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

Abstract

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, Ca 2+-activated BK Ca channels formed by hSlo1 (α) 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 μM intracellular free Ca 2+ ([Ca 2+] in). No effect on the single-channel conductance was observed. KB130015-mediated activation of BK Ca channels was half-maximal at 20 μM with a Hill coefficient of 2.8. BK Ca activation by KB130015 did not require the presence of Ca 2+ and still occurred with saturating (100 μM) [Ca 2+] in. Effects of the prototypic BK Ca activator NS1619 (1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2 H-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 BK Ca channels from insects and for human BK Ca channels with already profoundly left-shifted voltage-dependence. In contrast, human intermediate conductance Ca 2+-activated channels were inhibited by KB130015. Using segments of porcine pulmonary arteries, KB130015 induced endothelium-independent vasorelaxation, half-maximal at 43 μM KB130015. Relaxation was inhibited by 1 mM tetraethylammonium, suggesting that KB130015 can activate vascular smooth muscle type BK Ca channels under physiological conditions. Interestingly, the shift in the P o/ V relationship was considerably stronger (− 90 mV in 3 μM [Ca 2+] in) for BK Ca channels containing Slo-β1 subunits. Thus, KB130015 belongs to a novel class of BK Ca channel openers that exert an effect depending on the subunit composition of the channel complex.