Determination of ATP‐Induced45Calcium Uptake in Rat Mast Cells

Radioactively labelled calcium (45Ca) was taken up into isolated rat mast cells incubated with adenosine 5′‐triphosphate (ATP). This uptake did not occur in the presence of lanthanum. After a short period of incubation of the mast cells with ATP and45Ca, the addition of lanthanum markedly decreased the amounts of45Ca associated with the cells. Exchange between cellular45Ca and added unlabelled calcium in the medium was counteracted by the presence of lanthanum. It was concluded that lanthanum inhibited calcium fluxes and in addition displaced bound45Ca from sites accessible to the trivalent cation. From indirect evidence it was concluded that the ATP‐induced uptake of45Ca in rat mast cells contained one component of net increase in the cellular calcium contents and one component of increased exchange of calcium.