Calcium-activated hydrolysis of phosphatidyl-myo-inositol 4-phosphate and phosphatidyl-myo-inositol 4,5-bisphosphate in guinea-pig synaptosomes

摘要

p1. Addition of the bivalent ionophore A23187 to synaptosomes isolated from guinea-pig brain cortex and labelled with [sup32/supP]phosphate iin vitro/i or iin vivo/i caused a marked loss of radioactivity from phosphatidyl-imyo/i-inositol 4-phosphate (diphosphoinositide) and phosphatidyl-imyo/i-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 Mgsup2+/sup concentrations, the effect of ionophore A23187 on labelling of synaptosomal di- and tri-phosphoinositide was dependent on Casup2+/sup and was apparent at Casup2+/sup concentrations in the medium as low as 10sup?5/supm. 3. An increase in internal Mgsup2+/sup concentration stimulated incorporation of [sup32/supP]phosphate into di- and tri-phosphoinositide, whereas lowering internal Mgsup2+/sup decreased labelling. 4. Increased labelling of phosphatidate was independent of medium Mgsup2+/sup concentration and apparently only partly dependent on medium Casup2+/sup 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 Casup2+/sup caused a rapid resynthesis of di- and tri-phosphoinositide and a further stimulation of phosphatidate labelling. 7. Addition of ionophore A23187 to synaptosomes labelled iin vivo/i with [sup3/supH]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. sup32/supP-labelled di- and tri-phosphoinositides appeared to be located in the synaptosomal plasma membrane. 9. The results indicate that increased Casup2+/sup influx into synaptosomes markedly activates triphosphoinositide phosphatase and diphosphoinositide phosphodiesterase, but has little or no effect on phosphatidylinositol phosphodiesterase./p