Ion transport studies with H+-K+-ATPase-rich vesicles: implications for HCl secretion and parietal cell physiology

A summary of recent studies on relations between the properties of the membrane incorporating the H+-K+-ATPase, the H+ motive force in gastric acid secretion, and the secretory state of the parietal cell is presented. Depending on tissue secretory state, two distinct H+-K+-ATPase-rich membranes predominate in tissue homogenates, the gastric microsomes derived from the intracellular tubulovesicles of the resting cell and the stimulation-associated (SA) vesicle derived from the apical membrane of the acid-secreting cell. Structural and chemical differences between both vesicular types lend support to the notion that the formation of an expanded, elaborated apical membrane in the secreting parietal cell results from fusion of tubulovesicles containing the H+-K+-ATPase to an apical membrane of different chemical composition. Comparison of polypeptide composition of microsomes and SA membranes provides a way to identify and isolate membrane and cytoskeletal components putatively involved in the membrane interconversion process. Comparison of transport properties between gastric microsomes and SA vesicles demonstrates that stimulation triggers the appearance of rapid K+ and Cl- permeabilities in the H+-K+-ATPase membrane, allowing efficient acid accumulation in SA vesicles by the combination of rapid KCl influx followed by ATPase-driven H+ for K+ exchange, i.e., by K+ recycling. These stimulation-triggered conductances are functionally independent. Nevertheless, their concurrent inhibition by certain divalent cations (Mn2+,Zn2+) suggests their location within a single physical domain. The compatibility of the K+-recycling model for HCl accumulation in SA vesicles with gastric HCl secretion and selected electrophysiological observations and certain implications of the findings for cellular mechanisms of transport regulation in the context of a membrane fusion and recycling model are discussed.