Effects of calcium channel antagonists on the phosphorylation of major protein kinase C substrates in the rat hippocampus.

K(+)-induced depolarization of rat hippocampal slices resulted in significant increases in the phosphorylation state of myristoylated, alanine-rich C kinase substrate (MARCKS; also known as 87K, pp80) and neuromodulin [also known as growth associated protein 43 (GAP43), B50, F1] as determined by back-phosphorylation using protein kinase C. The effect of organic and inorganic Ca2+ antagonists on the phosphorylation of these major protein kinase C substrates in the rat hippocampus was studied to determine whether Ca2+ influx through L- or N-type voltage-sensitive Ca2+ channels was required for the phosphorylation changes observed. The depolarization-induced changes appeared to be dependent on extracellular Ca2+, based on evidence indicating that the chelation of extracellular Ca2+ with ethylene glycol-bis (beta-amino-ethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) inhibited these changes. In addition, pretreatment of the slices with 500 microM Cd2+, but not 300 nM nimodipine, 10 microM omega-conotoxin GVIA or 10 microM MK-801, blocked the K(+)-induced change in phosphorylation. These results suggest that K(+)-induced changes in the phosphorylation of MARCKS and neuromodulin are mediated by Ca(2+)-dependent mechanisms other than, or in addition to, those sensitive to the organic Ca2+ channel antagonists employed.