Simulations of NHE1 Na/H exchange function using the model described in Fig. 8 . Data from the cited papers was extracted digitally and fits were generated as described in the supplemental material. (A) Depletion of ATP is assumed to deplete anionic lipids that maintain NHE1 in an active state, thereby favoring inactivation and deocclusion or protons bound to regulatory sites (states 5 to 7 in Fig. 8). Proton dependence reflects mostly binding to transport sites in the control condition, although a 0.3 log unit shift to higher pH is still possible. When ATP is depleted, NHE1 inactivation shifts the secondary activation to a lower pH range so that secondary activation by protons dominates the activation curve. (B) Agonists such as pentagastrin in the AR42J pancreatic cell line promote the transitions to active states with occluded protons and sift the activation curve to a higher pH range. (C) The left panel illustrates that in cardiac Purkinje fibers acidification of the extracellular solution shifts the activation of NHE activity by cytoplasmic protons to a lower pH range (Hill slope 3.2). These shifts are recreated by assuming that inactivation of NHE activity occurs preferentially when extracellular transport sites are occupied by protons. The right panel shows that the inhibition of NHE1 activity by extracellular acidification follows simple Michaelis–Menten kinetics. (D) The left panel shows that in CHO cells activation of NHE1 activity by extracellular Na occurs with a Hill slope of 1 when the cytoplasm is acidic and exchangers are saturated with protons. The Hill slope increases to 2 when cytoplasmic pH is increased. The steeper Na concentration dependence, when cytoplasmic protons are not saturating, reflects a shift of NHE transporters from inactive to active states when transport sites are Na-occupied, thereby stabilizing active transport configurations. The right graph in D shows a more complete dataset for activation of NHE activity by cytoplasmic acidification in a linear plot. Decreasing extracellular Na shifts the activation by protons to a higher concentration range and reveals cooperativity of the proton activation, as lower extracellular Na favors occupation of inactive states.