Circadian regulation of uroguanylin and guanylin in the rat intestine

Uroguanylin (UGN) and guanylin (GN) are the endogenous intestinal ligands f or guanylyl cyclase C (GC-C). We examined the circadian expression of UGN, GN, and GC-C in the jejunum, ileum, and proximal colon of young adult rats by Northern blot analyses. These assays revealed that UGN is more abundant in the proximal small intestine, whereas GN and GC-C are more abundant in t he proximal colon. mRNA levels showed significant circadian variation for U GN (3- to 18-fold peak/trough difference), GN (2.1- to 2.8-fold peak/trough difference), and GC-C (3- to 5-fold peak/trough difference). The maximal a bundance occurred in the dark period for all three mRNAs, although peak UGN and GN expression occurred later in the dark period in the jejunum relativ e to the ileum and colon. Immunoblot analyses using monospecific polyclonal antibodies against UGN and GN prohormones confirmed the regional and circa dian variation detected by Northern assays. Thus the expression of these ge nes is regulated not only by histological position but also by circadian ti me.