Figure 8: Signaling pathways of guanylin (GN) and uroguanylin (UGN) in the principal cell of the cortical collecting ducts. GN present from glomerular filtration or from paracrine secretion via proximal parts of the nephron segments is degradated by peptidase and therefore not present in the urine. Guanylin peptides (GN and UGN) activate phospholipase A2 (PLA2), which leads to an increase in the intracellular concentration of arachidonic acid and an inhibition of luminal ROMK K+ channels. ENaC: epithelial sodium channels.