Synergistic effects of inositol 1,3,4,5-tetrakisphosphate on inositol 2,4,5-triphosphate-stimulated Ca²⁺ release do not involve direct interaction of inositol 1,3,4,5-tetrakisphosphate with inositol triphosphate-binding sites

We have previously found that for permeabilized L1210 cells, low micromolar concentrations of Ins(1,3,4,5)P₄ added prior to Ins(2,4,5)P₃ enhance the effects of suboptimal concentrations of Ins(2,4,5)P₃ in causing Ca²⁺ release from InsP₃-sensitive Ca²⁺ stores [Cullen, Irvine and Dawson (1990) Biochem. J. 271, 549–553]. If this was due either to some conversion of added Ins(1,3,4,5)P₄ into Ins(1,4,5)P₃ by the 3-phosphatase, or to Ins(1,3,4,5)P₄ acting as a weak (or partial) agonist on the InsP₃ receptor it would be expected that, in the presence of thimerosal to sensitize the InsP₃ receptor, the dose–response curve to Ins(1,3,4,5)P₄ would be left-shifted by the same extent as that of Ins(1,4,5)P₃. This was found not to be the case; the dose–response curve to Ins(1,3,4,5)P₄ was not shifted at all by thimerosal. Furthermore, L-Ins(1,3,4,5)P₄, which can displace radiolabelled D-Ins(1,3,4,5)P₄ but not D-Ins(1,4,5)P₃ from their respective high-affinity binding sites, mimicked the effects of D-Ins(1,3,4,5)P₄ in enhancing the slow phase of Ins(2,4,5)P₃-stimulated Ca²⁺ release. Ins(1,3,4,5)P₄ caused an increase in magnitude of the slow phase of InsP₃-stimulated Ca²⁺ release leaving the magnitude of the fast phase unaltered, in contrast to increasing Ins(2,4,5)P₃ concentrations which increased the size of both phases. In addition, Ins(1,3,4,5)P₄ decreased the rate constant for the slow phase of Ca²⁺ release These findings point strongly to the conclusion that InsP₄ is not working directly via the InsP₃ receptor but indirectly via an InsP₄ receptor.