Comparative Effects of Halogenated Inhaled Anesthetics on Voltage-gated Na⁺Channel Function

Chinese hamster ovary cells stably transfected with rat Na^v1.4 α-subunit (a gift from S. Rock Levinson, Ph.D., Professor, Department of Physiology and Biophysics, University of Colorado Health Sciences Center, Denver, Colorado) were cultured in 90% (v/v) Dulbecco’s Modified Eagle Medium, 10% (v/v) fetal bovine serum, 300 μg/ml G418 (Invitrogen, Carlsbad, CA), 100 units/ml penicillin, and 100 μg/ml streptomycin (Biosource, Rockville, MD) under 95% air/5% CO²at 37°C. Cells were plated on glass coverslips in 35-mm plastic dishes (Becton Dickinson, Franklin Lakes, NJ) 1–3 days before electrophysiological recording.

Cells attached to coverslips were transferred to a plastic Petri dish (35 × 10 mm) on the stage of a Nikon ECLIPSE TE300 inverted microscope (Melville, NY). The culture medium was replaced, and cells were superfused at 1.5–2 ml/min with extracellular solution containing (in mm): NaCl 140; KCl 4; CaCl²1.5; MgCl²1.5; HEPES 10; d-glucose 5; pH 7.30 with NaOH. Studies were conducted at room temperature (24 ± 1°C) using conventional whole cell patch-clamp techniques.18Patch electrodes (tip diameter < 1 μm) were made from borosilicate glass capillaries (Drummond Scientific, Broomall, PA) using a micropipette puller (P-97; Sutter Instruments, Novato, CA) and fire polished (Narishige Microforge, Kyoto, Japan). Electrode tips were coated with SYLGARD (Dow Corning Corporation, Midland, MI) to lower background noise and reduce capacitance; electrode resistance was 2–5 MΩ. The pipette electrode solution contained (in mM): CsF 80; CsCl 40; NaCl 15; HEPES 10; EGTA 10; pH 7.35 with CsOH. Currents were sampled at 10 kHz and filtered at 1–3 kHz using an Axon 200B amplifier, digitized via  a Digidata 1321A interface, and analyzed using pClamp 8.2 (Axon/Molecular Devices, Sunnyvale, CA). Capacitance and 60–85% series resistance were compensated, and leak current was subtracted using P/4 or P/5 protocols. Cells were held at –80 mV between recordings. Only cells with Na⁺currents of 0.5–3.5 nA were analyzed to minimize increasing series resistance and contributions of endogenous Na⁺currents (< 50 pA) occasionally observed in Chinese hamster ovary cells.19 

Thymol-free halothane was obtained from Halocarbon Laboratories (River Edge, NJ); isoflurane and sevoflurane were from Abbott Laboratories (Abbott Park, IL); enflurane was from Anaquest Inc. (Liberty Corner, NJ); desflurane was from Baxter Healthcare Corporation (Deerfield, IL). Anesthetics were diluted from saturated aqueous stock solutions made in extracellular solution (14–16 mm halothane, 10–12 mm isoflurane, 4–6 mm sevoflurane, 10–12 mm enflurane, 8–10 mm desflurane) prepared 12–24 h before experiments into airtight glass syringes and applied locally to attached cells at 50–70 μl/min using an ALA-VM8 pressurized perfusion system (ALA Scientific, Westbury, NY) through a perfusion pipette (diameter, 0.15 mm) positioned 30–40 μm away from patched cells. Concentrations of volatile anesthetics were determined by local sampling of the perfusate at the site of the recording pipette tip and analysis by gas chromatography as described.5 

IC⁵⁰values were calculated by least squares fitting of data to the Hill equation: Y = 1/(1 + 10((logIC⁵⁰− X) ×h )), where Y is the effect, X is measured anesthetic concentration, and h  is Hill slope. Activation curves were fitted to a Boltzmann equation of the form G/G^max= 1/(1 + e(V^1/2a– V)/k ), where G/G^maxis normalized fractional conductance, G^maxis maximum conductance, V^1/2ais voltage for half-maximal activation, and k  is the slope factor. Na⁺conductance (G^Na) was calculated using the equation: G^Na=I  ^Na/(V^t– V^r), where I  ^Nais peak Na⁺current, V^tis test potential, and V^ris Na⁺reversal potential (E^Na= 69 mV). Fast inactivation curves were fitted to a Boltzmann equation of the form I /I  ^max= 1/(1 + e(V^1/2in– V)/k ), where I /I  ^maxis normalized current, I  ^maxis maximum current, V^1/2inis voltage of half maximal inactivation, and k  is slope factor. I  ^Nacurrent decay was analyzed by fitting the decay phase of the current trace between 90% and 10% of maximal I  ^Nato the monoexponential equation I  ^Na= A · exp(−t/τⁱⁿ) + C, where A is maximal I  ^Naamplitude, C is plateau I  ^Na, t is time, and τⁱⁿis time constant of current decay. Channel recovery from fast inactivation was fitted to the monoexponential function Y = A × (1 – exp(−τ^r× X)), where Y is fractional current recovery constrained to 1.0 at infinity time, A is normalized control amplitude, X is recovery time, and τ^ris time constant of recovery, and the goodness of fit was compared to that of a biexponential function. The effects of anesthetics were compared to control using sum-of-squares F test between curve fits of mean data. The time course of use-dependent decay of normalized I  ^Nawas analyzed by fitting to the monoexponential equation I  ^Na= exp(−τ^use· n) + C, where n is pulse number, C is plateau I  ^Na, and τ^useis time constant of use-dependent decay. Data were analyzed using pClamp 8.2 (Axon/Molecular Devices), Prism 4.0 (GraphPad Software Inc., San Diego, CA), and SigmaPlot 6.0 (SPSS Science Software Inc., Chicago, IL). Curve fits were compared by sum-of-squares F test. Statistical significance was assessed by analysis of variance with Newman-Keuls post hoc  test or paired or unpaired t  tests, as appropriate; P < 0.05 was considered statistically significant.

Average peak Na⁺current (I  ^Na) in Chinese hamster ovary cells transfected with the Na^v1.4 α-subunit was –3.0 ± 0.2 nA (n = 11) from a holding potential of –120 mV. Peak I  ^Nawas rapidly (onset < 1.5 min) and reversibly inhibited by all five inhaled anesthetics tested (fig. 1) and by the specific Na⁺channel blocker tetrodotoxin (data not shown). Inhibition was greater from the more physiologic holding potential of –80 mV than from the hyperpolarized potential of –120 mV (fig. 2), indicative of significant voltage-dependent inhibition. At aqueous concentrations equivalent to 1 minimum alveolar concentration (MAC) for rat (0.35 mm for halothane and isoflurane, 0.46 mm for sevoflurane, 0.75 mm for enflurane, and 0.80 mm for desflurane),20desflurane showed the greatest inhibition of peak I  ^Nafrom a holding potential of –80 mV. The rank order of inhibition was desflurane (57 ± 6.2% at 0.83 ± 0.06 mm, n = 4) > halothane (32 ± 3.5% at 0.42 ± 0.05 mm, n = 6) ≈ enflurane (32 ± 6.7% at 0.82 ± 0.06 mm, n = 5) > isoflurane (19 ± 1.9% at 0.46 ± 0.04 mm, n = 4) ≈ sevoflurane (17 ± 3.3% at 0.44 ± 0.04 mm, n = 4; mean ± SEM). The degree of voltage-dependent inhibition (difference between efficacy at –80 mV vs . –120 mV) varied between anesthetics and was greatest for desflurane and least for halothane (fig. 2).

Voltage-gated Na⁺channels have at least three distinct conformational states: resting, open, and inactivated.13The potency of each anesthetic for tonic inhibition was investigated using a holding potential of –120 mV to maintain channels in the resting state, allowing assessment of resting channel block with minimal interference from voltage-dependent inactivation. All five anesthetics exhibited concentration-dependent inhibition of peak I  ^Nawith IC⁵⁰values in the millimolar range and Hill slopes of 2, except for halothane, which had a Hill slope of 1 (fig. 3); this suggests the possibility of two sites of interaction with Na^v1.4 for the ethers versus  a single site of interaction for the alkane.

None of the anesthetics tested altered the current-voltage relationship or reversal potential for I  ^Na(fig. 4; data not shown). At concentrations equivalent to ∼1 MAC, all five anesthetics produced no significant shift in the voltage dependence of activation (fig. 5), with minor effects on slope factors (data not shown). The voltage dependence of fast inactivation was determined for the prototypical anesthetic isoflurane and for desflurane and halothane, which exhibited extremes in voltage sensitivity (fig. 2). Representative current traces, which reflect channel availability at various holding potentials, obtained using a protocol designed to minimize the influence of slow inactivation are shown in figure 6A. Isoflurane, halothane, and desflurane strongly enhanced inactivation in a concentration-dependent manner. Desflurane produced a greater negative shift in V^1/2inthan isoflurane or halothane as seen in curve fits of the mean data (fig. 6, B and C). At concentrations of ∼1 MAC, isoflurane, halothane, and desflurane shifted V^1/2inby –7 mV, –9 mV, and –13 mV, respectively (fig. 6B). Similar effects were evident when the data were analyzed by calculating the mean values of V^1/2inderived from curve fits of the individual data sets (table 1). Slope factors, which reflect the voltage sensitivity of the inactivation gate,21were not significantly affected by isoflurane or halothane, but they were slightly increased by desflurane (table 1). Macroscopic current inactivation was examined by fitting the rate of decay of current elicited by depolarization from –120 mV to V^maxto a mono-exponential equation. Time constants of current decay (τⁱⁿ) were reduced by desflurane > halothane > isoflurane (table 2). The effects of isoflurane and halothane on recovery of I  ^Nafrom fast inactivation were evaluated by a two-pulse protocol with varying time intervals (fig. 7). Both anesthetics slowed recovery from inactivation by increasing the time constant of recovery (τ^r, ms) derived from monoexponential fits of the fractional current (fig. 7).

Use-dependent block of Na^v1.4 was evident as a reduction in normalized I  ^Narelative to the peak of the first pulse evaluated in a series of rapid depolarizing pulses (fig. 8). In the absence of anesthetic, repetitive pulses produced only small reductions in peak I  ^Na. Both halothane and isoflurane at ∼2 MAC reduced the time constant of use-dependent decay (τ^use). Halothane produced a greater reduction in steady-state normalized I  ^Naamplitude than isoflurane (P < 0.05 by paired t  test, n = 3). These results are consistent with contributions of open channel block and/or enhanced inactivation to inhibition of Na^v1.4.

We compared the actions of five potent halogenated inhaled anesthetics on a single Na⁺channel isoform (Na^v1.4) expressed in a uniform mammalian cellular environment to enhance detection of possible agent–specific effects on state-dependent inhibition. All five of these clinically used inhaled anesthetics, with representatives from both alkane and ether subclasses, inhibited currents conducted by the α-subunit of the Na^v1.4 voltage-gated Na⁺channel isoform at clinically relevant concentrations consistent with a role for blockade of Na^vin anesthetic immobilization.1There were differences between agents in their potencies for inhibition of Na^v1.4 relative to their anesthetic potencies, and there were differences in the voltage-dependence of their inhibition. For example, at equianesthetic concentrations, desflurane was the most effective inhibitor of peak I  ^Nafrom a near physiologic holding potential of –80 mV, and halothane was most effective from a hyperpolarized holding potential of –120 mV. Members of the same drug class have agent-specific differences in their effects on a single target with potential pharmacological implications. A clinical implication of these findings is that inhibition of Na^v1.4 could contribute to skeletal muscle-relaxing effects of anesthetics given the high density of Na^v1.4 at the neuromuscular junction.12Indeed the greater inhibition of Na^v1.4 by desflurane at 1 MAC correlates with its relatively greater enhancement of nondepolarizing neuromuscular blocking drug potency during anesthesia in vivo  in human subjects.22 

Inhibition of Na^v1.4 from a hyperpolarized holding potential is consistent with anesthetic block of the closed resting state.21Our results are comparable to those reported for the cardiac isoform Na^v1.5, for which equianesthetic concentrations of halothane are more potent than isoflurane in tonic blockade of human9and guinea pig23cardiac I  ^Na. Isoflurane, halothane, and desflurane, which were analyzed in more detail, exhibited state-dependent block and a negative shift in the voltage dependence of inactivation, consistent with enhancement of inactivation at physiologic holding potentials. State-dependent block has been reported previously for isoflurane effects on I  ^Nain rat neurohypophysial nerve terminals,7guinea pig cardiomyocytes,23and heterologously expressed Na^v1.2, Na^v1.4, Na^v1.5, and Na^v1.6.8–11Moreover, isoflurane and halothane affected channel-gating, evident as accelerated current decay and use-dependent block (halothane > isoflurane). The similarity of these effects of inhaled anesthetics to the effects of local anesthetics, antidepressants, and anticonvulsants on Na^v1.2 and Na^v1.413–17currents suggests conserved or overlapping drug binding sites and/or allosteric conformational mechanisms for these chemically diverse Na⁺channel antagonists. The relative contributions of open state block and enhanced fast and/or slow inactivation to use-dependent block by inhaled anesthetics, which differs between various Na⁺channel blockers, should be resolvable by examining anesthetic effects on fast inactivation-deficient Na⁺channels.

Inhaled anesthetics are known to inhibit various isoforms of Na^vα-subunits heterologously expressed in Chinese hamster ovary cells (rat Na^v1.2, Na^v1.4, and Na^v1.5),11human embryonic kidney cells (HEK 293, human Na^v1.5),9and Xenopus  oocytes (rat Na^v1.2 and 1.6, human Na^v1.4).10Small differences in potencies reported between various studies probably result from differences in isoform sensitivity, species, expression systems, β-subunit coexpression, recording conditions, stimulation protocols, anesthetic concentration determinations, etc . Isoflurane at clinically relevant concentrations inhibits rat neuronal (Na^v1.2), skeletal muscle (Na^v1.4), and cardiac muscle (Na^v1.5) voltage-gated Na⁺channel α-subunits studied under identical conditions with isoform-dependent differences in state-dependent block.11Significantly lower IC⁵⁰values for isoflurane were observed at more physiologic holding potentials11due to marked voltage-dependent effects on channel-gating compared to the hyperpolarized potential used to characterize tonic block in the current study. The IC⁵⁰for inhibition of Na^v1.4 by isoflurane reported previously for a holding potential of –100 mV (IC⁵⁰= 0.99 mm)11compares well with the value obtained in the present study for a holding potential of –120 mV (IC⁵⁰= 1.16 mm). Rat Na^v1.8 in Xenopus  oocytes has been reported to be insensitive to isoflurane,10but recent evidence indicates that rat Na^v1.8 expressed in a mammalian neuroblastoma cell line is inhibited by isoflurane at concentrations comparable to those effective on other isoforms (unpublished data, 2008; Karl F. Herold, M.D., and Hugh C. Hemmings, M.D., Ph.D., New York NY). All mammalian Na^visoforms tested so far are susceptible to inhibition by the prototypical inhaled anesthetic isoflurane with minor isoform-specific differences in relative potency and mechanism.

Human Na^v1.4 heterologously expressed in Xenopus  oocytes is inhibited by isoflurane and halothane,10whereas rat Na^v1.4 in the same expression system was reported to be insensitive to halothane unless coexpressed with protein kinase C (PKC).24Our results indicate that rat Na^v1.4 expressed in a mammalian cell line is inhibited by multiple inhaled anesthetics in the absence of overexpression or pharmacological activation of PKC, indicating that PKC activation is apparently not required for inhibition. A requirement for activation of endogenous PKC, which can be activated by halogenated inhaled anesthetics,25cannot be excluded, however. Attempts to test this possibility by inhibition of endogenous PKC using small-molecule PKC inhibitors have been unsuccessful because the PKC inhibitors themselves inhibit Na⁺channels.26 

Inhaled anesthetics negatively shift the voltage dependence of I  ^Nafast inactivation. Similar shifts in inactivation of I  ^Naare produced in ventricular cardiomyocytes by isoflurane and halothane9and in rat neurohypophysial nerve terminals7and heterologously expressed Na^visoforms by isoflurane.10,11Negative shifts in the voltage dependence of fast inactivation suggest greater anesthetic binding affinity and selective stabilization of inactivated states. The large negative shift in V^1/2inby desflurane contributes to its greater potency compared to isoflurane and halothane for inhibition of Na^v1.4 from the more positive (physiologic) holding potential of –80 mV vs . –120 mV. Preferential anesthetic interaction with the nonconducting inactivated state of Na^v1.4 is also consistent with anesthetic slowing of recovery from fast inactivation. The slightly greater slowing effect of halothane compared to isoflurane is consistent with a greater shift in V^1/2inand hence stronger interaction with the fast inactivated state for halothane.

Enhanced inactivation is critical to inhibition of Na^v1.4 by inhaled anesthetics at more positive membrane potentials. This has classically been attributed to fast inactivation, but recent evidence implicates slow inactivation in activity-dependent attenuation of Na^vcurrents.4The possible role of slow inactivation in the Na⁺channel-blocking effects of inhaled anesthetics is not clear, but it is an important area for future investigation that will be facilitated using mutations in Na^v1.4 that enhance or remove inactivation.27Possible mechanisms underlying state-dependent effects include facilitated transitions from the closed to inactivated state (tonic block), open to inactivated state (enhanced inactivation), and/or stabilization of inactivated states (delayed recovery). Small differences between anesthetics in their state-dependent effects on various Na^visoforms11could underlie anesthetic- and isoform-specific differences in pharmacological profiles of specific anesthetics in vivo , e.g. , differential effects on brain, heart, and skeletal muscle.

All five inhaled anesthetics tested produced insignificant shifts in the voltage dependence of Na^v1.4 activation, similar to the findings of Stadnicka et al.  9but in contrast to those of Weigt et al. ,23who found small negative shifts for Na^v1.5. These findings suggest isoform-selective interactions between volatile anesthetics and the activation process similar to those observed for the local anesthetic lidocaine.28Another class of anesthetics, the n-alkanols, also inhibits voltage-gated Na⁺channels at anesthetic concentrations, but with somewhat distinct mechanisms from those of inhaled anesthetics that involve primarily open channel block with relatively small effects on both activation and inactivation.29Thus the n-alkanols and inhaled anesthetics both inhibit Na⁺channels, but they differ in the relative involvement of open channel block and activation (greater for the alkanols) versus  inactivation mechanisms (greater for inhaled anesthetics).

Both halothane and isoflurane exhibited use-dependent block with repetitive stimuli. The fraction of open versus  inactivated channels increases during fast repetitive depolarizations; therefore, the presence of use-dependent block suggests a possible role for open-channel block and/or slow inactivation mechanisms by both anesthetics.17,21Halothane was more efficacious than isoflurane in inhibiting normalized current amplitude with repetitive stimuli consistent with its greater tonic I  ^Nablocking effect. Open-channel block is particularly important in pathologic conditions such as myotonia and periodic paralysis, which involve Na^v1.4 inactivation gating defects,30inflammatory and neuropathic pain states that involve repetitive activation of Na^v1.7 and Na^v1.8,31,32and ischemia, which leads to resting membrane depolarization.33 

Accessory subunits can have important effects on the pharmacological and gating properties of voltage-gated Na⁺channels that must be considered in pharmacological studies, although α-subunit expression is sufficient to mimic native Na⁺channel-gating properties.4Modulation of Na^vfunction by β-subunits depends on both the α-subunit isoform and the cell type used for expression. Coexpression of the β1-subunit has no effect on inhibition by isoflurane of Na^v1.2, Na^v1.4, Na^v1.6, or Na^v1.8 α-subunits expressed in Xenopus  oocytes.10In mammalian expression systems, β1-subunit coexpression has no major effects on local anesthetic sensitivity, current kinetics, or activation and inactivation properties of tetrodotoxin-sensitive currents in ND7/23 cells,34but it produces positive shifts of channel activation and inactivation of Na^v1.2 in tsA-201 cells35and positive shifts of inactivation but no change in cocaine affinity of Na^v1.4 and Na^v1.5 in human embryonic kidney 293t cells.36Although we have not ruled out possible effects of β-subunit coexpression on inhaled anesthetic sensitivity of Na^v1.4 in Chinese hamster ovary cells, these findings make a major effect unlikely.

The role of voltage-gated Na⁺channels in the mechanisms of inhaled anesthetics is an important and unresolved question, but a number of factors impede its resolution.1Correlations between in vitro  effects on Na⁺currents and anesthetic potencies in vivo  are limited by our ignorance regarding the specific cells, networks, and molecular targets involved in the behavioral effects of inhaled anesthetics (immobilization in the case of MAC). We are thus unable to define the degree of Na⁺channel inhibition critical for an anesthetic effect that must be taken into consideration when evaluating correlations between potencies measured in vitro  and in vivo . Interestingly, experiments with local anesthetics suggest that relatively small degrees of Na^vblockade (10–20% inhibition of peak current amplitude) can have profound effects on neuronal firing rate.37Moreover, determination of anesthetic effects on ion channels in vitro  is subject to numerous experimental variables, including the species and isoform of the channel, expression system used, accessory subunits, modulation by cellular signaling pathways, temperature, experimental conditions, including holding potentials, and stimulus protocols, etc.  These and other factors can have profound effects on channel function and pharmacological sensitivity. Given these reservations, the observation that all five inhaled anesthetics tested inhibit Na^v1.4 supports, but does not prove, an important role for Na^vinhibition in anesthesia. Additional support for this hypothesis is provided by the recent observation that intrathecal administration of the Na^vagonist veratridine increases MAC in rats.38 

In summary, halogenated inhaled anesthetics all inhibit heterologously expressed Na^v1.4 at clinical concentrations by state-dependent mechanisms. These findings support inhibition of Na^vas a common mechanism for inhaled anesthetic action. Small agent-specific differences in relative potency and gating effects are consistent with subtle interagent variability in pharmacodynamic profiles, such as skeletal muscle relaxant effects. Agent-specific differences in potency for Na^v1.4 inhibition at normal resting membrane potential were determined primarily by differences in state-dependent block reflected in effects on inactivation gating. These gating effects of inhaled anesthetics are remarkably similar to those of local anesthetics, and they suggest the possibility of overlapping binding sites,13–17an interesting hypothesis that can now be tested by detailed structure-function studies in Na^v1.4 using mutations that affect gating mechanisms and local anesthetic sensitivity.