Depolarization-dependent protein phosphorylation in rat cortical synaptosomes: the effects of calcium, strontium and barium.

Depolarization of synaptosomes increases the phosphorylation of a number of proteins in a calcium-dependent manner. The concentration of calcium required for optimum stimulation was 0.1 mM, with higher concentrations up to 2.5 mM being progressively less effective. Calcium was significantly better than strontium at increasing depolarization-dependent protein phosphorylation, while barium had no stimulating effect at concentrations above 0.1 mM. The order of potency of these ions is consistent with a calmodulin-stimulated protein kinase being activated on entry of calcium into synaptosomes, but is not consistent with the known efficacy of these ions in stimulating neurotransmitter release. The data show for the first time that phosphorylation of proteins may not be a prerequisite for neurotransmitter release.