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 recen tly described a new messenger that is released by empty Ca2+ stores an d that activates Ca2+ influx in heterologous cells (Randriamampita, C. & Tsien, R. Y. (1993) Nature 364, 809-814). This factor, provisionall y named CIF (for Ca2+ influx factor), seems to be a small nonprotein f actor possessing a phosphate group. Meanwhile Parekh et al, reported t hat okadaic acid, an inhibitor of protein phosphatases 1 and 2A, poten tiates Ca2+ influx in Xenopus oocytes (Parekh, A. B., Terlau, H. and S tuhmer, W. (1993) Nature 364, 814-818). A link between these two obser vations is presented in this paper, We show that in astrocytoma cells, okadaic acid and cyclosporin A (an inhibitor of calcineurin) both pot entiate the Ca2+ elevations due to low doses of CIF, thapsigargin, or carbachol. In lymphocytes, okadaic acid potentiates the Ca2+ elevation s due to low doses of phytohemagglutinin and increases the amount of e xtractable CIF. CIF degradation can be observed in cell-free homogenat es 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+ conc entrations, which could be a feedback mechanism to enhance Ca2+ influx when cells are depleted of Ca2+.