Inhibition of Na+ ‐K+ pump activity by divalent cations in intact peritoneal mast cells of the rat

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Abstract
The inhibition by the divalent cations magnesium, barium and strontium and the trivalent ion lanthanum of the Na+‐K+ pump in the plasma membrane of rat peritoneal mast cells was studied in pure mast cell populations by measurement of the ouabain‐sensitive uptake of the radioactive potassium analogue, 86rubidium (86Rb+). Exposure of the cells to magnesium induced a time‐ and concentration‐dependent decrease in the ouabain‐sensitive K+(86Rb+)‐uptake of the cells without influencing the ouabain‐resistant uptake. The time‐dependent decrease was apparent after incubation of the cells for 10 min or more, but no decrease was observed after 2 min incubation when the cells are supposed to be loaded with sodium due to the cell isolation procedure. Barium and strontium caused concentration‐dependent decreases in the ouabain‐sensitive K+‐(86Rb+) ‐uptake of the cells but the ouabain‐resistant uptake was not changed. Half maximum decrease in the ouabain‐sensitive K+(86Rb+)‐uptake was observed with 1.8 mm magnesium, 1.2 mm barium and 0.7 mm strontium. The trivalent ion lanthanum blocked almost completely the ouabain‐sensitive K+(86Rb+)‐uptake at a concentration of 1 μm as does 1 mm calcium. Combining either of these ions with magnesium had no further inhibitory effect on the ouabain‐sensitive uptake. In conclusion, in addition to the previously suggested modulation by calcium of the activity of the Na+‐K+ pump, evidence is provided in this investigation that the modulation may be a more general effect of divalent and polyvalent cations present in the extracellular space through their influence on the sodium permeability of the plasma membrane.