Demonstration of an electrogenic Na+-K+ pump in mouse spleen macrophages

The effect of ouabain on the membrane potential of cultured mouse spleen macrophages was examined. Ouabain (10(-3) M) induced a membrane depolarization (6.6 mV) in 18 of 19 cells studied that occurred within several minutes after exposure and was not associated with significant changes in current-voltage relationships. In related studies, cells were placed in K+-free medium in the cold for 2 h to block pump activity. Subsequent exposure to K+ at 37 degrees C resulted in a membrane hyperpolarization. However, addition of K+ also enhanced inward rectification. To differentiate between the effect of K+ on the Na+-K+ pump and its action on inward rectification, two types of experiments were done. Studies performed in the presence of barium, which blocks inward rectification, demonstrated a K+-induced hyperpolarization with no changes in rectification. Additional studies examined the effects of rubidium on Na+-loaded cells. Rubidium, which blocks inward rectification but substitutes for K+ in activating the Na+-K+ pump, induced membrane hyperpolarizations that were reversed by addition of ouabain. These data indicate that macrophages exhibit an electrogenic Na+-K+ pump, which probably contributes to the resting membrane potential under steady-state conditions and can be activated under conditions designed to Na+ load the cells. In addition, they demonstrate that increasing extracellular K+ enhances inward rectification in macrophages.