Myo‐inositol 1,3,4,5‐tetrakisphosphate can independently mobilise intracellular calcium, via the inositol 1,4,5‐trisphosphate receptor: studies with myo‐inositol 1,4,5‐trisphosphate‐3‐phosphorothioate and myo‐inositol hexakisphosphate

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Abstract
Myo‐inositol 1,3,4,5‐tetrakisphosphate [Ins(1,3,4,5)P4] acts as a full agonist for Ca2+ release in saponin‐permeabilised SH‐SY5Y neuroblastoma cells. Studies were conducted in the presence of myo‐inositol hexakisphosphate (InsP6, 10 μM), to inhibit the Ins(1,3,4,5)P4‐3‐phosphatase catalysed back conversion of Ins(1,3,4,5)P4 to Ins(1,4,5)P3. HPLC analysis confirmed that Ins(1,3,4,5)P4 releases the entire content of Ins(1,4,5)P3‐sensitive intracellular Ca2+ stores, independent of 3‐phosphatase activity. Further we utilised racemic myo‐inositol 1,4,5‐trisphosphate‐3‐phosphorothioate [dl‐Ins(1,3,4,5)P4‐3S], a novel intrinsically Ins(1,3,4,5)P4‐3‐phosphatase resistant Ins(1,3,4,5)P4 analogue. DL‐Ins(1,3,4,5)P4‐3S specifically displaced [3H]Ins(1,4,5)P3 from bovine adrenal cortex Ins(1,4,5)P3 binding sites (IC50 = 889 nM, compared to Ins(1,4,5)P3,1C50 = 4.4 nM and Ins(1,3,4,5)P4, IC50 = 152 nM). dl‐Ins(1,3,4,5)P4‐3S was a full agonist for Ca2+ release (EC50 = 4.7 μM), being 90‐ and 2‐fold less potent than Ins(1,4,5)P3 and Ins(1,3,4,5)P4 (with InsP6), respectively. dl‐Ins(1,3,4,5)P4‐3S will be an important tool for identification of potentially exclusive Ins(1,3,4,5)P4 second messenger functions, since its resistance to 3‐phosphatase action precludes the inconvenient artefact of steady state Ins(1,4,5)P3 generation.