Neurosecretory PC 12 cells were exposed in a variety of experimental conditions to nanomolar concentrations of α-latrotoxin purified from the venom of the black widow spider. When applied in a modified Ringer medium containing millimolar Ca2+ the toxin rapidly elicited a marked stimulation of exocytosis, as indicated by the appearance of typical images of granule-plasmalemma interaction and by the decreased density (number/unit area) of secretion granules in the cytoplasm. Without Ca2+ in the medium this early toxin effect was delayed and evolved less rapidly, but was still clearly appreciable. These morphological results appear in good quantitative agreement with the biochemical data on dopamine release reported in the preceding article.28 The stimulation of exocytosis was followed after a short delay by a stimulation of endocytosis, as revealed by an increased accumulation of the extracellular tracer, [14C]sucrose, within the toxin-treated cells. At later times after the application of α-latrotoxin other effects appeared, but only in the presence of Ca2+: these included changes in cell shape; focal alterations of the mitochondrial matrix (clear discrete areas and dense precipitates) and frank signs of cytotoxicity (rupture of the plasmalemma, clearing of the cytoplasmatic matrix). The toxin-induced cell death was studied quantitatively by using trypan blue exclusion as well as the 51Cr test, and was found to be dependent on α-latrotoxin concentration, temperature of incubation and Ca2+ concentration in the medium. Ionic substitutions concerning anions as well as cations other than Ca2+ had minor or no consequences. Thus, the early effect of α-latrotoxin in PC12 cells (stimulation of exocytosis, at least partially Ca2+-independent) can be dissociated from the late 'toxic' effect (strictly Ca2+-dependent).
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