Characterization of basolateral K+ channels underlying anion secretion in the human airway cell line Calu‐3

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Abstract
Transepithelial anion secretion in many tissues depends upon the activity of basolateral channels. Using monolayers of the Calu‐3 cell line, a human submucosal serous cell model mounted in an Ussing chamber apparatus, we investigated the nature of the K+ channels involved in basal, cAMP‐ and Ca2+‐stimulated anion secretion, as reflected by the transepithelial short circuit current (Isc). The non‐specific K+ channel inhibitor Ba2+ inhibited the basal Isc by either 77 or 16 % when applied directly to the basolateral or apical membranes, respectively, indicating that a basolateral K+ conductance is required for maintenance of basal anion secretion. Using the K+ channel blockers clofilium and clotrimazole, we found basal Isc to be sensitive to clofilium, with a small clotrimazole‐sensitive component. By stimulating the cAMP and Ca2+ pathways, we determined that cAMP‐stimulated anion secretion was almost entirely abolished by clofilium, but insensitive to clotrimazole. In contrast, the Ca2+‐stimulated response was sensitive to both clofilium and clotrimazole. Thus, pharmacologically distinct basolateral K+ channels are differentially involved in the control of anion secretion under different conditions. Isolation of the basolateral K+ conductance in permeabilized monolayers revealed a small basal and forskolin‐stimulated Isc. Finally, using the reverse transcriptase‐polymerase chain reaction, we found that Calu‐3 cells express the K+ channel genes KCNN4 and KCNQ1 and the subunits KCNE2 and KCNE3. We conclude that while KCNN4 contributes to Ca2+‐activated anion secretion by Calu‐3 cells, basal and cAMP‐activated secretion are more critically dependent on other K+ channel types, possibly involving one or more class of KCNQ1‐containing channel complexes.