In addition to inhibiting the excitation conduction process in peripheral nerves, local anesthetics (LAs) cause toxic effects on the central nervous system, cardiovascular system, neuromuscular junction, and cell metabolism. Different postoperative neurological complications are ascribed to the cytotoxicity of LAs, but the underlying mechanisms remain unclear. Because the clinical concentrations of LAs far exceed their EC50 for inhibiting ion channel activity, ion channel block alone might not be sufficient to explain LA-induced cell death. However, it may contribute to cell death in combination with other actions. In this study, we compared the cytotoxicity of six frequently used LAs and will discuss the possible mechanism(s) underlying their toxicity.METHODS:
In human SH-SY5Y neuroblastoma cells, viability upon exposure to six LAs (bupivacaine, ropivacaine, mepivacaine, lidocaine, procaine, and chloroprocaine) was quantitatively determined by the MTT-(3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetra-odium bromide) colorimetry assay and qualitatively confirmed by fluorescence imaging, using the LIVE/DEAD® assay reagents (calcein/AM and ethidium homodimer-1). In addition, apoptotic activity was assessed by measuring the activation of caspase-3/-7 by imaging using a fluorescent caspase inhibitor (FLICA™). Furthermore, LA effects on depolarization- and carbachol-stimulated intracellular Ca2+-responses were also evaluated.RESULTS:
1) After a 10-min treatment, all six LAs decreased cell viability in a concentration-dependent fashion. Their killing potency was procaine ≤ mepivacaine < lidocaine < chloroprocaine < ropivacaine < bupivacaine (based on LD50, the concentration at which 50% of cells were dead). Among these six LAs, only bupivacaine and lidocaine killed all cells with increasing concentration. 2) Both bupivacaine and lidocaine activated caspase-3/-7. Caspase activation required higher levels of lidocaine than bupivacaine. Moreover, the caspase activation by bupivacaine was slower than by lidocaine. Lidocaine at high concentrations caused an immediate caspase activation, but did not cause significant caspase activation at concentrations lower than 10 mM. 3) Procaine and chloroprocaine concentration-dependently inhibited the cytosolic Ca2+-response evoked by depolarization or receptor-activation in a similar manner as a previous observation made with bupivacaine, ropivacaine, mepivacaine, and lidocaine. None of the LAs caused a significant increase in the basal and Ca2+-evoked cytosolic Ca2+-level.CONCLUSION:
LAs can cause rapid cell death, which is primarily due to necrosis. Lidocaine and bupivacaine can trigger apoptosis with either increased time of exposure or increased concentration. These effects might be related to postoperative neurologic injury. Lidocaine, linked to the highest incidence of transient neurological symptoms, was not the most toxic LA, whereas bupivacaine, a drug causing a very low incidence of transient neurological symptoms, was the most toxic LA in our cell model. This suggests that cytotoxicity-induced nerve injury might have different mechanisms for different LAs and different target(s) other than neurons.