Acid secretion, physiological response

Histamine A chemical produced by various cells in the body that is involved in the modulation of certain physiologic responses (e.g., secretion of gastric acid), as well as in the mediation of hypersensitivity (allergic) responses. 

Glucose-dependent insulinotropic polypeptide is originally known as gastric inhibitory polypeptide (GIP), which is a 42-residue peptide first isolated by Brown and Dryburgh (14). It is secreted from the duodenum and proximal jejunum in response to food. Two major physiological effects of GIP are inhibition of gastric acid secretion and stimulation of insulin release. 

Histamine released from mast cells plays an important physiological role in immediate hypersensitivity and allergic responses. In addition, histamine functions as a neurotransmitter in the CNS and it is a potent stimulus for gastric acid secretion. These actions depend on the interaction of histamine with two types of receptors, Hi and H2. Hi and H2 receptors are coupled via G proteins to phospholipase C and adenylyl cyclase, respectively. The principal H3 receptor response is stimulation of gastric acid secretion, whereas other actions of histamine (e.g., smooth muscle contraction, vasodilation, increased capillary permeability, pain, and itching) are prunarily mediated by Hi receptors. 

Histamine is formed from the amino acid histidine and is stored in high concentrations in vesicles in mast cells. Histamine is metabolized by the enzymes monoamine oxidase and diamine oxidase. Excess production of histamine in the body (by. for example, systemic mastocytosis) can be detected by measurement of imidazoleacetic acid (its major metabolite) in tbe urine. Because it is released from mast cells in response to IgE-mediated (immediate) allergic reactions, this autacoid plays an important pathophysiologic role in seasonal rhinitis (hay fever), urticaria, and angioneurotic e ma. Histamine also plays an important physiologic role in the control of acid secretion in the stomach and as a neurotransmitter. 

It is unlikely that secretin or cholecystokinin are important inhibitors of acid secretion under physiological conditions because the doses needed to produce this effect probably produce higher blood levels than are likely to occur in response to a meal. Because GIF is released by fat (as well as by glucose) and because it inhibits both gastrin and histamine stimulated secretion, thus mimicking fat, it is the best candidate for the inhibitor of acid secretion and motility released by fat. It must now be shown that the amount and... 

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