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Cholecystokinin (CCK or CCK-PZ; from Greek chole, "bile"; cysto, "sac"; kinin, "move"; hence, move the bile-sac (gallbladder)) is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein. Cholecystokinin, previously called pancreozymin, is synthesized by I-cells in the mucosal epithelium of the small intestine and secreted in the duodenum, the first segment of the small intestine, and causes the release of digestive enzymes and bile from the pancreas and gallbladder, respectively. It also acts as a hunger suppressant.
CCK is composed of varying numbers of amino acids depending on post-translational modification of the CCK gene product, preprocholecystokinin. Thus CCK is actually a family of hormones identified by number of amino acids, e.g., CCK58, CCK33, and CCK8. CCK58 assumes a helix-turn-helix configuration. Its existence was first suggested in 1905 by the British physiologist Joy Simcha Cohen. CCK is very similar in structure to gastrin, another of the gastrointestinal hormones. CCK and gastrin share the same five amino acids at their C-termini.
CCK mediates digestion in the small intestine by inhibiting gastric emptying and gastric acid secretion. It stimulates the acinar cells of the pancreas to release digestive enzymes and stimulates the secretion of a juice rich in pancreatic digestive enzymes, hence the old name pancreozymin. Together these enzymes catalyze the digestion of fat, protein, and carbohydrates. Thus, as the levels of the substances that stimulated the release of CCK drop, the concentration of the hormone drops as well. The release of CCK is also inhibited by somatostatin. Trypsin, a protease released by pancreatic acinar cells hydrolyzes CCK-releasing peptide and monitor peptide effectively turning off the additional signals to secrete CCK.
Cholecystokinin has been shown to interact with the Cholecystokinin A receptor located mainly on pancreatic acinar cells and Cholecystokinin B receptor mostly in the brain and stomach. CCKB receptor also binds gastrin, a gastrointestinal hormone involved in stimulating gastric acid release and growth of the gastric mucosa.
CCK has also been shown to interact with calcineurin in the pancreas. Calcineurin will go on to activate the transcription factors NFAT 1–3, which will stimulate hypertrophy and growth of the pancreas. CCK can be stimulated by a diet high in protein, or by protease inhibitors.
Role in appetite and satietyEdit
CCK mediates a number of physiological processes, including digestion and satiety. It is released by I cells located in the mucosal epithelium of the small intestine (mostly in the duodenum and jejunum), neurons of the enteric nervous system and neurons in the brain. Release of CCK is stimulated by monitor peptide released by pancreatic acinar cells as well as CCK-releasing protein, a paracrine factor secreted by enterocytes in the gastrointestinal mucosa. In addition, release of acetylcholine by the parasympathetic nerve fibers of the vagus nerve also stimulate its secretion. The presence of fatty acids and/or certain amino acids in the chyme entering the duodenum is the greatest stimulator of CCK release.
As a neuropeptide, CCK mediates satiety by acting on the CCK receptors distributed widely throughout the central nervous system. In humans, it has been suggested that CCK administration causes nausea and anxiety, and induces a satiating effect. CCK-4 is routinely used to induce anxiety in humans though certainly different forms of CCK are being shown to have highly variable effects. The mechanism for this hunger suppression is thought to be a decrease in the rate of gastric emptying.
The effects of CCK vary between individuals. For example, in rats, CCK administration significantly reduces hunger in young males, but is slightly less effective in older subjects, and even slightly less effective in females. The hunger-suppressive effects of CCK also are reduced in obese rats.
Role in anxietyEdit
CCK also has stimulatory effects on the vagus nerve, effects that can be inhibited by capsaicin. The stimulatory effects of CCK oppose those of ghrelin, which has been shown to inhibit the vagus nerve. The CCK tetrapeptide fragment CCK-4 (Trp-Met-Asp-Phe-NH2) reliably causes anxiety when administered to humans, and is commonly used in scientific research to induce panic attacks for the purpose of testing new anxiolytic drugs.
Role in drug toleranceEdit
Recent evidence has suggested that it also plays a major role in inducing drug tolerance to opioids like morphine and heroin, and is partly implicated in experiences of pain hypersensitivity during opioid withdrawal.
- Cholecystokinin antagonist
- Cholecystokinin receptor
- Cholecystokinin A receptor
- Cholecystokinin B receptor
- Cholecystokinin tetrapeptide
- ↑ Reeve JR Jr, Eysselein VE, Rosenquist G, Zeeh J, Regner U, Ho FJ, Chew P, Davis MT, Lee TD, Shively JE, Brazer SR, Liddle RA (1996). Evidence that CCK-58 has structure that influences its biological activity. Am. J. Physiol. 270 (5 Pt 1): G860-8.
- ↑ Harikumar KG, Clain J, Pinon DI, Dong M, Miller LJ (January 2005). Distinct molecular mechanisms for agonist peptide binding to types A and B cholecystokinin receptors demonstrated using fluorescence spectroscopy. J. Biol. Chem. 280 (2): 1044–50.
- ↑ Aloj L, Caracò C, Panico M, Zannetti A, Del Vecchio S, Tesauro D, De Luca S, Arra C, Pedone C, Morelli G, Salvatore M (March 2004). In vitro and in vivo evaluation of 111In-DTPAGlu-G-CCK8 for cholecystokinin-B receptor imaging. J. Nucl. Med. 45 (3): 485–94.
- ↑ Galés C, Poirot M, Taillefer J, Maigret B, Martinez J, Moroder L, Escrieut C, Pradayrol L, Fourmy D, Silvente-Poirot S (May 2003). Identification of tyrosine 189 and asparagine 358 of the cholecystokinin 2 receptor in direct interaction with the crucial C-terminal amide of cholecystokinin by molecular modeling, site-directed mutagenesis, and structure/affinity studies. Mol. Pharmacol. 63 (5): 973–82.
- ↑ Gurda GT, Guo L, Lee SH, Molkentin JD, Williams JA (January 2008). Cholecystokinin activates pancreatic calcineurin-NFAT signaling in vitro and in vivo. Mol. Biol. Cell 19 (1): 198–206.
- ↑ Greenough A, Cole G, Lewis J, Lockton A, Blundell J (1998). Untangling the effects of hunger, anxiety, and nausea on energy intake during intravenous cholecystokinin octapeptide (CCK-8) infusion. Physiol. Behav. 65 (2): 303–10.
- ↑ Shillabeer G, Davison JS (1987). Proglumide, a cholecystokinin antagonist, increases gastric emptying in rats. Am. J. Physiol. 252 (2 Pt 2): R353–60.
- ↑ Tsujino N, Yamanaka A, Ichiki K, Muraki Y, Kilduff TS, Yagami K, Takahashi S, Goto K, Sakurai T (August 2005). Cholecystokinin activates orexin/hypocretin neurons through the cholecystokinin A receptor. J. Neurosci. 25 (32): 7459–69.
- ↑ Template:Cite thesis
- ↑ Fink H, Rex A, Voits M, Voigt JP (1998). Major biological actions of CCK—a critical evaluation of research findings. Exp Brain Res 123 (1–2): 77–83.
- ↑ Holzer P (July 1998). Neural injury, repair, and adaptation in the GI tract. II. The elusive action of capsaicin on the vagus nerve. Am. J. Physiol. 275 (1 Pt 1): G8–13.
- ↑ Bradwejn J (July 1993). Neurobiological investigations into the role of cholecystokinin in panic disorder. J Psychiatry Neurosci 18 (4): 178–88.
- ↑ Kissin I, Bright CA, Bradley EL (2000). Acute tolerance to continuously infused alfentanil: the role of cholecystokinin and N-methyl-D-aspartate-nitric oxide systems. Anesth. Analg. 91 (1): 110–6.
- ↑ Fukazawa Y, Maeda T, Kiguchi N, Tohya K, Kimura M, Kishioka S (2007). Activation of spinal cholecystokinin and neurokinin-1 receptors is associated with the attenuation of intrathecal morphine analgesia following electroacupuncture stimulation in rats. J. Pharmacol. Sci. 104 (2): 159–66.
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