[Effect of extracellular calcium on enhancement of norepinephrine release in hippocampus by phorbol ester]

Rabbit hippocampal slices were preloaded with [3H]norepinephrine ([3H]NE) and superfused with medium. Tetrodotoxin(TTX) inhibited [3H]NE release evoked by high K+, but did not affect on that evoked by Ca(2+)-pulse. The facilitatory effect of phorbol ester (PDB) on high K(+)-evoked [3H]NE release was elevated with increasing extracellular Ca2+ concentration. In the case of Ca(2+)-pulse-evoked [3H]NE release, the enhancement by PDB was decreased with increasing Ca2+ concentration in the extracellular space. It was suggested that the facilitation of Ca(2+)-influx was not involved in the mechanism of action of protein kinase C.