Depolarisation‐Dependent Protein Phosphorylation in Rat Cortical Synaptosomes: Factors Determining the Magnitude of the Response

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Abstract
The sequence of molecular events linking depolarization-dependent Ca influx to the release of neurotransmitters from nerve terminals is unknown; Ca-stimulated protein phosphorylation may play a role. The incorporation of phosphate into proteins was investigated using an intact postmitochondrial pellet isolated from rat cerebral cortex. The rate and relative incorporation of label into individual phosphoproteins depended on the prelabeling time and buffer concentrations of Ca and P. After prelabeling for 45 min, depolarization caused a > 20% increase in the labeling of 10 phosphoproteins, and this initial increase was maximal with 41 mM K+ for 5 s, or 30 .mu.M veratridine for 15 s, in the presence of 1 mM Ca. Both agents also led to an initial dephosphorylation of 4 phosphoproteins. Depolarization for 5 min led to a significant decrease in the labeling of all phosphoproteins. All of the depolarization-stimulated changes in protein phosphorylation were Ca-dependent. The depolarization conditions found to optimally alter the phosphorylation of synaptosomal proteins find many parallels in studies on Ca uptake and neurotransmitter release. The uniform responses of such a large number of phosphoproteins to the multitude of depolarization conditions studied suggest that the changes could equally well relate to recovery events such as biosynthesis of neurotransmitters and regulation of intraterminal metabolic activity.