Effects of intravenous and local anesthetic agents on ω-conotoxin MVIIA binding to rat cerebrocortex

PURPOSE: The cellular target site(s) for anesthetic action remain controversial. In this study we have examined any interaction of i.v. anesthetics (thiopental, pentobarbital, ketamine, etomidate, propofol, alphaxalone), local anesthetics (lidocaine, prilocaine, procaine and tetracaine), and the non anesthetic barbiturate, barbituric acid with the omega-conotoxin MVII(A) binding site on N-type voltage sensitive Ca2+ channels in rat cerebrocortical membranes. METHODS: [125I] omega-conotoxin MVII(A) binding assays were performed in 0.5 ml volumes of Tris.HCl buffer containing BSA 0.1% for 30 min at 20 degrees C using fresh cerebrocortical membranes (5 microg of protein). Non-specific binding was defined in the presence of excess (10(-8) M) omega-conotoxin MVII(A). The interaction of i.v. (alphaxolone, etomidate, propofol, pentobarbitone, ketamine and thiopentone), local (lidocaine, prilocaine, procaine and tetracaine) anesthetics and barbituric acid was determined by displacement of [125I] omega-conotoxin MVII(A) (approximately 1 pM). RESULTS: The binding of [125I] omega-conotoxin was concentration-dependent and saturable with Bmax and Kd of 223 +/- 15 fmol/mg protein and 2.13 +/- 0.14 pM, respectively. Unlabelled omega-conotoxin MVII(A) displaced [125I] omega-conotoxin MVII(A) yielding a pKd of 11.04 +/- 0.04 (9.2 pM). All i.v. and local anesthetics at clinically relevant concentrations did not show any interaction with the omega-conotoxin MVII(A) binding site. CONCLUSION: The present study suggests that omega-conotoxin MVII(A) binding site on N-type voltage sensitive Ca2+ channels may not be a target for i.v. and local anesthetic agents.     

Differential modulation of human N-methyl-D-aspartate receptors by structurally diverse general anesthetics

N-Methyl-d-aspartate (NMDA) receptors have a presumed role in excitatory synaptic transmission and nociceptive pathways. Although previous studies have found that inhaled anesthetics inhibit NMDA receptor-mediated currents at clinically relevant concentrations, the use of different experimental protocols, receptor subtypes, and/or tissue sources confounds quantitative comparisons of the NMDA receptor inhibitory potencies of inhaled anesthetics. In the present study, we sought to fill this void by defining, using the two-electrode voltage-clamp technique, the extent to which diverse clinical and aromatic inhaled anesthetics inhibit the NR1/NR2B subtype of the human NMDA receptor expressed in Xenopus laevis oocytes. At 1 minimum alveolar anesthetic concentration (MAC), anesthetic compounds reversibly inhibited NMDA receptor currents by 12 +/- 6% to 74 +/- 6%. These results demonstrate that equianesthetic concentrations of inhaled anesthetics can differ considerably in the extent to which they inhibit NMDA receptors. Such differences may be useful for defining the role that this receptor plays in producing the in vivo actions of general anesthetics.     

The effects of intravenous lidocaine administration on the minimum alveolar concentration of isoflurane in cats

Lidocaine decreases the minimum alveolar concentration (MAC) of inhaled anesthetics and has been used clinically to reduce the requirements for other anesthetic drugs. In this study we examined the effects of lidocaine on isoflurane MAC in cats. Six cats were studied. In Experiment 1, the MAC of isoflurane was determined. An IV bolus of lidocaine 2 mg/kg was then administrated and venous plasma lidocaine concentrations were measured to determine pharmacokinetic values. In Experiment 2, lidocaine was administered to achieve target plasma concentrations between 1 and 11 microg/mL and the MAC of isoflurane was determined at each lidocaine plasma concentration. Actual lidocaine plasma concentrations were 1.06 +/- 0.12, 2.83 +/- 0.39, 4.93 +/- 0.64, 6.86 +/- 0.97, 8.86 +/- 2.10, and 9.84 +/- 1.34 microg/mL for the target concentrations of 1, 3, 5, 7, 9, and 11 microg/mL, respectively. The MAC of isoflurane in this study was 2.21% +/- 0.17%, 2.14% +/- 0.14%, 1.88% +/- 0.18%, 1.66% +/- 0.16%, 1.47% +/- 0.13%, 1.33% +/- 0.23%, and 1.06% +/- 0.19% at lidocaine target plasma concentrations of 0, 1, 3, 5, 7, 9, and 11 microg/mL, respectively. Lidocaine, at target plasma concentrations of 1, 3, 5, 7, 9, and 11 microg/mL, linearly decreased isoflurane MAC by -6% to 6%, 7% to 28%, 19% to 35%, 28% to 45%, 29% to 53%, and 44% to 59%, respectively. We conclude that lidocaine decreases the MAC of isoflurane.     

Subcutaneous injection of inhaled anesthetics produces cutaneous analgesia

PURPOSE: Previous investigations suggest that inhaled anesthetics may produce cutaneous analgesia. The objective of this study was to evaluate whether inhaled anesthetics have a direct analgesic effect on skin. METHODS: We conducted subcutaneous injections of one of three inhaled anesthetics (halothane, isoflurane, and enflurane) or one of two local anesthetics (lidocaine and procaine) at various dosages in rats (n=6 rats, for each dose of each drug). Subcutaneous injections of vehicles (saline or olive oil) were used as controls (n=6 rats for each vehicle). We constructed concentration-response curves, wherein the concentrations of drugs tested in subcutaneous tissue fluid were estimated by calculation, and the cutaneous analgesic effects of drugs were evaluated by pinprick tests on skin. RESULTS: Like local anesthetics, subcutaneous injection of inhaled anesthetics produced concentration-dependent, cutaneous analgesia which attained maximum (complete cutaneous analgesia) at high concentration. This effect was reversible and localized in the area of injection. On the basis of 50% effective concentration, the ranking of potencies was lidocaine>halothane>isoflurane>enflurane>procaine (P<0.05 for all differences). Subcutaneous injections of vehicles did not produce cutaneous analgesia. CONCLUSIONS: Like local anesthetics (lidocaine and procaine), subcutaneous injections of inhaled anesthetics (halothane, isoflurane, and enflurane) produced a concentration-dependent, cutaneous, analgesic effect at the site of injection. Inhaled anesthetics have a direct analgesic effect on skin.     

The influence of heparin administration on the plasma protein binding of lidocaine during epidural anesthesia]

The influence of heparin administration on the protein binding of lidocaine was investigated in this study. Epidural anesthesia was conducted in 20 patients undergoing vasoconstructive surgery, and the changes of lidocaine concentration and its protein binding ratio after heparin administration were estimated. Protein binding ratio of lidocaine decreased rapidly after the commencement of heparin injection until attaining the minimal rate of 50.3 +/- 8.8%, and it then recovered gradually. There was a distinct negative correlation between the reduction in protein binding ratio of lidocaine and the increase of non-esterified fatty acid (NEFA) based on the activated lipoprotein lipase after heparin administration. This reduction of protein binding ratio after heparin was in safety range so that practically no dangerous adverse reaction was seen in these patients. The detailed mechanism was elucidated in an in vitro study. Normal concentration of NEFA (300 microEq.l-1) increased the protein binding ratio of lidocaine, and high concentration of NEFA (1800 microEq.l-1) decreased the ratio on human serum albumin (HSA). Heparin (1.0 unit.ml-1) itself, however, showed no influence for protein binding ratio onto HSA. Concerning alpha 1-acid glycoprotein (AAG), NEFA did not show any influence on the binding ratio, and heparin decreased it little. Consequently, it can be said that the protein binding ratio of lidocaine was mainly affected by NEFA concentration.     

Airway anesthesia alone does not explain attenuation of histamine-induced bronchospasm by local anesthetics: a comparison of lidocaine, ropivacaine, and dyclonine

BACKGROUND: Lidocaine inhalation attenuates histamine-induced bronchospasm while evoking airway anesthesia. Because this occurs at plasma concentrations much lower than those required for intravenous lidocaine to attenuate bronchial reactivity, this effect is likely related to topical airway anesthesia and presumably independent of the specific local anesthetic used. Therefore, the authors tested the effect of dyclonine, lidocaine, and ropivacaine inhalation on histamine-induced bronchospasm in 15 volunteers with bronchial hyperreactivity. METHODS: Bronchial hyperreactivity was verified by an inhalational histamine challenge. Histamine challenge was repeated after inhalation of dyclonine, lidocaine, ropivacaine, or placebo on 4 different days in a randomized, double-blind fashion. Lung function, bronchial hyperreactivity to histamine, duration of local anesthesia, and lidocaine and ropivacaine plasma concentrations were measured. Statistical analyses were performed with the Friedman and Wilcoxon rank tests. Data are presented as mean +/- SD. RESULTS: The inhaled histamine concentration necessary for a 20% decrease of forced expiratory volume in 1 s (PC20) was 7.0 +/- 5.0 mg/ml at the screening evaluation. Lidocaine and ropivacaine inhalation increased PC20 significantly to 16.1 +/- 12.9 and 16.5 +/- 13.6 mg/ml (P = 0.007), whereas inhalation of dyclonine and saline did not (9.1 +/- 8.4 and 6.1 +/- 5.0 mg/ml, P = 0.7268). Furthermore, in contrast to saline and lidocaine, inhalation of both ropivacaine and dyclonine significantly decreased forced expiratory volume in 1 s from baseline (P = 0.0016 and 0.0018, respectively). The longest lasting and most intense anesthesia developed after dyclonine inhalation (48 +/- 13 vs. 28 +/- 8 [lidocaine] and 25 +/- 4 min [ropivacaine]). CONCLUSION: Both lidocaine and the new amide local anesthetic ropivacaine significantly attenuate histamine-induced bronchospasm. In contrast, dyclonine, despite its longer lasting and more intense local anesthesia, does not alter histamine-evoked bronchoconstriction and irritates the airways. Thus, airway anesthesia alone does not necessarily attenuate bronchial hyperreactivity. Other properties of inhaled local anesthetics may be responsible for attenuation of bronchial hyperreactivity.     

Pregnancy does not alter the threshold for lidocaine-induced seizures in the rat.

Although altered effects of various anesthetics have been demonstrated during pregnancy, published studies have incompletely defined potential pregnancy-induced changes in the central nervous system toxicity of lidocaine. Accordingly, the seizure threshold for lidocaine was measured in three groups of mechanically ventilated rats breathing 70% N2O-30% O2: male (n = 21), nonpregnant female (n = 19), and pregnant female (n = 23). Lidocaine was administered intravenously at a constant rate of 2.3 mg.kg-1.min-1 while the electroencephalogram was monitored continuously. Total doses of lidocaine and the durations of lidocaine infusion necessary to induce seizure activity were similar among groups. Plasma lidocaine concentrations at the onset of electroencephalographic seizure activity were also similar among groups (male = 10.7 +/- 5.5, nonpregnant female = 12.1 +/- 4.9, pregnant female = 10.8 +/- 4.1 micrograms/mL). In a subset of each group, brain lidocaine concentrations at the onset of seizure activity were also measured, and again no differences among groups were observed (male = 17.4 +/- 6.3, nonpregnant female = 16.8 +/- 4.5, pregnant female = 16.7 +/- 4.2 micrograms/100 g wet wt). The authors conclude that there are no pregnancy-specific alterations in either plasma or brain concentration thresholds for central nervous system toxicity of lidocaine in rats.     

Transfer of lidocaine across the dual perfused human placental cotyledon

Factors affecting lidocaine transfer across the normal term human placenta were studied using the dual perfused isolated single cotyledon. Experiments were performed using perfusates which provided equal protein binding in both the maternal and fetal circuits as well as perfusates that approached the actual in vivo maternal/fetal protein binding gradient. Additional experiments were performed to investigate the effects of increasing maternal lidocaine concentration (5, 10, 40, 80 microg/mL) on maternal to fetal (M-->F) lidocaine transfer across the human placenta. Lidocaine crossed the placenta rapidly in both the M-->F and fetal to maternal (F-->M) directions. When protein binding was similar in the two circuits, M-->F transfer ratios (lidocaine transfer/antipyrine transfer) were significantly lower than the transfer ratios seen in the F-->M direction (0.59+/-0.04 versus 0.84+/-0.06, P<0.05). Transfer ratios (M-->F: 0.83+/-0.06, F-->M: 0.96+/-0.06) were not reduced when the physiological maternal/fetal protein binding gradient was present. Lidocaine transfer was not diminished by increasing maternal concentrations and, in contrast to bupivacaine, was not significantly affected by its binding.     

Excretion of lidocaine and bupivacaine in breast milk following epidural anesthesia for cesarean delivery

BACKGROUND: There is a lack of information and knowledge about the practical importance of even low concentrations of the excretion of local anesthetics into breast milk, particularly concerning bupivacaine. The present work aims to confirm, under practical clinical conditions of admission of parturients, the passage of local anesthetics (lidocaine and bupivacaine) into breast milk after an epidural anesthesia. METHODS: Twenty-seven pregnant women admitted for cesarean delivery received epidural anesthesia with 0.5% bupivacaine and 2% lidocaine. Blood and milk samples were simultaneously collected at 2, 6 and 12 h after the beginning of the epidural infusion. Lidocaine, bupivacaine and its main metabolite, pipecolylxylidide (PPX), were determined in serum and milk by a gas-liquid chromatographic technique. APGAR scores were systematically performed at delivery and a clinical examination was done 24 h after delivery. RESULTS: Our data indicate that lidocaine and bupivacaine as well as PPX are excreted into breast milk. The milk/serum ratio based upon area under the curve values were 1.07 +/- 0.82, 0.34 +/- 0.24 and 1.37 +/- 0.61 mean +/- SD for lidocaine, bupivacaine and PPX, respectively. Most of the newborns had a maximal APGAR score. Our study does not reveal any adverse reactions related to the excretion of local anesthetics into breast milk. CONCLUSION: This study documents the magnitude of excreted lidocaine, bupivacaine and PPX in breast milk, and indicates that the use of both lidocaine and bupivacaine for epidural anaesthesia is safe with regard to breast-feeding.     

Cardiovascular safety and actions of high concentrations of I-653 and isoflurane in swine

The ratio of lethal-to-anesthetic concentration can be used to define the margin of safety of an inhaled anesthetic. In mechanically ventilated swine the fatal concentration of I-653, a new inhaled anesthetic, was 23.9 +/- 0.06% (mean +/- SE), and of isoflurane, 6.22 +/- 0.23%. The ratio of fatal anesthetic concentration-to-MAC for I-653 (2.45 +/- 0.11) was less than that determined for isoflurane (3.02 +/- 0.13; P less than 0.01) but relatively greater than that reported previously for other inhaled anesthetics. As with other inhaled anesthetics, the concentration of I-653 causing cardiovascular collapse exceeds that producing apnea, making cardiovascular collapse during spontaneous ventilation unlikely. Mean aortic blood pressure and cardiac output decreased as linear functions of anesthetic concentration. Values for these variables for isoflurane were greater than those for I-653 at concentrations exceeding 1.5 MAC. Heart rate, blood lactate concentration, and base-deficit did not change with anesthetic depth. Mixed venous PO2, mixed venous oxyhemoglobin saturation, and the ratio of oxygen transport to oxygen consumption remained at or above values in conscious swine but decreased similarly with both anesthetics when anesthetic concentration increased to within 0.5 MAC of the fatal concentration. Thus, the latter three variables, reflecting the fraction of delivered oxygen that is consumed, and "mean" tissue PO2 appear to be useful indices of anesthetic concentrations approaching those producing cardiovascular collapse.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intravenous Lidocaine and Bupivacaine Dose-dependently Attenuate Bronchial Hyperreactivity in Awake Volunteers

Background: In standard textbooks, intravenous lidocaine is recommended for intubation of patients with bronchial hyperreactivity. However, whether and to what extent intravenous local anesthetics attenuate bronchial hyperreactivity in humans is unknown. Accordingly, nine awake volunteers with known bronchial hyperreactivity were subjected to an inhalational challenge with acetylcholine before and during intravenous infusion of lidocaine, bupivacaine, or placebo in a randomized, double-blinded fashion.

Methods: Baseline acetylcholine threshold concentrations were determined 3-5 days before initiation of the investigation. The response to the acetylcholine challenge was defined as hyperreactive, if forced expiratory volume in 1 s decreased by at least 20%. In addition, the acetylcholine threshold for a 100% increase in airway resistance was obtained by body plethysmography. On seven different days, the acetylcholine challenge was repeated at the end of a 30-min intravenous infusion period of three doses of lidocaine (1, 3, and 6 mg *symbol* min sup -1) or bupivacaine (0.25, 0.75, and 1.5 mg *symbol* min sup -1), during saline placebo infusion, respectively. Acetylcholine-threshold concentrations were presented with the respective plasma concentrations of the local anesthetic.

Results: The infusion of lidocaine and bupivacaine resulted in plasma concentrations (means+/-SD) of 0.29+/-0.11, 1.14 +/-0.39, and 2.02+/-0.5 micro gram *symbol* ml sup -1 for lidocaine and 0.11+/-0.04, 0.31+/-0.09, and 0.80 +/-0.18 micro gram *symbol* ml sup -1 for bupivacaine, respectively. Compared to baseline, the acetylcholine threshold for a 20% decrease of forced expiratory volume in 1 s as well as the threshold for a 100% increase in total airway resistance increased significantly with increasing plasma concentrations of both local anesthetics. Compared to placebo, acetylcholine threshold was almost quadrupled for lidocaine and tripled for bupivacaine with the highest plasma concentration of each local anesthetic.     

Time-dependent inhibition of G protein-coupled receptor signaling by local anesthetics

BACKGROUND: Several beneficial effects of local anesthetics (LAs) were shown to be due to inhibition of G protein-coupled receptor signaling. Differences in exposure time might explain discrepancies in concentrations of LAs required to achieve these protective effects in vivo and in vitro (approximately 100-fold higher). Using Xenopus oocytes and human neutrophils, the authors studied time-dependent effects of LAs on G protein-coupled receptor signaling and characterized possible mechanisms and sites of action. METHODS: Measurement of agonist-induced Ca2+-activated Cl currents, using a two-electrode voltage clamp technique, and determination of superoxide anion production by cytochrome c assay were used to assess the effects of LAs on G protein-coupled receptor signaling in oocytes and primed and activated human neutrophils, respectively. Antisense knockdown of G alpha q protein and inhibition of various proteins within the signaling pathway served for defining mechanisms and sites of action more specifically. RESULTS: LAs attenuated G protein-coupled receptor signaling in both models in a time-dependent and reversible manner (lidocaine reduced lysophosphatidic acid signaling to 19 +/- 3% after 48 h and 25 +/- 2% after 6 h of control response in oocytes and human neutrophils, respectively). Whereas no effect was observed after extracellularly applied or intracellularly injected QX314, a lidocaine analog, using G alpha q-depleted oocytes, time-dependent inhibition also occurred after intracellular injection of QX314 into undepleted oocytes. Inhibition of phosphatases or protein kinases and agonist-independent G-protein stimulation, using guanosine 5'-O-3-thiotriphosphate or aluminum fluoride, did not affect time-dependent inhibition by LAs. CONCLUSION: Inhibition of G protein-coupled receptor signaling by LAs was found to be time dependent and reversible. Critically requiring G alpha q-protein function, this effect is located downstream of guanosine diphosphate-guanosine triphosphate exchange and is not dependent on increased guanosine triphosphatase activity, phosphatases, or protein kinases.     

Local anesthetics have different mechanisms and sites of action at recombinant 5-HT3 receptors

BACKGROUND AND OBJECTIVES: In addition to their blockade of voltage-dependent sodium channels, the action of local anesthetics at 5-hydroxytryptamine-3 (5-HT3) receptors may be clinically relevant. Because local anesthetics have different clinical properties, we have tested the hypothesis that differences in interactions at the 5-HT3 receptor may be clinically relevant by investigating the effects of 4 local anesthetics on recombinant wild-type and 4 mutant 5-HT3A receptors. METHODS: The cRNAs from human wild-type and 4 mutant 5-HT3A subunit clones were synthesized in vitro and expressed in Xenopus oocytes. Four mutant receptors were obtained by site-directed mutagenesis in the N-terminal extracellular region, which contains the agonist binding domain. Tryptophan (W) at positions 62 and 155 were replaced by tyrosine (Y) and glutamate (E) at position 101 by aspartate (D) or asparagine (N). The 2-electrode voltage clamp technique was used to measure peak currents induced by 5-HT in these receptors in the presence and absence of local anesthetics. RESULTS: All local anesthetics inhibited 5-HT-induced currents in a dose-dependent manner in the wild-type receptor. Inhibition by procaine and tetracaine were competitive whereas those of bupivacaine and lidocaine were both noncompetitive and competitive. The 4 mutants (W62Y, W155Y, E101D, E101N) could all form functional receptors. All mutant receptors exhibited a major increase (> 10-fold) in the half-maximum inhibitory concentration for procaine. The half-maximum inhibitory concentrations of tetracaine, bupivacaine, and lidocaine in mutant receptors were increased 2- to 3-fold except that of tetracaine in W62Y receptor (6-fold). CONCLUSIONS: The ester type local anesthetics, procaine and tetracaine, may act at a different site on the 5-HT(3A) receptor and with a different mechanism than the amide-type local anesthetics. Clinical differences between local anesthetics may be at least partially due to differences in interactions at the 5-HT3A receptor.     

Lidocaine exerts its effect on induced bronchospasm by mitigating reflexes, rather than by attenuation of smooth muscle contraction

BACKGROUND: Lidocaine inhalation attenuates histamine-induced bronchoconstriction, as well as bronchoconstriction elicited by mechanical irritation. This effect could be mediated by direct effects on smooth muscle or by reflex attenuation. Therefore, we evaluated whether lidocaine attenuated the bronchial response of direct smooth muscle stimulation with methacholine. METHODS: In 15 volunteers with bronchial hyperreactivity, a methacholine challenge was performed following the inhalation of lidocaine, dyclonine (which does not attenuate bronchial reactivity) or saline on three different days in a randomized, double-blind fashion. Lung function, response to methacholine, and lidocaine and dyclonine plasma concentrations were measured. RESULTS: The inhaled methacholine concentration (PC20) necessary for a 20% decrease in the forced expiratory volume in 1 s (FEV1) was 8.8 +/- 6.1 mg/ml at the screening evaluation. The sensitivity to methacholine challenge (PC20) remained unchanged regardless of which solution was inhaled (9.1 +/- 7.5 mg/ml for lidocaine, 10.2 +/- 9.0 mg/ml for dyclonine and 9.8 +/- 8.3 mg/ml for saline; P = 0.58, means +/- standard deviation). Furthermore, the inhalation of all three solutions caused a significant decrease in FEV1 from baseline (P = 0.0007), with a significantly larger effect for dyclonine than lidocaine (P = 0.0153). CONCLUSIONS: Although both inhaled and intravenous lidocaine attenuates histamine-evoked bronchoconstriction, it does not alter the response to methacholine. Therefore, the attenuation of bronchial reactivity by lidocaine appears to be related solely to neurally mediated reflex attenuation, rather than to the attenuation of direct constriction of airway smooth muscle.     

Blood glucose control during selective arterial stimulation and venous sampling for localization of focal hyperinsulinism lesions in anesthetized children

Surgical management of congenital hyperinsulinism is improved by accurate localization of small, focal dysregulated pancreatic lesions using the arterial stimulation and venous sampling (ASVS) test, which can demonstrate increased hepatic venous insulin concentrations after selective arterial injections of calcium. However, anesthesia-related increases in blood glucose can induce insulin secretion, making it difficult to interpret ASVS test data. In this retrospective study, we examined the effect of anesthetic interventions on blood glucose concentrations in 68 children undergoing ASVS testing. We considered only the glucose concentrations observed before calcium stimulation in the final analysis. The choice of drugs for induction (sevoflurane, propofol, or thiopentone), maintenance inhaled anesthetics (sevoflurane, desflurane, or isoflurane), and the use of caudal epidural bupivacaine were not associated with significant differences in the mean blood glucose concentration before ASVS. However, patients receiving remifentanil infusions had smaller mean glucose concentrations (80 +/- 18 versus 100 +/- 44 mg x dl(-1), P = 0.01). These concentrations were also significantly smaller if tracheal intubation was delayed for at least 10 min after induction while patients received inhaled anesthetics via a face mask along with remifentanil infusions (79 +/- 14 for delayed intubation versus 95 +/- 39 mg x dl(-1) for early intubation, respectively, P = 0.03). The percentage increase in glucose concentrations from preintubation values was significantly smaller in these subjects (3.7% +/- 21.9% for delayed intubation versus 31.7% +/- 60.4% for early intubation, P = 0.02). We conclude that the anesthetic management protocol for these patients should include the use of remifentanil infusions and the administration of inhaled anesthetics and remifentanil infusions for a minimum of 10 min to establish a deep plane of anesthesia before tracheal intubation.     

Epinephrine increases the extracellular lidocaine concentration in the brain: a possible mechanism for increased central nervous system toxicity

BACKGROUND: Local anesthetics exert central nervous system (CNS) toxicity by inhibiting intracerebral neuronal activity, while epinephrine augments the CNS toxicity of intravenously administered local anesthetics. Viewed together, increases of extracellular concentrations of local anesthetics in the brain may be directly associated with increased CNS toxicity. The authors examined the hypothesis that epinephrine enhances the CNS toxicity of lidocaine by increasing the extracellular concentration in the brain. METHODS: An awake, spontaneously breathing rat model was used. Twenty male Sprague-Dawley rats received an intravenous infusion of lidocaine (3 mg x kg x min; group C) or lidocaine with epinephrine (3 mg x kg x min and 2 microg x kg x min, respectively; group E) for 10 min (n = 10 in each group). Effects of epinephrine on the convulsive dose and concentrations of total (protein-bound and unbound) and unbound lidocaine in plasma were examined. Concentrations of extracellular lidocaine in the cerebral nucleus accumbens were quantitatively determined by a microdialysis method. RESULTS: The convulsive dose of lidocaine was significantly lower in group E than in group C (22.4 +/- 5.5 vs. 27.9 +/- 3.1 mg/kg, respectively; P < 0.05). Overall concentrations and area under the plasma concentration-versus-time curve of unbound lidocaine in group E were significantly higher than those in group C. Concentrations of extracellular lidocaine in the nucleus accumbens in group E were comparable to those of unbound fraction in plasma and were also significantly higher than those in group C. CONCLUSIONS: Concomitant administration of epinephrine significantly enhanced the CNS toxicity of intravenously administered lidocaine. Increased extracellular concentration in the brain would be related to this mechanism.     

Patch-clamp analysis of anesthetic interactions with recombinant SK2 subtype neuronal calcium-activated potassium channels

Small conductance calcium-activated potassium channels (SK) mediate spike frequency adaptation and underlie the slow afterhyperpolarization in central neurons. We tested the actions of several anesthetics on the SK2 subtype of recombinant SK channels, cloned from rat brain and functionally expressed in a mammalian cell line. Butanol, ethanol, ketamine, lidocaine, and methohexital blocked recombinant SK2 channel currents, measured in the whole-cell patch clamp recording mode. The block was reversible, dose-dependent, and of variable efficacy. The inhaled anesthetics chloroform, desflurane, enflurane, halothane, isoflurane, and sevoflurane produced little or no block when applied at 1 minimum alveolar anesthetic concentration; varying degrees of modulation were observed at very large concentrations (10 minimum alveolar concentration). The extent of block by inhaled anesthetics did not appear to depend on concentration or membrane voltage. IMPLICATIONS: We describe differential effects of anesthetics on cloned small conductance calcium-activated potassium channels from brain that may play a role in generating the effects or side effects of anesthetics.     

Effects of local anesthetics upon human natural killer cytotoxicity in vitro]

Effects of 3 local anesthetics, bupivacaine, mepivacaine and lidocaine, upon natural killer cytotoxicity were studied in vitro. Mononuclear cell layer was recovered by Ficoll-Paque sedimentation from heparinized venous blood obtained prior to the induction of anesthesia. The mononuclear cells were divided into three groups: control group was incubated in medium only: low concentration group incubated in medium with 2.0 micrograms.ml-1 of mepivacaine (n = 20) or lidocaine (n = 20), or 0.5 micrograms.ml-1 of bupivacaine (n = 21); high concentration group in medium with 20 micrograms.ml-1 of mepivacaine or lidocaine, or 5 micrograms.ml-1 of bupivacaine. These three groups were incubated simultaneously in humidified atmosphere with 5% CO2 in incubator for 2 hours. NK cell cytotoxicity was determined in a chromium release assay against K 562 cell as a target cell. An effector to target cell ratio of 40:1 was used. Comparison among 3 local anesthetics showed no significant difference at high concentration, but a significant difference at low concentration. This was due to the differences between bupivacaine and lidocaine. Neither bupivacaine nor mepivacaine inhibited % NK cytotoxicity at both low and high concentrations compared with control. Lidocaine significantly inhibited % NK cytotoxicity at low concentration, but did not inhibit at high concentration compared with control. We concluded that neither bupivacaine nor mepivacaine inhibited % NK cytotoxicity at concentration of clinical dose compared with control in vitro, but lidocaine inhibited % NK cytotoxicity at a concentration of 2.0 micrograms.ml-1 compared with control.     

Time-dependent Inhibition of G Protein-coupled Receptor Signaling by Local Anesthetics

Background: Several beneficial effects of local anesthetics (LAs) were shown to be due to inhibition of G protein-coupled receptor signaling. Differences in exposure time might explain discrepancies in concentrations of LAs required to achieve these protective effects in vivo and in vitro (approximately 100-fold higher). Using Xenopus oocytes and human neutrophils, the authors studied time-dependent effects of LAs on G protein-coupled receptor signaling and characterized possible mechanisms and sites of action.

Methods: Measurement of agonist-induced Ca2+-activated Cl- currents, using a two-electrode voltage clamp technique, and determination of superoxide anion production by cytochrome c assay were used to assess the effects of LAs on G protein-coupled receptor signaling in oocytes and primed and activated human neutrophils, respectively. Antisense knockdown of G[alpha]q protein and inhibition of various proteins within the signaling pathway served for defining mechanisms and sites of action more specifically.

Results: LAs attenuated G protein-coupled receptor signaling in both models in a time-dependent and reversible manner (lidocaine reduced lysophosphatidic acid signaling to 19 +/- 3% after 48 h and 25 +/- 2% after 6 h of control response in oocytes and human neutrophils, respectively). Whereas no effect was observed after extracellularly applied or intracellularly injected QX314, a lidocaine analog, using G[alpha]q-depleted oocytes, time-dependent inhibition also occurred after intracellular injection of QX314 into undepleted oocytes. Inhibition of phosphatases or protein kinases and agonist-independent G-protein stimulation, using guanosine 5'-O-3-thiotriphosphate or aluminum fluoride, did not affect time-dependent inhibition by LAs.     

Contrasting roles of the N-methyl-D-aspartate receptor in the production of immobilization by conventional and aromatic anesthetics

We hypothesized that N-methyl-d-aspartate (NMDA) receptors mediate some or all of the capacity of inhaled anesthetics to prevent movement in the face of noxious stimulation, and that this capacity to prevent movement correlates directly with the in vitro capacity of such anesthetics to block the NMDA receptor. To test this hypothesis, we measured the effect of IV infusion of the NMDA blockers dizocilpine (MK-801) and (R)-4-(3-phosphonopropyl) piperazine-2-carboxylic acid (CPP) to decrease the MAC (the minimum alveolar concentration of anesthetic that prevents movement in 50% of subjects given a noxious stimulation) of 8 conventional anesthetics (cyclopropane, desflurane, enflurane, halothane, isoflurane, nitrous oxide, sevoflurane, and xenon) and 8 aromatic compounds (benzene, fluorobenzene, o-difluorobenzene, p-difluorobenzene, 1,2,4-trifluorobenzene, 1,3,5-trifluorobenzene, pentafluorobenzene, and hexafluorobenzene) and, for comparison, etomidate. We postulated that MK-801 or CPP infusions would decrease MAC in inverse proportion to the in vitro capacity of these anesthetics to block the NMDA receptor. This notion proved correct for the aromatic inhaled anesthetics, but not for the conventional anesthetics. At the greatest infusion of MK-801 (32 microg x kg(-1) x min(-1)) the MACs of conventional anesthetics decreased by 59.4 +/- 3.4% (mean +/- sd) and at 8 microg x kg(-1) x min(-1) by 45.5 +/- 4.2%, a decrease not significantly different from a 51.4 +/- 19.0% decrease produced in the EC50 for etomidate, an anesthetic that acts solely by enhancing gamma-amino butyric acid (GABA) receptors. We conclude that some aromatic anesthetics may produce immobility in the face of noxious stimulation by blocking the action of glutamate on NMDA receptors but that conventional inhaled anesthetics do not.