Identification of partial agonists with low intrinsic activity at the inositol-1,4,5-trisphosphate receptor

The interactions of synthetic L-chiro-inositol-2,3,5-trisphosphorothioate [L-ch-Ins(2,3,5)PS₃] and D-6-deoxy-myo-inositol-1,4,5-trisphosphorothioate [D-6-deoxy-Ins(1,4,5)PS₃] with D-myo-inositol-1,4,5-trisphosphate [Ins(1,4,5)P₃] receptors have been examined using radioligand binding assays and Ca²⁺ mobilization from permeabilized SH-SY5Y cells. The ability of these analogues to compete with [³H]Ins(1,4,5)P₃ for specific sites on adrenal cortical membranes indicated that, although weaker than Ins(1,4,5)P₃, both ligands competed fully for these sites [L-ch- Ins(2,3,5)PS₃, K(i) = 0.5 μM; D-6-deoxy-Ins(1,4,5)PS₃, K(i) = 5.3 μM]. However, in assays examining the amount of Ca²⁺ mobilized from the stores of permeabilized SH-SY5Y cells, both of these synthetic analogues displayed low intrinsic activity [L-ch-Ins(2,3,5)PS₃, 34%; D-6-deoxy-Ins(1,4,5)PS₃, 42% of that of Ins(1,4,5)P₃]. Moreover, L-ch-Ins(2,3,5)PS₃ and D-6-deoxy- Ins(1,4,5)PS₃ were able to inhibit the response to Ins(1,4,5)P₃ with K(i) values (6 μM and 33 μM, respectively) virtually identical to their EC₅₀ values for Ca²⁺ release. This is consistent with partial agonist behavior, because these compounds exhibit low maximal responses when the extent of Ca²⁺ release is examined. These compounds represent the first examples of inositol-based analogues with low intrinsic activity and may point the way towards the design of selective antagonists at Ins(1,4,5)P₃ receptors. It also seems probable that these may represent the first true affinity values of inositol phosphates at the active receptor.