Liddle's syndrome mutations disrupt cAMP-mediated translocation of the epithelial Na+ channel to the cell surface

The epithelial Na+ channel (ENaC) plays a critical role in Na+ absorption, and mutations in this channel cause diseases of Na+ homeostasis, including a genetic form of hypertension (Liddle's syndrome). To investigate cAMP-med iated stimulation of ENaC, alpha, beta, and gamma ENaC were coexpressed in Fischer rat thyoid epithelia to generate apical Na+ channels and transepith elial Na+ current. cAMP agonists stimulated Na+ current by 70%. Following c ovalent modification of cysteines introduced into ENaC, cAMP increased the rate of appearance of unmodified channels at the cell surface. In addition, cAMP increased the fluorescent labeling of ENaC at the apical cell surface . Inhibition of vesicle trafficking by incubating epithelia at 15 degrees C prevented the cAMP-mediated stimulation of ENaC. These results suggest tha t cAMP stimulates Na+ absorption in part by increasing translocation of ENa C to the cell surface. Stimulation of ENaC by cAMP was dependent on a seque nce (PPPXY) in the COOH terminus of each subunit. This sequence is the targ et for mutations that cause Liddle's syndrome, suggesting that cAMP-mediate d translocation of ENaC to the cell surface is defective in this genetic fo rm of hypertension.