1. Addition of the bivalent ionophore A23187 to synaptosomes isolated from guinea-pig brain cortex and labelled with [32P]phosphate in vitro or in vivo caused a marked loss of radioactivity from phosphatidyl-myo-inositol 4-phosphate (diphosphoinositide) and phosphatidyl-myo-inositol 4,5-bisphosphate (triphosphoinositide) and stimulated labelling of phosphatidate. No change occurred in the labelling of other phospholipids. 2. In conditions that minimized changes in internal Mg2+ concentrations, the effect of ionophore A23187 on labelling of synaptosomal di- and tri-phosphoinositide was dependent on Ca2+ and was apparent at Ca2+ concentrations in the medium as low as 10−5m. 3. An increase in internal Mg2+ concentration stimulated incorporation of [32P]phosphate into di- and tri-phosphoinositide, whereas lowering internal Mg2+ decreased labelling. 4. Increased labelling of phosphatidate was independent of medium Mg2+ concentration and apparently only partly dependent on medium Ca2+ concentration. 5. The loss of label from di- and tri-phosphoinositide caused by ionophore A23187 was accompanied by losses in the amounts of both lipids. 6. Addition of excess of EGTA to synaptosomes treated with ionophore A23187 in the presence of Ca2+ caused a rapid resynthesis of di- and tri-phosphoinositide and a further stimulation of phosphatidate labelling. 7. Addition of ionophore A23187 to synaptosomes labelled in vivo with [3H]inositol caused a significant loss of label from di- and tri-phosphoinositide, but not from phosphatidylinositol. There was a considerable rise in labelling of inositol diphosphate, a small increase in that of inositol phosphate, but no significant production of inositol triphosphate. 8. 32P-labelled di- and tri-phosphoinositides appeared to be located in the synaptosomal plasma membrane. 9. The results indicate that increased Ca2+ influx into synaptosomes markedly activates triphosphoinositide phosphatase and diphosphoinositide phosphodiesterase, but has little or no effect on phosphatidylinositol phosphodiesterase.

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