Molecular Mechanisms Underlying Ketamine-mediated Inhibition of Sarcolemmal Adenosine Triphosphate-sensitive Potassium Channels

cDNAs (The human Kir6.2, rat Kir6.1, rat SUR1, rat SUR2A, and rat SUR2B) and expression vector pCMV6C were kindly provided by Susumu Seino, MD., Ph.D. (Professor and Chairman, Department of Cellular and Molecular Medicine, Chiba University, Chiba, Japan). Coexpressing SUR1 and Kir6.2 (SUR1/Kir6.2) forms the pancreatic β cell K^ATPchannel, SUR2A and Kir6.2 (SUR2A/Kir6.2) forms the cardiac K^ATPchannel, SUR2B and Kir6.2 (SUR2B/Kir6.2) forms the nonvascular smooth muscle K^ATPchannel, and SUR2B and Kir6.1 (SUR2B/Kir6.1) forms the vascular smooth muscle K^ATPchannel.3,20,21A truncated form of human Kir6.2 lacking the last 36 amino acids at the C terminus was obtained by polymerase chain reaction amplification as previously described.17Chimeric cDNA constructs were produced by splicing, using the overlap extension polymerase chain reaction technique.22The exact amino acid composition of the SUR1-SUR2A chimeric constructs was: chimera SUR1–2A = (1–1035, SUR1)-(1013–1261, SUR2A)-(1297–1581, SUR1); SUR2A-1 = (1–1013, SUR2A)-(1035–1277, SUR1)-(1241–1545, SUR2A). All DNA products were sequenced using BigDye terminator cycle sequencing kit (Applied Biosystems, Foster City, CA), and an ABI PRISM 377 DNA sequencer (Applied Biosystems) was used to confirm the sequence.

K^ATPchannel-deficient COS-7 cells were plated at a density of 3 × 10⁵per dish (35 mm diameter) and cultured in Dulbecco’s modified Eagle’s medium supplemented with 10% fetal calf serum. A full-length Kir cDNA and a full-length SUR cDNA were subcloned into the mammalian expression vector pCMV6c. For electrophysiological recordings, mutated pCMV6c Kir alone (1 μg) or either wild-type or mutated pCMV6c Kir (1 μg) plus pCMV6c SUR (1 μg) were transfected into COS-7 cells with green fluorescent protein cDNA (pEGFP-N1; Clontech Laboratories, Palo Alto, CA) as a reporter gene by using lipofectamine and Opti-MEM 1 reagents (Life Technologies Inc., Rockville, MD) according to the manufacturer’s instructions. After transfection, cells were cultured for 48 to 72 h before being subjected to electrophysiological recordings.

Membrane currents were recorded in the inside-out configurations using a patch-clamp amplifier as described previously.9,10,17Transfected cells were identified by their green fluorescence under a microscope. The intracellular solution contained 140 mm KCl, 2 mm EGTA, 2 mm MgCl², and 10 mm HEPES (pH = 7.3). The pipette solution contained 140 mm KCl, 1 mm CaCl², 1 mm MgCl², and 10 mm HEPES (pH = 7.4). Recordings were made at 36°± 0.5°C. Patch pipettes were pulled with an electrode puller (PP-830; Narishige, Tokyo, Japan). The resistance of pipettes filled with internal solution and immersed in the Tyrode’s solution was 5–7 MΩ. The sampling frequency of the single-channel data were 5 KHz with a low-pass filter (1 KHz).

Channel currents were recorded with a patch clamp amplifier (CEZ 2200; Nihon Kohden, Tokyo, Japan) and stored in a personal computer (Aptiva; IBM, Armonk, NY) with an analog-to-digital converter (DigiData 1200; Axon Instruments, Foster City, CA). pClamp version 7 software (Axon Instruments) was used for data acquisition and analysis. The open probability (P^o) was determined from current amplitude histograms and was calculated as follows:

where t^jis the time spent at current levels corresponding to j = 0, 1, 2, N channels in the open state, T^dis the duration of the recording, and N is the number of the channels active in the patch. Recordings of 2–3 min were analyzed to determine P^o. The channel activity was expressed as NPo. The NPo in the presence of drugs was normalized to the baseline NPo value obtained before drug administration and presented as the relative channel activity. When the concentration-dependent effects of drugs were studied, the superfusion was stopped for approximately 1 min at each concentration, and these drugs were injected into the cell bath using a glass syringe to five final concentrations in a cumulative manner (total volume injected was approximately 10–20 μl). Therefore, the superfusion was stopped for approximately 5 min; preliminary studies showed that the stopping of superfusion for approximately 5 min had no significant effects on electrophysiological measurements. The average percent recovery of K^ATPchannel activities after washout of Ketamine racemate or S-(+)-ketamine was 94 ± 6% of the NPo measured before drug treatment.

The drug concentration needed to induce half-maximal inhibition of the channels (IC⁵⁰) and the Hill coefficient were calculated as follows:

where y is the relative NP^o, [D] is the concentration of drug, Kⁱis IC⁵⁰, and H is the Hill coefficient.

The following drugs were used: ketamine racemate, S-(+)-ketamine, glibenclamide, and pinacidil (Sigma- Aldrich Japan, Tokyo, Japan). Glibenclamide and pinacidil were dissolved in dimethylsulfoxide (the final concentration of solvent was 0.01%); preliminary studies showed that 0.02% of dimethylsulfoxide, a twofold higher concentration than we used in the current study, had no significant effects on all types of reassociated K^ATPchannel currents.

All data were presented as means ± SD. Differences between data sets were evaluated either by repeated-measures one-way analysis of variance followed by the SchefféF  test or by Student t  test. P < 0.05 was considered significant.

Sarcolemmal K^ATPchannels SUR2A/Kir6.2 (cardiac type), SUR2B/Kir6.1 (vascular smooth muscle type), SUR2B/Kir6.2 (nonvascular smooth muscle type), and SUR1/Kir6.2 (pancreatic β-cell type) were heterologously expressed in COS-7 cells. Our previous experiments have shown that the single-channel characteristics of all types of reassociated K^ATPchannels were similar to those of native K^ATPchannels.17 

To assess the effects of ketamine racemate and S-(+)-ketamine on reassociated K^ATPchannels in the absence of intracellular ADP, we measured single-channel currents on inside-out patches in the presence of these drugs. Application of 100 μm ketamine racemate to the intracellular membrane surface inhibited the SUR2A/Kir6.2, SUR2B/Kir6.1, and SUR2B/Kir6.2 channel currents, with relative channel activities decreasing to 0.45 ± 0.11, 0.41 ± 0.07, and 0.47 ± 0.18, respectively (fig. 1A). However, 100 μm ketamine racemate did not significantly inhibit the SUR1/Kir6.2 channel currents. On the contrary, S-(+)-ketamine at 100 μm did not significantly inhibit all of the reassociated K^ATPchannel currents (fig. 1B). The inhibitory effects of ketamine racemate and S-(+)-ketamine on K^ATPchannel activities were readily reversible (fig. 1A).

The concentration-dependent effects of ketamine racemate and S-(+)-ketamine on the activities of various types of reassociated K^ATPchannels, in the absence of intracellular ADP, are shown in figure 2. The IC⁵⁰values and Hill coefficients of ketamine and S-(+)-ketamine for the SUR2A/Kir6.2, SUR2B/Kir6.1, SUR2B/Kir6.2, and SUR1/Kir6.2 channels are summarized in table 1. The IC⁵⁰value of ketamine racemate for SUR2A/Kir6.2 (83 ± 8 mμ) (table 1) was very similar to that obtained by Ko et al.  8for the native cardiac K^ATPchannel in the absence of intracellular ADP (63 μm). Ketamine racemate inhibited the activity of all types of reassociated K^ATPchannels with higher potency than S-(+)-ketamine. In addition, ketamine inhibited the SUR2A/Kir6.2, SUR2B/Kir6.1, and SUR2B/Kir6.2 channels with higher potency than the SUR1/Kir6.2 channel.

A C-terminal truncated pore-forming subunit of Kir6.2 (Kir6.2ΔC36), lacking the last 36 amino acids, is capable of forming a functional channel in the absence of SUR.23This has proved to be a useful tool for discriminating the site of action of various agents on K^ATPchannels.

Ketamine racemate and S-(+)-ketamine inhibited the Kir6.2ΔC36 channel current with very low affinity, with 1 mm concentrations of both anesthetics producing less than 30% inhibition (fig. 3, A and B). This result indicates that the SUR subunit, rather than Kir6.2, is primarily responsible for the effects of ketamine racemate and S-(+)-ketamine on wild-type K^ATPchannels, a view that is supported by the differential actions of ketamine on K^ATPchannels containing different types of SUR subunit.

The effects of several sulfonylureas and K^ATPchannel openers are modified by interaction of MgADP with SURs.18,19Indeed, it has been previously reported that MgADP simultaneously activates the K^ATPchannels strongly via  SUR and inhibits them weakly via  the ATP binding site on Kir6.2.23,24Therefore, we next examined whether the inhibitory effects of ketamine on reassociated K^ATPchannel currents are affected in the presence of intracellular MgADP.

Figure 4shows the effects of 100 μm ketamine racemate on SUR1/Kir6.2, SUR2A/Kir6.2, and SUR2B/Kir6.2 channel activities in the absence or presence of 100 μm MgADP. The IC⁵⁰for these channels are given in table 2. MgADP was significantly enhanced the inhibitory potency of ketamine racemate on all types of reassociated K^ATPchannels.

The differential effects of ketamine on SUR1 and SUR2 enabled us to use a chimeric approach in identifying regions of SUR2 critical for inhibition by ketamine racemate. SUR is a member of the ATP-binding cassette transporter family and is predicted to possess two intracellular nucleotide binding domains and three transmembrane domains (TMD0, TMD1, and TMD2) that contain five, six, and six transmembrane helices (TMs), respectively.25It has been reported recently that TMD2 of SUR1 and SUR2 is crucial for the action of several SUR1-selective sulfonylureas, K^ATPchannel blocker, and SUR2-selective K^ATPchannel openers, respectively.22,26Thus, we hypothesized that TMD2 of SUR might be involved in the SUR2-selective inhibition of ketamine. To test this hypothesis, we constructed chimeric SURs, in which portions of TMD2 were swapped between SUR1 and SUR2A and coexpressed with Kir6.2 in COS-7 cells (fig. 5A).

Ketamine racemate sensitivity could be introduced into SUR1 by transferring TMs 13–17 from SUR2A to SUR1 (chimera SUR1–2) (fig. 5B). The reverse chimera, in which TMs 13–16 were swapped from SUR1 to SUR2A (chimera SUR2–1), was also sensitive to inhibition by ketamine racemate but to a lesser extent than wild-type SUR2A. These results suggest that the region within TM 13–17 is essential for the full inhibitory effect of ketamine racemate.

In the current study, we transiently expressed the different subtypes of reassociated sarcolemmal K^ATPchannels in COS-7 cells and demonstrated that ketamine racemate and S-(+)-ketamine inhibited these channels in a concentration-dependent manner. Our results also indicated that ketamine racemate specifically inhibits cardiovascular type K^ATPchannels. Notably, ketamine’s potency in blocking K^ATPchannels containing SUR2 was approximately 18-fold higher than its ability to block K^ATPchannels containing SUR1. Furthermore, ketamine racemate was significantly more potent than S-(+)-ketamine in blocking all types of reassociated K^ATPchannels. Because ketamine racemate is a mixture of S-(+) and R-(-)-ketamine stereoisomers, this observation would suggest that block of K^ATPchannels by ketamine enantiomers may be at least partly stereoselective.

K^ATPchannels are formed from pore-forming Kir6.0 and regulatory SUR subunits, arranged with 4:4 stoichio- metries.3The SUR subunit is encoded by two different genes, SUR1 and SUR2; SUR1 serves as the regulatory subunits of the pancreatic β-cell K^ATPchannel, and splice variants of SUR2 act as the cardiac (SUR2A) and smooth muscle (SUR2B) SURs.3K^ATPchannel activators and inhibitors show variable tissue specificity, the different types of K^ATPchannel exhibit differential ATP sensitivity and pharmacologic properties, which are endowed by their different molecular composition of Kir6 and SUR subunits.20In the current study, the effects of ketamine racemate and S-(+)-ketamine were influenced by the type of SUR subunits, indicating that these anesthetics may interact with the SUR2 rather than the SUR1 subunits of the channels (table 1, figs. 1 and 2). In addition, the results obtained in the current study showing that both ketamine racemate and S-(+)-ketamine at concentrations up to 1 mm had no significant effects on the current generated by expressing Kir6.2ΔC36 in the absence of SUR (fig. 3) suggest that the inhibitory effects of these drugs on K^ATPchannel activities are not mediated through Kir6.0 subunits. That is, it is unlikely that binding to SUR1 or Kir6.0 subunits contributes significantly to the inhibitory effect of these anesthetics. Furthermore, the inhibitory effects of ketamine racemate on SUR2/Kir6.0 channels were significantly larger than those of S-(+)-ketamine (table 1, figs. 1 and 2). It is, therefore, suggested that specific binding to SUR2A and SUR2B subunits may be the major mechanism underlying the tissue-specific and stereoselective inhibitory effects of ketamine.

The specificity of ketamine for K^ATPchannels containing SUR2 isoforms enabled us to use a chimeric approach to identify regions of SURs important for activity of the drug. Chimeric sulfonylurea receptors were constructed in which isolated domains were swapped between SUR1 and SUR2 to identify regions of SUR2 that are required for the high affinity action of ketamine. In this way, we showed that high-affinity inhibition by ketamine racemate could be introduced into SUR1 by transferring parts of TMD2 (TMs 13–17) from SUR2A to SUR1 (fig. 5) (chimera SUR1–2). Furthermore, the reverse chimera, in which parts of TMD2 (TMs 13–16) were swapped from SUR1 to SUR2A (fig. 5) (chimera SUR2–1), abolished high-affinity ketamine racemate inhibition in SUR2. These results suggest that this region within TMD2 of SUR2A is essential for high-affinity inhibition by ketamine racemate.

It has been reported that clinical plasma concentrations for ketamine racemate are 20–50 μm27or 3– 60 μm,8and the percentage of ketamine bound to plasma protein are 12%27or 45–50%,8suggesting that plasma concentrations of free ketamine racemate are 2.4–6 μm or 1.4–30 μm, respectively, during surgical anesthesia. The threshold concentrations at which ketamine racemate inhibits reassociated K^ATPchannels containing SUR2 are close to this range (>10–100 μm) (fig. 2, A–C), but this is not the case for SUR1-containing channels (>10 m) (fig. 2D). It is possible, therefore, that inhibition of sarcolemmal K^ATPchannels containing SUR2 (cardiovascular type) by ketamine might have adverse consequences in clinical practice. Recently, physiologic studies on mice lacking different K^ATPchannel subunits have begun to clarify the roles of cardiac and vascular K^ATPchannels in cardiovascular pathophysiology. Cardiac K^ATPchannel (Kir6.2^−/−)-deficient mice had a number of cardiac abnormalities during myocardial ischemia or severe stress, include impaired ischemic preconditioning and attenuated electrocardiographic ST changes.28,29Impaired vascular smooth muscle function was a feature of Kir6.1-deficient and SUR2-deficient mice and manifested as episodic coronary artery vasospasm and a high rate of sudden death.6,30Increased systolic and diastolic blood pressure was also observed in SUR2-deficient mice.30Indeed, racemic ketamine, but not the S-(+)-ketamine stereoisomer, was found to block early and late preconditioning in rabbit hearts and inhibit vasorelaxation induced by a K^ATPchannel opener.13–16In heart, however, mitochondrial rather than sarcolemmal K^ATPchannels might play an important role in ischemic and anesthetic preconditioning. Because the molecular identity of the channel has not been established, molecular biologic approaches such as reassociation of cloned channels or gene targeting technique are not yet applicable for the study of mitochondria K^ATPchannel function. Recently, Zaugg et al.  31reported that 10 μm R-(-)-ketamine, but not S-(+)-ketamine, inhibited diazoxide-induced flavoprotein oxidation of rat myocytes, an index of mitochondrial K^ATPchannel activation. Thus, these observations may point to the similarity of ketamine’s inhibitory effects on sarcolemmal and mitochondrial K^ATPchannels.

It is well established that MgADP simultaneously activates the K^ATPchannels strongly via  SUR and inhibits them weakly via  the ATP binding site on Kir6.2.23,24In addition, recent studies demonstrated that intracellular MgADP could modulate the sensitivity of K^ATPchannel activators and inhibitors.18,19The current study indicated that ketamine racemate inhibits K^ATPchannels via  SUR subunits and that a physiologic concentration of MgADP (100 μm) significantly enhanced the inhibitory effects of ketamine racemate on all three types of SUR1/Kir6.2, SUR2A/Kir6.2, and SUR2B/Kir6.2 channels (table 2, fig. 4B). It is very difficult to explain the precise mechanisms of the interaction between MgADP and ketamine racemate, it might be possible to speculate that ketamine racemate attenuates (or abolishes) the MgADP-induced activation mediated by the high-affinity site on SUR, thereby exposing the inhibitory effect of MgADP on Kir6.2 such that the overall inhibitory potency is enhanced. Therefore, it might be possible to estimate that ketamine racemate may inhibit K^ATPchannels containing SUR2 when the channels are opened in vivo  by increased ADP concentrations, such as those occurring during ischemia.

Our study has several limitations. First, although we used the same amount of SUR cDNA and Kir cDNA for transfection, the genomic integration of the various constructs may have been different, and a varying ratio of SUR versus  Kir may affect electrophysiological findings. Therefore, it might be better for us to establish the level of expression as well as the ratio of SUR versus  Kir subunits by polymerase chain reaction method and Western blot analyses. However, in our previous study, we confirmed that the sensitivity to ATP, diazoxide, and glibenclamide and the single-channel conductance of all kinds of reassociated K^ATPchannels were similar to those of native K^ATPchannels.17Therefore, we expect that the reassociated K^ATPchannels in the current study can be used as experimental models to characterize the function of the native K^ATPchannels and that we can draw conclusions from our experimental model. Second, as we discussed above, the threshold concentrations at which ketamine racemate inhibited reassociated K^ATPchannels containing SUR2 (10–100 μm) are close to free plasma concentrations for ketamine (1.4–30 μm). In contrast, considering that intracellular concentrations of ketamine racemate should be much lower than this range, the concentrations of ketamine racemate we used in the current study should be much higher than the clinically relevant concentrations, suggesting the inhibitory effects of ketamine racemate on sarcolemmal K^ATPchannels were observed at very high concentrations in the inside-out patch clamp configurations. Third, although we studied the effects of ketamine racemate on sarcolemmal K^ATPchannels using the inside-out patch clamp configurations, there is a possibility that ketamine racemate acts on these channels from outside of the membrane. To confirm this possibility, it is necessary to study the effects of ketamine racemate using outside-out patch clamp configurations. Fourth, although we studied direct effects of ketamine racemate on sarcolemmal K^ATPchannel activities, it should be noted that ketamine could affect K^ATPchannel activities through alteration in nitric oxide concentrations, for example.32 

In conclusion, ketamine-induced inhibition of sarcolemmal K^ATPchannels is mediated by SUR subunits. These inhibitory effects of ketamine exhibit a high degree of specificity for cardiovascular K^ATPchannels, the most critical binding site for ketamine being in the C-terminal set of TMs of SUR2A, and they exhibit at least some degree of stereoselectivity; ketamine racemate was more potent than S-(+)-ketamine. Our results further suggest that MgADP enhances the inhibitory effects of ketamine racemate on sarcolemmal K^ATPchannel activity under conditions of metabolic inhibition, for example, during ischemia.

We thank Susumu Seino, M.D., Ph.D. (Professor and Chairman, Department of Cellular and Molecular Medicine, Chiba University, Chiba, Japan), for providing cDNAs (Kir6.2, Kir6.1, SUR1, SUR2A, and SUR2B) and expression vector pCMV6C.