AbstractIn a permeable neoplastic rat liver epithelial (261 B) cell system, inositol 1,3,4,5‐tetrakisphosphate—Ins (1,3,4,5)P4—induces sequestration of Ca2+released by inositol 2,4,5‐trisphosphate—Ins(2,4,5)P3; a non‐metabolized inositol trisphosphate (InsP3) isomer—and Ca2+added exogenously in the form of CaCl2. Studies were performed to identify the Ca2+pool filled after Ins(1,3,4,5)P4treatment. Both Ins(2,4,5)P3and inositol 1,4,5‐trisphosphate—Ins(1,4,5)P3—dose‐dependently release Ca2+from permeable 261B cells‐Ins(1,4,5)P3having a threefold greater potency—but differ in that Ca2+released by Ins(1,4,5)P3is readily sequestered, while the Ca2+released by Ins(2,4,5)P3is not. Maximal release of Ca2+by 6 μM Ins(2,4,5)P3blocked the action of Insd (2,4,5)P3, demonstrating that these two isomers influence the same intracellular Ca2+pool through a shared membrane receptor. Addition of 2 μM Ins(2,4,5)P3to discharge partially the Ca2+pool reduced the amount of Ca2+released by a maximal dose of Ins(1,4,5)P3(2 μM). Ins(1,3,4,5)P4combined with Ins(2,4,5)P3produced a Ca2+release and sequestration response, which replenished the InsP3‐sensitive pool as indicated by a recovery of full Ca2+release by 2 μM Ins(1,4,5)P3. Induction of Ca2+sequestration by Ins(1,3,4,5)P4occurred 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)P4apart from the influence of Ins(2,4,5)P3using a model in which the Ca2+levels are raised by an exogenous addition of CaCl2showed that Ins(1,4,5)P4released twice the amount of Ca2+from the storage pool following Ins(1,3,4,5)P4‐induced Ca2+sequestration. These results demonstrate that the Ca2+uptake induced by Ins(1,3,4,5)P4preferentially replenishes the intracellular Ca2+storage sites regul