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.
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.
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.
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 commonly used volatile anesthetics halothane and isoflurane may exert negative inotropic and proarrhythmic side effects on the heart. Negative inotropic effects have also been demonstrated in vitro in humans 1and in various animal models. 2–4The noble gas xenon has recently been introduced in clinical trials in several countries. It offers the principal advantage of appearing to have virtually no relevant cardiac side effects. 5,6Possible limitations beyond the high costs and limited availability include an unfavorably high minimum alveolar concentration (MAC) required for narcosis. The MAC of xenon has been agreed to be 71%, 7although very recently this figure has been in doubt, and a MAC of 63% has been suggested. 8
To understand the cellular and molecular basis of cardiac side effects of halothane and isoflurane, electrophysiologic studies on cardiomyocytes have been performed and revealed that both volatile anesthetics depress calcium currents through L-type calcium channels in the animal myocardium. 9,10Along with effects on the Ca2+sensitivity of the contractile proteins and on cytosolic Ca2+transients, the inhibition of the transsarcolemmal Ca2+influx can explain the negative inotropic effect of the volatile anesthetics. 11,12In addition, inhibition of Ca2+channels is in line with the observation that halothane shortens the duration of action potentials, 13because this duration crucially depends on L-type Ca2+channel activity. 14It is noteworthy that xenon did not exhibit any measurable inhibition of Ca2+currents in ventricular myocytes from the guinea pig heart. 15
Furthermore, halothane and isoflurane (but not xenon) have been investigated in animal studies for their impact on some voltage-gated potassium currents and were found to depress them. 16,17Because these currents are responsible for the repolarization, any interference with them may have consequences on the timing of the refractory period and on the electrical stability of the myocardium.
In experiments on effects of gases on ion currents, loss of partial pressure by diffusion presents a major technical problem. In particular, the typical set-up of the patch clamp technique predisposes to severe underestimation of actual gas concentrations. The problem may be even more important in studies with xenon because the high partial pressure required to reach relevant anesthetic concentrations and the low molecular weight of the noble gas facilitate diffusion.
Therefore, the aim of the current patch clamp study was to establish experimental conditions in which the gas concentrations at the cells are well defined and to analyze the effects on Ca2+and K+currents that the three anesthetics halothane, isoflurane, and xenon exert on human atrial cardiomyocytes. Human cells were chosen since it has previously been demonstrated that there are subtle but distinct species differences with regard to the regulation and modulation of ion channels in the heart. 18–20Atrial cells were used because specimens from patients without atrial dysfunction could be obtained during cardiac surgery performed with the aid of cardiopulmonary bypass.
Isolation of Single Atrial Myocytes
Right atrial appendages were obtained as surgical specimens from patients (age range, 48–79 yr; mean, 61.2 yr; n = 20) undergoing heart surgery because of coronary artery insufficiency (n = 19) or aortic valve disease (n = 1). All patients were in sinus rhythm and had no evidence of right atrial dysfunction. The cardiovascular medication that most patients received (β-adrenergic antagonists, diuretics, angiotensin-converting enzyme inhibitors, or nitric oxide donors) was stopped at least 12 h before surgery. The investigations were performed in accordance with the principles outlined in the Declaration of Helsinki and approved by the local ethical board. All patients gave informed consent prior to surgery.
Atrial myocytes were prepared, as described previously in detail, 21by digestion with collagenase (type V; Sigma, St. Louis, MO), followed by gradual recalcification. Only well-striated, bleb-free, rod-shaped myocytes were used for the electrophysiologic studies, done within 4 h after the isolation.
Patch Clamp Experiments
Membrane currents were recorded by means of the whole cell patch clamp technique 22with use of an amplifier (EPC-9; HEKA, Lambrecht, Germany) and a personal computer equipped with Pulse 8.0 software (HEKA) for data acquisition and analysis.
The patch pipettes were pulled from borosilicate glass (Hilgenberg, Malsfeld, Germany) with a vertical puller and heat-polished with a microforge. Pipette resistances ranged from 3.0 to 7.0 MΩ when they were filled with pipette solution. The EPC-9 provides automatic procedures for both fast and slow capacitance subtraction. Cell capacitance was determined before any series of depolarizing or hyperpolarizing pulses. The capacitance did not noticeably change during any experiment. Series resistance compensation with supercharging was not performed.
Cardiac myocytes were allowed to adhere to a glass coverslip, which was transferred into a bath chamber. The bath chamber was filled with bath solution to a height of approximately 2 mm (total volume, approximately 400 μl; surface area, approximately 200 mm2). In some experiments, the bath solution was covered with a thin paraffin oil layer (DAB10; Wasserfuhr, Bonn, Germany) saturated with xenon. The chamber was continuously perfused at a rate of 3 to 4 ml/min. The inflow occurred through high-performance-liquid-chromatography-grade steel capillaries.
After obtaining the whole cell configuration, an equilibrium period of 5 min was allowed, during which current amplitudes and access resistance stabilized. Afterward, rundown of Ca2+and K+currents was negligible (less than 5%) over the time course of the experiments (less than 15 min). For measurements of L-type Ca2+currents (ICa,L), a resting membrane potential of −60 mV was chosen to minimize Na+currents; K+currents were blocked with intracellular Cs+. ICa,Lwas elicited with depolarizing pulses for 400 ms. Ca2+currents are given here as peak current amplitude of the respective depolarizing step. For measurements of K+currents, Ca2+currents were blocked with 200 μm Cd2+and extracellular Na+was replaced by N -methyl-d-glucamine. Inward rectifying K+currents (IK,ir) were measured during stepwise hyperpolarizing pulses 50 ms in duration, from a holding potential of −60 mV to test potentials ranging from −80 to −130 mV. Outward K+currents were measured during stepwise depolarizing pulses 300 ms in duration, from a holding potential of −90 mV to test potentials ranging from −60 to +60 mV. All experiments were performed at room temperature (21°C).
Preparation of Solutions and Gases
For measurements of ICa,L, the bath solution contained tetraethylammonium chloride, 136 mm; CaCl2, 1.8 mm; MgCl2, 1.8 mm; glucose, 10 mm; and HEPES, 10 mm; this was pH-adjusted to 7.40 with tetraethylammonium hydroxide. The pipette solution contained CsCl, 140 mm; MgCl2, 2 mm; adenosine 5′-triphosphate, 0.3 mm; guanosine 5′-triphosphate, 0.3 mm; ethylene glycol-bis(β-aminoethylether)-N,N,N′,N′-tetraacetic acid, 10 mm; HEPES, 10 mm. This was pH-adjusted to 7.20 with CsOH.
For measurements of K+currents, the bath solution contained N -methyl-d-glucamine, 132 mm; CaCl2, 1 mm; MgCl2, 2 mm; KCl, 5 mm; CdCl2, 0.2 mm; glucose, 10 mm; and HEPES, 10 mm. This was pH-adjusted to 7.40 with HCl. The pipette solution contained K-aspartate, 110 mm; KCl, 20 mm; MgCl2, 1 mm; Na2-phosphocreatine, 5 mm; adenosine 5′-triphosphate, 5 mm; guanosine 5′-triphosphate, 0.1 mm; ethylene glycol-bis(β-aminoethylether)-N,N,N′,N′-tetraacetic acid, 10 mm; and HEPES, 10 mm. This was pH-adjusted to 7.20 with KOH.
Bath solutions containing xenon or volatile anesthetics were prepared by passing an appropriate gas mixture through the solutions at a flow of 30–200 ml/min in a 50-ml glass flask equipped with a glass frit and a membranous septum. Gas left the flask through a valve. After at least 30 min of gassing, a fraction (approximately 5–8 ml) of the solutions was taken through the septum into gas-tight glass syringes (1010 TLL; Hamilton, Bonaduz, Switzerland) and immediately perfused through the bath chamber. Each solution was presented to the cells for at least 60–120 s until current amplitudes remained stable.
For the preparation of solutions containing halothane or isoflurane, the gas composition used for gassing was set with a calibrated vaporizer (Fluotec3 and Isotec3; Ohmeda, Steeton, UK). The respective gas concentrations were as follows: halothane, 0.75% (vol/vol; corresponding to 1 MAC) or 1.5% (2 MAC); and isoflurane, 1.2% (1 MAC). The O2concentration was 35%; the remainder was N2. An anesthetic gas analyzer (AGM II; Heyer, Bad Ems, Germany) was used to continuously monitor the concentrations of halothane (Rüsch, Böblingen, Germany) and of isoflurane (Abbott, Wiesbaden, Germany) in the gas phase.
During gassing with xenon, the gas composition was set with a mass flowmeter–controller (E-7000; Bronkhorst HI-TEC, AK Ruurlo, Netherlands) as a mixture of xenon (80% or 95%) and O2. Xenon was provided by Messer-Griesheim (Krefeld, Germany).
Analysis of Gas Concentrations in the Bath Solution
Samples of 50 μl were taken from the perfused bath chamber with a gas-tight glass syringe (1710 RN, Hamilton) equipped with an integrated steel needle. Care was taken to obtain the sample from a spot where cells would have been in an experiment. Each sample was injected into gas-tight vials (1 ml; type N 11-1 [Macherey-Nagel, Düren, Germany]). The gas concentrations were determined with head space–gas chromatography/mass spectrometry/selected ion monitoring (GC/MS/SIM; SSQ 7000; Finnegan, Bremen, Germany), as described by Maruyama et al. 23As standards, samples were taken from the glass flask after gassing of the bath solution with various concentrations of halothane, isoflurane, or xenon. The precision of the determination of all concentrations was better than 5% of the mean. Measurements of O2concentrations of solutions were performed with a conventional blood gas analyzer (ABL 505; Radiometer, Copenhagen, Denmark).
Data Analysis and Statistics
Current densities were calculated by dividing current amplitudes by the whole cell capacitance. Values are expressed as mean ± SD. Curve-fitting and statistical comparisons were made with commercially available software (Prism 2, San Diego, CA). Paired Student t tests (two-tailed) were used to determine the statistical significance of differences between raw data obtained before and after the application of a given anesthetic. Differences with P < 0.05 were considered significant.
Concentrations of Xenon, Halothane, and Isoflurane in the Bath Solution
When the bath solution was gassed with 80% xenon, the xenon concentration in the bath chamber during perfusion was 60.9 ± 7.8% (n = 6) when the bath solution was not covered with paraffin oil. In the presence of oil, the xenon concentration was 68.6 ± 4.5% (n = 4). Thus, a loss of xenon occurred during perfusion that amounted to approximately 19% in the absence and to 11% in the presence of paraffin oil. Accordingly, the patch clamp experiments were performed either with bath solutions gassed with 80% xenon and covered with oil or with bath solutions gassed with 95% xenon, to make sure that one MAC of xenon was presented to the cells.
In preliminary experiments, we noticed the importance of gas-tight syringes and steel capillaries. With a plastic syringe instead of a gas-tight glass syringe, a reduction in the xenon concentration occurred by 21%, even when steel capillaries and paraffin oil were used. For comparison, we determined the loss of oxygen from the bath solution gassed with 100% O2. No loss of O2was detected through plastic syringes during the 5-min period. A loss of O2by approximately 5% occurred through plastic perfusion tubes, which consequently were not used for any experiments. A further loss of 5% through the surface of the bath could be prevented by covering the bath solution with paraffin oil.
The mean concentrations of halothane in the bath solution at room temperature (0.75% and 1.5% in the gas phase) were 0.39 mm and 0.85 mm, respectively, whereas the mean concentration of isoflurane in the solution (1.2% in the gas phase) was 0.51 mm. No measurable loss of halothane and of isoflurane was demonstrated under the standard experimental conditions (gas-tight glass syringes, steel capillaries, no paraffin oil). Changes in the concentration during passage from the syringe to the bath chamber were in the range of precision of the analysis method.
Effects of Anesthetics on L-type Ca2+Currents
Representative tracings of ICa,Lin human atrial myocytes are shown in figures 1A and B. Exposure to xenon (in the range of 1 MAC) left the ICa,Lvirtually unchanged with respect to the peak amplitude as well as to the time course of the decay of ICa,Lafter the peak (fig. 1A). Current densities of ICa,Lat various membrane potentials, before and during exposure to xenon, are shown in figure 2A. The maximal activation was observed between 10 and 20 mV. During depolarizing steps to +20 mV, the peak ICa,Lamplitude in the presence of xenon was 97.5 ± 4.8% (range, 92–107%; n = 7; 4 patients), as related to the peak ICa,Lamplitude of −2.5 ± 0.7 picoamperes (pA)/picofarad (pF) obtained immediately before xenon exposure. The values after washout of xenon were 99.2 ± 2.3% (n = 7) of those prior to xenon exposure. Even when xenon was applied at a higher concentration (approximately 80%; solution gassed with 95% xenon; bath covered with oil; n = 4), no effect on ICa,Lwas noticed (data not shown). For comparison, a solution gassed with 95% N2and 5% O2did not change ICa,Lto a noticeable amount either (data not shown).
In contrast to xenon, halothane produced a concentration-dependent depression of ICa,L. A representative tracing for the effect of halothane is shown in figure 1B. Halothane (1.5%, corresponding to 2 MAC) depressed peak ICa,Lamplitudes at +20 mV to 53.9 ± 8.0% (P < 0.05; n = 10; 4 patients), from −2.3 ± 0.6 pA/pF to −1.2 ± 0.3 pA/pF (fig. 2B). When a halothane concentration of 0.75% (1 MAC) was used, peak ICa,Lamplitude was reduced to 68.1 ± 4.1% (P < 0.05; n = 4; 3 patients) of control. After washout, a rapid and complete recovery of peak ICa,Lto 95.9 ± 7.0% (n = 14) of control was observed.
Similar to halothane, isoflurane (1.2%, corresponding to 1 MAC) induced a depression of the peak ICa,Lamplitude to 78.3 ± 9.2% (P < 0.05; n = 7; 2 patients), from −1.6 ± 0.7 pA/pF to −1.2 ± 0.6 pA/pF. This effect was reversible; washout of isoflurane restored peak ICa,Lto 96.8 ± 6.3% (n = 7) of the current values before the application of isoflurane.
Effects of Anesthetics on K+Currents
Three subtypes of K+currents in human atrial cardiomyocytes were discriminated: inward rectifying K+currents (IK,ir), measured as inward currents during hyperpolarization, and the two voltage-gated K+outward currents of the transient (IK,to) and sustained (IK,sus) type.
A representative tracing of IK,iris shown in figure 3A. The apparent reversal potential of the currents (approximately −65 mV) is different from the value expected from the NERNST equation, indicating some voltage error, such as a liquid junction potential, not corrected in the current experiments. IK,irremained constant within 7% over the time of one experiment in all cells studied (n = 19). No significant effect on IK,irwas demonstrated for any of the three anesthetics tested. In the presence of xenon, IK,ir(as measured during hyperpolarizing pulses to −130 mV) were reduced to 97.5 ± 2.8% (n = 5; 3 patients), from −4.0 ± 2.2 pA/pF to −3.9 ± 2.2 pA/pF (fig. 3B). The respective values for halothane 0.75% were 102.9 ± 4.4% (n = 4; 2 patients); for halothane 1.5%, they were 98.8 ± 3.8% (n = 4; 4 patients); and for isoflurane 1.2%, they were 96.4 ± 4.3% (n = 6; 3 patients) of control.
Voltage-gated K+outward currents exhibited a remarkable variability between different cell populations, with respect to the current amplitude and the relative contribution of IK,toand IK,sus. The sustained component was measured at the end of depolarizing pulses 300 ms in duration. It was present in all cells studied at membrane potentials of −10 mV and greater (fig. 4A). The current density at +60 mV (6.1 ± 2.3 pA/pF; n = 28; 8 patients) ranged from 2.8 to 12.0 pA/pF. In any one cell over the time of one experiment, however, IK,suswere sufficiently stable to test effects of anesthetics. No attempt was made to discriminate further subtypes of the sustained K+outward current.
The transient component IK,tothat produced a peak outward current within the first 10 ms after depolarization (fig. 4A) was quantified as the difference between this peak and the current sustained after 300 ms. The current densities as measured at +60 mV (7.8 ± 4.1 pA/pF; n = 27; 8 patients) ranged from 2.8 to 17.6 pA/pF. Again, IK,towas stable in individual cells over the duration of one experiment.
Effects of anesthetics on voltage-gated K+outward currents are shown in figures 4B and Cand figure 5. In the presence of xenon (figs. 4B and 5), IK,towas suppressed by 6.1 ± 3.7% (n = 10; 4 patients), from 8.2 ± 6.0 pA/pF to 7.7 ± 5.8 pA/pF. Even though this effect was small, it was consistently observed and fully reversible (IK,toafter washout of xenon: 100.4 ± 8.8%; n = 10). Therefore, the xenon-induced inhibition of IK,towas highly significant (P < 0.001). In contrast to IK,to, IK,suswas virtually unaffected by xenon. Halothane (0.75%) induced a reversible depression of IK,toas well as of IK,sus(figs. 4C and 5). Stronger effects on both currents (fig. 5) were evoked by a higher halothane concentration (1.5%). Isoflurane (1.2%) preferentially depressed the IK,to, whereas no significant effect was demonstrated on IK,sus(fig. 5).
In the current study, we analyzed the effects of the three anesthetic gases halothane, isoflurane, and xenon on Ca2+and K+currents in human myocardial cells prepared from the atria of patients without evidence of atrial dysfunction. Inhibitions of Ca2+as well as of K+currents were demonstrated for halothane and for isoflurane, whereas xenon depressed only K+currents. Anesthesia may be understood, according to one of several currently discussed hypotheses, as action on specific ion channels. 24,25It has been suggested that inhibition of the N -methyl-d-aspartate (NMDA) receptor is the main effect of the anesthetic drug ketamine 24; an inhibition of the same channel has been reported for xenon and perhaps may explain its anesthetic properties. 26For other anesthetics and sedatives such as the barbiturates and benzodiazepines, interactions with γ-aminobutyric acid type A (GABAA) receptor are considered essential. 24If interference with one particular ion channel is the principal action of any given anesthetic drug, it is not surprising when it affects other ion channels as well. The spectrum of side effects is then dependent, to a large part, on the type of channels involved, the physiologic role of these channels, and the potency of the drug to interfere with these channels. In any case, actions on myocardial channels may be the basis for negative inotropic and proarrhythmic effects on the heart. In this respect, the present study suggests that xenon may be superior to halothane and isoflurane since only xenon left L-type Ca2+currents unaltered and evoked the least inhibition of voltage-gated K+outward currents. Thus, clinical observations that favor xenon as the anesthetic drug with the least cardiac side effects are supported, at a cellular and molecular level, by the results of the current study.
To the best of our knowledge, this is the first study in which human myocardial cells have been used to determine whether anesthetic gases alter ion currents. Moreover, it is the first one to demonstrate that isoflurane and xenon modify IK,toin the heart; at the same time, it provides further evidence of an action of halothane similar to that recently been found in a study on rat ventricular myocytes. 27IK,tois characteristic for cells prepared from mammalian myocardial tissues. 28,29In the human heart, it has been found in the atria and in the ventricles, whereas IK,susis less prominent in the ventricles. 30IK,toconsists of two components, 31which were not discriminated in the current study. Moreover, Cd2+used to block contaminating Ca2+currents is known to shift the steady-state activation and inactivation curves of IK,to. 32Therefore, our study does not assess IK,toin its full complexity but demonstrates a significant albeit slight inhibition of its amplitude by xenon. The inhibition was so small that IK,tovaried between different cells to a much higher degree than the amount by which it was depressed by xenon in any one individual cell. The variability of voltage-gated K+currents in the human heart has been consistently high in many studies. 30,33–35Obviously, there are regional differences within the heart, 30but even within cells of the same regional origin, 34,36the variability was comparably high, as in our cells from the auricles. In spite of the variability of IK,to, the effects of xenon could be resolved because they occurred consistently and were fully reversible.
A markedly stronger inhibition of IK,towas observed during exposure to halothane and isoflurane. At equipotent anesthetic concentrations, corresponding to 1 MAC of either gas, isoflurane evoked a more pronounced effect on IK,tothan halothane. This order of relative potencies of the two volatile anesthetics was turned around when IK,susand ICa,Lwere considered. Halothane was the only drug that depressed IK,susand evoked the largest inhibitions of ICa,L.
The inhibitory effects on both K+and Ca2+currents are likely to be of pathophysiologic relevance. Slowly inactivating K+channels that together produce IK,susare important for the repolarization and the timing and duration of the refractory period. 37An even more important role is attributed to the L-type Ca2+channels. Diminished expression of this channel type is not only a hallmark of the ventricular myocardium in several different states of insufficiency 38but also a characteristic finding of the atrial myocardium in patients with persistent atrial fibrillation. 39The reduced contractile force and the electrical instability of persistent atrial fibrillation are accounted for by the reduced number of L-type Ca2+channels. 40A comparison of L-type Ca2+channels between the atrium and the ventricle reveals that ventricular cells express no other α1 subunit mRNA than α1C, whereas α1C as well as a small portion of α1D is found in atrial cells. 41However, as the α1C subunit mRNA was approximately 40-fold more abundant, there is no reason to expect different effects of anesthetics on Ca2+currents in ventricles than in atria. The situation may be more complex in the case of voltage-gated K+channels, because an exact assignment of currents to particular gene products is not possible. Given the considerable regional heterogeneity of members of the K v gene family within the heart, 42especially with respect to differences between atria and ventricles, 43the observations of the effects of the anesthetic gases in atrial cells should not be extrapolated without caution to other cardiac regions.
Because we studied human cells, it is of interest to relate our results to those obtained in animal studies. In our investigation, halothane inhibited ICa,Lby 32% and 46% at concentrations corresponding to 1 and 2 MAC. In various studies of ICa,Lin guinea pig 10,15,16,44and canine 9,45ventricular myocytes, halothane at approximately 2 MAC inhibited by 40–70%. The strongest inhibition was observed when the experiments were performed at 37°C instead of at room temperature;44a similarly strong inhibition was observed in atrial myocytes at 37°C by 2.6 MAC halothane. 16Thus, the susceptibility to halothane does not seem to vary markedly between the investigated species. The situation may be different with isoflurane, as far as can be deduced from the few published studies. Isoflurane at 1 MAC inhibited ICa,Lby only 10% in guinea pig cardiomyocytes 10but by approximately 30% in canine cardiomyocytes. 9,45Incidentally, our own experiments reveal an inhibition just in between those two extremes. Moreover, when IK,susare considered, the inhibition evoked by halothane (2 MAC) on IK,susin human atrial cells (21%) appears moderate in relation to the reported 54%16or 57%13in the guinea pig at 2.6 MAC.
A further point with regard to which our study of human cells has produced results contrasting with those previously reported for cells from other mammals is the apparent lack of any inhibition of IK,irby all three anesthetic gases. Although the same finding has been reported with regard to xenon in guinea pig cardiomyocytes, 15there was significant inhibition by halothane as well as by isoflurane in another study of guinea pig cardiomyocytes. 46However, it is not certain whether this reflects true species differences or whether the differences may be related to other experimental conditions, because another study of guinea pig cardiomyocytes did not show any alteration of IK,irin the presence of halothane. 16Inwardly rectifying K+channels are not voltage-gated, and their major role is in controlling the resting potential. Therefore, our results suggest that the electrophysiologic influence of anesthetic gases on the human myocardium preferentially bears impact on the action potential, particularly its duration and repolarization.
A limitation of our study is the fact that we performed all experiments at room temperature. Although ion currents in general tend to be much larger at 37°C than at 21°C, it would be interesting to know whether the extent of inhibitory effects exerted by anesthetic gases might be temperature-dependent. To the best of our knowledge, no study has compared the effects of volatile anesthetics on cardiac ion currents in dependence of the temperature. In our laboratory, exposure of cells to solutions with defined concentrations of gases, especially xenon, could be performed only at room temperature. On the other hand, application of xenon and of the two volatile anesthetics at defined concentrations was a major aim of our study. We consider this particularly important with regard to xenon, because few or no effects were observed, and the high MAC value precludes the determination of the concentration–effect relation over a large range. The technical problems that arose in preliminary experiments came as a surprise. For example, we would not have expected that a sizeable loss of gases occurs through plastic tubes. Even with all the precautions that we took for the final series of experiments, we could not completely prevent loss of xenon. Therefore, we had to resort to gassing with mixtures with a higher xenon percentage than was finally provided to the cells. In contrast, no such loss was observed for halothane, isoflurane, or oxygen. Therefore, we believe that xenon concentrations should be rigorously measured and controlled in all further in vitro studies.
In conclusion, our study demonstrates inhibition of voltage-gated ion currents in human atrial myocytes by the three anesthetic gases halothane, isoflurane, and xenon. Each gas had its individual profile of effects. The results contribute to our understanding of how the three anesthetics exert their cardiac effects at the cellular and molecular level.
The authors thank Wolfgang Bettray, Ph.D. (Assistant Professor, Department of Chemistry, Rheinisch-Westfällische Technische Hochschule [RWTH] Aachen, Aachen, Germany), and Manfred Erkens, M.D. (Senior Fellow, Institute of Forensic Medicine, RWTH Aachen), for their analysis of gas concentrations with GC/MS/SIM. The authors acknowledge the excellent cooperation of the surgeons and the staff of the Department of Cardiothoracic Surgery, University Hospital RWTH Aachen. Part of the xenon supply was a kind gift of Messer-Griesheim (Krefeld, Germany).