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Physiology and pharmacology of the parietal cell
Abstract:
The parietal cells possess the unique capacity to produce large quantities of acid at a high concentration, and this is reflected in unique properties at the cellular level. The cells are comparatively large, and they are equipped with secretory canaliculi, a multitude of mitochondria, and cytoplasmic tubulovesicles. During secretion many of the tubulovesicles merge with the secretory canaliculi, which then expand. In the process H+, K+-ATPase is transferred from the tubulovesicular membrane to the secretory membrane. This enzyme catalyses the final step in the production of HCl. Parietal cell activity is regulated through receptors on the basolateral cell surfaces. In the isolated gland and in the isolated parietal-cell fractions, stimulation of receptors for histamine evokes higher secretion than receptor stimulation with cholinergic compounds or with gastrin. In these experimental models, specific inhibitors are required to block acid secretion; for example histamine H2-receptor antagonists will block histamine-induced secretion but will be inactive when secretion is evoked by gastrin or by cholinergic stimulation. These stimuli cause a more or less marked increase in the intracellular levels of Ca2+, which acts as a second messenger, leading to the activation of phosphokinases and, ultimately, to morphological transformation of the parietal cells and acid secretion. Another such intracellular messenger is cAMP, which is formed in response to histamine stimulation only; prostaglandins may prevent this process and block acid secretion. The final step in the production of acid requires K+ and Cl- channels in the secretory membrane and the H+, K+-ATPase-catalysed exchange of K+ for H+ across this membrane. This reaction consumes large amounts of energy and depends on the aerobic production of ATP by the parietal cells. Substituted benzimidazoles, such as omeprazole, accumulate in the acid compartments of the parietal cells and inhibit the H+, K+-ATPase, thereby blocking acid production.