Figure 1. Critical interplay of calcium in the HAF-mediated (displayed as “AF”) regulation of the gastric H, K-ATPase pump showing oscillation between its H- and Ca-transporting modes depending on the local Ca level.

In a similar fashion, the H, K-ATPase will also act as a Na-pump (not shown in the diagram) at the basolateral membrane depending on the local Na-concentration and pH. Following our current evidence, the critical interplay among the HAF, H, K-ATPase and Ca in parietal cells is depicted in this diagram. While the pump molecules integral to the tubulovesicle (TV) are stimulated appreciably by the HAF, those associated with the apical plasma membrane (APM) are absolutely dependent on the HAF for their function, revealing the essential nature of the HAF in gastric proton-pump function ¹⁷. For the ATPase assay ⁹ the desired amount of HAF (as indicated by the prior dose response study) was first pre-incubated with 5 µg of APM for 10 minutes at 37°C in 2 mM Pipes buffer (pH 7.4). The concentration of free Ca was regulated by varying Ca at a fixed concentration of 0.5 mM EGTA.