Local anesthetics depolarize mitochondrial membrane potential by intracellular alkalization in rat dorsal root ganglion neurons.

BACKGROUND: Although it has been reported that local anesthetics, especially lidocaine, are cytotoxic, the mechanism is unclear. Depolarization of the mitochondrial membrane potential (Delta Psi m), one of the markers of mitochondrial failure, is regulated by the proton electrochemical gradient (Delta H(+)). Therefore, intracellular pH ([pH]in) and mitochondrial pH ([pH]m) are important factors for modifying Delta Psi m. However, the effects of local anesthetics on [pH]in and [pH]m are unclear. To investigate mitochondrial responses to local anesthetics, we simultaneously measured [pH]m and [pH]in, along with Delta Psi m. METHODS: The ratiometric fluorescent probe JC-1 and HPTS were used for the simultaneous measurements of Delta Psi m with [pH]in in rat dorsal root ganglion neurons. A carboxy-SNARF-1 fluorescent probe was used to measure [pH]m. Lidocaine, mepivacaine, bupivacaine, procaine, QX-314, a charged form of lidocaine, and ammonium chloride (NH(4)Cl) were evaluated. RESULTS: Delta Psi m was depolarized and [pH]in was increased by lidocaine, mepivacaine, bupivacaine, and procaine in a dose-dependent manner. Significantly, a relationship between Delta Psi m and [pH]in was observed for lidocaine, mepivacaine, bupivacaine, procaine, and NH(4)Cl perfusion. In contrast, QX-314 did not change Delta Psi m or [pH]in. In low-pH saline (pH6) and in the presence of a weak acid, lidocaine failed to increase [pH]in or depolarize Delta Psi m. The [pH]m was also increased by lidocaine, mepivacaine, bupivacaine, procaine, and NH(4)Cl. CONCLUSION: These results demonstrate that uncharged (base) forms of local anesthetics induce Delta Psi m depolarization. One of the causes is intracellular and mitochondrial alkalization. (Anesth Analg 2010;111:775-83)