Final stage of acid production by parietal cell

(B) H+-K+ ATPase pump > Gastric parietal cells are highly specialized for their unusual task of secreting concentrated acid. The cells are packed with mitochondria that supply energy to drive the apical H+,K+-ATPase, or proton pump, that moves H+ ions out of the parietal cell against a concentration gradient of more than a million-fold. At rest, the proton pumps are sequestered within the parietal cell in a series of membrane compartments known as tubulovesicles. When the parietal cell begins to secrete, on the other hand, these vesicles fuse with invaginations of the apical membrane known as canaliculi, thereby substantially amplifying the apical membrane area and positioning the proton pumps to begin acid secretion. The apical membrane also contains potassium channels, which supply the K+ ions to be exchanged for H+, and Cl- channels that supply the counterion for HCI secretio. The secretion of protons is also accompanied by the release of equivalent numbers of bicarbonate ions into the blood-stream, which are later used to neutralize gastric acidity once its function is complete.