Humans are exposed to complex mixtures of pesticides and other compounds, mainly via food. Azole fungicides are broad spectrum antifungal compounds used in agriculture and in human and veterinary medicine. The mechanism of antifungal action relies on inhibition of CYP51, resulting in inhibition of fungal cell growth. Known adverse health effects of azole fungicides are mainly linked to CYP inhibition. Additionally, azole fungicide-induced neurotoxicity has been reported, though the underlying mechanism(s) are largely unknown. We therefore investigated the effects of a group of six azole fungicides (imazalil, flusilazole, fluconazole, tebuconazole, triadimefon, and cyproconazole) on cell viability using a combined alamar Blue/CFDA-AM assay and on oxidative stress using a H2-DCFDA fluorescent assay. As calcium plays a pivotal role in neuronal survival and functioning, effects of these six azole fungicides and binary and quaternary mixtures of azole fungicides on the intracellular calcium concentration ([Ca(2+)]i) were investigated using single-cell fluorescence microscopy in dopaminergic PC12 cells loaded with the calcium-sensitive fluorescent dye Fura-2. Only modest changes in cell viability and ROS production were observed. However, five out of six azole fungicides induced a nonspecific inhibition of voltage-gated calcium channels (VGCCs), though with varying potency. Experiments using binary IC20 and quaternary IC10 mixtures indicated that the inhibitory effects on VGCCs are additive. The combined findings demonstrate modulation of intracellular Ca(2+) via inhibition of VGCCs as a novel mode of action of azole fungicides. Furthermore, mixtures of azole fungicides display additivity, illustrating the need to take mixture effects into account in human risk assessment.
Keywords: Fura-2 Ca2+-imaging; calcium homeostasis; dose addition; in vitro neurotoxicology; triazole/imidazole fungicides.; voltage-gated calcium channels.