BACKGROUND: Lipid resuscitation has become a standard treatment for local anesthetic (LA) systemic toxicity, but its mechanisms remain to be fully elucidated. Although the partitioning effect is one of the proposed mechanisms, it is difficult to evaluate its impact independently from several other mechanisms or to examine the intracellular concentration of a LA, which is primarily responsible for LA systemic toxicity. We recently reported that LAs as weak bases reduced voltage-gated proton currents by increasing intracellular pH, which could be estimated from the reversal potentials of the channels (Vrev). Using this characteristic, we examined the partitioning effect in detail and showed its impact on lipid resuscitation.
METHODS: A whole-cell voltage clamp technique was used to record proton channel currents in a rat microglial cell line (GMI-R1). We used Intralipid® 20% as lipid emulsion. The effects of lipid emulsion on the intracellular concentrations of LAs were evaluated by measuring the current amplitude and the Vrev. The intracellular concentrations of LAs were calculated by the Henderson–Hasselbalch equation, using estimated intracellular pH. To confirm the importance of partitioning, we separated lipid by centrifugation. Data are means ± SD unless otherwise stated.
RESULTS: Bupivacaine (1 mM) decreased proton currents to 43% ± 10% of the control and shifted the Vrev to positive voltages (from −88.0 ± 4.1 to −76.0 ± 5.5 mV, n = 5 each, P = 0.02). An addition of the lipid emulsion recovered the currents to 79% ± 2% of the control and returned the Vrev toward the control value (to −86.0 ± 7.1 mV, n = 5, P = 0.03). Both recoveries of the current and Vrev in the centrifuged aqueous extract were almost the same as in the 4% lipid solution (−85.6 ± 4.9 mV, n = 5, P = 0.9, 95% confidence interval for difference = −9.3 to 8.6). When 1 mM bupivacaine was applied extracellularly, the intracellular concentration of the charged form of bupivacaine was estimated to reach about 18.1 ± 3.9 mM but decreased to 5.4 ± 1.8 mM by the 4% lipid solution.
CONCLUSIONS: Here we quantitatively evaluated for the first time the partitioning effect of lipid emulsion therapy on the intracellular concentration of bupivacaine in real-time settings by analyzing behaviors of voltage-gated proton channels. Our results suggested that lipid emulsion markedly reduced the intracellular concentration of bupivacaine, which was mostly due to the partitioning effect. This could contribute to our understanding of the mechanisms underlying lipid resuscitation, especially the importance of the partitioning effect.