Inhibition of amiloride-sensitive epithelial Na+ absorption by extracellular nucleotides in human normal and cystic fibrosis airways

Cystic fibrosis (CF) airway epithelia are characterized by enhanced Na+ absorption probably due to a lack of downregulation of epithelial Na+ channels by mutant CF transmembrane conductance regulator. Extracellular nucleotides adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) have been shown to activate alternative Ca2+-dependent CI- channels in normal and CF respiratory epithelia, Recent studies suggest additional modulation of Na+ absorption by extracellular nucleotides. In this study we examined the role of mucosal ATP and UTP in regulating Na+ transport in native human upper airway tissues from patients with 16 patients with CF and 32 non-CF control subjects. To that end, transepithelial voltage and equivalent short-circuit current (I-sc) were assessed by means of a perfused micro-Ussing chamber. Mucosal ATP and UTP caused an initial increase in lumen-negative I-sc that was followed by a sustained decrease of I-sc in both non-CF and CF tissues. The amiloride-sensitive portion of I-sc was inhibited significantly in normal and CF tissues in the presence of either ATP or UTP, Both basal Na+ transport and nucleotide-dependent inhibition of amiloride-sensitive I-sc were significantly enhanced in CF airways compared with non-CF, Nucleotide-mediated inhibition of Na+ absorption was attenuated by pretreatment with the Ca2+-adenosine triphosphatase inhibitor cyclopiazonic acid but not by inhibition of protein kinase C with bisindolylmaleimide, These data demonstrate sustained inhibition of Nat transport in non-CF and CF airways by mucosal ATP and UTP and suggest that this effect is mediated by an increase of intracellular Ca2+. Because ATP and UTP inhibit Na+ absorption and stimulate CI- secretion simultaneously, extracellular nucleotides could have a dual therapeutic effect, counteracting the ion transport defect in CF lung disease.