Inhibition of catecholamine secretion by iron-rich and iron-deprived multiwalled carbon nanotubes in chromaffin cells

The assay of the toxic effects of carbon nanotubes (CNTs) on human health is a stringent need in view of their expected increasing exploitation in industrial and biomedical applications. Most studies so far have been focused on lung toxicity, as the respiratory tract is the main entry of airborne particulate, but there is also recent evidence on the existence of toxic effects of multiwalled carbon nanotubes (MWCNTs) on neuronal and neuroendocrine cells (Belyanskaya et al., 2009; Xu et al., 2009; Gavello et al., 2012). Commercial MWCNTs often contain large amounts of metals deriving from the catalyst used during their synthesis. Since metals, particularly iron, may contribute to the toxicity of MWCNTs, we compared here the effects of two short MWCNTs samples (<5μm length), differing only in their iron content (0.5 versus 0.05% w/w) on the secretory responses of neurotransmitters in mouse chromaffin cells. We found that both iron-rich (MWCNT+Fe) and iron-deprived (MWCNT-Fe) samples enter chromaffin cells after 24h exposure, even though incorporation was attenuated in the latter case (40% versus 78% of cells). As a consequence of MWCNT+Fe or MWCNT-Fe exposure (50-263μg/ml, 24h), catecholamine secretion of chromaffin cells is drastically impaired because of the decreased Ca2+-dependence of exocytosis, reduced size of ready-releasable pool and lowered rate of vesicle release. On the contrary, both MWCNTs were ineffective in changing the kinetics of neurotransmitter release of single chromaffin granules and their quantal content. Overall, our data indicate that both MWCNT samples dramatically impair secretion in chromaffin cells, thus uncovering a true depressive action of CNTs mainly associated to their structure and degree of aggregation. This cellular "loss-of-function" is only partially attenuated in iron-deprived samples, suggesting a minor role of iron impurities on MWCNTs toxicity in chromaffin cells exocytosis.