Model of human colon secretory and cAMP turnover systems. A: Schematic presentation of the secretory system of the colon as revealed by this study. The subcellular location of ABCB1 was not determined in this study but has been localized to the apical membrane by others. * Indicates significantly increased expression in CRN patients. B: Schematic presentation of the cAMP-turnover system in a colonic epithelial cell. AC: adenylate cyclase. PDE: phosphodiesterase.