In a permeable neoplastic rat liver epithelial (261 B) cell system, inositol 1,3,4,5tetrakisphosphate-lns(l,3,4,5)P4-induces sequestration of CaL+ released by inositol 2,4,5-trisphosphate-lns(2,4,5)P3; a non-metaboiized inositol trisphosphate (InsP,) isomer-and Cazf added exogenously in the form of CaCI,. Studies were performed to identify the CaL+ pool filled atter Ins(l,3,4,5)P4 treatment.
Both lns(2,4,5)P3 and inositol 1,4,5-trisphosphate--lns(l ,4,5)P3-dose-dependently release Ca*+ from permeable 261 B cells-Ins(l,4,5)P3 having a threefold greater potency-but differ in that Ca2 ' released by Ins(l,4,5jP3 is readily sequestered, while the Ca2 t released by lns(2,4,5)P3 is not. Maximal release of Ca2 ' by 6 pM lns(2,4,5)P3 blocked the action of Ins(l ,4,5)P3, demonstrating that these two isomers influence the same intracellular Ca2+ pool through a shared membrane receptor. Addition of 2 pM lns(2,4,5)P3 to discharge partially the Ca2+ pool reduced the amount of Ca2+ released by a maximal dose of lns(1 ,4,5jP3 (2 pMj. Ins(1 ,3,4,5)P, combined with lns(2,4,5)P3 produced a Ca2 ' release and sequestration response, which replenished the InsP,-sensitive pool as indicated by a recovery of full Ca2 ' release by 2 pM Ins(1,4,5)P3. Induction of Ca2 + sequestration by lns(l,3,4,5)P4 occurred dose-dependently, with a halfmaximal response elicited at a dose of 0.9 pM. Further studies of the effect of Ins(l,3,4,5)P4 apart from the influence of lns(2,4,5)P3 using a model in which the Ca2+ levels are raised by an exogenous addition of CaCI, showed that Ins(1,4,5)P3 released twice the amount of Ca2+ from the storage pool following Ins(l,3,4,5)P4-induced Ca'+ sequestration. These results demonstrate that the CaL+ uptake induced by Ins(l,3,4,5)P4 preferentially replenishes the intracellular Ca2+ storage sites regulated by Ins(1 ,4,5)P3 and lns(2,4,5)P3.