cAMP-stimulated Na+transport in H441 distal lung epithelial cells: role of PKA, phosphatidylinositol 3-kinase, and sgk1

H441 cells, a bronchiolar epithelial cell line, develop a cAMP-regulated benzamil-sensitive Na + transport pathway on permeable supports (Itani OA, Auerbach SD, Husted RF, Volk KA, Ageloff S, Knepper MA, Stokes JB, Thomas CP. Am J Physiol Lung Cell Mol Physiol 282: L631–L641, 2002). To understand the molecular basis for the stimulation of Na + transport, we delineated the role of specific intracellular pathways and examined the effect of cAMP on αβγ-epithelial Na + channel (ENaC) and sgk1 expression. Na + transport increases within 5 min of cAMP stimulation and is sustained for >24 h. The sustained effect of cAMP on Na + transport is abolished by LY-294002, an inhibitor of phosphatidylinositol 3-kinase, by H89, an inhibitor of PKA, or by SB-202190, an inhibitor of p38 MAP kinase. The sustained effect of cAMP was associated with increases in α-ENaC mRNA and protein but without a detectable increase in βγ-ENaC and sgk1. The early effect of cAMP on Na + transport is brefeldin sensitive and is mediated via PKA. These results are consistent with a model where the early effect of cAMP is to increase trafficking of Na + channels to the apical cell surface whereas the sustained effect requires the synthesis of α-ENaC.