A new small molecule, 4-(2-methoxy-phenylcarbamoyloxymethyl)-piperidine-1-carboxylic acid tert-butyl ester (GW542573X), is presented as an activator of small-conductance Ca2+-activated K+ (SK, KCa2) channels and distinguished from previously published positive modulators of SK channels, such as 1-ethyl-2-benzimidazolinone (1-EBIO) and cyclohexyl-[2-(3,5-dimethylpyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine (CyPPA), in several aspects. GW542573X is the first SK1-selective compound described: an EC50 value of 8.2 ± 0.8 μM (n = 6, [Ca2+]i = 200 nM) was obtained from inside-out patches excised from hSK1-expressing HEK293 cells. Whole-cell experiments showed that hSK2 and hSK3 channels were more than 10 times, and hIK channels even more than 100 times, less sensitive to GW542573X. The Ca2+-response curve of hSK1 was left-shifted from an EC50(Ca2+) value of 410 ± 20 nM (n = 9) to 240 ± 10 nM (n = 5) in the presence of 10 μM GW542573X. In addition to this positive modulation, GW542573X activated SK1 in the absence of Ca2+ and furthermore induced a 15% increase in the maximal current at saturating Ca2+. Thus, GW542573X also acts as a genuine opener of the hSK1 channels, a mechanism of action (MOA) not previously obtained with SK channels. The differential potency on hSK1 and hSK3 enabled a chimera approach to elucidate site(s) important for this new MOA and selectivity property. A single amino acid (Ser293) located in S5 of hSK1 was essential, and substituting the corresponding Leu476 in hSK3 with serine conferred hSK1-like potency (EC50 = 9.3 ± 1.4 μM, n = 5). GW542573X may activate SK channels via interaction with “deep-pore” gating structures at the inner pore vestibule or the selectivity filter in contrast to 1-EBIO and CyPPA that exert positive modulation via the intracellular calmodulin binding domain.
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Smith et al. (1955) studied this question.