8-(4-Chlorophenylthio)-guanosine-3′,5′-cyclic monophosphate-Na stimulates human alveolar fluid clearance by releasing external Na + self-inhibition of epithelial Na + channels

Salt absorption via alveolar epithelial Na+ channels (ENaC) is a critical step for maintaining an airspace free of flooding. Previously, we found that 8-(4-chlorophenylthio)-guanosine-3',5'-cyclic monophosphate-Na (CPT-cGMP) activated native and heterologous ENaC. To investigate the potential pharmacological relevance, we applied this compound intratracheally to human lungs and found that ex vivo alveolar fluid clearance was increased significantly. Furthermore, this compound eliminated self-inhibition in human lung H441 cells and in oocytes expressing human alpha beta gamma but not delta beta gamma channels. To further elucidate this novel mechanism, we constructed mutants abolishing (beta(Delta V348) and gamma(H233R)) or augmenting (alpha(Y458A) and gamma(M432G)) self-inhibition. The mutant seliminating self-inhibition lost their responses to CPT-cGMP, whereas those enhancing self-inhibition facilitated the stimulatory effects of this compound. CPT-cGMP was unable to activate a high P-o mutant (beta(S520C)) and plasmin proteolytically cleaved channels. Our data suggest that elimination of self-inhibition of abg ENaC may be a novel mechanism for CPT-cGMP to stimulate salt reabsorption in human lungs.