevier B.V. All rights reserved.