Weak polar interactions confer albumin binding site selectivity for haloether anesthetics

BACKGROUND

Enflurane and isoflurane are structural isomers with different anesthetic potencies and side effects. It is not clear whether these differences are produced by differing occupancy of common protein binding sites or by occupancy of different sites, but the very similar molecular properties make the latter possibility unlikely. In this study, the authors examined binding site selectivity of these anesthetics in human serum albumin (HSA).

METHODS

Binding of isoflurane and enflurane with HSA was determined with isothermal titration calorimetry. Competition with known ligands (propofol) allowed localization of binding sites within the HSA molecule. Molecular properties of isoflurane and enflurane were calculated.

RESULTS

Isoflurane binds HSA with higher affinity but smaller total enthalpy than enflurane. Enthalpogram analysis suggested that isoflurane bound a single site, whereas enflurane bound two. Competition experiments indicated that enflurane and isoflurane share one binding site, which also binds propofol. The additional enflurane site binds propofol but not isoflurane. Increased salt concentration decreased the affinity for isoflurane but not for enflurane. The dipole moment of isoflurane is higher than that of enflurane, and the isoflurane binding site is more polar.

CONCLUSION

These data indicate two binding sites of different character for the haloether anesthetics on HSA. One site is more polar and prefers isoflurane, presumably because of its larger dipole. The second site prefers the less polar enflurane. Therefore, weak polar interactions confer considerable selectivity, and differences in drug action may arise from occupancy of different protein sites.

Fluoride metabolism in smokers and non-smokers following enflurane anaesthesia

BACKGROUND

Inorganic fluoride is released by the metabolism of enflurane and the increased serum fluoride concentrations may impair renal function. Tobacco smoke consists of numerous reactive compounds that can either induce or inhibit drug metabolism. Studies on the interaction of smoking with anaesthetic drug metabolism and possible toxicity are warranted.

METHODS

Sixteen non-smoking and 17 smoking (>10 cigarettes day(-1)) generally healthy women undergoing elective gynaecological surgery were given 1 MAC (minimum alveolar concentration)-hour standardized anaesthesia with enflurane in oxygen-air mixture. The serum inorganic fluoride and renal function markers beta(2)-microglobulin, tumour-associated trypsin inhibitor (TATI) and serum creatinine were measured for 48 h.

RESULTS

The greatest inorganic fluoride concentration was between 8.4 and 21.0 (mean 13.8 (SD 3.4)) micromol litre(-1) in the non-smokers and between 8.6 and 38.0 (18.7 (7.0)) micromol litre(-1) in the smokers; the mean difference was 4.9 micromol litre(-1) (95% confidence interval (CI) 1.0-8.8, P<0.05). Serum beta(2)-microglobulin, TATI and creatinine were not increased. Serum inorganic fluoride concentrations were significantly greater in the smokers compared with the non- smokers 1, 2, 3 and 6 h after 1 MAC-hour inhalation with enflurane (P<0.05). Inorganic fluoride concentrations were still increased 24 h after anaesthesia in both groups. Urine beta(2)-microglobulin and TATI creatinine ratio remained at low values during the whole 48-h period in both groups.

CONCLUSIONS

Regular smoking is associated with an increase in serum inorganic fluoride concentration after anaesthesia with enflurane, but there are no signs of renal damage.

Antagonism of inhalant and volatile anesthetic enhancement of glycine receptor function

Recent studies suggest that alcohols, volatile anesthetics, and inhaled drugs of abuse, which enhance gamma-aminobutyric acid, type A, and glycine receptor-activated ion channel function, may share common or overlapping molecular sites of action on these receptors. To investigate this possibility, these compounds were applied singly and in combination to wild-type glycine alpha(1) receptors expressed in Xenopus laevis oocytes. Data obtained from concentration-response curves of the volatile anesthetic enflurane constructed in the presence and absence of ethanol, chloroform, or toluene were consistent with competition for a common binding pocket on these receptors. A mutant glycine receptor, insensitive to the enhancing effects of ethanol but not anesthetics or inhalants, demonstrated antagonism of anesthetic and inhalant effects on this receptor. Although ethanol (25-200 mm) had no effect on its own in this receptor, it was able to inhibit reversibly the enhancing effect of enflurane, toluene, and chloroform in a concentration-dependent manner. These data suggest the existence of overlapping molecular sites of action for ethanol, inhalants, and volatile anesthetics on glycine receptors and illustrate the feasibility of pharmacological antagonism of the effects of volatile anesthetics.

Propofol inhibits renal cytochrome P450 activity and enflurane defluorination in vitro in hamsters

PURPOSE

To determine the effect of propofol on renal cytochrome P450 activity and defluorination of enflurane.

METHODS

Renal microsomes were prepared by homogenization and differential centrifugation from pooled hamster kidneys. Defluorination of enflurane was assessed by measuring free fluoride metabolites after reacting enflurane with renal microsomes incubated with various concentrations, 0.05 - 1.0 mmol x L(-1) propofol in the NADPH-generating system. Drug metabolizing activities of renal cytochrome P450 mono-oxygenase enzymes were evaluated within microsomes preincubated with propofol and reacted with the specific marker substrates, aniline, benzo(a)pyrene, erythromycin and pentoxyresorufin, for cytochrome P450 2E1, 1A1, 3A4 and 2B1, respectively.

RESULTS

Renal defluorination of enflurane was inhibited by clinical concentrations, 0.05 mmol x L(-1) of propofol (P < 0.05). Dose-dependent inhibition of defluorination, aniline and benzo(a)pyrene hydroxylase within kidney microsomes was related to propofol concentration. Propofol demonstrated a profound inhibition of renal pentoxyresorufin dealkylase activity even at low concentrations, 0.05 mmol x L(-1) (P < 0.01). Propofol did not exhibit inhibition of erythromycin N-demethylation of kidney microsomes except at high concentration, 1.0 mmol x L(-1). Spectral analyses of key coenzymes of renal cytochrome P450 monooxygenase, cytochrome b5 and cytochrome c reductase, demonstrated an inhibition when incubated with high concentrations of propofol (P < 0.05).

CONCLUSION

In an in vitro study in an NADPH-generating system of hamster kidney microsomes, propofol, in clinical concentrations, exhibited a broad-spectrum of inhibition to renal monooxygenase activities and enflurane defluorination.

Modulation of cytochrome P450-dependent monooxygenases in streptozotocin-induced diabetic hamster: II. Reverse role of insulin in P450 activity and defluorination

BACKGROUND

Metabolic activities of cytochrome (cyt) P450-dependent monooxygenase could be modulated by diabetic state in experimental diabetic animals. The purpose of this study is to validate the effect of insulin on the modulation of the metabolic activity of cyt P450 and the defluorination ability to inhalational anesthetics in diabetic animals.

METHODS

Diabetic state in golden Syrian hamsters was achieved by intraperitoneal injection of streptozotocin 40 mg/kg once a day for 4 days. After stabilization of diabetic state for 6 weeks, a regimen of insulin treatment given subcutaneously was carried out. Metabolic activities of cyt P450 were assessed by the reaction with benzo(a) pyrene, pentoxyresorufin, aniline and erythromycin (specific substrates). The metabolic activities of cyt 1A1, 2B1, 2E1 and 3A4 respectively in a NADPH-generating system in microsomal preparations of the diabetic hamsters were observed before and after insulin treatment, and were compared with the control group. The ability of defluorination was evaluated by measuring the free fluoride metabolites after incubating the microsomes with enflurane in diabetic and insulin-treated hamsters. Contents of cyt P450 isozymes were measured by electrophoresis and immunoblotting before and after insulin treatment. Pathological features of hepatocytes in diabetic hamsters were evaluated microscopically before and after insulin treatment.

RESULTS

The defluorination of enflurane and activity of aniline hydroxylase (cyt 2E1) were successfully induced by diabetic state (P < 0.01). The pentoxyresorufin O-dealkylase (cyt 2B1) was inhibited nearly 50% in the diabetic hamster liver when compared with that of control (P < 0.01). While the activities of benzo(a)pyrene hydroxylase (cyt 1A1) and the erythromycin N-demethylase (cyt 3A4) were basically unaffected by diabetes, alterations in content of cyt P450 were parallel to the alterations in enzyme activities. Microscopically, diabetes induced vacuolization with fatty droplets in the hepatocytes. After treatment with insulin injection, the enzyme activities, protein content and pathologic features returned to the baseline similar to the control.

CONCLUSIONS

Our data demonstrated that under diabetic state, metabolic activities of cyt P450 and its extent of defluorination would be polymorphically modulated. After administration of insulin, the activities of cyt P450 and defluorination of enflurane returned to baseline as the blood sugar level had been normalized. This could remind the clinicians of the importance of insulin treatment in the potential drug-to-drug interactions in the diabetic patients.

Modulation of cytochrome P-450 dependent monooxygenases in streptozotocin-induced diabetic hamster: I. Effects of propofol on defluorination and cytochrome P-450 activities

BACKGROUND

Diabetes mellitus could induce polymorphic alterations of metabolic activities of cytochrome P-450 dependent monooxygenases in chemical-induced diabetic animals. The purpose of this study is to define the functional impact of clinical concentrations of propofol on the metabolic activities of cytochrome P-450 in the diabetic animals.

METHODS

In order to validate the effect of propofol on cytochrome P-450 activities, especially the cytochrome P-450 2E1 and its defluorination activity, we applied NADPH-generating system to measure the metabolizing activities of cytochrome P-450 isozymes of streptozotocin-induced diabetic hamsters within the microsomes preincubated with various concentrations of propofol. The extent of defluorination and activity of cytochrome P-450 2E1 were assessed by reacting the propofol-treated microsomes in NADPH-generating system with enflurane and aniline as substrates respectively. Drug metabolizing activities of cytochrome 1A1, 2B1, and 3A4 were evaluated by metabolizing specific substrates, benzo(a)pyrene, pentoxyresorufin and erythromycin, within the microsomes of diabetic hamsters preincubated with various concentrations of propofol.

RESULTS

The hepatic and renal defluorination of enflurane was significantly inhibited by 0.05 and 0.10 mM propofol in the microsomes of diabetic hamster (P < 0.05). The activities of aniline hydroxylase (cytochrome 2E1), pentoxyresorufin O-dealkylase (cytochrome 2B1) and benzo(a)pyrene hydroxylase (cytochrome 1A1) were inhibited by propofol in a concentration-dependent manner from 0.05 to 0.10 mM. However, propofol showed no significant effect to the erythromycin N-demethylase (cytochrome 3A4) at its concentration of 0.05-0.10 mM in the diabetic hamsters.

CONCLUSIONS

Our data demonstrated that propofol in therapeutic concentrations of 0.05 and 0.10 mM, could inhibit both liver and kidney defluorination and cytochrome P-450's activities of the diabetic hamsters in vitro of different extent. This should remind clinicians of propofol's potential drug-to-drug interactions in the diabetic patients.

Metabolic characteristics and enflurane defluorination of cytochrome P450-dependent monooxygenases in human hepatocellular carcinoma

BACKGROUND

Xenobiotic metabolism and defluorination capacity of microsomal monooxygenases were investigated in vitro through the surgical specimens of liver resected from patients with hepatocellular carcinoma and patients of extrahepatic pathology as control.

METHODS

In microsomes of hepatocellular carcinoma tissues, the activities of cytochrome P450-dependent monooxygenase isoenzymes 1A1, 2B1, and 2E1 were evaluated in vitro by reacting with the specific marker substrates benzo(a)pyrene, benzphetamine and aniline, respectively, in the generating incubation system. The distant normal liver tissues and tissues from control patients with extrahepatic lesion were also investigated for comparison. The ability of enflurane defluorination was assessed by Orion combined for detection of free fluoride ion production.

RESULTS

Concentrations of P450 total content, cytochrome b5, and NADPH-cytochrome c reductase showed parallel and marked reduction in tumor tissues when compared with its distant normal regions or normal livers. The monooxygenase functions displayed significant decreases within the tumor tissues as benzo(a)pyrene hydroxylation > or = benzphetamine demethylation > aniline hydroxylation in magnitude. Defluorination of enflurane also markedly decreased in tumor tissues comparing with normal livers.

CONCLUSIONS

These marked reductions in the compositions and in vitro metabolic activities, including defluorination of anesthetics, in the cytochrome P450-dependent monooxygenases within the tumor tissues characterize the unique pattern of xenobiotic metabolism in patients with hepatocellular carcinoma.

Mass spectrometry provides warning of carbon monoxide exposure via trifluoromethane

BACKGROUND

The chemical breakdown of isoflurane, enflurane, or desflurane in dried carbon dioxide absorbents may produce carbon monoxide. Some mass spectrometers can give false indications of enflurane during anesthetic breakdown.

METHODS

During clinical anesthesia with isoflurane or desflurane, the presence of carbon monoxide in respiratory gas was confirmed when enflurane was inappropriately indicated by a clinical mass spectrometer that identified enflurane at mass to charge ratio = 69. In vitro, isoflurane, enflurane, or desflurane in oxygen was passed through dried carbon dioxide absorbents at 35, 45, and 55 degrees C. Gases were analyzed by gas chromatography and by mass spectrometry.

RESULTS

Mass spectrometry identified several clinical incidents in which 30-410 ppm carbon monoxide was measured in respiratory gas. Trifluoromethane was produced during in vitro breakdown of isoflurane or desflurane. Although these inappropriately indicated quantities of "enflurane" correlated (r2 > 0.95) to carbon monoxide concentrations under a variety of conditions, this ratio varied with temperature, anesthetic agent, absorbent type, and water content.

CONCLUSIONS

Trifluoromethane causes the inappropriate indication of enflurane by mass spectrometry, and indicates isoflurane and desflurane breakdown. Because the ratio of carbon monoxide to trifluoromethane varies with conditions, this technique cannot be used to quantitatively determine the amount of carbon monoxide to which a patient is exposed. If any warning of anesthetic breakdown results from this technique then remedial steps should be taken immediately to stop patient exposure to carbon monoxide. No warning can be provided for the breakdown of enflurane by this technique.

Stereoselective metabolism of enflurane by human liver cytochrome P450 2E1

The toxicity of the chiral fluorinated volatile anesthetics halothane, enflurane, and isoflurane is closely related to their metabolism by hepatic cytochrome P450. Although individual anesthetic enantiomers have been shown to exhibit a difference in anesthetic efficacy, metabolism of anesthetic enantiomers has not been reported. Human liver enflurane metabolism to difluoromethoxydifluoroacetic acid (DFMDFA) and inorganic fluoride is catalyzed in vivo and in vitro by cytochrome P450 2E1. The purpose of this investigation was to characterize enflurane racemate and enantiomer metabolism to test the hypothesis that fluorinated ether anesthetic metabolism by cytochrome P450 2E1 exhibits substrate stereoselectivity. Enflurane metabolism by microsomes from three human livers and by microsomes containing cDNA-expressed human P450 2E1 was measured at saturating enflurane concentrations. DFMDFA was quantitated with gas chromatography-mass spectrometry by detection of the ethanolamide derivative. In microsomes from all three livers, (R)-enflurane metabolism was significantly greater than that of (S)-enflurane, whereas rates of racemic enflurane metabolism were generally between those seen for the R- and S-enantiomers. The ratio of (R)-enflurane to (S)-enflurane metabolism in the three livers studied was 2.1:1, 1.9:1, and 1.4:1. (R)-, (S)-, and racemic enflurane were all metabolized by expressed P450 2E1. The ratio of (R)-enflurane to (S)-enflurane metabolism was 1.9:1. The metabolic enantiomeric selectivity of human liver P450 2E1 for (R)-enflurane suggests that enflurane metabolism by P450 2E1 occurs by direct substrate oxidation, rather than indirectly through the generation of a P450-dependent reactive oxygen species, and supports the hypothesis that the P450 2E1 active site is somewhat restrictive and capable of stereochemical discrimination.

Halogenated anesthetics form liver adducts and antigens that cross-react with halothane-induced antibodies

Two halogenated anesthetics, enflurane and isoflurane, have been associated with an allergic-type hepatic injury both alone and following previous exposure to halothane. Halothane hepatitis appears to involve an aberrant immune response. An antibody response to a protein-bound biotransformation product (trifluoroacetyl adduct) has been detected on halothane hepatitis patients. This study was performed to determine cross-reactivity between enflurane and isoflurane with the hypersensitivity induced by halothane. The subcellular and lobular production of hepatic neoantigens recognized by halothane-induced antibodies following enflurane and isoflurane, and the biochemical nature of these neoantigens was investigated in two animal models. Enflurane administration resulted in neoantigens detected in both the microsomal and cytosolic fraction of liver homogenates and in the centrilobular region of the liver. In the same liver, biochemical analysis detected fluorinated liver adducts that were up to 20-fold greater in guinea pigs than in rats. This supports and extends previous evidence for a mechanism by which enflurane and/or isoflurane could produce a hypersensitivity condition similar to that of halothane hepatitis either alone or subsequent to halothane administration. The guinea pig would appear to be a useful model for further investigations of the immunological response to these antigens.

Anesthetic and hemodynamic interactions of dexmedetomidine and fentanyl in dogs

BACKGROUND

Anesthetic doses of dexmedetomidine (DMED), a highly selective alpha 2 agonist, are not well tolerated hemodynamically. The combination of an opioid with DMED might reduce the dosage requirements for each drug and thereby allow the same anesthetic depth to be achieved with lesser degrees of their individual side effects.

METHODS

Dogs were anesthetized with enflurane. One group (n = 5) received intravenous doses of DMED from 0.1 to 10 micrograms/kg. Two other groups of five dogs each received fentanyl 15 micrograms/kg plus 0.05 microgram.kg-1.min-1 or fentanyl 45 micrograms/kg plus 0.2 micrograms.kg-1.min-1. Thereafter, they received DMED doses of 0.03-3 micrograms/kg. After the effects of the last DMED dose were measured, atipamezole 0.3 mg/kg was infused intravenously and all measurements were repeated. Then, naloxone (1 mg/kg) was injected intravenously and a final set of measurements obtained. Anesthetic effects were assessed by determining enflurane minimum alveolar concentration (MAC). Hemodynamics and plasma fentanyl concentrations were measured at each determination of MAC.

RESULTS

DMED and fentanyl individually produced dose-related reductions of enflurane MAC. During the lower rate infusion of fentanyl (plasma fentanyl concentration 1.0 +/- 0.3 ng/ml), DMED reduced enflurane MAC more than could be attributed to a simple additive interaction. During the higher rate infusion of fentanyl (plasma fentanyl concentration 4.4 +/- 0.7 ng/ml), DMED reduced enflurane MAC to greater degrees than were achievable by fentanyl alone. DMED caused a dose-dependent increase in arterial pressure concomitantly with a decrease in cardiac output, and these changes were not modified by fentanyl. The bradycardia following DMED was augmented by fentanyl.

CONCLUSIONS

There was a positive interaction, additive or synergistic, between DMED and fentanyl with respect to their enflurane-sparing effects. The interaction allowed the same depth of anesthesia to be achieved by lower doses of all three drugs, potentially limiting the intensity of their individual side effects. However, the presence of fentanyl increased the degree of bradycardia induced by DMED.

Clinical enflurane metabolism by cytochrome P450 2E1

BACKGROUND

Fluorinated ether anesthetic hepatotoxicity and nephrotoxicity are mediated by cytochrome P450-catalyzed oxidative metabolism. Metabolism of the volatile anesthetic enflurane to inorganic fluoride ion by human liver microsomes in vitro is catalyzed predominantly by the cytochrome P450 isoform CYP2E1. This investigation tested the hypothesis that P450 2E1 is also the isoform responsible for human enflurane metabolism in vivo. Disulfiram, which is converted in vivo to a selective inhibitor of P450 2E1, was used as a metabolic probe for P450 2E1.

METHODS

Twenty patients undergoing elective surgery were randomized to receive disulfiram (500 mg orally; n = 10) or nothing (control subjects; n = 10) the evening before surgery. All patients received a standard anesthetic of enflurane (2.2% end-tidal) in oxygen for 3 hours. Blood enflurane concentrations were measured by gas chromatography. Plasma and urine fluoride concentrations were quantitated by ion-selective electrode.

RESULTS

Patient groups were similar with respect to age, weight, gender, duration of surgery, and blood loss. Total enflurane dose, measured by cumulative end-tidal enflurane concentrations (3.9 to 4.1 MAC-hr) and by blood enflurane concentrations, was similar in both groups. Plasma fluoride concentrations increased from 3.6 +/- 1.5 mumol/L (baseline) to 24.3 +/- 3.8 mumol/L (peak) in untreated patients (mean +/- SE). Disulfiram treatment completely abolished the rise in plasma fluoride concentration. Urine fluoride excretion was similarly significantly diminished in disulfiram-treated patients. Fluoride excretion in disulfiram-treated patients was 62 +/- 10 and 61 +/- 12 mumol on days 1 and 2, respectively, compared with 1090 +/- 180 and 1200 +/- 220 mumol in control subjects (p < 0.05 on each day).

CONCLUSIONS

Disulfiram prevented fluoride ion production after enflurane anesthesia. These results suggest that P450 2E1 is the predominant P450 isoform responsible for human clinical enflurane metabolism in vivo.

Intrapulmonary gas mixing and pulmonary gas exchange in artificially ventilated dogs

To investigate the effect of incomplete gas mixing between tidal air and residual gas on pulmonary gas exchange, anaesthetized dogs were ventilated artificially with breathing patterns with different durations of the post-inspiratory apnoea (ta = 0, 0.5, 1.0 and 2.0 s), where tidal volume, breathing frequency, inspiratory and expiratory flow patterns were kept constant. We determined the alveolar ventilations (VA) of He and SF6 from the product of end-expiratory lung volume (VL,E') and specific ventilation (VA/VL,E'). VL,E' was determined by the dilution technique and the specific ventilations of the two gases were obtained from their multiple-breath washout. Further, tracer amounts of acetone, ether and enflurane were infused continuously into a peripheral vein and a bolus of a gas mixture of krypton, Freon12 and SF6 was introduced into the peritoneal cavity. We determined the Excretion (E) and Retention (R) of these six gases according to the multiple-inert-gas-elimination technique (MIGET). VA increased with increasing ta, where VA,He was about 14% larger than VA,SF6. For both gases, however, the increase in VA relative to control (VA for ta = 0) was virtually the same: 9, 11 and 19% (mean values) for ta = 0.5, 1.0 and 2.0 s respectively. For all dogs the E/R curve shifted to larger E values with increasing ta. E for the most soluble tracer gas (acetone) increased by 11, 21 and 25% for ta = 0.5, 1.0 and 2.0 s respectively. VA, determined with MIGET from the ventilation/perfusion distribution, increased by almost the same percentages.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of propofol on cytochrome P450 activities in rat hepatic microsomes

The effects of propofol on cytochrome P450 activity in rat hepatic microsomes were evaluated to determine the potential influence of this anesthetic on the metabolism of coadministered agents. In microsomes from untreated and isoniazid-treated rats, propofol was a weak inhibitor of enflurane metabolism, inhibiting activity only at 0.35 mM propofol. In contrast, toluene, a related compound, effectively impaired enflurane defluorination in microsomes from untreated, and isoniazid- and phenobarbital-treated rats at concentrations as low as 0.025 mM. Propofol, in contrast to toluene, was an effective inhibitor of benzphetamine demethylation where it inhibited this activity at propofol concentrations as low as 0.025 mM in microsomes from phenobarbital-treated rats. In microsomes from phenobarbital-treated rats, propofol potently inhibited the metabolism of aniline. Sixty-four percent inhibition was achieved at 0.03 mM propofol, whereas toluene had no effect at 1 mM. These data demonstrate that propofol does not effectively inhibit enflurane metabolism performed by the isoniazid-inducible cytochrome P450IIE1 but effectively impairs activities of the phenobarbital-inducible cytochrome P450 isozymes.

Intrathecal morphine during thoracotomy, Part I: Effect on intraoperative enflurane requirements

The ability of intrathecal morphine to reduce the anesthetic requirements during thoracotomy was investigated. Twenty-four patients scheduled for thoracic surgery were studied. Anesthesia was induced with thiamylal sodium, 4 mg/kg, fentanyl, 100 micrograms, and 100 mg of succinylcholine. Prior to skin incision, 12 patients received intrathecal injection of 12 micrograms/kg of preservative-free morphine sulfate (ITM), while the remaining 12 patients served as controls. The ITM was given undiluted at the L3-4 or L4-5 level. Anesthesia was maintained solely with enflurane, titrated to keep mean arterial pressure within 15% of the preoperative values. Vecuronium was given as required for relaxation. No additional narcotics were administered. Throughout the procedure, end-tidal (ET) enflurane concentration was recorded at 15-minute intervals from the mass spectrometer (Perkin Elmer). The intraoperative mean ET concentration of enflurane was significantly reduced in the ITM group beginning 1 hour after the injection (1.19 +/- .45% in the control group versus 0.73 +/- 0.08% in the ITM group). The enflurane requirements, expressed as percent end-tidal enflurane/hour, were significantly less in the ITM group for the duration of the procedure (0.8 +/- .17 v 1.08 +/- .22, respectively). In conclusion, when administered prior to skin incision for post-thoracotomy pain control, intrathecal morphine reduces intraoperative enflurane requirements.

Human liver microsomal enflurane defluorination catalyzed by cytochrome P-450 2E1

The volatile anesthetic agent enflurane undergoes oxidative metabolism in human liver, yielding both inorganic and organic fluoride metabolites. Numerous studies conducted in animals indicate that the enzyme cytochrome P-450 2E1 is a major catalyst for the defluorination reaction. However, the P-450 enzyme catalyzing enflurane metabolism in humans has not been identified. Therefore, experiments were conducted to determine whether hepatic P-450 2E1 is a catalyst for the reaction in humans, and whether other constitutive or inducible isoforms might also be involved. Purified human liver P-450 2E1, reconstituted with cytochrome b5 and P-450 reductase, catalyzed enflurane defluorination at a rate of 9.3 nmol F-/nmol P-450/30 min, in contrast to a mean liver microsomal rate of 2.0 nmol F-/nmol P-450/30 min. The microsomal rate of defluorination for individual human livers correlated significantly with the microsomal content of P-450 2E1 protein (r = 0.92), the rate of p-nitrophenol hydroxylation (r = 0.86), and the rate of chlorzoxazone 6-hydroxylation (r = 0.90). In addition, specific anti-P-450 2E1 IgG, at a concentration of 10 mg IgG/nmol P-450 inhibited the microsomal reaction by 80%. Finally, a series of P-450 isoform-specific chemical inhibitors of oxidative metabolism--furafylline (1A2), sulfaphenazole (2C9/10), quinidine (2D6), troleandomycin (3A3/4), and diethyldithiocarbamate (2E1)--were screened for their ability to block human microsomal enflurane defluorination. Only diethyldithiocarbamate, a mechanism-based inhibitor of P-450 2E1, inhibited the reaction; this occurred to a degree comparable to the effect of anti-P-450 2E1 antibody.(ABSTRACT TRUNCATED AT 250 WORDS)

Does hypothermia or hyperventilation affect enflurane MAC reduction following partial cardiopulmonary bypass in dogs?

This study in dogs determined the effect of systemic cooling and arterial hypocarbia during cardiopulmonary bypass (CPB) on the requirements for enflurane anaesthesia (MAC) before and after CPB. Twelve mongrel dogs were each anaesthetized with enflurane in oxygen on two separate occasions. End-tidal enflurane concentration was measured with a Puritan-Bennett Anaesthesia Agent Monitor. Using the tail-clamp method, MAC was determined twice with a one-hour interval between measurements (MAC 1 and MAC 2). Partial CPB was then initiated using femoral arterio-venous cannulation and maintained for one hour. Following separation from CPB, MAC was again determined twice with a one hour interval between measurements (MAC 3 and MAC 4). Dogs were randomly assigned according to PaCO2 management during CPB (low, 17.6 +/- 8.6 mmHg vs high, 38.9 +/- 11.5 mmHg), and then subjected to two experimental conditions. The first experiment on each dog was undertaken using normothermia during CPB (warm, 35-37 degrees C) while the second experiment (at least two weeks later) was conducted using hypothermia during CPB (cold, 30 degrees C). Analysis of the data, using ANOVA for repeated measures, revealed MAC 3 (1.95 +/- 0.33%, post-CPB) to be reduced when compared with MAC 1 (2.18 +/- 0.28%, P < 0.01) or MAC 2 (2.10 +/- 0.22%, P < 0.01), determined before CPB. Multivariate repeated measures analysis revealed no independent effects of hypothermia or arterial hypocarbia during CPB, on MAC reduction. By the time of the second experiment in each dog (two weeks later), MAC had returned to baseline levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of hepatic microsomal cytochrome P450IIE1 level by dietary lipids and carbohydrates in rats

The present work tests the hypothesis that high fat/low carbohydrate diets elevate the level of liver microsomal cytochrome P450IIE1. Male Sprague-Dawley rats were fed liquid diets containing varied ratios of corn oil/carbohydrate for 4 d. Rats fed diets with higher fat/carbohydrate ratios produced higher serum acetone levels and higher hepatic microsomal P450IIE1 content and N-nitrosodimethylamine demethylase activity than those fed diets with lower fat/carbohydrate ratios. This dietary fat/carbohydrate effect on P450IIE1 also was observed with modified semipurified AIN-76A diets. In addition, both the quantity and the extent of unsaturation of dietary lipids affected P450IIE1 regulation. At moderate fat levels (5 and 20% diet), rats fed corn oil and menhaden oil diets produced higher P450IIE1 activity than those fed lard and olive oil diets. Rats fed a diet containing 20% corn oil or an amount of linoleic acid equivalent to the 20% corn oil diet showed twofold to threefold increases in the level of P450IIE1 over those fed a fat-free diet. Rats fed a 25% corn oil diet showed twofold higher enflurane metabolism in vivo than those fed a 0.5% corn oil diet. The present results suggest that the constitutive P450 enzyme level is regulated by dietary fat/carbohydrate ratios.

Blood/gas solubility coefficient and blood concentration of enflurane during normothermic and hypothermic cardiopulmonary bypass

The blood/gas solubility coefficient and blood concentration of enflurane were measured at intervals in 10 patients undergoing coronary artery revascularization with cardiopulmonary bypass (CPB) and moderate hypothermia. A constant end-tidal concentration of enflurane was maintained throughout the study. Blood/gas solubility coefficient was determined at 37 degrees C, which when combined with an initial single-step equilibration of the blood sample with air, permitted the accurate measurement of blood concentration. Blood/gas solubility coefficient and blood concentration both decreased significantly with the onset of CPB. During the period of hypothermia, blood/gas solubility as measured at 37 degrees C showed little change; however, there was a progressive, marked increase in blood concentration with a mean increase of 80% prior to rewarming. Therefore, the level of anesthesia provided by enflurane may lighten with the onset of CPB, and a deeper level will accompany any decrease in blood temperature. On rewarming, blood concentration levels rapidly returned to levels similar to those measured before cooling. The increased uptake and accumulation of volatile anesthetic agent that occurred as a result of the period of hypothermic CPB was rapidly cleared. The rapidity with which blood concentration responded to the changes occurring during CPB make it unlikely that there was any significant increase in myocardial depression in response to the raised blood concentration secondary to the hypothermia.

The interaction between halogenated anaesthetics and bacteriorhodopsin in purple membranes as examined by intrinsic ultraviolet fluorescence

In the presence of halogenated general anaesthetics such as enflurane and halothane, the spectral properties of the bacteriorhodopsin pigment contained in the purple membranes of Halobacterium halobium are strongly modified. It is reversibly transformed into a red-coloured species absorbing maximally at 480 nm, at the expense of its characteristic 570-nm absorption band. The ultraviolet fluorescence of bacteriorhodopsin has been used to probe the structural modifications that are reflected by this spectral change. Our results show that they are very small and do not perturb the energy transfer dynamics which take place between the aromatic amino acid residues and the retinyl chromophore. The fluorescence properties of anaesthetic-treated bacteriorhodopsin are dominated by the quenching properties of the halogenated hydrocarbon, which are obvious even at anaesthetic concentrations under those needed to induce a spectral change in the bacteriorhodopsin chromophore. This does not rule out direct interaction between anaesthetics and bacteriorhodopsin, but it indicates that the chromophoric site might well not be their primary target.

Serum fluoride in children anaesthetized with enflurane

The serum inorganic fluoride concentration (SF) was measured in 40 children aged 22 days to 11 yrs (five infants) undergoing enflurane anaesthesia lasting 20-200 min, at an inspiratory concentration of 0.8 or 1.0%. Regardless of age, SF peaked at 2-8 mumol l-1 after 20-40 min of enflurane exposure, and at 4-10 mumol 1-1 and 6-10 mumol l-1 after 41-90 min and 91-200 min of exposure, respectively. The highest individual value was 12.5 mumol l-1. Another 23 children, aged 1-16 yrs, received 0.8% enflurane for 60 min. The increase in SF was 3-9 mumol l-1, with no clear dependence on age. Altogether, the increase in SF was comparable to that detected in adults after anaesthesia of equal duration.

Isoflurane inhibits enflurane metabolism in man

Halogenated inhalation anaesthetics interfere with each other's hepatic microsomal metabolism. The increase in plasma inorganic fluoride concentration, caused by the metabolism of a standardised dose of enflurane, was attenuated by isoflurane given either before or after the enflurane exposure. It is concluded that isoflurane inhibits the metabolism of enflurane in man, a fact that might be advantageous in certain clinical situations.

Ethanol-inducible cytochrome P450 in rabbits metabolizes enflurane

Following anaesthesia with enflurane, some patients receiving isoniazid have increased serum concentrations of fluoride ion, presumably because of induction of an isozyme of cytochrome P450 which is responsible for enflurane biodegradation. In rats, isoniazid and ethanol enhance metabolism of enflurane and also induce a form of cytochrome P450 which is homologous with a form of rabbit liver cytochrome P450 known as 3a. Isoniazid, ethanol and imidazole increase the concentration of cytochrome P450 3a in hepatic microsomes. We have pretreated rabbits with imidazole, the most potent of the three inducers of isozyme 3a, to determine if the hepatic microsomal metabolism of enflurane is enhanced and if purified isozyme 3a catalyses the oxidation of enflurane. Imidazole produced a 250% increase in the hepatic microsomal metabolism of enflurane, sevoflurane, methoxyflurane and the control substrate, aniline. Polyclonal antibodies to cytochrome P450 3a inhibited 90% of enflurane metabolism, but only 40% of methoxyflurane biotransformation in the microsomes from imidazole-pretreated rabbits. Thus isozyme 3a or a structurally similar cytochrome P450 seemed to catalyse almost all microsomal metabolism of enflurane. In addition, purified cytochrome P450 3a catalysed the metabolism of enflurane, sevoflurane and methoxyflurane, and the oxidation of these anaesthetics by cytochrome P450 3a was stimulated four-fold by cytochrome b5, a protein which serves as an alternate source of electrons for some cytochrome P450 reactions.

Saturable and unsaturable binding of a volatile anesthetic enflurane with model lipid vesicle membranes

Presence of specific receptors for volatile anesthetics has recently been proposed (Evers, A.S. et al. (1987) Nature 328, 157-160) by a finding that halothane uptake by the rat brain was characterized, in part, by saturable binding. We report here that volatile anesthetics bind model lipid membranes also with saturable and unsaturable kinetics. Binding of enflurane to dipalmitoylphosphatidylcholine vesicle membranes was measured by gas chromatography. At low anesthetic concentrations, comparable to the clinical level, the interaction was saturable. After reaching a temporary saturation, a sudden increase in the anesthetic binding to the membrane occurred, when the anesthetic concentration in the aqueous phase exceeded 2.7 mM, or 6.3 x 10(-2) atm partial pressure in the gas phase in equilibrium with the aqueous phase. The secondary binding was linear to the aqueous anesthetic concentrations and was unsaturable to the limit of this study. We also found that enflurane self-aggregated in water above 4 mM. When the aqueous concentration exceeded 6 mM, the aggregation number was about 8. We conclude that the saturable binding indicates adsorption onto the vesicle surface, and the unsaturable binding indicates multilayer stacking of the enflurane molecules, where the initially adsorbed molecules provide the binding sites to the succeeding molecules according to the multilayer condensation kinetics. The tendency of enflurane to self-aggregate in water promotes the multilayer stacking at the surface of the membrane.

Cimetidine and ranitidine do not affect enflurane metabolism in surgical patients

Hepatic cytochrome-P-450-linked microsomal metabolism is inhibited by cimetidine, and to a lesser extent by ranitidine. Such an inhibition might protect against the metabolite-related toxicity of inhalation anesthetics. However, in comparison with the values measured in a control group, neither cimetidine (600 mg p.o. + 200 mg i.m.) nor ranitidine (150 mg p.o. + 50 mg i.m.), both administered 11-12 h before anesthesia, inhibited enflurane metabolism as assessed by the increase in plasma inorganic fluoride concentration and urinary fluoride excretion in 21 ASA I patients anesthetized with enflurane (end-tidal concentration 0.5 +/- 0.05% for 2-6 h). The inorganic fluoride concentration in the gastric juice remained low in all groups.

Enflurane metabolism produces covalently bound liver adducts recognized by antibodies from patients with halothane hepatitis

The existence of a rare syndrome of "enflurane hepatitis" similar to that described for halothane and of a cross-sensitization between halothane and enflurane has been controversial, largely due to equivocal clinical case reports and a lack of a plausible molecular mechanism for the hepatotoxicity. The present study suggests a possible hypersensitivity basis for enflurane hepatitis and the apparent cross-sensitization between halothane and enflurane involving covalently bound liver microsomal adducts. Immunoblotting studies have revealed that antibodies in the sera of six patients with halothane hepatitis recognize liver microsomal antigens of Mr = 100,000, or both 100,000 and 76,000, formed in rats treated with enflurane or halothane. These antigens were not detected in microsomes from isoflurane- or sesame oil-treated rats. The recognition of these antigens could be abolished by preincubation of the sera with microsomes from halothane-treated rats. These data suggest that the difluoromethoxydifluoroacetyl halide metabolite of enflurane, as well as the trifluoroacetyl halide metabolite of halothane, covalently bind to similar hepatic proteins, and may become immunogens in susceptible patients. This mechanism may also account for the apparent cross-sensitization between halothane and enflurane anesthesia, and the development of hepatic necrosis.

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.

Contrasting effects of etomidate and propylene glycol upon enflurane metabolism and adrenal steroidogenesis in Fischer 344 rats

This study was designed to investigate the effects of etomidate and its solubilizing agent (propylene glycol) upon enflurane metabolism and adrenal steroidogenesis in Fischer 344 rats. A central venous catheter was placed using pentobarbital anesthesia, and rats were randomized to one of four groups for treatment several days later. Group 1 animals received normal saline, 3 ml/kg, given via the central venous catheter. The other three groups were administered equivalent volumes of either: crystalline etomidate (group 2), 0.4 mg/ml, in saline and 1.1% ethanol; propylene glycol (group 3), 7%, in saline; or etomidate (group 4), 0.4 mg/ml in saline with 7% propylene glycol. In the first part of this study, after an intravenous bolus of one of these four solutions, animals were immediately placed in a 200-liter chamber and received 1 h of 2% enflurane. Serum and urine were assayed for inorganic fluoride (F-) before and after anesthesia. Two hours after enflurane anesthesia, groups 1 and 2 had the highest mean peak serum F- concentrations (13.2 and 13.5 uM, respectively). Groups 3 and 4 had significantly lower mean peak serum F- concentrations (4.7 and 4.5 uM, respectively). In the second part of this study, additional animals were randomized into four groups and received the same intravenous medications as above. Thirty minutes later, they received an intravenous bolus of ACTH. Blood samples were drawn and serum aldosterone levels were measured. Animals in groups 1 and 3 had significantly greater increases in peak serum aldosterone levels 30 minutes after ACTH (peak levels: 0.80 and 0.77 ng/ml, respectively) than animals in groups 2 and 4 (peak levels: 0.60 and 0.58 ng/ml, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Potential metabolic basis for enflurane hepatitis and the apparent cross-sensitization between enflurane and halothane

Clinical case reports of unexplained hepatic dysfunction following enflurane and isoflurane anesthesia led to the hypothesis that oxidative metabolism of these drugs by cytochromes P-450 produces immunoreactive, covalently bound acylated protein adducts similar to those implicated in the genesis of halothane-induced hepatic necrosis. Microsomal adducts were detected by enzyme-linked immunosorbent assay and immunoblotting techniques utilizing specific anti-trifluoroacetyl (TFA) IgG hapten antibodies in rat liver following enflurane, isoflurane, or halothane administration. Preincubation of the antibodies with microsomes from halothane-pretreated rats or with 500 microM TFA-lysine, markedly inhibited adduct recognition, while preincubation with 500 microM acetyllysine had no effect. The relative amounts of immunoreactive protein adducts formed were halothane much greater than enflurane much greater than isoflurane and correlates directly with the relative extents of metabolism of these agents. These results support the view that acyl metabolites of the volatile anesthetics may become covalently bound to hepatic proteins, thus serving as antigens, and thereby account for the apparent cross-sensitization and idiosyncratic hepatotoxicity reported for these drugs.

Dezocine-MAC reduction and evidence for myocardial depression in the presence of enflurane

The effect of dezocine, an agonist-antagonist opioid analgesic, on enflurane MAC (EMAC) was measured in dogs and, in a separate study, the hemodynamic effects of IV bolus doses of dezocine in the presence of a stable end-tidal enflurane concentration were measured. Study 1 (n = 8)--EMAC Reduction: Dezocine reduced EMAC in a dose-dependent fashion by a maximum of 58 +/- 3% (mean +/- SEM) after injection of 20 mg/kg. Cardiac toxicity prevented administration of higher doses. Study 2--Hemodynamics: Group 1 (n = 7) received dezocine 0.2, 1.5, 5, and 20 mg/kg IV each as a bolus over 30 sec. Group 2 (n = 5) was studied in exactly the same manner except that instead of dezocine, each dog received drug carrier solution alone (carrier). Significant hemodynamic differences between carrier and drug groups were observed only at the 20 mg/kg dose level, which produced death in one dog and a decrease in mean aortic pressure to 39 +/- 5% of baseline, in cardiac output to 60 +/- 9% of baseline, and in stroke volume to 69 +/- 9% of baseline in the remaining dogs. It is concluded that dezocine produces a dose-dependent reduction in EMAC limited by cardiovascular toxicity. This toxicity appears to be related to direct myocardial depression by high doses of dezocine in the presence of enflurane.

The enflurane sparing effect of sufentanil in dogs

There is a ceiling to the reduction of enflurane MAC by fentanyl in the dog. Sufentanil (SUF), a more potent narcotic, may be more efficacious in reducing enflurane MAC. To test this hypothesis, 25 mongrel dogs were studied in three groups. Group 1 (n = 8) received SUF in progressively increasing infusion rates from 0.005 micrograms . kg-1 . min-1 to a maximum of 1.215 micrograms . kg-1 . min-1. MAC was determined at stable SUF concentrations in plasma [SUF] during each infusion rate. Group 2 (n = 10) received SUF at a dose rate (0.007 micrograms . kg-1 . min-1) designed to produce approximately 35% MAC reduction, and MAC determinations were made at regular intervals over a mean infusion time of 7.6 +/- 0.43 h (mean +/- SEM). Group 3 (n = 7) received 1.215 micrograms . kg-1 . min-1 and were studied as in group 2 over an infusion time of 6.7 +/- 0.42 h. In group 1, the highest infusion rate (1.215 micrograms . kg-1 . min-1) produced [SUF] = 48 ng/ml and reduced MAC by 71 +/- 6%. This was not statistically different from the reduction which occurred at [SUF] = 0.92 ng/ml (57 +/- 7%; infusion rate 0.015 micrograms . kg-1 . min-1; P = 0.21). In group 2, the degree of MAC reduction achieved by stable [SUF] (0.54 +/- 0.08 ng/ml) declined over time (MAC reduction at start = 34 +/- 2% versus 18 +/- 4.0% at the end of the infusion; P = 0.001), suggesting the development of tolerance.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Serum fluoride levels in morbidly obese patients: enflurane compared with isoflurane anaesthesia

Obese patients are known to metabolise anaesthetic agents more than patients of normal weight. The extent of this was investigated by the measurement of serum fluoride concentrations in 10 morbidly obese patients undergoing gastroplasty. Five were allocated to receive enflurane and five to receive isoflurane supplemented anaesthesia. The mean peak serum fluoride concentrations after enflurane anaesthesia were greater (22.7 mumol/litre, SE 2.9) than after isoflurane anaesthesia (6.5 mumol/litre, SE 0.6). The mechanisms and implications of this finding are discussed.

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.

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.

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.

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.

Liver function following hypovolemic hypotension in rats anaesthetized with halothane or enflurane

Rats which had approximately 25-30% of their calculated blood volume removed were exposed to halothane (1%) or enflurane (2%) in 33% oxygen for 30 min. Hepatic function was evaluated by determining, at various time intervals, serum activities of glutamic-oxalacetic and glutamic-pyruvic transaminase, acid phosphatase and gamma-glutamyl-transpeptidase. In this model serum enzyme activities and animal mortality were significantly increased when hypovolemic hypotension was induced during halothane anaesthesia. The same events did not occur in bleeding animals anaesthetized with enflurane. The marked disparity in hepatic dysfunction and mortality between halothane and enflurane-anaesthetized rats during hypovolemic hypotension may be explained by the more pronounced decrease of oxygen available for the liver and production of reductive toxic intermediates in animals exposed to halothane.

The extent of metabolism of inhaled anesthetics in humans

To determine the percentage of anesthetic metabolized and to assess the role of metabolism in the total elimination of inhaled anesthetics, the authors administered isoflurane, enflurane, halothane, and methoxyflurane simultaneously, for 2 h, to nine healthy patients. Total anesthetic uptake during the 2 h of washin and total recovery of unchanged anesthetic in exhaled gases during 5 to 9 days of washout were measured, and from these the per cent of anesthetic uptake that was recovered was calculated. Of the isoflurane taken up, 93 +/- 4% (mean +/- SE) was recovered. To compensate for factors other than metabolism that limit complete recovery of unchanged anesthetic, the percentage recovery of each anesthetic was normalized to the percentage recovery of isoflurane (which it was assumed undergoes no metabolism). Deficits in normalized recovery were assumed to be due to metabolism of the anesthetics. The resulting estimates of metabolism of anesthetic taken up were: enflurane 8.5 +/- 1.0%, halothane 46.1 +/- 0.9%, and methoxyflurane 75.3 +/- 1.6%. These results indicate that elimination is primarily via the lungs for isoflurane and enflurane, equally via the lungs and via metabolism for halothane, and primarily via metabolism for methoxyflurane.

Effect of cimetidine on biotransformation of enflurane in man

This study was designed to test the hypothesis that administration of clinical doses of cimetidine could affect the metabolic degradation of enflurane to inorganic fluoride via inhibition of the mixed function oxidase enzyme (MFOE) system. In Part 1 of the study 38 female patients undergoing gynaecologic surgery received, double blind, either cimetidine, 300 mg PO the night prior to surgery and 300 mg IV 30 minutes prior to anaesthesia induction or a placebo. In Part 2, 24 patients received either cimetidine as in Part 1, but with continued administration for 24 hours into the postoperative period, or a placebo. Anaesthesia in all cases was with enflurane in oxygen, via a closed circuit. In both Parts 1 and 2 of the study there were no statistically significant differences between the two groups in serum fluoride levels at baseline, four hours or 24 hours postoperatively, or in the total urinary fluoride excretion during the first or second postoperative days. The authors speculate that this is due either to separate interactions of cimetidine and enflurane with the MFOE system or to the relatively low rate of enflurane metabolism.

Pharmacokinetics of inhaled anesthetics in humans: measurements during and after the simultaneous administration of enflurane, halothane, isoflurane, methoxyflurane, and nitrous oxide

To determine the relative washin and washout characteristics of isoflurane, enflurane, halothane, and methoxyflurane, we administered all four anesthetics simultaneously (total = 1.1 MAC) to nine healthy patients for 2 hr. Concentrations of anesthetics in end-tidal gases were measured during washin and for 5-9 days during washout. Multiexponential (multicompartment) models were fit to the washin and washout curves using least-squares analysis. Slowly equilibrating compartments could only be identified during washout. For 27 of the 36 data sets, five-compartment models fit the washout curves significantly better than four-compartment models. The time constant for our first compartment is consistent with that predicted for washout of the lungs. Time constants for the second, third, and fifth compartments were consistent with current data for blood flows and solubilities of vessel-rich, muscle, and fat tissue groups, respectively. The fourth compartment has a time constant that lies between the time constants predicted for muscle and fat.

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)