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

The human intermediate-conductance, Ca²⁺-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 Ca²⁺-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 (IC₅₀ 28 nM) and clotrimazole (IC₅₀ 153 nM) as well as by nitrendipine (IC₅₀ 27 nM),Stichodactyla toxin (IC₅₀ 291 nM), margatoxin (IC₅₀ 459 nM), miconazole (IC₅₀ 785 nM), econazole (IC₅₀ 2.4 μM), and cetiedil (IC₅₀ 79 μM). Finally, 1-ethyl-2-benzimidazolinone, an opener of the T84 cell IK channel, activated hIK with an EC₅₀ of 74 μM.