Two possible cellular pathways of catecholamines from the chromaffin vesicles of PC 12 cells

to the surrounding medium are explored in this study. The direct one circumventing the cytoplasm can

be activated in a-toxin-permeabilized cells with micromolar levels of free Ca2+. Catecholamine metabolites

formed in the cytoplasm (i.e., 3,4-dihydroxyphenylacetic acid and 3,4-dihydroxyphenylethanol) are neither

formed nor released from the cells under these conditions. However, when vesicular catecholamines were

discharged into the cytoplasm by addition of the ionophore nigericin, such metabolites are formed and released

into the medium independent of Ca2+. Both types of experiments provide direct evidence for the operation

of Ca2+-induced exocytosis of dopamine and noradrenaline in permeabilized PC12 cells. The Ca2+ dependence

of dopamine or noradrenaline release, as measured by the determination of the endogenous catecholamines

using the high-performance liquid chromatography technique, exhibits two different phases. One is already

activated below 1 pM free Ca2+ and plateaus at 1-5 pM free Ca2+, while a second occurs in the presence

of larger amounts of free Ca2+ (10-100 pM). Ca2+-induced catecholamine release from the permeabilized

cells can be modulated in different ways: It is enhanced by the phorbol ester 12-0-tetradecanoylphorbol

13-acetate and the diacylglycerol 1 -oleyl-2-acetylglycerol provided Mg*+/ATP is present, and it is inhibited

by guanosine 5’-0-(3-thiotriphosphate). The latter effect is abolished by pretreatment of the cells with

pertussis toxin but not by cholera toxin. Thus, it appears that Ca2+-induced exocytosis can be modulated

via the protein kinase C system, as well as via GTP binding proteins.