Degradation of a calcium influx factor (CIF) can be blocked by phosphatase inhibitors or chelation of Ca2+.

In many cell types, depletion of Ca2+ stores causes activation of Ca2+ influx by a mechanism whose molecular basis remains unclear. We recently described a new messenger that is released by empty Ca2+ stores and that activates Ca2+ influx in heterologous cells (Randriamampita, C. & Tsien, R. Y. (1993) Nature 364, 809-814). This factor, provisionally named CIF (for Ca2+ influx factor), seems to be a small nonprotein factor possessing a phosphate group. Meanwhile Parekh et al. reported that okadaic acid, an inhibitor of protein phosphatases 1 and 2A, potentiates Ca2+ influx in Xenopus oocytes (Parekh, A. B., Terlau, H. & Stühmer, W. (1993) Nature 364, 814-818). A link between these two observations is presented in this paper. We show that in astrocytoma cells, okadaic acid and cyclosporin A (an inhibitor of calcineurin) both potentiate the Ca2+ elevations due to low doses of CIF, thapsigargin, or carbachol. In lymphocytes, okadaic acid potentiates the Ca2+ elevations due to low doses of phytohemagglutinin and increases the amount of extractable CIF. CIF degradation can be observed in cell-free homogenates of lymphocytes and is prevented by the above phosphatase inhibitors, an effect that can at least partly explain their potentiation of Ca2+ influx. CIF degradation is also prevented by lowering free Ca2+ concentrations, which could be a feedback mechanism to enhance Ca2+ influx when cells are depleted of Ca2+.