Synthetic inositol 1,3,4,5-tetrakisphosphate analogs and their effect on the binding to microsomal fraction of rat cerebellum

In vitro and in vivo effects of lead, methyl mercury and mercury on inositol 1,4,5-trisphosphate and 1,3,4,5-tetrakisphosphate receptor bindings in rat brain

In vitro and in vivo effects of mercury (Hg), methyl mercury (MM) and lead (Pb) on [3H]inositol 1,4,5-trisphosphate (IP3) and [3H]inositol 1,3,4,5-tetrakisphosphate (IP4) receptor binding in the Sprague-Dawley rat brain cerebellar membranes were studied. In vitro studies indicate that binding of [3H]IP3 and [3H]IP4 to cerebellar membranes was inhibited by Hg while they were stimulated by MM or Pb in a concentration-dependent manner. MM was more potent (EC50 3.4 microM) than Pb (EC50 18.2 microM) in stimulating the [3H]IP3 receptor binding activity whereas Pb (IC50 30 microM) was more potent than MM (IC50 133 microM) in stimulating the [3H]IP4 receptor binding. When the rats were treated (i.p) with Hg (5 mg/kg body wt.) or MM (5 mg/kg body wt.) or Pb (25 mg/kg body wt.) for 3 or 24 h, no significant alterations in [3H]IP3 receptor binding were observed in cerebellum and cerebral cortex. But the above treatment of Pb or MM for 3 or 24 h to rats resulted in an increase of [3H]IP4 receptor binding in the membranes of cerebral cortex. However, the rats treated with Hg (1 mg/kg body wt./day) or Pb (25 mg/kg body wt./day) for 7 days did not show any alteration in binding of [3H]IP3 to its receptors in cerebellar membranes but an increase in this receptor binding was noticed with the treatment of MM (2.5 mg/kg body wt./day) for 7 days. The cerebellum and cerebral cortex of rats with the above treatment of MM or Pb for 7 days exhibited an increase in [3H]IP4 receptor binding. These in vitro and in vivo data suggest that alterations in inositol polyphosphate receptor binding by metals could result in alterations in intracellular calcium levels which may influence neuronal activity.

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 Ca2+ concentrations. Since toxic metal cations like Pb2+ are known to alter Ca(2+)-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 (InsP4) receptor binding and InsP3-mediated Ca(2+)-release. Rat cerebellar membrane and microsomal fractions were incubated with various concentrations of Pb2+ (0.01-100 microM). Pb2+ significantly stimulated [3H]-InsP3 and [3H]-InsP4 receptor binding (EC50 22.7 and 13.5 microM 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 Ca(2+)-pump by micromolar concentration of Pb2+ was also observed. ATP at 5-1000 microM 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 Pb(2+)-mediated increase in [3H]-InsP3 and [3H]-InsP4 receptor binding. The kinetic analysis of ATP effect on Pb(2+)-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.