Effects of ATP on Phosphatidylinositol‐Phospholipase C and Inositol 1‐Phosphate Accumulation in Rat Brain Synaptosomes

Abstract Incubation of rat brain synaptosomes prelabeled with [2‐ 3 H]inositol resulted in a time‐dependent release of labeled inositol 1 ‐phosphate. This process was Ca 2+ dependent, and ATP (1 m M ) enhanced the inositol 1‐phosphate formation three‐ to fivefold. Using [l‐ 14 C]arachidonoyl‐phosphatidylinositol which was introduced into saponin‐permeabilized synaptosomes, ATP (1 m M ) and free Ca 2+ (˜20 μ M ) enhanced the phospholipase C hydrolysis of this substrate to form labeled diacylglycerol. When the same permeabilized synaptosomal preparation was incubated with [2‐ 3 H]inositol‐phosphatidylinositol, ATP not only enhanced the formation of labeled inositol 1‐phosphate, but also inhibited the conversion of inositol 1‐phosphate to inositol. Furthermore, ATP appeared to reduce the Ca 2+ requirement of the phosphatidylinositol‐phospholipase C. Inhibition of the conversion of inositol 1‐phosphate to inositol could not be overcome by increasing the Mg 2+ concentration in the incubation medium. Although the ATP effect is not viewed as a receptor‐mediated event, it is possible that such an event may occur in synaptosomes under conditions in which intrasynaptic Ca 2+ concentration becomes elevated.