Lead Alters Inositol Polyphosphate Receptor Activities: Protection by ATP

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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]‐InsP4receptor binding (EC5022.7 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]‐InsP4binding to the specific receptors. [3H]‐InsP4receptor 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]‐InsP3and [3H]‐InsP4receptor binding. The kinetic analysis of ATP effect on Pb2+‐stimulated [3H]‐InsP4receptor binding revealed non‐competitive type of interaction. The results of the present study suggest that Pb2+may be increasing the binding of [3H]‐InsP3and [3H]‐InsP4to 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.