Hormones cholecystokinin

Most pancreatic secretion takes place during the intestinal phase. The intestinal hormone secretin stimulates release of a large volume of pancreatic juice with a high concentration of bicarbonate ion. Secretin is released in response to acidic chyme in the duodenum (maximal release at pH < 3.0). The intestinal hormone cholecystokinin is released in response to the presence of the products of protein and lipid digestion. Cholecystokinin then stimulates the release of digestive enzymes from the pancreas. 

Bile is secreted by the liver, stored in the gallbladder, and used in the small intestine. It is transported toward the small intestine by the hepatic duct (from the liver) and the cystic duct (from the gallbladder), which join to form the common bile duct. Pancreatic juice is transported toward the small intestine by the pancreatic duct. The common bile duct and the pancreatic duct join to form the hepatopancreatic ampulla, which empties into the duodenum. The entrance to the duodenum is surrounded by the Sphincter of Oddi. This sphincter is closed between meals in order to prevent bile and pancreatic juice from entering the small intestine it relaxes in response to the intestinal hormone cholecystokinin, thus allowing biliary and pancreatic secretions to flow into the duodenum. 

In the GI tract, different hydrodynamic conditions are present, depending on the fasted or the fed state. Contraction patterns are controlled in terms of electromechanical impulses (myoelectric activity) as well as by various hormones (cholecystokinin, secretin, glucagon, motilin, and insulin, for example). In the fasted state, the motility pattern is regulated by the (interdigestive) migrating myoelectric complex [(I)... 

The flow of bile is lowest during fasting, mostly being diverted to the gall bladder for concentrating. When chyme from an ingested meal enters the small intestine, acid and partially digested fats and proteins stimulate secretion of the enteric hormones cholecystokinin and secretin. 

Special mucosal endocrine cells secret the peptide hormones cholecystokinin (CCK) and secretin into the circulatory system. 

The chemical phase of digestion also involves the hormone cholecystokinin (CCK). Dietary fats and proteins elicit the release of CCK from cells of the intestines. The versatility of this hormone in digestion is revealed, later in this chapter, via studies involving dogs, rats, and humans. These studies address the influence of CCK on the release of pancreatic enzymes, bile salts, and pancreatic bicarbonate into the lumen of the small intestines. 

Magnesium salts are also believed to act by stimulating the secretion of the hormone cholecystokinin, which promotes fluid secretion and motility in the intestine. 

Dietary fat leaves the stomach and enters the small intestine, where it is emulsified (suspended in small particles in the aqueous environment) by bile salts (Fig. 32.2). The bile salts are amphipathic compounds (containing both hydrophobic and hydrophilic components), synthesized in the liver (see Chapter 34 for the pathway) and secreted via the gallbladder into the intestinal lumen. The contraction of the gallbladder and secretion of pancreatic enzymes are stimulated by the gut hormone cholecystokinin, which is secreted by the intestinal cells when stomach contents enter the intestine. Bile salts act as detergents, binding to the globules of dietary fat as they are broken up by the peristaltic action of the intestinal muscle. This emulsified fat, which has an increased surface area as compared with unemulsified fat, is attacked by digestive enzymes from the pancreas (Fig. 32.3). 

The duodenum releases secretin, which is a hormone that suppresses gastric acid secretion. This results in the intestinal juices having a higher pH than the gastric juices in the stomach. The hormone cholecystokinin is also released. It simulates the flow of bile into the duodenum. Hormones, bile, and pancreatic enzymes tr5 sin, chymotrypsin, lipase, and amylase digest carbohydrates, protein, and fat in preparation for absorption in the small intestine. 

The conclusion that a peptide such as bradykinin consisting of nine amino acids does not have a preferred conformation but that a well defined geometry develops from a certain chain length on (secretin contains twenty-seven amino acids, glucagon with fairly similar sequence and somewhat similar spectra twenty-nine amino acids), would be attractive but is not well founded. Examination of the ORD and CD spectra of the intestinal hormone cholecystokinin-pancreozymin (CCK), a single chain peptide of 33 amino acids, gives no evidence for structural rigidity which would express itself in spectra similar to those of helical molecules. 

At one time there were thought to be two separate hormones, cholecystokinin which produced contraction of the gall bladder and pancreozymin which increased the secretion of pancreatic enzymes. However it is now known that a single hormone secreted by the upper small intestine performs both activities. 

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