Calcium-dependent turnover of brain polyphosphoinositides in vitro after prelabelling in vivo

Rat brain phospholipids were labelled in vivo by an intraventricular injection of 32P. The radioactivity was found to accumulate predominantly in limbic structures, particularly hippocampus and diencephalon. A rapid and high specific labelling of the inositol phospholipids and phosphatidic acid was observed. The rate of incorporation into a crude myelin fraction was similar to that into a mitochondrial/synaptosomal fraction although phosphatidylmyo-inositol 4, 5-diphosphate was especially enriched in myelin. Upon incubation in vitro of the brain fractions after 2 h prelabelling in vivo, both phosphatidylvnyo-inositol 4-phosphate and phosphatidyl-myo-inositol 4, 5-diphosphate rapidly lost their radioactivity. Half of the labile fraction of the incorporated 32P was removed within 2 min. None of the other phospholipids changed in the 30 min in vitro incubation period. The metabolism of the polyphosphoinositide proceeded at a lower rate when the temperature was lowered, and was Ca2+-dependent. Further subcellular fractionation revealed that purified synaptosomes and myelin contained highly labelled phosphatidyl-myo-inositol 4, 5-diphosphate. Mitochondria contained highly labelled phosphatidyl-myo-inositol but no phosphatidyl-myo-inositol 4-phosphate or phosphatidyl-myo-inositol 4, 5-diphosphate. ACTUH1–24 did not inhibit the in vitro dephosphorylation of prelabelled polyphosphoinositide, confirming previous findings that the peptide affects the polyphosphoinositide kinases and not the respective phosphatases.