Lead Alters Inositol Polyphosphate Receptor Activities: Protection by ATP

Abstract: Receptor‐mediated phosphoinositide signaling pathway which generates a variety of second messengers is regulated by intracellular free Ca²⁺ concentrations. Since toxic metal cations like Pb²⁺ are known to alter Ca²⁺‐dependent processes, the present study was initiated to study the effects of Pb²⁺ on inositol 1,4,5‐trisphosphate (InsP₃) and inositol 1,3,4,5‐tetrakisphosphate (InsP₃) receptor binding and InsP₃‐mediated Ca²⁺‐release. Rat cerebellar membrane and microsomal fractions were incubated with various concentrations of Pb²⁺ (0.01‐100 μM). Pb²⁺ significantly stimulated [³H]‐InsP₃, and [³H]‐InsP₄ receptor binding (EC₅₀ 22.7 and 13.5 μM respectively) as a function of metal concentrations. However, InsP₃‐mediated Ca²⁺ release, determined by measuring the changes in fluorescence intensity of Fura‐2, was significantly inhibited by varying concentrations of Pb²⁺. Re‐uptake of Ca²⁺ into the microsomes was also inhibited by Pb²⁺. A significant inhibition of microsomal Ca²⁺‐pump by micromolar concentration of Pb²⁺ was also observed. ATP at 5‐1000 μM concentration range inhibited [³H]‐InsP₃, and [³H]‐InsP₄ binding to the specific receptors. [³H]‐InsP₄ receptor binding was more sensitive to ATP inhibition as compared to [³H]‐InsP₃, receptor binding. Furthermore, varying concentrations of ATP also inhibited Pb²⁺‐mediated increase in [³H]‐InsP₃ and [³H]‐InsP₄ receptor binding. The kinetic analysis of ATP effect on Pb²⁺‐stimulated [³H]‐InsP₄ receptor binding revealed non‐competitive type of interaction. The results of the present study suggest that Pb²⁺ may be increasing the binding of [³H]‐InsP₃ and [³H]‐InsP₄ to the specific receptors by modulating the conformation of the receptor sites. ATP may be playing a protective role in Pb²⁺ induced alteration of the receptor sites. 1994 Nordic Pharmacological Society