Phorbol Ester Interferes with [3H]Norepinephrine Accumulation into Monolayers of PC 12 Cells by Stimulating the Release of Endogenous Norepinephrine.

In this study we have observed an inhibition of the [3H]norepinephrine ([3H]NE) accumulation into rat phaeochromocytoma PC 12 cells by phorbol 12-myristate 13-acetate (PMA), whereas a biologically inactive phorbol ester was without effect. The inhibition was no longer observed when the PC 12 cells were preincubated with 10 microM reserpine, which inhibits the uptake of norepinephrine into the storage vesicles. This suggested a role of the intact vesicles in the inhibitory phenomenon. Subsequently we could show that PMA increased the efflux of [3H]NE from PC 12 cells in a dose dependent manner. It seemed likely, therefore, that the increased release of endogenous norepinephrine from PC 12 cells by the phorbol ester competed with exogenous [3H]NE for uptake by the plasma membrane NE (uptake1) carrier into the cells. Two observations were in agreement with such a model: (i) an increase in the external NE concentration decreased the effect of PMA on the [3H]NE uptake; and (ii) desensitization of the protein kinase C by long term treatment of the PC 12 cells with phorbol ester abolished the PMA effect on both [3H]NE efflux and [3H]NE uptake.