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+ m obilization 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 ex hibited an EC50 of 2.5 mum, compared with 52 nm for Ins(1,4,5)P3, with the maximally effective concentration of Ins(1,3,4,5)P4 (100 muM) mob ilizing the entire Ins(1,4,5)P3-sensitive pool. Both Ins(1,3,4,5)P4- a nd Ins(1,4,5)P3-induced Ca2+ mobilizations were heparin sensitive. Fur ther, L-chiro-inositol-2,3,5-trisphosphorothioate, a recently identifi ed low intrinsic activity Ins(1,4,5)P3 receptor partial agonist, shift ed both the Ins(1,4,5)P3 and Ins(1,3,4,5)P4 concentration-response cur ves significantly rightward, with similar potencies. However, binding studies demonstrate that L-chiro-inositol-2,3,5-trisphosphorothioate i nteracts very poorly (IC50 > 30 mum) with specific Ins(1,3,4,5)P4 bind ing sites that have been previously characterized in pig cerebellum. C arbachol-pretreated SH-SY5Y cells (1 mm, greater-than-or-equal-to 6 hr ) exhibit a decrease in Ins(1,4,5)P3 receptor number, accompanied by b oth 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 identic ally reduced maximal Ca2+ release responses and about 4-fold rightward shifts in EC50 values. Together, these observations provide compellin g evidence for our hypothesis that Ins(1,3,4,5)P4 exhibits weak but fu ll agonist status at Ins(1,4,5)P3 receptor-operated Ca2+ channels in S H-SY5Y cells.