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Pain Medicine  |   June 2001
Local Anesthetic Inhibition of Voltage-activated Potassium Currents in Rat Dorsal Root Ganglion Neurons
Author Affiliations & Notes
  • Hirochika Komai, Ph.D.
    *
  • Thomas S. McDowell, M.D., Ph.D.
  • * Associate Scientist, Emeritus, † Assistant Professor.
  • Received from the Department of Anesthesiology, University of Wisconsin, Madison, Wisconsin.
Article Information
Pain Medicine
Pain Medicine   |   June 2001
Local Anesthetic Inhibition of Voltage-activated Potassium Currents in Rat Dorsal Root Ganglion Neurons
Anesthesiology 6 2001, Vol.94, 1089-1095. doi:
Anesthesiology 6 2001, Vol.94, 1089-1095. doi:
DURING spinal and epidural anesthesia, the axon terminals of dorsal root ganglion (DRG) neurons and the dorsal horn neurons with which they synapse are both exposed to relatively high concentrations of local anesthetics because of their proximity to the injection site. Blockade of Na+channels by local anesthetics has been studied extensively and likely represents the main mechanism by which these agents block impulse propagation in axons. 1–2 Local anesthetic effects on other ion channels in sensory neurons may modulate the ability of these agents to produce anesthesia and analgesia. For example, alteration of K+channel function by local anesthetics may influence the transmission of sensory impulses by altering the resting membrane potential, the timing of repolarization after an action potential, neuronal excitability, and the firing pattern of a neuron.
Olschewski et al.  3 reported that local anesthetics inhibit transient but not sustained K+currents of rat dorsal horn neurons. Using frog sensory neurons, Guo et al.  4 reported that bupivacaine inhibits IK(the delayed rectifier K+current). In outside-out patches obtained from amphibian sciatic nerve, which should include axons of DRG, Bräu et al.  5 reported that local anesthetics inhibit a sustained K+current. These authors did not report the effects of local anesthetics on transient K+currents. The effects of local anesthetics on mammalian sensory neurons is not known. Therefore, we studied the effects of local anesthetics on transient and sustained K+currents in isolated rat DRG neurons.
Methods and Materials
Weanling Sprague-Dawley rats of either sex were deeply anesthetized with an intraperitoneal injection of pentobarbital (200 mg/kg), after which the spinal column was removed. The procedure was approved by the Animal Care and Use Committee of the University of Wisconsin, Madison, Wisconsin. DRGs were isolated in Dulbecco’s Modified Eagle Medium and treated with trypsin (2.5 mg/ml) and collagenase (2 mg/ml) for 45 min at 35°C. After the enzyme treatment, DRGs were dissociated by trituration with fire-polished Pasteur pipettes. DRG neurons were collected by low-speed centrifugation; resuspended in Dulbecco’s Modified Eagle Medium containing fetal bovine serum (10%), penicillin (50 U/ml), and streptomycin (50 μg/ml); and allowed to settle on cover glass coated with polylysine. Cells were maintained at 35°C in a humidified incubator equilibrated with a gas mixture containing 5% CO2. The cells were used within two days after isolation.
Patch pipettes were pulled from borosilicate glass tubing. Whole cell K+currents were measured from DRG neurons at room temperature (21–25°C) using an Axopatch 200B patch clamp amplifier (Axon Instruments, Foster City, CA) and acquired using pCLAMP 6 software (Axon Instruments). Currents were filtered at 5 kHz and acquired at 10 kHz. A P/−4 protocol was used for leak subtraction. Cell capacitance and series resistance were read from the dials of the patch clamp amplifier after cancellation of the capacitative current transient obtained during a small depolarizing test pulse. Cell diameter was estimated before patch clamping using an eyepiece micrometer at 400× magnification. Gigaseal formation and whole cell configuration were achieved in a medium containing the following (in mm): NaCl, 130; KCl, 5; MgCl2, 1; CaCl2, 2; HEPES, 10 (pH 7.4 with NaOH). Control external solution for the measurements of K+currents contained the following (in mm): choline Cl, 130; KCl, 5; MgCl2, 1; CoCl2, 2; HEPES, 10 (pH 7.4 with choline base). The recording chamber (0.7 ml) was perfused with 5 ml of the external solution containing various concentrations of local anesthetics before the current measurements. The pipette solution contained the following (in mm): KCl, 120; MgCl2, 2.5; EGTA, 10; HEPES, 10; MgATP, 2; LiGTP, 0.3 (pH 7.3 with NaOH).
Dulbecco’s Modified Eagle Medium was obtained from Life Technologies (Grand Island, NY). All other reagents were obtained from Sigma Chemical Company (St. Louis, MO). Stock solutions of bupivacaine (10 mm) were prepared in water. Because of the high concentrations used in the experiments, stock solutions of lidocaine (20 mm) and tetracaine (10 mm) were prepared in the control external solution, and pH was adjusted to 7.4 by the addition of choline base.
The resting potential was maintained at −80 mV. Based on the study of Gold et al.  , 6 we used test pulses to +30 mV with a duration of 400 ms preceded by 1 s conditioning prepulses to −100 mV or −30 mV to separate the different types of K+currents. A test pulse preceded by a prepulse to −100 mV was used to elicit all types of voltage-activated K+currents (total current). A prepulse to −30 mV eliminated the currents that undergo inactivation, leaving only a noninactivating sustained current (IKn). Subtraction of IKnfrom the total current revealed either a fast-inactivating transient current (IAf) or a slow-inactivating transient current (IAs). The magnitudes of the transient currents (IAfand IAs) were measured at the peak amplitude, whereas the sustained currents (IKn) was measured at its plateau amplitudes at the latter part of the test pulse.
The inhibitory effect of local anesthetics on the amplitude of each type of K+current was analyzed using a Hill equation of the form:
where IC50is the concentration of local anesthetic for half-maximal inhibition, and n is the Hill coefficient. Under control conditions, the rates of inactivation of IAfand IAswere determined by fitting the decaying phase of the current to a single exponential of the form:
where A is the magnitude, C is the offset constant, and τ is the time constant. With bupivacaine present, the decaying phase of IAswas fit to a biexponential equation of the form:
where A1represents the magnitude of fast phase with the time constant τ1, A2represents the magnitude of slow phase with the time constant τ2, and C is the offset constant. The relative proportion (%) of the fast phase was 100 × A1/(A1+ A2). The fast phase of current decay in the presence of bupivacaine most likely reflects the bupivacaine block of the open K+channel. 7 The effect of bupivacaine on the magnitude of the fast-inactivating component of IAswas analyzed using the Hill equation of the form:
where Kd is the dissociation constant. We also have obtained Kd for bupivacaine binding from the plot of a reciprocal of time constant of the fast-inactivating inactivating component (τB) of the current against bupivacaine concentration using the relation:
where k  is the rate constant for association (m1· s1), and l  is the rate constant for dissociation (s1). 7 Then, Kd is calculated 7 from the relation Kd =l  / k  .
Data are expressed as mean ± SD. K+current decaying phases were fit to either single or double exponential equations as described using the Chebyshev algorithm (pCLAMP6, Axon Instruments). Dose–response relations were fit using the Quasi-Newton algorithm (MacCurveFit; Kevin Raner Software, Mt. Waverley, Victoria, Australia). Data were excluded from analysis if the amplitude of the K+current after washout of local anesthetic was less than approximately 50% or greater than approximately 150% of control. An unpaired t  test was used to evaluate the difference between two mean values. The concentration dependence of the effects of local anesthetics was evaluated by repeated measures analysis of variance followed by the Dunnett t  test. Differences were considered significant when P  < 0.05.
Results
Voltage-activated K+Currents Expressed in Two Subpopulations of DRG cells
From the patterns of K+current expression, two types of cells were recognized (fig. 1). One cell type (type 1) generally expressed two different K+currents. These were a fast-transient current (IAf; inactivation time constant of 12 ± 8 ms, n = 24) and a sustained current that does not undergo steady-state inactivation (IKn). The current elicited after a prepulse to −30 mV is IKn(fig. 1A). The difference between the total current (measured after a prepulse to −100 mV) and IKnwas IAf(fig. 1A). The other cell type (type 2) expressed a slow-transient current (IAs; inactivation time constant of 279 ± 86 ms, n = 20) and a noninactivating sustained current (IKn). As in type 1 cells, the current elicited after a prepulse to −30 mV was IKn(fig. 1B). However, in type 2 cells, the difference between the total current and IKnwas IAs(fig. 1B). Type 1 cells had significantly larger diameters (36 ± 4 vs.  28 ± 3 μm;P  < 0.05) and whole cell capacitance (59 ± 18 vs.  26 ± 7 pF;P  < 0.05) compared with type 2 cells.
Fig. 1. Distribution of K+currents observed in two major classes of dorsal root ganglion neurons. Representative K+currents from a type 1 cell and a type 2 cell are shown in panels A  and B  , respectively. The total current is measured during a voltage step to +30 mV after a conditioning prepulse to −100 mV for 1 s. Both types of cells express IKn, the noninactivating current remaining after a conditioning prepulse to −30 mV, although the magnitude of this current was much greater in the type 1 cell. The current components that undergo steady-state inactivation are obtained by subtracting the current measured after the prepulse to −30 mV (IKn) from the current measured after the prepulse to −100 mV (total current). These subtracted currents are the fast-inactivating transient current (IAf) for type 1 cells and the slow-inactivating transient current (IAs) for type 2 cells.
Fig. 1. Distribution of K+currents observed in two major classes of dorsal root ganglion neurons. Representative K+currents from a type 1 cell and a type 2 cell are shown in panels A 
	and B 
	, respectively. The total current is measured during a voltage step to +30 mV after a conditioning prepulse to −100 mV for 1 s. Both types of cells express IKn, the noninactivating current remaining after a conditioning prepulse to −30 mV, although the magnitude of this current was much greater in the type 1 cell. The current components that undergo steady-state inactivation are obtained by subtracting the current measured after the prepulse to −30 mV (IKn) from the current measured after the prepulse to −100 mV (total current). These subtracted currents are the fast-inactivating transient current (IAf) for type 1 cells and the slow-inactivating transient current (IAs) for type 2 cells.
Fig. 1. Distribution of K+currents observed in two major classes of dorsal root ganglion neurons. Representative K+currents from a type 1 cell and a type 2 cell are shown in panels A  and B  , respectively. The total current is measured during a voltage step to +30 mV after a conditioning prepulse to −100 mV for 1 s. Both types of cells express IKn, the noninactivating current remaining after a conditioning prepulse to −30 mV, although the magnitude of this current was much greater in the type 1 cell. The current components that undergo steady-state inactivation are obtained by subtracting the current measured after the prepulse to −30 mV (IKn) from the current measured after the prepulse to −100 mV (total current). These subtracted currents are the fast-inactivating transient current (IAf) for type 1 cells and the slow-inactivating transient current (IAs) for type 2 cells.
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Effects of Local Anesthetics on IAf
Figure 2shows the effects of local anesthetics on IAf. Bupivacaine and lidocaine but not tetracaine reduced the amplitude of IAf. Higher concentrations of tetracaine were not tested because of the nonspecific cytotoxicity 8 of this local anesthetic. Bupivacaine also increased the rate of inactivation of IAf(figs. 2A and 3A). In contrast to bupivacaine, lidocaine and tetracaine did not decrease the inactivation time constant of IAf. In some cells, lidocaine and tetracaine slowed the inactivation rate of IAf(figs. 2B and C). This effect showed statistical significance at high concentrations of lidocaine but not at any concentration of tetracaine tested (figs. 3B and C).
Fig. 2. Representative traces from three cells illustrating the effects of local anesthetics on the fast-inactivating transient current. Effects of bupivacaine (BUP, 100 μm, A  ), lidocaine (LID, 5 mm, B  ), and tetracaine (TET, 1.5 mm, C  ). Only bupivacaine and lidocaine reduced the amplitude of the fast-inactivating transient current.
Fig. 2. Representative traces from three cells illustrating the effects of local anesthetics on the fast-inactivating transient current. Effects of bupivacaine (BUP, 100 μm, A 
	), lidocaine (LID, 5 mm, B 
	), and tetracaine (TET, 1.5 mm, C 
	). Only bupivacaine and lidocaine reduced the amplitude of the fast-inactivating transient current.
Fig. 2. Representative traces from three cells illustrating the effects of local anesthetics on the fast-inactivating transient current. Effects of bupivacaine (BUP, 100 μm, A  ), lidocaine (LID, 5 mm, B  ), and tetracaine (TET, 1.5 mm, C  ). Only bupivacaine and lidocaine reduced the amplitude of the fast-inactivating transient current.
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Fig. 3. Effects of local anesthetics on the inactivation time constant (τ) of the fast-inactivating transient current. Mean ± SD are shown. Effects of bupivacaine (n = 12, A  ), lidocaine (n = 7, B  ), and tetracaine (n = 5, C  ). For some data points, the error bars are too small to be seen. Control values of inactivation time constant were as follows: bupivacaine series, 14 ± 10 ms; lidocaine series, 11 ± 6 ms; and tetracaine series, 12 ± 7 ms. * P  < 0.05 compared with the corresponding control value.
Fig. 3. Effects of local anesthetics on the inactivation time constant (τ) of the fast-inactivating transient current. Mean ± SD are shown. Effects of bupivacaine (n = 12, A 
	), lidocaine (n = 7, B 
	), and tetracaine (n = 5, C 
	). For some data points, the error bars are too small to be seen. Control values of inactivation time constant were as follows: bupivacaine series, 14 ± 10 ms; lidocaine series, 11 ± 6 ms; and tetracaine series, 12 ± 7 ms. * P 
	< 0.05 compared with the corresponding control value.
Fig. 3. Effects of local anesthetics on the inactivation time constant (τ) of the fast-inactivating transient current. Mean ± SD are shown. Effects of bupivacaine (n = 12, A  ), lidocaine (n = 7, B  ), and tetracaine (n = 5, C  ). For some data points, the error bars are too small to be seen. Control values of inactivation time constant were as follows: bupivacaine series, 14 ± 10 ms; lidocaine series, 11 ± 6 ms; and tetracaine series, 12 ± 7 ms. * P  < 0.05 compared with the corresponding control value.
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Effects of Local Anesthetics on IAs
Figure 4shows the effects of local anesthetics on IAs. All three local anesthetics that were tested decreased the amplitude of IAs. In the presence of bupivacaine, the time course of decay consisted of an initial rapid phase followed by a slow phase (fig. 4A). Lidocaine and tetracaine did not induce the pronounced rapid phase of inactivation observed with bupivacaine (figs. 4B and C). With bupivacaine present, the decay could be described by the sum of two exponentials, with the time constant of the fast phase an order of magnitude smaller than the time constant for the slow component (fig. 5A). As the bupivacaine concentration was increased, the magnitude of the fast component of decay increased, with the half-maximal effect at the bupivacaine concentration of 41 μm (fig. 5B). From the plot of the reciprocal of τBagainst bupivacaine concentration (fig. 5C), we obtained k = 0.89 × 106m−1· s1, l = 58 s1, and Kd = 65 μm.
Fig. 4. Representative traces from three cells illustrating the effects of local anesthetics on the slow-inactivating transient current. Effects of bupivacaine (BUP, A  ), lidocaine (LID, B  ), and tetracaine (TET, C  ). All three local anesthetics decrease the slow-inactivating transient current in a concentration-dependent manner. In the presence of bupivacaine, the decay was well-described by a biexponential equation as described in the Methods.
Fig. 4. Representative traces from three cells illustrating the effects of local anesthetics on the slow-inactivating transient current. Effects of bupivacaine (BUP, A 
	), lidocaine (LID, B 
	), and tetracaine (TET, C 
	). All three local anesthetics decrease the slow-inactivating transient current in a concentration-dependent manner. In the presence of bupivacaine, the decay was well-described by a biexponential equation as described in the Methods.
Fig. 4. Representative traces from three cells illustrating the effects of local anesthetics on the slow-inactivating transient current. Effects of bupivacaine (BUP, A  ), lidocaine (LID, B  ), and tetracaine (TET, C  ). All three local anesthetics decrease the slow-inactivating transient current in a concentration-dependent manner. In the presence of bupivacaine, the decay was well-described by a biexponential equation as described in the Methods.
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Fig. 5. Effects of bupivacaine (BUP) on the inactivation of the slow-inactivating transient current. All values are mean ± SD (n = 6). (A  ) Time constants of the decay of the slow-inactivating transient current obtained from either a single exponential fit (in control conditions only) or a biexponential fit of the current measured in different concentrations of bupivacaine. Note that the ordinate is logarithmic. For some data points, the error bars are too small to be seen. (B  ) Bupivacaine increased the relative proportion of the current displaying rapid inactivation. The curve describes the fit to the Hill equation with an IC50of 41 μm. (C  ) The time constants of the rapid decay phase shown in A  are plotted as the reciprocal of τBagainst bupivacaine concentration and fit with a straight line to obtain an estimate of binding parameters as described in the Methods. The equation for the line and the correlation coefficient (r2) are shown. The estimate for the Kd of bupivacaine obtained from these data was 65 μm.
Fig. 5. Effects of bupivacaine (BUP) on the inactivation of the slow-inactivating transient current. All values are mean ± SD (n = 6). (A 
	) Time constants of the decay of the slow-inactivating transient current obtained from either a single exponential fit (in control conditions only) or a biexponential fit of the current measured in different concentrations of bupivacaine. Note that the ordinate is logarithmic. For some data points, the error bars are too small to be seen. (B 
	) Bupivacaine increased the relative proportion of the current displaying rapid inactivation. The curve describes the fit to the Hill equation with an IC50of 41 μm. (C 
	) The time constants of the rapid decay phase shown in A 
	are plotted as the reciprocal of τBagainst bupivacaine concentration and fit with a straight line to obtain an estimate of binding parameters as described in the Methods. The equation for the line and the correlation coefficient (r2) are shown. The estimate for the Kd of bupivacaine obtained from these data was 65 μm.
Fig. 5. Effects of bupivacaine (BUP) on the inactivation of the slow-inactivating transient current. All values are mean ± SD (n = 6). (A  ) Time constants of the decay of the slow-inactivating transient current obtained from either a single exponential fit (in control conditions only) or a biexponential fit of the current measured in different concentrations of bupivacaine. Note that the ordinate is logarithmic. For some data points, the error bars are too small to be seen. (B  ) Bupivacaine increased the relative proportion of the current displaying rapid inactivation. The curve describes the fit to the Hill equation with an IC50of 41 μm. (C  ) The time constants of the rapid decay phase shown in A  are plotted as the reciprocal of τBagainst bupivacaine concentration and fit with a straight line to obtain an estimate of binding parameters as described in the Methods. The equation for the line and the correlation coefficient (r2) are shown. The estimate for the Kd of bupivacaine obtained from these data was 65 μm.
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Concentration-dependent Effects of Local Anesthetics on Amplitudes of IAfand IAs
Figure 6shows the concentration–response curves for the effects of the three local anesthetics on the current amplitudes of IAfand IAs. The values for the IC50and Hill coefficients are summarized in table 1. Tetracaine, in the range of concentration tested, did not significantly alter the amplitude of IAf(fig. 6A). Bupivacaine had a small effect on the amplitude of IAf, and the IC50value was estimated by assuming the data represent a portion of the dose–response relation that can be described by a Hill equation. The ratio of IC50values indicates that bupivacaine was 30 times more potent than lidocaine in its effects on IAf(table 1). All three local anesthetics also reduced the magnitude of IAs(fig. 6B), with bupivacaine being 20 times more potent than lidocaine and tetracaine being 2 times more potent than lidocaine (table 1).
Fig. 6. Concentration–response curves for the effects of bupivacaine (BUP), tetracaine (TET), and lidocaine (LID) on the amplitudes of two different types of transient K+currents in dorsal root ganglion neurons. All values are mean ± SD. * P  < 0.05 compared with control (no local anesthetic). Solid lines through the data points represent the fits to the Hill equation where appropriate. Parameters for the fits are given in table 1. (A  ) Effects of local anesthetics on the fast-inactivating transient current (IAf). Control amplitudes were 3.07 ± 1.64 nA for bupivacaine (n = 12), 2.26 ± 0.96 nA for tetracaine (n = 5), and 2.34 ± 0.73 nA for lidocaine (n = 7). (B  ) Effects of local anesthetics on the slow-inactivating transient current (IAs). Control amplitudes were 9.09 ± 2.41 nA for bupivacaine (n = 6), 9.24 ± 3.08 nA for tetracaine (n = 7), and 7.16 ± 4.41 nA for lidocaine (n = 7).
Fig. 6. Concentration–response curves for the effects of bupivacaine (BUP), tetracaine (TET), and lidocaine (LID) on the amplitudes of two different types of transient K+currents in dorsal root ganglion neurons. All values are mean ± SD. * P 
	< 0.05 compared with control (no local anesthetic). Solid lines through the data points represent the fits to the Hill equation where appropriate. Parameters for the fits are given in table 1. (A 
	) Effects of local anesthetics on the fast-inactivating transient current (IAf). Control amplitudes were 3.07 ± 1.64 nA for bupivacaine (n = 12), 2.26 ± 0.96 nA for tetracaine (n = 5), and 2.34 ± 0.73 nA for lidocaine (n = 7). (B 
	) Effects of local anesthetics on the slow-inactivating transient current (IAs). Control amplitudes were 9.09 ± 2.41 nA for bupivacaine (n = 6), 9.24 ± 3.08 nA for tetracaine (n = 7), and 7.16 ± 4.41 nA for lidocaine (n = 7).
Fig. 6. Concentration–response curves for the effects of bupivacaine (BUP), tetracaine (TET), and lidocaine (LID) on the amplitudes of two different types of transient K+currents in dorsal root ganglion neurons. All values are mean ± SD. * P  < 0.05 compared with control (no local anesthetic). Solid lines through the data points represent the fits to the Hill equation where appropriate. Parameters for the fits are given in table 1. (A  ) Effects of local anesthetics on the fast-inactivating transient current (IAf). Control amplitudes were 3.07 ± 1.64 nA for bupivacaine (n = 12), 2.26 ± 0.96 nA for tetracaine (n = 5), and 2.34 ± 0.73 nA for lidocaine (n = 7). (B  ) Effects of local anesthetics on the slow-inactivating transient current (IAs). Control amplitudes were 9.09 ± 2.41 nA for bupivacaine (n = 6), 9.24 ± 3.08 nA for tetracaine (n = 7), and 7.16 ± 4.41 nA for lidocaine (n = 7).
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Table 1. Effects of Local Anesthetics on IAfand IAs
Image not available
Table 1. Effects of Local Anesthetics on IAfand IAs
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Effects of Local Anesthetics on IKn
The magnitude of IKnin type 1 cells under control conditions (10.2 ± 4.3 nA, n = 30) was significantly greater than that of IKnfrom type 2 cells (5.1 ± 1.4 nA, n = 19;P  < 0.05). Figure 7shows that lidocaine more potently inhibited the IKncurrent of type 1 cells than that of type 2 cells. This preferential inhibition was found with all three local anesthetics (fig. 8). Table 2shows the values for IC50and Hill coefficients that describe the inhibition of IKnin type 1 and type 2 cells for all three local anesthetics. For IKnof type 1 cells, bupivacaine was 42 times more potent than lidocaine, and tetracaine was 3 times more potent than lidocaine. For IKnof type 2 cells, bupivacaine was 39 times more potent than lidocaine, and tetracaine was 4 times more potent than lidocaine.
Fig. 7. Effects of lidocaine (LID) on the noninactivating sustained current of type 1 cell and type 2 cell. (A  ) Type 1 cell with diameter 35 μm. (B  ) Type 2 cell with diameter 28 μm.
Fig. 7. Effects of lidocaine (LID) on the noninactivating sustained current of type 1 cell and type 2 cell. (A 
	) Type 1 cell with diameter 35 μm. (B 
	) Type 2 cell with diameter 28 μm.
Fig. 7. Effects of lidocaine (LID) on the noninactivating sustained current of type 1 cell and type 2 cell. (A  ) Type 1 cell with diameter 35 μm. (B  ) Type 2 cell with diameter 28 μm.
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Fig. 8. Concentration-response curves for the effects of local anesthetics on the amplitudes of the noninactivating sustained current (IKn) in type 1 cells (open symbols) and in type 2 cells (closed symbols). All values are mean ± SD. * P  < 0.05 compared with control (no local anesthetic). The lines through the data points show the fits to the Hill equation. Parameters for the fits are given in table 2. (A  ) Effect of bupivacaine. Control amplitudes were 11.07 ± 5.17 nA for type 1 cells (n = 12) and 4.07 ± 0.76 nA for type 2 cells (n = 7). (B  ) Effect of lidocaine. Control amplitudes were 10.83 ± 3.74 nA for type 1 cells (n = 7) and 5.71 ± 1.32 nA for type 2 cells (n = 7). (C  ) Effect of tetracaine. Control amplitudes were 8.97 ± 3.71 nA for type 1 cells (n = 11) and 5.56 ± 1.06 nA for type 2 cells (n = 5). IKnrecorded from type 2 cells was more sensitive to all three local anesthetics than was IKnrecorded from type 1 cells.
Fig. 8. Concentration-response curves for the effects of local anesthetics on the amplitudes of the noninactivating sustained current (IKn) in type 1 cells (open symbols) and in type 2 cells (closed symbols). All values are mean ± SD. * P 
	< 0.05 compared with control (no local anesthetic). The lines through the data points show the fits to the Hill equation. Parameters for the fits are given in table 2. (A 
	) Effect of bupivacaine. Control amplitudes were 11.07 ± 5.17 nA for type 1 cells (n = 12) and 4.07 ± 0.76 nA for type 2 cells (n = 7). (B 
	) Effect of lidocaine. Control amplitudes were 10.83 ± 3.74 nA for type 1 cells (n = 7) and 5.71 ± 1.32 nA for type 2 cells (n = 7). (C 
	) Effect of tetracaine. Control amplitudes were 8.97 ± 3.71 nA for type 1 cells (n = 11) and 5.56 ± 1.06 nA for type 2 cells (n = 5). IKnrecorded from type 2 cells was more sensitive to all three local anesthetics than was IKnrecorded from type 1 cells.
Fig. 8. Concentration-response curves for the effects of local anesthetics on the amplitudes of the noninactivating sustained current (IKn) in type 1 cells (open symbols) and in type 2 cells (closed symbols). All values are mean ± SD. * P  < 0.05 compared with control (no local anesthetic). The lines through the data points show the fits to the Hill equation. Parameters for the fits are given in table 2. (A  ) Effect of bupivacaine. Control amplitudes were 11.07 ± 5.17 nA for type 1 cells (n = 12) and 4.07 ± 0.76 nA for type 2 cells (n = 7). (B  ) Effect of lidocaine. Control amplitudes were 10.83 ± 3.74 nA for type 1 cells (n = 7) and 5.71 ± 1.32 nA for type 2 cells (n = 7). (C  ) Effect of tetracaine. Control amplitudes were 8.97 ± 3.71 nA for type 1 cells (n = 11) and 5.56 ± 1.06 nA for type 2 cells (n = 5). IKnrecorded from type 2 cells was more sensitive to all three local anesthetics than was IKnrecorded from type 1 cells.
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Table 2. IC50and Hill Coefficient for the Effects of Local Anesthetics on IKnof Type 1 and Type 2 Cells
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Table 2. IC50and Hill Coefficient for the Effects of Local Anesthetics on IKnof Type 1 and Type 2 Cells
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Discussion
Of the three voltage-activated K+currents we studied, IAfwas least sensitive to the inhibitory action of local anesthetic. Tetracaine, up to 1.5 mm, did not inhibit IAf. With bupivacaine and lidocaine, the values of IC50for IAfwere about 5 times higher than IC50for the most sensitive currents, IAsand IKnof type 2 cells (tables 1 and 2). In this regard, the effects of local anesthetics we observed resemble those of tetraethyl ammonium ion. 9 The effects we observed with DRG cells contrast with the selective inhibition of a transient current reported in cells of the spinal dorsal horn. 3 Although concentrations of lidocaine higher than 1 mm were required to inhibit IKncurrent of type 1 cells (fig. 8), it should be noted that 1% lidocaine (molecular weight of 271 for lidocaine hydrochloride) corresponds to 37 mm. One possibility that can account for the Hill coefficient of approximately 2 observed for the effects of lidocaine and tetracaine on the transient currents (table 1) is that binding of more than one molecule of these local anesthetic is involved in reduction of the current amplitude.
In addition to the relative insensitivity of IAfto the action of local anesthetics, the results indicated a preferential inhibition of K+currents expressed in type 2 cells. This is well-demonstrated for IKn, which was expressed in both type 1 and type 2 cells (table 2). The IC50values for local anesthetic inhibition of IKnin type 2 cells were approximately half of those for inhibition of IKnin type 1 cells. In fact, both of the K+currents expressed by type 2 cells (IAsand IKn) were generally more sensitive to local anesthetics than the two currents expressed by type 1 cells (IAfand IKn). Because type 2 cells are smaller and express IAs, similar to a subpopulation of DRG cells reported by Gold et al.  6 that also respond to capsaicin, these cells may represent C-fiber-type nociceptive neurons. Therefore, the preferential inhibition of K+currents in type 2 cells may reflect selective potentiation of local anesthetic-induced impulse inhibition in nociceptive nerves. This is because the K+channel blocker, tetraethyl ammonium ion, is known to increase the lidocaine inhibition of compound action potentials of sciatic nerve. 10 
The observed effects of three local anesthetics on noninactivating sustained K+current (IKn) were similar to those reported for patches of amphibian sciatic nerve. 5 Therefore, bupivacaine was much more potent than lidocaine, and tetracaine, despite similar lipid solubility, 11 was less effective than bupivacaine. In dorsal horn, on the other hand, Olschewski et al.  3 found that sustained K+currents were more resistant to local anesthetics than were transient K+currents. Whether selective inhibition of presynaptic sustained K+currents and postsynaptic transient K+currents by local anesthetics influences their ability to alter sensory nerve transmission has not been tested.
We have studied the effects of local anesthetics on three different types of K+currents in DRG neurons using a simple inactivation protocol based on the method used by Gold et al.  6 However, more than three types of K+currents are thought to exist in DRG neurons. Gold et al.  , 6 using biophysical parameters as well as specific K+channel blockers, identified a total of six different types of voltage-activated K+currents. 6 Using immunocytochemical methods, Ishikawa et al.  12 showed as many as seven voltage-activated K+channels in DRG (Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5, Kv1.6, Kv2.1). The three currents we studied likely correspond to three of the currents described by Gold et al.  6 Although we observed two of the other three currents described by these authors (IAhtand IKi), we did not report the effects of local anesthetics on these currents because of the small number of observations (IAht) or because of our inability to isolate the current (IKi) adequately. Therefore, it is possible that the K+currents we recorded were contaminated by currents through other K+channels that we could not distinguish using our protocol. Similarly, the current we identified as IKnin type 1 and type 2 cells actually may be two different currents.
The effects of bupivacaine on the time course of inactivation we observed with IAfresembles those reported for Ito13 and for rKv1.4 and rKv4.3. 14 It is possible that bupivacaine block contributes to the faster inactivation in the presence of this local anesthetic. As a possible mechanism for the pronounced decrease in the rate of inactivation of IAfobserved in the presence of lidocaine, we speculate that this local anesthetic may interfere with the endogenous fast-inactivation mechanism. The effect of bupivacaine on the time course of the decrease in current amplitude of IAswas similar to the effect of this local anesthetic on hKv1.5. 7 The values of Kd (41 and 65 μm) obtained by two different methods for racemic bupivacaine in this study, however, were higher than the values reported for S  (−)-bupivacaine (24.9 μm) and for R  (+)-bupivacaine (5.0 μm) with hKv1.5 expressed in mouse Ltk  -cell line. 7 The initial fast phase of inactivation (blocking) is also reported for bupivacaine action on rat brain Kv1.1, which does not inactivate under control conditions, and is considered to reflect slow open channel block. 15 The lack of the fast decrease of current observed with lidocaine was attributed to fast open channel block, so that the time course of block could not be measured. 15 It is possible that the minimal effect of lidocaine and tetracaine on the time course of inactivation of IAsin the current study also may reflect fast open channel block.
In summary, the greater sensitivity of IKnfrom small type 2 cells suggests that local anesthetics may preferentially inhibit impulses in primary nociceptive neurons. Furthermore, all the local anesthetics used in this study inhibited the sustained K+current (IKn) of DRG neurons, an effect different from that reported in dorsal horn neurons by Olschewski et al.  3 It is likely that the overall modulatory effects of local anesthetics at the level of the DRG–dorsal horn synapse are influenced by their differential effects on presynaptic (DRG) and postsynaptic (dorsal horn) K+currents.
The authors thank Craig V. Levenick, B.S., Department of Anesthesiology, University of Wisconsin, Madison, Wisconsin, for his excellent technical assistance.
References
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Gold MS, Reichling DB, Hampl KF, Drasner K, Levine JD: Lidocaine toxicity in primary afferent neurons from the rat. J Pharmacol Exp Ther 1998; 285: 413–21Gold, MS Reichling, DB Hampl, KF Drasner, K Levine, JD
Olschewski A, Hempelmann G, Vogel W, Safronov BV: Blockade of Na+and K+currents by local anesthetics in the dorsal horn neurons of the spinal cord. A nesthesiology 1998; 88: 172–9Olschewski, A Hempelmann, G Vogel, W Safronov, BV
Guo X, Castle NA, Chernoff DM, Strichartz GR: Comparative inhibition of voltage-gated cation channels by local anesthetics. Ann N Y Acad Sci 1991; 625: 181–99Guo, X Castle, NA Chernoff, DM Strichartz, GR
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Weinberg G, VadeBoncouer T, Wang X, Palmer JW: A cell culture model of local anesthetic neurotoxicity (abstract). A nesthesiology 1998; 89: A1412Weinberg, G VadeBoncouer, T Wang, X Palmer, JW
Safronov BV, Bischoff U, Vogel W: Single voltage-gated K+channels and their functions in small dorsal root ganglion neurons of rat. J Physiol (Lond) 1996; 493.2: 393–408Safronov, BV Bischoff, U Vogel, W
Drachman D, Strichartz G: Potassium channel blockers potentiate impulse inhibition by local anesthetics. A nesthesiology 1991; 75: 1051–61Drachman, D Strichartz, G
Strichartz GR, Sanchez V, Arthur GR, Chafetz R, Martin D: Fundamental properties of local anesthetics: II. Measured octanol:buffer partition coefficients and pKavalues of clinically used drugs. Anesth Analg 1990; 71: 158–70Strichartz, GR Sanchez, V Arthur, GR Chafetz, R Martin, D
Ishikawa K, Tanaka M, Black JA, Waxman SG: Changes in expression of voltage-gated potassium channels in dorsal root ganglion neurons following axotomy. Muscle Nerve 1999; 22: 502–7Ishikawa, K Tanaka, M Black, JA Waxman, SG
Castle NA: Bupivacaine inhibits the transient outward K+current but not the inward rectifier in rat ventricular myocytes. J Pharmacol Exp Ther 1990; 255: 1038–46Castle, NA
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Elliott AA, Harrold JA, Newman JP, Elliott JR: Open channel block and open channel destabilization: Contrasting effects of phenol, TEA+and local anaesthetics on Kv1.1 K+channels. Toxicol Lett 1998; 100–1:277–85
Fig. 1. Distribution of K+currents observed in two major classes of dorsal root ganglion neurons. Representative K+currents from a type 1 cell and a type 2 cell are shown in panels A  and B  , respectively. The total current is measured during a voltage step to +30 mV after a conditioning prepulse to −100 mV for 1 s. Both types of cells express IKn, the noninactivating current remaining after a conditioning prepulse to −30 mV, although the magnitude of this current was much greater in the type 1 cell. The current components that undergo steady-state inactivation are obtained by subtracting the current measured after the prepulse to −30 mV (IKn) from the current measured after the prepulse to −100 mV (total current). These subtracted currents are the fast-inactivating transient current (IAf) for type 1 cells and the slow-inactivating transient current (IAs) for type 2 cells.
Fig. 1. Distribution of K+currents observed in two major classes of dorsal root ganglion neurons. Representative K+currents from a type 1 cell and a type 2 cell are shown in panels A 
	and B 
	, respectively. The total current is measured during a voltage step to +30 mV after a conditioning prepulse to −100 mV for 1 s. Both types of cells express IKn, the noninactivating current remaining after a conditioning prepulse to −30 mV, although the magnitude of this current was much greater in the type 1 cell. The current components that undergo steady-state inactivation are obtained by subtracting the current measured after the prepulse to −30 mV (IKn) from the current measured after the prepulse to −100 mV (total current). These subtracted currents are the fast-inactivating transient current (IAf) for type 1 cells and the slow-inactivating transient current (IAs) for type 2 cells.
Fig. 1. Distribution of K+currents observed in two major classes of dorsal root ganglion neurons. Representative K+currents from a type 1 cell and a type 2 cell are shown in panels A  and B  , respectively. The total current is measured during a voltage step to +30 mV after a conditioning prepulse to −100 mV for 1 s. Both types of cells express IKn, the noninactivating current remaining after a conditioning prepulse to −30 mV, although the magnitude of this current was much greater in the type 1 cell. The current components that undergo steady-state inactivation are obtained by subtracting the current measured after the prepulse to −30 mV (IKn) from the current measured after the prepulse to −100 mV (total current). These subtracted currents are the fast-inactivating transient current (IAf) for type 1 cells and the slow-inactivating transient current (IAs) for type 2 cells.
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Fig. 2. Representative traces from three cells illustrating the effects of local anesthetics on the fast-inactivating transient current. Effects of bupivacaine (BUP, 100 μm, A  ), lidocaine (LID, 5 mm, B  ), and tetracaine (TET, 1.5 mm, C  ). Only bupivacaine and lidocaine reduced the amplitude of the fast-inactivating transient current.
Fig. 2. Representative traces from three cells illustrating the effects of local anesthetics on the fast-inactivating transient current. Effects of bupivacaine (BUP, 100 μm, A 
	), lidocaine (LID, 5 mm, B 
	), and tetracaine (TET, 1.5 mm, C 
	). Only bupivacaine and lidocaine reduced the amplitude of the fast-inactivating transient current.
Fig. 2. Representative traces from three cells illustrating the effects of local anesthetics on the fast-inactivating transient current. Effects of bupivacaine (BUP, 100 μm, A  ), lidocaine (LID, 5 mm, B  ), and tetracaine (TET, 1.5 mm, C  ). Only bupivacaine and lidocaine reduced the amplitude of the fast-inactivating transient current.
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Fig. 3. Effects of local anesthetics on the inactivation time constant (τ) of the fast-inactivating transient current. Mean ± SD are shown. Effects of bupivacaine (n = 12, A  ), lidocaine (n = 7, B  ), and tetracaine (n = 5, C  ). For some data points, the error bars are too small to be seen. Control values of inactivation time constant were as follows: bupivacaine series, 14 ± 10 ms; lidocaine series, 11 ± 6 ms; and tetracaine series, 12 ± 7 ms. * P  < 0.05 compared with the corresponding control value.
Fig. 3. Effects of local anesthetics on the inactivation time constant (τ) of the fast-inactivating transient current. Mean ± SD are shown. Effects of bupivacaine (n = 12, A 
	), lidocaine (n = 7, B 
	), and tetracaine (n = 5, C 
	). For some data points, the error bars are too small to be seen. Control values of inactivation time constant were as follows: bupivacaine series, 14 ± 10 ms; lidocaine series, 11 ± 6 ms; and tetracaine series, 12 ± 7 ms. * P 
	< 0.05 compared with the corresponding control value.
Fig. 3. Effects of local anesthetics on the inactivation time constant (τ) of the fast-inactivating transient current. Mean ± SD are shown. Effects of bupivacaine (n = 12, A  ), lidocaine (n = 7, B  ), and tetracaine (n = 5, C  ). For some data points, the error bars are too small to be seen. Control values of inactivation time constant were as follows: bupivacaine series, 14 ± 10 ms; lidocaine series, 11 ± 6 ms; and tetracaine series, 12 ± 7 ms. * P  < 0.05 compared with the corresponding control value.
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Fig. 4. Representative traces from three cells illustrating the effects of local anesthetics on the slow-inactivating transient current. Effects of bupivacaine (BUP, A  ), lidocaine (LID, B  ), and tetracaine (TET, C  ). All three local anesthetics decrease the slow-inactivating transient current in a concentration-dependent manner. In the presence of bupivacaine, the decay was well-described by a biexponential equation as described in the Methods.
Fig. 4. Representative traces from three cells illustrating the effects of local anesthetics on the slow-inactivating transient current. Effects of bupivacaine (BUP, A 
	), lidocaine (LID, B 
	), and tetracaine (TET, C 
	). All three local anesthetics decrease the slow-inactivating transient current in a concentration-dependent manner. In the presence of bupivacaine, the decay was well-described by a biexponential equation as described in the Methods.
Fig. 4. Representative traces from three cells illustrating the effects of local anesthetics on the slow-inactivating transient current. Effects of bupivacaine (BUP, A  ), lidocaine (LID, B  ), and tetracaine (TET, C  ). All three local anesthetics decrease the slow-inactivating transient current in a concentration-dependent manner. In the presence of bupivacaine, the decay was well-described by a biexponential equation as described in the Methods.
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Fig. 5. Effects of bupivacaine (BUP) on the inactivation of the slow-inactivating transient current. All values are mean ± SD (n = 6). (A  ) Time constants of the decay of the slow-inactivating transient current obtained from either a single exponential fit (in control conditions only) or a biexponential fit of the current measured in different concentrations of bupivacaine. Note that the ordinate is logarithmic. For some data points, the error bars are too small to be seen. (B  ) Bupivacaine increased the relative proportion of the current displaying rapid inactivation. The curve describes the fit to the Hill equation with an IC50of 41 μm. (C  ) The time constants of the rapid decay phase shown in A  are plotted as the reciprocal of τBagainst bupivacaine concentration and fit with a straight line to obtain an estimate of binding parameters as described in the Methods. The equation for the line and the correlation coefficient (r2) are shown. The estimate for the Kd of bupivacaine obtained from these data was 65 μm.
Fig. 5. Effects of bupivacaine (BUP) on the inactivation of the slow-inactivating transient current. All values are mean ± SD (n = 6). (A 
	) Time constants of the decay of the slow-inactivating transient current obtained from either a single exponential fit (in control conditions only) or a biexponential fit of the current measured in different concentrations of bupivacaine. Note that the ordinate is logarithmic. For some data points, the error bars are too small to be seen. (B 
	) Bupivacaine increased the relative proportion of the current displaying rapid inactivation. The curve describes the fit to the Hill equation with an IC50of 41 μm. (C 
	) The time constants of the rapid decay phase shown in A 
	are plotted as the reciprocal of τBagainst bupivacaine concentration and fit with a straight line to obtain an estimate of binding parameters as described in the Methods. The equation for the line and the correlation coefficient (r2) are shown. The estimate for the Kd of bupivacaine obtained from these data was 65 μm.
Fig. 5. Effects of bupivacaine (BUP) on the inactivation of the slow-inactivating transient current. All values are mean ± SD (n = 6). (A  ) Time constants of the decay of the slow-inactivating transient current obtained from either a single exponential fit (in control conditions only) or a biexponential fit of the current measured in different concentrations of bupivacaine. Note that the ordinate is logarithmic. For some data points, the error bars are too small to be seen. (B  ) Bupivacaine increased the relative proportion of the current displaying rapid inactivation. The curve describes the fit to the Hill equation with an IC50of 41 μm. (C  ) The time constants of the rapid decay phase shown in A  are plotted as the reciprocal of τBagainst bupivacaine concentration and fit with a straight line to obtain an estimate of binding parameters as described in the Methods. The equation for the line and the correlation coefficient (r2) are shown. The estimate for the Kd of bupivacaine obtained from these data was 65 μm.
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Fig. 6. Concentration–response curves for the effects of bupivacaine (BUP), tetracaine (TET), and lidocaine (LID) on the amplitudes of two different types of transient K+currents in dorsal root ganglion neurons. All values are mean ± SD. * P  < 0.05 compared with control (no local anesthetic). Solid lines through the data points represent the fits to the Hill equation where appropriate. Parameters for the fits are given in table 1. (A  ) Effects of local anesthetics on the fast-inactivating transient current (IAf). Control amplitudes were 3.07 ± 1.64 nA for bupivacaine (n = 12), 2.26 ± 0.96 nA for tetracaine (n = 5), and 2.34 ± 0.73 nA for lidocaine (n = 7). (B  ) Effects of local anesthetics on the slow-inactivating transient current (IAs). Control amplitudes were 9.09 ± 2.41 nA for bupivacaine (n = 6), 9.24 ± 3.08 nA for tetracaine (n = 7), and 7.16 ± 4.41 nA for lidocaine (n = 7).
Fig. 6. Concentration–response curves for the effects of bupivacaine (BUP), tetracaine (TET), and lidocaine (LID) on the amplitudes of two different types of transient K+currents in dorsal root ganglion neurons. All values are mean ± SD. * P 
	< 0.05 compared with control (no local anesthetic). Solid lines through the data points represent the fits to the Hill equation where appropriate. Parameters for the fits are given in table 1. (A 
	) Effects of local anesthetics on the fast-inactivating transient current (IAf). Control amplitudes were 3.07 ± 1.64 nA for bupivacaine (n = 12), 2.26 ± 0.96 nA for tetracaine (n = 5), and 2.34 ± 0.73 nA for lidocaine (n = 7). (B 
	) Effects of local anesthetics on the slow-inactivating transient current (IAs). Control amplitudes were 9.09 ± 2.41 nA for bupivacaine (n = 6), 9.24 ± 3.08 nA for tetracaine (n = 7), and 7.16 ± 4.41 nA for lidocaine (n = 7).
Fig. 6. Concentration–response curves for the effects of bupivacaine (BUP), tetracaine (TET), and lidocaine (LID) on the amplitudes of two different types of transient K+currents in dorsal root ganglion neurons. All values are mean ± SD. * P  < 0.05 compared with control (no local anesthetic). Solid lines through the data points represent the fits to the Hill equation where appropriate. Parameters for the fits are given in table 1. (A  ) Effects of local anesthetics on the fast-inactivating transient current (IAf). Control amplitudes were 3.07 ± 1.64 nA for bupivacaine (n = 12), 2.26 ± 0.96 nA for tetracaine (n = 5), and 2.34 ± 0.73 nA for lidocaine (n = 7). (B  ) Effects of local anesthetics on the slow-inactivating transient current (IAs). Control amplitudes were 9.09 ± 2.41 nA for bupivacaine (n = 6), 9.24 ± 3.08 nA for tetracaine (n = 7), and 7.16 ± 4.41 nA for lidocaine (n = 7).
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Fig. 7. Effects of lidocaine (LID) on the noninactivating sustained current of type 1 cell and type 2 cell. (A  ) Type 1 cell with diameter 35 μm. (B  ) Type 2 cell with diameter 28 μm.
Fig. 7. Effects of lidocaine (LID) on the noninactivating sustained current of type 1 cell and type 2 cell. (A 
	) Type 1 cell with diameter 35 μm. (B 
	) Type 2 cell with diameter 28 μm.
Fig. 7. Effects of lidocaine (LID) on the noninactivating sustained current of type 1 cell and type 2 cell. (A  ) Type 1 cell with diameter 35 μm. (B  ) Type 2 cell with diameter 28 μm.
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Fig. 8. Concentration-response curves for the effects of local anesthetics on the amplitudes of the noninactivating sustained current (IKn) in type 1 cells (open symbols) and in type 2 cells (closed symbols). All values are mean ± SD. * P  < 0.05 compared with control (no local anesthetic). The lines through the data points show the fits to the Hill equation. Parameters for the fits are given in table 2. (A  ) Effect of bupivacaine. Control amplitudes were 11.07 ± 5.17 nA for type 1 cells (n = 12) and 4.07 ± 0.76 nA for type 2 cells (n = 7). (B  ) Effect of lidocaine. Control amplitudes were 10.83 ± 3.74 nA for type 1 cells (n = 7) and 5.71 ± 1.32 nA for type 2 cells (n = 7). (C  ) Effect of tetracaine. Control amplitudes were 8.97 ± 3.71 nA for type 1 cells (n = 11) and 5.56 ± 1.06 nA for type 2 cells (n = 5). IKnrecorded from type 2 cells was more sensitive to all three local anesthetics than was IKnrecorded from type 1 cells.
Fig. 8. Concentration-response curves for the effects of local anesthetics on the amplitudes of the noninactivating sustained current (IKn) in type 1 cells (open symbols) and in type 2 cells (closed symbols). All values are mean ± SD. * P 
	< 0.05 compared with control (no local anesthetic). The lines through the data points show the fits to the Hill equation. Parameters for the fits are given in table 2. (A 
	) Effect of bupivacaine. Control amplitudes were 11.07 ± 5.17 nA for type 1 cells (n = 12) and 4.07 ± 0.76 nA for type 2 cells (n = 7). (B 
	) Effect of lidocaine. Control amplitudes were 10.83 ± 3.74 nA for type 1 cells (n = 7) and 5.71 ± 1.32 nA for type 2 cells (n = 7). (C 
	) Effect of tetracaine. Control amplitudes were 8.97 ± 3.71 nA for type 1 cells (n = 11) and 5.56 ± 1.06 nA for type 2 cells (n = 5). IKnrecorded from type 2 cells was more sensitive to all three local anesthetics than was IKnrecorded from type 1 cells.
Fig. 8. Concentration-response curves for the effects of local anesthetics on the amplitudes of the noninactivating sustained current (IKn) in type 1 cells (open symbols) and in type 2 cells (closed symbols). All values are mean ± SD. * P  < 0.05 compared with control (no local anesthetic). The lines through the data points show the fits to the Hill equation. Parameters for the fits are given in table 2. (A  ) Effect of bupivacaine. Control amplitudes were 11.07 ± 5.17 nA for type 1 cells (n = 12) and 4.07 ± 0.76 nA for type 2 cells (n = 7). (B  ) Effect of lidocaine. Control amplitudes were 10.83 ± 3.74 nA for type 1 cells (n = 7) and 5.71 ± 1.32 nA for type 2 cells (n = 7). (C  ) Effect of tetracaine. Control amplitudes were 8.97 ± 3.71 nA for type 1 cells (n = 11) and 5.56 ± 1.06 nA for type 2 cells (n = 5). IKnrecorded from type 2 cells was more sensitive to all three local anesthetics than was IKnrecorded from type 1 cells.
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Table 1. Effects of Local Anesthetics on IAfand IAs
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Table 1. Effects of Local Anesthetics on IAfand IAs
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Table 2. IC50and Hill Coefficient for the Effects of Local Anesthetics on IKnof Type 1 and Type 2 Cells
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Table 2. IC50and Hill Coefficient for the Effects of Local Anesthetics on IKnof Type 1 and Type 2 Cells
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