Extracellular ATP induced a prompt increase in sodium and decrease in potassium contents of isolated rat mast cells. The effects on the cation composition of the cells preceded the histamine release induced by ATP and, in contrast to the release, was not dependent on the presence of calcium in the medium. When mast cells were incubated with ATP and CaCl2 for more than 1 min, the accumulated sodium gradually decreased. This decrease was counteracted by addition of LaCl3, prior to the centrifugation and washing procedure which followed incubation, presumably through a membrane-stabilizing effect of the trivalent cation. The optimal concentration of calcium for histamine release induced by ATP was 0.25 mm. Increasing concentrations of calcium concomitantly depressed histamine release and the changes of the cellular cation composition induced by ATP. Mast cells pretreated with ATP (which allowed the cells to accumulate sodium) released histamine upon addition of calcium at a concentration that was inhibitory to both sodium uptake and histamine release when added together with ATP. It is concluded that exogenous ATP exerts an effect on the mast cell membrane leading to an increase in its permeability to sodium and potassium. These permeability changes of the membrane are necessary but not sufficient for histamine release to be induced by ATP, since the release is also dependent on the presence of calcium.

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