Mechanism of Action of Cholecystokinin : A Not Atypical Brain-Gut Peptide
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Brain-gut peptide action has been best studied in certain target cells of the gastrointestinal tract such as isolated pancreatic acini. Cholecystokinin (CCK) activation of pancreatic digestive enzyme secretion is initiated by specific receptors present in the basolateral membrane of the acinar cell. These receptors are highly selective for CCK and readily dis criminate it from the homologous peptide, gastrin. Studies covalently crosslinking<SUP>125</SUP>I-CCK to its receptor have revealed a binding glycoprotein subunit of M<SUB>r</SUB>= 76,000 attached by a disulfide bridge to a M<SUB>r</SUB>=40,000 nonbinding subunit. Receptor occupancy leads to phosphotidylinositide breakdown and Ca<SUP>2+</SUP> mobilization. Recent studies with the fluorescent chelate probe Quin-2 have shown that CCK increases cytosolic Ca<SUP>2+</SUP> from a basal level of 100 nM to 500-1000 nM. The effects of Ca<SUP>2+</SUP>, and diacylglycerol produced by the breakdown of phosphoinositides, are believed mediated by activation of a group of protein kinases and phosphatases.<BR>CCK in the brain is present in neurons and is released from nerve endings by depolarization. The cellular mechanism of action of CCK, however, is essentially unknown. CCK application excites certain neurons but attempts to demonstrate effects on ion fluxes, phospholipid metabolism and protein phosphorylation have been negative to date. A possible explanations is provided by the finding that the brain CCK receptor shows differences in binding specificity from peripheral CCK receptors. Moreover, crosslinking studies reveal a single binding protein of M<SUB>r</SUB>= 51,000. Thus, CCK may act differently in the brain and pancreas.
- 一般社団法人 日本内分泌学会の論文
一般社団法人 日本内分泌学会 | 論文
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