Subcellular compartmentalization of 1-methyl-4-phenylpyridinium with catecholamines in adrenal medullary chromaffin vesicles may explain the lack of toxicity to adrenal chromaffin cells.

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Abstract
Cultures of bovine adrenomedullary chromaffin cells accumulated 1-methyl-4-phenylpyridinium (MPP+) in a time- and concentration-dependent manner by a process that was prevented by desmethylimipramine. The subcellular localization of the incorporated [methyl-3H]MPP+ was examined by differential centrifugation and sucrose density gradient fractionation and was found to be predominantly colocalized with catecholamines in chromaffin vesicles, and negligible amounts were detected within the mitochondrial fraction. When chromaffin cell membranes were made permeable with the detergent digitonin in the absence of calcium, there was no increase in the release of [3H]MPP+, indicating that there is negligible accumulation of the neurotoxin in the cytosol. Simultaneous exposure to digitonin and calcium induced cosecretion of MPP+ and catecholamines. Stimulation of the cells with nicotine released both catecholamines and MPP+ at identical rates and percentages of cellular content in a calcium-dependent manner. Last, when cells were incubated with MPP+ in the presence of tetrabenazine (an inhibitor of vesicular uptake), the chromaffin cell toxicity of MPP+ was potentiated. We submit that the ability of the chromaffin cells to take up and store MPP+ in the chromaffin vesicle prevents the toxin''s interaction with other structures and, thus, prevents cell damage. As an extension of this hypothesis, the relative resistance of some brain monoaminergic neurons to the toxic actions of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine may result from the subcellular sequestration of MPP+ in the storage vesicle.