Noninvasive observations of fluorinated anesthetics in rabbit brain by fluorine-19 nuclear magnetic resonance

Fluorinated anesthetics were observed noninvasively in the brain of intact rabbits with fluorine-19 nuclear magnetic resonance spectroscopy. High-resolution fluorine-19 spectra of halothane, methoxyflurane, and isoflurane were obtained with a surface coil centered over the calvarium. Elimination of halothane from the brain was also monitored by this technique. Residual fluorine-19 signals from halothane (or a metabolite) could be detected as long as 98 hours after termination of anesthesia. These observations demonstrate the feasibility of using this technique to study the fate of fluorinated anesthetics in live mammals.

Multiple environments of fluorinated anesthetics in intact tissues observed with 19F NMR spectroscopy

The incorporation of two fluorine-containing general anesthetic agents, halothane and methoxyflurane, into erythrocytes (from three different species), rabbit muscle and rabbit nerve, was followed with 19F NMR spectroscopy. Two major findings emerged from these studies: (1) multiple environments indicative of domain structure in the membrane can be observed depending on the anesthetic and the tissue type; and (2) the 19F chemical shifts of a given anesthetic were characteristic for the tissue examined. Halothane showed a single resonance in erythrocytes and multiple resonances in muscle and nerve, while methoxyflurane showed multiple resonances in both muscle and erythrocytes. The range of the 19F chemical shifts for the multiple peaks was as great as 6 ppm.

Metabolism by rat hepatic microsomes of fluorinated ether anesthetics following ethanol consumption

The possibility that the metabolism of volatile inhalational anesthetics is altered following chronic ethanol consumption was investigated in male Fischer 344 rats. The hepatic microsomal defluorination rates of methoxyflurane, enflurane, and sevoflurane were determined for pair-fed rats receiving ethanol with normal caloric or with 50% of normal caloric intake. For comparison, the effects of phenobarbital treatment on anesthetic defluorination rates also were examined. Fourteen days of ad libitum consumption of 16% ethanol resulted in maximal defluorination rates of the above anesthetics. No overt signs of ethanol toxicity were observed. Ethanol-treated rats with a normal caloric intake had significantly increased microsomal defluorination rates per mg protein compared with pair-fed control rats as follows: methoxyflurane, 190% of control; enflurane, 298% of control; and sevoflurane, 301% of control. Ethanol-treated animals with 50% of normal caloric intake showed similar elevations in microsomal defluorination rates when compared with pair-fed controls. Phenobarbital treatment significantly increased the rate of methoxyflurane defluorination (673% of control), whereas the rates of sevoflurane defluorination (127% of control) and enflurane defluorination (86% of control) were not altered significantly. Phenobarbital treatment increased the microsomal content of cytochrome P-450, while ethanol treatment did not. This study demonstrated that regardless of total caloric intake, chronic ethanol consumption increases defluorination of inhalation anesthetics in Fischer 344 rats. It also illustrated that the two enzyme-inducing agents are unique with respect to the degree to which they enhance anesthetic defluorination.

Enflurane, isoflurane, and methoxyflurane metabolism in rat hepatic microsomes from ethanol-treated animals

The effects of ethanol on the metabolism of enflurane, isoflurane, and methoxyflurane were investigated to determine if alterations in biotransformation of these agents occur as a result of this treatment. In vitro incubations of hepatic microsomes from rats pretreated with 10 days' ethanol vapor inhalation revealed a fourfold increase in inorganic fluoride from enflurane when compared with incubations of microsomes from unpretreated rats and from phenobarbital-pretreated rats. Isoflurane, while metabolized to a lesser extent than enflurane, showed a similar stimulation of metabolism. Methoxyflurane, while metabolized to a greater extent than either enflurane or isoflurane, had lesser fluoride production by the microsomes from ethanol-pretreated rats than microsomes from phenobarbital-pretreated rats, but greater fluoride production than that found in microsomes from unpretreated rats. Ethanol pretreatment did not alter the levels of cytochrome P-450 which is the enzyme responsible for such metabolism. This suggests that the altered metabolism involves either a specific P-450 isozyme or an unidentified enzyme.

Dependence of microsomal methoxyflurane O-demethylation on cytochrome P-450 reductase and the stoichiometry of fluoride ion and formaldehyde release

In order to characterize further the in vitro liver microsomal O-demethylation and defluorination of the volatile anesthetic methoxyflurane, and obtain additional information regarding the participation of cytochrome P-450 in the oxidation, the stoichiometry of the reaction was determined and the effect of antibody to cytochrome P-450 reductase on this unique biotransformation was examined. Liver microsomes were isolated from rabbits and rats in which enzyme induction had previously been produced by phenobarbital. The O-demethylation of methoxyflurane by phenobarbital-induced microsomes results in the production of 1 mol of formaldehyde for every 2 mol of fluoride ion produced. Dichloroacetic acid is also a product of methoxyflurane O-demethylation. Antibody to cytochrome P-450 reductase inhibits by 85% the amount of fluoride ion produced by the microsomal metabolism of methoxyflurane. Thus critical indirect supportive data are contributed to the hypothesis that at least one, but perhaps more, cytochrome P-450 is indeed responsible for methoxyflurane O-demethylation and defluorination.

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.

The partial molar volumes of anesthetics in lipid bilayers

The excess volumes of mixing of benzyl alcohol, halothane, and methoxyflurane in water and in suspensions of several lipid bilayers have been determined at 25 degrees C using a novel excess volume dilatometer. The excess volumes of mixing in water were all found to be negative, whereas in lipid suspensions they were all more positive than those in water alone. From known partition coefficients the partial molar volumes of these three solutes in the lipid bilayers were calculated. These values were all close to the molar volumes of the pure anesthetics, as was a value determined for halothane in olive oil. Halothane was studied in dipalmitoylphosphatidylcholine below its phase transition, and was found to exhibit a much larger excess volume than in any other system we studied. The potency of these three anesthetics was determined in tadpoles. It was calculated that at equi-anesthetic doses these three agents caused an expansion in egg lecithin/cholesterol (2:1) bilayers of 0.21 +/- 0.015%. This result is consistent with the hypothesis that general anesthetics act by expanding membranes.

Partition equilibrium of inhalation anesthetics and alcohols between water and membranes of phospholipids with varying acyl chain-lengths

From the depression of the phase-transition temperature of phospholipid membranes, the partition coefficients of inhalation anesthetics (methoxyflurane, halothane, enflurane, chloroform and diethyl ether) and alcohols (benzyl alcohol and homologous n-alcohols up to C = 7) between phospholipid vesicle membranes and water were determined. The phospholipids used were dimyristoyl-, dipalmitoyl- and distearoylphosphatidylcholines. It was found that the difference in the acyl chain length of the three phospholipids did not affect the partition coefficients of the inhalation anesthetics and benzyl alcohol. The actions of these drugs are apparently directed mainly to the interfacial region. In contrast, n-alcohols tend to bind more tightly to the phospholipid vesicles with longer acyl chains. The absolute values of the transfer free energies of n-alcohols increased with the increase of the length of the alkyl chain of the alcohols. The increment was 3.43 kJ per each carbon atom. The numerical values of the partition coefficients are not identical when different expressions for solute concentrations (mole fraction, molality and molarity) are employed. The conversion factors among these values were estimated from the molecular weights and the partial molal volumes of the phospholipids in aqueous solution determined by oscillation densimetry.

Effects of liver injury and cholestasis on microsomal enzyme activities and metabolism of halothane, enflurane and methoxyflurane in vivo in rats

1. Cholestasis (bile-duct ligation 24 h before) had no effect on rat liver microsomal protein content, cytochrome P-450 or cytochrome c reductase activity, but depressed aniline hydroxylase activity and aminopyrine demethylase less so. Pretreatment with CCl4 (24 h before) decreased rat liver cytochrome P-450, aniline hydroxylase and aminopyrine demethylase. 2. Halothane, enflurane and methoxyflurane are metabolized via different pathways, resulting in different metabolic elimination rates in our exposure system (methoxyflurane greater than halothane greater than enflurane). Elimination half-lives of the three compounds from the atmosphere of the exposure system were three times longer in CCl4-injured rats; cholestasis had a weaker effect (30-50% increase). 3. Dehalogenation of enflurane, which is the preferred pathway, is affected to the same extent as the cytochrome P-450-dependent hydroxylation of halothane and the O-dealkylation of methoxyflurane.

Molecular aspects of inhalational anaesthetic interaction with excitable and non-excitable membranes

The interaction of three volatile general anaesthetics (halothane, enflurane and methoxyflurane) with erythrocyte membranes at concentrations causing protection of intact erythrocytes against hypotonic lysis was investigated in the hope of deriving fundamental information regarding the membrane perturbational characteristics of these substances as compared with those of local anaesthetics studied previously. The volatile agents increased the susceptibility of membrane proteins and, to a somewhat lesser extent, of phospholipids to trinitrophenylation of picryl chloride or trinitrobenzenesulfonic acid but decreased the accessibility of membrane protein sulfhydryl groups to modification by 5,5'-dithio-bis-(2-nitrobenzoic acid). These observations stood in marked contrast to our previous findings with local anaesthetics, in that these substances, when compared to general anaesthetics at concentrations producing equivalent erythrocyte stabilization, caused a greater enhancement of trinitrophenylation, largely restricted to the phospholipid component and an increased exposure of membrane sulfhydryl groups. Further evidence for alterations in membrane proteins produced by concentrations of volatile anaesthetics relevant to surgical anaesthesia was obtained from the observation that all three agents produced significant decreases in the activation energy of membrane-bound p-nitrophenylphosphatases. Preliminary experiments with brain synaptic membranes suggested that the structural and functional consequences of membrane-anaesthetic interaction in erythrocytes are relevant to the situation in excitable tissues. Our results indicate, therefore, that general and local anaesthetics cause distinctly different alterations in the properties of model membrane systems and this may reflect corresponding differences in the molecular mechanisms by which these groups of agents produce their anaesthetic actions.

Extrahepatic biotransformation of halothane and enflurane

The rates of biotransformation of halothane and enflurane by rabbit kidney and lung microsomal preparations were compared t the hepatic microsomal biotransformation of these agents. All three microsomal preparations (pulmonary, renal, and hepatic) were found capable of performing oxidative demethylation reactions as well as epoxidation. This was evidenced by the ability of these three microsomal preparations to metabolize benzphetamine, methoxyflurane, and trichloroehylene. Only the liver microsomal preparations were capable of defluorinating enflurane at any appreciable rate (6 +/- 3 pmoles/min/mg of microsomal protein). The three microsomal preparations performed reductive biotransformation of halothane, and the liver microsomes produced more than 3 times as much product as the other tissues. Pulmonary and renal microsomal preparations metabolized halothane reductively about equally. Differences in the solubility of halothane and enflurane in the rabbit pulmonary and hepatic microsomes were not found to be a cause of the differences in biotransformation in these two organs. Extrahepatic biotransformation may be an important factor in the disposition of volatile anesthetics.

[Inhibition and acceleration of the metabolism of enflurane and methoxyflurane in rats (author's transl)]

Rats exposed to enflurane (100 ppm) or methoxyflurane (300 ppm) in a closed all glass-system eliminated these anesthetics from the atmosphere of the system with a half-life of 6.84 h for enflurane and 0.64 h for methoxyflurane. 24 h-fasting had no influence on these elimination half-lives. An oral load of ethanol (4.8 g/kg p.o.) only prolonged the half-life for methoxyflurane. Pretreatment with diethyl maleate (1 ml/kg i.p.), dimethylsulfoxide (DMSO, 1 g/kg i.p.) or dithiocarb (100 mg/kg i.p.) prolonged the elimination half-life of both enflurane and methoxyflurane. An accelerated metabolic elimination was only observed in DDT-pretreated rats exposed to enflurane; other inducers of the microsomal mixed-function oxidase system like phenobarbital or rifampicine had no significant influence on the in vivo metabolism of both enflurane or methoxyflurane.

Effects of thyroid dysfunction on the metabolism of halothane, enflurane and methoxyflurane in rats

The effect of thyroid dysfunction on the metabolism of halothane (100 ppm), enflurane (100 ppm) and methoxyflurane (300 ppm) was investigated during application by inhalation. In male rats the elimination half-lives from the atmosphere of the exposure system amounted to 0.76h for halothane, 6.84h for enflurane and 0.64h for methoxyflurane. Hyperthyroidism due to three daily injections of 0.1 mg/kg triiodothyronine i.p. significantly shortened the half-lives of all three inhalation anesthetics. Hypothyroidism due to operative removal of the thyroid gland affected the metabolism of halothane only as evidenced by a prolongation of the elimination half-life while enflurane and methoxyflurane half-lives remained unchanged. The observed differences in metabolic rates are explained by different metabolic pathways of the three compounds. They may be important for the manifestation of toxic effects.

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.

Hepatic drug metabolism and anesthesia

Anesthetic agents, including most inhalation anesthetics, the barbiturates, narcotics, local anesthetic amides and curare-like compounds are metabolized inside the liver cell. Consequently, drug metabolism in the liver has become an increasingly important consideration in the practice of anesthesiology. Hepatic metabolism is, first and foremost, a mechanism that converts drugs and other compounds into products that are more easily excreted and that usually have a lower pharmacologic activity than the partent compound. Thus, duration and intensity of drug action are limited. However, there are exceptions. In certain instances a metabolite may have higher activity and/or greater toxicity than the original drug. Intrahepatic metabolism hinges upon the oxidative reactions that are catalyzed by a group of mixed oxidases, the P-450 cytochromes. Their concentration and activity can be enhanced by certain drugs or environmental chemicals that are ingested by the individual. This usually is beneficial, in that this mechanism of enzyme induction promotes the detoxification of pharmaca, which is the normal aspect of drug metabolism. If, however, the normal metabolite is more toxic than the parent compound, or there exists an alternate, abnormal metabolic pathway that produces a toxic metabolite, then enzyme induction may have serious consequences. Inorganic fluoride is a normal metabolite of methoxyflurane. It is responsible for the high-output renal failure that can be observed after anesthesia with this inhalation agent. A patient with induced enzyme activity is especially at risk to develop methoxyflurane-related renal failure. The picture of halothane toxicity is not as clear. There are indications that an abnormal metabolite, produced in sufficient quantities via an alternate pathway with induced enzyme activity, may be capable of causing liver damage.

Early appearance of methoxyflurane fluorometabolites in mice

The biotransformation of methoxyflurane produces two fluorometabolites: inorganic fluoride (F) and an acid-labile fluorocompound (OALF). These have been assayed in the serum of male mice 10, 30, and 100 minutes after either ip or sc injections of 0.7 microliter of methoxyflurane (MOF) per gram of body weight, as either the volatile liquid itself or as a 1:10 dilution in corn oil. During the 10- to 100-minute intervals, the mice were observed for loss of the righting reflex. Corn oil dilution significantly altered the production of MOF fluorometabolites, usually decreasing them, but after 100 minutes, the serum concentration of OALF was 25% greater. Loss of righting reflex was also altered; 11 of 18 mice given undiluted MOF experienced it, whereas only 2 of 18 given the diluted agent did. Compared to the tip route, the sc route resulted in less fluoride at all three time intervals, but more of the OALF after 30 and 100 minutes. None of the sc-dosed mice lost his righting reflex.

PaCO2 for optimum washout of inhalational anesthetics from the brain. A model study

A hypothesis was established that, during emergence of inhalational anesthesia, hyperventilation and accompanying hypocapnia beyond a certain limit may actually disturb rather than enhance the washout of inhalational anesthetics from the brain because of a decreased cerebral blood flow. Two mathematical models were constructed and the washout of nitrous oxide, halothane and methoxyflurane were studied. In model I, the whole body consisted of a single compartment, and blood flow to this compartment was assumed to change proprotionally with the PaCO2. In model 2, the body was divided into two compartments, brain and the rest of the body. It was assumed that the blood flow to the brain compartment varies proportionally with the PaCO2, while that to the rest of the body remains constant. The analysis indicated that there indeed existed the PaCO2 values at which the washout of anesthetics from the brain can be maximally achieved. In model 1, they were 49.0, 22.1 and 9.7 mmHg for nitrous oxide, halothane, and methoxyflurane, respectively. In model 2, these PaCO2 values varied with time. While the hypothesis was proven to be valid, we conclude that it is of limited clinical significance. For halothane and methoxyflurane, these theoretically optimum PaCO2 values are sufficiently low. For nitrous oxide, the variation of PaCO2 makes little difference clinically, because its washout is fast enough regardless of PaCO2.

Assessment of the anaesthetic and metabolic activities of dioxychlorane, a new halogenated volatile anaesthetic agent

The ability of dioxychlorane to depress cortical activity in rats with implanted electrodes was compared to that reported previously for methoxyflurane, halothane and enflurane. Dioxychlorane was eight times more potent than enflurane, five times more potent than halothane and twice as potent as methoxyflurane. Serum fluoride concentrations after the administration of dioxychlorane and enflurane were not different from controls. In contrast, serum fluoride concentrations after methoxyflurane reached a value of 105 mumol litre-1 and remained increased for at least the next 48 h. Urine fluoride concentrations in the dioxychlorane and enflurane groups were a half and a quarter, respectively, of those recorded in the methoxyflurane group. Polyuria and polydipsia were observed only in the methoxyflurane group. Dilatation of the proximal convoluted tubules was noted in the rats anesthetized with methoxyflurane. These changes were most marked at the 6- and 24-h periods following anaesthesia. Haemorrhage and ulcerative cystitis were noted in the bladders of the rats subjected to methoxyflurane. Cellular swelling in the proximal tubule was observed in the rats sacrificed 24 h after the administration of dioxychlorane. Enflurane produced no pathological changes.