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

摘要

> 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 [ class="smallCaps">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). class="smallCaps">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. class="smallCaps">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.