Depolarisation‐Dependent Protein Phosphorylation and Dephosphorylation in Rat Cortical Synaptosomes Is Modulated by Calcium

Abstract

The effect of Ca protein phosphorylation was investigated using intact synaptosomes isolated from rat cerebral cortex and prelabeled with 32Pi. For nondepolarized synaptosomes a group of Ca-sensitive phosphoproteins were maximally labeled in the presence of 0.1 mM Ca. The phosphorylation of these proteins was sliglhtly decreased in the presence of strontium and absent in the presence of barium, consistent with the decreased ability of these cations to activate Ca-stimulated protein kinases. Addition of Ca alone to synaptosomes prelabeled in its absence increased phosphorylation of a number of proteins. On depolarization in the presence of Ca certain of the Ca-sensitive phosphoproteins were further increased in labeling above nondepolarized levels. These increases were maximal and most sustained after prelabeling at 0.1 mM Ca. On prolonged depolarization at this Ca concentration a slow decrease in labeling was observed for most phosphoproteins, whereas a greater rate and extent of decrease occurred at higher Ca concentrations. At 2.5 mM Ca rapid and then a subsequent slow dephosphorylation was observed, indicating 2 distinct phases of dephosphorylation. Of all the phosphoproteins normally stimulated by depolarization, only phosphoprotein 59 did not exhibit the rapid phase of dephosphorylation at high Ca concentrations. Replacing Ca with strontium markedly decresed the extent of change observed on depolarization, whereas barium decreased phosphorylation changes even further. An influx of Ca into synaptosomes initially activates protein phosphorylation, but as the levels of intrasynaptosomal Ca rise protein dephosphorylation predominates. Other phosphoproteins were dephosphorylated predominates. Other phosphoproteins were dephosphorylated immediately on depolarization in the presence of Ca. The synaptosomal phosphoproteins could play a role in modulating events such as neurotransmitter release in the nerve terminal.