Abstract
Small- and intermediate-conductance Ca(2+)-activated potassium (K(Ca)2.x and K(Ca)3.1, also called SK and IK) channels are activated exclusively by a Ca(2+)-calmodulin gating mechanism. Wild-type K(Ca)2.3 channels have a Ca(2+) EC(50) value of ∼0.3 μM, while the apparent Ca(2+) sensitivity of wild-type K(Ca)3.1 channels is ∼0.27 μM. Heterozygous genetic mutations of K(Ca)2.3 channels have been associated with Zimmermann-Laband syndrome and idiopathic noncirrhotic portal hypertension, while K(Ca)3.1 channel mutations were reported in hereditary xerocytosis patients. K(Ca)2.3_S436C and K(Ca)2.3_V450L channels with mutations in the S(45)A/S(45)B helices exhibited hypersensitivity to Ca(2+). The corresponding mutations in K(Ca)3.1 channels also elevated the apparent Ca(2+) sensitivity. K(Ca)3.1_S314P, K(Ca)3.1_A322V and K(Ca)3.1_R352H channels with mutations in the HA/HB helices are hypersensitive to Ca(2+), whereas K(Ca)2.3 channels with the equivalent mutations are not. The different effects of the equivalent mutations in the HA/HB helices on the apparent Ca(2+) sensitivity of K(Ca)2.3 and K(Ca)3.1 channels may imply distinct modulation of the two channel subtypes by the HA/HB helices. AP14145 reduced the apparent Ca(2+) sensitivity of the hypersensitive mutant K(Ca)2.3 channels, suggesting the potential therapeutic usefulness of negative gating modulators.