Inositol tetrakisphosphate‐induced sequestration of Ca 2+ replenishes an intracellular pool sensitive to inositol trisphosphate

In a permeable neoplastic rat liver epithelial (261 B) cell system, inositol 1,3,4,5‐tetrakisphosphate—Ins (1,3,4,5)P 4 —induces sequestration of Ca 2+ released by inositol 2,4,5‐trisphosphate—Ins(2,4,5)P 3 ; a non‐metabolized inositol trisphosphate (InsP 3 ) isomer—and Ca 2+ added exogenously in the form of CaCl 2 . Studies were performed to identify the Ca 2+ pool filled after Ins(1,3,4,5)P 4 treatment. Both Ins(2,4,5)P 3 and inositol 1,4,5‐trisphosphate—Ins(1,4,5)P 3 —dose‐dependently release Ca 2+ from permeable 261B cells‐Ins(1,4,5)P 3 having a threefold greater potency—but differ in that Ca 2+ released by Ins(1,4,5)P 3 is readily sequestered, while the Ca 2+ released by Ins(2,4,5)P 3 is not. Maximal release of Ca 2+ by 6 μM Ins(2,4,5)P 3 blocked the action of Insd (2,4,5)P 3 , demonstrating that these two isomers influence the same intracellular Ca 2+ pool through a shared membrane receptor. Addition of 2 μM Ins(2,4,5)P 3 to discharge partially the Ca 2+ pool reduced the amount of Ca 2+ released by a maximal dose of Ins(1,4,5)P 3 (2 μM). Ins(1,3,4,5)P 4 combined with Ins(2,4,5)P 3 produced a Ca 2+ release and sequestration response, which replenished the InsP 3 ‐sensitive pool as indicated by a recovery of full Ca 2+ release by 2 μM Ins(1,4,5)P 3 . Induction of Ca 2+ sequestration by Ins(1,3,4,5)P 4 occurred dose‐dependently, with a halfmaximal response elicited at a dose of 0.9 μM. Further studies of the effect of Ins(1,3,4,5)P 4 apart from the influence of Ins(2,4,5)P 3 using a model in which the Ca 2+ levels are raised by an exogenous addition of CaCl 2 showed that Ins(1,4,5)P 4 released twice the amount of Ca 2+ from the storage pool following Ins(1,3,4,5)P 4 ‐induced Ca 2+ sequestration. These results demonstrate that the Ca 2+ uptake induced by Ins(1,3,4,5)P 4 preferentially replenishes the intracellular Ca 2+ storage sites regulated by Ins(1,4,5)P 3 and Ins(2,4,5)P 3 .