Parietal cells morphology

The apical membrane of the parietal cell forms, under nonsecretory conditions, tubulovesicular structures and, under secretory conditions, secretory cannaliculi. Its morphology depends on the secretory state of the parietal cell [14]. The lumen of the cannaliculi belongs to the extracellular compartment under secretory conditions Figure 4.1) and contains hydrochloric acid with a pH of about 1. H /K -ATPase exchanges protons for potassium ions across the apical surface. Thereby, the enzyme pumps out the protons against a proton gradient of 1 1,000,000. 

Morphological studies have demonstrated the presence of cells in the GI tract that contain dopamine and express components of dopamine signaling pathways, including catecholamine biosynthetic enzymes and specific dopamine receptors and transporters. In tlie stomach, tyrosine hydroxylase is expressed in epithelial cells, including acid-secreting parietal cells. In the small intestine, cells of the lamina propria, including immune cells, also express tyro-... 

The parietal cell is transformed morphologically when acid secretion is stimulated. The tubulovesicuiar membranes, prominent in the resting cell, diminish, whereas there is a marked increase in the apical plasma membrane (the secretory surface of the cell) together with the appearance of long apical microvilli. 

Cimetidine (Tagamet) and other H2-receptor antagonists (such as ranitidine [Zantac] and famotidine [Pepcid]) block botli the morphological transformation of the parietal cell and secretion. Proton pump inhibitors (PPIs) have a different mechanism of action. Omeprazole (a PPI) is taken up by the parietal cell and converted to an active metaboHte that inactivates the parietal H", IC-ATPase. Hydrogen ion secretion is inhibited until new ATPase is synthesized—a process that requires at least 24 hours. 

These morphologic events in the parietal cell reflect stimulation-secretion coupling in the parietal cell much of this chapter will be devoted to a discussion of the mechanisms underlying activation of acid secretion. 

---