Briefly describe the functions of secretin and CCK the factors regulating their secretions.

Essay:

            Secretin is secreted by S cells that are located deep in the glands of the mucosa of the upper portion of the small intestine. Secretin increases the secretion of bicarbonate by the duct cells of the pancreas and the biliary tract. It thus causes the secretion of a watery, alkaline pancreatic juice. Its action on pancreatic ducts is mediated by cAMP. It also augments the action of CCK in producing pancreatic secretion of digestive enzymes. It decreases gastric acid secretion and may cause contraction of the pyloric sphincter. The secretion of secretin is increased by the products of protein digestion and by acid bathing the mucosa of the upper small intestine.

            CCK is a peptide hormone secreted by cells in the mucosa of the upper small intestine. The intracellular actions of CCK is mediated by the phospholipase C second messenger system. CCK causes the contraction of the gallbladder and the relaxation of the sphincter of Oddi, allowing bile to be released into the duodenum for digestion of fats. CCK stimulates the secretion of a pancreatic juice rich in enzymes and augments the actions of secretin in producing secretion of an alkaline pancreatic juice. It also inhibits gastric emptying, exerts a trophic effect on the pancreas, increases the secretion of enterokinase and may enhance the motility of the small intestine and colon. Both gastrin and CCK stimulates glucagon secretion. The secretion of CCK is increased by contact of the intestinal mucosa with the products of digestion, particularly the peptides, amino acids and fatty acids.