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

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