Evidence for hippocampal calcium channel regulation by PKC based on comparison of diacylglycerols and phorbol esters.

Studies using phorbol esters imply that hippocampal Ca2+ channels are regulated by protein kinase C (PKC); however concerns have been raised because in some circumstances phorbol esters have non-specific effects on ion channels. We have tested the hypothesis that PKC modulates Ca2+ channel activity in hippocampal neurons by conducting a detailed comparison of the effects of the diacylglycerols, diC8 and OAG, with those of the phorbol ester, PDBu, on whole-cell and single-channel Ca2+ currents. Close similarity of action of these different activators would support the hypothesis. We found that, like PDBu, the diacylglycerols (DAGs) suppressed whole-cell Ba2+ current (IBa) in a dose-dependent and reversible manner and caused a hyperpolarizing shift in the voltage dependence of steady-state IBa inactivation. Suppression of IBa by diC8 and OAG was not mimicked by an enzymatically inactive diacylglycerol isomer, EGD. The effects of both PDBu and DAGs could be blocked by a specific peptide inhibitor of PKC, and both types of activator depressed IBa when it was recorded in the nystatin perforated-patch mode. In single-channel recordings, DAGs enhanced L-type Ca2+ channel activity in a manner indistinguishable from that of PDBu. Finally, DAGs as well as PDBu markedly increased spontaneous synaptic activity in tissue-cultured hippocampal neurons. The numerous similarities between the effects of DAGs and PDBu strongly support the general conclusion that PKC mediates the effects of these activators and the specific conclusion that PKC modulates Ca2+ channel activity in hippocampal neurons.