BACKGROUND: Although it has been reported that local anesthetics, especially lidocaine, are cytotoxic, the mechanism is unclear. Depolarization of the mitochondrial membrane potential (DeltaPsim), 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 DeltaPsim. 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 DeltaPsim. METHODS: The ratiometric fluorescent probe JC-1 and HPTS were used for the simultaneous measurements of DeltaPsim 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: DeltaPsim was depolarized and [pH]in was increased by lidocaine, mepivacaine, bupivacaine, and procaine in a dose-dependent manner. Significantly, a relationship between DeltaPsim and [pH]in was observed for lidocaine, mepivacaine, bupivacaine, procaine, and NH(4)Cl perfusion. In contrast, QX-314 did not change DeltaPsim or [pH]in. In low-pH saline (pH6) and in the presence of a weak acid, lidocaine failed to increase [pH]in or depolarize DeltaPsim. 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 DeltaPsim depolarization. One of the causes is intracellular and mitochondrial alkalization.
BACKGROUND: Although it has been reported that local anesthetics, especially lidocaine, are cytotoxic, the mechanism is unclear. Depolarization of the mitochondrial membrane potential (DeltaPsim), 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 DeltaPsim. 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 DeltaPsim. METHODS: The ratiometric fluorescent probe JC-1 and HPTS were used for the simultaneous measurements of DeltaPsim 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: DeltaPsim was depolarized and [pH]in was increased by lidocaine, mepivacaine, bupivacaine, and procaine in a dose-dependent manner. Significantly, a relationship between DeltaPsim and [pH]in was observed for lidocaine, mepivacaine, bupivacaine, procaine, and NH(4)Cl perfusion. In contrast, QX-314 did not change DeltaPsim or [pH]in. In low-pH saline (pH6) and in the presence of a weak acid, lidocaine failed to increase [pH]in or depolarize DeltaPsim. 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 DeltaPsim depolarization. One of the causes is intracellular and mitochondrial alkalization.