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

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