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]hemicholinium-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 N-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.