Correlation of parietal cell structure and function.

The apical surface of the gastric parietal cell is greatly expanded (5-10-fold) during maximal HCl secretion, as compared to the resting cell. The membrane recycling hypothesis has been proposed to account for the extensive, functionally related, rearrangement of cell membranes. Cytoplasmic membranes within the resting cell, the tubulovesicles, contain the H+/K+-ATPase. Fusion of tubulovesicles with the apical plasma membrane occurs when the cells are stimulated, thus providing the increased surface area and proper disposition of the H+ pump enzyme. Microfilaments, composed of actin and other regulatory proteins, serve to direct the reordering of the apical surface during stages of the secretory cycle. Cell fractionation of resting oxyntic mucosa reveals that virtually all of the H+/K+-ATPase activity is associated with light microsomal membrane vesicles, presumably derived from tubulovesicles. Although the enzyme from resting tissue is fully competent (e.g. ATP-driving pump, H+-K+ exchange), the microsomal vesicles lack an endogenous pathway to provide rapid access for K+ to its intravesicular activity site. In stimulated stomach, there is a redistribution of H+/K+-ATPase to a larger, denser membrane fraction, the so-called stimulation-associated vesicles. Morphological features and chemical content (e.g. microfilament proteins) suggest that the stimulation-associated vesicles are derived from the expanded apical surface of the stimulated oxyntic cell. A KCl cotransport system has been identified in the stimulation-associated membranes, which operates in parallel with the ATP-driven H+-K+ exchange pump. These two transport systems operate in concert within the apical membrane to provide the machinery for net HCl transport by the parietal cell.