TY - JOUR T1 - Evidence for a Common Pharmacological Interaction Site on K<sub>Ca</sub>2 Channels Providing both Selective Activation and Selective Inhibition of the Human K<sub>Ca</sub>2.1 Subtype JF - Molecular Pharmacology JO - Mol Pharmacol DO - 10.1124/mol.111.074252 SP - mol.111.074252 AU - Charlotte Hougaard AU - Sofia Hammami AU - Birgitte L. Eriksen AU - Ulrik S. Sorensen AU - Marianne L. Jensen AU - Dorte Strobaek AU - Palle Christophersen Y1 - 2011/01/01 UR - http://molpharm.aspetjournals.org/content/early/2011/11/01/mol.111.074252.abstract N2 - We have previously identified Ser293 in transmembrane segment 5 as a determinant for selective KCa2.1 channel activation by GW542573X (4-(2-methoxyphenylcarbamoyloxymethyl)-piperidine-1-carboxylic acid tert-butyl ester). Now we show that Ser293 mediates both activation and inhibition of KCa2.1: CM-TPMF (N-{7-[1-(4-chloro-2-methylphenoxy)ethyl]-[1,2,4]triazolo[1,5-a]pyrimidin-2-yl}-N'-methoxy-formamidine) and B-TPMF (N-{7-[1-(4-tert-butyl-phenoxy)ethyl]-[1,2,4]triazolo[1,5-a]pyrimidin-2-yl}-N'-methoxy-formamidine), two newly identified and structurally related [1,2,4]triazolo[1,5-a]pyrimidines, act either as an activator or as an inhibitor of the human KCa2.1 channel. While (-)-CM-TPMF activates KCa2.1 with an EC50 value of 24 nM, (-)-B-TPMF inhibits the channel with an IC50 value of 31 nM. In contrast, their (+)-enantiomers are 40-100 times less active. Both (-)-CM-TPMF and (-)-B-TPMF are subtype-selective with 10-20 fold discrimination towards other KCa2 channels and the KCa3 channel. Co-application experiments reveal competitive-like functional interactions between the effects of (-)-CM-TPMF and (-)-B-TPMF. Despite belonging to a different chemical class than GW542573X the KCa2.1 selectivity of (-)-CM-TPMF and (-)-B-TPMF depend critically on Ser293 as revealed by loss- and gain-of-function mutations. We conclude that compounds occupying the TPMF site may either positively or negatively influence the gating process depending on their substitution pattern. Notably, (-)-CM-TPMF is 10 times more potent on KCa2.1 than NS309 (6,7-dichloro-1H-indole-2,3-dione 3-oxime), an unselective but hitherto the most potent KCa3/KCa2 channel activator. (-)-B-TPMF is the first small molecule inhibitor with significant selectivity among the KCa2 channel subtypes, and in contrast to peptide blockers like apamin and scyllatoxin, which preferentially affect KCa2.2, (-)-B-TPMF exhibits KCa2.1 selectivity. These high-affinity compounds, which exert opposite effects on KCa2.1 gating, may help define physiological or pathophysiological roles of this channel. ER -