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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Apical adenosine regulates basolateral Ca²⁺-activated potassium channels in human airway Calu-3 epithelial cells

Department of Biology, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People's Republic of China

Submitted 21 November 2007 ; accepted in final form 25 March 2008

In airway epithelial cells, apical adenosine regulates transepithelial anion secretion by activation of apical cystic fibrosis transmembrane conductance regulator (CFTR) via adenosine receptors and cAMP/PKA signaling. However, the potent stimulation of anion secretion by adenosine is not correlated with its modest intracellular cAMP elevation, and these uncorrelated efficacies have led to the speculation that additional signaling pathways may be involved. Here, we showed that mucosal adenosine-induced anion secretion, measured by short-circuit current (I_sc), was inhibited by the PLC-specific inhibitor U-73122 in the human airway submucosal cell line Calu-3. In addition, the I_sc was suppressed by BAPTA-AM (a Ca²⁺ chelator) and 2-aminoethoxydiphenyl borate (2-APB; an inositol 1,4,5-trisphosphate receptor blocker), but not by PKC inhibitors, suggesting the involvement of PKC-independent PLC/Ca²⁺ signaling. Ussing chamber and patch-clamp studies indicated that the adenosine-induced PLC/Ca²⁺ signaling stimulated basolateral Ca²⁺-activated potassium (K_Ca) channels predominantly via A_2B adenosine receptors and contributed substantially to the anion secretion. Thus, our data suggest that apical adenosine activates contralateral K⁺ channels via PLC/Ca²⁺ and thereby increases the driving force for transepithelial anion secretion, synergizing with its modulation of ipsilateral CFTR via cAMP/PKA. Furthermore, the dual activation of CFTR and K_Ca channels by apical adenosine resulted in a mixed secretion of chloride and bicarbonate, which may alter the anion composition in the secretion induced by secretagogues that elicit extracellular ATP/adenosine release. Our findings provide novel mechanistic insights into the regulation of anion section by adenosine, a key player in the airway surface liquid homeostasis and mucociliary clearance.

phospholipase C; cystic fibrosis transmembrane conductance regulator; anion secretion; mucociliary clearance