Evolution of the gastrointestinal endocrine system (with special reference to gastrin and CCK).
The evolution of gut endocrine cells can be seen to have depended in the first instance on the expression of genes encoding regulatory peptides in cells that had evolved the regulated pathway of secretion. It seems probable that the endocrine cells made use of molecules and mechanisms that first emerged in early nervous systems. However, by the start of the vertebrate line of evolution, most of the major families of gut hormones were already found in association with endocrine cells. A single common class of receptor with seven transmembrane domains and acting via association with G-proteins transduces many (perhaps all) gut peptide actions. The duplication and divergence of receptors and peptides can now be traced, in outline at least, for gastrin and CCK in vertebrates. Even in phylogenetically similar groups such as birds and mammals, quite different molecular approaches have been applied to solving the same physiological problem. Evolution of the modern gastrointestinal control system evidently depended in this case both on molecular evolution of peptides and receptors and on cells expressing the genes encoding them.