Differential modulation of the cardiac adenosine triphosphate-sensitive potassium channel by isoflurane and halothane

BACKGROUND: The cardiac adenosine triphosphate-sensitive potassium (K(ATP)) channel is activated during pathophysiological episodes such as ischemia and hypoxia and may lead to beneficial effects on cardiac function. Studies of volatile anesthetic interactions with the cardiac K(ATP) channel have been limited. The goal of this study was to investigate the ability of volatile anesthetics halothane and isoflurane to modulate the cardiac sarcolemmal K(ATP) channel. METHODS: The K(ATP) channel current (I(KATP)) was monitored using the whole cell configuration of the patch clamp technique from single ventricular cardiac myocytes enzymatically isolated from guinea pig hearts. I(KATP) was elicited by extracellular application of the potassium channel openers 2,4-dinitrophenol or pinacidil. RESULTS: Volatile anesthetics modulated I(KATP) in an anesthetic-dependent manner. Isoflurane facilitated the opening of the K(ATP) channel. Following initial activation of I(KATP) by 2,4-dinitrophenol, isoflurane at 0.5 and 1.3 mm further increased current amplitude by 40.4 +/- 11.1% and 58.4 +/- 20.6%, respectively. Similar results of isoflurane were obtained when pinacidil was used to activate I(KATP). However, isoflurane alone was unable to elicit K(ATP) channel opening. In contrast, halothane inhibited I(KATP) elicited by 2,4-dinitrophenol by 50.6 +/- 5.8% and 72.1 +/- 11.6% at 0.4 and 1.0 mm, respectively. When I(KATP) was activated by pinacidil, halothane had no significant effect on the current. CONCLUSIONS: The cardiac sarcolemmal K(ATP) channel is differentially modulated by volatile anesthetics. Isoflurane can facilitate the further opening of the K(ATP) channel following initial channel activation by 2,4-dinitrophenol or pinacidil. The effect of halothane was dependent on the method of channel activation, inhibiting I(KATP) activated by 2,4-dinitrophenol but not by pinacidil.     

Inhibition of HERG channels by the local anaesthetic articaine

BACKGROUND AND OBJECTIVE: Articaine is an amide local anaesthetic widely used in dentistry. Human ether-a-go-go-related gene (HERG) potassium channels constitute potential targets involved in cardiotoxic side-effects of various pharmacological agents including amide local anaesthetics. The aim of this study was to determine the sensitivity of HERG channels to the inhibitory action of articaine and to further evaluate the effect of the mutations Y652A and F656A in the putative drug-binding region of HERG on the sensitivity for articaine. METHODS: We examined the inhibition of wild-type and mutant HERG channels, transiently expressed in Chinese hamster ovary cells by articaine. Whole cell patch-clamp recordings were performed at room temperature. RESULTS: Inhibition of HERG wild-type and HERG Y652A channels by articaine was concentration dependent and reversible. The concentration-response data were described by Hill functions (wild type: IC50 = 224 +/- 6 micromol L-1, Hill coefficient h = 1.17 +/- 0.03, n = 23; Y652A: IC50 = 360 +/- 48 micromol L-1, h = 0.93 +/- 0.08, n = 26). The mutation Y5652A decreased the sensitivity by factor 1.6. The mutation F656A decreased inhibition of inward tail currents by 300 micromol L-1 articaine in 100 mmol extracellular K+ 3-fold. CONCLUSIONS: Our results indicate that the local anaesthetic articaine does not inhibit HERG channels at clinically relevant concentrations. Articaine may therefore constitute a safer alternative for local and regional anaesthesia. The aromatic amino acid F656 rather than Y652 in the S6 region might play a role in interaction of the drug with the channel.     

Local anesthetic inhibition of baseline potassium channels with two pore domains in tandem

BACKGROUND: Recently, a new structural family of potassium channels characterized by two pore domains in tandem within their primary amino acid sequence was identified. These tandem pore domain potassium channels are not gated by voltage and appear to be involved in the control of baseline membrane conductances. The goal of this study was to identify mechanisms of local anesthetic action on these channels. METHODS: Oocytes of Xenopus laevis were injected with cRNA from five cloned tandem pore domain baseline potassium channels (TASK, TREK-1, TOK1, ORK1, and TWIK-1), and the effects of several local anesthetics on the heterologously expressed channels were assayed using two-electrode voltage-clamp and current-clamp techniques. RESULTS: Bupivacaine (1 mM) inhibited all studied tandem pore potassium channels, with TASK inhibited most potently. The potency of inhibition was directly correlated with the octanol: buffer distribution coefficient of the local anesthetic, with the exception of tetracaine, to which TASK is relatively insensitive. The approximate 50% inhibitory concentrations of TASK were 709 microM mepivacaine, 222 microM lidocaine, 51 microM R(+)-ropivacaine, 53 microM S(-)-ropivacaine, 668 microM tetracaine, 41 microM bupivacaine, and 39 microM etidocaine. Local anesthetics (1 mM) significantly depolarized the resting membrane potential of TASK cRNA-injected oocytes compared with saline-injected control oocytes (tetracaine 22+/-6 mV rs. 7+/-1 mV, respectively, and bupivacaine 31+/-7 mV vs. 6+/-4 mV). CONCLUSIONS: Local anesthetics inhibit tandem pore domain baseline potassium channels, and they could depolarize the resting membrane potential of cells expressing these channels. Whether inhibition of these channels contributes to conduction blockade or to the adverse effects of local anesthetics remains to be determined.     

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.     

Rapid and direct modulation of GABAA receptors by halothane

BACKGROUND: Hypotheses regarding the nature of channel modulation by volatile anesthetics have focused primarily on "membrane actions" of anesthetics and more recently on direct actions of volatile agents on receptor proteins themselves. With the recognition that many channels are subject to modulation by intracellular enzymes, such as protein kinases and phosphatases, and recent demonstrations that the activity of these modulators themselves may be altered by anesthetic agents, a third possibility has been suggested:-anesthetic actions on channels may be indirect, produced, for example, via direct effects on intracellular enzyme systems. METHODS: To determine the contribution of indirect versus direct modulation, the authors compared effects of the volatile anesthetic halothane on gamma-aminobutyric acid A receptors under two conditions: in the whole cell configuration with intact intracellular regulatory systems, and in the excised patch configuration, in which intracellular signaling systems have been disrupted. They also evaluated the effects of rapid application and withdrawal of anesthetic to determine the time course of onset and offset of the anesthetic actions on these channels. RESULTS: Characteristic changes in gamma-aminobutyric acid A receptor function occurred in excised patches as in whole cells, did not require alteration of receptor phosphorylation, and were rapid (onset and offset of anesthetic action occurred within milliseconds). CONCLUSIONS: These results are not consistent with indirect modulation but rather indicate that volatile agents modulate gamma-aminobutyric acid A receptors by direct action on the channel complex or surrounding lipid membrane.     

Reduction in halothane anesthetic requirement by clonidine, an alpha-adrenergic agonist

The effects of clonidine, a potent central alpha-adrenergic agonist, and of tolazoline, an alpha-adrenergic antagonist, on the minimal anesthetic concentration (MAC) of halothane were studied in male mongrel dogs. Control halothane MAC was 0.8 +/- 0.04 vol% (determined in each dog by gas chromatography of arterial blood, n = 30). Clonidine, 5 microgram/kg (n = 10) and 20 microgram/kg (n = 10), give slowly intravenously, maximally reduced MAC by 42% (at 2.3 hours after clonidine) and 48% (at 2.6 hours after clonidine) for each dose. In another set of animals (n = 5) an alpha-adrenergic antagonist, tolazoline, 5 mg/kg IV, reversed the clonidine-induced reduction in halothane MAC rapidly and completely. Tolazoline alone, 5 mg/kg, (n = 5) had no significant effect on halothane MAC. Thus, the administration of the central alpha-adrenergic agonist clonidine decreased the required anesthetic concentration of halothane, as defined by MAC, by almost half. This effect, as it is reversed by tolazoline, is likely to be mediated through a central alpha-adrenergic receptor mechanism.     

Enhancement of anesthetic effect of halothane by spiradoline, a selective kappa-agonist

Reduction in the anesthetic requirement of halothane by narcotics has been studied extensively in humans and animals. Problems of respiratory depression, cardiovascular depression, muscle rigidity, and abuse potential make narcotics less than ideal as supplements to general anesthesia with inhalational agents. Spiradoline, a clinical candidate, is a highly potent and selective kappa-agonist. As such it was considered important to study the effects of spiradoline on the minimum anesthetic concentration (MAC) of halothane required to block responses to noxious stimulation. The results of these experiments in rats showed a dose and plasma concentration-dependent reduction in halothane MAC over a wide range of subcutaneous doses of spiradoline (0.03 to 300 mg/kg). A maximum MAC reduction of 70% was obtained. Plasma levels of spiradoline (6 to 1800 ng/ml) were linearly related to dose. Measurement of blood pressure, heart rate, and PCO2 determined over the course of each experiment showed minor variations which would be acceptable if observed in a clinical setting. It is concluded that spiradoline has promise as an anesthetic supplement.     

Effects of the anesthetic gases xenon, halothane, and isoflurane on calcium and potassium currents in human atrial cardiomyocytes

BACKGROUND: Negative inotropic and proarrhythmic side effects on the heart are well known for the volatile anesthetics halothane and isoflurane but not for the noble gas xenon. We investigated the effects of halothane, isoflurane, and xenon on calcium and potassium currents in human atrial myocytes to elucidate the cellular and molecular basis of their cardiac actions. METHODS: Atrial myocytes were prepared from the right auricles obtained from patients undergoing heart surgery. Ion currents were measured with the whole cell patch clamp technique during superfusion of the cells with solutions that contained halothane, isoflurane, or xenon at concentrations corresponding to their respective minimum alveolar concentration (MAC); gas concentrations were determined with the head space-gas chromatography/mass spectrometry/selected ion monitoring method. RESULTS: L-type calcium currents were significantly depressed by 31.9 +/- 4.1%, from -1.8 +/- 0.3 to -1.2 +/- 0.4 picoampere (pA)/picofarad (pF) (n = 4; P < 0.05) at 1 MAC halothane and by 21.7 +/- 9.2%, from -1.6 +/- 0.7 to -1.2 +/- 0.6 pA/pF (n = 7; P < 0.05) at 1 MAC isoflurane, but not affected by 70% xenon (1 MAC). Inwardly rectifying potassium currents were not influenced by any anesthetic. Halothane (1 MAC) significantly inhibited the transient as well as the sustained part of voltage-gated potassium outward currents, by 19.4 +/- 6.7%, from 6.7 +/- 2.1 to 5.4 +/- 1.6 pA/pF (n = 8; P < 0.05), and by 8.6 +/- 4.8%, from 5.5 +/- 1.7 to 5.0 +/- 1.5 pA/pF (n = 8; P < 0.05), respectively. Transient K+ outward currents were even more inhibited, by 25.8 +/- 4.8%, from 9.8 +/- 3.1 to 7.3 +/- 2.1 pA/pF (n = 5; P < 0.05) at 1 MAC isoflurane, whereas xenon evoked only a slight (albeit significant) inhibition, by 6.1 +/- 3.7%, from 8.2 +/- 6.0 to 7.7 +/- 5.8 pA/pF (n = 10; P < 0.05). Isoflurane and xenon did not affect sustained potassium currents. All effects of the anesthetics were fully reversible after washout. CONCLUSIONS: Halothane and isoflurane exhibited considerable inhibitory effects on voltage-gated cardiac Ca2+ and K+ currents important for the duration of action potentials and the repolarization. Xenon, in contrast, did not affect Ca2+ currents and only slightly inhibited transient K+ outward currents, in line with the almost absent cardiac side effects of the noble gas.     

The effect of levodopa on halothane anesthetic requirements.

Acute and chronic levodopa (L-dopa) administration alters halothane minimum alveolar concentration (MAC) in dogs. L-dopa 5, 10, and 25 mg./kg. intravenously (I.V.) reduced halothane MAC 15, 20, and 25 percent, respectively, 1 hour after injection, with return toward normal after 3 hours. Fifty mg./kg. increased halothane MAC 40 percent at 1 hour, but produced at 30 percent decrease from control MAC at 3 hours. Two groups of 4 dogs each given oral L-dopa in doses increasing from 50 mg./kg. daily the 1st week to 300 mg./kg. daily the 5th week, decreased MAC an insignificant 9.5 plus or minus 25.2 percent at 4 hours after the last dose and increased MAC 3.5 plus or minus 7.3 percent at 12 hours after the last dose. A small, insignificant decrease of 9 percent in MAC was measured in 4 dogs given I.V. L-dopa 25 mg./kg. q.i.d for 4 days.     

Comparison of halothane and isoflurane for rapid anesthetic induction

To study the hypothesis that isoflurane will induce anesthesia faster than halothane when given by a single vital capacity breath technique, we studied 20 ASA I and II adults who breathed approximately 4.5 MAC equivalents of either vapor. The patients, randomly assigned to receive either agent, were fully preoxygenated and monitored for cardiovascular, respiratory, and EEG parameters. All subjects were premedicated with 5 micrograms/kg fentanyl IV 5 min before induction. Time to loss of consciousness was significantly longer with halothane than with isoflurane (86 +/- 4 vs 38 +/- 2 sec, respectively) although there were no clinically remarkable differences in cardiovascular or respiratory variables. Patients given halothane had a greater excitatory phase on EEG, whereas those given isoflurane had low frequency predominance. Overall rapid inhalation induction was well-received by all patients and was significantly faster with isoflurane.     

Mutagenicity studies with volatile metabolites of halothane.

The mutagenicities of two volatile metabolites of halothane, 2-chloro-1,1,1-trifluoroethane (CF3CH2Cl) and 2-chloro-1,1-difluoro-ethylene (CF2CHCl), and a presumed halothane metabolite, 2-bromo-2-chloro-1,1-difluoroethylene (CF2CBrCl), were investigated in the bacterial assay system developed by Ames and co-workers. Both gas-phase and liquid culture tests were made. In addition, mutagenicity of CF2CBrCl was studied in a modified Ames test system using rapidly growing cells in enriched liquid medium. The purity of tested compounds was verified by combined gas chromatography and mass spectrometry. In the standard Ames test, CF3CH2Cl and CF2CBrCl were not mutagenic, and CF2CHCl was only weakly mutagenic. CF2CBrCl was detectable as weakly mutagenic, however, in the modified Ames test using rapidly growing cells. Although these results are reassuring, the effects of long-term exposure to halogenated anesthetics are still not fully known.     

Local anesthetic interaction with human ether-a-go-go-related gene (HERG) channels: role of aromatic amino acids Y652 and F656

BACKGROUND: Human ether-a-go-go-related gene (HERG) potassium channels constitute a potential target involved in cardiotoxic side effects of amino-amide local anesthetics. The molecular interaction site of these low-affinity blockers with HERG channels is currently unknown. The aim of this study was to determine the effect of the mutations Y652A and F656A in the putative drug binding region of HERG on the inhibition by bupivacaine, ropivacaine, and mepivacaine. METHODS: The authors examined the inhibition of wild-type and mutant HERG channels, transiently expressed in Chinese hamster ovary cells by bupivacaine, ropivacaine, and mepivacaine. Whole cell patch clamp recordings were performed at room temperature. RESULTS: Inhibition of HERG wild-type and mutant channels by the different local anesthetics was concentration dependent, stereoselective, and reversible. The sensitivity decreased in the order bupivacaine > ropivacaine > mepivacaine for wild-type and mutant channels. The mutant channels were approximately 4-30 times less sensitive to the inhibitory action of the different local anesthetics than the wild-type channel. The concentration-response data were described by Hill functions (bupivacaine: wild-type IC50 = 22 +/- 2 microm, n = 38; Y652A IC50 = 95 +/- 5 microm, n = 31). The mutations resulted in a change of the stereoselectivity of HERG channel block by ropivacaine. The potency of the local anesthetics to inhibit wild-type and mutant channels correlated with the lipophilicity of the drug (r > 0.9). CONCLUSIONS: These results indicate that local anesthetics specifically but not exclusively interact with the aromatic residues Y652 and F656 in S6 of HERG channels.     

吸入麻醉药预处理对心肌缺血“第二保护窗”的作用机制

吸入麻醉药预处理对心肌缺血侑罐注损伤具有急性期和“第二保护窗”两个时间段的保护作用。“第二保护窗”起效缓慢而持久,临床上有充分的时间在手术前给予,能更方便有效地预防围术期心肌缺血的并发症。它可诱导一些触发因子如腺苷、一氧化氮的产生,通过信号转导通路蛋白激酶C、核因子-κB等,作用于ATP敏感性钾通道及活性氧族等终末效应离子通道或保护蛋白而发挥迟发性心肌保护。现就近年来关于吸入麻醉药预处理对心肌“第二保护窗”的作用机制作一综述。     

Circulatory effects of volatile anesthetic agents.

Modern general anesthesia includes the use of multimodal techniques, mixing volatile or intravenous agents with opioids and/or regional anesthesia. Therefore, most circulatory side effects of inhaled agents are seldom observed. They all produce dose-dependent decrease of systolic and diastolic function and depress baroreflexes to a varying degree. Desflurane, and to a much lesser degree, isoflurane, may cause sympathetic nervous system activation when inhaled concentration is rapidly increased to above 1.5 MAC. This appears to be the result of central activation rather than airway irritation. The older volatile anesthetics halothane and enflurane have no or minimal effect on coronary vascular tone. In contrast, the more recent agents all limit coronary vasodilator reserve by direct effect upon coronary resistance vessels, sevoflurane less than isoflurane and desflurane at equipotent anesthetic dose. Although isoflurane and desflurane appear to produce myocardial ischemia by non-hemodynamically related mechanisms more frequently than halothane or fentanyl in patients at risk, no particular agent has been identified to carry greater risk for adverse cardiac outcome than others.