LEAD ALTERS INOSITOL POLYPHOSPHATE RECEPTOR ACTIVITIES - PROTECTION BY ATP

Receptor-mediated phosphoinositide signaling pathway which generates a variety of second messengers is regulated by intracellular free Ca2concentrations. Since toxic metal cations like Pb2+ are known to alter Ca2+-dependent processes, the present study was initiated to study th e effects of Pb2+ on inositol 1,4,5-trisphosphate (InsP(3)) and inosit ol 1,3,4,5-tetrakisphosphate (InsP(4)) receptor binding and InsP(3)-me diated Ca2+-release. Rat cerebellar membrane and microsomal fractions were incubated with various concentrations of Pb2+ (0.01-100 mu M). Pb 2+ significantly stimulated [H-3]-InsP(3) and [H-3]-InsP(4) receptor b inding (EC(50) 22.7 and 13.5 mu M respectively) as a function of metal concentrations. However, InsP(3)-mediated Ca2+ release, determined by measuring the changes in fluorescence intensity of Fura-2, was signif icantly inhibited by varying concentrations of Pb2+. Re-uptake of Ca2 into the microsomes was also inhibited by Pb2+. A significant inhibit ion of microsomal Ca2+-pump by micromolar concentration of Pb2+ was al so observed. ATP at 5-1000 mu M concentration range inhibited [H-3]-In sP(3) and [H-3]-InsP(4) binding to the specific receptors. [H-3]-InsP( 4) receptor binding was more sensitive to ATP inhibition as compared t o [H-3]-InsP(3) receptor binding. Furthermore, varying concentrations of ATP also inhibited Pb2+-mediated increase in [H-3]-InsP(3) and [H-3 ]-InsP(4) receptor binding. The kinetic analysis of ATP effect on Pb2-stimulated [H-3]-InsP(4) receptor binding revealed non-competitive ty pe of interaction. The results of the present study suggest that Pb2may be increasing the binding of [H-3]-InsP(3) and [H-3]-InsP(4) to th e specific receptors by modulating the conformation of the receptor si tes. ATP may be playing a protective role in Pb2+ induced alteration o f the receptor sites.