Kinetic Analysis of the Regulation of the Na+/H+ Exchanger NHE-1 by Osmotic Shocks

NHE-1 is a ubiquitous, mitogen-activatable, mammalian Na+/H+ exchanger that maintains cytosolic pH and regulates cell volume. We have previously shown that the kinetics of NHE-1 positive cooperative activation by intracellular acidifications fit best with a Monod−Wyman−Changeux mechanism, in which a dimeric NHE-1 oscillates between a low- and a high-affinity conformation for intracellular protons. The ratio between these two forms, the allosteric equilibrium constant L0, is in favor of the low-affinity form, making the system inactive at physiological pH. Conversely the high-affinity form is stabilized by intracellular protons, resulting in the observed positive cooperativity. The aim of the present study was to investigate the kinetics and mechanism of NHE-1 regulation by osmotic shocks. We show that they modify the L0 parameter (865 ± 95 and 3757 ± 328 for 500 and 100 mOsM, respectively, vs 1549 ± 57 in isotonic conditions).This results in an activation of NHE-1 by hypertonic shocks and, conversely, in ...