High resolution

Module 7: Figure intestinal secretion

Control of fluid secretion by the small intestine.

Acetylcholine controls secretion by stimulating the inositol 1,4,5-trisphosphate (InsP3)/Ca2+ signalling pathway that controls two channels: K+ efflux across the basolateral membrane and Cl flux into the lumen. Vasoactive intestinal polypeptide (VIP) uses the cyclic AMP signalling pathway to control the co-transporter that brings Cl into the cell and the cystic fibrosis transmembrane conductance regulator (CFTR) channel that allows it to pass into the lumen. Guanylin acts through the particulate guanylyl cyclase (pGC) to produce cyclic GMP, which also acts to open CFTR. Various toxins such as STa and cholera toxin (CT) induce secretion by activating the cyclic AMP and cyclic GMP signalling systems respectively.