Residency, Stanford University Department of Anesthesia, Anesthesiology (1993)
fellowship, Stanford University Department of Anesthesia, Critical Care Medicine (1994)
Molecular structures of drugs related to clinical, anesthesia; molecular modeling related to biological activity.
OBJECTIVES: To compare the 1-year survival for different age strata of intensive care unit (ICU) patients after receipt of packed red blood cell (PRBC) transfusions. BACKGROUND: Despite guidelines documenting risks of PRBC transfusion and data showing that increasing age is associated with ICU mortality, little data exist on whether age alters the transfusion-related risk of decreased survival. METHODS: We retrospectively examined data on 2393 consecutive male ICU patients admitted to a tertiary-care hospital from 2003 to 2009 in age strata: 21-50, 51-60, 61-70, 71-80 and >80 years. We calculated Cox regression models to determine the modifying effect of age on the impact of PRBC transfusion on 1-year survival by using interaction terms between receipt of transfusion and age strata, controlling for type of admission and Charlson co-morbidity indices. We also examined the distribution of admission haematocrit and whether transfusion rates differed by age strata. RESULTS: All age strata experienced statistically similar risks of decreased 1-year survival after receipt of PRBC transfusions. However, patients age >80 were more likely than younger cohorts to have haematocrits of 25-30% at admission and were transfused at approximately twice the rate of each of the younger age strata. DISCUSSION: We found no significant interaction between receipt of red cell transfusion and age, as variables, and survival at 1 year as an outcome.
View details for DOI 10.1111/tme.12010
View details for Web of Science ID 000321975300005
View details for PubMedID 23480030
Cys-loop receptors constitute a superfamily of pentameric ligand-gated ion channels (pLGICs), including receptors for acetylcholine, serotonin, glycine and ?-aminobutyric acid. Several bacterial homologues have been identified that are excellent models for understanding allosteric binding of alcohols and anesthetics in human Cys-loop receptors. Recently, we showed that a single point mutation on a prokaryotic homologue (GLIC) could transform it from a channel weakly potentiated by ethanol into a highly ethanol-sensitive channel. Here, we have employed molecular simulations to study ethanol binding to GLIC, and to elucidate the role of the ethanol-enhancing mutation in GLIC modulation. By performing 1-µs simulations with and without ethanol on wild-type and mutated GLIC, we observed spontaneous binding in both intra-subunit and inter-subunit transmembrane cavities. In contrast to the glycine receptor GlyR, in which we previously observed ethanol binding primarily in an inter-subunit cavity, ethanol primarily occupied an intra-subunit cavity in wild-type GLIC. However, the highly ethanol-sensitive GLIC mutation significantly enhanced ethanol binding in the inter-subunit cavity. These results demonstrate dramatic effects of the F(14')A mutation on the distribution of ligands, and are consistent with a two-site model of pLGIC inhibition and potentiation.
View details for DOI 10.1371/journal.pcbi.1002710
View details for Web of Science ID 000310568800011
View details for PubMedID 23055913
The accompanying articles in this issue of the journal's special collection describe attempts to improve on the dynamics of distribution and reduce side effects of analogs of etomidate and benzodiazepines. Both classes of drugs have their principal sites of action on ?-aminobutyric acid type A receptors, although at very different binding sites and by different mechanisms of action. Herein, we review the structure of ?-aminobutyric acid type A receptors and describe the location of the 2 likely binding sites. In addition, we describe how these drugs can interact with the nervous system at a systems level. We leave it to other reviewers to discuss whether these new drugs offer true clinical improvements.
View details for DOI 10.1213/ANE.0b013e31824a0b3c
View details for Web of Science ID 000306769500012
View details for PubMedID 22344244
Although general anesthetics have been provided effectively for many years, their exact molecular underpinnings remain relatively unknown. In this article, we discuss the recent findings associated with resistance to anesthetic effects as a way of shedding light on these mechanisms.The original theories of anesthetic action based upon their effects on cellular membranes have given way to specific theories concerning direct effects on ion channel proteins. These molecular targets are intimately involved in the conduct of neuronal signaling within the central nervous system and are thought to be essential in the modulation of conscious states. It is the lack of a thorough understanding of unperturbed consciousness that fosters great difficulty in understanding how anesthetics alter this conscious state. However, one very fruitful line of analysis in the quest for such answers lies in the examination of both in-vitro and in-vivo ion channel systems that seem to maintain variable levels of resistance to anesthetics.Information about the possible targets and molecular nature of anesthetic action is being derived from studies of anesthetic resistance in ? aminobutyric acid receptors, tandem pore potassium channels, and an apparently wide variety of protein systems within the nematode, Caenorhabditis elegans.
View details for DOI 10.1097/ACO.0b013e328354fda8
View details for Web of Science ID 000306273900002
View details for PubMedID 22614247
This study examined the association of hematocrit (Hct) levels measured upon intensive care unit (ICU) admission and red blood cell transfusions to long-term (1-year or 180-day) mortality for both surgical and medical patients.Administrative and laboratory data were collected retrospectively on 2393 consecutive medical and surgical male patients admitted to the ICU between 2003 and 2009. We stratified patients based on their median Hct level during the first 24 hours of their ICU stay (Hct < 25.0%, 25% ? Hct < 30%, 30% ? Hct < 39%, and 39.0% and higher). An extended Cox regression analysis was conducted to identify the time period after ICU admission (0 to <180, 180 to 365 days) when low Hct (<25.0) was most strongly associated with mortality. The unadjusted and adjusted relationship between admission Hct level, receipt of a transfusion, and 180-day mortality was assessed using Cox proportional hazards regression modeling.Patients with an Hct level of less than 25% who were not transfused had the worst mortality risk overall (hazard ratio [HR], 6.26; 95% confidence interval [CI], 3.05-12.85; p < 0.001) during the 6 months after ICU admission than patients with a Hct level of 39.0% or more who were not transfused. Within the subgroup of patients with a Hct level of less than 25% only, receipt of a transfusion was associated with a significant reduction in the risk of mortality (HR, 0.40; 95% CI, 0.19-0.85; p = 0.017).Anemia of a Hct level of less than 25% upon admission to the ICU, in the absence of a transfusion, is associated with long-term mortality. Our study suggests that there may be Hct levels below which the transfusion risk-to-benefit imbalance reverses.
View details for DOI 10.1111/j.1537-2995.2011.03134.x
View details for Web of Science ID 000295917700014
View details for PubMedID 21985048
Cys-loop receptors constitute a superfamily of ion channels gated by ligands such as acetylcholine, serotonin, glycine, and ?-aminobutyric acid. All of these receptors are thought to share structural characteristics, but due to high sequence variation and limited structure availability, our knowledge about allosteric binding sites is still limited. These sites are frequent targets of anesthetic and alcohol molecules, and are of high pharmacological importance. We used molecular simulations to study ethanol binding and equilibrium exchange for the homomeric ?1 glycine receptor (GlyR?1), modeled on the structure of the Gloeobacter violaceus pentameric ligand-gated channel. Ethanol has a well-known potentiating effect and can be used in high concentrations. By performing two microsecond-scale simulations of GlyR with/without ethanol, we were able to observe spontaneous binding in cavities and equilibrium ligand exchange. Of interest, it appears that there are ethanol-binding sites both between and within the GlyR transmembrane subunits, with the intersubunit site having the highest occupancy and slowest exchange (?200 ns). This model site involves several residues that were previously identified via mutations as being crucial for potentiation. Finally, ethanol appears to stabilize the GlyR model built on a presumably open form of the ligand-gated channel. This stabilization could help explain the effects of allosteric ligand binding in Cys-loop receptors.
View details for DOI 10.1016/j.bpj.2011.02.032
View details for Web of Science ID 000289494200009
View details for PubMedID 21463577
Ligand-gated ion channels (LGICs) significantly modulate anesthetic effects. Their exact molecular structure remains unknown. This has led to ambiguity regarding the proper amino acid alignment within their 3D structure and, in turn, the location of any anesthetic binding sites. Current controversies suggest that such a site could be located in either an intra- or intersubunit locale within the transmembrane domain of the protein. Here, we built a model of the glycine alpha one receptor (GlyRa1) based on the open-state structures of two new high-resolution ion channel templates from the prokaryote, Gloebacter violaceus (GLIC). Sequence scoring suggests reasonable homology between GlyRa1 and GLIC. Three of the residues notable for modulating anesthetic action are on transmembrane segments 1-3 (TM1-3): (ILE229, SER 267, and ALA 288). They line an intersubunit interface, in contrast to previous models. However, residues from the fourth transmembrane domain (TM4) that are known to modulate a variety of anesthetic effects are quite distant from this putative anesthetic binding site. While this model can account for a large proportion of the physicochemical data regarding such proteins, it cannot readily account for the alterations on anesthetic effects that are due to mutations within TM4.
View details for DOI 10.1021/ci100266e
View details for Web of Science ID 000285559900017
View details for PubMedID 21117677
We have previously used molecular modeling and normal-mode analyses combined with experimental data to visualize a plausible model of a transmembrane ligand-gated ion channel. We also postulated how the gating motion of the channel may be affected by the presence of various ligands, especially anesthetics. As is typical for normal-mode analyses, those studies were performed in vacuo to reduce the computational complexity of the problem. While such calculations constitute an efficient way to model the large scale structural flexibility of transmembrane proteins, they can be criticized for neglecting the effects of an explicit phospholipid bilayer or hydrated environment. Here, we show the successful calculation of normal-mode motions for our model of a glycine ?-1 receptor, now suspended in a fully hydrated lipid bilayer. Despite the almost uniform atomic density, the introduction of water and lipid does not grossly distort the overall gating motion. Normal-mode analysis revealed that even a fully immersed glycine ?-1 receptor continues to demonstrate an iris-like channel gating motion as a low-frequency, high-amplitude natural harmonic vibration consistent with channel gating. Furthermore, the introduction of periodic boundary conditions allows the examination of simultaneous harmonic vibrations of lipid in synchrony with the protein gating motions that are compatible with reasonable lipid bilayer perturbations. While these perturbations tend to influence the overall protein motion, this work provides continued support for the iris-like motion model that characterizes gating within the family of ligand-gated ion channels.
View details for DOI 10.1021/cn100026t
View details for Web of Science ID 000281052400005
View details for PubMedID 22816018
Proper regulation of neurotransmission requires that ligand-activated ion channels remain closed until agonist binds. How channels then open remains poorly understood. Glycine receptor (GlyR) gating is initiated by agonist binding at interfaces between adjacent subunits in the extracellular domain. Aspartate-97, located at the alpha1 GlyR interface, is a conserved residue in the cys-loop receptor superfamily. The mutation of D97 to arginine (D97R) causes spontaneous channel opening, with open and closed dwell times similar to those of maximally activated WT GlyR. Using a model of the N-terminal domain of the alpha1 GlyR, we hypothesized that an arginine-119 residue was forming intersubunit electrostatic bonds with D97. The D97R/R119E charge reversal restored this interaction, stabilizing channels in their closed states. Cysteine substitution shows that this link occurs between adjacent subunits. This intersubunit electrostatic interaction among GlyR subunits thus contributes to the stabilization of the closed channel state, and its disruption represents a critical step in GlyR activation.
View details for DOI 10.1073/pnas.1001845107
View details for Web of Science ID 000277088700069
View details for PubMedID 20385800
The present study tested the hypothesis that several residues in Loop 2 of alpha1 glycine receptors (GlyRs) play important roles in mediating the transduction of agonist activation to channel gating. This was accomplished by investigating the effect of cysteine point mutations at positions 50-60 on glycine responses in alpha1GlyRs using two-electrode voltage clamp of Xenopus oocytes. Cysteine substitutions produced position-specific changes in glycine sensitivity that were consistent with a beta-turn structure of Loop 2, with odd-numbered residues in the beta-turn interacting with other agonist-activation elements at the interface between extracellular and transmembrane domains. We also tested the hypothesis that the charge at position 53 is important for agonist activation by measuring the glycine response of wild type (WT) and E53C GlyRs exposed to methanethiosulfonate reagents. As earlier, E53C GlyRs have a significantly higher EC(50) than WT GlyRs. Exposing E53C GlyRs to the negatively charged 2-sulfonatoethyl methanethiosulfonate, but not neutral 2-hydroxyethyl methanethiosulfonate, positively charged 2-aminoethyl methanethiosulfonate, or 2-trimethylammonioethyl methanethiosulfonate, decreased the glycine EC(50) to resemble WT GlyR responses. Exposure to these reagents did not significantly alter the glycine EC(50) for WT GlyRs. The latter findings suggest that the negative charge at position 53 is important for activation of GlyRs through its interaction with positive charge(s) in other neighboring agonist activation elements. Collectively, the findings provide the basis for a refined molecular model of alpha1GlyRs based on the recent x-ray structure of a prokaryotic pentameric ligand-gated ion channel and offer insight into the structure-function relationships in GlyRs and possibly other ligand-gated ion channels.
View details for DOI 10.1074/jbc.M802384200
View details for Web of Science ID 000259719200041
View details for PubMedID 18658152
Recent crystal structures of the acetylcholine binding protein (AChBP) have revealed surprisingly small structural alterations upon ligand binding. Here we investigate the extent to which ligand binding may affect receptor dynamics. AChBP is a homologue of the extracellular component of ligand-gated ion channels (LGICs). We have previously used an elastic network normal-mode analysis to propose a gating mechanism for the LGICs and to suggest the effects of various ligands on such motions. However, the difficulties with elastic network methods lie in their inability to account for the modest effects of a small ligand or mutation on ion channel motion. Here, we report the successful application of an elastic network normal mode technique to measure the effects of large ligand binding on receptor dynamics. The present calculations demonstrate a clear alteration in the native symmetric motions of a protein due to the presence of large protein cobratoxin ligands. In particular, normal-mode analysis revealed that cobratoxin binding to this protein significantly dampened the axially symmetric motion of the AChBP that may be associated with channel gating in the full nAChR. The results suggest that alterations in receptor dynamics could be a general feature of ligand binding.
View details for DOI 10.1021/ci700456s
View details for Web of Science ID 000255448400015
View details for PubMedID 18348519
Gamma-aminobutyric acid type A receptors (GABA(A)-R) containing alpha1beta2gamma2 subunits are weakly inhibited by Zn2+, whereas receptors containing only the alpha1beta2 subunits are strongly inhibited. We built homology models of the ion pores of alpha1beta2 and alpha1beta2gamma2 GABA(A)-R using coordinates of the nicotinic acetylcholine receptor as a template. Threading the GABA(A)-R beta2 sequence onto this template placed the 17' histidine and the 20' glutamate residues at adjacent locations in the mouth of the pore, such that a nearly ideal tetradentate site for Zn2+ was formed from two histidine and two glutamate residues between adjacent beta subunits in the alpha1beta2 GABA(A)-R. Following optimization with CHARMM, the distance between the alpha-carbons of the adjacent histidine residues was approximately 9.2 A, close to the ideal distance for a Zn2+ binding site. Loss of inhibition by Zn2+ in alpha1beta2gamma2 GABA(A)-R can be explained by the geometry of these residues in the arrangement alpha1beta2gamma2alpha1beta2, in which the nearest C-alpha-C-alpha distance between the histidine residues is 15.5 A, too far apart for an energetically optimal Zn2+ binding site. We then mutated the gamma subunit at the 17' and/or 20' positions. Zn2+ inhibition was not restored in alpha1beta2gamma2 (I282H) receptors. A novel finding is that the modeling shows the native 20' lysine in gamma2 can compete with Zn2+ for binding to the inserted 17' histidine. Sensitivity to Zn2+ was restored in the double mutant receptor, alpha1beta2gamma2 (I282H; K285E), in which the competition with lysine was removed and a more favorable Zn2+ binding site was formed.
View details for DOI 10.1021/ci700324a
View details for Web of Science ID 000253428400011
View details for PubMedID 18197653
Considerable evidence indicates that ethanol acts on specific residues in the transmembrane domains of glycine receptors (GlyRs). In this study, we tested the hypothesis that the extracellular domain is also a target for ethanol action by investigating the effect of cysteine substitutions at positions 52 (extracellular domain) and 267 (transmembrane domain) on responses to n-alcohols and propyl methanethiosulfonate (PMTS) in alpha1GlyRs expressed in Xenopus oocytes. In support of the hypothesis: (i) The A52C mutation changed ethanol sensitivity compared to WT GlyRs; (ii) PMTS produced irreversible alcohol-like potentiation in A52C GlyRs; and (iii) PMTS binding reduced the n-chain alcohol cutoff in A52C GlyRs. Further studies used PMTS binding to cysteines at positions 52 or 267 to block ethanol action at one site in order to determine its effect at other site(s). In these situations, ethanol caused negative modulation when acting at position 52 and positive modulation when acting at position 267. Collectively, these findings parallel the evidence that established the TM domain as a target for ethanol, suggest that positions 52 and 267 are part of the same alcohol pocket and indicate that the net effect of ethanol on GlyR function reflects the summation of its positive and negative modulatory effects on different targets.
View details for DOI 10.1111/j.1471-4159.2007.04680.x
View details for Web of Science ID 000249452000035
View details for PubMedID 17561937
Predicting collective dynamics and structural changes in biological macromolecules is pivotal toward a better understanding of many biological processes. Limitations due to large system sizes and inaccessible time scales have prompted the development of alternative techniques for the calculation of such motions. In this work, we present the results of a normal-mode analysis technique based on molecular mechanics that enables the calculation of accurate force-field based vibrations of extremely large molecules and compare it with two elastic network approximate models. When applied to the glycine alpha1 receptor, all three normal-mode analysis algorithms demonstrate an "iris-like" gating motion. Such gating motions have implications for understanding the effects of anesthetic and other ligand binding sites and for the means of transducing agonist binding into ion channel opening. Unlike the more approximate methods, molecular mechanics based analyses can also reveal approximate vibrational frequencies. Such analyses may someday allow the use of protein dynamics elucidated via normal-mode calculations as additional endpoints for future drug design.
View details for DOI 10.1021/ci600566j
View details for Web of Science ID 000248192200029
View details for PubMedID 17602605
It is not yet possible to obtain crystal structures of anesthetic molecules bound to proteins that are plausible neuronal targets; for example, ligand-gated ion channels. However, there are x-ray crystal structures in which anesthetics are complexed with proteins that are not directly related to anesthetic action. Much useful information about anesthetic-protein interactions can be derived from the x-ray crystal structures of halothane-cholesterol oxidase, bromoform-luciferase, halothane-albumin, and dichloroethane-dehalogenase. These structures show anesthetic-protein interactions at the atomic level.We obtained the known coordinate files for bromoform-luciferase, halothane- albumin, dichloroethane-dehalogenase, and halothane-cholesterol oxidase. These were then modified by adding hydrogens, edited into subsets, and underwent a series of restrained molecular mechanics optimizations. Final analysis of anesthetic polarization within the anesthetic binding site occurred via combined molecular mechanics-quantum mechanics calculations.The anesthetic binding sites within these well-characterized anesthetic-protein complexes possess a set of common characteristics that we refer to as "binding motifs." The common features of these motifs are polar and nonpolar interactions within an amphiphilic binding cavity, including the presence of weak hydrogen bond interactions with amino acids and water molecules. Calculations also demonstrated the polarizing effect of the amphipathic binding sites on what are otherwise considered quite hydrophobic anesthetics. This polarization appears energetically favorable.Anesthetic binding to proteins involves amphipathic interactions.
View details for DOI 10.1213/01.ane.0000253029.67331.8d
View details for Web of Science ID 000243661000018
View details for PubMedID 17242087
The superfamily of ligand-gated ion channels (LGICs) has been implicated in anesthetic and alcohol responses. Mutations within glycine and GABA receptors have demonstrated that possible sites of anesthetic action exist within the transmembrane subunits of these receptors. The exact molecular arrangement of this transmembrane region remains at intermediate resolution with current experimental techniques. Homology modeling methods were therefore combined with experimental data to produce a more exact model of this region. A consensus from multiple bioinformatics techniques predicted the topology within the transmembrane domain of a glycine alpha one receptor (GlyRa1) to be alpha helical. This fold information was combined with sequence information using the SeqFold algorithm to search for modeling templates. Independently, the FoldMiner algorithm was used to search for templates that had structural folds similar to published coordinates of the homologous nAChR (1OED). Both SeqFold and Foldminer identified the same modeling template. The GlyRa1 sequence was aligned with this template using multiple scoring criteria. Refinement of the alignment closed gaps to produce agreement with labeling studies carried out on the homologous receptors of the superfamily. Structural assignment and refinement was achieved using Modeler. The final structure demonstrated a cavity within the core of a four-helix bundle. Residues known to be involved in modulating anesthetic potency converge on and line this cavity. This suggests that the binding sites for volatile anesthetics in the LGICs are the cavities formed within the core of transmembrane four-helix bundles.
View details for DOI 10.1021/ci0497399
View details for Web of Science ID 000227982800016
View details for PubMedID 15667138
We built a model of a GABAA alpha1 receptor (GABAAR) that combines the ligand binding (LBD) and the transmembrane domains (TMD). We used six steps: (1) a four-alpha helical bundle in the crystal structure of bovine cytochrome c oxidase (2OCC) was identified as a template for the TMD of a single subunit. (2) The five pore-forming alpha helices of a bacterial mechanosensitive channel (1MSL) served as a template for the pentameric ion channel. (3) Five copies of the tetrameric template from 2OCC were superimposed on 1MSL to produce a homopentamer containing 20 alpha helices arranged around a funnel-shaped central pore. (4) Five copies of the GABAAR sequence were threaded onto the alpha-helical segments of this template and inter-helical loops were generated to produce the TMD model. (5) A model of the LBD was built by threading the aligned sequence of GABAAR onto the crystal structure of the acetylcholine binding protein (1I9B). (6) The models of the LBD and the TMD were aligned along a common five-fold axis, moved together along that axis until in vdW contact, merged, and then optimized with restrained molecular dynamics. Our model corresponds closely with recently published coordinates of the acetylcholine receptor (1OED) but also explains additional features. Our model reveals structures of loops that were not visible in the cryoelectron micrograph and satisfies most labeling and mutagenesis data. It also suggests mechanisms for ligand binding transduction, ion selectivity, and anesthetic binding.
View details for DOI 10.1016/j.jmgm.2004.03.004
View details for Web of Science ID 000224010100004
View details for PubMedID 15331052
Several studies relating electroencephalogram parameter values to clinical endpoints using a single (mostly hypnotic) drug at relatively low levels of central nervous system depression (sedation) have been published. However, the usefulness of a parameter derived from the electroencephalogram for clinical anesthesia largely depends on its ability to predict the response to stimuli of different intensity or painfulness under a combination of a hypnotic and an (opioid) analgesic. This study was designed to evaluate the predictive performance of spectral edge frequency 95 (SEF95), BIS, and approximate entropy for the response to increasingly intense stimuli under different concentrations of both propofol and remifentanil in the therapeutic range.Ten healthy male and ten healthy female volunteers were studied during coadministration of propofol and remifentanil. After having maintained a specific target concentration for 10 min, the depth of sedation-anesthesia was assessed using the responsiveness component of the Observer's Assessment of Alertness/Sedation (OAA/S) rating scale, which was modified by adding insertion of a laryngeal mask and laryngoscopy. The electroencephalogram derived parameters approximate entropy, bispectral index, and SEF95 were recorded just before sedation level was assessed.The prediction probability values for approximate entropy were slightly, but not significantly, better than those for bispectral index, SEF95, and the combination of drug concentrations. A much lower prediction ability was observed for tolerance of airway manipulation than for hypnotic endpoints.Approximate entropy revealed informations on hypnotic and analgesic endpoints using coadministration of propofol and remifentanil comparable to bispectral index, SEF95, and the combination of drug concentrations.
View details for Web of Science ID 000181290800007
View details for PubMedID 12606904
The pharmacokinetics of both propofol and remifentanil have been described extensively. Although they are commonly administered together for clinical anesthesia, their pharmacokinetic interaction has not been investigated so far. The purpose of the current investigation was to elucidate the nature and extent of pharmacokinetic interactions between propofol and remifentanil.Twenty healthy volunteers aged 20-43 yr initially received either propofol or remifentanil alone in a stepwise incremental and decremental fashion a target controlled infusion. Thereafter, the respective second drug was infused to a fixed target concentration in the clinical range (0-4 microg/ml and 0-4 ng/ml for propofol and remifentanil, respectively) and the stepwise incremental pattern repeated. Frequent blood samples were drawn for up to 6 h for propofol and 40 min for remifentanil after the end of administration and assayed for the respective drug concentrations with gas chromatography-mass spectrometry. The time courses of the measured concentrations were fitted to standard compartmental models. Calculations were performed with NONMEM. After having established the individual population models for both drugs and an exploratory analysis for hypothesis generation, pharmacokinetic interaction was identified by including an interaction term into the population model and comparing the value of the objective function in the presence and absence of the respective term.The concentration-time courses of propofol and remifentanil were described best by a three- and two-compartment model, respectively. In the concentration range examined, remifentanil does not alter propofol pharmacokinetics. Coadministration of propofol decreases the central volume of distribution and distributional clearance of remifentanil by 41% and elimination clearance by 15%. This effect was not concentration-dependent in the examined concentration range of propofol.Coadministration of propofol decreases the bolus dose of remifentanil needed to achieve a certain plasma-effect compartment concentration but does not alter the respective maintenance infusion rates and recovery times to a clinically significant degree.
View details for Web of Science ID 000179590500004
View details for PubMedID 12459659
There has been rapid progress in molecular modelling in recent years. The convergence of improved software for molecular mechanics and dynamics, techniques for chimeric substitution and site-directed mutations, and the first x-ray structures of transmembrane ion channels have made it possible to build and test models of anaesthetic binding sites. These models have served as guides for site-directed mutagenesis and as starting points for understanding the molecular dynamics of anaesthetic-site interactions. Ligand-gated ion channels are targets for inhaled anaesthetics and alcohols in the central nervous system. The inhibitory strychnine-sensitive glycine and gamma-aminobutyric acid type A receptors are positively modulated by anaesthetics and alcohols; site-directed mutagenesis techniques have identified amino acid residues important for the action of volatile anaesthetics and alcohols in these receptors. Key questions are whether these amino acid mutations form part of alcohol- or anaesthetic-binding sites or if they alter protein stability in a way that allows anaesthetic molecules to act remotely by non-specific mechanisms. It is likely that molecular modelling will play a major role in answering these questions.
View details for Web of Science ID 000176825400004
View details for PubMedID 12173239
The opioid agonist meperidine has actions, such as antishivering, that are more pronounced than those of other opioid agonists and that are not blocked with nonselective opioid antagonists. Agonists at the alpha(2) adrenoceptors, such as clonidine, are very effective antishivering drugs. Preliminary evidence also indicates that meperidine interacts with alpha(2) adrenoceptors. The authors therefore studied the ability of meperidine to bind and activate each of the alpha(2)-adrenoceptor subtypes in a transfected cell system.The ability of meperidine to bind to and inhibit forskolin-stimulated cyclic adenosine monophosphate formation as mediated by the three alpha(2)-adrenoceptor subtypes transiently transfected into COS-7 cells has been tested. The ability of the opioid antagonist naloxone and the alpha(2)-adrenoceptor antagonists yohimbine and RX821002 to block the analgesic action of meperidine in the hot-plate test was also assessed. The ability of meperidine to fit into the alpha(2B) adrenoceptor was assessed using molecular modeling techniques.Meperidine bound to all alpha2-adrenoceptor subtypes, with alpha(2B) having the highest affinity (alpha(2B), 8.6 +/- 0.3 microm; alpha(2C), 13.6 +/- 1.5 microm, P < 0.05; alpha(2A), 38.6 +/- 0.7 microm). Morphine was ineffective at binding to any of the receptor subtypes. Meperidine inhibited the production of forskolin-stimulated cyclic adenosine monophosphate mediated by all receptor subtypes but was most effective at the alpha(2B) adrenoceptor (alpha(2B), 0.6 microm; alpha(2A), 1.3 mm; alpha(2C), 0.3 mm), reaching the same level of inhibition (approximately 70%) as achieved with the alpha2-adrenoceptor agonist dexmedetomidine. The analgesic action of meperidine was blocked by naloxone but not by the alpha 2-adrenoceptor antagonists yohimbine and RX821002. The modeling studies demonstrated that meperidine can fit into the alpha(2B)-adrenoceptor subtype.Meperidine is a potent agonist at the alpha2 adrenoceptors at its clinically relevant concentrations, especially at the alpha(2B)-adrenoceptor subtype. Activation of the alpha(2B) receptor does not contribute significantly to the analgesic action of meperidine. This raises the possibility that some of its actions, such as antishivering, are transduced by this mechanism.
View details for Web of Science ID 000175965500021
View details for PubMedID 12170055
Recent mutational analyses of ligand-gated ion channels (LGICs) have demonstrated a plausible site of anesthetic action within their transmembrane domains. Although there is a consensus that the transmembrane domain is formed from four membrane-spanning segments, the secondary structure of these segments is not known. We utilized 10 state-of-the-art bioinformatics techniques to predict the transmembrane topology of the tetrameric regions within six members of the LGIC family that are relevant to anesthetic action. They are the human forms of the GABA alpha 1 receptor, the glycine alpha 1 receptor, the 5HT3 serotonin receptor, the nicotinic AChR alpha 4 and alpha 7 receptors and the Torpedo nAChR alpha 1 receptor. The algorithms utilized were HMMTOP, TMHMM, TMPred, PHDhtm, DAS, TMFinder, SOSUI, TMAP, MEMSAT and TOPPred2. The resulting predictions were superimposed on to a multiple sequence alignment of the six amino acid sequences created using the CLUSTAL W algorithm. There was a clear statistical consensus for the presence of four alpha helices in those regions experimentally thought to span the membrane. The consensus of 10 topology prediction techniques supports the hypothesis that the transmembrane subunits of the LGICs are tetrameric bundles of alpha helices.
View details for Web of Science ID 000176585100001
View details for PubMedID 12082162
The GABA(A) receptor is an important target for a variety of general anesthetics (Franks and Lieb, 1994) and for benzodiazepines such as diazepam. Specific point mutations in the GABA(A) receptor selectively abolish regulation by benzodiazepines (Rudolph et al., 1999; McKernan et al., 2000) and by anesthetic ethers (Mihic et al., 1997; Krasowski et al., 1998; Koltchine et al., 1999), suggesting the existence of discrete binding sites on the GABA(A) receptor for these drugs. Using anesthetics of different molecular size (isoflurane > halothane > chloroform) together with complementary mutagenesis of specific amino acid side chains, we estimate the volume of a proposed anesthetic binding site as between 250 and 370 A(3). The results of the "cutoff" analysis suggest a common site of action for the anesthetics isoflurane, halothane, and chloroform on the GABA(A) receptor. Moreover, the data support a crucial role for Leu232, Ser270, and Ala291 in the alpha subunit in defining the boundaries of an amphipathic cavity, which can accommodate a variety of small general anesthetic molecules.
View details for Web of Science ID 000167422200002
View details for PubMedID 11245705
The mechanisms of general anesthesia in the central nervous system are finally yielding to molecular examination. As a result of research during the past several decades, a group of ligand-gated ion channels have emerged as plausible targets for general anesthetics. Molecular biology techniques have greatly accelerated attempts to classify ligand-gated ion channel sensitivity to general anesthetics, and have identified the sites of receptor subunits critical for anesthetic modulation using chimeric and mutated receptors. The experimental data have facilitated the construction of tenable molecular models for anesthetic binding sites, which in turn allows structural predictions to be tested. In vivo significance of a putative anesthetic target can now be examined by targeted gene manipulations in mice. In this review, we summarize from a molecular perspective recent advances in our understanding of mechanisms of action of general anesthetics on ligand-gated ion channels.
View details for Web of Science ID 000168439400002
View details for PubMedID 11264449
There has been rapid progress in molecular modeling of LGICs in recent years. The convergence of improved software for molecular mechanics/dynamics, techniques of chimeric substitution and site-directed mutations, and the first X-ray structures of transmembrane ion channels will make it possible to build reasonable models of neuronal ion channels well in advance of publication of their crystal structures. These models will not only serve as guides for future site-directed mutagenesis, but they will also be a starting point for understanding the dynamics of ion channel gating.
View details for Web of Science ID 000170995200004
View details for PubMedID 11526737
(1) Successful application of molecular mechanics and molecular dynamics calculations to the binding of halogenated anesthetics requires forcefields with correct parameters for halocarbons. (2) Unfortunately, our survey of six popular forcefields revealed that some of them provide a very poor representation of electrostatic interactions for the halogens. (3) This problem is due to poor or missing assignments of partial atomic charges to the halogen atoms. (4) We describe the forcefields most appropriate for use with halogenated anesthetics and suggest a general method for editing the assignment of partial atomic charges by performing an initial quantum mechanics calculation.
View details for PubMedID 10049174
To probe the hypothesis of a lipid-mediated mechanism of general anesthetic action on a molecular level, and to help elucidate the nature of the interactions of bioactive compounds with membranes, the effects of trichloroethylene (TCE), an inhalational general anesthetic, on a dioleoylphosphatidylcholine (DOPC) lipid bilayer have been investigated by molecular dynamics (MD) simulations at 37 degrees C and 1 atm and the results compared with 31P and 2H NMR experimental studies (Ref 1). The model used included a single TCE molecule embedded in a lipid bilayer consisting of 24 DOPC molecules and an 8 A layer of explicit water of solvation in each polar head group region of the bilayer, together with constant-pressure periodic boundary conditions in three dimensions. A comparison of the bilayer properties calculated in the presence and absence of the anesthetic led to the detection of three major perturbations of the bilayer caused by the anesthetic at 1 atm: i) an increase in the ratio of the effective areas of hydrocarbon tails and the head group per lipid, predicting the tendency of lipids near the anesthetic site of action to form a hexagonal phase (HII); ii) a slight increase in the frequency of chain dihedral angles found in the gauche conformation; and iii) a significant increase in the lateral mean-square displacement of lipid molecules, an indication of increased lipid lateral diffusion and membrane fluidity. The pressure antagonism of these effects was also studied by MD simulations at pressures of 200 and 400 atm. The study of the pressure reversibility of these effects at 200 and 400 atm indicated that they were partially prevented at 200 atm and essentially blocked at 400 atm, suggesting their probable relevance to the pressure reversal effect seen with general anesthesia. These results may thus provide insights into the interaction between general anesthetics and similar small organic molecules with membranes.
View details for Web of Science ID A1995QJ14600001
View details for PubMedID 7779297
An 11-month-old, 11-kg infant presented to the emergency department after ingesting 130 to 150 mg/kg of elemental iron. Emesis was induced twice and the child was lavaged throughout a 4-hour period with some tablet return. An abdominal radiograph after gastrointestinal decontamination showed at least 16 whole iron tablets remaining in the stomach. Serum iron drawn 2 hours postingestion was 46.7 mumol/L. Blood glucose was 7.7 mmol/L and white blood count was 21,800 mm3. Despite a second lavage 8 hours postingestion, a large number of whole tablets were visualized in the stomach per radiograph. Whole bowel irrigation with polyethylene glycol electrolyte lavage solution (Golytely, Braintree Laboratories, Inc, Braintree, MA) was begun via nasogastric tube 14 hours after the ingestion. Serial abdominal radiographs showed tablet movement out of the stomach within 4 hours after initiating whole bowel irrigation. This case demonstrates the safety and efficacy of WBI in an infant when conventional gastrointestinal decontamination has failed.
View details for Web of Science ID A1991FW04100016
View details for PubMedID 1675852
Benzodiazepines, such as lorazepam and midazolam, are frequently administered to surgical intensive care unit (ICU) patients for postoperative sedation. To date, the pharmacology of lorazepam in critically ill patients has not been described. The aim of the current study was to characterize and compare the pharmacokinetics and pharmacodynamics of lorazepam and midazolam administered as continuous intravenous infusions for postoperative sedation of surgical ICU patients.With Institutional Review Board approval, 24 consenting adult surgical patients were given either lorazepam or midazolam in a double-blind fashion (together with either intravenous fentanyl or epidural morphine for analgesia) through target-controlled intravenous infusions titrated to maintain a moderate level of sedation for 12-72 h postoperatively. Moderate sedation was defined as a Ramsay Sedation Scale score of 3 or 4. Sedation scores were measured, together with benzodiazepine plasma concentrations. Population pharmacokinetic and pharmacodynamic parameters were estimated using nonlinear mixed-effects modeling.A two-compartment model best described the pharmacokinetics of both lorazepam and midazolam. The pharmacodynamic model predicted depth of sedation for both midazolam and lorazepam with 76% accuracy. The estimated sedative potency of lorazepam was twice that of midazolam. The predicted C50,ss (plasma benzodiazepine concentrations where P(Sedation > or = ss) = 50%) values for midazolam (sedation score [SS] > or = n, where n = a Ramsay Sedation Score of 2, 3, ... 6) were 68, 101, 208, 304, and 375 ng/ml. The corresponding predicted C50,ss values for lorazepam were 34, 51, 104, 152, and 188 ng/ml, respectively. Age, fentanyl administration, and the resolving effects of surgery and anesthesia were significant covariates of benzodiazepine sedation. The relative amnestic potency of lorazepam to midazolam was 4 (observed). The predicted emergence times from sedation after a 72-h benzodiazepine infusion for light (SS = 3) and deep (SS = 5) sedation in a typical patient were 3.6 and 14.9 h for midazolam infusions and 11.9 and 31.1 h for lorazepam infusions, respectively.The pharmacology of intravenous infusions of lorazepam differs significantly from that of midazolam in critically ill patients. This results in significant delays in emergence from sedation with lorazepam as compared with midazolam when administered for ICU sedation.
View details for Web of Science ID 000170237800003
View details for PubMedID 11506097