Inositol-1,3,4,5-tetrakisphosphate induces calcium mobilization via the inositol-1,4,5-trisphosphate receptor in SH-SY5Y neuroblastoma cells

myo-Inositol-1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P₄]-induced Ca²⁺ mobilization was examined in saponin-permeabilized SH-SY5Y cells using myo- inositol hexakisphosphate-supplemented buffer to prevent Ins(1,3,4,5)P₄-3- phosphatase-catalyzed back-conversion of exogenous Ins(1,3,4,5)P₄ to myo- inositol-1,4,5-trisphosphate [Ins(1,4,5)P₃]. The Ins(1,3,4,5)P₄ concentration-response curve for Ca²⁺ release in SH-SY5Y cells exhibited an EC₅₀ of 2.5 μM, compared with 52 nM for Ins(1,4,5)P₃, with the maximally effective concentration of Ins(1,3,4,5)P₄ (100 μM) mobilizing the entire Ins(1,4,5)P₃-sensitive pool. Both Ins(1,3,4,5)P₄- and Ins(1,4,5)P₃- induced Ca²⁺ mobilizations were heparin sensitive. Further, L-chiro- inositol-2,3,5-trisphosphorothioate, a recently identified low intrinsic activity Ins(1,4,5)P₃ receptor partial agonist, shifted both the Ins(1,4,5)P₃ and Ins(1,3,4,5)P₄ concentration-response curves significantly rightward, with similar potencies. However, binding studies demonstrate that L-chiro-inositol-2,3,5-trisphosphorothioate interacts very poorly (IC₅₀ > 30 μM) with specific Ins(1,3,4,5)P₄ binding sites that have been previously characterized in pig cerebellum. Carbachol-pretreated SH-SY5Y cells (1 mM, ≥6 hr) exhibit a decrease in Ins(1,4,5)P₃ receptor number, accompanied by both a rightward shift and a reduced maximal Ca²⁺ release in their Ins(1,4,5)P₃ concentration-response curve. Here both Ins(1,4,5)P₃ and Ins(1,3,4,5)P₄ concentration-response curves were found to exhibit identically reduced maximal Ca²⁺ release responses and about 4-fold rightward shifts in EC₅₀ values. Together, these observations provide compelling evidence for our hypothesis that Ins(1,3,4,5)P₄ exhibits weak but full agonist status at Ins(1,4,5)P₃ receptor-operated Ca²⁺ channels in SH- SY5Y cells.