Ⅵ LABORATORY REPORT Anesthesiology 2003; 99:748–51 © 2003 American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins, Inc. Combination of Xenon and Isoflurane Produces a Synergistic Protective Effect against Oxygen–Glucose Deprivation Injury in a Neuronal–Glial Co-culture Model Daqing Ma, M.D., Ph.D.,* Mahmuda Hossain, Ph.D.,† Nishanthan Rajakumaraswamy, B.Sc.,‡ Nicholas P. Franks, Ph.D.,§ Mervyn Maze, FRCP, FRCA, FMedSci࿣ SUSTAINED exposure to glutamate causes neuronal combination, their efficacy would be enhanced. To test death by overactivation of its receptors, particularly our hypothesis, we studied the neuroprotective effect of 1,2 Downloaded from http://pubs.asahq.org/anesthesiology/article-pdf/99/3/748/337011/0000542-200309000-00034.pdf by guest on 27 September 2021 those of the N-methyl-D-aspartate (NMDA) subtype. the combination of xenon and isoflurane versus oxygen– This process, denoted by the term excitotoxicity, is be- glucose deprivation (OGD) injury in a neuronal–glial lieved to play an important role in ongoing neuronal co-culture model. injury and death in acute insults, such as ischemic stroke and head trauma.3,4 Consequently, the neuroprotective effects of NMDA receptor antagonists, including xenon, Materials and Methods have been investigated in a variety of both in vitro and in vivo models of neuronal injury5–9 of the type that may The methodology for preparing mixed cortical cell occur perioperatively.10 Notwithstanding their putative cultures containing both neuronal and glial cell elements beneficial effects, the clinical use of NMDA antagonists from mice has been reported elsewhere.5 Before OGD has been hindered11 by the observation that several exposure, the cell cocultures were carefully washed drugs of this class of compound produce neurotoxicity, twice with HEPES buffer, after which prewarmed characterized by distinctive behavioral and morphologic (37°C), deoxygenated balanced salt solution containing effects. Although xenon does not seem to have this side (in mM): 116 NaCl, 5.4 KCl, 0.8 MgSO4, 1.0 NaH2PO4, 8 effect, it may be desirable to use lower concentrations 1.8 CaCl2, and 26 NaHCO3, pH 7.4, was added into each because of the prohibitive cost of this gas and to permit well, and the plates were placed in a temperature-con- adequate oxygenation. Therefore, we sought to investi- trolled anaerobic chamber for 75 min at 37°C. Pilot gate whether neuroprotective efficacy can be observed studies showed that the maximal neuronal injury could when this NMDA receptor antagonist is combined with be achieved by OGD for between 60 and 100 min. OGD another putative neuroprotective agent that acts by mod- was terminated by washing with Eagle’s minimal essen- ulating the ␥-aminobutyric acid (GABA) receptor. The tial medium enhanced with 25 mM glucose and 38 mM neuronal damage from ischemia may also be a result of NaHCO3, and the culture plates were returned to nor- loss of inhibitory influences. In the brain, GABA acts as moxic conditions containing a minimum of 20% O2 for the major inhibitory neurotransmitter; an agonist of the 6 h at 37°C. The balance of the gas mixture depended on A subtype of the GABA receptor (GABAA) has been the experimental group. Culture media for the OGD and shown to be neuroprotective in a transient forebrain the 6 h post-OGD periods were prepared by bubbling 12 ischemia model. Isoflurane potentiates activation of pure gases (O2,N2,CO2, xenon, or isoflurane) through 13 the GABAA receptors, and its neuroprotective effect fine sintered-glass bubblers in Drechsel bottles filled has been demonstrated previously.14–18 Because these with either balanced salt solution or Eagle’s minimal anesthetics exert their neuroprotectant properties essential medium for 20 min, a period that was found to through different mechanisms, we hypothesized that in be sufficient to allow equilibration as assessed by gas chromatography. For each set of experimental condi- tions, flow rates of gases were adjusted so that the gas * Lecturer, Department of Anaesthetics and Intensive Care, Imperial College, concentrations in the solutions were equivalent to the † Research Assistant, Department of Anaesthetics and Intensive Care, Imperial College, ‡ Medical Student, University College, § Professor of Biophysics and corresponding gas composition in a purpose-built air- Anaesthetics, Departments of Biological Sciences and Anaesthetics and Intensive tight, temperature-controlled cell-culture chamber Care, Imperial College, ࿣ Sir Ivan Magill Professor of Anaesthetics, Departments of Anaesthetics and Intensive Care and Biological Sciences, Imperial College. equipped with inlet and outlet valves and an internal 9 Received from the Magill Department of Anaesthesia, Chelsea & Westminster electric fan (as described previously ). Total gas flow Hospital, Imperial College of Science, Technology and Medicine, London, United was 100 ml/min, and the chamber was flushed for 40 Kingdom. Submitted for publication January 8, 2003. Accepted for publication April 30, 2003. Supported by the Medical Research Council and the Joint min before a closed system was established. With the Research Committee of the Chelsea and Westminster NHS HealthCare Trust, above-mentioned protocol, the final gas concentrations London, United Kingdom. Address reprint requests to Dr. Maze: Magill Department of Anaesthesia, (determined by use of gas chromatography) were found Chelsea & Westminster Hospital, Imperial College of Science, Technology and to be stable, and there was no measurable leakage over Medicine, 369 Fulham Road, London SW10 9NH, United Kingdom. Address electronic mail to: [email protected]. Individual article reprints may be purchased 24 h. For the delivery of isoflurane, a Cyprane isoflurane through the Journal Web site, www.anesthesiology.org. vaporizer (AE Service and Supplies Ltd., Keighley, United Anesthesiology, V 99, No 3, Sep 2003 748 LABORATORY REPORT 749 Kingdom) was used; because this vaporizer is not cali- brated to operate at these low flow rates, we monitored isoflurane concentration (Datex-Engstrom AS/3; Instru- mentarium Corp, Helsinki, Finland) continuously to es- tablish that the appropriate concentration was being delivered. Neuronal injury was assessed by lactate dehy- drogenase (LDH) released into the medium, as previ- ously described.9 The data were fitted to a logistic equa- tion of the form: ␣n ͑ ͒ ϭ E c , Downloaded from http://pubs.asahq.org/anesthesiology/article-pdf/99/3/748/337011/0000542-200309000-00034.pdf by guest on 27 September 2021 ␣n ϩ cn where E(c) is the LDH release expressed as a fraction of the control at a xenon or isoflurane concentration c, ␣ is the IC50, and n is a slope factor. The results are ex- Ϯ pressed as mean SEM. The IC50 value is defined as the concentration of xenon or isoflurane that reduced the LDH release to 50% of the maximal value in control OGD exposures (i.e., in the absence of either anesthetic). To determine whether or not the combined effects of xenon and isoflurane were additive when administered as a mixture, we compared the observed IC50 value for the mixture with that predicted assuming additivity. In general, for additivity, the following condition holds: a b ϩ ϭ 1, 1 Ϫ E͑c ͒ 1/na 1 Ϫ E͑c ͒ 1/nb Fig. 1. Xenon and isoflurane alone and in combination inhib- ␣ ͩ a ͪ ␣ ͩ b ͪ ited lactate dehydrogenase (LDH) release, expressed as a frac- a ͑ ͒ b ͑ ͒ E ca E cb tion of the maximum LDH release in the absence of agents or 4) induced by 75 min oxygen–glucose 3 ؍ mean ؎ SEM, n) where a and b are the concentrations of xenon and deprivation. The values for control LDH release in this series of isoflurane, respectively, that are present in a mixture, experiments were in the range between 327 and 380 Berger- xenon alone, 35.9 ؎ 2.2% ؍ Broida units/ml. (A) Closed circles ␣ ␣ ؍ ؍ and a and b are the IC50 concentrations of xenon and atm; open circles in the present of 0.6% isoflurane, IC50 ؍ ؍ ؎ isoflurane when applied alone, with associated slope 18.9 2.5% atm. (B) Closed circles isoflurane alone, IC50 ,in the presence of 14% xenon ؍ open circles ;0.35% ؎ 2.72 factors na and nb. For the special case of 50% inhibition .%0.13 ؎ %0.92 ؍ IC of LDH release (i.e.,Eϭ 0.5), this equation reduces to: 50 a b values (mean Ϯ SEM) were 35.9 Ϯ 2.2% atm for xenon ϩ ϭ ␣ ␣ 1. and 2.72 Ϯ 0.35% atm for isoflurane. We next deter- a b mined the effects of a low concentration of one agent on ␣ ␣ Thus, having determined a and b from the concentra- the concentration–response curve for the other agent. In tion–response curves of each agent individually, the con- the presence of 0.6% isoflurane, the concentration–re- centration of a (xenon) in the presence of a fixed con- sponse curve for xenon was changed significantly (fig. centration of b (isoflurane), or vice versa, to achieve a 1A), and the IC50 value for xenon-mediated neuroprotec- 50% reduction in LDH release from the control value can tion decreased to 18.9 Ϯ 2.5% atm, which is significantly be predicted, assuming that additivity holds. This pre- Ϯ Ͻ lower than 28 2.0% atm (P 0.05), the predicted IC50 dicted value can then be compared with the observed value for xenon in the presence of 0.6% isoflurane, value. Statistical significance was assessed by use of the assuming that the effects of isoflurane and xenon are Student t test with Bonferroni correction for multiple additive (see Materials and Methods). Similarly, in the Ͻ comparisons. A value of P 0.05 was considered to be presence of 14% xenon, the concentration–response statistically significant. curve for isoflurane was changed significantly (fig. 1B), Ϯ with the IC50 value being decreased to 0.92 0.13% Ͻ Ϯ Results (P 0.05), which is significantly lower than 1.66 0.22% atm, the predicted IC50 value for isoflurane, as- Neuronal damage induced by a 75-min duration of suming additivity.