Selective modification of the kinetic properties of Na+/H+ exchanger by cell shrinkage and swelling

The effect of volume perturbation on the interaction of Na+ and H+ with the intracellular and extracellular faces of the Na+/H+ exchanger was studied in UMR-106 cells, a rat osteosarcoma cell line. Osmotic shrinkage of the cells stimulated the activity of the Na+/H+ exchanger. Kinetic analysis of this stimulation demonstrated that in hyperosmotically stressed cells, the apparent affinities for intracellular H+ and intracellular Na+ are modified in opposite directions. While there is an increased apparent affinity for protons from 0.275 +/- 0.03 to 0.107 +/- 0.025 microM in isotonic and hypertonic conditions, respectively, the apparent affinity for intracellular Na+ decreases from 83 +/- 9 to 126 +/- 6 mM under the same conditions. Osmotic swelling induced a decreased exchanger activity which appeared to involve reduction in Vmax only without changes in the apparent affinities of either H+i or Na+i. We conclude that: 1) osmotic shrinkage and swelling modify the kinetic behavior of the Na+/H+ exchanger in different modes; 2) in hyperosmotically stressed cells, the interactions of intracellular H+ and Na+ are modified in a selective mode. The described phenomenon may serve as a general mechanism for activation of the exchanger by various stimuli.