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We studied the molecular events underlying K(+)-induced phosphorylation of the neuron-specific protein kinase C substrate B-50. Rat cortical synaptosomes were prelabelled with 32P-labelled orthophosphate. B-50 phosphorylation was measured by an immunoprecipitation assay. In this system, various phorbol esters, as well as a synthetic diacylglycerol derivative, enhance B-50 phosphorylation. K+ depolarization induces a transient enhancement of B-50 phosphorylation, which is totally dependent on extracellular Ca2+. Also, the application of the Ca2+ ionophore A23187 induces B-50 phosphorylation, but the magnitude and kinetics of A23187-induced B-50 phosphorylation differ from those induced by depolarization. The protein kinase inhibitors 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), and staurosporine antagonize K(+)- as well as PDB-induced B-50 phosphorylation, whereas trifluoperazine and calmidazolium are ineffective under both conditions. We suggest that elevation of the intracellular Ca2+ level after depolarization is a trigger for activation of protein kinase C, which subsequently phosphorylates its substrate B-50. This sequence of events could be of importance for the mechanism of depolarization-induced transmitter release.
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