Expression of intermediate-conductance, Ca2+-activated K+ channel (KCNN4) in H441 human distal airway epithelial cells
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Electrophysiological studies of H441 human distal airway epithelial cells showed that thapsigargin caused a Ca2+-dependent increase in membrane conductance (GTot) and hyperpolarization of membrane potential (Vm). These effects reflected a rapid rise in cellular K+ conductance (GK) and a slow fall in amiloride-sensitive Na+ conductance (GNa). The increase in GTot was antagonized by Ba2+, a nonselective K+ channel blocker, and abolished by clotrimazole, a KCNN4 inhibitor, but unaffected by other selective K+ channel blockers. Moreover, 1-ethyl-2-benzimidazolinone (1-EBIO), which is known to activate KCNN4, increased GK with no effect on GNa. RT-PCR-based analyses confirmed expression of mRNA encoding KCNN4 and suggested that two related K+ channels (KCNN1 and KCNMA1) were absent. Subsequent studies showed that 1-EBIO stimulates Na+ transport in polarized monolayers without affecting intracellular Ca2+ concentration ([Ca2+]i), suggesting that the activity of KCNN4 might influence the rate of Na+ absorption by contributing to GK. Transient expression of KCNN4 cloned from H441 cells conferred a Ca2+- and 1-EBIO-sensitive K+ conductance on Chinese hamster ovary cells, but this channel was inactive when [Ca2+]i was <0.2 µM. Subsequent studies of amiloride-treated H441 cells showed that clotrimazole had no effect on Vm despite clear depolarizations in response to increased extracellular K+ concentration ([K+]o). These findings thus indicate that KCNN4 does not contribute to Vm in unstimulated cells. The present data thus establish that H441 cells express KCNN4 and highlight the importance of GK to the control of Na+ absorption, but, because KCNN4 is quiescent in resting cells, this channel cannot contribute to resting GK or influence basal Na+ absorption.