NA+-H+ EXCHANGE IN RESIDENT ALVEOLAR MACROPHAGES - ACTIVATION BY OSMOTIC CELL SHRINKAGE

Intracellular pH (pH(i)) homeostasis in resident alveolar macrophages (m phi) under nominally CO2-free conditions is mediated primarily by the activity of plasmalemmal H+-ATPase, The m phi also possess an Na+-H+ exchanger (NHE) but this mechanism has no detectable role in pH(i) regulation in the physiologic range, To further explore the physiological significance of the NHE in this cell type, resident alveolar m phi from rabbits were subjected to a hyperosmotic challenge (similar to 620 mOsm/kg) in the nominal absence of CO2-HCO3-. Osmotic cell shrinkage was accompanied by an amiloride-sensitive increase in baseline pH(i). The NHE-mediated rate of pH(i) recovery from intracellular acid loads also increased under hyperosmotic conditions, Cell shrinkage caused an alkaline shift in the pH(i) set point of the NHE without altering the exchanger's affinity for extracellular Na+. The results indicate that Na+-H+ exchange in resident alveolar m phi is activated by osmotic cell shrinkage and imply that the NHE may be involved in volume regulatory responses of the cell.