Ans. D. S cells. (Ref. Guyton's Medical Physiology 11th/pg. 801; Ganong 23rd/ Chapter 36).GI secretory products1. Nitrous oxide: Causes smooth muscle relaxation (e.g., elower oesophageal sphincter (LES) relaxtion).2. Gastrin: Secreted in response to gastric distention, vagal stimulation, and amino acid entering the stomach; causes gastric H+ secretion.3. Secretin: Secreted in response to H+ and fatty acids entering the duodenum; causes pancreatid secretion of of HCO3 and inhibits gastrin H+ secretion. It is known as Nature's antacid as after being secreted by 'S' cells of duodenum it causes, secretion of bicarbonate from pancreas, which neutralises gastric acid.4. Cholecystokinin (CCK): Secreted in response to amino acids and fatty acids entering the duodenum; causes contraction of gallbladder and pancreatic secretion of enzymes and HC03 (AIIMS-99).5. Parasympathetic (ACh): Increases production of saliva; increased gastric H+ secretion; increases pancreatic enzyme and HCO3+ secretion; stimulates enteric nervous system to create intestinal peristalisis; relaxes sphincters.6. Vasoactive intestinal peptide (VIP): Secreted by smooth muscle and nerves of intestines. Relaxes intestinal smooth muscle causes pancreatic HC03 secretion and inhibits gastric H+ secretion.7. Sympathetic (NE): Increases production of saliva; deceases spalnchnic blood flow in fight-or-flight response: decreases motility; constricts sphincters.HormoneSourceActionRegulationNotesGastrinG cells, antrum of stomach| gastric H+ secretion, | growth of gastric mucosa, | gastric motility| by stomach distention, amino acids peptides, vagal stimulation; | by H+ secretion and stomach acid pH <1.5| | in Zollinger-Ellison syndrome; phenylalanine and tryptophan are potent stimulatorsCholecy- stokinin| cells, duodenum, jejunum| pancreatic secretion, | gallbladder contraction; |gasctic emptying, | growth of exocrine pancreas and gallbladder| by secretin and stomach pH < 1.5; | by fatty acids, amino acidsIncholelithiasis, pain woesens after fatty food ingestion due to| CCKSecretinS cells, duodenum| pancreatic HCO3' secretion, | gastric acid secretion| by acid, fatty acids in lumen of duodenum| HCO3 neutralizes gastric acid in duodenum, allowing pancrestic enzymes to functionSomatostainD cells, pancreatic islets; GI mucosa| gastric acid and pepsinogen secretion | pancreatic and small intestine fluid secretion | insulin and glucagon release| by acid; | by vagal stimulationInhibitory hormone; antigrowth hormone effects (digestion and absorption of substances needed for growth)Gastric inhibitory peptide (GIP)K cells, duodenum and jejunumExocrine: | gastric H+ secretionEndocrine: | insulin release| by fatty acids, amino acids, oral glucose (only GI hormone stimulated by all 3 nutrient classes)| GIP is why an oral glucose load is used more rapidly than the equivalent given by IVSecretin# Secretin is a polypeptide, containing 27 amino acids (molecular weight about 3400), present in an inactive form, prosecretin, in so-called S cells in the mucosa of the duodenum and jejunum. When acid chyme with pH less than 4.5 to 5.0 enters the duodenum from the stomach, it causes duodenal mucosal release and activation of secretin, which is then absorbed into the blood. The one truly potent constituent of chyme that causes this secretin release is the hydrochloric acid from the stomach.# Secretin in turn causes the pancreas to secrete large quantities of fluid containing a high concentration of bicarbonate ion (up to 145 mEq/L) but a low concentration of chloride ion. The secretin mechanism is especially important for two reasons: First, secretin begins to be released from the mucosa of the small intestine when the pH of the duodenal contents falls below 4.5 to 5.0, and its release increases greatly as the pH falls to 3.0. This immediately causes copious secretion of pancreatic juice containing abundant amounts of sodium bicarbonate.Cholecystokinin# The presence of food in the upper small intestine also causes a second hormone, cholecystokinin, a polypeptide containing 33 amino acids, to be released 'I cells', in the mucosa of the duodenum and upper jejunum. This release of cholecystokinin results especially from the presence of proteoses and peptones and long-chain fatty acids in the chyme coming from the stomach.# The differences between the pancreatic stimulatory effects of secretin and Cholecystokinin:- intense sodium bicarbonate secretion in response to acid in duodenum, stimulated by secretin,- a dual effect in response to soap (a fat), and- intense digestive enzyme secretion (when peptones enter duodenum) stimulated by CCK.Educational points:Basic Stimuli That Cause Pancreatic SecretionThree basic stimuli are important in causing pancreatic secretion:1. Acetylcholine, which is released from the parasympathetic vagus nerve endings and from other cholinergic nerves in the enteric nervous system2. Cholecystokinin, which is secreted by duodenal and upper jejunal mucosa when food enters small bowel3. Secretin, which is also secreted by duodenal and jejunal mucosa when highly acid food enters small bowel.The presence of acid, fat, protein breakdown products, hyperosmotic or hypo-osmotic fluids, or any irritating factor in the upper small intestine causes release of several intestinal hormones. One of these is secretin, which is especially important for control of pancreatic secretion. However, secretin opposes stomach secretion. Three other hormones--gastric inhibitory peptide, vasoactive intestinal polypeptide, and somatostatin--also have slight to moderate effects in inhibiting gastric secretion.