Effect of Isoflurane and Other Potent Inhaled Anesthetics on Minimum Alveolar Concentration, Learning, and the Righting Reflex in Mice Engineered to Express α1 γ-Aminobutyric Acid Type A Receptors Unresponsive to Isoflurane

Background: Enhancement of the function of [gamma]-aminobutyric acid type A receptors containing the [alpha]1 subunit may underlie a portion of inhaled anesthetic action. To test this, the authors created gene knock-in mice harboring mutations that render the receptors insensitive to isoflurane while preserving sensitivity to halothane.

Methods: The authors recorded miniature inhibitory synaptic currents in hippocampal neurons from hippocampal slices from knock-in and wild-type mice. They also determined the minimum alveolar concentration (MAC), and the concentration at which 50% of animals lost their righting reflexes and which suppressed pavlovian fear conditioning to tone and context in both genotypes.

Results: Miniature inhibitory postsynaptic currents decayed more rapidly in interneurons and CA1 pyramidal cells from the knock-in mice compared with wild-type animals. Isoflurane (0.5-1 MAC) prolonged the decay phase of miniature inhibitory postsynaptic currents in neurons of the wild-type mice, but this effect was significantly reduced in neurons from knock-in mice. Halothane (1 MAC) slowed the decay of miniature inhibitory postsynaptic current in both genotypes. The homozygous knock-in mice were more resistant than wild-type controls to loss of righting reflexes induced by isoflurane and enflurane, but not to halothane. The MAC for isoflurane, desflurane, and halothane did not differ between knock-in and wild-type mice. The knock-in mice and wild-type mice did not differ in their sensitivity to isoflurane for fear conditioning.     

Effect of isoflurane and other potent inhaled anesthetics on minimum alveolar concentration, learning, and the righting reflex in mice engineered to express alpha1 gamma-aminobutyric acid type A receptors unresponsive to isoflurane

BACKGROUND: Enhancement of the function of gamma-aminobutyric acid type A receptors containing the alpha1 subunit may underlie a portion of inhaled anesthetic action. To test this, the authors created gene knock-in mice harboring mutations that render the receptors insensitive to isoflurane while preserving sensitivity to halothane. METHODS: The authors recorded miniature inhibitory synaptic currents in hippocampal neurons from hippocampal slices from knock-in and wild-type mice. They also determined the minimum alveolar concentration (MAC), and the concentration at which 50% of animals lost their righting reflexes and which suppressed pavlovian fear conditioning to tone and context in both genotypes. RESULTS: Miniature inhibitory postsynaptic currents decayed more rapidly in interneurons and CA1 pyramidal cells from the knock-in mice compared with wild-type animals. Isoflurane (0.5-1 MAC) prolonged the decay phase of miniature inhibitory postsynaptic currents in neurons of the wild-type mice, but this effect was significantly reduced in neurons from knock-in mice. Halothane (1 MAC) slowed the decay of miniature inhibitory postsynaptic current in both genotypes. The homozygous knock-in mice were more resistant than wild-type controls to loss of righting reflexes induced by isoflurane and enflurane, but not to halothane. The MAC for isoflurane, desflurane, and halothane did not differ between knock-in and wild-type mice. The knock-in mice and wild-type mice did not differ in their sensitivity to isoflurane for fear conditioning. CONCLUSIONS: gamma-Aminobutyric acid type A receptors containing the alpha1 subunit participate in the inhibition of the righting reflexes by isoflurane and enflurane. They are not, however, involved in the amnestic effect of isoflurane or immobilizing actions of inhaled agents.     

Blockade of AMPA receptors and volatile anesthetics: reduced anesthetic requirements in GluR2 null mutant mice for loss of the righting reflex and antinociception but not minimum alveolar concentration

BACKGROUND: The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype of glutamate receptor mediates fast excitatory neurotransmission in the central nervous system. Many general anesthetics inhibit AMPA receptors in vitro; however, it is not certain if this inhibition contributes to the behavioral properties of these drugs. AMPA receptors lacking the GluR2 subunit are resistant to blockade by barbiturates in vitro. Paradoxically, GluR2 null mutant (-/-) mice are more sensitive to barbiturate-induced loss of the righting reflex (LORR) compared with wild-type (+/+) littermates. To determine if interactions between anesthetics and AMPA receptors account for the increased sensitivity of (-/-) mice, the effects of volatile anesthetics that do not directly inhibit AMPA receptors were examined. METHODS: Isoflurane, halothane, desflurane, or sevoflurane were administered to (-/-) and (+/+) littermate controls. Anesthetic requirements for LORR, movement to tail clamp (minimum alveolar concentration [MAC]), and hind-paw withdrawal latency (HPWL) were determined. Electrophysiologic methods examined the inhibition of AMPA receptors by isoflurane and halothane. RESULTS: Anesthetic requirements for LORR and HPWL were decreased, whereas MAC values were unchanged in (-/-) mice. Isoflurane and halothane caused minimal inhibition of AMPA receptors at clinically relevant concentrations. CONCLUSIONS: Direct blockade of AMPA receptors did not account for the increased sensitivity to volatile anesthetics in GluR2 null mutant mice for HPWL or LORR. Thus, the deficiency of GluR2-containing AMPA receptors increases the sensitivity of neuronal circuitry mediating these end points, but not MAC. GluR2-containing receptors do not contribute appreciably to MAC in this mouse model. These results illustrate the difficulties in attributing behavioral responses to drug-receptor interactions in genetically engineered animals.     

Blockade of AMPA Receptors and Volatile Anesthetics: Reduced Anesthetic Requirements in GluR2 Null Mutant Mice for Loss of the Righting Reflex and Antinociception but Not Minimum Alveolar Concentration

Background: The [alpha]-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype of glutamate receptor mediates fast excitatory neurotransmission in the central nervous system. Many general anesthetics inhibit AMPA receptors in vitro; however, it is not certain if this inhibition contributes to the behavioral properties of these drugs. AMPA receptors lacking the GluR2 subunit are resistant to blockade by barbiturates in vitro. Paradoxically, GluR2 null mutant (-/-) mice are more sensitive to barbiturate-induced loss of the righting reflex (LORR) compared with wild-type (+/+) littermates. To determine if interactions between anesthetics and AMPA receptors account for the increased sensitivity of (-/-) mice, the effects of volatile anesthetics that do not directly inhibit AMPA receptors were examined.

Methods: Isoflurane, halothane, desflurane, or sevoflurane were administered to (-/-) and (+/+) littermate controls. Anesthetic requirements for LORR, movement to tail clamp (minimum alveolar concentration [MAC]), and hind-paw withdrawal latency (HPWL) were determined. Electrophysiologic methods examined the inhibition of AMPA receptors by isoflurane and halothane.

Results: Anesthetic requirements for LORR and HPWL were decreased, whereas MAC values were unchanged in (-/-) mice. Isoflurane and halothane caused minimal inhibition of AMPA receptors at clinically relevant concentrations.     

Mutation of KCNK5 or Kir3.2 potassium channels in mice does not change minimum alveolar anesthetic concentration

Several reports suggest that clinically used concentrations of inhaled anesthetics can increase conductance through noninactivating potassium channels and that the resulting hyperpolarization might decrease excitability, thereby leading to the anesthetic state. We speculated that animals deficient in such potassium channels might be resistant to the effects of anesthetics. Thus, in the present study, we measured the minimum alveolar anesthetic concentration (MAC) needed to prevent movement in response to a noxious stimulus in 50% of adult mice lacking functional KCNK5 potassium channel subunits and compared these results with those for heterozygous and wild-type mice. We also measured MAC in weaver mice that had a mutation in the potassium channel Kir3.2 and compared the resulting values with those for wild-type mice. MAC values for desflurane, halothane, and isoflurane for KCNK5-deficient mice and isoflurane MAC values for weaver mice did not differ from MAC values found in control mice. Our results do not support the notion that these potassium channels mediate the capacity of inhaled anesthetics to produce immobility. In addition, we found that the weaver mice did not differ from control mice in their susceptibility to convulsions from the nonimmobilizers flurothyl [di-(2,2,2,-trifluoroethyl)ether] or 2N (1,2-dichlorohexafluorocyclobutane). IMPLICATIONS: Mice harboring mutations in either of two different potassium channels have minimum alveolar anesthetic concentration (MAC) values that do not differ from MAC values found in control mice. Such findings do not support the notion that these potassium channels mediate the capacity of inhaled anesthetics to produce immobility in the face of noxious stimulation.     

Glycine receptors mediate part of the immobility produced by inhaled anesthetics

Many inhaled anesthetics potentiate the effect of glycine on inhibitory strychnine-sensitive glycine receptors in vitro, supporting the view that this receptor could mediate the immobility produced by inhaled anesthetics during noxious stimulation (i.e., would underlie minimum alveolar anesthetic concentration [MAC]). There are quantitative differences between anesthetics in their capacity to potentiate glycine's effect in receptor expression systems: halothane (most potentiation), isoflurane (intermediate), and cyclopropane (minimal). If glycine receptors mediate MAC, then their blockade in the spinal cord should increase the MAC of halothane more than that of isoflurane and isoflurane MAC more than cyclopropane MAC; the increases in MAC should be proportional to the receptor potentiation produced in vitro. Rats with chronically implanted intrathecal catheters were anesthetized with halothane, isoflurane, or cyclopropane. During intrathecal infusion of artificial cerebrospinal fluid, MAC was determined. Then MAC was re-determined during an infusion of 3, 12, 24, or 48 (isoflurane only) micro g/min of strychnine (strychnine blocks glycine receptors) in artificial cerebrospinal fluid. Strychnine infusion increased MAC in proportion to the enhancement of glycine receptors found in vitro. The maximum effect was with an infusion of 12 micro g/min. For the combined results at 12 and 24 micro g/min of strychnine, the increase in MAC correlated with the extent of in vitro potentiation (r(2) = 0.82). These results support the hypothesis that glycine receptors mediate part of the immobilization produced by inhaled anesthetics. IMPLICATIONS: In vitro, halothane potentiates glycine's effect on strychnine-sensitive glycine receptors more than isoflurane and isoflurane more than cyclopropane. The present in vivo work indicates that antagonism of the glycine receptor with strychnine increases minimum alveolar anesthetic concentration for halothane more than isoflurane and isoflurane more than cyclopropane. Such results support the notion that glycine receptors may mediate part of the immobility produced by inhaled anesthetics.     

Administration of epinephrine does not increase learning of fear to tone in rats anesthetized with isoflurane or desflurane

Previous reports suggest that the administration of epinephrine increases learning during deep barbiturate-chloral hydrate anesthesia in rats but not during anesthesia with 0.4% isoflurane in rabbits. We revisited this issue, using fear conditioning to a tone in rats as our experimental model for learning and memory and isoflurane and desflurane as our anesthetics. Expressed as a fraction of the minimum alveolar anesthetic concentration (MAC) preventing movement in 50% of rats, the amnestic 50% effective dose (ED(50)) for fear to tone in control rats inhaling isoflurane and injected with saline intraperitoneally (i.p.) was 0.32 +/- 0.03 MAC (mean +/- se) compared with 0.37 +/- 0.06 MAC in rats injected with 0.01 mg/kg of epinephrine i.p. and 0.38 +/- 0.03 MAC in rats injected with 0.1 mg/kg of epinephrine i.p. For desflurane, the amnestic ED(50) were 0.32 +/- 0.05 MAC in control rats receiving a saline injection i.p. versus 0.36 +/- 0.04 MAC in rats injected with 0.1 mg/kg of epinephrine i.p. We conclude that exogenous epinephrine does not decrease amnesia produced by inhaled isoflurane or desflurane, as assessed by fear conditioning to a tone in rats.     

Patch-clamp analysis of anesthetic interactions with recombinant SK2 subtype neuronal calcium-activated potassium channels

Small conductance calcium-activated potassium channels (SK) mediate spike frequency adaptation and underlie the slow afterhyperpolarization in central neurons. We tested the actions of several anesthetics on the SK2 subtype of recombinant SK channels, cloned from rat brain and functionally expressed in a mammalian cell line. Butanol, ethanol, ketamine, lidocaine, and methohexital blocked recombinant SK2 channel currents, measured in the whole-cell patch clamp recording mode. The block was reversible, dose-dependent, and of variable efficacy. The inhaled anesthetics chloroform, desflurane, enflurane, halothane, isoflurane, and sevoflurane produced little or no block when applied at 1 minimum alveolar anesthetic concentration; varying degrees of modulation were observed at very large concentrations (10 minimum alveolar concentration). The extent of block by inhaled anesthetics did not appear to depend on concentration or membrane voltage. IMPLICATIONS: We describe differential effects of anesthetics on cloned small conductance calcium-activated potassium channels from brain that may play a role in generating the effects or side effects of anesthetics.     

Naturally occurring variability in anesthetic potency among inbred mouse strains

We measured the naturally occurring variability in anesthetic potency, defined by the minimum alveolar anesthetic concentrations (MACs) of inhaled anesthetics required to produce immobility in response to noxious stimuli, in seven widely used laboratory mouse strains. To these data, we added similar data for eight other mouse strains. The average MAC values for each anesthetic for the 15 strains were normally distributed, with a coefficient of variation (ratio of SD to mean) of 0.1. The range of MAC values was 39% for desflurane, 44% for isoflurane, and 55% for halothane. MAC values were highly reliable, with approximately 1% of the variance in MAC measurements for the strains being explained by measurement error. One hundred forty-six statistically significant differences among the 15 strains were found for the three inhaled anesthetics (isoflurane, desflurane, and halothane). Our results suggest that multiple genes underlie the observed variability in anesthetic potency. IMPLICATIONS: Laboratory mouse strains differ significantly in susceptibility to anesthetics. These phenotypic differences may be exploited to help determine the genetic basis of anesthetic-induced immobility.     

Halothane and isoflurane have additive minimum alveolar concentration (MAC) effects in rats

Studies suggest that at concentrations surrounding MAC (the minimum alveolar concentration suppressing movement in 50% of subjects in response to noxious stimulation), halothane depresses dorsal horn neurons more than does isoflurane. Similarly, these anesthetics may differ in their effects on various receptors and ion channels that might be anesthetic targets. Both findings suggest that these anesthetics may have effects on movement in response to noxious stimulation that would differ from additivity, possibly producing synergism or even antagonism. We tested this possibility in 20 rats. MAC values for halothane and (separately) for isoflurane were determined in duplicate before and after testing the combination (also in duplicate; six determinations of MAC for each rat). The sum of the isoflurane and halothane MAC fractions for individual rats that produced immobility equaled 1.037 +/- 0.082 and did not differ significantly from a value of 1.00. That is, the combination of halothane and isoflurane produced immobility in response to tail clamp at concentrations consistent with simple additivity of the effects of the anesthetics. These results suggest that the immobility produced by inhaled anesthetics need not result from their capacity to suppress transmission through dorsal horn neurons. IMPLICATIONS: Despite differences in their capacities to inhibit spinal dorsal horn cells, isoflurane and halothane are additive in their ability to suppress movement in response to a noxious stimulus.     

Beta3-containing gamma-aminobutyric acidA receptors are not major targets for the amnesic and immobilizing actions of isoflurane

Mice bearing an N265M point mutation in the gamma-aminobutyric acid (GABA)(A) receptor beta3 subunit resist various anesthetic effects of propofol and etomidate. They also require a 16% larger concentration of enflurane and a 21% larger concentration of halothane to abolish the withdrawal reflex than do wild-type mice. Using a Pavlovian test, we measured whether this mutation increased the concentration of isoflurane required to impair learning and memory relative to wild-type mice. We found that the concentration was not significantly increased. We also measured MAC (the minimum alveolar concentration required to eliminate movement in response to noxious stimulation in 50% of subjects). Isoflurane MAC for mutant mice (1.93% +/- 0.0.03%; mean +/- se; n = 14) was 17.0% larger than MAC for wild-type mice (1.65 +/- 0.04; n = 14; P < 0.001). Similarly, the cyclopropane MAC for mutant mice (27.6% +/- 0.55%; n = 16) was 13.6% larger than MAC for wild-type mice (24.3 +/- 0.46; n = 8; P < 0.01). The increase in MAC for cyclopropane was unexpected, because published reports find only minimal actions at alpha1beta2gamma2 GABA(A) receptors whereas isoflurane provides a large enhancement. Consistent with previous work on alpha1beta2gamma2 GABA(A) receptors, we found in Xenopus oocytes that 5 MAC cyclopropane enhanced the effect of GABA on alpha1beta2gamma2 GABA(A) receptors by only 76%, and by a nearly identical enhancement in alpha1beta3gamma2, and alpha6beta3gamma2 receptors. In contrast, a much smaller concentration of isoflurane (1 MAC) produced a 160% to 310% enhancement in these receptors. If, relative to isoflurane, cyclopropane minimally increases GABA-induced chloride currents at any GABA(A) receptor subtype, the present data for MAC are consistent with the notion that GABA(A) receptors do not mediate the immobility produced by inhaled anesthetics. IMPLICATIONS: The results of the present study indicate that beta3-containing gamma-aminobutyric acidA receptors do not mediate the amnesia produced by isoflurane and do not mediate, or only partially mediate, the immobility produced by inhaled anesthetics.     

Effects of Volatile Anesthetics on Atrial and AV Nodal Electrophysiological Properties in Guinea Pig Isolated Perfused Heart

Background: Knowledge of the anesthetic effects on atrial and atrioventricular (AV) nodal electrophysiologic properties is fundamental to understand the modulatory role of anesthetics on the pathogenesis of supraventricular tachycardias, and to individualize the perioperative management of patients with supraventricular tachycardias or AV nodal conduction disturbances. Therefore the authors studied the effects of three commonly used volatile anesthetics on the electrophysiologic properties of the atrium and AV node.

Methods: The concentration-dependent electrophysiologic effects of halothane, isoflurane, and desflurane (0 - 2 minimum alveolar concentration [MAC]) were studied in guinea pig Langendorff-perfused hearts fit with instruments to simultaneously measure atrial and AV nodal conduction times and atrial monophasic action potential duration. Atrial and AV nodal effective refractory periods were measured simultaneously using a computer-assisted premature stimulation protocol. The concentrations of anesthetics in the gas phase were monitor by an infrared gas analyzer.

Results: Volatile anesthetics caused markedly different concentration-dependent effects on atrial conduction, repolarization, and refractoriness, and on AV nodal function. At equianesthetic concentrations, halothane depressed atrial conduction the most, whereas desflurane caused the greatest shortening of atrial monophasic action potential duration. Halothane had no significant effect on atrial refractoriness, whereas at 2 MAC desflurane significantly shortened and isoflurane significantly prolonged atrial effective refractory periods by 18.1 +/- 13.5% and 13.2 +/- 14.7%, respectively. On an equi-MAC basis, the rank order of potency for the anesthetics to prolong AV nodal conduction time and AV nodal ERP was halothane > desflurane > isoflurane.     

Minimum alveolar anesthetic concentration of volatile anesthetics in normal and cardiomyopathic hamsters

Minimum alveolar anesthetic concentrations (MAC) values of volatile anesthetics in cardiovascular diseases remain unknown. We determined MAC values of volatile anesthetics in spontaneously breathing normal and cardiomyopathic hamsters exposed to increasing (0.1%-0.3% steps) concentrations of halothane, isoflurane, sevoflurane, or desflurane (n = 30 in each group) using the tail-clamp technique. MAC values and their 95% confidence interval were calculated using logistic regression. In normal hamsters, inspired MAC values were: halothane 1.15% (1.10%-1.20%), isoflurane 1.62% (1.54%-1.69%), sevoflurane 2.31% (2.22%-2.40%), and desflurane 7.48% (7.30%-7.67%). In cardiomyopathic hamsters, they were: halothane 0.89% (0.83%-0.95%), isoflurane 1.39% (1.30%-1.47%), sevoflurane 2.00% (1.85%-2.15%), and desflurane 6.97% (6.77%-7.17%). Thus, MAC values of halothane, isoflurane, sevoflurane, and desflurane were reduced by 23% (P < 0.05), 14% (P < 0.05), 13% (P < 0.05), and 7% (P < 0.05), respectively in cardiomyopathic hamsters. IMPLICATIONS: Minimum alveolar anesthetic concentrations of volatile anesthetics were significantly lower in cardiomyopathic hamsters than in normal hamsters.     

Mouse strain modestly influences minimum alveolar anesthetic concentration and convulsivity of inhaled compounds.

In this study, we measured the minimum alveolar anesthetic concentration (MAC) in several mouse strains, including strains used in the construction of genetically engineered mice. This is important because defined genetic modifications are used increasingly to test mechanisms of inhaled anesthetic action, and background variability in MAC can potentially influence the interpretation of these studies. We investigated the effect of strain on MAC for desflurane, isoflurane, halothane, ethanol, the experimental anesthetic 1-chloro-1,2,2-trifluorocyclobutane, and convulsive 50% effective dose (the dose required to produce convulsions in 50% of animals) of the nonimmobilizer 1,2-dichlorohexafluorocyclobutane. These drugs were studied in eight inbred strains, including both laboratory and wild mouse strains (129/J, 129/SvJ, 129/Ola Hsd, C57BL/6NHsd, C57BL/6J, DBA/2J, Spret/Ei, and Cast/Ei), one hybrid strain (B6129F2/J, derived from the C57BL/6J and 129/J strains), and one outbred strain (CD-1). To test our ability to detect effects in a genetically modified mouse, we compared these data with those for a mouse lacking the gamma (neuronal) isoform of the protein kinase C gene (PKCgamma). We also assessed whether amputating the tail tip of mice (a standard method of obtaining tissue for genetic analysis) increased MAC (e.g., by sensitization of the spinal cord). MAC and convulsant 50% effective dose values differed modestly among strains, with a range of 17% to 39% from the lowest to highest values for MAC using conventional anesthetics, and up to 48% using the experimental anesthetic 1-chloro-1,2,2-trifluorocyclobutane. Convulsivity to the nonimmobilizer varied by 47%. Amputating the tail tip did not affect MAC. PKCgamma knockout mice had significantly higher MAC values than control animals for isoflurane, but not for halothane or desflurane, which implies that protein phosphorylation by PKCgamma can alter sensitivity to isoflurane. IMPLICATIONS: Anesthetic potency differs by modest amounts among inbred, outbred, wild, and laboratory mouse strains. Absence of the neural form of protein kinase C increases minimum alveolar anesthetic concentration for isoflurane, indicating that protein phosphorylation by the gamma-isoform of protein kinase C (PKCgamma) can influence the potency of this anesthetic.     

In Vitro Effects of Desflurane, Sevoflurane, Isoflurane, and Halothane in Isolated Human Right Atria

Background: Direct myocardial effects of volatile anesthetics have been studied in various animal species in vitro. This study evaluated the effects of equianesthetic concentrations of desflurane, sevoflurane, isoflurane, and halothane on contractile parameters of isolated human atria in vitro.

Methods: Human right atrial trabeculae, obtained from patients undergoing coronary bypass surgery, were studied in an oxygenated (95% O2-5% CO2) Tyrode's modified solution ([Ca2+]o = 2.0 mM, 30[degrees]C, stimulation frequency 0.5 Hz). The effects of equianesthetic concentrations (0.5, 1, 1.5, 2, and 2.5 minimum alveolar concentration [MAC]) of desflurane, sevoflurane, isoflurane, and halothane on inotropic and lusitropic parameters of isometric twitches were measured.

Results: Isoflurane, sevoflurane, and desflurane induced a moderate concentration-dependent decrease in active isometric force, which was significantly lower than that induced by halothane. In the presence of adrenoceptor blockade, the desflurane-induced decrease in peak of the positive force derivative and time to peak force became comparable to those induced by isoflurane. Halothane induced a concentration-dependent decrease in time to half-relaxation and a contraction-relaxation coupling parameter significantly greater than those induced by isoflurane, sevoflurane and desflurane.     

The effects of volatile anesthetics on spontaneous contractility of isolated human pregnant uterine muscle: a comparison among sevoflurane, desflurane, isoflurane, and halothane

We examined the effects of equianesthetic concentrations of sevoflurane, desflurane, isoflurane, and halothane on the spontaneous contractility of isolated human pregnant uterine muscles. We also determined if their action was related to potassium channels. Uterine specimens were obtained from normal full-term pregnant women undergoing elective lower-segment cesarean delivery. Longitudinal muscle strips were mounted vertically in tissue chambers. Their isometric tension was recorded while they were exposed to 0.5-3 minimum alveolar concentration (MAC) of volatile anesthetics in the absence and presence of the high conductance calcium-activated potassium channel blocker, tetraethylammonium, or the adenosine triphosphate-sensitive potassium channel (K(ATP))-blocker, glibenclamide. The anesthetics examined produced a dose-dependent depression of contractility. The inhibitory potency of sevoflurane and desflurane was comparable to, whereas that of isoflurane was smaller than, that of halothane: concentrations causing 50% inhibition of the contractile amplitude (ED(50)) were 1.72, 1.44, 2.35, and 1.66 MAC (P < 0.05), respectively. Tetraethylammonium and glibenclamide did not affect the uterine response to the anesthetics, except for glibenclamide, which attenuated the response to isoflurane. These results indicate that the volatile anesthetics have inhibitory effects on the contractility of the human uterus. The inhibitory effect of isoflurane may in part be mediated through activation of K(ATP) channels.     

In vitro effects of desflurane, sevoflurane, isoflurane, and halothane in isolated human right atria

BACKGROUND: Direct myocardial effects of volatile anesthetics have been studied in various animal species in vitro. This study evaluated the effects of equianesthetic concentrations of desflurane, sevoflurane, isoflurane, and halothane on contractile parameters of isolated human atria in vitro. METHODS: Human right atrial trabeculae, obtained from patients undergoing coronary bypass surgery, were studied in an oxygenated (95% O2-5% CO2) Tyrode's modified solution ([Ca2+]o = 2.0 mM, 30 degrees C, stimulation frequency 0.5 Hz). The effects of equianesthetic concentrations (0.5, 1, 1.5, 2, and 2.5 minimum alveolar concentration [MAC]) of desflurane, sevoflurane, isoflurane, and halothane on inotropic and lusitropic parameters of isometric twitches were measured. RESULTS: Isoflurane, sevoflurane, and desflurane induced a moderate concentration-dependent decrease in active isometric force, which was significantly lower than that induced by halothane. In the presence of adrenoceptor blockade, the desflurane-induced decrease in peak of the positive force derivative and time to peak force became comparable to those induced by isoflurane. Halothane induced a concentration-dependent decrease in time to half-relaxation and a contraction-relaxation coupling parameter significantly greater than those induced by isoflurane, sevoflurane and desflurane. CONCLUSIONS: In isolated human atrial myocardium, desflurane, sevoflurane, and isoflurane induced a moderate concentration-dependent negative inotropic effect. The effect of desflurane on time to peak force and peak of the positive force derivative could be related to intramyocardial catecholamine release. At clinically relevant concentrations, desflurane, sevoflurane, and isoflurane did not modify isometric relaxation.     

The actions of sevoflurane and desflurane on the gamma-aminobutyric acid receptor type A: effects of TM2 mutations in the alpha and beta subunits

BACKGROUND: Previous studies have shown that specific amino acid residues in the putative second transmembrane segment (TM2) of the gamma-aminobutyric acid receptor type A (GABAA) receptor play a critical role in the enhancement of GABAA receptor function by halothane, enflurane, and isoflurane. However, very little is known about the actions of sevoflurane and desflurane on recombinant GABAA receptors. The aim of this study was to examine the effects of sevoflurane and desflurane on potentiation of GABA-induced responses in the wild-type GABAA receptor and in receptors mutated in TM2 of the alpha1, alpha 2, or beta 2 subunits. METHODS: GABAA receptor alpha 1 or alpha 2, beta 2 or beta 3, and gamma 2s subunit cDNAs were expressed for pharmacologic study by transfection of human embryonic kidney 293 cells and assayed using the whole cell voltage clamp technique. Concentration-response curves and EC50 values for agonist were determined in the wild-type alpha 1 beta 2 gamma 2s and alpha 2 beta 3 gamma 2s receptors, and in receptors harboring mutations in TM2, such as alpha1(S270W)beta 2 gamma 2s, alpha 1 beta 2(N265W)gamma 2s, and alpha2(S270I)beta 3 gamma 2s. The actions of clinically relevant concentration of volatile anesthetics (isoflurane, sevoflurane, and desflurane) on GABA activated Cl- currents were compared in the wild-type and mutant GABAA receptors. RESULTS: Both sevoflurane and desflurane potentiated submaximal GABA currents in the wild-type GABAA alpha 1 beta 2 gamma 2s receptor and alpha 2 beta 3 gamma 2s receptor. Substitution of Ser270 in TM2 of the alpha subunit by a larger amino acid, tryptophan (W) or isoleucine (I), as in alpha1(S270W)beta 2 gamma 2s and alpha 2(S270I)beta 3 gamma 2s, completely abolished the potentiation of GABA-induced currents by these anesthetic agents. In contrast, mutation of Asn265 in TM2 of the beta subunit to tryptophan (W) did not prevent potentiation of GABA-induced responses. The actions of sevoflurane and desflurane in the wild-type receptor and in mutated receptors were qualitatively and quantitatively similar to those observed for isoflurane. CONCLUSIONS: Positions Ser270 of the GABAA alpha1 and alpha2 subunits, but not Asn265 in the TM2 of the beta2 subunit, are critical for regulation of the GABAA receptor by sevoflurane and desflurane, as well as isoflurane, consistent with the idea that these three volatile anesthetics share a common site of actions on the alpha subunit of the GABAA receptor.     

Pharmacokinetics of desflurane, sevoflurane, isoflurane, and halothane in pigs

We tested the prediction that the alveolar washin and washout, tissue time constants, and pulmonary recovery (volume of agent recovered during washout relative to the volume taken up during washin) of desflurane, sevoflurane, isoflurane, and halothane would be defined primarily by their respective solubilities in blood, by their solubilities in tissues, and by their metabolism. We concurrently administered approximately one-third the MAC of each of these anesthetics to five young female swine and determined (separately) their solubilities in pig blood and tissues. The blood/gas partition coefficient of desflurane (0.35 +/- 0.02) was significantly smaller (P less than 0.01) than that of sevoflurane (0.45 +/- 0.02), isoflurane (0.94 +/- 0.05), and halothane (2.54 +/- 0.21). Tissue/blood partition coefficients of desflurane and halothane were smaller than those for the other two anesthetics (P less than 0.05) for all tissue groups. As predicted from their blood solubilities, the order of washin and washout was desflurane, sevoflurane, isoflurane, and halothane (most to least rapid). As predicted from tissue solubilities, the tissue time constants for desflurane were smaller than those for sevoflurane, isoflurane, and halothane. Recovery (normalized to that of isoflurane) of the volume of anesthetic taken up was significantly greater (P less than 0.05) for desflurane (93% +/- 7% [mean +/- SD]) than for halothane (77% +/- 6%), was not different from that of isoflurane (100%), but was less than that for sevoflurane (111% +/- 17%). The lower value for halothane is consistent with its known metabolism, but the lower (than sevoflurane) value for desflurane is at variance with other presently available data for their respective biodegradations.     

Acetylcholine receptors do not mediate the immobilization produced by inhaled anesthetics

Acetylcholine receptors transmit excitatory impulses, are broadly distributed throughout the central nervous system, and are particularly sensitive to the depressant effects of inhaled anesthetics. Thus these receptors are potential mediators of the immobility produced by inhaled anesthetics. We tested this potential in rats by giving intraperitoneal atropine, scopolamine, and mecamylamine to block muscarinic (atropine and scopolamine) and neuronal nicotinic (mecamylamine) acetylcholine receptors. Block with scopolamine (up to 100 mg/kg), atropine (10 mg/kg), mecamylamine (up to 4 mg/kg), or atropine (10 mg/kg) plus mecamylamine (up to 4 mg/kg) did not significantly decrease the isoflurane concentration required to suppress movement to noxious stimulation (minimum alveolar anesthetic concentration). We also gave atropine intrathecally, finding that the infusions that did not cause permanent paralysis produced slight or no decreases in the minimum alveolar anesthetic concentration. We conclude that acetylcholine receptors do not seem to play a role as mediators of immobilization by inhaled anesthetics. IMPLICATIONS: Inhaled anesthetics produce two crucial effects: amnesia and immobility in the face of noxious stimulation. Block of muscarinic and neuronal nicotinic acetylcholine receptors in rats does not significantly decrease the isoflurane concentration required to suppress movement to stimulation. Thus, acetylcholine receptors do not seem to play a major role as mediators of the immobilization produced by inhaled anesthetics. Their capacity to mediate other effects of inhaled anesthetics (e.g., amnesia) remains to be tested.