Gastric gland

Figure 1-7 The central role of the ECL cell in regulation of acid secretion by the parietal cell is depicted. As shown, following ingestion of a meal, vagal fibers are stimulated and release acetylcholine (cephalic phase). Acetylcholine binds to M3 receptors located on the ECL cell, parietal cell, and G-cell to cause the release of histamine, hydrochloric acid, and gastrin, respectively. Acetylcholine also interacts with M3 receptors located on the D-cell to inhibit somatostatin release. Food within the gastric lumen also stimulates the G-cell to release gastrin, which in turn binds to type B CCK receptors located on the ECL cell and parietal cell causing the release of histamine and hydrochloric acid, respectively (gastric phase). Somatostatin released from the D-cell inhibits histamine release from the ECL cell and gastrin release from the G-cell. Somatostine also inhibits acid secretion by the parietal cell (not shown). The principal stimulus for activation of the D-cell is antral luminal acidification (not shown).

Figure 1-8 The intracellular events following ligand binding to the parietal cell are depicted. Gastrin binds to the type B CCK receptor and acetylcholine binds to M3 receptors to stimulate phospholipase C through a G-protein linked mechanism. Activated phospholipase C converts membrane bound phospholipids into inositol triphosphate (IP3 ), which stimulates the release of intracellular calcium from intracellular calcium stores. The increase in intracellular calcium leads to activation of protein kinases which activate the H/K-AtPase. Histamine binds to its H2 receptor to stimulate adenylate cyclase, which also occurs through a G protein-linked mechanism. Activation of adenylate cyclase leads to an increase in intracellular cyclic AMP levels, which activates protein kinases. Activated protein kinases stimulate a phosphorylation cascade that results in increased levels of phosphoproteins which activate the proton pump. Activation of the proton pump leads to extrusion of cytosolic hydrogen in exchange for extracytoplasmic potassium. In addition, chloride is secreted through a chloride channel located at the luminal side of the membrane. ATP, adenosine triphosphate; ATPase, adenosine triphosphatase; c-AMP, cyclic adenosine monophosphate; G5protein, stimulatory guanine nucleotide protein; Gj, inhibitory guanine nucleotide protein; PLC, phospholipase C: PIP2, phosphatidylinositol 4,5 diphosphate.

TABLE 1-2 -- Gastric Electrolyte Composition in the Human Whole Stomach

Parietal

[H]

[Na]

[K]

[All cations]

[HCO3 ]

[Cl]

[All anions]

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