Mechanisms of the inhibition of epithelial Na⁺ channels by CFTR and purinergic stimulation. The epithelial Na⁺ channel ENaC is inhibited when the cystic fibrosis transmembrane conductance regulator (CFTR) coexpressed in the same cell is activated by the cyclic adenosine monophosphate (cAMP)-dependent pathway. Regulation of ENaC by CFTR has been studied in detail in epithelial tissues from intestine and trachea and is also detected in renal cells. In the kidney, regulation of other membrane conductances might be the predominant function of CFTR. A similar inhibition of ENaC takes place when luminal purinergic receptors are activated by 5′-adenosine triphosphate (ATP) or uridine triphosphate (UTP). Because both stimulation of purinergic receptors and activation of CFTR induce a Cl⁻ conductance, it is likely that Cl⁻ ions control ENaC activity.