No evidence for inhibition of ENaC through CFTR-mediated release of ATP

Both purinergic stimulation and activation of cystic fibrosis transmembrane conductance regulator (CFTR) increases Cl⁻ secretion and inhibit amiloride-sensitive Na⁺ transport. CFTR has been suggested to conduct adenosine 5′-triphosphate (ATP) or to control ATP release to the luminal side of epithelial tissues. Therefore, a possible mechanism on how CFTR controls the activity of epithelial Na⁺ channels (ENaC) could be by release of ATP or uridine 5′-triphosphate (UTP), which would then bind to P2Y receptors and inhibit ENaC. We examined this question in native tissues from airways and colon and in Xenopus oocytes. Inhibition of amiloride-sensitive transport by both CFTR and extracellular nucleotides was observed in colon and trachea. However, nucleotides did not inhibit ENaC in Xenopus oocytes, even after coexpression of P2Y₂ receptors. Using different tools such as hexokinase, the P2Y inhibitor suramin or the Cl⁻ channel blocker 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS), we did not detect any role of a putative ATP secretion in activation of Cl⁻ transport or inhibition of amiloride sensitive short circuit currents by CFTR. In addition, N²,2′-O-dibutyrylguanosine 3′,5′-cyclic monophosphate (cGMP) and protein kinase G (PKG)-dependent phosphorylation or the nucleoside diphosphate kinase (NDPK) do not seem to play a role for the inhibition of ENaC by CFTR, which, however, requires the presence of extracellular Cl⁻.