Involvement of phospholipase A₂ in the regulation of [³H]hemicholinium-3 binding

We have examined the effects of exogenous phospholipase A₂ (PLA₂) on the sodium- dependent high-affinity choline uptake mechanism as assessed by the specific binding of [³H]hemi- cholinium-3 ([³H]HCh-3). Incubation of striatal synaptic membranes with bee venom PLA₂ resulted in a concentration-dependent increase in the specific binding of [³H]HCh-3. The effect of PLA₂ on [³H]HCh-3 binding was inhibited by quinacrine, a PLA₂ inhibitor, and by removal of calcium. Scatchard analysis revealed that the observed changes in binding reflected a 2-fold increase in both the capacity and affinity of [³H]HCh-3 for its binding site. Choline and tN-butylcholine inhibited the specific binding of [³H]HCh-3 in both control and PLA₂-treated membranes with similar potency. When a low concentration of PLA₂ was incubated with the striatal synaptosomes, a small but significant increase in high-affinity [³H]choline uptake was observed. However, higher concentrations of PLA₂, which further increased the specific binding of [³H]HCh-3, caused a reduction of [³H]choline uptake, apparently due to disruption of synaptosomal integrity by PLA₂. Finally, potassium depolarization- and PLA₂-induced increases in specific [³H]HCh-3 binding were not additive. These results suggest a possible role for endogenous PLA₂ in the calcium-dependent regulation of sodium-dependent high-affinity choline uptake.