Block 6 Gastrophysiology BRS W Expl

Cholecystokinin (CCK) inhibits gastric emptying and therefore helps to slow the delivery of food from the stomach to the intestine during periods of high digestive activity. CCK stimulates both functions of the exocrine pancreas—HCO 3- secretion and digestive enzyme secretion. It also stimulates the delivery of bile from the gallbladder to the small intestinal lumen by causing contraction of the gallbladder while relaxing the sphincter of Oddi.

The gastric parietal cells secrete HCl and intrinsic factor. The chief cells secrete pepsinogen

Vasoactive intestinal peptide (VIP) is a gastrointestinal (GI) neurocrine that causes relaxation of GI smooth muscle. For example, VIP mediates the relaxation response of the lower esophageal sphincter when a bolus of food approaches it, allowing passage of the bolus into the stomach.

Saliva is characterized by hypotonicity and a high HCO3- concentration (relative to plasma) and by the presence of a-amylase and lingual lipase (not proteases). The high HCO3- concentration is achieved by secretion of HCO3- into saliva by the ductal cells (not reabsorption of HCO3). Because control of saliva production is parasympathetic, it is abolished by vagotomy.

Gastrin's principal physiologic action is to increase H+ secretion. H+ secretion decreases the pH of the stomach contents. The decreased pH, in turn, inhibits further secretion of gastrin—a classic example of negative feedback.

Cimetidine is a reversible inhibitor of H2 receptors on parietal cells and blocks H+ secretion. Cyclic adenosine monophosphate (cAMP) [the second messenger for histamine] levels would be expected to decrease, not increase. Cimetidine also blocks the action of acetylcholine (ACh) to stimulate secretion. Omeprazole blocks H+,K+-adenosine triphosphatase (ATPase) directly.

Cholecystokinin (CCK) is the most important hormone for digestion and absorption of dietary fat. In addition to causing contraction of the gallbladder, it inhibits gastric emptying. As a result, chyme moves more slowly from the stomach to the small intestine, thus allowing more time for fat digestion and absorption.
With respect to the digestive system, VIP seems to induce smooth muscle relaxation (lower esophageal sphincter, stomach, gallbladder), stimulate secretion of water into pancreatic juice and bile, and cause inhibition of gastric acid secretion and absorption from the intestinal lumen. Its role in the intestine is to greatly stimulate secretion of water and electrolytes, as well as stimulating contraction of enteric smooth muscle, dilating peripheral blood vessels, stimulating pancreatic bicarbonate secretion, and inhibiting gastrin-stimulated gastric acid secretion. These effects work together to increase motility

GIP is synthesized by K cells, which are found in the mucosa of the duodenum and the jejunum of the gastrointestinal tract. Like all endocrine hormones, it is transported by blood.
GIP receptors are seven-transmembrane proteins found on beta-cells in the pancreas.
the function of GIP is to induce insulin secretion, which is stimulated primarily by hyperosmolarity of glucose in the duodenum. GIP is also thought to have significant effects on fatty acid metabolism through stimulation of lipoprotein lipase activity in adipocytes.

Peristalsis is contractile activity that is coordinated by the enteric nervous system [not the central nervous system (CNS)] and propels the intestinal contents forward. Normally, it takes place after sufficient mixing, digestion, and absorption have occurred. To propel the food bolus forward, the smooth muscle must simultaneously contract behind it and relax in front of it.

Both the internal and external anal sphincters must be relaxed to allow feces to be expelled from the body. Rectal smooth muscle contracts, and intra-abdominal pressure is elevated by expiring against a closed glottis (Valsalva maneuver). Segmentation contractions are prominent in the small intestine during digestion and absorption.

The major anion in pancreatic secretions is HCO3- (which is found in higher concentration than in plasma), and the Cl- concentration is lower than in plasma. Pancreatic secretion is stimulated by the presence of fatty acids in the duodenum. Secretin (not gastrin) stimulates pancreatic HCO 3- secretion, and cholecystokinin (CCK) stimulates pancreatic enzyme secretion. Pancreatic secretions are always isotonic, regardless of flow rate.

Cholera toxin activates adenylate cyclase and increases cyclic adenosine monophosphate (cAMP) in the intestinal crypt cells. In the crypt cells, cAMP activates the C1--secretory channels and produces a primary secretion of Cl-, with Na t and 1120 following.

Bile salts are recirculated to the liver in the enterohepatic circulation via a Na+-bile acid cotransporter located in the ileum of the small intestine.

Only monosaccharides can be absorbed by intestinal epithelial cells. Disaccharides, such as sucrose, must be digested to monosaccharides before they are absorbed. On the other hand, proteins are hydrolyzed to amino acids, dipeptides, or tripeptides, and all three forms are transported into intestinal cells for absorption.

Intrinsic factor is secreted by the parietal cells of the gastric fundus (as is HC1). It is absorbed, with vitamin B 12, in the ileum.

"Receptive relaxation" of the oral region of the stomach is initiated when food enters the stomach from the esophagus. This parasympathetic (vagovagal) reflex is abolished by vagotomy.

Gastrin is secreted by the G cells of the gastric antrum. HCI and intrinsic factor are secreted by the fundus. (antrum is the portion between body and pyloris- Yin's slides do not include antrum and just have pyloris secreting gastrin---but we should know this for the STEP)

Gastric inhibitory peptide (GIP) is the only gastrointestinal (GI) hormone that is released in response to all three categories of nutrients—fat, protein, and carbohydrate. Oral glucose releases GIP which, in turn, causes the release of insulin from the endocrine pancreas. This action of GIP explains why oral glucose is more effective than intravenous glucose in releasing insulin.

Fructose is the only monosaccharide that is not absorbed by Na+ dependent cotransport; it is transported by facilitated diffusion. Amino acids are absorbed by Na+ dependent cotransport, but oligopeptides (larger peptide units) are not. Triglycerides are not absorbed without further digestion. The products of lipid digestion, such as fatty acids, are absorbed by simple diffusion.

Zollinger-Ellison syndrome (gastrinoma) is a tumor of the non-B-cell pancreas. The tumor secretes gastrin, which then circulates to the gastric parietal cells to produce increased H+ secretion, peptic ulcer, and parietal cell growth (trophic effect of gastrin). Because the tumor does not involve the pancreatic B cells, insulin levels should be unaffected. Absorption of lipids is decreased (not increased) because increased H + secretion decreases the pH of the intestinal lumen and inactivates pancreatic lipases.

The two hormones have five identical amino acids at the C terminus. Biologic activity of cholecystokinin (CCK) is associated with the seven C-terminal amino acids, and biologic activity of gastrin is associated with the four C-terminal amino acids. Because this CCK heptapeptide contains the five common amino acids, it is logical that CCK should have some gastrin-like properties. G cells secrete gastrin. I cells secrete CCK. The secretin family includes glucagon.

Micelles provide a mechanism for solubilizing fat-soluble nutrients in the aqueous solution of the intestinal lumen until the nutrients can be brought into contact with and absorbed by the intestinal epithelial cells. Because vitamin D is fat-soluble, it is absorbed in the same way as other dietary lipids. Glycerol is one product of lipid digestion that is water-soluble and is not included in micelles. Galactose and leucine are absorbed by Na+ dependent cotransport. Although bile acids are a key ingredient of micelles, they are absorbed by a specific Na+ dependent cotransporter in the ileum. Vitamin B 12 is water-soluble; thus, its absorption does not require micelles.

Slow waves are oscillating resting membrane potentials of the gastrointestinal (GI) smooth muscle. The slow waves bring the membrane potential toward or to threshold, but are not themselves action potentials. If the membrane potential is brought to threshold by a slow wave, then action potentials occur, followed by contraction.

ileal resection removes the portion of the small intestine that normally transports bile acids from the lumen of the gut and recirculates them to the liver. Because this process maintains the bile acid pool, new synthesis of bile acids is needed only to replace those bile acids that are lost in the feces. With ileal resection, most of the bile acids secreted are excreted in the feces, and the liver pool is significantly diminished. Bile acids are needed for micelle formation in the intestinal lumen to solubilize the products of lipid digestion so that they can be absorbed. Chylomicrons are formed within the intestinal epithelial cells and are transported to lymph vessels.