Michaela Scheller

Isoflurane and Sevoflurane Interact with the Nicotinic Acetylcholine Receptor Channels in Micromolar Cconcentrations

Background This study was performed to elucidate and compare the effects of sevoflurane and of isoflurane on the nicotinic acetylcholine receptor of mouse myotubes. The experiments were done with special reference to anesthetic concentrations considerably less than those used for clinical anesthesia. Methods The patch‐clamp technique was used to record acetylcholine‐activated currents from the embryonic type of the nicotinic acetylcholine receptor in the outside‐out mode. A piezo‐driven liquid filament switch was used for the ultrafast application of acetylcholine alone or in combination with isoflurane or sevoflurane. In addition, the patches were preexposed to either anesthetic, preceding the activation with acetylcholine. Results The current elicited by acetylcholine was reduced reversibly and in a concentration‐dependent manner by both anesthetics, which were equally effective. Preexposure of the patches to isoflurane or sevoflurane showed an additional inhibition that was present at micromolar concentrations. The time courses of current decay could be fitted by single exponentials for isoflurane. At higher concentrations of sevoflurane, the current decay became biexponential. In contrast to isoflurane, sevoflurane increased the time constants of desensitization when applied in low concentrations. Conclusion At the nicotinic acetylcholine receptor, isoflurane and sevoflurane act primarily through the same mechanisms: Both affect the open and the closed state of the channels in concentrations equal to and less than those encountered clinically. The kinetics of desensitization, however, are altered in a different manner. Thus there may be several different sites of interaction.

Nonhalogenated Alkane Anesthetics Fail to Potentiate Agonist Actions on Two Ligand-gated Ion Channels

Background Although ether, alcohol, and halogenated alkane anesthetics potentiate agonist actions or increase the apparent agonist affinity of ligand-gated ion channels at clinically relevant concentrations, the effects of nonhalogenated alkane anesthetics on ligand-gated ion channels have not been studied. The current study assessed the abilities of two representative nonhalogenated alkane anesthetics (cyclopropane and butane) to potentiate agonist actions or increase the apparent agonist affinity of two representative ligand-gated ion channels: the nicotinic acetylcholine receptor and &ggr;-aminobutyric acid type A (GABAA) receptor. Methods Nicotinic acetylcholine receptors were obtained from the electroplax organ of Torpedo nobiliana, and human GABAA receptors (&agr;1&bgr;2&ggr;2L) were expressed in human embryonic kidney 293 cells. The Torpedo nicotinic acetylcholine receptors apparent agonist affinity in the presence and absence of anesthetic was assessed by measuring the apparent rates of desensitization induced by a range of acetylcholine concentrations. The GABAA receptor’s apparent agonist affinity in the presence and absence of anesthetic was assessed by measuring the peak currents induced by a range of GABA concentrations. Results Neither cyclopropane nor butane potentiated agonist actions or increased the apparent agonist affinity (reduced the apparent agonist dissociation constant) of the Torpedo nicotinic acetylcholine receptor or GABAA receptor. At clinically relevant concentrations, cyclopropane and butane reduced the apparent rate of Torpedo nicotinic acetylcholine receptor desensitization induced by low concentrations of agonist. Conclusions Our results suggest that the in vivo central nervous system depressant effects of nonhalogenated alkane anesthetics do not result from their abilities to potentiate agonist actions on ligand-gated ion channels. Other targets or mechanisms more likely account for the anesthetic activities of nonhalogenated alkane anesthetics.

The gamma subunit determines whether anesthetic-induced leftward shift is altered by a mutation at alpha1S270 in alpha1beta2gamma2L GABA(A) receptors.

BackgroundA major action of volatile anesthetics is enhancement of γ-aminobutyric acid receptor type A (GABAAR) currents. In recombinant GABAARs consisting of several subunit mixtures, mutating the α1 subunit serine at position 270 to isoleucine [α1(S270I)] was reported to eliminate anesthetic-induc

Anesthesia for Cesarean Section and Acid Aspiration Prophylaxis: A German Survey

UNLABELLED We surveyed routine anesthetic practice and measures to prevent acid aspiration syndrome (AAS) in patients undergoing cesarean section (CS) throughout Germany. Of 1061 questionnaires, 81.9% were returned. For scheduled CS, general anesthesia was used in 63% of cases, and for urgent CS, it was used in 82% of cases. Regional anesthesia was used less often for both scheduled and urgent CS in smaller (< or =500 deliveries/yr; 28% and 16%, respectively) than in medium-sized (500-1000 deliveries/yr; 42% and 19%, respectively) or major obstetric departments (>1000 deliveries/yr; 45% and 21%, respectively). Among the regional techniques, epidural anesthesia (59%) was preferred more than spinal anesthesia (40%) in scheduled CS. In urgent CS, spinal anesthesia predominated (56% vs 42%). Pharmacological AAS prophylaxis is routinely used in 69% (68%) of departments before elective (urgent) CS under general anesthesia and in 52% under regional anesthesia. H2-blocking drugs are preferred for AAS prophylaxis over H2-blocker plus sodium citrate and sodium citrate alone. Both the incidence of and the mortality from AAS at CS are very low in Germany (<1 fatality per year). Nevertheless, AAS prophylaxis deserves more widespread use in obstetric anesthesia and in other patients at risk (e.g., children, outpatients). IMPLICATIONS According to a countrywide survey, the use of regional anesthesia for cesarean section and pharmacological prophylaxis of acid aspiration syndrome is considerably less common in Germany than in the United States, United Kingdom, or other European countries.

Dual action of isoflurane on the γ-aminobutyric acid (GABA)-mediated currents through recombinant α1β2γ2L-GABAA-Receptor channels

Isoflurane (ISO) increased the agonist-induced chloride flux through the &ggr;-aminobutyric acid A receptor (GABAAR). This may reflect an anesthetic-induced increase in the apparent agonist affinity. A dual effect of anesthetics was postulated for both the nicotinic acetylcholine receptor (nAChR) and the GABAAR. We tested the hypothesis that, in addition to a blocking effect, ISO increases &ggr;-aminobutyric acid (GABA)-gated currents through recombinant GABAAR channels. HEK293 cells were transfected with rat cDNA for &agr;1,&bgr;2,&ggr;2L subunits. Currents elicited by 1 mM or 0.01 mM GABA, respectively, alone, or with increasing concentrations of ISO, were recorded by using standard patch clamp techniques. ISO reduced the peak current elicited by 1 mM GABA. Currents induced by 0.01 mM GABA were potentiated by small ISO (twofold at 0.5 mM ISO) and inhibited by larger concentrations. Withdrawal of ISO and GABA induced rebound currents, suggesting an open-channel block by ISO. These currents increased with increasing concentrations of ISO. At large concentrations of ISO, the inhibitory effect predominated and was caused by, at least partly, an open-channel block. At small concentrations of ISO, potentiation of the GABA-gated currents was more prominent. This dual action of ISO indicates different binding sites at the GABAAR. The balance between potentiation and block depends on the concentrations of both ISO and GABA. Implications Isoflurane (ISO) interacts with the inhibitory &ggr;-aminobutyric acid (GABA) receptor. This patch clamp study demonstrates that it may block or potentiate the type A of GABA receptor studied, depending on the concentrations of ISO and of GABA used. At clinically relevant concentrations, ISO considerably potentiates this receptor. This may partly explain its clinical effect.

Acid aspiration prophylaxis and caesarean section.

Acid aspiration syndrome still contributes to the few anaesthesia-related deaths in caesarean section. Although none of the numerous measures intended to prevent such fatalities is based on clear evidence, many different regimens are being used. As obstetric acid aspiration syndrome occurs mainly in general anaesthesia, using regional techniques wherever possible may be the most effective prophylactic measure.

Butanol effects on γ-amino butyric acid concentration-responses in human α1β2γ2L γ-amino butyric acid type A receptors with a mutation at α1S270

Abstract Alcohol enhancement of γ-amino butyric acid type A receptor (GABA A R) gating at low GABA is reduced by a serine-to-isoleucine mutation at residue αS270, suggesting that αS270 forms an enhancement site. However, whether the αS270I mutation strengthens alcohol inhibition of GABA A Rs remains unexplored. Furthermore, αS270 mutations have not been studied in the most prevalent form of mammalian GABA A Rs consisting of α 1 , β 2 , and γ 2 subunits. In voltage-clamped Xenopus oocytes expressing recombinant α 1 β 2 γ 2L GABA A Rs, electrophysiological analysis of GABA concentration-responses demonstrates that the α 1 (S270I) mutation increases apparent GABA affinity and significantly reduces the Hill coefficient of GABA A R activation. Butanol-induced leftward-shifts in GABA concentration-responses for both wild-type α 1 β 2 γ 2L and α 1 (S270I)β 2 γ 2L GABA A Rs are equal. At high GABA, butanol neither enhances nor inhibits α 1 (S270I)β 2 γ 2L responses. Thus, in the dominant mammalian GABA A R isoform, the αS270I mutation affects neither enhancement nor inhibition by butanol, but alters the gating mechanism by reducing cooperativity, producing an apparent reduction in alcohol enhancement at low GABA.