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+ abs orption probably due to a lack of downregulation of epithelial Na+ channels by mutant CF transmembrane conductance regulator. Extracellular nucleotide s adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) have be en shown to activate alternative Ca2+-dependent CI- channels in normal and CF respiratory epithelia, Recent studies suggest additional modulation of N a+ absorption by extracellular nucleotides. In this study we examined the r ole of mucosal ATP and UTP in regulating Na+ transport in native human uppe r airway tissues from patients with 16 patients with CF and 32 non-CF contr ol subjects. To that end, transepithelial voltage and equivalent short-circ uit current (I-sc) were assessed by means of a perfused micro-Ussing chambe r. Mucosal ATP and UTP caused an initial increase in lumen-negative I-sc th at was followed by a sustained decrease of I-sc in both non-CF and CF tissu es. 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 Natransport and nucleotide-dependent inhibition of amiloride-sensitive I-sc w ere significantly enhanced in CF airways compared with non-CF, Nucleotide-m ediated inhibition of Na+ absorption was attenuated by pretreatment with th e Ca2+-adenosine triphosphatase inhibitor cyclopiazonic acid but not by inh ibition of protein kinase C with bisindolylmaleimide, These data demonstrat e 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 int racellular Ca2+. Because ATP and UTP inhibit Na+ absorption and stimulate C I- secretion simultaneously, extracellular nucleotides could have a dual th erapeutic effect, counteracting the ion transport defect in CF lung disease .