Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.57 no.3 Campinas May/June 2007
Influence of lithium on the neuromuscular blockade produced by atracurium and cisatracurium. Study on rat phrenic nerve-diaphragm preparations*
Influencia del litio en el bloqueo neuromuscular producido por el atracurio y por el cisatracurio. Estudio en preparo nervio frénico-diafragma del ratón
Samanta Cristina Antoniassi FernandesI; Angélica de Fátima de Assunção Braga, TSAII; Franklin Sarmento da Silva BragaIII; Yolanda Christina S. LoyolaIV; Silmara Rodrigues de SouzaV; Caroline Coutinho de BarcelosI
do Curso de Pós-Graduação do Departamento de Farmacologia
IIProfessora Associada do Departamento de Anestesiologia da FCM-UNICAMP
IIIProfessor Doutor do Departamento de Anestesiologia da FCM-UNICAMP
IVDoutora em Farmacologia pelo Departamento de Farmacologia da FCM-UNICAMP
VMestra em Farmacologia pelo Departamento de Farmacologia da FCM-UNICAMP
AND OBJECTIVES: Lithium is widely used for the treatment of bipolar disorders
and can interact with neuromuscular blockers. There is a controversy about the
mechanisms by which it affects neuromuscular transmission and its interaction
with neuromuscular blockers. The objective of this study was to evaluate, on
the rat diaphragm, the effects of lithium on the muscular response and indirect
stimulation, and the possible interaction with neuromuscular blockers.
METHODS: Rats weighing between 250 and 300 g were sacrificed under urethane anesthesia. The phrenic nerve-diaphragm preparation was assembled according to the Bulbring technique. The diaphragm was kept under tension, connected to an isometric transducer, and submitted to indirect stimulation with a frequency of 0.1 Hz. The contractions of the diaphragm were registered on a physiograph. The analysis of the amplitude of the muscular responses evaluated: the effects of the isolated drugs: lithium (1.5 mg.mL-1); atracurium (20 µg.mL-1), and cisatracurium (3 µg.mL-1); the lithium-neuromuscular blockers association; and the effects of lithium on the neuromuscular blockade produced by atracurium (35 µg.mL-1) and cisatracurium (5 µg.mL-1). The effects were evaluated before and 45 minutes after the addition of the drugs. The effects of lithium on membrane potentials (MP) and miniature end-plate potentials (MEPP) were also evaluated.
RESULTS: Lithium by itself did not change the amplitude of the muscular responses, but it decreased significantly the neuromuscular blockade produced by atracurium and cisatracurium. It did not change MP and caused an initial increase in MEPP.
CONCLUSIONS: Lithium by itself did not compromise neuromuscular transmission and increased the resistance to the effects of atracurium and cisatracurium. It did not show any action on the muscle fiber, and the changes in miniature end-plate potentials indicated pre-synaptic action.
Key Words: ANIMALS: rats; MOOD STABILIZERS: Lithium; NEUROMUSCULAR BLOCKERS, Nondepolarizing: atracurium, cisatracurium.
Y OBJETIVOS: El litio, fármaco ampliamente utilizado en los disturbios
bipolares, puede interactuar con los bloqueadores neuromusculares. Los mecanismos
para explicar sus efectos en la transmisión neuromuscular y en la interacción
con bloqueadores neuromusculares son controvertidos. El objetivo de este trabajo
fue evaluar, en diafragma de ratón, los efectos del litio sobre la respuesta
muscular al estímulo indirecto y la posible interacción con los
MÉTODO: Se utilizaron ratones con peso entre 250 y 300 g, sacrificados bajo anestesia con uretana. La preparación nervio frénico-diafragma se montó de acuerdo con la técnica de Bulbring. El diafragma se mantuvo bajo tensión, ligado a un transductor isométrico y sometido a la estimulación indirecta de 0,1 Hz de frecuencia. Las contracciones del diafragma fueron registradas en un fisiógrafo. Del análisis de la amplitud de las respuestas musculares se evaluaron los efectos de los fármacos: litio (1,5 mg.mL-1); atracurio (20 µg.mL-1) y cisatracurio (3 µg.mL-1) empleados aisladamente; de la asociación litio-bloqueadores neuromusculares; y del litio en el bloqueo neuromuscular producido por el atracurio (35 µg.mL-1) y cisatracurio (5 µg.mL-1). Los efectos se evaluaron antes y 45 minutos después de la adición de los fármacos. También se estudiaron los efectos del litio en los potenciales de membrana (PM) y potenciales de placa terminal en miniatura (PPTM).
RESULTADOS: El litio aisladamente no alteró la amplitud de las respuestas musculares, pero sí que redujo significativamente el bloqueo neuromuscular producido por el atracurio y el cisatracurio. No alteró el PM y ocasionó un aumento inicial de la frecuencia de los PPTM.
CONCLUSIONES: El litio empleado aisladamente no comprometió la transmisión neuromuscular y aumentó la resistencia al efecto del atracurio y del cisatracurio. No mostró acción sobre la fibra muscular, siendo que las alteraciones en los potenciales de placa terminal en miniatura mostraron una acción presináptica.
Lithium, a drug widely used in patients with bipolar disorder, is the main drug to prevent mood changes 1. Although several studies have been conducted to determine the effects of lithium on muscle responses and its interaction with other neuromuscular blockers (NMB), the results are conflicting 2,3. These studies indicate that there is an interaction between lithium and neuromuscular blockers, potentiating its effects, or even no significant influence on those agents 4-7. The influence of lithium on the synthesis and/or release of acetylcholine (Ach), interfering with the neuromuscular transmission and the consequent potentiation of depolarizing and non-depolarizing neuromuscular blockers, can be expected 5. The objective of this study was to evaluate the effects of lithium on neuromuscular transmission and its influence on the neuromuscular blockade produced by atracurium and cisatracurium.
Since this is an experimental study, the procedures followed the ethical principles on animal experiments adopted by the Colégio Brasileiro de Experimentação Animal (COBEA) and were approved by the Ethics Committee on Animal Experiments of the Instituto de Biologia of Universidade Estadual de Campinas (UNICAMP).
Rats weighing between 250 and 300 g were sacrificed under urethane anesthesia. After bleeding by sectioning the neck vessels, the preparation was assembled according to the technique of Bulbring 8. The hemidiaphragms and corresponding phrenic nerves were removed and fixed in a container with 40 mL of nutrient Tyrode's solution with the following composition in mM: NaCl 137; NaH2PO4 0.3; CaCl2 1.8; KCl 2.7; glucose 11; MgCl2 0.25; and NaHCO3 11.9. The solution was aerated constantly with carbogen (95% O2 + 5% CO2) and kept at 37°C. The nerve was placed over platinum electrodes connected to a Grass S48 stimulator.
The diaphragm was maintained under constant tension (5 g) by its tendinous portion through a wire connected to a Load Cell BG50 GMS isometric transducer, undergoing indirect stimulation of 0.1 Hz of frequency and 0.2 msec of duration. The tension variation produced by the contractions of the diaphragm was recorded by a Gould RS 3400 physiograph.
To evaluate the effects of lithium on neuromuscular transmission and its influence on the neuromuscular blockade produced by atracurium and cisatracurium, 5 groups of rats were used and the experiments were conducted in 3 distinct stages: 1) effects of lithium (1.5 mg.mL-1) and NMB atracurium (20 µg.mL-1) and cisatracurium (3 µg.mL-1) administered separately; 2) lithium and NMB administered simultaneously; and 3) lithium administered right after obtaining neuromuscular blockade (> 80%) with atracurium (35 µg.mL-1) and cisatracurium (5 µg.mL-1). The amplitude of the responses of the diaphragm to indirect stimulation were evaluated before and 45 minutes after the addition of the drugs separately or in association. The rat phrenic nerve-diaphragm (PND) was also used to study miniature end-plate potentials and membrane potentials before and after the addition of lithium to the preparation.
Results were expressed as mean and standard deviation. The test t Student, paired test t Student, and Signal testing were used for the statistical analysis. A level of 5% (a = 5%) was considered significant. The power of the test was calculated, obtaining a b > 20% (power > 80%).
Lithium, in the concentration used in this study (1.5 mg.mL-1) and used by itself, did no cause any changes in the amplitude of muscular responses (Figure 1). The neuromuscular blockade produced by atracurium (20 µg.mL-1) and cisatracurium (3 µg.mL-1), used isolatedly, was 41.28% ± 7.29% and 55.56% ± 9.10%, respectively (Figures 2 and 3). On the preparations exposed to the association lithium-atracurium and lithium-cisatracurium, a neuromuscular blockade of 17.74% ± 7.17% and 12.58% ± 3.42%, respectively, was produced, which was significantly smaller (p < 0.05) than what was observed in the experiments in which neuromuscular blockers were used by themselves.
In the preparations exposed to higher doses of neuromuscular blockers (atracurium 35 µg.mL-1 and cisatracurium 5 µg.mL-1), the blockades were 86.44% ± 4.92% and 86.43% ± 3.65%, respectively. The addition of lithium (1.5 mg.mL-1) to the preparation promoted a significant recovery (p < 0.05) in the amplitude of muscular responses (Figure 4). Lithium did not change membrane potentials. Initially, it caused an increase in the frequency of miniature end-plate potentials, observed 10 minutes after the addition of lithium, followed by a blockade at 90 minutes (Figure 5).
Some drugs, in high concentrations, may hinder neuromuscular transmission or, in low concentrations, potentiate the effects of neuromuscular blockers by decreasing the safety margin of muscular transmission 1,2,4-7,9-12. There are several hypotheses to explain the mechanism of action of lithium and the possibility to affect neuromuscular transmission, as well as its interaction with neuromuscular blockers. The literature has conflicting data, with studies demonstrating inhibition of muscular contraction, potentiation or insignificant interaction with neuromuscular blockers, and even no effects on neuromuscular transmission 1,2,4,6,7,9-15.
In view of the controversial results, the objective of this work was to evaluate the effects of lithium on rat neuromuscular junction; its interaction with neuromuscular blockers, and the possible mechanisms that might elucidate this interaction. Experiments that evaluate the muscular responses and electrophysiological experiments were conducted. In the isolated rat phrenic nerve-diaphragm preparation with indirect stimulation, after adding lithium (1.5 mg.mL-1) we observed initially a discrete increase in muscular contractions, corresponding to the facilitation of the neuromuscular transmission, followed by a return to values close to baseline. This facilitation of the neuromuscular transmission might be due to the release of acetylcholine, reflecting a pre-junctional action of lithium. This effect, similar to what has been reported by other authors, could be corroborated by the electrophysiological studies, in which we observed that lithium, in the concentration used in this study, initially caused an increase in the frequency of miniature end-plate potentials (Figure 5), demonstrating an increase in the release of the neurotransmitter. These results indicate that the release of the neurotransmitter could be attributed to the accumulation of lithium in nerve endings, leading to an increase in intracellular calcium 10.
On the other hand, other studies described the reduction in the synthesis and release of acetylcholine to justify the potentiating effects of lithium on the degree and difficulty in reducing the blockade produced by several neuromuscular blockers 1-3,7,11,13,14,16. The controversial results could be due to the differences in methodology and experimental models used, and even be related to the concentrations used and the route of lithium administration 2,3,6.
The similarity between the atomic weights of lithium and sodium could explain the intracellular transport of lithium, along with sodium, during cellular depolarization. In the sequence of this process, the extracellular transport of lithium is slower than that of sodium; thus, lithium accumulates inside the cell, causing a more positive reduction in the membrane potential, in the height of the action potential, and in the efficacy of the sodium-potassium pump 13,14,16,17.
In vitro and in vivo experiments demonstrated the inhibitory effects of lithium on muscular contraction resulting from direct and indirect stimulation, with a concentration-dependent reduction of the muscular responses and potentiation of the effects of depolarizing and non-depolarizing blockers 2. The author suggests that the inhibitory effect of lithium on the neuromuscular transmission and muscular contraction is mediated by the activation of potassium channels sensitive to adenosine triphosphate (KATP) in the membranes of nerve endings and muscle cells, which has been previously described 9,18. The activation of KATP in the membranes of nerve endings reduces the duration of the action potential and inhibits the influx of calcium through voltage-dependent calcium channels, essential for the release of acetylcholine. In the muscular membrane, it inhibits the release of calcium from the sarcoplasmic reticulum during the action potential, hindering the excitation-contraction process.
In this study, lithium administered by itself, in the concentration indicated, did not cause significant change in neuromuscular transmission, but it influenced the effects of neuromuscular blockers. The reduction in the amplitude of the muscular responses in the preparations exposed to the association of lithium and atracurium or cisatracurium was significantly smaller when compared with what was observed on the preparations exposed only to the neuromuscular blockers. These results might suggest that lithium hampers the action of some neuromuscular blockers and, as reported by some authors, it can be attributed to an increase in the release of acetylcholine by this ion 10,22,23. The probable resistance to the action of neuromuscular blockers in the preparations exposed to lithium was also confirmed by other experiments in which lithium was added to the preparation after obtaining an 80% blockade with the different neuromuscular blockers. In those experiments, lithium reversed the neuromuscular blockade, with a significant recovery in the amplitude of the muscular responses, which was similar to the effect observed when anticholinesterase agents are used to revert the blockade produced by depolarizing drugs.
The results of these experiments do not agree with those reported by other authors. They demonstrated, in preparations exposed to lithium, potentiation and an increase in the duration of the blockade, or even no change at all on the effects of depolarizing or non-depolarizing neuromuscular blockers 2,5,6,24. In other studies, lithium alone also did not cause any changes in the neuromuscular transmission, but it was capable of potentiating the blockade produced by pancuronium, gallamine, and vecuronium, but it did not change the effects of d-tubocurarine, succinylcholine, and decamethonium 5,6.
The interaction lithium-neuromuscular blockers was also described in psychiatric patients, with an increase in the duration of the neuromuscular blockade produced by succinylcholine and pancuronium 4,24. Besides, it was also observed, in human skeletal muscle, a negative effect of lithium on the excitation-contraction process 15.
The responses regarding the influence of lithium on different neuromuscular blockers suggest that the mechanism of action of those blockers might not be identical. These results could also reflect different sites of action of lithium, pre or postsynaptic, besides the effects on the synthesis and release of acetylcholine, and the reduction in the number of acetylcholine receptors 5,14,25.
In the electrophysiological studies, we observed that lithium, in the concentration used in this study, did not change the membrane potential of muscle fibers, remaining between 82 and 88 millivolts, i.e., within normal limits. Therefore, it is difficult to explain why lithium potentiates, hampers, or does not influence the effects of some neuromuscular blockers. It is possible that, depending on their chemical structure, these agents could have a greater effect on the cellular distribution of sodium and potassium. Pancuronium, due to its steroidal structure, has mineralocorticoid activity similar to many steroids, causing accumulation of sodium and depletion of potassium in the cell, and these effects are similar to those produced by lithium 5,16,17,26. The probability of clinically significant interaction with neuromuscular blockers is increased in patients taking several drugs in the preoperative period, which can affect neuromuscular transmission and muscular contraction. Based on the conflicting results found in the literature, the interaction of lithium with neuromuscular blockers is not given much importance in clinical practice. However, due to the possibility of an interaction, preoperative care, such as monitoring serum levels of lithium or even its temporary suspension, seems prudent. Intraoperatively, cautious use of neuromuscular blockers and continuous monitoring of the neuromuscular blockade and recovery of the neuromuscular transmission in patients treated with lithium or other alkaline metals are also recommended.
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Dra. Angélica de Fátima de Assunção Braga
Rua Luciano Venere Decourt, 245 Cidade Universitária
13084-040 Campinas, SP
em 19 de junho de 2006
Accepted para publicação em 23 de fevereiro de 2007
* Received from Departamento de Farmacologia da Faculdade de Ciências Médicas da Universidade Estadual de Campinas (FCM-UNICAMP), Campinas, São Paulo