Plasma membrane adenosine triphosphatases in rat peritoneal mast cells and macrophages--the relation of the mast cell enzyme to histamine release.

Adenosine triphosphatases activated by calcium or magnesium have been demonstrated on the outer surface of rat peritoneal mast cells and macrophages. The plasma membrane ATPases in the two types of cells have similar but not identical properties. Mg²⁺ is somewhat more effective than Ca²⁺ in stimulating both the enzymes. They are not influenced by sodium and potassium and not inhibited by ouabain and oligomycin. Ethacrynic acid inhibits both, but the mast cell enzyme is more sensitive to it. The enzyme on the macrophage has five to thirty-seven times higher activity (average seventeen times) than that on the mast cell. The apparent K_m of the enzymes in intact cells, incubated with adenosine triphosphate for 5 min, is estimated to be 36 μM for mast cells and 30 μM for macrophages. The optimal pH for the mast cell and the macrophage enzymes is 6.7 and 7.1 respectively. The activities of the two enzymes rise similarly with temperature up to 37° but differ at 47°, the macrophage enzyme being less active at this temperature than at 37°. Phosphatidyl serine, which stimulates anaphylactic and dextran-induced histamine release, causes about 40 per cent stimulation of the plasma membrane ATPase of mast cells in the absence of Ca²⁺ and Mg²⁺ but has no appreciable effect in their presence. No change in the mast cell enzyme could, however, be observed in relation to histamine release induced by dextran, compound $\text{48}\text{80}$ and ATP. But ethacrynic acid, which in 1 mM concentration inhibits 50 per cent of the mast cell enzyme activity, also causes pronounced inhibition of histamine release induced by all the three agents in the same concentration. The inhibition is not influenced by the presence of glucose, suggesting that ethacrynic acid does not inhibit histamine release by blocking energy metabolism. Ethacrynic acid apparently acts at another site. The site of action could very well be plasma membrane ATPase. There is also a correlation between the inhibition of the mast cell enzyme by sodium fluoride and lack of calcium and their inhibitory effect on histamine release. The possible involvement of the plasma membrane ATPase of mast cells in the process of exocytosis leading to histamine release is discussed.