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Recent discoveries suggest that a peptide produced by the stomach and duodenum plays an important role in regulation of hunger and body weight. Ghrelin is a 28-amino acid peptide produced by endocrine cells of the stomach and small intestine. Circulating levels increase before each meal and decrease after each meal. Ingested food rapidly suppresses ghrelin levels. This hormone is the only yet identified stimulant of hunger in humans. Circulating levels of ghrelin are inversely proportional to the level of body fat stores. Levels appear to increase after voluntary weight loss and decrease after weight gain, triggering changes in appetite and energy expenditure that persist until the level of body fat is restored to the previous level. The peptide appears to act centrally; it is readily detectable in the hypothalamus where it interacts with receptors of anabolic neurons in the arcuate nucleus.
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Salivary amylase (until denatured)
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Salivary amylase can digest starch in the stomach until the acidity of gastric contents inactivates the enzyme. Pepsinogens hydrolyze in the gastric lumen to yield pepsin which is optimally active at pH 2, reversibly inactivated at pH 5, and is denatured at pH 7.0. Pepsin hydrolyzes protein to a mixture of polypeptides. Gastric digestion of protein and carbohydrate is incomplete.
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Gastric lipase is ~ 15% as effective as pancreatic lipase, but may be important in pancreatic insufficiency.
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Gastric lipase is secreted by fundal chief cells which has activity at acidic pH. The potential lipolytic activity towards triglycerides is about 20% of pancreatic lipase for tributyrin, and 10% for triolein, but this activity of gastric lipase may become important in pancreatic insufficiency.
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Gastric mucosa is really only permeable to substances that are both lipo- & hydro-philic.
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The gastric mucosa is thought to be relatively impermeable to water soluble substances. However, substances which are water-soluble such as acetylsalicylic acid (pKa 3.5) can be readily absorbed because aspirin becomes lipophilic when gastric pH is less than 3.5.
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Gastric HCl keeps Fe in solution & kills bacteria. Stomach delivers weakly polar metals to duodenum slowly for absorption
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One function of gastric HCl secretion is to keep Fe in solution (presumably as a chloride). The stomach also contributes to the absorption of Fe++, Fe+++, Ca++, and Mg++ by delivering them slowly to the duodenum.
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Next Section (H): Acute Disruptions of Gastric Mucosa »
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