Inositol 1,4,5-trisphosphate receptor subtypes differentially recognize regioisomers of D-myo-inositol 1,4,5-trisphosphate

The Ins(1,4,5)P₃ regioisomers, Ins(1,4,6)P₃ and Ins(1,3,6)P₃, which can mimic the 1,4,5-arrangement on the inositol ring of Ins(1,4,5)P₃, were examined for Ca²⁺ release by using four types of saponin-permeabilized cell possessing various abundances of receptor subtypes, with special reference to the relation of potency to receptor subtype. Ins(1,4,6)P₃ and Ins(1,3,6)P₃ were weak agonists in rat basophilic leukaemic cells (RBL cells), which possess predominantly subtype II receptors, with respective potencies of 1/200 and less than 1/500 that of Ins(1,4,5)P₃ [the EC₅₀ values were 0.2, 45 and more than 100 μM for Ins(1,4,5)P₃, Ins(1,4,6)P₃ and Ins(1,3,6)P₃ respectively]. Similar rank order potencies were also evaluated for the displacement of [³H]Ins(1,4,5)P₃ bound to RBL cell membranes by these regioisomers. However, they caused Ca²⁺ release from GH₃, rat pituitary cells possessing predominantly subtype I receptors more potently; Ins(1,4,6)P₃ and Ins(1,3,6)P₃ evoked release at respective concentrations of only one-third and one-twentieth that of Ins(1,4,5)P₃ (the EC₅₀ values were 0.4, 1.2 and 8 μM for Ins(1,4,5)P₃, Ins(1,4,6)P₃ and Ins(1,3,6)P₃ respectively). In COS-1 African green-monkey kidney cells, with the relative abundances of 37% of the subtype II and of 62% of the subtype III receptor, potencies of 1/40 and approx. 1/200 for Ins(1,4,6)P₃ and Ins(1,3,6)P₃ respectively were exhibited relative to Ins(1,4,5)P₃ (the EC₅₀ values were 0.4, 15 and approx. 80 μM for Ins(1,4,5)P₃, Ins(1,4,6)P₃ and Ins(1,3,6)P₃ respectively). In HL-60 human leukaemic cells, in spite of the dominant presence of subtype I receptors (71%), similar respective potencies to those seen with COS-1 cells were exhibited (the EC₅₀ values were 0.3, 15 and approx. 100 μM for Ins(1,4,5)P₃, Ins(1,4,6)P₃ and Ins(1,3,6)P₃ respectively). These results indicate that these regioisomers are the first ligands that distinguish between receptor subtypes; the present observations are of significance for the future design of molecules with enhanced selectivity.