Intramuscular injection of sevoflurane detects malignant hyperthermia predisposition in susceptible pigs

BACKGROUND

The authors hypothesized that intramuscular sevoflurane injection allows diagnostic differentiation between malignant hyperthermia-susceptible (MHS) and -nonsusceptible (MHN) pigs by measurement of intramuscular lactate and carbon dioxide partial pressure (PCO2), and that dantrolene reduces the sevoflurane-induced PCO2 increase.

METHODS

With approval of the local animal care committee, microdialysis probes with attached microtubing for sevoflurane injection were placed in the adductor muscles of nine MHS and six MHN pigs, and PCO2 probes with microtubing were positioned in the triceps muscle of eight MHS and six MHN pigs. After equilibration, sevoflurane boluses at different concentrations and a sevoflurane-dantrolene bolus were injected synchronously. Lactate, pyruvate, and glucose as well as PCO2 were measured spectrophotometrically, and the rate of PCO2 increase was calculated.

RESULTS

Intramuscular sevoflurane injection increased local lactate and PCO2 dose dependently, and significantly higher in MHS than in MHN pigs. Measurement of the rate of PCO2 increase allowed a distinct differentiation between single MHS and MHN pigs. No significant increase in PCO2 was found with sevoflurane and dantrolene.

CONCLUSIONS

Local sevoflurane induces a hypermetabolic reaction measured by PCO2 and lactate increases. The reduced PCO2 increase in MHS after sevoflurane and dantrolene injection is likely to be a result of the sevoflurane-mediated calcium release and its antagonism by dantrolene. Sevoflurane may be useful for a less invasive diagnostic test for malignant hyperthermia in humans.

The beta-adrenoceptor antagonist propranolol counteracts anti-inflammatory effects of isoflurane in rat endotoxemia

BACKGROUND

Recent studies suggest that volatile anaesthetics have anti-inflammatory and preconditioning properties and that beta-adrenoceptors are involved in the signalling pathways for these effects. Concurrently, the blockade of beta-adrenoceptors has been shown to augment the release of inflammatory mediators in response to pro-inflammatory stimuli. We therefore aimed to investigate whether the beta-adrenoceptor antagonist propranolol might modulate the anti-inflammatory effects of isoflurane on the systemic and pulmonary release of pro-inflammatory cytokines in endotoxemic rats.

METHODS

Forty anaesthetized and ventilated Sprague-Dawley rats were randomly treated as follows. Lipopolysaccharide (LPS) only (n = 8), endotoxemia with LPS [5 mg/kg, intravenously (i.v.)]. LPS-isoflurane (n = 8): endotoxemia and continuous inhalation of 1 minimum alveolar concentration (MAC) of isoflurane. LPS-isoflurane-propranolol (n = 8): administration of propranolol (3 mg/kg) before continuous inhalation of isoflurane and induction of endotoxemia. LPS-propranolol (n = 8): administration of propranolol (3 mg/kg) before endotoxemia without inhalation of isoflurane. Sham (n = 8): control-group only with surgical preparation. After 4 h of endotoxemia, levels of tumour necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta) and interleukin-10 (IL-10) in plasma and bronchoalveolar fluid (BALF) were analysed. Release of nitric oxide (NO) and amount of inducible nitric oxide synthase (iNOS) protein in alveolar macrophages was measured by Griess assay or determined by Western Blotting, respectively.

RESULTS

Inhalation of isoflurane reduced the release of TNF-alpha (P < 0.05) and IL-1beta (P < 0.05) in plasma and IL-1beta (P < 0.05) in BALF. Co-administration of propranolol significantly inhibited these effects. During inhalation of isoflurane, the increased release of NO and iNOS protein from alveolar macrophages was also completely inhibited by propranolol.

CONCLUSION

Our results indicate for the first time, that blockade of beta-adrenoceptors counteracts the anti-inflammatory effects of isoflurane in endotoxemic rats.

Inhibitory effect of alprostadil against sevoflurane-induced myometrial relaxation in rats

PURPOSE

For anesthetic management of cesarean sections, regardless of the use of regional or general anesthesia, it is crucial to achieve sufficient uterine contraction immediately following the delivery of an infant in order to reduce excessive bleeding. No previous study has investigated the ability of alprostadil, a synthesized prostaglandin, to inhibit myometrial relaxation induced by volatile anesthetics. The aim of the present study was to investigate the inhibitory effects of alprostadil on sevoflurane-induced myometrial relaxation using myometrial strips isolated from pregnant rats.

METHODS

Myometrial strips were isolated from Sprague-Dawley rats (300-400 g) in the late stage of gestation (19-21 days). The time course of changes in spontaneous myometrium contraction was studied in the presence and absence of sevoflurane. Additionally, alprostadil was titrated at three different concentrations during continuous introduction of sevoflurane 2%, and myometrium contraction was studied. As an index of contraction, the area under the contraction curve was used, and data were analyzed by repeated measure one-way analysis of variance.

RESULTS

We have shown a significant decrease in myometrium contraction as a result of the use of sevoflurane (2%). Additionally, alprostadil has been shown to inhibit myometrial relaxation induced by sevoflurane in a dose-dependent manner. The areas under the contraction curve were 87%, 87%, 129%, and 172% of the baseline value for the control and at low, medium, and high concentrations of alprostadil, respectively.

CONCLUSION

The ability of alprostadil to inhibit myometrial relaxation induced by sevoflurane suggests that the use of alprostadil during general anesthesia for cesarean section may be advantageous for the reduction of postpartum bleeding.

Isoflurane postconditioning prevents opening of the mitochondrial permeability transition pore through inhibition of glycogen synthase kinase 3beta

BACKGROUND

Postischemic administration of volatile anesthetics activates reperfusion injury salvage kinases and decreases myocardial damage. However, the mechanisms underlying anesthetic postconditioning are unclear.

METHODS

Isolated perfused rat hearts were exposed to 40 min of ischemia followed by 1 h of reperfusion. Anesthetic postconditioning was induced by 15 min of 2.1 vol% isoflurane (1.5 minimum alveolar concentration) administered at the onset of reperfusion. In some experiments, atractyloside (10 microm), a mitochondrial permeability transition pore (mPTP) opener, and LY294002 (15 microm), a phosphatidylinositol 3-kinase inhibitor, were coadministered with isoflurane. Western blot analysis was used to determine phosphorylation of protein kinase B/Akt and its downstream target glycogen synthase kinase 3beta after 15 min of reperfusion. Myocardial tissue content of nicotinamide adenine dinucleotide served as a marker for mPTP opening. Accumulation of MitoTracker Red 580 (Molecular Probes, Invitrogen, Basel, Switzerland) was used to visualize mitochondrial function.

RESULTS

Anesthetic postconditioning significantly improved functional recovery and decreased infarct size (36 +/- 1% in unprotected hearts vs. 3 +/- 2% in anesthetic postconditioning; P < 0.05). Isoflurane-mediated protection was abolished by atractyloside and LY294002. LY294002 inhibited isoflurane-induced phosphorylation of protein kinase B/Akt and glycogen synthase kinase 3beta and opened mPTP as determined by nicotinamide adenine dinucleotide measurements. Atractyloside, a direct opener of the mPTP, did not inhibit phosphorylation of protein kinase B/Akt and glycogen synthase kinase 3beta by isoflurane but reversed isoflurane-mediated cytoprotection. Microscopy showed accumulation of the mitochondrial tracker in isoflurane-protected functional mitochondria but no staining in mitochondria of unprotected hearts.

CONCLUSIONS

Anesthetic postconditioning by isoflurane effectively protects against reperfusion damage by preventing opening of the mPTP through inhibition of glycogen synthase kinase 3beta.

Antioxidants reverse reduction of the human hypoxic ventilatory response by subanesthetic isoflurane

BACKGROUND

In subanesthetic concentrations, volatile anesthetics reduce the acute hypoxic response (AHR), presumably by a direct action on the carotid bodies but by an unknown molecular mechanism. To examine a possible involvement of reactive oxygen species or changes in redox state in this inhibiting effect, the authors studied the effect of antioxidants on the isoflurane-induced reduction of the AHR in humans.

METHODS

In 10 volunteers, the authors studied the effect of antioxidants (intravenous ascorbic acid and oral alpha-tocopherol) on the reduction by isoflurane (0.12% end-tidal concentration) of the AHR on a 3-min isocapnic hypoxic stimulus (hemoglobin oxygen saturation 86 +/- 4%). All subjects participated in three separate sessions in which the effects of the antioxidants (session 1), placebo (session 2), and sham isoflurane plus antioxidants (session 3) were tested on the (sham) isoflurane-induced effect on the AHR.

RESULTS

Isoflurane reduced the acute hypoxic response from 0.82 +/- 0.41 l . min . % to 0.49 +/- 0.23 l . min . % and from 0.89 +/- 0.43 l . min . % to 0.48 +/- 0.28 l . min . % in sessions 1 and 2, respectively (mean +/- SD; P < 0.05 vs. control). This reduction of the AHR was completely reversed by antioxidants (AHR = 0.76 +/- 0.39 l . min . %; not significantly different from control, session 1) but not by placebo in session 2 (AHR = 0.50 +/- 0.30 l . min . %; P < 005 vs. control). Sham isoflurane or antioxidants per se had no effect on the hypoxic response.

CONCLUSIONS

The data indicate that isoflurane may depress the AHR by influencing the redox state of oxygen-sensing elements in the carotid bodies. This finding may have clinical implications for patients who are prone to recurrent hypoxic episodes, e.g., due to upper airway obstruction, in the postoperative period when low-dose isoflurane may persist in the body for some time.

Comparison of antagonizing potencies of dodecane analogues to isoflurane in goldfish

PURPOSE

We reported previously that long-chain fatty acids (carbon atoms > or =12) antagonize volatile anesthetics in goldfish. To examine the contribution of the carboxyl group to the antagonizing potency of fatty acids in vivo, we compared antagonizing potencies to isoflurane in goldfish among terminally substituted dodecane analogues.

METHODS

Dodecane (carbon atoms = 12) analogues [fatty acid (DoAC), alcohol (DoAL), alkane (DoAK), sulfate (DoSF), trimethylammonium (DoTA)] were examined. We determined the EC50 (the anesthetic concentration producing a 50% effect) values of isoflurane in the absence or presence of these chemical compounds in goldfish by observing the escape reaction of goldfish from an electrical stimulus.

RESULTS

DoAC at higher than 10 microM and DoAL at higher than 20 microM increased the EC50 values of isoflurane in a concentration-dependent manner compared with the control (P < 0.05). DoAC at 50 microM and DoAL at 100 microM increased the EC50 1.7- and 1.6 fold, respectively. DoAK, DoSF, and DoTA showed no significant differences from the control. In the comparison of DoAC and DoAL at the same concentration, DoAC was more effective than DoAL (P < 0.001).

CONCLUSION

DoAC and DoAL showed antagonizing potencies to isoflurane, whereas DoAK, DoSF, and DoTA had no effect. DoAC was more effective than DoAL. The findings suggest that polarity of the chemical compounds may be necessary to exert antagonizing potency to isoflurane. Furthermore, a highly negative charge density of the carboxyl group may be responsible for the effective antagonization of DoAC to isoflurane.

General anesthetics inhibit the nitrous-oxide-induced activation of corticotropin releasing factor containing neurons in rats

The activation of intracerebral corticotropin releasing factor (CRF) system is involved in nitrous oxide analgesia. We evaluated the effect of general anesthetics on nitrous-oxide-induced CRF activation and antinociception. Male Sprague-Dawley rats inhaled isoflurane (0%, 0.6%, 1.0% and 1.5%) or were administered with intravenous propofol (0, 0.1 and 0.2 mg/kg/min), with or without 75% nitrous oxide inhalation, for 90 min. The brain was fixed with fixative, and brain sections, including the paraventricular nucleus of the hypothalamus, were double immunostained with c-Fos and CRF antibodies to assess the activation of CRF-containing neurons. In other groups of rats, the effect of propofol on nitrous oxide antinociception was evaluated with tail flick latency tests. Both inhaled isoflurane and intravenous propofol inhibited nitrous-oxide-induced activation of CRF neurons, suggesting that these general anesthetics may inhibit one of the analgesic mechanisms of nitrous oxide. Indeed, propofol inhibited the antinociceptive action of nitrous oxide, as evaluated with tail flick latencies (TFL).

Role of tyrosine kinase in desflurane-induced preconditioning

BACKGROUND

Short administration of volatile anesthetics preconditions myocardium and protects the heart against the consequences of subsequent ischemia. Activation of tyrosine kinase is implicated in ischemic preconditioning. The authors investigated whether desflurane-induced preconditioning depends on activation of tyrosine kinase.

METHODS

Sixty-four rabbits were instrumented for measurement of left ventricular pressure, cardiac output, and myocardial infarct size (IS). All rabbits were subjected to 30 min of occlusion of a major coronary artery and 2 h of subsequent reperfusion. Rabbits underwent a treatment period consisting of either no intervention for 35 min (control group, n = 12) or 15 min of 1 minimum alveolar concentration desflurane inhalation followed by a 10-min washout period (desflurane group, n = 12). Four additional groups received the tyrosine kinase inhibitor genistein (5 mg/kg) or lavendustin A (1.3 mg/kg) at the beginning of the treatment period with (desflurane-genistein group, n = 11; desflurane-lavendustin A group, n = 12) or without desflurane inhalation (genistein group, n = 9; lavendustin A group, n = 8).

RESULTS

Hemodynamic values were similar in all groups during baseline (left ventricular pressure, 87 +/- 14 mmHg (mean +/- SD]; cardiac output, 198 +/- 47 ml/min), during coronary artery occlusion (left ventricular pressure, 78 +/- 12 mmHg; cardiac output, 173 +/- 39 ml/min), and after 2 h of reperfusion (left ventricular pressure, 59 +/- 17; cardiac output, 154 +/- 43 ml/min). IS in the control group was 55 +/- 10% of the area at risk. The tyrosine inhibitors had no effect on IS (genistein group, 56 +/- 13%; lavendustin A group, 49 +/- 13%; each P = 1.0 vs. control group). Desflurane preconditioning reduced IS to 40 +/- 15% (P = 0.04 vs. control group). Tyrosine kinase inhibitor administration had no effect on IS reduction (desflurane-genistein group, 44 +/- 13%; desflurane-lavendustin A group, 44 +/- 16%; each P = 1.0 vs. desflurane group).

CONCLUSION

Desflurane-induced preconditioning does not depend on tyrosine kinase activation.

Antagonizing potencies of saturated and unsaturated long-chain free fatty acids to isoflurane in goldfish

PURPOSE

We have previously reported that myristate, a saturated free fatty acid (FFA) with 14 carbons (C14), antagonizes volatile anesthetics in goldfish. The hydrophobicity and molecular configuration of FFAs may play an important role in the antagonizing effect. To examine their contribution, we investigated the antagonizing potencies of saturated and unsaturated long-chain FFAs in goldfish.

METHODS

Saturated and monounsaturated FFAs of C14-18 were tested. We determined the anesthetic concentration producing a 50% effect (EC50) of isoflurane in the absence or presence of FFA by observing the escape reaction of goldfish against an electrical stimulus.

RESULTS

All FFAs increased the EC50 of isoflurane dose-dependently compared with reactions in the absence of FFA ( P < 0.05). For saturated FFAs, the relationship between chain lengths and antagonizing potencies was not linear. C18 was the most effective and C16 was the least effective antagonist ( P < 0.05). Among unsaturated FFAs, C14 was the most effective antagonist ( P < 0.05). In a comparison of saturated and unsaturated FFAs, saturated C14 and C18 were more effective antagonists than unsaturated FFAs of the same carbon numbers ( P <<0.05).

CONCLUSION

The hydrophobicity of FFAs increases as the chain length increases. Therefore, our findings suggest that the antagonizing effect of long-chain FFAs in goldfish, in terms of their capacity to perturb the lipid membrane structure, may be determined not solely by their hydrophobicity but also by their molecular configuration.

Cholinergic reversal of isoflurane anesthesia in rats as measured by cross-approximate entropy of the electroencephalogram

BACKGROUND

Pharmacologic modulation of the state of consciousness is of interest for clinical practice and for a better understanding of anesthetic mechanisms. The cholinergic activating system is an important regulator of the state of consciousness during general anesthesia. Entropy of the electroencephalogram has been proposed as a promising measure of anesthetic depth. The authors have shown that volatile anesthetics decrease cross-approximate entropy (C-ApEn) of the bihemispheric frontal electroencephalogram in rats. The effect of cholinergic agents on C-ApEn has not been examined. Here, the authors test the hypothesis that cholinergic activation reverses the effect of isoflurane anesthesia on C-ApEn.

METHODS

An electroencephalogram in the 1- to 100-Hz range was recorded bipolarly, with epidural leads from the frontal cortex of both hemispheres, and used to calculate C-ApEn, which reflects statistical independence of bihemispheric electroencephalographic activity. Cholinesterase inhibitor, neostigmine (25 mug), or the muscarinic agonist oxotremorine (25 mug) were infused intracerebroventricularly while the rats were inhaling 1.0% (0.7 minimum alveolar concentration) isoflurane. In other animals, isoflurane was lowered to 0.4% (0.3 minimum alveolar concentration) to assess the electroencephalogram in a sedated, waking state.

RESULTS

At 1.0% isoflurane, C-ApEn decreased by 54% compared with that at 0.4%, but the motor reflex response to tail pinch was still present. Cholinergic agents reversed the electroencephalogram-depressant effect of isoflurane, i.e., C-ApEn rose to the level measured at 0.4% isoflurane. The rise in C-ApEn was paralleled by the appearance of spontaneous limb and orofacial explorative movements, suggesting a return of consciousness. In contrast, cholinergic agents fully blocked the motor reflex to tail pinch.

CONCLUSIONS

C-ApEn of the bihemispheric electroencephalogram correlates with the return of spontaneous motor signs but not with the nociceptive reflex. Cerebral cholinergic activation dissociates central and peripheral anesthetic effects. C-ApEn, a novel measure of interhemispheric electroencephalogram independence, is a promising correlate of depth of sedation and state of consciousness.

Antagonism of sevoflurane anaesthesia by physostigmine: effects on the auditory steady-state response and bispectral index

BACKGROUND

Physostigmine, a centrally acting anticholinesterase, antagonizes the hypnotic effect of propofol, as shown by the return of consciousness (response to commands) or wakefulness (spontaneous eye-opening without response to commands) and by recovery of auditory evoked potentials (40 Hz auditory steady-state response (ASSR)) and the bispectral index (BIS). We measured the effects of physostigmine on the hypnotic effect of inhaled volatile anaesthetics, using sevoflurane as the representative agent.

METHODS

Eight healthy volunteers received sevoflurane adjusted to produce loss of consciousness. Physostigmine (plus glycopyrrolate) was given while the end-tidal concentration of sevoflurane was kept constant.

RESULTS

Loss of consciousness was accompanied by a significant (P<0.02) decrease in ASSR amplitude (to 21% of awake value) and BIS (to 70% of awake value). Five subjects had return of consciousness or wakefulness after physostigmine. The others showed no behavioural change. Physostigmine caused a significant increase of the mean ASSR amplitude from 0.11 (SD 0.04) to 0.17 (0.06) microV (P<0.05). The BIS also increased, from 66 (12) to 74 (12), but the difference was not significant.

CONCLUSIONS

Physostigmine can antagonize, at least partially, the hypnotic effect of sevoflurane and changes in arousal after physostigmine are shown by ASSR measurements. However, the antagonism is not as clear or reliable as with propofol.

Pronociceptive actions of isoflurane: a protective role for estrogen

BACKGROUND

Low concentrations of inhaled anesthetics present in the early postoperative period can increase pain sensitivity. Changes in pain threshold associated with inhaled anesthetics have been reported in male mice, rats, and humans.

METHODS

The authors compared the pain-enhancing effects of isoflurane in male and female mice in response to a thermal stimulus and studied the consequences of hormonal manipulation.

RESULTS

Isoflurane produced a larger increase in pain sensitivity in female mice. Both castration and oophorectomy resulted in an increase in baseline pain sensitivity and potentiated pain enhancement by isoflurane. At stages of the estrus cycle when estrogen was low, female mice showed greater pain enhancement from isoflurane than at high estrogen stages. Treatment with exogenous estrogen reduced isoflurane-induced pain sensitivity. Exogenous testosterone treatment had a similar effect, which did not occur when enzymatic conversion to estrogen was prevented.

CONCLUSIONS

Because both estrogen and testosterone reduce the pronociceptive action of isoflurane, intact females may exhibit a larger increase in pain sensitivity because of their cyclical estrogen levels. Testosterone is effective in the male because of conversion to estrogen. Enhanced pain sensitivity is clearly undesirable in the postoperative setting. If these findings also apply to humans, the menstrual cycle may be an important factor in determining pain levels after emergence from general anesthesia.

A comparison of airway responses during desflurane and sevoflurane administration via a laryngeal mask airway for maintenance of anesthesia

IMPLICATIONS

Although sevoflurane is less pungent than desflurane at larger concentrations, neither anesthetic seems to irritate the airway when administered at the smaller concentrations often used during maintenance of anesthesia. Both anesthetics may be delivered effectively via a laryngeal mask airway, with minimal evidence of airway irritation.

GABA(A) receptor blockade antagonizes the immobilizing action of propofol but not ketamine or isoflurane in a dose-related manner

The enhancing action of propofol on gamma-amino-n-butyric acid subtype A (GABA(A)) receptors purportedly underlies its anesthetic effects. However, a recent study found that a GABA(A) antagonist did not alter the capacity of propofol to depress the righting reflex. We examined whether the noncompetitive GABA(A) antagonist picrotoxin and the competitive GABA(A) antagonist gabazine affected a different anesthetic response, immobility in response to a noxious stimulus (a tail clamp in rats), produced by propofol. This effect was compared with that seen with ketamine and isoflurane. Picrotoxin increased the 50% effective dose (ED(50)) for propofol by approximately 379%; gabazine increased it by 362%, and both antagonists acted in a dose-related manner with no apparent ceiling effect (i.e., no limit). Picrotoxin maximally increased the ED(50) for ketamine by approximately 40%-50%, whereas gabazine increased it by 50%-60%. The isoflurane minimum alveolar anesthetic concentration increased by approximately 60% with the picrotoxin and 70% with the gabazine infusion. The ED(50) for propofol was also antagonized by strychnine, a non-GABAergic glycine receptor antagonist and convulsant, to determine whether excitation of the central nervous system by a non-GABAergic mechanism could account for the increases in propofol ED(50) observed. Because strychnine only increased the immobilizing ED(50) of propofol by approximately 50%, GABA(A) receptor antagonism accounted for the results seen with picrotoxin and gabazine. We conclude that GABA(A) antagonism can influence the ED(50) for immobility of propofol and the non-GABAergic anesthetic ketamine, although to a different degree, reflecting physiologic antagonism for ketamine (i.e., an indirect effect via a modulatory effect on the neural circuitry underlying immobility) versus physiologic and pharmacologic antagonism for propofol (i.e., a direct effect by antagonism of propofol's mechanism of action). This study also suggests that the immobilizing action of isoflurane probably does not involve the GABA(A) receptor because antagonism of GABA(A) receptors for animals anesthetized with isoflurane produces results quantitatively and qualitatively similar to ketamine and markedly different from propofol.

IMPLICATIONS

IV picrotoxin and gabazine antagonized the immobilizing action of propofol in a dose-related manner, whereas antagonism of the immobilizing action of ketamine and isoflurane was similar, smaller than for propofol, and not dose-related. These results are consistent with a role for gamma-amino-n-butyric acid subtype A receptors in mediating propofol anesthesia but not ketamine or isoflurane anesthesia.

Antioxidants prevent depression of the acute hypoxic ventilatory response by subanaesthetic halothane in men

We studied the effect of the antioxidants (AOX) ascorbic acid (2 g, I.V.) and alpha-tocopherol (200 mg, P.O.) on the depressant effect of subanaesthetic doses of halothane (0.11 % end-tidal concentration) on the acute isocapnic hypoxic ventilatory response (AHR), i.e. the ventilatory response upon inhalation of a hypoxic gas mixture for 3 min (leading to a haemoglobin saturation of 82 +/- 1.8 %) in healthy male volunteers. In the first set of protocols, two groups of eight subjects each underwent a control hypoxic study, a halothane hypoxic study and finally a halothane hypoxic study after pretreatment with AOX (study 1) or placebo (study 2). Halothane reduced the AHR by more than 50 %, from 0.79 +/- 0.31 to 0.36 +/- 0.14 l min(-1) %(-1) in study 1 and from 0.79 +/- 0.40 to 0.36 +/- 0.19 l min(-1) %(-1) in study 2, P < 0.01 for both. Pretreatment with AOX prevented this depressant effect of halothane in the subjects of study 1 (AHR returning to 0.77 +/- 0.32 l min(-1) %(-1), n.s. from control), whereas placebo (study 2) had no effect (AHR remaining depressed at 0.36 +/- 0.27 l min(-1) %(-1), P < 0.01 from control). In a second set of protocols, two separate groups of eight subjects each underwent a control hypoxic study, a sham halothane hypoxic study and finally a sham halothane hypoxic study after pretreatment with AOX (study 3) or placebo (study 4). In studies 3 and 4, sham halothane did not modify the control hypoxic response, nor did AOX (study 3) or placebo (study 4). The 95 % confidence intervals for the ratio of hypoxic sensitivities, (AOX + halothane) : halothane in study 1 and (AOX - sham halothane) : sham halothane in study 3, were [1.7, 2.6] and [1.0, 1.2], respectively. Because the antioxidants prevented the reduction of the acute hypoxic response by halothane, we suggest that this depressant effect may be caused by reactive species produced by a reductive metabolism of halothane during hypoxia or that a change in redox state of carotid body cells by the antioxidants prevented or changed the binding of halothane to its effect site. Our findings may also suggest that reactive species have an inhibiting effect on the acute hypoxic ventilatory response.

A Caenorhabditis elegans pheromone antagonizes volatile anesthetic action through a go-coupled pathway

Volatile anesthetics (VAs) disrupt nervous system function by an ill-defined mechanism with no known specific antagonists. During the course of characterizing the response of the nematode C. elegans to VAs, we discovered that a C. elegans pheromone antagonizes the VA halothane. Acute exposure to pheromone rendered wild-type C. elegans resistant to clinical concentrations of halothane, increasing the EC(50) from 0.43 +/- 0.03 to 0.90 +/- 0.02. C. elegans mutants that disrupt the function of sensory neurons required for the action of the previously characterized dauer pheromone blocked pheromone-induced resistance (Pir) to halothane. Pheromone preparations from loss-of-function mutants of daf-22, a gene required for dauer pheromone production, lacked the halothane-resistance activity, suggesting that dauer and Pir pheromone are identical. However, the pathways for pheromone's effects on dauer formation and VA action were not identical. Not all mutations that alter dauer formation affected the Pir phenotype. Further, mutations in genes not known to be involved in dauer formation completely blocked Pir, including those altering signaling through the G proteins Goalpha and Gqalpha. A model in which sensory neurons transduce the pheromone activity through antagonistic Go and Gq pathways, modulating VA action against neurotransmitter release machinery, is proposed.

Physostigmine does not antagonize sevoflurane anesthesia assessed by bispectral index or enhances recovery

In this double-blinded study, we investigated the effect of physostigmine on sevoflurane anesthesia and recovery. Forty female patients scheduled for breast biopsy were randomly assigned to receive either physostigmine 2 mg IV or an equal volume of normal saline after skin closure. Anesthesia was induced and maintained with sevoflurane in oxygen. After skin closure, a steady state of 0.6% inspired and end-tidal sevoflurane concentration was obtained, heart rate, blood pressure, and Bispectral index (BIS) baseline values were recorded, and physostigmine or normal saline was administered. Hemodynamics and BIS values were also recorded 5, 8, and 10 min after treatments. Anesthesia was discontinued, and orientation, sedation, sitting ability, and "picking up matches" scores were recorded immediately after extubation and 15 and 30 min later. No differences were found between the two groups in BIS (69, 70, 70, and 71 in the Physostigmine group versus 70, 74, 75, and 76 in the Control group) or blood pressure. Only heart rate was increased 8 min after physostigmine (P < 0.05 versus the control). Scores assessing early recovery were similar in the two groups at all time points. We conclude that physostigmine does not change BIS or enhance recovery after sevoflurane anesthesia.

IMPLICATIONS

This double-blinded, randomized study investigated the impact of physostigmine of BIS values during 0.6% sevoflurane anesthesia as well as in the postoperative recovery, when sevoflurane is administered as a sole anesthetic. Physostigmine has no effect on BIS values or on the tests assessing recovery.

Hepatobiliary scintigraphy for evaluating the hepatotoxic effect of halothane and the protective effect of catechin in comparison with histo-chemical analysis of liver tissue

Halothane and its metabolites cause liver damage by decreasing liver blood flow and generating free-radical species. Catechin suppresses lipid peroxidation and increases enzyme activity, therefore it seems to be capable of protecting liver parenchyma against the direct toxic effect of halothane. The aim of this study was to investigate the role of hepatobiliary scintigraphy in detecting liver damage after halothane anaesthesia and the protective effect of catechin in comparison with histo-chemical analysis. Thirty rabbits, divided into three groups (A, controls; B, halothane; and C, catechin+halothane), were investigated. In group A no anaesthesia was administered. Group B only received halothane, while group C was pretreated with catechin and halothane anaesthesia was administered for 2 h. Dynamic scintigrams were taken for 60 min after injecting 99mTc-mebrofenin, and the time of peak uptake (TPU) and the time for half of the activity to clear from the liver (T(1/2)) were calculated. Rabbits were killed, and malondialdehyde (MDA), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase (CAT) levels were measured in hepatic tissue. The TPU and T(1/2) values of group A is significantly lower than in groups B and C (P<0.0002 and P<0.0002, respectively, for TPU; and P<0.0002 and P<0.0003, respectively, for T(1/2)). The TPU and T(1/2) values of group B were significantly higher than in group C (P<0.0003 and P<0.0003, respectively). The hepatic MDA level of group A was significantly lower than in groups B and C (P<0.0002 and P<0.0002, respectively). SOD, GSH-Px and CAT levels of group A were significantly higher than in groups B and C (P<0.0002, P<0.0001 and P<0.003, respectively, for group A vs group B; and P<0.0005, P<0.0002 and P<0.03, respectively, for group A vs group C). The MDA level of group B was significantly higher than that in group C (P<0.0002). SOD, GSH-Px and CAT levels of group B were significantly lower than in group C (P<0.0002, P<0.0002 and P<0.003, respectively). According to these results, we suggest that catechin protects liver parenchyma against the toxic effect of halothane and its metabolites, and that, compared to invasive histo-chemical analysis, hepatobiliary scintigraphy is a useful and alternative non-invasive method for detecting the protective effect of catechin on liver parenchyma after halothane anaesthesia.

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.

Both Cerebral GABAA Receptors and Spinal GABAA Receptors Modulate the Capacity of Isoflurane to Produce Immobility

We previously demonstrated that intrathecal administration of the noncompetitive gamma-aminobutyric acid type A (GABA(A)) receptor antagonist picrotoxin increased isoflurane MAC (the minimum alveolar concentration of anesthetic producing immobility in 50% of animals) by a maximum (ceiling effect) of approximately 40%. We also found that IV administration of picrotoxin increased MAC by more than 60%, without evidence of a ceiling effect. The larger increase with IV administration suggested a role of cerebral GABA(A) receptors. Accordingly, in this study we examined the effect of intracerebroventricular administration of picrotoxin in rats, finding that picrotoxin infusion into the third ventricle increased isoflurane MAC by a maximum of approximately 40%, without finding a ceiling effect. In addition, we concurrently infused picrotoxin into the intrathecal and intracerebroventricular spaces, producing an increase in MAC in excess of 70%, also with no evidence of a ceiling effect. The dose-response relationship for the intrathecal-intraventricular infusion paralleled that of the IV infusion but was shifted to the left by an order of magnitude. We conclude that both cerebral and spinal GABA(A) receptors modulate the capacity of inhaled anesthetics to produce immobility. Because other studies have shown that the spinal cord, and not the brain, mediates the capacity of inhaled anesthetics to produce immobility, these results call into question the relevance of GABA(A) receptors to the immobilizing action of isoflurane.

Local GABAA Receptor Blockade Reverses Isoflurane’s Suppressive Effects on Thalamic Neurons In Vivo

Many in vitro effects of volatile anesthetics are known, but the mechanisms of action are still under debate. Because suppression of sensory perception is one of the major goals of general anesthesia, we studied the effects of isoflurane on the processing of somatosensory information in anesthetized rats. Local iontophoretic administration of the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist bicuculline in the thalamic ventral posteromedial nucleus reversed suppressive effects of isoflurane on thalamocortical relay neurons (TCNs). The action potential discharges of TCNs (n = 23) in response to defined mechanical stimulation of receptive fields seen with small concentrations of isoflurane (0.79% +/- 0.01%, mean +/- SEM) were suppressed under large concentrations (1.44% +/- 0.04%). In addition, the tonic response pattern was lost, which initially encoded the information about the stimulus features. In 70% of TCNs, bicuculline administration reestablished the initially present tonic response pattern under large isoflurane concentrations. These results indicate that isoflurane suppresses somatosensory information transfer at the thalamic level in vivo, apparently by enhancing thalamic GABA(A) receptor-mediated inhibition.

Suppressive effect of nitrous oxide on motor evoked potentials can be reversed by train stimulation in rabbits under ketamine/fentanyl anaesthesia, but not with additional propofol

The effect of nitrous oxide on myogenic motor evoked potentials (MEPs) after multipulse stimulation is controversial. We investigated the effects of propofol in this paradigm. MEPs were elicited electrically by a single pulse and by trains of three and five pulses in rabbits anaesthetized with ketamine and fentanyl. Nitrous oxide 30-70% was given and MEPs were recorded. After washout of nitrous oxide, propofol was given as a bolus of 10 mg kg(-1) followed by 0.8 (n=9) or 1.6 mg kg(-1) min(-1) (n=8) as a continuous infusion. Nitrous oxide was then re-administered and MEPs were recorded. Without propofol, nitrous oxide significantly reduced the amplitude of MEPs dose-dependently, but this effect was reversed by multipulse stimulation. Administration of low-dose propofol enhanced nitrous oxide-induced suppression, and this effect was reversed by five-pulse stimulation. However, high-dose propofol produced a greater increase in suppression, such that even five-pulse stimulation did not overcome the suppression. The results suggest that the degree of reversal of nitrous oxide-induced MEP suppression produced by multipulse stimulation is affected by the administration of propofol.

Effects of nonimmobilizers and halothane on Caenorhabditis elegans

We studied the effects of two nonimmobilizers, a transitional compound, and halothane on the nematode, Caenorhabditis elegans, by using reversible immobility as an end point. By themselves, the nonimmobilizers did not immobilize any of the four strains of animals tested. Toluene appears to be a transitional compound for all strains tested. The additive effects of the nonimmobilizers with halothane were also studied. Similar to results seen in studies of mice, the nonimmobilizers were antagonistic to halothane in the wild type nematode. However, the nonimmobilizers did not affect the 50% effective concentrations of halothane for two other mutant strains. For halothane, the slopes of the dose response curves were smaller in more sensitive strains compared with the wild type. As in mammals, nonimmobilizers antagonize the effects of halothane on the nematode, C. elegans. The variation in slopes in the response to halothane in different strains is consistent with multiple sites of action. These results support the use of C. elegans as a model for the study of anesthetics.

Sarcolemmal and mitochondrial adenosine triphosphate- dependent potassium channels: mechanism of desflurane-induced cardioprotection

BACKGROUND

Volatile anesthetic-induced preconditioning is mediated by adenosine triphosphate-dependent potassium (KATP) channels; however, the subcellular location of these channels is unknown. The authors tested the hypothesis that desflurane reduces experimental myocardial infarct size by activation of specific sarcolemmal and mitochondrial KATP channels.

METHODS

Barbiturate-anesthetized dogs (n = 88) were acutely instrumented for measurement of aortic and left ventricular pressures. All dogs were subjected to a 60-min left anterior descending coronary artery occlusion followed by 3-h reperfusion. In four separate groups, dogs received vehicle (0.9% saline) or the nonselective KATP channel antagonist glyburide (0.1 mg/kg intravenously) in the presence or absence of 1 minimum alveolar concentration desflurane. In four additional groups, dogs received 45-min intracoronary infusions of the selective sarcolemmal (HMR 1098; 1 microg. kg-1. min-1) or mitochondrial (5-hydroxydecanoate [5-HD]; 150 microg. kg-1. min-1) KATP channel antagonists in the presence or absence of desflurane. Myocardial perfusion and infarct size were measured with radioactive microspheres and triphenyltetrazolium staining, respectively.

RESULTS

Desflurane significantly (P < 0.05) decreased infarct size to 10 +/- 2% (mean +/- SEM) of the area at risk as compared with control experiments (25 +/- 3% of area at risk). This beneficial effect of desflurane was abolished by glyburide (25 +/- 2% of area at risk). Glyburide (24 +/- 2%), HMR 1098 (21 +/- 4%), and 5-HD (24 +/- 2% of area at risk) alone had no effects on myocardial infarct size. HMR 1098 and 5-HD abolished the protective effects of desflurane (19 +/- 3% and 22 +/- 2% of area at risk, respectively).

CONCLUSION

Desflurane reduces myocardial infarct size in vivo, and the results further suggest that both sarcolemmal and mitochondrial KATP channels could be involved.

Colchicine inhibits isoflurane-induced preconditioning

BACKGROUND

When administered before prolonged myocardial ischemia and reperfusion, isoflurane exerts potent cardioprotective effects similar to those inferred by ischemic preconditioning. To determine whether an intact cytoskeleton is critically important in isoflurane-induced preconditioning, the authors used a rabbit model in which isoflurane-induced myocardial preconditioning decreases myocardial infarct size (IS) substantially. In this model, the authors tested whether the microtubule depolymerizing agent, colchicine, would inhibit isoflurane-induced myocardial preconditioning.

METHODS

Myocardial IS was measured in four groups of propofol-anesthetized rabbits, each subjected to 30 min of anterolateral coronary occlusion followed by 3 h of reperfusion. Groups differed only in the pretreatments given, and only the control group received no pretreatment. An isoflurane-preconditioned group was pretreated with 15 min of end-tidal isoflurane, 1.1%, and then 15 min of washout. An isoflurane-plus-colchicine group was administered 2 mg/kg colchicine intravenously before isoflurane pretreatment. A colchicine-control group was administered 2 mg/kg colchicine but no isoflurane pretreatment. Myocardial IS and area at risk (AR) were defined by staining. Data were analyzed by analysis of variance or covariance.

RESULTS

Infarct size, expressed as a percentage of AR (IS:AR) was 33.6%+/-8.8% (SD) in the control group. Isoflurane preexposure reduced myocardial IS:AR significantly, to 11.8%+/-9.1%. Colchicine pretreatment eliminated the preconditioning-like effect of isoflurane (IS:AR = 32.6%+/-8.7%). Colchicine alone did not alter IS (IS:AR = 27.6%+/-7.1%; P = not significant).

CONCLUSIONS

Colchicine abolished the preconditioning effect of isoflurane but did not increase IS when administered alone. An intact microtubular cytoskeleton is critically important in the process of volatile anesthetic-induced preconditioning.

5-HT2 receptor antagonist-mediated inhibition of halothane-induced contractures in skeletal muscle specimens from malignant hyperthermia susceptible patients

Administration of 5-HT2 receptor agonists induced malignant hyperthermia (MH) in susceptible pigs. Furthermore, the 5-HT2 receptor antagonist ritanserin prevented 5-HT-induced porcine MH. It has been shown that 5-HT2 receptor agonists induce marked contractures in skeletal muscle specimens from MH susceptible (MHS) but not in specimens from normal patients. The purpose of this study was to investigate the effects of ritanserin on halothane-induced contractures in muscle specimens from MHS patients. Twenty-five patients aged 8-56 years (29.5+/-13.6) classified as MHS by the in vitro contracture test (IVCT) with halothane and caffeine according to the protocol of the European MH Group participated in this study. Muscle specimens were pretreated with ritanserin 10 micromol/l (n= 14), 20 micromol/l (n=14) and 100 micromol/l (n=12) for 10 min and subsequently halothane was added incrementally (0.11-0.22-0.44 mmol/l) to the tissue bath as described in the European MH protocol. The results of the halothane contracture test were used as control. Following administration of halothane, muscle contractures reached a maximum of 16.9+/-4.2 mN. Ritanserin led to a significant inhibition of halothane-induced contractures in MHS muscles. Following pretreatment with ritanserin, halothane-induced contracture maximum was significantly smaller with 7.5+/-3.1 mN after 10 micromol/l ritanserin, 4.9+/-1.5 mN after 20 micromol/l ritanserin and 0.5+/- 0.2 mN after 100 micromol/l ritanserin than without pretreatment. Administration of ritanserin induced at all concentrations a decrease in muscle twitch height. Increase in muscle twitch following halothane was reduced in a concentration-dependent manner by ritanserin. The presented findings indicate that 5-HT might be involved in the mechanisms of halothane-induced MH in humans. Further studies have to determine the pathophysiological role of the 5-HT system in MH, and whether ritanserin could be an alternative for treatment or prevention of halothane-induced MH.

[Effects of anesthetics on somatosensory evoked potentials by median nerve stimulation in human--analyses after single administration in volunteers]

In the present study, effects of midazolam, thiopental sodium, propofol, and nitrous oxide upon SEP in a clinically used dose were investigated on 24 male volunteers. In addition, antagonistic actions of flumazenil and naloxone against effects of midazolam and nitrous oxide, respectively, on SEP were studied. Midazolam had no effect on latencies of N 20 and P 25, but increased latency of P 45 and attenuated P 100 amplitude. Flumazenil reversed these effects of midazolam of P 45 latency and P 100 amplitude to their control values. While thiopental sodium and propofol suppressed P 100 amplitude, they had no effect on N 20, P 25, P 45 latencies. Nitrous oxide elongated latencies of N 20, P 25, P 45 and decreased P 100 amplitude. Naloxone reversed the effects of nitrous oxide on N 20 and P 25 latencies without affecting increased P 45 latency and attenuated P 100 amplitude. These results suggest that midazolam might have an analgesic action of suppressing cortical sensory neurons, whereas thiopental sodium and propofol have no effect on neurons in the primary sensory cortex. The finding that naloxone antagonized the increased latencies of N 20 and P 25 by nitrous oxide could be explained by the analgesic action of nitrous oxide that could be mediated by opioid receptors. The results also indicate that electrical activities of the cortical neurons in the associated area are more susceptible to psychotropic agents than those in the primary sensory cortex. The effects of anesthetics on SEP appear to reflect their characteristics of functioning mechanisms on cortical neurons. Analysis of SEP is, therefore, useful for the assessment of the mechanism and the acting site of anesthetics in the sensory cortex.

Does nitrous oxide antagonize sevoflurane-induced hypnosis?

We have studied 64 ASA I and II patients (aged 20-60 yr) to determine if nitrous oxide affects sevoflurane requirement for achieving 50% probability of no movement in response to verbal commands (MACawake). Patients were allocated randomly to one of four nitrous oxide concentration groups (0, 20, 40 and 60 vol.%). Patients in each group received sevoflurane at two different end-tidal concentrations according to a predetermined randomization table. After steady state sevoflurane and nitrous oxide concentrations had been maintained for at least 15 min, patients were assessed as being awake or asleep. The MACawake for sevoflurane was 0.63% and this was reduced significantly in a non-linear manner by increasing nitrous oxide concentration. A 50% reduction in MACawake was produced by a nitrous oxide concentration of 45%. The reduction in MACawake by nitrous oxide was non-linear; the interaction coefficient between nitrous oxide and sevoflurane being significantly less than zero (P = 0.0238), indicating that the reduction in MACawake by nitrous oxide was smaller than would be expected from simple additivity and that nitrous oxide antagonized the effects of sevoflurane in preventing response to verbal commands.

Halothane reduces reperfusion injury after regional ischaemia in the rabbit heart in vivo

In addition to having anti-ischaemic effects, halothane can protect isolated rat hearts and isolated cardiomyocytes against reperfusion injury of the "oxygen paradox" type. The aim of this study was to investigate if halothane can also protect against myocardial reperfusion injury in vivo. Twenty-two rabbits anaesthetized with alpha-chloralose underwent 30 min of occlusion of a major coronary artery and 2 h of subsequent reperfusion. Seven animals received 1 MAC of halothane for the first 15 min of reperfusion (halothane group), and eight animals served as untreated controls (controls group). In seven additional animals, the haemodynamic effects of halothane were antagonized by an i.v. infusion of noradrenaline (halothane-noradrenaline group). We measured cardiac output (CO) by an ultrasonic flow probe around the ascending aorta, left ventricular pressure (LVP) by a tip manometer and infarct size by triphenyltetrazolium staining. Baseline LVP was mean 92 (SEM 4) mm Hg and CO was 289 (16) ml min-1. During coronary occlusion, LVP was reduced to 86 (4)% of baseline and CO to 84 (4)% (similar in all groups). During halothane administration at reperfusion, LVP declined further to 55 (6)% of baseline and CO to 66 (9)% (P < 0.05 halothane group vs control group). Noradrenaline prevented the reduction in LVP (halothane-noradrenaline group 87 (5)% of baseline, control group 84 (6)% and reduction in CO (halothane-noradrenaline group 89 (5)%, control group 83 (6)%. Infarct size was 49 (6)% of the area at risk in controls and was reduced markedly by administration of halothane to 32 (3)% in the halothane group (P < 0.05) and to 30 (3)% in the halothane-noradrenaline group (P < 0.05). Treatment with halothane during the early reperfusion period after myocardial ischaemia protected the myocardium against infarction in vivo, independent of the haemodynamic effect of halothane.

Administration of sufentanil and nitrous oxide blunts cardiovascular effects of desflurane but does not prevent an increase in middle cerebral artery blood flow velocity

Desflurane has been reported to cause tachycardia and hypertension during induction of anaesthesia. The aim of this study was to determine the effects of desflurane on cerebral blood flow (CBF) velocity using transcranial Doppler ultrasonography in a setting that closely resembled usual clinical practice. In two groups (n = 9 in each) ASA Grade I or II patients, anaesthesia was induced with etomidate and vecuronium intravenously (i.v.), sufentanil (0.3 microgram kg-1 i.v.) was added in the second group. Patients were ventilated by facemask for 2 min before desflurane was administered in steps of 0.5 MAC min-1 until 1.5 MAC was reached and maintained for 7 min. Haemodynamic variables and CBF velocity in the middle cerebral artery (MCA) were monitored throughout the study period. In group 1 heart rate increased to 108 +/- 2 b.p.m. (37% increase) whereas MAP increased to 114 +/- 6 mmHg after administration of desflurane (33% increase). CBF velocity increased to 86 +/- 7 cm s-1 (69% increase). In group 2 no significant changes in systemic haemodynamic responses were measured after desflurane administration; however, CBF velocity increased to 73 +/- 5 cm s-1 (59% increase). The results indicate that desflurane increases CBF velocity concurrently with induction of tachycardia and hypertension. Although sufentanil and N2O attenuate the systemic haemodynamic alterations caused by desflurane, the CBF velocity increases. These data suggest that the abrupt addition of desflurane may have adverse consequences in patients at risk for intracranial hypertension.

Visual evoked potentials and nitrous oxide-induced neuronal depression: role for benzodiazepine receptors

We have examined the role of benzodiazepine receptors in nitrous oxide-induced neuronal depression in rats. The changes in neuronal excitability induced by nitrous oxide and the benzodiazepine inverse agonist, Ro15-4513, were monitored by measurement of visual evoked potentials (VEP). Administration of Ro15-4513 10 mg kg-1 i.p., in rats breathing air, did not affect the amplitude or latency of VEP. However, the same concentrations of Ro15-4513 antagonized nitrous oxide-induced depression of VEP amplitudes. We conclude that antagonism of nitrous oxide-induced depression by Ro15-4513 indicates that at least part of the decreased neuronal excitability caused by nitrous oxide could be ascribed to interactions with the GABAA receptor complex.

Antagonism of the antinocifensive action of halothane by intrathecal administration of GABAA receptor antagonists

BACKGROUND

The hind brain and the spinal cord, regions that contain high concentrations of gamma-aminobutyric acid (GABA) and GABA receptors, have been implicated as sites of action of inhalational anesthetics. Previous studies have established that general anesthetics potentiate the effects of gamma-aminobutyric acid at the GABAA receptor. It was therefore hypothesized that the suppression of nocifensive movements during anesthesia is due to an enhancement of GABAA receptor-mediated transmission within the spinal cord.

METHODS

Rats in which an intrathecal catheter had been implanted 1 week earlier were anesthetized with halothane. Core temperature was maintained at a steady level. After MAC determination, the concentration of halothane was adjusted to that at which the rats last moved in response to tail clamping. Saline, a GABAA, a GABAB, or glycine receptor antagonist was then injected intrathecally. The latency to move in response to application of the tail clamp was redetermined 5 min later, after which the halothane concentration was increased by 0.2%. Response latencies to application of the noxious stimulus were measured at 7-min intervals during the subsequent 35 min. To determine whether these antagonists altered baseline response latencies by themselves, another experiment was conducted in which the concentration of halothane was not increased after intrathecal administration of GABAA receptor antagonists.

RESULTS

Intrathecal administration of the GABAA receptor antagonists bicuculline (0.3 micrograms) or picrotoxin (0.3, 1.0 micrograms) antagonized the suppression of nocifensive movement produced by the small increase in halothane concentration. In contrast, the antinocifensive effect of the increase in halothane concentration was not attenuated by the GABAB receptor antagonist CGP 35348 or the glycine receptor antagonist strychnine. By themselves, the GABAA receptor antagonists did not alter response latency in rats anesthetized with sub-MAC concentrations of halothane.

CONCLUSIONS

Intrathecal administration of bicuculline or picrotoxin, at doses that do not change the latency to pinch-evoked movement when administered alone, antagonized the suppression of noxious-evoked movement produced by halothane concentrations equal to or greater than MAC. These results suggest that enhancement of GABAA receptor-mediated transmission within the spinal cord contributes to halothane's ability to suppress nocifensive movements.

Alfentanil modifies the neurocirculatory responses to desflurane

Activation of the sympathetic nervous system occurs in response to desflurane, causing tachycardia and hypertension. Fentanyl partially blunts the hemodynamic effects of desflurane but fails to attenuate the sympathetic response. This study determined the clinical effectiveness and dose response of alfentanil on the neurocirculatory responses to desflurane. Twenty-five healthy, male volunteers were randomized into one of three groups to receive either placebo (n = 9), 10 micrograms/kg intravenous (IV) bolus alfentanil (n = 9), or 20 micrograms/kg IV bolus alfentanil (n = 7) in conjunction with anesthetic induction by propofol, 2.5 mg/kg. Mean arterial pressure (MAP, radial artery), heart rate (HR), and efferent muscle sympathetic nerve activity (SNA, peroneal nerve) were recorded. After conscious baseline measurements, anesthesia was induced by propofol and alfentanil/placebo. One minute later, the desflurane vaporizer was activated at 11%. Neurocirculatory measurements were recorded for 11 min. There were no differences between the groups at conscious baseline. Induction of anesthesia was associated with significantly decreased MAP in the placebo and the 10 micrograms/kg alfentanil groups and increased HR in all groups with little change in SNA. In placebo subjects, desflurane administration increased HR and MAP above baseline. In both alfentanil groups, during desflurane administration HR and MAP never increased significantly above baseline. However, SNA was significantly increased in both groups. Alfentanil effectively blunts the hemodynamic changes but not the sympathetic responses associated with rapid increases in the inspired concentration of desflurane.

Spermidine attenuation of volatile anesthetic inhibition of glutamate-stimulated [3H](5D,10S)-(+)-methyl-10,11-dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine ([3H]MK-801) binding to N-methyl-D-aspartate (NMDA) receptors in rat brain

The influence of spermidine, a polyamine agonist, on volatile anesthetic inhibition of N-methyl-D-aspartate (NMDA) receptor activation, as indicated by glutamate stimulation of [3H]MK-801 ([3H](5D,10S)-(+)-methyl-10,11-dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine) binding, was studied in rat brain. Spermidine reserved the inhibition caused by four volatile anesthetics (enflurane, halothane, methoxyflurane and chloroform) at the same concentrations (EC50 approximately 3 microM) at which it potentiated glutamate opening of the NMDA ion channel. The anesthetics had no effect on the direct stimulation of channel opening by spermidine, which occurred at concentrations of spermidine greater than 30 microM in the absence of receptor agonist. In these actions, spermidine closely resembled the allosteric co-agonist glycine. The present results suggest that anesthetic action on NMDA receptors involves a set of sites on the channel complex that is distinct from the recognition sites for glutamate, glycine, and channel blockers, and are consistent with the idea that blockade of NMDA receptors contributes to the development of the anesthetic state.