Isoflurane or halothane for cesarean section: comparative maternal and neonatal effects

The maternal and neonatal effects of isoflurane and halothane combined with 50% N2O - 50% O2 were compared in 60 healthy parturients undergoing primary or repeat cesarean section. All patients had rapid sequence induction of anesthesia with sodium thiamylal 4 mg/kg followed by succinylcholine for tracheal intubation. Patients were randomly assigned to one of three groups of 20 each (inspired 0.5% isoflurane, 1% isoflurane or 0.5% halothane), combined with 50% N2O and O2. After delivery, 67% N2O in O2 was used, supplemented by butorphanol. Maternal blood loss did not differ significantly among the three groups and none of the patients developed intraoperative awareness. At the time of delivery, maternal plasma epinephrine levels were significantly above preinduction levels in the 0.5% isoflurane group but unchanged in the other two groups. Neonatal status as ascertained by Apgar scores, cord acid base status and the Neurologic and Adaptive Capacity Scores (NACS) was equally good in the three groups of patients. Serum inorganic fluoride concentrations in the mother after anesthesia were not significantly above preanesthetic levels in any of the groups and there was no biochemical evidence of renal toxicity. In all neonates fluoride ion concentrations in the first voided urine sample were less than 7 mumol/l, a value well below that associated with nephrotoxicity. It is concluded that isoflurane is a safe supplement to N2O - O2 mixture for cesarean section and is a safer alternative to halothane in situations when patients receiving beta-adrenergic therapy require cesarean section since halothane might potentiate arrhythmias caused by beta adrenergic agonists.

Solubility of I-653, sevoflurane, isoflurane, and halothane in human tissues

Tissue/blood partition coefficients of anesthetics are important indicators of the rate of tissue wash-in and wash-out, and wash-in and wash-out are determinants of the rates of induction of and recovery from anesthesia. In the present study of human tissues, we found that the tissue/blood partition coefficients (for brain, heart, liver, kidney, muscle, and fat) for the new anesthetic I-653 were smaller than those for isoflurane, sevoflurane, and halothane (anesthetics listed in order of increasing tissue/blood partition coefficients). For example, the respective brain/blood partition coefficients were 1.29 +/- 0.05 (mean +/- SD); 1.57 +/- 0.10; 1.70 +/- 0.09; and 1.94 +/- 0.17. This indicates that induction of and recovery from anesthesia with I-653 should be more rapid than with the other agents. The finding of a lower tissue/blood partition coefficient for I-653 parallels the previous finding of a lower blood/gas partition coefficient.

Atracurium, vecuronium, and pancuronium do not alter the minimum alveolar concentration of halothane in humans

The authors studied 64 unpremedicated, healthy surgical patients, aged 42 +/- 14 yr, to determine the effects of atracurium, vecuronium, and pancuronium on the minimum alveolar concentration (MAC) of halothane. Anesthesia was induced using halothane/nitrous oxide/oxygen via a mask without the administration of other drugs. Nitrous oxide was discontinued, the trachea was intubated without prior administration of neuromuscular blocking drugs, and anesthesia was maintained with halothane in oxygen. Participating patients were assigned to one of five groups: 1) no neuromuscular blocking drug (control group, n = 9); 2) atracurium 0.5 mg/kg (n = 10); 3) atracurium 1.0 mg/kg (n = 15); 4) vecuronium 0.1 mg/kg (n = 20); or, 5) pancuronium 0.1 mg/kg (n = 10). Tourniquets, inflated to 300 mmHg immediately before iv administration of neuromuscular blocking drug and 15-30 min prior to skin incision, were used to isolate extremities from circulating neuromuscular blocking drug in all patients. A positive response to stimulation was defined as movement of at least one extremity occurring distal to the tourniquet within 1 min following skin incision. The first patients in the control and atracurium groups were studied at an end-tidal halothane concentration of 0.95%. The first patient in the pancuronium group was studied at a halothane concentration of 0.75%, and the first patient in the vecuronium group at 0.70%. Subsequent patients were studied at end-tidal halothane concentrations 0.10% above or below that of the preceding patient, depending on the presence or absence of movement with skin incision. Control MAC for halothane was 0.74% +/- 0.09% (mean +/- SEM).(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of halothane-induced antigen in situ by specific anti-halothane metabolite antibodies

Multiple or single halothane exposure of rabbits or guinea pigs induces an antibody reactive with trifluoroacetylated (TFA) proteins. The antigen that initiates this immune response was investigated in halothane-exposed rabbits and guinea pigs for its anatomical location in the liver, the chronology of its expression in situ and exposure conditions which would modulate its expression. Using an immuno-staining technique, binding by an anti-TFA antibody to the antigen was detected in liver tissue from all halothane-exposed rabbits and guinea pigs. Antigen could be detected only in the centrilobular area around the central vein where staining intensity was concentrated in an area seven to nine cells deep. In halothane-exposed rabbits, the appearance of TFA antigen was most predominant on the first and second days following a single exposure. Multiple exposures induced TFA antigen in a larger area around the central vein than did a single exposure. Though maximal expression of TFA antigen occurred following two or three exposures, subsequent exposures did not potentiate antigen expression. In halothane-exposed guinea pigs, exposure to deuterated halothane, which reduces the extent and metabolites of oxidative halothane metabolism, elicited the appearance of TFA antigen around the central veins, although to a lesser extent than during halothane exposure. Halothane-induced antigen was evident in guinea pigs as early as 6 h post-exposure and was still apparent 90 h later. Thus, halothane exposure by inhalation elicits the appearance of TFA protein conjugates which may, in turn, evoke the anti-TFA immune response.

Halothane-induced hepatic microsomal lipid peroxidation in guinea pigs and rats

Halothane-induced hepatic microsomal lipid peroxidation in guinea pigs and rats was examined with respect to the mixed function oxidase system, anaerobic dehalogenation activity of halothane, and the antioxidant system. The levels of cytochrome P-450 and NADPH-cytochrome P-450 reductase were significantly higher in guinea pigs than in rats. There was no difference between the two animals in anaerobic dehalogenation activity of halothane per cytochrome P-450 in microsomes. Microsomal alpha-tocopherol was significantly lower in guinea pigs than in rats, and was increased by multiple exposure to halothane in guinea pigs but remained lower than in rats. Microsomal alpha-tocopherol was decreased in rats by multiple exposure. The concentration of reduced glutathione and ascorbic acid was decreased significantly by multiple exposure to halothane in guinea pigs but not in rats. These results suggest that the higher level of halothane-induced hepatic microsomal lipid peroxidation in guinea pigs is due to the large production of radical metabolites resulting from the large amounts of cytochrome P-450, the high activity of NADPH-cytochrome P-450 reductase, and the low concentration of microsomal alpha-tocopherol.

Determination of halothane distribution in the rat head using 19F NMR technique

The distribution of halothane in the rat head was examined with 19F NMR rotating-frame zuegmatography and 2DFT 19F NMR imaging. The rotating frame experiments were conducted at varying times following anesthesia to assess the time dependence of the halothane distribution. The results of these experiments demonstrate that halothane and halothane metabolite are unequally partitioned through the rat tissues examined. 19F spin-echo imaging experiments were conducted immediately following anesthesia. The results of these experiments are compared with those of the spectroscopic technique.

Halothane, enflurane, and isoflurane decrease calcium sensitivity and maximal force in detergent-treated rat cardiac fibers

This study was designed to test the hypothesis that the volatile anesthetics directly affect cardiac contractile proteins. For this purpose, the effects of various anesthetic doses of halothane, enflurane, and isoflurane on myocardial calcium sensitivity and maximal calcium-activated force were examined in rat cardiac fibers skinned with Triton X-100. In this preparation, all membranes are chemically destroyed, and the sarcoplasmic reticulum is not functional. The three anesthetics shifted the pCa/tension curves (pCa = -log10[Ca2+]) toward higher calcium concentrations and decreased pCa for half-maximum activation (pCa50) in a dose-dependent and reversible fashion without changing the slope of this relationship (Hill coefficient). No differences between agents were observed at equipotent anesthetic concentrations. In addition, the three anesthetics decreased both maximal activated tension and tension at half-maximal activation in a dose-dependent fashion. Both the decrease in calcium sensitivity and the decrease in maximum activated tension may contribute to the negative inotropic effects of these agents. The relative importance of such effects compared with the other mechanisms of action remains to be determined, however.

Halothane interaction with guanine nucleotide binding proteins in mouse heart

Volatile anesthetics have been shown to decrease hormone-induced adenosine cyclic monophosphate (cAMP) formation and to increase guanosine cyclic monophosphate (cGMP) content in mouse ventricular myocardium. Hormone-induced inhibition of adenylate cyclase, the enzyme that synthesizes cAMP, and the cGMP response to alpha adrenergic agonists are mediated by a guanine nucleotide binding protein (N) sensitive to pertussis toxin. To evaluate the involvement of N proteins in the action of halothane on cyclic nucleotides in the heart, mice were pretreated with pertussis toxin, 50 micrograms/kg, ip, 72 h prior to exposure to halothane, 1.2 vol%. Pretreatment with the toxin decreased the cGMP response to halothane by 65% but was without effect on the decrease in myocardial cAMP induced by the anesthetic. The results indicate that a functionally active pertussis toxin-sensitive N protein is involved in the cGMP response to halothane, but not in the cAMP response.

A gas chromatographic mass spectrometric method for the analysis of trifluoroacetic acid: application to the metabolism of halothane by in vitro preparations

A selected ion monitoring gas chromatographic mass spectrometric assay for trifluoroacetic acid was developed for the study of the metabolism of the volatile anesthetic agent halothane by in vitro preparations. The assay uses a headspace sampling technique after formation of methyl esters with dimethyl sulfate and sulfuric acid. Pentafluoropropionic acid proved to be a suitable internal standard, although care is required in the preparation of the calibration standards so that they reflect the composition and treatment of the samples. Contamination of the samples, possibly with trifluoroacetic acid itself, was found to be the primary factor in limiting the sensitivity of the assay for the metabolism of halothane. Generally, trifluoroacetic acid could be determined at levels as low as 1 microM in 50-100 microliters of incubate.

Resonance Raman study of the cytochrome P-450 LM2-halothane intermediate complex

Resonance Raman (RR) and absorption spectroscopic studies of purified rabbit liver cytochromes P-450 show that the form 2 isomer (LM2) but not the form 4 isomer (LM4) forms a long-lived complex with halothane after dithionite reduction, absorbing light at 470 nm, in which ferric 6-coordinated heme iron in the low-spin configuration is liganded to 2-chloro-1,1-difluoroethylene. The RR data exclude the possibility that the CF3CHCl- carbanion is a ligand and are consistent with the involvement of an active-site pocket in the cytochrome P-450 polypeptide.

Characterization of halothane oxidation by hepatic microsomes and purified cytochromes P-450 using a gas chromatographic mass spectrometric assay

A sensitive assay for trifluoroacetic acid, the major product of the oxidative metabolism of halothane, has been developed to study the biotransformation of halothane. A selected ion monitoring gas chromatographic mass spectrometric assay measured trifluoroacetic acid levels as low as 1 microM in 100 microliter of reaction mixture. This assay was used to quantitate halothane metabolism in human and rabbit microsomal systems and with purified proteins. Trifluoroacetic acid production was examined as a function of the concentration of substrate present, the amount of microsomal protein used and the length of reaction time. Halothane metabolism in microsomes was linear for at least 30 min, and up to a microsomal protein concentration of 1 mg/ml. In rabbits, phenobarbital and imidazole induced the microsomal metabolism of halothane 7.36- and 18.2-fold, respectively. Imidazole was used because it is a potent inducer of cytochrome P-450 isozyme 3a which is also induced by ethanol. The cytochrome P-450 in microsomes from a single human subject metabolized halothane at a rate comparable to that found in microsomes from phenobarbital- and imidazole-pretreated rabbits. The purified phenobarbital and imidazole inducible cytochromes P-450, isozymes 2 and 3a, catalyzed the oxidation of halothane to trifluoroacetic acid. Cytochrome b5 stimulated the isozyme 3a-catalyzed oxidation of halothane by 19-fold, whereas isozyme 2 catalyzed oxidation was increased 4.3-fold. Antibodies to cytochrome P-450 3a inhibited halothane metabolism by 90% in microsomes from imidazole-pretreated rabbits, suggesting that isozyme 3a catalyzes halothane metabolism in imidazole-pretreated rabbits. In conclusion, the oxidation of halothane to trifluoroacetic acid by cytochrome P-450 isozymes 3a and 2 is enhanced markedly by cytochrome b5.

Comparison of covalent binding from halothane metabolism in hepatic microsomes from phenobarbital-induced and hyperthyroid rats

1. Hepatic microsomal suspensions from rats pretreated with saline, phenobarbital or triiodothyronine were incubated with 14C-halothane under aerobic and anerobic conditions. 2. Metabolism of halothane by microsomes from phenobarbital-induced rats under anaerobic conditions resulted in covalent binding of 14C to microsomal lipids, and to a lesser extent, microsomal proteins, as seen in previous studies. Covalent binding was decreased with incubation under aerobic conditions. 3. Metabolism of halothane by microsomal suspensions from hyperthyroid rats produced much less covalent binding to microsomal lipids and proteins, with binding similar to, or less than, that observed with microsomes from saline-treated rats. The covalent binding of halothane to protein of microsomes from hyperthyroid rats was dependent upon metabolism, and was inhibited by SKF 525A, reduced glutathione, or cytosol. 4. The in vitro observations with respect to covalent binding are inconsistent with previous reports on halothane hepatotoxicity in hyperthyroid rats in vivo. This inconsistency and the relatively small extent of covalent binding with microsomes from hyperthyroid rats observed, suggests that covalent binding is not an important mechanism of halothane hepatotoxicity in the hyperthyroid rat model.

Reductive metabolism of halothane by purified cytochrome P-450

The reductive metabolism of halothane was determined using purified RLM2, PBRLM4 and PBRLM5 forms of rat liver microsomal cytochrome P-450. The metabolites, 2-chloro-1,1,1-trifluoroethane (CTE) and 2-chloro-1,1-difluoroethylene (CDE), were determined. All three forms of cytochrome P-450 produced CTE with relatively small differences in its production among the various forms. There were major differences, however, in the production of CDE, with PBRLM5 being the most active. PBRLM5 was also the only form to show the development of a complex between halothane and cytochrome P-450. This complex absorbed light maximally at 470 nm. The complex formation and the production of CDE by PBRLM5 were stimulated by the addition of cytochrome b5. Cytochrome b5 had no effect on CDE production by PBRLM4 and inhibited the production of both CTE and CDE by RLM2. These results show that the two-electron reduction of halothane by cytochrome P-450 was catalyzed by the PBRLM5 form and that cytochrome b5 stimulated the transfer of the second electron to halothane through PBRLM5, but not RLM2 or PBRLM4.

I-653 resists degradation in rats

The ability of rats pretreated with phenobarbital to metabolize a new volatile anesthetic, I-653, was compared with the metabolism of halothane, isoflurane, and methoxyflurane. Each anesthetic was administered for 2 hours at 1.6 MAC (inspired). Control rats were given phenobarbital but not exposed to an anesthetic. In rats pretreated with phenobarbital and exposed to I-653, fluoride ion concentrations in serum and excretion of fluoride ion and organic fluoride in the urine were almost indistinguishable from values measured in control rats. In contrast, rats pretreated with phenobarbital metabolized small but significant amounts of isoflurane. In rats pretreated with ethanol and exposed to I-653, the 24-hour excretion of urinary organic fluoride was nearly ten times greater than that observed in control rats. Marked increases in organic fluoride (as high as 1000 times control values) and/or fluoride ion were found in serum and/or urine after anesthesia of phenobarbital-pretreated rats with halothane or methoxyflurane. The relative stability of I-653 indicates that it may possess minimal toxic properties.

Effects of piperonyl butoxide on halothane hepatotoxicity and metabolism in the hyperthyroid rat

A series of experiments were conducted to examine the potential role of phase I metabolism in halothane-induced liver injury in the hyperthyroid rat. The metabolism of halothane was determined in both hyperthyroid (triiodothyronine, 3 mg/kg per day, for 6 days) and euthyroid rats and in animals pre-treated with the cytochrome P-450 inhibitor piperonyl butoxide (75-100 mg/kg, i.p.). It was found that the hyperthyroid state, which is associated with a substantial increase in sensitivity to the hepatotoxic effects of halothane, decreases both oxidative and reductive routes of halothane metabolism in the rat. The production of trifluoroacetic acid (TFA), an oxidative metabolite, as well as that of chlorodifluoroethylene (CDF) and chlorotrifluoroethane (CTF), 2 reductive metabolites, was significantly reduced in hyperthyroid animals. Consistent with these findings serum and urinary bromide levels resulting from the formation of TFA, CDF or CTF were significantly reduced. The only route of halothane metabolism significantly increased by the hyperthyroid condition was the defluorination of halothane. Piperonyl butoxide administration did not render euthyroid animals sensitive to the halothane-induced hepatotoxicity and had no effect on the defluorination of halothane in euthyroid animals. However, piperonyl butoxide markedly increased the hepatotoxicity of halothane in hyperthyroid rats and, except for a modest increase in debromination reactions, decreased all measured indices of halothane metabolism including the defluorination of halothane. Thus, none of the observed changes in halothane metabolism produced by triiodothyronine or piperonyl butoxide treatment could be consistently correlated to the increases in hepatotoxicity linked to these 2 treatments. Based on these studies we suggest that the halothane hepatotoxicity induced in the hyperthyroid rat results from effects produced by either the parent compound or an as yet unidentified metabolite. In addition, these studies further demonstrate that considerable mechanistic differences exist for halothane-induced hepatotoxicity when comparing euthyroid and hyperthyroid animal models.

Rifampicin, halothane and glucose as mediators of lysosomal enzyme release and tissue damage

It is suggested that the important drugs rifampicin and halothane and the raised glucose levels in diabetes mellitus exert injurous effects on cells through a lysosomal mechanism. Further evidence is given of by time rifampicin induction of beta-glucuronidase and beta-N acetylglucosaminidase and its possible relation to hepatitis and pancreatitis. On the basis of preliminary data halothane may cause hepatitis connected to lysosomal enzyme release in the presence of other aggravating factors common to the perioperative period. The onset of diabetic vascular complications may be related to the similar raised levels of lysosomal enzymes found in insulin, drug and diet controlled disease. Release of these enzymes into plasma may be a marker of important changes in the lysosome, whether due to enzyme induction or damage, and could be a primary mechanism of many disease processes including some thought to be mainly autoimmune in character. Routine estimation in the clinical laboratory along with existing cytoplasmic and microsomally derived enzymes in the chemical screen would be a useful way of surveying lysosomal changes in the wide spectrum of disease in a general hospital.

Metabolic basis for a drug hypersensitivity: antibodies in sera from patients with halothane hepatitis recognize liver neoantigens that contain the trifluoroacetyl group derived from halothane

Previous studies have demonstrated that antibodies in sera from patients with halothane hepatitis recognize halothane-induced liver microsomal polypeptide neoantigens, and have suggested that these antibodies may play a role in the pathogenesis of the hepatitis. In the present study, the mechanism of neoantigen generation was investigated. Liver microsomes from rats treated in vivo with halothane or deuterated halothane were tested by immunoblotting for reactivity with patients' sera and with an antiserum specific for the covalently bound trifluoroacetyl (TFA) halide metabolite of halothane. Rat liver microsomes incubated aerobically or anaerobically with halothane or deuterated halothane in vitro, +/- NADPH and/or NADH, were also analyzed. The results obtained demonstrate that neoantigen expression involves oxidative halothane metabolism by cytochromes P-450 to TFA halide and covalent binding of the TFA group to the proteins. Incubation of microsomes from halothane-treated rats with 1 M piperidine cleaved the TFA groups from the proteins and abolished antigenicity, confirming this conclusion. Recognition of the neoantigens by the patients' antibodies was inhibited only partially using the hapten derivative N-E-TFA-L-lysine. It appears that the patients' antibodies recognize epitopes consisting of the TFA group plus associated structural features of the protein carriers (100 kDa, 76 kDa, 59 kDa, 57 kDa and 54 kDa), not the TFA hapten alone. To our knowledge, this constitutes the first characterization of drug metabolite-tissue protein neoantigens implicated in a drug hypersensitivity. The approach described may be of general utility for characterization of drug-induced neoantigens associated with other drug hypersensitivities.

Halothane: inhibition and activation of rat hepatic glutathione S-transferases

Multiple halothane anesthesias (1.25 MAC for 1 hr on 3 alternate days) of male Long-Evans rats initially decreased by up to 30% and subsequently increased to up to 185% liver cytosolic glutathione S-transferase activity toward 1-chloro-2,4-dinitrobenzene, 3,4-dichloro-1-nitrobenzene and trans-4-phenyl-3-buten-2-one and glutathione peroxidase activity. Halothane rapidly and reversibly activated hepatic cytosolic glutathione S-transferases and purified isoenzyme 1-2 but not isoenzymes 1-1 and 3-3. At high concentrations of halothane (ca. 22 mM), maximal activation was ca. 25%. Halothane, enflurane, isoflurane and methoxyflurane, but not the halothane metabolite 1-chloro-2,2-difluoroethylene, inhibited a mixture of liver cytosolic glutathione S-transferases with time (ca. 30% inhibition/15 min). The inhibition exhibited pseudo-first order kinetics (kobs = 0.13 min-1) and an I50 for halothane of greater than or equal to 15 mM. Halothane inhibited glutathione S-transferases 3-3, 3-4, and 4-4 by 50-60%, but did not affect isoenzymes 1-1 and 1-2. The ability of halothane to diminish hepatic glutathione S-transferase activity in vivo may in part reflect the time-dependent inhibition of glutathione S-transferase isoenzymes containing the 3- and 4-subunits.

Halothane hepatotoxicity and reductive metabolism of halothane in acute experimental liver injury in rats

Reductive metabolism of halothane was measured after acute liver injury induced by galactosamine (1.0 g/kg, IP) in rats. On the seventh day of liver injury, when previously elevated serum alanine aminotransferase levels had returned to near normal range, anaerobic release of fluoride from halothane by hepatic microsomes, which appears to reflect the reductive pathway of halothane metabolism, was still remarkably decreased (1.36 +/- 0.56 nmol/mg protein/h vs 5.88 +/- 0.58 in controls, P less than 0.001). In another set of experiments, rats (n = 8) given galactosamine 7 days earlier and saline-treated control rats were given halothane anesthesia (1.0%) under mildly hypoxic conditions (F1O2 0.14). In saline controls, halothane anesthesia resulted in a mild but statistically significant increase in serum alanine aminotransferase levels (32 +/- 4 vs 59 +/- 6 U/ml, P less than 0.001). In contrast, serum levels of this enzyme were not changed by halothane anesthesia in galactosamine-treated rats (45 +/- 3 vs 49 +/- 4 U/ml). Although care should be taken in extrapolating the importance of these animal data to humans, the results of this study suggest that halothane hepatotoxicity can be attenuated in the presence of minor liver injury as a result of decreased hepatic biotransformation of the anesthetic. The data support the view that halothane anesthesia is not necessarily contraindicated in subjects with impaired liver function.

Extrahepatic sites of metabolism of halothane in the rat

Rats were given 14C-halothane intravenously and whole-body autoradiography with freeze-dried sections, or with sections extracted in trichloroacetic acid, water, and organic solvents, was carried out to trace tissues accumulating halothane metabolites. In vitro incubations of tissue homogenates were performed to examine the capacity by the various organs to form tissue-bound 14C from the 14C-halothane. Autoradiography of isolated organs after incubation with 14C-halothane was performed to study the tissue localization of halothane metabolites formed under in vitro conditions. A localization of halothane metabolites was observed in several extrahepatic tissues in vivo, and the in vitro experiments showed a capacity by the same tissues to transform 14C-halothane to metabolites that bind strongly to tissue components. In addition to the liver, the other tissues shown to have a marked halothane-metabolizing capacity were the nasal mucosa, lateral nasal gland, mucosa of the tongue, cheek, soft palate (but not the hard palate), pharynx, larynx, oesophagus, and the tracheo-bronchial mucosa. The in vivo data obtained indicated a diffusion of the halothane over the walls of the large intestine and the caecum, followed by the formation of apparently reductive metabolites by intestinal microbes and a binding of the metabolites to the intestinal contents. The localization of halothane metabolites in the upper alimentary and respiratory pathways is correlated to the presence of cytochrome P-450 at these sites.

Metabolic activation of the halothane metabolite, [14C]2-chloro-1,1-difluoroethene, in hepatic microsomes

Halothane is reduced to 2-chloro-1,1,1-trifluoroethane (CTE) and 2-chloro-1,1-difluoroethene (CDE) by cytochrome P-450. These compounds may potentially undergo secondary metabolism in vivo, but their capacity to undergo metabolic activation and bind to macromolecules is unknown. This study, therefore, compared the abilities of CDE and CTE to bind to microsomal components in relation to that of halothane in hepatic microsomes. The results show that CDE, in addition to halothane, binds to microsomes under conditions of cytochrome P-450 activity. While halothane bound predominantly to lipids under nitrogen, CDE bound mainly to protein under oxygen. No CTE binding under any conditions could be detected. On an equimolar basis, CDE binding to protein was approximately one-third of that of halothane under oxidative conditions, however, CDE binding was enhanced in the presence of halothane. The results support the hypothesis that CDE metabolism may contribute to the metabolic binding due to halothane exposures.

Modulation of the reductive metabolism of halothane by microsomal cytochrome b5 in rat liver

To study the modulation of the reductive metabolism of halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) by microsomal cytochrome b5, formation of 2-chloro-1,1,1-trifluoroethane (CTE) and 2-chloro-1,1-difluoroethylene (CDE), major reduced metabolites of halothane, was analyzed in vivo and in vitro. Rats were pretreated with both malotilate (diisopropyl-1,3-dithiol-2-ylidenemalonate) and sodium phenobarbital (malotilate-treated rats) or only with sodium phenobarbital (control rats). The microsomes of malotilate-treated rats had significantly more cytochrome b5 than the controls, whereas the cytochrome P-450 content was not different between the two groups. At the end of 2-h exposure to 1% halothane in 14% oxygen, the ratio of CDE to CTE in arterial blood was significantly higher in malotilate-treated rats than in the controls. Under anaerobic conditions, the formation of CDE and the ratio of CDE to CTE were significantly greater in microsomal preparations of malotilate-treated rats than those of the controls. In a reconstituted system containing cytochrome P-450PB purified from rabbit liver, addition of cytochrome b5 to the system enhanced the formation of CDE and increased the ratio of CDE to CTE. These results suggested that cytochrome b5 enhances the formation ratio of CDE to CTE by stimulating the supply of a second electron to cytochrome P-450, which might reduce radical reactions in the reductive metabolism of halothane.

Halothane metabolism in cirrhotic rats

A rat model was used to determine whether the metabolism of halothane is changed in the presence of cirrhosis and whether exacerbation of liver dysfunction is correlated with such a change. Cirrhosis was produced by gavaging enzyme-induced male Wistar rats with carbon tetrachloride in corn oil once weekly for 12 weeks. Control rats received corn oil only. After a 3-week period without treatment, blood and urine were collected from each rat for determination of background levels of inorganic fluoride, bromide, and trifluoroacetic acid (halothane metabolites) and for assessment of liver function. Rats were then anesthetized with 1.05% halothane in 50% oxygen for 3 h. Following anesthesia, serial blood and urine samples were taken to monitor halothane metabolism and liver function. No differences were observed between cirrhotic and non-cirrhotic rats in serum levels and urinary excretion of halothane metabolites. However, serum levels of SGOT and SGPT were significantly increased about 1.5-fold in the noncirrhotic group and about 2.5-fold in the cirrhotic group after anesthesia. The increased levels observed in the cirrhotic group were significantly greater than in the noncirrhotic group. The results imply that the exacerbation of liver dysfunction after halothane anesthesia is most likely related to an indirect effect, such as change in liver blood flow, rather than to toxic metabolites.

Stimulatory effects of halothane and isoflurane on fluoride release and cytochrome P-450 loss caused by metabolism of 2-chloro-1,1-difluoroethene, a halothane metabolite

The structural similarity of the halothane metabolite, 2-chloro-1,1-difluoroethene (CDE), to haloethenes that are metabolized by and inactivate cytochrome P-450, suggests that CDE may undergo secondary metabolism and degrade these isozymes. This possibility was examined in hepatic microsomes by determining fluoride release and cytochrome P-450 loss due to CDE metabolism in the presence of several anesthetics. CDE alone decreased cytochrome P-450 from phenobarbital-treated rats by as much as 37%, but the addition of isoflurane or halothane to incubations containing CDE increased the loss of cytochrome P-450 nearly twofold. Fluoride release was enhanced approximately 2.5 to 3 times by halothane or isoflurane; however, fluroxene inhibited fluoride release and did not enhance the loss of cytochrome P-450. Extrapolation of these results to the clinical situation suggests that the metabolism of CDE produced during halothane anesthesia and the accompanying cytochrome P-450 loss may contribute to the inhibition of drug metabolism produced by halothane.

Equilibration of halothane with brain tissue in vitro: comparison to brain concentrations during anesthesia

A method was devised for reproducing anesthetic concentrations of halothane in slice and membrane preparations of rat brain in vitro. Rats were anesthetized with varying concentrations of halothane, responsiveness was tested, and brain halothane content was determined by heptane extraction and gas chromatography. The inspired concentration of halothane at which half of all animals were unresponsive was 1.05%. At 1.25% halothane, all animals were unresponsive and brain halothane was determined to be 41 +/- 1.3 nmol/mg lipid. No significant differences in halothane concentration between whole brain and a variety of brain regions were detected. To obtain similar concentrations in vitro, membranes or slices of cerebral cortex were incubated in Krebs-Ringer bicarbonate buffer (KRB) that had been preequilibrated with anesthetic. Halothane equilibrated rapidly with the buffer and the tissues. The partition coefficient between gas and KRB was found to be 0.78, and between brain slices and KRB approximately 12. Slightly lower gas concentrations were necessary in vitro than in vivo to obtain the same tissue levels of anesthetic. Using this method, it was shown that there was no effect of anesthetic concentrations of halothane on the uptake of [3H]norepinephrine or [3H]choline into slices of rat cerebral cortex.

Halogenated anesthetics increase oxygen consumption in isolated hepatocytes from phenobarbital-treated rats

Using suspensions of hepatocytes isolated from phenobarbital-treated and untreated rats (+PB cells and -PB cells, respectively), the authors examined the effects of halothane, enflurane, and isoflurane on O2 consumption (VO2) and on extracellular PO2 and energy status at steady states of O2 and energy metabolism. In +PB cells, all three agents produced increases in VO2 which were largest at 1 MAC and progressively smaller at 2 and 3 MAC. At all three doses, VO2 increases were largest with enflurane (48% at 1 MAC), intermediate with halothane (24%), and smallest with isoflurane (11%). These anesthetic-induced VO2 increases were abolished by prior addition of a cytochrome P450 inhibitor (metyrapone) to the incubations. In -PB cells, all three agents produced slight and comparable decreases in VO2 at 1 MAC, with further decreases at 2 and 3 MAC. In +PB cell suspensions at steady states of O2 and energy metabolism, 1 MAC enflurane or halothane, but not isoflurane, produced significant declines in steady state PO2 (from initial values of 24 mmHg to values less than 10 mmHg) and reductions in adenosine triphosphate/adenosine diphosphate ratio (ATP/ADP). These changes were absent in -PB cells exposed to the same conditions or in +PB cells not exposed to anesthetic. The authors conclude that clinical doses of enflurane and, to a lesser extent, halothane produce statistically significant increases in O2 consumption, reflecting enhanced cytochrome P450 activity, in liver cells isolated from phenobarbital-treated rats. Such increases in O2 demand represent a mechanism by which anesthetic metabolism could contribute to intrahepatic hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

An in vivo study of halothane uptake and elimination in the rat brain with fluorine nuclear magnetic resonance spectroscopy

A recent NMR study reported the elimination of halothane from the brain of rabbits to be ten times slower than expected, based on known anesthetic solubility and cerebral blood flow. The authors conducted a study in five rats using fluorine nuclear magnetic resonance (NMR) spectroscopy to see if major pharmacokinetic discrepancies are associated with the uptake, maintenance, and elimination of halothane from the brain. The rats underwent a 60-min period of halothane anesthesia. They employed a spatially selective NMR spectroscopy technique known as surface coil "depth-pulsing" to assure that the fluorine NMR signals originated in brain tissue, and not in the scalp, muscle, adipose tissue, and bone marrow that surround the brain. After the inspired anesthetic concentration was decreased to zero, the amplitude of the fluorine NMR signal decreased to 40% of its maximum value within 34 +/- 8.0 minutes (n = 5), rather than after 7 h as in the recent study, where the fluorine signal may have contained substantial contributions from metabolites or tissues outside the brain. Fluorine was barely detectable in all of the animals 90 min after stopping the administration of halothane. The authors' results are in agreement with model calculations and several other investigations.

In vivo 19F-NMR study of halothane distribution in brain

Halothane distribution and elimination from rabbit brain was studied in vivo using 19F-NMR spectroscopy. Two exponential decay functions for the anesthetic were observed in the clearance curve. They are assigned to halothane in brain held in two distinct chemical environments characterized by different chemical shifts, and half-lives (25 and 320 min). A nonvolatile halothane metabolite with a half-life of several days was found to be present in rabbit brains. The in vivo results were corroborated by ex vivo experiments on excised brain tissue. Halothane was distributed in all of the major cell subfractions, whereas the metabolite was present predominantly in the cytoplasm.

Correlation between the anaesthetic effect of halothane and saturable binding in brain

Two theories of the molecular mechanism of volatile anaesthetic action suggest either that anaesthetics cause a generalized perturbation of neuronal membrane structure, probably through a nonspecific interaction with membrane lipids, or that anaesthetics bind to sets of sites of appropriate molecular dimension on membrane proteins. Based on the recent finding that fluorinated anaesthetics can be observed in animal tissue by 19F nuclear magnetic resonance (19F-NMR) spectroscopy, we have used 19F-NMR to quantify the interaction between the volatile anaesthetic halothane and rat brain tissue. Steady-state brain halothane concentration was found to be a non-linear function of inspired concentration, with apparent saturation of brain occurring at inspired halothane concentrations above 2.5% by volume. Using a spin-echo pulse sequence it was found that halothane exists in two distinct chemical environments in brain, characterized by different spin-spin relaxation times (T2), chemical shifts and kinetics of occupancy. Halothane concentration in one of these environments (T2 = 3.6 ms) was saturated at approximately 2.5% inspired halothane; occupancy of this environment was found to correlate with the anaesthetic effect of the drug. In the other environment (T2 = 43 ms), brain halothane concentration was a linear function of inspired concentration. These data suggest the existence of a saturable anaesthetic site for halothane in brain and do not support the concept that anaesthetics act by nonspecific membrane perturbation.

Influence of cimetidine and diethyldithiocarbamate on the metabolism of halothane and methoxyflurane in vitro

The metabolism of halothane and methoxyflurane was measured in vitro by the vial equilibration method using the S-9-fraction from rat liver as source of enzymes. Kinetic values were measured for halothane: Vmax = 11.6 nmol/g.min, KM = 19.6 mumol/l and methoxyflurane: Vmax = 12.0 nmol/g.min, KM = 17.5 mumol/l. Dithiocarb showed strong inhibitory activity on halothane and methoxyflurane metabolism; inhibition constants were calculated as Ki = 0.051 mmol/l and Ki = 0.004 mmol/l, respectively. Cimetidine inhibited the metabolism of both anesthetics to a lesser extent. Inhibition constants were calculated as Ki = 16.2 mmol/l and Ki = 8.2 mmol/l for halothane and methoxyflurane, respectively. The observed inhibitory properties of dithiocarb and cimetidine on the metabolism of halothane and methoxyflurane may be of interest in connection with the problem of toxic liver and kidney injury after anesthesia with these agents.

Halothane and enflurane metabolite elimination during anaesthesia in man

Ten patients received 0.75% halothane and 12 received 1.5% enflurane for 1 h in a 50:50 nitrous oxide/oxygen mixture. Plasma and end-tidal concentrations were measured by gas-liquid chromatography (GLC) using the head-space method. Fluoride ion assay was performed with a specific electrode by HPLC, trifluoroacetate and oxalate ion levels were determined after extraction, by GLC. Comparison of the evolution of the non-metabolized forms showed that enflurane was more rapidly eliminated: by the third hour after starting, enflurane plasma concentrations were 3.6 micrograms ml-1 compared with 6.3 micrograms ml-1 for halothane. Fluoride plasma levels were nearly constant in the halothane group, but a significant increase up to 14.9 microM was observed in the enflurane group. The ratio of 10:1 in peak urinary concentrations was linked to the molecular structure and the metabolic pathways.

Metabolism of halothane in children having repeated halothane anaesthetics

Metabolism of halothane was studied in nine children receiving daily halothane anaesthetics (from 10 up to a maximum of 31) over periods of two to seven weeks. Serum bromide concentrations never exceeded 3.5 mmol/l, a concentration below the toxic threshold. Repeated halothane anaesthetics at short intervals did not induce the reductive metabolism of halothane as assessed by 2-chloro-1,1,1-trifluoroethane (CTF) in the expired breath. One patient developed viral hepatitis A during the course of anaesthetic administration; this patient was the only one whose serum bromide concentrations fell substantially and whose exhaled CTF concentration increased as more anaesthetics were administered.

A randomized prospective controlled study of the metabolism and hepatotoxicity of halothane in humans

In a randomized prospective controlled study in humans, the metabolism and hepatic effects of a single administration of halothane were compared with enflurane and meperidine. Pre- and postoperative antipyrine pharmacokinetics, intraoperative indocyanine green clearance, liver histology, and postoperative liver function tests were determined in 24 patients undergoing abdominal surgery who were randomly allocated to receive either halothane (0.5%, group I), enflurane (0.8%, group II), or meperidine (group III) as a supplement to a common basal anesthetic regimen consisting of thiopental, nitrous oxide/oxygen/muscle relaxant. In addition, end-tidal concentrations of the volatile reductive metabolites of halothane, chlorodifluoroethylene (CDF), and chlorotrifluoroethane (CTF) were determined in group I patients and serum and urinary inorganic fluoride were determined in both group I and II patients. Indocyanine green clearance was measured before anesthesia (stage I), during basal anesthesia (stage II), in the presence of surgical stimuli (stage III), and after introduction of the selected anesthetic agent (stage IV). CDF and CTF were detectable within 20 min of the start of halothane anesthesia in every patient receiving halothane. Peak serum fluoride concentrations occurred at 2 and 24 hr in the enflurane and halothane groups, respectively, whereas urinary fluoride excretion was elevated postanesthesia in the enflurane group only. There was no difference between the pre- and postoperative disposition of antipyrine in group II or III, but after anesthesia, antipyrine clearance was significantly decreased (P less than 0.02) and plasma half-life increased (P less than 0.05) in group I patients (halothane). Concentrations of serum alanine aminotransferase (ALT) and bilirubin were significantly elevated (P less than 0.5) postoperatively in groups I and II but unchanged from preoperative values in group III patients. Three of the 24 liver biopsies taken at the end of stage IV showed several foci of acute liver cell necrosis; of these, two patients were from group I and one from group II. There were no significant differences in liver cell morphology (P greater than 0.5) in biopsies taken at the end of stage IV compared with biopsies at the end of stage III, from groups I and II. The results of this study show that reductive metabolism of halothane occurs routinely in patients undergoing halothane anesthesia under conditions of normoxia. This may be the cause of the changes in antipyrine clearance after halothane anesthesia.

Halothane hepatotoxicity in Fischer 344 rats pretreated with isoniazid

Male Fischer 344 rats were used to investigate the hepatic effects of exposure to halothane under normoxic conditions (FIO2 = 0.21) in isoniazid-treated rats. Animals were treated with saline or isoniazid (50 mg/kg) for 7 days and then were exposed to either 1% halothane or air for 2 hr. One-half of the rats from each treatment and exposure group were killed 24 hr postexposure; the remaining were killed 4 days postexposure. Twenty-four hours following halothane exposure, serum transaminase levels were significantly elevated in isoniazid- compared with saline-treated rats (i.e., aspartate aminotransferase = twofold; alanine aminotransferase = seven-fold). Cholesterol levels were significantly depressed by halothane exposure in both saline- and isoniazid-treated rats. Other serum parameters indicative of hepatic and renal function were not different: alkaline phosphatase, total protein, total bilirubin, hematocrit, uric acid, creatinine, urea nitrogen, Na+, K+, Ca2+, and inorganic phosphate. Neither saline-treated nor isoniazid-treated rats exposed to air exhibited histologic evidence of hepatic damage. Halothane-exposed rats, however, showed a circumscribed disruption of cellular morphology. The most severe lesions were observed with isoniazid-treated animals with extensive pericentral hepatocellular necrosis and infiltration by leucocytes and Kupffer cells. Serum concentrations of two products of the oxidative metabolism of halothane, trifluoroacetic acid and bromide, were significantly elevated in isoniazid- compared with saline-treated rats. Serum levels of fluoride, a product of reductive metabolism, were not different. These results strongly suggest that hepatic injury following halothane administration can be produced by intermediates of oxidative metabolism.

Clinical pharmacokinetics of the inhalational anaesthetics

At present, the most widely used inhalational anaesthetics are the halogenated, inflammable vapours halothane, enflurane, isoflurane and the gas nitrous oxide. The anaesthetic effect of these agents is related to their tension or partial pressure in the brain, represented at equilibrium by the alveolar concentration. The minimum alveolar concentration for a specific agent is remarkably constant between individuals. The uptake and distribution of inhalational anaesthetics depends on inhaled concentration, pulmonary ventilation, solubility in blood, cardiac output and tissue uptake. Inhalational anaesthetics are mainly eliminated by pulmonary exhalation, but significant amounts of halothane are removed by hepatic metabolism. Inhalational agents currently in use have acceptable pharmacokinetic characteristics, and clinical acceptance depends on their potential for adverse effects. Induction of anaesthesia with halothane is rapid and relatively pleasant and it is the agent of choice for paediatric anaesthesia. Between 20 and 50% is metabolised, and the parent drug is a potent inhibitor of drug metabolism. Post-operatively enzyme induction may follow. The major disadvantages of halothane are myocardial depression, propensity to evoke cardiac arrhythmias and the rare but serious halothane hepatitis. Induction and recovery from enflurane anaesthesia is rapid. Metabolism accounts for 5 to 9% of the elimination. The metabolic product inorganic fluoride may in rare cases cause renal toxicity. Enflurane is a weak inhibitor of drug metabolism at anaesthetic concentrations. Enflurane depresses circulation more than halothane by reducing both myocardial contractility and systemic vascular resistance, but cardiac rhythm is stable. Enflurane anaesthesia may, unlike the other agents, induce epileptic activity. Enflurane is widely used as replacement for halothane in adults. Despite its low blood-gas solubility, the airway irritability of isoflurane precludes a faster induction of anaesthesia than with halothane. Isoflurane is almost resistant to biodegradation. Myocardial contractility is maintained during isoflurane anaesthesia and cardiac rhythm is stable except for the occurrence of tachycardia in some patients. Isoflurane is the inhalational agent of choice for neurosurgical operations. Sevoflurane is an experimental ether vapour: induction and recovery is fast and pleasant. It is metabolised to the same extent as enflurane and subnephrotoxic concentrations of inorganic fluoride may result. Sevoflurane has fewer respiratory and cardiovascular depressant effects than halothane and may be a future alternative for paediatric anaesthesia.(ABSTRACT TRUNCATED AT 400 WORDS)

Does the duration of anesthetic administration affect the pharmacokinetics or metabolism of inhaled anesthetics in humans?

To define the effect of anesthetic duration on the pharmacokinetics of inhaled anesthetics, we determined the pharmacokinetics of isoflurane, enflurane, halothane, and methoxyflurane given simultaneously to seven healthy subjects for exactly 30 min and compared the results with data from a previous study in which these four anesthetics were administered for 120 min. End-tidal and mixed-expired anesthetic concentrations were measured during washin of anesthetic and for 3-9 days of washout. Multiexponential (multicompartment) models were fit by least squares to the alveolar washin and washout curves. We estimated the percentage of anesthetic that was metabolized from total uptake and recovery of anesthetic. Alveolar washout was more rapid after the shorter period of anesthetic administration. However, duration of administration did not affect the time constants determined, the number of compartments identified (i.e., five compartments were identified in both studies), or the percentages of anesthetic metabolized.

Adverse effects of volatile anaesthetics

The volatile inhalation anaesthetics have been implicated in a variety of adverse viscerotoxic reactions. In general, they have been proven to produce very few non-predicted toxicities. Hepatitis caused by halothane now seems to be the only major problem in this regard with these drugs in current practice. The evidence is convincing that this reaction is based initially on biotransformation. Thus decreases in the amount of biotransformation and lessened production of reactive metabolic products would theoretically produce a safer anaesthetic. While not perfect in all circumstances, enflurane and isoflurane come close to achieving the goal of decreased adverse viscerotoxic events.

Effect of streptozotocin-induced diabetes in the rat on the metabolism of fluorinated volatile anesthetics

Three weeks after dosing male Fischer 344 rats with streptozotocin to induce diabetes, enflurane was administered ip, and 1 h later, fluoride levels were measured in plasma and livers were removed. Hepatic microsomes were prepared, and the oxidative defluorination of enflurane, isoflurane, and methoxyflurane and the reductive defluorination of halothane were measured in vitro. In diabetic rats the defluorination of enflurane was increased 3.4-fold over control levels in vivo and 2.7-fold in vitro. Insulin treatment prevented these effects. In vitro metabolism of isoflurane by livers from diabetic rats was 2.5-fold greater than by livers from control rats, but defluorination of methoxyflurane and of halothane was not altered. The results show that streptozotocin-induced diabetes in rats enhances the defluorination of enflurane and of isoflurane but not of methoxyflurane or halothane.

Halothane metabolism in acyanotic and cyanotic patients undergoing open heart surgery

The metabolism of halothane was examined in patients with acyanotic and cyanotic congenital heart disease undergoing open heart surgery. Statistically significant (P less than 0.05) pre-surgical differences between acyanotic and cyanotic groups included pH (7.46 +/- 0.02 vs 7.36 +/- 0.02), PaO2 (277 +/- 58 vs 51 +/- 3 torr), O2 saturation (97 +/- 1 vs 74 +/- 4%), and hematocrit (45 +/- 3 vs 58 +/- 2%). Serum fluoride levels were significantly greater in cyanotic than in acyanotic groups 2-4 hours after initial exposure to halothane. Both groups had significant intragroup increases in serum levels of fluoride, bromide, and trifluoroacetic acid. Significant increases in serum levels of lactate dehydrogenase, creatinine phosphokinase, and glutamic oxaloacetate transaminase were observed in both groups, whereas, the cyanotic patients had additional significant increases in blood urea nitrogen and direct bilirubin. The cyanotic group also had higher total and direct serum bilirubin levels than the acyanotic group. Therefore, patients with cyanotic congenital heart disease had greater reductive metabolism of halothane than acyanotics. However, cyanotic and acyanotic patients had essentially similar postoperative derangements in hepatic and renal function.

Genetic predisposition to liver damage after halothane anesthesia in guinea pigs

Three 4-hr normoxic (21% oxygen) exposures to 1% halothane administered 3 days apart were associated with elevations in serum alanine aminotransferase (ALT) activity in four of 20 guinea pigs after the initial and third exposures. Serum alanine aminotransferase values were not measured after the second anesthetic. Susceptibility was defined as an ALT level greater than 300 IU/L after halothane. Nonsusceptible animals, that is, animals without significant increases in ALT values after halothane, remained nonsusceptible after reexposure. Serum alanine aminotransferase values after the first and third anesthesias were significantly correlated (rs = 0.86, P less than 0.001). Two exposures of another 30 guinea pigs at a 5-week interval resulted in high elevations of ALT in the same eight animals after both anesthetics. In contrast, after an initial exposure nonsusceptible animals remained nonsusceptible upon reexposure. Serum alanine aminotransferase levels after the first and second anesthetics were significantly correlated (rs = 0.85, P less than 0.001). The proportion of first generation (F1) males with elevated ALTs whose parents were susceptible to halothane hepatotoxicity (HH) was significantly higher than the proportion of males with elevated ALTs in a random group of 90 males (P less than 0.005). First generation males and females of nonsusceptible parents had ALTs within the normal range after halothane exposure. These studies suggest that in the guinea pig genetic predisposition is an important determinant of susceptibility to HH, although other contributing factors are not excluded.

Distribution of halothane and the metabolites trifluoroacetic acid and bromide in the conceptus after halothane inhalation by pregnant mice

Mice in late stage of gestation were exposed to halothane at various concentrations for 1 hr, and were killed at different time intervals after discontinuance of inhalation. The concentration of halothane in maternal plasma decreased rapidly, and in the amniotic fluid the halothane never reached more than 20% of maternal plasma levels. Trifluoroacetic acid (TFA) and bromide, formed mainly by maternal metabolism of halothane, accumulated in foetus and amniotic fluid with time, and reached plateau levels in amniotic fluid between 4 and 24 hrs. TFA infused intravenously to the mother reached higher levels in amniotic fluid after long survival times, than in maternal plasma. Equilibrium dialysis experiments showed that TFA and trichloroacetic acid (TCA) (previously shown to accumulate in amniotic fluid) were bound to amniotic fluid macromolecules only to approximately 20-30 percent. This was at the same magnitude (or lower) as compared to binding in maternal plasma, suggesting that such binding did not contribute to the observed retention in the amniotic fluid. Other possible explanations for the slow accumulation and long-term retention in amniotic fluid are transport by bulk flow via foetus, excretion via foetal urine, or paraplacentally through endometrium and foetal membranes, followed by trapping in the amniotic fluid. The significance of this accumulation of metabolites of halogenated organic solvents and halothane for their foetotoxicity is not clear.