Cholecystokinin
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Cholecystokinin (CCK, previously pancreozymin) is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein. CCK is secreted by the duodenum, the first segment of the small intestine, and causes the release of digestive enzymes and bile from the pancreas and gall bladder, respectively. It also acts as a hunger suppresant.
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Structure
CCK is composed of 33 amino acids, and is very similar in structure to gastrin, another of the gut hormones, so much so that the last five C-terminal amino acids are same as gastrin. It is activated post-translationally from the precursor preprocholecystokinin.
Function
CCK mediates a number of physiological processes, including digestion and satiety.
Digestion
CCK is secreted by the duodenum when fat- or protein-rich chyme leaves the stomach and enters the duodenum. The hormone acts on the pancreas to stimulate the secretion of the enzymes lipase, amylase, trypsin, and chymotrypsin. Together these pancreatic enzymes catalyze the digestion of fat and protein.
CCK also stimulates the contraction of the gall bladder, which secretes bile into the small intestine. Bile salts serve to emulsify fats, thereby increasing the effectiveness with which enzymes can digest them.
Neurobiology
As a neuropeptide, CCK mediates satiety by acting on the CCK receptors distributed widely throughout the central nervous system. In humans, CCK administration causes nausea and anxiety, and weakly decreases the desire to eat [1] (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9855480&dopt=Abstract).
The effects of CCK vary between individuals. For example, in rats, CCK administration significantly reduces hunger in young males, but is less effective in older subjects, and even less effective in females. The hunger-suppressive effects of CCK also diminish in obese rats [2] (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=9835394).
Nomenclature
The term "cholecystokinin" originates from the Latin chole for "bile", cysto for "sac", and kinin for "move". Hence cholecysto refers to the gall bladder, the small sac that stores bile produced by the liver. Cholecystokinin thus refers to the movement, or contraction of the gallbladder.
Previously, CCK was also called "pancreozymin", referring to its stimulation of the pancreas to release digestive enzymes.
References
- Greenough A, Cole G, Lewis J, Lockton A, Blundell J. "Untangling the effects of hunger, anxiety, and nausea on energy intake during intravenous cholecystokinin octapeptide (CCK-8) infusion". Physiol Behav. 1998 Nov 15;65(2):303-10. PMID 9855480
- Fink H, Rex A, Voits M, Voigt JP. "Major biological actions of CCK--a critical evaluation of research findings". Exp Brain Res. 1998 Nov;123(1-2):77-83. PMID 9835394
- Cholecystokinin, NIH/NLM Medical Subject Headings (http://www.nlm.nih.gov/cgi/mesh/2K/MB_cgi?term=Cholecystokinin)pl:Cholecystokinina