CFTR Regulation of Intracellular Calcium in Normal and Cystic Fibrosis Human Airway Epithelia

In cystic fibrosis airway epithelia, mutation of the CFTR protein causes a reduced response of Cl− secretion to secretagogues acting via cAMP. Using a Ca2+ imaging system, the hypothesis that CFTR activation may permit ATP release and regulate [Ca2+]i via a receptor-mediated mechanism, is tested in this study. Application of external nucleotides produced a significant increase in [Ca2+]i in normal (16HBE14o− cell line and primary lung culture) and in cystic fibrosis (CFTE29o− cell line) human airway epithelia. The potency order of nucleotides on [Ca2+]i variation was UTP ≫ ATP > UDP > ADP > AMP > adenosine in both cell types. The nucleotide [Ca2+]i response could be mimicked by activation of CFTR with forskolin (20 μm) in a temperature-dependent manner. In 16HBE14o− cells, the forskolin-induced [Ca2+]i response increased with increasing temperature. In CFTE29o− cells, forskolin had no effect on [Ca2+]i at body temperature-forskolin-induced [Ca2+]i response in CF cells could only be observed at low experimental temperature (14°C) or when cells were cultured at 26°C instead of 37°C. Pretreatment with CFTR channel blockers glibenclamide (100 μm) and DPC (100 μm), with hexokinase (0.5 U/mg), and with the purinoceptor antagonist suramin (100 μm), inhibited the forskolin [Ca2+]i response. Together, these results demonstrate that once activated, CFTR regulates [Ca2+]i by mediating nucleotide release and activating cell surface purinoceptors in normal and CF human airway epithelia.