Characterization of the cloned human intermediate-conductance Ca2+-activated K+ channel

1 NeuroSearch A/S, DK-2600 Glostrup; 2 Department of Medical Genetics, Institute of Medical Biochemistry and Genetics, University of Copenhagen, DK-2200 Copenhagen N, Denmark; and 3 Department of Genetics, Division of Biomedical Sciences, Cerrahpasa Medical Faculty, Istanbul University, TR-34303 Istanbul, Turkey The human intermediate-conductance, Ca 2+ -activated K + channel (hIK) was identified by searching the expressed sequence tag database. hIK was found to be identical to two recently cloned K + channels, hSK4 and hIK1. RNA dot blot analysis showed a widespread tissue expression, with the highest levels in salivary gland, placenta, trachea, and lung. With use of fluorescent in situ hybridization and radiation hybrid mapping, hIK mapped to chromosome 19q13.2 in the same region as the disease Diamond-Blackfan anemia. Stable expression of hIK in HEK-293 cells revealed single Ca 2+ -activated K + channels exhibiting weak inward rectification (30 and 11 pS at 100 and +100 mV, respectively). Whole cell recordings showed a noninactivating, inwardly rectifying K + conductance. Ionic selectivity estimated from bi-ionic reversal potentials gave the permeability ( P K / P X ) sequence K + = Rb + (1.0) > Cs + (10.4) Na + , Li + , N -methyl- D -glucamine (>51). NH + 4 blocked the channel completely. hIK was blocked by the classical inhibitors of the Gardos channel charybdotoxin (IC 50 28 nM) and clotrimazole (IC 50 153 nM) as well as by nitrendipine (IC 50 27 nM), Stichodactyla toxin (IC 50 291 nM), margatoxin (IC 50 459 nM), miconazole (IC 50 785 nM), econazole (IC 50 2.4 µM), and cetiedil (IC 50 79 µM). Finally, 1-ethyl-2-benzimidazolinone, an opener of the T84 cell IK channel, activated hIK with an EC 50 of 74 µM. intermediate-conductance calcium-activated potassium channel; charybdotoxin; clotrimazole; fluorescent in situ hybridization; radiation hybrid mapping; patch clamp; Diamond-Blackfan anemia