End-tidal to Arterial Gradients and Alveolar Deadspace for Anesthetic Agents.

BACKGROUND: According to the "three-compartment" model of ventilation-perfusion ((Equation is included in full-text article.)) inequality, increased (Equation is included in full-text article.)scatter in the lung under general anesthesia is reflected in increased alveolar deadspace fraction (VDA/VA) customarily measured using end-tidal to arterial (A-a) partial pressure gradients for carbon dioxide. A-a gradients for anesthetic agents such as isoflurane are also significant but have been shown to be inconsistent with those for carbon dioxide under the three-compartment theory. The authors hypothesized that three-compartment VDA/VA calculated using partial pressures of four inhalational agents (VDA/VAG) is different from that calculated using carbon dioxide (VDA/VACO2) measurements, but similar to predictions from multicompartment models of physiologically realistic "log-normal" (Equation is included in full-text article.)distributions. METHODS: In an observational study, inspired, end-tidal, arterial, and mixed venous partial pressures of halothane, isoflurane, sevoflurane, or desflurane were measured simultaneously with carbon dioxide in 52 cardiac surgery patients at two centers. VDA/VA was calculated from three-compartment model theory and compared for all gases. Ideal alveolar (PAG) and end-capillary partial pressure (Pc'G) of each agent, theoretically identical, were also calculated from end-tidal and arterial partial pressures adjusted for deadspace and venous admixture. RESULTS: Calculated VDA/VAG was larger (mean ± SD) for halothane (0.47 ± 0.08), isoflurane (0.55 ± 0.09), sevoflurane (0.61 ± 0.10), and desflurane (0.65 ± 0.07) than VDA/VACO2 (0.23 ± 0.07 overall), increasing with lower blood solubility (slope [Cis], -0.096 [-0.133 to -0.059], P < 0.001). There was a significant difference between calculated ideal PAG and Pc'G median [interquartile range], PAG 5.1 [3.7, 8.9] versus Pc'G 4.0[2.5, 6.2], P = 0.011, for all agents combined. The slope of the relationship to solubility was predicted by the log-normal lung model, but with a lower magnitude relative to calculated VDA/VAG. CONCLUSIONS: Alveolar deadspace for anesthetic agents is much larger than for carbon dioxide and related to blood solubility. Unlike the three-compartment model, multicompartment (Equation is included in full-text article.)scatter models explain this from physiologically realistic gas uptake distributions, but suggest a residual factor other than solubility, potentially diffusion limitation, contributes to deadspace.

Genomics and Proteomic Techniques.

Although general anesthesia induced by inhaled anesthetics produces definitive phenotypes (e.g., loss of mobility, amnesia, analgesia), the underlying targets of these drugs are still not clear. Genomics and proteomic techniques are discussed for measurement of global transcriptional and translational changes after inhaled anesthetic exposures. The current discussion focuses primarily on the genomic and proteomic technical methodology. We also include a discussion of network and pathway analyses for data interpretation after identification of the targets.

End-tidal sevoflurane and halothane concentrations during simulated airway occlusion in healthy humans.

BACKGROUND: In a patient whose airway is likely to become obstructed upon loss of consciousness, anesthesia may be induced using an inhaled vapor. If the airway occludes during such an inhalational induction, the speed of patient awakening is related to the rate at which anesthetic gas redistributes away from lung and brain to other body compartments. To determine whether redistribution occurs more rapidly with a more blood-soluble or a less blood-soluble agent, the authors used subanesthetic concentrations of halothane and sevoflurane to simulate inhalational induction and airway obstruction in eight healthy human volunteers. METHODS: Inhalational induction was simulated using stepwise increases in inspired halothane or sevoflurane concentration, sufficient to reach an end-tidal concentration of approximately 0.1 minimal alveolar concentration. Airway occlusion was then simulated by initiating a 90-s period of rebreathing from a 1-l bag. During rebreathing, end-tidal halothane or sevoflurane concentration was measured continuously by mass spectrometry, and a time constant for the decline in concentration was calculated using a monoexponential model. RESULTS: At the onset of rebreathing, end-tidal concentrations of halothane and sevoflurane were 0.10 +/- 0.03 and 0.11 +/- 0.03 minimal alveolar concentration, respectively (mean +/- SD; P > 0.1, Student t test). During rebreathing, the time constants for the decline in end-tidal halothane and sevoflurane concentration were 22 +/- 9 and 62 +/- 16 s, respectively (P < 0.0001). CONCLUSIONS: During simulated airway occlusion in healthy volunteers, the end-tidal concentration of halothane falls more rapidly than that of sevoflurane. Halothane may therefore lead to more rapid awakening, compared with sevoflurane, should the airway obstruct during an inhalational induction of anesthesia.

Determination of the minimum alveolar concentration for halothane, isoflurane and sevoflurane in the gerbil.

The present work determined the minimum alveolar concentrations (MAC) of halothane, isoflurane and sevoflurane in adult female gerbils (n=24). Animals were placed in a chamber for anaesthetic induction before performing tracheal intubation. The tracheal tube was connected to a non-rebreathing circuit with minimal dead space. Body temperature, blood pressure, heart and respiratory rates and end-tidal volatile anaesthetic levels were measured continuously. MAC was determined using a standard noxious stimulus (tail-clamp). All experiments were performed at the same time of the day, body temperature was maintained constant and blood-gas analysis was performed to confirm that values were within normal limits. The mean+/-SD MAC values were 1.06+/-0.11% halothane (n=8), 1.55+/-0.08% isoflurane (n=8) and 2.90+/-0.12% sevoflurane (n=7). Cardiovascular parameters at 1xMAC did not differ significantly among anaesthetics but the respiratory rate was significantly higher in the halothane group than in the isoflurane and sevoflurane groups. The SpO2 values recorded throughout anaesthesia and the pH and partial oxygen pressure values determined at the end of the study did not differ among the studied anaesthetics at 1xMAC. These data suggest that the MAC for halogenated inhalant anaesthetics in gerbils is lower than the average MAC values obtained in rats and mice.

Isoflurane and sevoflurane decrease entropy indices more than halothane at equal MAC values.

Recently, bispectral index (BIS) values were demonstrated to be different for various anesthetics as a result of differential effects on electroencephalographic (EEG) signals. Entropy is similar to the BIS monitor, as both process raw EEG to derive a number. We hypothesized that entropy may also be anesthetic agent-specific. Thirty adult patients undergoing spinal surgery were randomized to receive halothane, isoflurane, or sevoflurane. Entropy indices were recorded at various minimum alveolar concentration (MAC) values-0.5, 0.75, 1.0 and 1.5-both during wash-in and wash-out of the agent. Heart rate (HR), mean arterial blood pressure (MAP), response entropy (RE), and state entropy (SE) were noted. Statistical analysis was done using a one-way analysis-ofvariance test. P values less than 0.05 were considered significant. Ten patients in each group completed the study. The demographics and baseline values of HR, MAP, RE, and SE were comparable in all three groups. During the study period, for a given MAC value, both RE and SE remained low in the isoflurane and sevoflurane groups compared to the halothane group. For a given MAC, the RE and SE were comparable during wash-in and wash-out phases. Halothane produced higher entropy values as compared to isoflurane and sevoflurane at equivalent MAC levels.

Adverse drug reactions with halogenated anesthetics.

Problems with organ toxicity from the halogenated volatile anesthetics have influenced clinical practice, anesthetic selection, and drug development for half a century.(1,2) In turn, investigations that elucidated the mechanisms of volatile anesthetics adverse effects identified new mechanisms, provided remarkable insights into human toxicology, and represented seminal contributions to clinical pharmacology. This review focuses on adverse organ effects (hepatic, renal, and others) that are attributable to anesthetic metabolism and/or degradation. Routine reversible effects on pulmonary, cardiac, and other organ functions are not addressed.

Kinetics and potency of halothane, isoflurane, and desflurane in the Northern Leopard frog Rana pipiens.

Equilibration between delivered and effect site anesthetic partial pressure is slow in frogs. The use of less soluble agents or overpressure delivery may speed equilibration. Ten Northern leopard frogs were exposed to 3-4 constant concentrations of halothane, isoflurane or desflurane and their motor response to noxious electrical stimulation of the forelimb evaluated every 30 minutes until a stable proportion of frogs were immobile. Each frog received each anesthetic and concentration in random order and allowed at least 14 hours to recover between anesthetic exposures. An overpressure technique based upon the kinetics in the first study was then tested with 4 concentrations of desflurane. For halothane, isoflurane and desflurane respectively; the proportion of frogs immobile in response to stimulus became stable after 510, 480 and 180 minutes, and ED50 values were 1.36, 1.67 and 6.78 % atm. Desflurane ED50 delivered by overpressure was not significantly different at 6.85 % atm. Halothane, isoflurane and desflurane are effective general anesthetics in frogs with potencies similar to those reported in mammals. The time required for anesthetic equilibration is fastest with desflurane and can be hastened further by initial delivery of higher partial pressures.

Portable mass spectrometry for measurement of anaesthetic agents and methane in respiratory gases.

Monitoring the composition of gases breathed by anaesthetised patients requires measurement methods with fast responses, high accuracy and good reliability. There is also an increasing demand for systems to be able to monitor more than one target analyte simultaneously, but some gas analysers can be sensitive to the presence of methane gas in exhaled breath, consequently leading to inaccurate measurements of the anaesthetic agent. This study investigated the feasibility of employing portable quadrupole mass spectrometry to monitor volatile anaesthetic agents (halothane, isoflurane and sevoflurane), methane accumulation in anaesthetic rebreathing systems, and inspired and exhaled carbon dioxide and oxygen concentrations during equine anaesthesia in a clinical setting. The volatile anaesthetic agents were easily measurable and methane was detectable. The instrument had an advantage over short wavelength infrared absorption spectrometry analysers because it could monitor anaesthetic agents and other respiratory gases simultaneously and at extremely low concentrations, although further optimisation is required.

Thiopental and halothane dose-sparing effects of magnesium sulphate in dogs.

OBJECTIVE: To evaluate the effect of pre- and intraoperatively administered magnesium sulphate (MgSO(4)) on the induction dose of thiopental and of halothane for maintenance of anaesthesia in dogs undergoing ovariohysterectomy (OHE). STUDY DESIGN: Prospective, double-blind, randomized, placebo-controlled study. ANIMALS: Forty-six healthy, ASA physical status 1 dogs, scheduled for elective OHE. METHODS: The dogs were randomly assigned to receive a bolus of 50 mg kg(-1) MgSO(4) intravenously (IV), just before induction of anaesthesia, followed by a constant rate infusion (CRI) of 12 mg kg(-1) hour(-1) MgSO(4) intraoperatively (group Mg, n = 27) or a placebo bolus and CRI of 0.9% sodium chloride (NaCl) (group C, n = 19), approximately 30 minutes after premedication with acepromazine (0.05 mg kg(-1), intramuscularly, IM) and carprofen (4 mg kg(-1), subcutaneously, SC). Anaesthesia was induced with thiopental administered to effect and maintained with halothane in oxygen. End-tidal halothane (ET(hal)) was adjusted to achieve adequate depth of anaesthesia. Blood samples were obtained pre- and postoperatively for measurement of total serum magnesium concentration. RESULTS: The mean dose of thiopental was statistically lower (p < 0.0005) and the mean standardized ET(hal) concentration and end-tidal carbon dioxide partial pressure (Pe'CO(2)) areas under the curve were statistically smaller (p < 0.0005 and 0.014 respectively) in group Mg. Postoperatively the mean total serum magnesium concentration was statistically higher than the preoperative value (p < 0.0005) in group Mg, but not in group C. Nausea, associated with the MgSO(4) bolus injection, was observed in six dogs in group Mg, two of which vomited prior to induction of anaesthesia. CONCLUSIONS AND CLINICAL RELEVANCE: Magnesium sulphate administration reduced the induction dose of thiopental and ET(hal) concentration for maintenance of anaesthesia in dogs undergoing OHE. Observed side effects were nausea and vomiting.

Effects of pregnancy on the solubility of halogenated volatile anaesthetics in rat blood and tissues.

This study was designed to evaluate the effects of pregnancy on the solubility of halogenated volatile anaesthetics in rat blood and tissues. Tissue samples from 10 pregnant and 10 non-pregnant adult female Sprague Dawley rats, including the heart, liver, kidney and brain, were obtained and made into respective homogenates. Blood/gas and tissue/gas partition coefficients for halothane, sevoflurane and isoflurane were determined by the method of two-stage headspace equilibration by gas chromatography with each of the homogenates. Values were analysed by t-test or one-way analysis of variance. The solubility within blood and brain for halothane in the pregnant group (2.90 +/- 0.44, 5.55 +/- 0.73) was significantly lower than that of the non-pregnant group (3.42 +/- 023, 6.33 +/- 0.64; P < 0.05). However, there were no significant differences between the two groups for liver, kidney or heart solubility. For sevoflurane and isoflurane, there were no significant differences in solubility between the two groups. In conclusion, pregnancy decreased the solubility of halothane within the blood and brain, whereas the solubility of halothane in other tissues including the liver, kidney and heart showed no significant alteration. Pregnancy did not affect the solubility ofsevoflurane or isoflurane within blood or the other tissues studied.

Minimum alveolar concentration of halogenated volatile anaesthetics in left ventricular hypertrophy and congestive heart failure in rats.

BACKGROUND: Although many physiological and pathological conditions affect minimal alveolar concentration (MAC), there are no reliable data on the MAC for halogenated anaesthetics during left ventricular hypertrophy (LVH) and congestive heart failure (CHF). The aim of this experimental study was to determine the MAC values of halothane, isoflurane, and sevoflurane in rats, at early and later stages of cardiomyopathic hypertrophy. METHODS: LVH was induced by ascending aortic stenosis in 3-4-week-old rats. LVH and CHF in each animal were assessed weekly by echocardiography. MAC of halothane, isoflurane, and sevoflurane was determined using the tail-clamp technique in spontaneously breathing rats from each group. Response vs no-response data were analysed using logistic regression analysis. Data are medians (95% confidence interval). RESULTS: The MAC of halothane [1.30% (1.26-1.34)], isoflurane [1.52% (1.48-1.57)], and sevoflurane [2.93% (2.78-3.07)] in rats with LVH was not different from sham-operated rats [respectively, 1.23% (1.20-1.26), 1.52% (1.47-1.56), and 2.90% (2.79-3.00)]. Conversely, the MAC of halothane [1.44 (1.39-1.50)] and isoflurane [1.74 (1.69-1.78)], but not sevoflurane [2.99 (2.93-3.06)], was significantly increased in rats with CHF. CONCLUSIONS: MAC values for halothane, isoflurane, and sevoflurane were unchanged in rats with pressure-induced overload LVH. Conversely, the MAC for halothane and isoflurane, but not sevoflurane, was significantly increased in rats with CHF.

Equi-MAC concentrations of halothane and isoflurane do not produce similar bispectral index values.

Minimum alveolar concentration (MAC) has been traditionally used to measure the potency of an inhalational anesthetic agent. Recently, bispectral index (BIS) derived from the frontal cortical electroencephalogram has been used frequently for quantifying the hypnotic component of anesthesia. The present study was designed to examine the BIS values produced by equi-MAC concentrations of halothane and isoflurane. In 34 patients undergoing spinal surgery, BIS and spectral edge frequency (SEF95) were recorded at 3 different concentrations of halothane and isoflurane--namely 0.5, 0.75, and 1.0 MAC. The measurements were made both during wash-in and wash-out phases of the anesthetic agent. Eighteen patients received halothane and 16 received isoflurane. Heart rate, mean arterial pressure, oxygen saturation, and end tidal carbon dioxide pressure values were not different between the 2 groups at various MAC concentrations of the anesthetic agents. BIS and SEF95 values decreased significantly with increasing concentrations of both the anesthetic agents (P<0.001). At any given MAC concentration of the anesthetic, BIS and SEF(95) values were significantly lower under isoflurane compared with halothane anesthesia both during wash-in and wash-out phases (P<0.001). For a given anesthetic agent, BIS values were comparable at equi-MAC concentrations during wash-in and wash-out phases. In conclusion, BIS values are significantly lower under isoflurane compared with halothane anesthesia at similar MAC concentrations. For a given anesthetic agent and a given MAC concentration, the BIS values are similar during wash-in and wash-out phases of anesthesia.

The differential effects of halothane and isoflurane on electroencephalographic responses to electrical microstimulation of the reticular formation.

Isoflurane and halothane cause electroencephalographic (EEG) depression and neuronal depression in the reticular formation, a site critical to consciousness. We hypothesized that isoflurane, more than halothane, would depress EEG activation elicited by electrical microstimulation of the reticular formation. Rats were anesthetized with either halothane or isoflurane and stimulating electrodes were positioned in the reticular formation. In a crossover design, anesthetic concentration was adjusted to 0.8 and 1.2 minimum alveolar concentration (MAC) of halothane or isoflurane and electrical microstimulation was performed and the EEG responses were recorded. Microstimulation increased the spectral edge and median edge frequencies 2-2.5 Hz at 0.8 MAC for halothane and isoflurane and 1.2 MAC halothane. At 1.2 MAC isoflurane, burst suppression occurred and microstimulation decreased the period of isoelectricity (24% +/- 19% to 8% +/- 7%; P < 0.05), whereas the spectral edge and median edge frequencies were unchanged. At anesthetic concentrations required to produce immobility, the cortex remains responsive to electrical microstimulation of the reticular formation, although the EEG response is depressed in the transition from 0.8 to 1.2 MAC. These data indicate that cortical neurons remain responsive to synaptic input during isoflurane and halothane anesthesia.

Effects of tail fat on halothane biotransformation in fat-tailed sheep.

1. The aim of the present study was to evaluate the effects of tail fat on halothane biotransformation following similar anaesthetic exposure in intact sheep and sheep with a ligated median sacral artery. 2. A prospective randomized experimental study was performed using 12 healthy, 10-12-month-old female sheep. 3. Sheep were randomly divided into two groups of six animals each and were anaesthetized twice at 2 weekly intervals. After mask induction with halothane in 100% oxygen, sheep were intubated and anaesthesia was maintained for 3 h using a rebreathing system. Serum fluoride concentration (SFC) was measured at 0, 1, 3, 6, 12, 24, 48 and 72 h following the induction of anaesthesia. Serum biochemistry was also evaluated at baseline and 72 h after anaesthesia. Induction and extubation times and time to sternal recumbency were also recorded during anaesthetic induction and recovery. Prior to the second anaesthesia (2 weeks later), the median sacral artery (MSA) was ligated under epidural anaesthesia in the experimental group. Sheep in the control group underwent sham operation. All sheep were anaesthetized as before. 4. Following the first halothane anaesthesia, SFC was significantly increased from 3 to 48 h compared with baseline. In the second stage of the experiment, the increases in SFC in the control group were similar to those seen in the first stage of the experiment. However, in MSA-ligated sheep, the increases in SFC were only significant between 3 and 12 h compared with baseline. The SFC was significantly higher in intact sheep from 3 to 72 h compared with the MSA-ligated group. Extubation and sternal recumbency times were significantly longer in intact sheep. 5. Ligation of the MSA in fat-tailed sheep induced a significant reduction in SFC, suggesting that the presence of tail fat substantially affects halothane metabolism during the peri-anaesthetic period in sheep. The greater extent of halothane biotransformation may be clinically important in, otherwise normal, obese patients.

The effect of three inhaled anesthetics in mice harboring mutations in the GluR6 (kainate) receptor gene.

Combinations of GluR5-GluR7, KA1, and KA2 subunits form kainate receptors, a subtype of excitatory ionotropic glutamate receptors. Isoflurane enhances the action of kainate receptors comprising GluR6 subunits expressed in oocytes. To test whether alterations of the GluR6 subunit gene affect the actions of inhaled anesthetics in vivo, we measured the minimum alveolar concentration of desflurane, isoflurane, and halothane in mice lacking the kainate receptor subunit GluR6 (GluR6 knockout mice) and mice with a dominant negative glutamine/arginine (Q/R) editing mutation in membrane domain 2 of the GluR6 receptor (GluR6 editing mutants), which increases the calcium permeability of kainate receptors containing GluR6Q. We also measured the capacity of isoflurane to interfere with Pavlovian fear conditioning to a tone and to context. Absence of the GluR6 subunit did not change the minimum alveolar concentration of isoflurane, desflurane, or halothane. Possibly, kainate receptors assembled from the remaining kainate receptor subunits compensate for the absent subunits and thereby produce a normal minimum alveolar concentration. A Q/R mutation that dominantly affects kainate receptors containing the GluR6 subunit in mice increased isoflurane minimum alveolar concentration (by 12%; P < 0.01), decreased desflurane minimum alveolar concentration (by 18%; P < 0.001), and did not change halothane minimum alveolar concentration (P = 0.25). These data may indicate that kainate receptors containing GluR6Q subunits differently modulate, directly or indirectly, the mechanism by which inhaled anesthetics cause immobility. The mutations of GluR6 that were studied did not affect the capacity of isoflurane to interfere with fear conditioning.

Determining minimum alveolar anesthetic concentration of halothane in rats: the effect of incremental change in halothane concentration and number of crossovers.

Computer simulations for the technique of estimating minimum alveolar anesthetic concentration (MAC) in patients (quantal design) suggest that incremental concentration changes and the number of crossovers affect MAC. We hypothesized that these variables may also apply to estimating MAC in rats (bracketing design). This study tested that hypothesis and also examined whether these variables might mask differences in MAC between groups in which MAC might be expected to differ (pregnant [P] versus nonpregnant [NP]). There were 2 cohorts (n = 27 and n = 30 rats). Each cohort included NP females, females in early P, and females in late P. MAC was tested by using an incremental concentration change of 0.20% and one within-subject crossover in the first cohort and by using an increment size of 0.10% and four crossovers in the second cohort. MAC was statistically significantly increased in the three groups in the second cohort (NP, 1.16 +/- 0.12; early P, 1.14 +/- 0.10; late P, 1.07 +/- 0.10; mean +/- sd) compared with values in the three comparable groups in the first cohort (NP, 0.95 +/- 0.06; early P, 1.01 +/- 0.09; late P, 0.93 +/- 0.13). Values did not differ among groups within each cohort. Post hoc simulations indicated that up to 36% of the difference between cohorts was due to increment size, with the balance due to experimental factors. Our findings confirmed the hypothesis that increment size affects estimates of MAC when a bracketing design is used.

Malignant hyperthermia: pathophysiology, clinical presentation, and treatment.

Malignant hyperthermia (MH) was first described as an inherited highly lethal disorder in 1960. There has since been significant progress in the clinical management, identification of MH susceptible (MHS) persons, and understanding of the underlying pathophysiology. When patients are known to be MHS prior to surgery, an MH episode can easily be avoided by the use of safe nontriggering anesthetic agents. Current MH mortality is <10%, but many experts believe this can be significantly reduced by improved MH preparedness. MH is triggered in humans by an MH triggering anesthetic agent, which causes the release of calcium from the sarcoplastic reticulum of the skeletal muscle cell at an uncontrolled rate resulting in a hypermetabolic state. Recent molecular genetic studies have shown that MH is related to an abnormal ryanodine receptor that controls the release of calcium from the sarcoplastic reticulum. This article reviews the current understanding of the pathophysiology, diagnosis, clinical presentation, and treatment of MH.

Inhibition of spinal protein kinase C-epsilon or -gamma isozymes does not affect halothane minimum alveolar anesthetic concentration in rats.

Anesthetic effects on receptor or ion channel phosphorylation by enzymes such as protein kinase C (PKC) have been postulated to underlie some aspects of anesthesia. In vitro studies show that anesthetic effects on several receptors are mediated by PKC. To test the importance of PKC for the immobility produced by inhaled anesthetics, we measured the effect of intrathecal injections of PKC-epsilon and -gamma inhibitors on halothane minimum alveolar anesthetic concentration (MAC) in 7-day-old and 21-day-old Sprague-Dawley rats. The inhibitors were made as solutions of 100 pmol/5 microL and were given in a volume of 5 microL (7-day-old [P7] rats) or 10 microL (21-day-old [P21] rats). Controls were saline injections or injections of the peptide carrier at the same concentration and volumes; there were six animals in each group. In P7 rats, MAC values (in percentage of an atmosphere) were 1.63 +/- 0.0727 (mean +/- SEM) in saline controls, 1.55 +/- 0.141 in carrier controls, 1.54 +/- 0.0800 in rats given PKC-epsilon, and 1.69 +/- 0.0554 in rats given PKC-gamma. In P21 animals, the values were 1.20 +/- 0.0490, 1.31 +/- 0.0124, 1.27 +/- 0.0367, and 1.15 +/- 0.0483, respectively. Injection of the inhibitors did not change MAC in either age group. These results do not support an anesthetic effect on phosphorylation as a mechanism underlying the capacity of inhaled anesthetics to prevent movement in response to noxious stimulation, and they indirectly support a direct action on receptors or ion channels.

Comparative binding character of two general anaesthetics for sites on human serum albumin.

Propofol and halothane are clinically used general anaesthetics, which are transported primarily by HSA (human serum albumin) in the blood. Binding characteristics are therefore of interest for both the pharmacokinetics and pharmacodynamics of these drugs. We characterized anaesthetic-HSA interactions in solution using elution chromatography, ITC (isothermal titration calorimetry), hydrogen-exchange experiments and geometric analyses of high-resolution structures. Binding affinity of propofol to HSA was determined to have a K(d) of 65 microM and a stoichiometry of approx. 2, whereas the binding of halothane to HSA showed a K(d) of 1.6 mM and a stoichiometry of approx. 7. Anaesthetic-HSA interactions are exothermic, with propofol having a larger negative enthalpy change relative to halothane. Hydrogen-exchange studies in isolated recombinant domains of HSA showed that propofol-binding sites are primarily found in domain III, whereas halothane sites are more widely distributed. Both location and stoichiometry from these solution studies agree with data derived from X-ray crystal-structure studies, and further analyses of the architecture of sites from these structures suggested that greater hydrophobic contacts, van der Waals interactions and hydrogen-bond formation account for the stronger binding of propofol as compared with the less potent anaesthetic, halothane.