Excessive disgust caused by brain lesions or temporary inactivations: mapping hotspots of the nucleus accumbens and ventral pallidum

Disgust is a prototypical type of negative affect. In animal models of excessive disgust, only a few brain sites are known in which localized dysfunction (lesions or neural inactivations) can induce intense 'disgust reactions' (e.g. gapes) to a normally pleasant sensation such as sweetness. Here, we aimed to map forebrain candidates more precisely, to identify where either local neuronal damage (excitotoxin lesions) or local pharmacological inactivation (muscimol/baclofen microinjections) caused rats to show excessive sensory disgust reactions to sucrose. Our study compared subregions of the nucleus accumbens shell, ventral pallidum, lateral hypothalamus, and adjacent extended amygdala. The results indicated that the posterior half of the ventral pallidum was the only forebrain site where intense sensory disgust gapes in response to sucrose were induced by both lesions and temporary inactivations (this site was previously identified as a hedonic hotspot for enhancements of sweetness 'liking'). By comparison, for the nucleus accumbens, temporary GABA inactivations in the caudal half of the medial shell also generated sensory disgust, but lesions never did at any site. Furthermore, even inactivations failed to induce disgust in the rostral half of the accumbens shell (which also contains a hedonic hotspot). In other structures, neither lesions nor inactivations induced disgust as long as the posterior ventral pallidum remained spared. We conclude that the posterior ventral pallidum is an especially crucial hotspot for producing excessive sensory disgust by local pharmacological/lesion dysfunction. By comparison, the nucleus accumbens appears to segregate sites for pharmacological disgust induction and hedonic enhancement into separate posterior and rostral halves of the medial shell.

New insights into the mechanism of methoxyflurane nephrotoxicity and implications for anesthetic development (part 2): Identification of nephrotoxic metabolites

BACKGROUND

Methoxyflurane nephrotoxicity results from its metabolism, which occurs by both dechlorination (to methoxydifluoroacetic acid [MDFA]) and O-demethylation (to fluoride and dichloroacetic acid [DCAA]). Inorganic fluoride can be toxic, but it remains unknown why other anesthetics, commensurately increasing systemic fluoride concentrations, are not toxic. Fluoride is one of many methoxyflurane metabolites and may itself cause toxicity and/or reflect formation of other toxic metabolite(s). This investigation evaluated the disposition and renal effects of known methoxyflurane metabolites.

METHODS

Rats were given by intraperitoneal injection the methoxyflurane metabolites MDFA, DCAA, or sodium fluoride (0.22, 0.45, 0.9, or 1.8 mmol/kg followed by 0.11, 0.22, 0.45, or 0.9 mmol/kg on the next 3 days) at doses relevant to metabolite exposure after methoxyflurane anesthesia, or DCAA and fluoride in combination. Renal histology and function (blood urea nitrogen, urine volume, urine osmolality) and metabolite excretion in urine were assessed.

RESULTS

Methoxyflurane metabolite excretion in urine after injection approximated that after methoxyflurane anesthesia, confirming the appropriateness of metabolite doses. Neither MDFA nor DCAA alone had any effects on renal function parameters or necrosis. Fluoride at low doses (0.22, then 0.11 mmol/kg) decreased osmolality, whereas higher doses (0.45, then 0.22 mmol/kg) also caused diuresis but not significant necrosis. Fluoride and DCAA together caused significantly greater tubular cell necrosis than fluoride alone.

CONCLUSIONS

Methoxyflurane nephrotoxicity seems to result from O-demethylation, which forms both fluoride and DCAA. Because their co-formation is unique to methoxyflurane compared with other volatile anesthetics and they are more toxic than fluoride alone, this suggests a new hypothesis of methoxyflurane nephrotoxicity. This may explain why increased fluoride formation from methoxyflurane, but not other anesthetics, is associated with toxicity. These results may have implications for the interpretation of clinical anesthetic defluorination, use of volatile anesthetics, and the laboratory methods used to evaluate potential anesthetic toxicity.

New insights into the mechanism of methoxyflurane nephrotoxicity and implications for anesthetic development (part 1): Identification of the nephrotoxic metabolic pathway

BACKGROUND

Methoxyflurane nephrotoxicity results from biotransformation; inorganic fluoride is a toxic metabolite. Concern exists about potential renal toxicity from volatile anesthetic defluorination, but many anesthetics increase fluoride concentrations without consequence. Methoxyflurane is metabolized by both dechlorination to methoxydifluoroacetic acid (MDFA, which may degrade to fluoride) and O-demethylation to fluoride and dichloroacetatic acid. The metabolic pathway responsible for methoxyflurane nephrotoxicity has not, however, been identified, which was the aim of this investigation.

METHODS

Experiments evaluated methoxyflurane metabolite formation and effects of enzyme induction or inhibition on methoxyflurane metabolism and toxicity. Rats pretreated with phenobarbital, barium sulfate, or nothing were anesthetized with methoxyflurane, and renal function and urine methoxyflurane metabolite excretion were assessed. Phenobarbital effects on MDFA metabolism and toxicity in vivo were also assessed. Metabolism of methoxyflurane and MDFA in microsomes from livers of pretreated rats was determined in vitro.

RESULTS

Phenobarbital pretreatment increased methoxyflurane nephrotoxicity in vivo (increased diuresis and blood urea nitrogen and decreased urine osmolality) and induced in vitro hepatic microsomal methoxyflurane metabolism to inorganic fluoride (2-fold), dichloroacetatic acid (1.5-fold), and MDFA (5-fold). In contrast, phenobarbital had no influence on MDFA renal effects in vivo or MDFA metabolism in vitro or in vivo. MDFA was neither metabolized to fluoride nor nephrotoxic. Barium sulfate diminished methoxyflurane metabolism and nephrotoxicity in vivo.

CONCLUSIONS

Fluoride from methoxyflurane anesthesia derives from O-demethylation. Phenobarbital increases in methoxyflurane toxicity do not seem attributable to methoxyflurane dechlorination, MDFA toxicity, or MDFA metabolism to another toxic metabolite, suggesting that nephrotoxicity is attributable to methoxyflurane O-demethylation. Fluoride, one of many metabolites from O-demethylation, may be toxic and/or reflect formation of a different toxic metabolite. These results may have implications for interpreting anesthetic defluorination, volatile anesthetic use, and methods to evaluate anesthetic toxicity.

Effects of abused inhalants and GABA-positive modulators in dizocilpine discriminating inbred mice

There is in vitro evidence that some of the effects of abused volatile solvents may be produced by actions at the NMDA receptor. In addition, some solvents produce phencyclidine-like discriminative stimulus effects. The major goal of the present study was to further compare abused solvents to NMDA antagonists by testing them in two strains of mice trained to discriminate 0.17 mg/kg of the very selective uncompetitive NMDA antagonist, dizocilpine, from saline and contrast those results with several GABA(A)-positive modulators, PCP and ethanol. The results indicated that the discriminative stimulus produced by 0.17 mg/kg dizocilpine was highly specific in both mouse strains. PCP produced 91% dizocilpine-lever responding in C57BL/6J mice, but only 56% dizocilpine-lever responding in DBA/2J mice. Pentobarbital, midazolam and ethanol produced at least some overlap in discriminative stimulus effects with dizocilpine in one or both mouse strains. In contrast, toluene, 1,1,1-trichloroethane (TCE), xylene and methoxyflurane produced saline-appropriate responding almost exclusively. These data indicate that, at least under the specific conditions tested, abused volatile solvents do not have substantial dizocilpine-like discriminative stimulus effects in either C57BL/6J or DBA/2J mice, providing little support that NMDA antagonism plays a central role in the production of this abuse-related effect.

Fluorinated anesthetics differ in toxic effects on peritoneum and subjacent tissues

Intraperitoneal injection of fluorinated anesthetics produced anesthesia in rats. This was followed by toxic effects on peritoneal organs and surfaces, except for sevoflurane, which did not produce any lesions.

[Nephrotoxicity and fluoride from the viewpoint of the nephrologist]

Fluoride released from methoxyflurane (MOF) during its hepatic and extrahepatic metabolism has been regarded as the major culprit responsible for MOF-induced nephrotoxicity. In the isolated, perfused rat kidney model, admixture of 1500 mumol/l fluoride to the perfusate resulted in tubular and glomerular damage with concomitant anuria. Fluoride administration in Fischer 344 rats in vivo elicited a renal diabetes insipidus-like syndrome that had also been observed in patients after MOF anaesthesia. The renal concentrating defect is most probably due to both dissipation of the corticomedullary osmolality gradient in the interstitium and failure of water reabsorption due to ADH refractoriness of the distal tubular cells. Hypothetically, the underlying mechanism may be a fluoride-induced inhibition of enzymes involved in intracellular energy production such as ATPase or enolase. The degree of nephrotoxicity correlates loosely with maximal serum fluoride levels, but can probably be modulated by further factors like intrarenal in situ formation of fluoride, urinary pH and flow, and especially, the presence of other nephrotoxins. This mitigates the importance of maximal fluoride serum levels, especially the 50 mumol threshold, as predictors of clinically relevant nephrotoxicity. To date, no nephrotoxic effects of sevoflurane could be demonstrated.

Human kidney methoxyflurane and sevoflurane metabolism. Intrarenal fluoride production as a possible mechanism of methoxyflurane nephrotoxicity

BACKGROUND

Methoxyflurane nephrotoxicity is mediated by cytochrome P450-catalyzed metabolism to toxic metabolites. It is historically accepted that one of the metabolites, fluoride, is the nephrotoxin, and that methoxyflurane nephrotoxicity is caused by plasma fluoride concentrations in excess of 50 microM. Sevoflurane also is metabolized to fluoride ion, and plasma concentrations may exceed 50 microM, yet sevoflurane nephrotoxicity has not been observed. It is possible that in situ renal metabolism of methoxyflurane, rather than hepatic metabolism, is a critical event leading to nephrotoxicity. We tested whether there was a metabolic basis for this hypothesis by examining the relative rates of methoxyflurane and sevoflurane defluorination by human kidney microsomes.

METHODS

Microsomes and cytosol were prepared from kidneys of organ donors. Methoxyflurane and sevoflurane metabolism were measured with a fluoride-selective electrode. Human cytochrome P450 isoforms contributing to renal anesthetic metabolism were identified by using isoform-selective inhibitors and by Western blot analysis of renal P450s in conjunction with metabolism by individual P450s expressed from a human hepatic complementary deoxyribonucleic acid library.

RESULTS

Sevoflurane and methoxyflurane did undergo defluorination by human kidney microsomes. Fluoride production was dependent on time, reduced nicotinamide adenine dinucleotide phosphate, protein concentration, and anesthetic concentration. In seven human kidneys studied, enzymatic sevoflurane defluorination was minima, whereas methoxyflurane defluorination rates were substantially greater and exhibited large interindividual variability. Kidney cytosol did not catalyze anesthetic defluorination. Chemical inhibitors of the P450 isoforms 2E1, 2A6, and 3A diminished methoxyflurane and sevoflurane defluorination. Complementary deoxyribonucleic acid-expressed P450s 2E1, 2A6, and 3A4 catalyzed methoxyflurane and sevoflurane metabolism, in diminishing order of activity. These three P450s catalyzed the defluorination of methoxyflurane three to ten times faster than they did that of sevoflurane. Expressed P450 2B6 also catalyzed methoxyflurane defluorination, but 2B6 appeared not to contribute to renal microsomal methoxyflurane defluorination because the P450 2B6-selective inhibitor had no effect.

CONCLUSIONS

Human kidney microsomes metabolize methoxyflurane, and to a much lesser extent sevoflurane, to fluoride ion. P450s 2E1 and/or 2A6 and P450 3A are implicated in the defluorination. If intrarenally generated fluoride or other metabolites are nephrotoxic, then renal metabolism may contribute to methoxyflurane nephrotoxicity. The relative paucity of renal sevoflurane defluorination may explain the absence of clinical sevoflurane nephrotoxicity to date, despite plasma fluoride concentrations that may exceed 50 microM.

Effect of repeated administration of novel stressors on central beta adrenoceptors

Subtypes of beta adrenoceptors were measured in 17 different areas of brain in rats exposed for 12 days to novel stressors. Mild stress such as individual housing and handling caused no change in beta 1 and beta 2 adrenoceptors in comparison with that measured in rats that were group housed and never handled. Exposure of rats to more severe stressors did reduce significantly the binding of 125I-iodopindolol (125I-IPIN) to beta 1 adrenoceptors, but not beta 2 adrenoceptors, only in the lateral and basolateral nuclei of the amygdala.

The effect of euthanasia technique on vascular arachidonic acid metabolism and vascular and intestinal smooth muscle contractility

This study was designed to determine the effects that specific euthanasia methods have on vascular arachidonic acid metabolism and vascular and intestinal smooth muscle contractility. Rats were euthanatized by decapitation (DC), pentobarbital overdose (PB), or anesthesia with CO2, methoxyflurane or ether followed by DC (CO2-DC, Met-DC, Ether-DC, respectively). Rabbits were killed by a similar protocol, but CO2 overexposure replaced Ether-DC. The rat and rabbit aortas produced mainly 6-keto PGF1 alpha, the prostacyclin metabolite, and lesser amounts of PGE2. No qualitative differences were seen in arachidonate metabolites. However, aortic tissue from rabbits and rats killed by Met-DC produced more prostacyclin. In contrast, aorta from rabbits euthanatized by CO2-DC produced less prostacyclin than controls, whereas aorta from rats killed in the same way yielded greater amounts of prostacyclin. Aortic tissue from rabbits killed by Met-DC and CO2-OD was less responsive to acetylcholine (ACH). Intestinal contractility to ACH was increased in rabbits when Met-DC was used as the method of euthanasia, while colon from rats sacrificed by Met-DC showed decreased responsiveness to ACH. Colon from rats killed by intraperitoneal PB exhibited altered contractility to ACH and norepinephrine. The results of this study show that methoxyflurane, carbon dioxide (rabbit) and pentobarbital (rat) alter the vascular synthesis of prostacyclin and smooth muscle contractility. We conclude that the method of euthanasia affects certain physiologic parameters and careful consideration should be given to the selection of a particular euthanasia technique.

Similarities of toluene and o-cresol neuroexcitation in rats

Exposure to high concentrations of toluene vapors, or to intravenous o-cresol (a toluene metabolite) at about 0.9 mg/min, caused excitation of the somatosensory evoked potential (SEP) and EEG of Fischer 344 rats. SEP excitation was characterized by a large increase in a positive waveform at about 20-50 msec. Prolonged exposure to either compound caused numerous oscillations to appear from 20 msec to the end of the recording (150 msec). Both substances induced an increase in EEG beta activity and caused a large increase in activity at 5 Hz. Toluene exposed rats were lightly anesthetized, while o-cresol rats were conscious but hyperreactive. If exposure was continued, both sets of rats had involuntary muscle movements and tremors. Benzoic acid and hippuric acid, also metabolites of toluene, were similarly tested. Neither caused neuroexcitation (about 2.4 mg/min IV, 144 mg total dose). It was concluded, therefore, that metabolically derived cresols are plausible candidates for the neuroexcitatory properties of toluene.

The effect of cimetidine on anesthetic metabolism and toxicity

Because the H2-receptor antagonist cimetidine has been shown to inhibit drug metabolism, the effects of cimetidine on anesthetic metabolism and toxicity were investigated in a rat model. Cimetidine decreased inorganic plasma fluoride production after methoxyflurane administration both in 21% oxygen (P less than 0.001) and in 100% oxygen (P less than 0.001). Phenobarbital produces an increased fluoride formation after methoxyflurane anesthesia, and this fluoride formation is also reduced by cimetidine (P less than 0.005). There was no significant difference between the plasma fluoride levels in rats anesthetized with halothane or enflurane. Although cimetidine inhibited the in vivo defluorination of methoxyflurane, fluoride levels were still within the nephrotoxic range, and cimetidine is not likely to play a role as part of a preanesthetic regimen that would permit the increased clinical use of methoxyflurane. Cimetidine also inhibited the oxidative metabolism of halothane; cimetidine decreased (P less than 0.05) trifluoroacetic acid concentrations after halothane anesthesia in 21% oxygen and in 100% oxygen and decreased (P less than 0.05) bromide concentrations after halothane anesthesia in 100% oxygen. Trifluoroacetic acid levels were less (P less than 0.02) after halothane anesthesia in 14% oxygen as compared with 100% oxygen, indicating a reduction in oxidative metabolism under hypoxic conditions. However, bromide concentrations were maximal after halothane anesthesia in 21% oxygen, and significantly (P less than 0.001) less after halothane anesthesia in 14% and 100% oxygen. Bromide production, therefore, seems to be inhibited by both hypoxia and hyperoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultra-long-duration local anesthesia produced by injection of lecithin-coated methoxyflurane microdroplets

This study was designed to evaluate a new drug delivery system. The authors undertook to determine if microdroplets prepared by encapsulating volatile anesthetics with a membrane of lecithin could be used for local anesthesia. Local anesthesia was determined by monitoring the response of the rat to tail clamping and electrical stimulation of the skin following the intradermal injection of the microdroplets. Microdroplets were prepared from isoflurane, enflurane, halothane, methoxyflurane, diethyl ether, chloroform, and heptane. Although all microdroplet preparations produced local anesthesia, only methoxyflurane microdroplets produced an ultra-long duration of local anesthesia (approximately 24 h). Further characterization of the methoxyflurane microdroplets revealed two important differences from conventional local anesthetics. First, the local anesthetic effect of methoxyflurane reached a plateau that did not change significantly for 20 h while the injection of lidocaine and bupivacaine resulted in a peak effect that returned to baseline within 1 and 3 h, respectively. Second, the anesthetic effect of methoxyflurane remained essentially localized to the site of injection, while the anesthetic effect of lidocaine and bupivacaine migrated 15 cm in less than 1 h. The toxicity and safety of methoxyflurane were evaluated. When administered over the dosage range 1-16% (v/v) intradermally, or by injections into muscle, or by repeat injections every 4 days for 16 days, all animals regained their pretreatment response to painful stimulations, and there was no evidence of gross injury to tissue. Deliberate intravenous injection of 0.8 ml of 6.7% (v/v) methoxyflurane microdroplets had no apparent anesthetic or toxic effect. The present study demonstrates that methoxyflurane microdroplets produce an anesthetic effect that is highly localized, stable in intensity, ultra-long in duration, and reversible.

Comparative effects of halothane, enflurane, and methoxyflurane on the incidence of abnormal development using sea urchin gametes as an in vitro model system

The incidence of sea urchin embryos developing abnormally after their exposure to equimolar concentrations of halothane, enflurane, and methoxyflurane has been determined. Halothane concentrations in the 0.6-1.25 mM range caused 18-96% of the embryos to undergo abnormal cleavage at the first cell division. This is important because embryos exhibiting atypical cleavage patterns at the first cell division eventually involute and die before reaching gastrulation. Over the same range of concentrations, enflurane and methoxyflurane have minimal effects on development. However, when exposed to 2.5 mM methoxyflurane, nearly 40% of the cells did not fertilize. These results show that volatile anesthetic agents have decidedly different effects on development and suggest that the incidence of abnormal development may not correlate directly with the anesthetic potency of inhalational agents.

Hepatic and renal effects of prolonged exposure of rats to 50 p.p.m. methoxyflurane

Male Fischer 344 rats were exposed to air or to 50 p.p.m. methoxyflurane vapour for a period of 14 weeks. At the end of this period, half of the rats in each group were killed; the remainder breathed air only for a further four weeks (recovery period) before being killed. During the exposure period, growth of the methoxyflurane-exposed rats was markedly depressed, though food consumption was similar in the two groups. Both water consumption and urine volume were increased by methoxyflurane, possibly due to the nephrotoxic effect of fluoride, the concentration of which exceeded 50 micromolar in the sera of all exposed rats. At the end of the exposure period, livers of all exposed rats, but no controls, showed focal hepatocellular degeneration and necrosis, and evidence of liver cell regeneration. Fatty change was prominent. During the recovery period, water consumption and urine volumes returned to near-normal levels. At the end of the recovery period, focal necrosis was still observed in the livers, although fatty change was no longer present. No histological abnormalities were observed in the kidneys of any rats.

Deuterated methoxyflurane anesthesia and renal function in Fischer 344 rats

Inorganic fluoride (F-) production and renal function were assessed in six groups of Fischer 344 rats administered either methoxyflurane (MOF) or deuterated methoxyflurane (d4-MOF). One untreated and one phenobarbital (PB)-treated group were exposed for two hours to either air, 0.5 per cent (V/v) MOF, or 0.5 per cent (v/v) d4-MOF. Serum and urinary F- and serum urea nitrogen and creatinine were measured. Urine volume and urinary F- excretion were only slightly greater among MOF than among d4-MOF exposed animals. Pretreatment with PB, however, greatly enhanced F- production in MOF-exposed animals leading to marked renal impairment but only slightly enhanced F- production in d4-MOF animals leading to mild renal impairment. Thus, only in PB-pretreated animals could a biologically significant difference in nephrotoxicity be demonstrated for MOF and d4-MOF.

Morphologic changes in mouse spermatozoa after exposure to inhalational anesthetics during early spermatogenesis

The authors studied anesthetic mutagenesis following exposure in vivo by use of an adaptation of the mouse spermatozoa morphology assay of Wyrobek and Bruce. The epididymal spermatozoa of (C57B1/C3H)F1 mice were examined for morphologic abnormalities following exposure to near-0.1 MAC and greater concentrations of general anesthetics. Twenty exposure hours (4 hr/day x 5 days) were conducted for nitrous oxide, diethyl ether, chloroform, trichlorethylene, halothane, methoxyflurane, enflurane, and isoflurane, each at two concentrations. Twenty-eight days after exposure, epididymal spermatozoa were examined. Statistically significant increases in the percentages of abnormal spermatozoa were found for chloroform, trichloroethylene, and enflurane, compared with controls. These data suggest that direct examination of reproductive cells following exposure to general anesthetics in vivo may be useful in the investigation of the genetic toxicities of these compounds.

Metabolic activation of nephrotoxic haloalkanes

Worldwide industrialization and environmental pollution have increased the incidence of human exposure to halogenated aliphatic hydrocarbons, many of which are injurious to the mammalian kidney. Evaluation of human risk from haloalkane exposure requires knowledge about the mechanisms of the nephrotoxic effects of these agents so that appropriate animal models of human response can be developed. Recent studies indicate that nephropathy following methoxyflurane (2,2-dichloro-1,1-difluoroethyl methyl ether) anesthesia is caused by hepatic enzymatic release of inorganic fluoride ion, a nephrotoxic component of the parent molecule. Thus, the toxic effect is dependent upon hepatic metabolism of methoxyflurance. Acute chloroform injury to the kidney also may be caused by a toxic metabolite. In this case, however, the metabolite is most likely produced within the kidney. Chloride ion is relatively innocuous, suggesting that a carbon fragment of chloroform is the nephrotoxic agent. These results indicate that haloalkane metabolism, both renal and hepatic, can be important determinants of haloalkane nephropathy.

Developmental toxicity of methoxyflurane in mice

The developmental toxicity of trace, subanesthetic, and anesthetic exposure to methoxyflurane was examined in Swiss/ICR mice. No adverse effects on reproduction or fetal development were demonstrated following exposure to trace (2 ppm) and subanesthetic (60 ppm) concentrations of methoxyflurane for 4 hours daily on days 6 through 15 of pregnancy. Exposure to an anesthetic concentration (2000 ppm; 0.2%) for the same period resulted in decreased fetal weight, decreased ossification, and delayed renal maturation. Additionally, the incidence of minor skeletal anomalies was increased. It is concluded that gestational exposure of mice to trace of subanesthetic concentrations of methoxyflurane does not result in reproductive loss or morphologic abnormalities in their offspring.

The effect of exposure to halogenated anaesthetics on liver glutathione levels in rats. An index of hepatotoxicity

Having studied anaesthetic drug interactions in rats, we report the effects of halogenated anaesthetics on the liver glutathione levels and histology, as well as the results of the enhancement of these effects by microsomal enzyme induction. The anaesthetic agents studied included methoxyflurane, halothane, ethrane, chloroform and fluroxene. While exposure of rats to methoxyflurane, helothane and ethrane produced no significant changes in hepatic glutathione levels, or in liver histology, exposure to chloroform and fluroxene produced marked depression of liver glutathione, especially after microsomal enzyme induction. Furthermore, rats exposed to thses agents after enzyme induction developed gross centrilobular necrosis and died. It is suggested that the study of the effects of any new anaesthetic agent on liver glutathione levels could be a valuable screening test of its hepatotoxic potential, before its clinical trial.

Hepatocyte responses to volatile anesthetics: changes in surface scanning and enzyme leakage

The effects of volatile anesthetics were assessed in freshly isolated rat hepatocytes by surface-scanning electron microscopy and by measuring leakage of cellular enzymes, lactate dehydrogenase and beta-glucuronidase into the surrounding medium. The order of potency in regard to their capacity to produce alterations of these parameters was halothane = methoxyflurane greater than ether = control. The extent of enzyme leakage from hepatocytes exposed to halothane or methoxyflurane was both dose dependent and, for the first 30 minutes, time dependent. Surface scanning of the isolated hepatocytes showed that both halothane and methoxyflurane produced enzyme leakage and morphologic changes in cellular membranes, but ether did not. These studies demonstrate that scanning electron microscopy and enzyme leakage from cells are useful for the evaluation of drug-induced changes in lever cells in vitro. The relation between these drug-induced changes and clinical hepatotoxicity remains to be elucidated.

A test of the carcinogenicity of enflurane, isoflurane, halothane, methoxyflurane, and nitrous oxide in mice

We exposed Swiss ICR mice for 2-hour periods to 1/32, 1/8 and/or 1/2 MAC enflurane, halothane, isoflurane, methoxyflurane, or N2O both in utero during the last 1/2 of pregnancy (4 exposures at 2-day intervals) and after delivery (24 exposures at 2-to-3-day intervals). Anesthetics were delivered in air or in O2. Thus, 1973 mice were exposed and examined after 15 months of life for the development of neoplastic lesions. Neoplastic lesions (principally pulmonary adenomas, lymphomas, hepatocyte lesions, liver vascular lesions) were found in all treatment and control sets. There was no indication that a specific anesthetic or anesthetic dose was carcinogenic. Our results do not confirm the suggestion that isoflurane is a hepatocarcinogen, nor do our data suggest that the modern inhaled anesthetics pose a significant threat of carcinogenicity.

Fetotoxicity in rats following chronic exposure to halothane, nitrous oxide, or methoxyflurane

An animal model was used to investigate the comparative fetal toxicities of three inhalational anesthetics. Pregnant Sprague-Dawley rats were exposed for eight hours a day throughout the 21 days of gestation to graded concentrations of halothane (0.16-0.32 per cent), or nitrous oxide (1-50 per cent), or a nitrous oxide (10 per cent) and halothane (0.16 per cent) mixture, or methoxyflurane (0.01-0.08 per cent). High subanesthetic concentrations of all the inhalational anesthetics could cause fetal growth retardation (e.g., 3-21 per cent decreases in normal fetal weights), but this was unaccompanied by significant fetal loss (overall rate: 4.8 +/- 1.2 per cent, mean +/- SE, in anesthetic groups) or any evidence of skeletal or gross abnormalities related to treatment. It is concluded that these rodent studies do not implicate any specific inhalational anesthetic agent in fetal toxicity, and that the effects of additional factors, such as stress, must be considered.

Toxicity following methoxyflurane anaesthesia. IV. The role of obesity and the effect of low dose anaesthesia on fluoride metabolism and renal function

Seven obese and five normal weight patients were studied before, during and after one hour of methoxyflurane-nitrous oxide anaesthesia during peripheral surgical operations and compared with eight patients of normal weight anaesthetized with nitrous oxide-meperidine and d-tubocurare. Estimates were made of renal function, including serum and urinary electrolytes, osmolarity, uric acid, urea and creatinine. Renal clearances for the latter three substances were also calculated. Serum and urinary inorganic and organic fluoride concentrations were measured, as were renal clearances. This low dose methoxyflurane anaesthesia resulted only in a decrease in uric acid clearance among all the measures, when compared to the meperidine-nitrous oxide controls. The clearance of uric acid remained depressed for longer in the obese patients, but otherwise they did not differ from the normal weight patients. It is possible but not proven that depressed uric acid clearance may be related to the organic fluoride metabolite and an early indicator of methoxyflurane renal toxicity. The previously documented biotransformation of methoxyflurane was seen in this study. A double peak in serum inorganic fluoride was shown in all patients but one. Rather large differences in peak levels of serum inorganic fluoride occurred. The only significant difference between the obese and normal weight patients as far as fluoride metabolism was concerned was a greater variability in the serum inorganic fluoride levels in the obese patients. It would appear that the obese patient metabolizes methoxyflurane in a quantitatively if not qualitatively different fashion than the normal weight patient, perhaps because of fatty infiltration of the liver. Caution is advised in the use of methoxyflurane for more than 90 minutes of low concentration administration in view of the unpredictability of the biotransformation.

Methoxyflurane nephropathy

Investigations of methoxyflurane-induced nephrotoxicity in man have been extensively aided by the use of an animal model. To be of value the animal model must share similar metabolic pathways with man and have the same clinical manifestations of the diseases process. The Fischer 344 rat appears to meet these criteria. The predominant factors in the production of methoxyflurane nephrotoxicity appear to be high methoxyflurane dosage and serum inorganic fluoride concentration. It is likely that secondary factors include: (1) a high rate of methoxyflurane metabolism and sepsitivity of the kidney to inorganic fluoride toxicity: (2) concurrent treatment with other nephrotoxic drugs; (3) preexisting renal disease; (4) surgery of the urogenital tract, aorta, or renal vasculative; (5) repeat administration of methoxyflurane due to accumulation of inorganic fluoride and, perhaps, methoxyflurane induction of its own metabolism: and (6) concurrent treatment with enzyme-inducing drugs such as phenobarbital.