Electrophysiological properties of sodium current subtypes in small cells from adult rat dorsal root ganglia

1. Whole-cell and single-channel Na+ currents were recorded from small (ca. 20 micron diameter) cells isolated from adult rat dorsal root ganglia (DRG). Currents were classified by their sensitivity to 0.3 microM tetrodotoxin (TTX), electrophysiological properties and single-channel amplitude. Cells were classified according to the types of current recorded from them. 2. Type A cells expressed essentially pure TTX-sensitive (TTX-S) currents. Availability experiments with prepulse durations between 50 ms and 1 s gave a half-available voltage (Vh) of around -65 mV but the availability curves often had a complex shape, consistent with multiple inactivation processes. Measured inactivation time constants ranged from less than 1 ms to over 100 s, depending on the protocol used. 3. Cell types B and C each had, in addition to TTX-S currents, substantial and different TTX-resistant (TTX-R) currents that we have designated TTX-R1 and TTX-R2, respectively. TTX-R1 currents had a 1 s Vh of -29 mV, showed little 1 Hz use dependence at -67 mV and recovered from the inactivation induced by a 60 ms depolarizing pulse with time constants of 1.6 ms (91 %) and 908 ms. They also exhibited slow inactivation processes with component time constants around 10 and 100 s. TTX-R2 currents activated and inactivated at more negative potentials (1 s Vh = -46 mV), showed substantial 1 Hz use dependence and had inactivation (60 ms pulse) recovery time constants at -67 mV of 3.3 ms (58 %) and 902 ms. 4. Type D cells had little or no current in 0.3 microM TTX at a holding potential of -67 mV. Current amplitude increased on changing the holding potential to -107 mV. Type D cell currents had more hyperpolarized availability and I-V curves than even TTX-R2 currents and suggest the existence of TTX-R3 channels. 5. In outside-out patches with 250 mM external NaCl, the single-channel conductance (gamma) of TTX-S channels was 19.5 pS and the potential for half-maximal activation (Va) was -45 mV. One population of TTX-R channels had a gamma of 9.2 pS and a Va of -27 mV. A second population had a gamma of 16.5 pS and a more negative Va of -42 mV. The latter population may underlie the type D cell current. 6. Small DRG cells express multiple Na+ currents with varied time constants and voltage dependences of activation and inactivation. Nociceptive cells still fire when chronically depolarized by an increased external K+ concentration. TTX-R1 and TTX-R2 Na+ channels may support that firing, while the range of inactivation time constants described here would increase the repertoire of DRG cell burst firing behaviour generally.

Effect of drugs used for neuropathic pain management on tetrodotoxin-resistant Na(+) currents in rat sensory neurons

BACKGROUND

Tetrodotoxin-resistant Na(+) channels play an important role in generation and conduction of nociceptive discharges in peripheral endings of small-diameter axons of the peripheral nervous system. Pathophysiologically, these channels may produce ectopic discharges in damaged nociceptive fibers, leading to neuropathic pain syndromes. Systemically applied Na(+) channel--blocking drugs can alleviate pain, the mechanism of which is rather unresolved. The authors investigated the effects of some commonly used drugs, i.e., lidocaine, mexiletine, carbamazepine, amitriptyline, memantine, and gabapentin, on tetrodotoxin-resistant Na+ channels in rat dorsal root ganglia.

METHODS

Tetrodotoxin-resistant Na(+) currents were recorded in the whole-cell configuration of the patch-clamp method in enzymatically dissociated dorsal root ganglion neurons of adult rats. Half-maximal blocking concentrations were derived from concentration-inhibition curves at different holding potentials (-90, -70, and -60 mV).

RESULTS

Lidocaine, mexiletine, and amitriptyline reversibly blocked tetrodotoxin-resistant Na(+) currents in a concentration- and use-dependent manner. Block by carbamazepine and memantine was not use-dependent at 2 Hz. Gabapentin had no effect at concentrations of up to 3 mm. Depolarizing the membrane potential from -90 mV to -60 mV reduced the available Na(+) current only by 23% but increased the sensitivity of the channels to the use-dependent blockers approximately fivefold. The availability curve of the current was shifted by 5.3 mV to the left in 300 microm lidocaine.

CONCLUSIONS

Less negative membrane potential and repetitive firing have little effect on tetrodotoxin-resistant Na(+) current amplitude but increase their sensitivity to lidocaine, mexiletine, and amitriptyline so that concentrations after intravenous administration of these drugs can impair channel function. This may explain alleviation from pain by reducing firing frequency in ectopic sites without depressing central nervous or cardiac excitability.

Single-channel recording in myelinated nerve fibers reveals one type of Na channel but different K channels

Amphibian myelinated nerve fibers were treated with collagenase and protease. Axons with retraction of the myelin sheath were patch-clamped in the nodal and paranodal region. One type of Na channel was found. It has a single-channel conductance of 11 pS (15 degrees C) and is blocked by tetrodotoxin. Averaged events show the typical activation and inactivation kinetics of macroscopic Na current. Three potential-dependent K channels were identified (I, F, and S channel). The I channel, being the most frequent type, has a single-channel conductance of 23 pS (inward current, 105 mM K on both sides of the membrane), activates between -60 and -30 mV, deactivates with intermediate kinetics, and is sensitive to dendrotoxin. The F channel has a conductance of 30 pS, activates between -40 and 60 mV, and deactivates with fast kinetics. The former inactivates within tens of seconds; the latter inactivates within seconds. The third type, the S channel, has a conductance of 7 pS and deactivates slowly. All three channels can be blocked by external tetraethylammonium chloride. We suggest that these distinct K channel types form the basis for the different components of macroscopic K current described previously.

Autoimmune etiology of complex regional pain syndrome (M. Sudeck)

Sera of 12 patients with complex regional pain syndrome (CRPS) were tested for the occurrence of autoantibodies against nervous system structures. Immunohistochemistry revealed autoantibodies against autonomic nervous system structures in 5 of 12 (41.6%) of the patients. Western blot analysis showed neuronal reactivity in 11 of 12 (91.6%) patients. The authors hypothesize that CRPS can result from an autoimmune process against the sympathetic nervous system.

Ketamine impairs excitability in superficial dorsal horn neurones by blocking sodium and voltage-gated potassium currents

Ketamine shows, besides its general anaesthetic effect, a potent analgesic effect after spinal administration. We investigated the local anaesthetic-like action of ketamine and its enantiomers in Na+ and K+ channels and their functional consequences in dorsal horn neurones of laminae I-III, which are important neuronal structures for pain transmission receiving most of their primary sensory input from Adelta and C fibres. Combining the patch-clamp recordings in slice preparation with the 'entire soma isolation' method, we studied action of ketamine on Na+ and voltage-activated K+ currents. The changes in repetitive firing behaviour of tonically firing neurones were investigated in current-clamp mode after application of ketamine. Concentration-effect curves for the Na+ peak current revealed for tonic block half-maximal inhibiting concentrations (IC50) of 128 microM and 269 microM for S(+) and R(-)-ketamine, respectively, showing a weak stereoselectivity. The block of Na+ current was use-dependent. The voltage-dependent K+ current (K(DR)) was also sensitive to ketamine with IC50 values of 266 microM and 196 microM for S(+) and R(-)-ketamine, respectively. Rapidly inactivating K+ currents (K(A)) were less sensitive to ketamine. The block of K(DR) channels led to an increase in action potential duration and, as a consequence, to lowering of the discharge frequency in the neurones. We conclude that ketamine blocks Na+ and K(DR) channels in superficial dorsal horn neurones of the lumbar spinal cord at clinically relevant concentrations for local, intrathecal application. Ketamine reduces the excitability of the neurones, which may play an important role in the complex mechanism of its action during spinal anaesthesia.

Local anesthetics potently block a potential insensitive potassium channel in myelinated nerve

Effects of some local anesthetics were studied in patch clamp experiments on enzymatically demyelinated peripheral amphibian nerve fibers. Micromolar concentrations of external bupivacaine depolarized the excised membrane considerably. The flicker K+ channel was found to be the most sensitive ion channel to local anesthetics in this preparation. Half-maximum inhibiting concentrations (IC50) for extracellular application of bupivacaine, ropivacaine, etidocaine, mepivacaine, lidocaine, and QX-314 were 0.21, 4.2, 8.6, 56, 220, and > 10,000 microM, respectively. The corresponding concentration-effect curves could be fitted under the assumption of a 1:1 reaction. Application from the axoplasmic side resulted in clearly lower potencies with IC50 values of 2.1, 6.6, 16, 300, 1,200, and 1,250 microM, respectively. The log(IC50)-values of the local anesthetics linearly depended on the logarithm of their octanol:buffer distribution coefficients with two regression lines for the piperidine derivatives and the standard amino-amides indicating an inherently higher potency of the cyclic piperidine series. Amide-linked local anesthetics did not impair the amplitude of the single-channel current but prolonged the time of the channel to be in the closed state derived as time constants tau c from closed-time histograms. With etidocaine and lidocaine tau c was 133 and 7.2 ms, and proved to be independent of concentration. With the most potent bupivacaine time constants of wash in and wash out were 1.8 and 5.2 s for 600 nM bupivacaine. After lowering the extracellular pH from 7.4 to 6.6, externally applied bupivacaine showed a reduced potency, whereas at higher pH of 8.2 the block was slightly enhanced. Intracellular pH of 6.4, 7.2, 8.0 had almost no effect on internal bupivacaine block. It is concluded that local anesthetics block the flicker K+ channel by impeding its gating but not its conductance. The slow blocker bupivacaine and the fast blocker lidocaine compete for the same receptor. Lipophilic interactions are of importance for blockade but besides a hydrophobic pathway, there exists also a hydrophilic pathway to the binding site which could only be reached from the cytoplasmic side of the membrane. Under physiological conditions, blockade of the flicker K+ channel which is more sensitive to bupivacaine than the Na+ channel might lead via membrane depolarization and the resulting sodium channel inactivation to a pronounced block of conduction in thin fibers.

A K+ channel in Xenopus nerve fibres selectively blocked by bee and snake toxins: binding and voltage-clamp experiments

1. The effects of mast cell degranulating peptide (MCDP), a toxin from the honey bee, and of dendrotoxin (DTX), a toxin from the green mamba snake, were studied in voltage-clamp experiments with myelinated nerve fibres of Xenopus. 2. MCDP and DTX blocked part of the K+ current. About 20% of the K+ current, however, was resistant to the toxins even in high concentrations. In Ringer solution half-maximal block was reached with concentrations of 33 nM-MCDP and 11 nM-DTX. In high-K+ solution the potency of both toxins was lower. beta-Bungarotoxin (beta-BuTX), another snake toxin, also blocked part of the K+ current, but was less potent than MCDP and DTX. 3. Tail currents in high-K+ solution were analysed and three K+ current components were separated according to Dubois (1981 b). Both MCDP and DTX selectively blocked a fast deactivating, slowly inactivating K+ current component which steeply activates between E = -60 mV and E = -40 mV (component f1). In concentrations around 100 nM, MCDP and DTX blocked neither the slow K+ current (component s) nor the fast deactivating, rapidly inactivating K+ current which activates between E = -40 mV and E = 20 mV (component f2). Similar results could be derived from K+ outward currents in Ringer solution. In high-K+, IC50 of MCDP for component f1 was 99 nM, whereas it was 7.6 microM for f2. Corresponding values for DTX are 68 nM and 1.8 microM. 4. Binding studies with nerve fibre membranes of Xenopus reveal high-affinity binding sites for 125I-labelled DTX (KD = 22 pM in Ringer solution and 81 pM in high-K+ solution). 125I-labelled DTX can be displaced from its sites completely by unlabelled DTX, toxin I (black mamba toxin), MCDP, and partially by beta-BuTX. 5. Immunocytochemical staining demonstrates that binding sites for DTX are present in nodal and paranodal regions of the axonal membrane. 6. The axonal membrane of motor and sensory nerve fibres is equipped with three types of well-characterized K+ channels and constitutes so far the best preparation to study MCDP- and DTX-sensitive K+ channels with electrophysiological and biochemical methods.

Blocking mechanisms of ketamine and its enantiomers in enzymatically demyelinated peripheral nerve as revealed by single-channel experiments

BACKGROUND

Ketamine shows, besides its general anesthetic effect, a local anesthetic-like action that is due to blocking of peripheral nerve sodium currents. In this study, the stereoselectivity of the blocking effects of the ketamine enantiomers S(+) and R(-) was investigated in sodium and potassium channels in peripheral nerve membranes.

METHODS

Ion channel blockade of ketamine was investigated in enzymatically dissociated Xenopus sciatic nerves in multiple-channel and in single-channel outside-out patches.

RESULTS

Concentration-effect curves for the Na+ peak current revealed half-maximal inhibiting concentrations (IC50) of 347 microM and 291 microM for S(+) and R(-) ketamine, respectively. The potential-dependent K+ current was less sensitive than the Na+ current with IC50 values of 982 microM and 942 microM. The most sensitive ion channel was the flickering background K+ channel, with IC50 values of 168 microM and 146 microM for S(+) and R(-) ketamine. Competition experiments suggest one binding site at the flicker K+ channel, with specific binding affinities for each of the enantiomers. For the Na+ channel, the block was weaker in acidic (pH = 6.6) than in neutral (pH = 7.4) and basic (pH = 8.2) solutions; for the flicker K+ channel, the block was weaker in acidic and stronger in basic solutions.

CONCLUSIONS

Ketamine blockade of sodium and potassium channels in peripheral nerve membranes shows no stereoselectivity except for the flicker K+ channel, which showed a very weak stereoselectivity in favor of the R(-) form. This potential-insensitive flicker K+ channel may contribute to the resting potential. Block of this channel and subsequent depolarization of the resting membrane potential leads, besides to direct Na+ channel block, to inexcitability via Na+ channel inactivation.

A TEA-insensitive flickering potassium channel active around the resting potential in myelinated nerve

A novel potassium-selective channel which is active at membrane potentials between -100 mV and +40 mV has been identified in peripheral myelinated axons of Xenopus laevis using the patch-clamp technique. At negative potentials with 105 mM-K on both sides of the membrane, the channel at 1 kHz resolution showed a series of brief openings and closings interrupted by longer closings, resulting in a flickery bursting activity. Measurements with resolution up to 10 kHz revealed a single-channel conductance of 49 pS with 105 mM-K and 17 pS with 2.5 mM-K on the outer side of the membrane. The channel was selective for K ions over Na ions (PNa/PK = 0.033). The probability of being within a burst in outside-out patches varied from patch to patch (> 0.2, but often > 0.9), and was independent of membrane potential. Open-time histograms were satisfactorily described with a single exponential (tau o = 0.09 msec), closed times with the sum of three exponentials (tau c = 0.13, 5.9, and 36.6 msec). Sensitivity to external tetraethylammonium was comparatively low (IC50 = 19.0 mM). External Cs ions reduced the apparent unitary conductance for inward currents at Em = -90 mV (IC50 = 1.1 mM). Ba and, more potently, Zn ions lowered not only the apparent single-channel conductance but also open probability. The local anesthetic bupivacaine with high potency reduced probability of being within a burst (IC50 = 165 nM). The flickering K channel is clearly different from the other five types of K channels identified so far in the same preparation. We suggest that this channel may form the molecular basis of the resting potential in vertebrate myelinated axons.

ATP-dependent potassium channel in rat cardiomyocytes is blocked by lidocaine. Possible impact on the antiarrhythmic action of lidocaine

BACKGROUND

During myocardial ischemia, lidocaine has favorable antiarrhythmic properties. Malignant arrhythmias result from heterogeneity between ischemic and nonischemic regions in extracellular potassium concentration and action potential duration. These effects have been attributed to the activation of ATP-dependent potassium (KATP) channels. In this study, we investigated the action of lidocaine on the KATP channels to test the possible link between the antiarrhythmic properties of lidocaine and its action on the KATP channel.

METHODS AND RESULTS

The patch-clamp technique was employed on enzymatic dissociated cardiomyocytes of adult rats. Lidocaine was applied to the outer side of excised membrane patches by means of a multibarrel perfusion system. Lidocaine reversibly blocked the mean current of the KATP channels in a concentration-dependent manner (IC50 = 43 +/- 4.7 mumol/L, E = 0 mV, n = 6), while the amplitude of the single-channel current remained unchanged. The half-maximum blocking concentration corresponds to the therapeutic range for the antiarrhythmic application of a lidocaine bolus in humans.

CONCLUSIONS

The open probability but not the conductance of the KATP channel in the membrane of rat cardiomyocytes is blocked by lidocaine. This action may explain, in part, the favorable antiarrhythmic properties of lidocaine during acute myocardial ischemia.

Stereoselectivity of bupivacaine in local anesthetic-sensitive ion channels of peripheral nerve

BACKGROUND

The local anesthetic bupivacaine exists in two stereoisomeric forms, R(+)- and S(-)-bupivacaine. Because of its lower cardiac and central nervous system toxicity, attempts were made recently to introduce S(-)-bupivacaine into clinical anesthesia. We investigated stereoselective actions of R(+)-and S(-)-bupivacaine toward two local anesthetic-sensitive ion channels in peripheral nerve, the Na+ and the flicker K+ channel.

METHODS

In patch-clamp experiments on enzymatically demyelinated peripheral amphibian nerve fibers, Na+ and flicker K+ channels were investigated in outside-out patches. Half-maximum inhibiting concentrations (IC50) were determined. For the flicker K+ channel, simultaneous block by R(+)-bupivacaine and S(-)-bupivacaine was analyzed for competition and association (k1) and dissociation rate constants (k(-1)) were determined.

RESULTS

Both channels were reversibly blocked by R(+)- and S(-)-bupivacaine. The IC50 values (+/- SEM) for tonic Na+ channel block were 29+/-3 microM and 44+/-3 microM, respectively. IC50 values for flicker K+ channel block were 0.15+/-0.02 microM and 11+/-1 microM, respectively, resulting in a high stereopotency ratio (+/-) of 73. Simultaneously applied enantiomers competed for a single binding site. Rate constants k1 and k(-1) were 0.83+/-0.13x10(6) M(-1) x S(-1) and 0.13+/-0.03 s(-1), respectively, for R(+)-bupivacaine and 1.90+/-0.20x10(6) M(-1) x s(-1) and 8.3+/-1.0 s(-1), respectively, for S(-)-bupivacaine.

CONCLUSIONS

Bupivacaine block of Na+ channels shows no salient stereoselectivity. Block of flicker K+ channels has the highest stereoselectivity ratio of bupivacaine action known so far. This stereoselectivity derives predominantly from a difference in k(-1), suggesting a tight fit between R(+)-bupivacaine and the binding site. The flicker K+ channel may play an important role in yet unknown toxic mechanisms of R(+)-bupivacaine.

Incidence and risk calculation of inotropic support in patients undergoing cardiac surgery with cardiopulmonary bypass using an automated anaesthesia record-keeping system

BACKGROUND

This retrospective study analysed the effects of preoperative and intraoperative factors on the occurrence of inotropic support after cardiopulmonary bypass (CPB).

METHODS

The data sets of 1471 adult patients having received elective cardiac surgery with CPB were recorded using an online anaesthesia record-keeping system. Patients were judged to have required inotropic drug support if they had received one or a combination of the positive inotropic drugs, epinephrine, dobutamine and enoximone. The effects of age, height, weight, body mass index, gender, chronic heart failure, documented preoperative myocardial infarction, left main coronary artery disease, preoperative history of hypertension, chronic renal failure, diabetes mellitus, chronic obstructive pulmonary disease (COPD), preoperative medical treatment, type of surgical procedure, duration of CPB, duration of aortic clamping and reperfusion time were analysed by logistic regression for predictive power of the need for positive inotropic drugs.

RESULTS

Of the patients, 32.4% received positive inotropic drugs in the operating theatre after weaning from CPB. The overall 30-day mortality was 2.2%. Of non-survivors, 81.8% received inotropes compared with 18.2% of survivors (P < 0.01). The numbers of previous myocardial infarctions (odds ratio (OR), 2.01), congestive heart failures New York Heart Association class > 2 (OR, 1.85), COPD (OR, 1.85) and age > 65 yr (OR, 1.62), aortic cross clamping time of > 90 min (OR, 2.32) and coronary artery bypass surgery (OR, 0.43) all represented influential factors within the logistic regression model.

CONCLUSION

The knowledge of these risk factors should be useful in increasing the anaesthetist's vigilance in those patients most at risk for inotropic support and in providing for more timely therapeutic intervention and optimizing anaesthesia management.

Local anaesthetics block hyperpolarization-activated inward current in rat small dorsal root ganglion neurones

(1) Hyperpolarizing voltage steps evoke slowly activating inward currents in a variety of neurones and in cardiac cells. This hyperpolarization-activated inward current (I(h)) is thought to play a significant role in cell excitability, firing frequency, or in setting of the resting membrane potential in these cells. We studied the effects of lidocaine, mepivacaine, QX-314 and bupivacaine as well as its enantiomers on I(h) in the membrane of dorsal root ganglion neurones (DRG). (2) The patch-clamp technique was applied to small dorsal root ganglion neurones identified in 200 micro M thin slices of young rat DRGs. Under voltage-clamp conditions, the whole-cell I(h) current was recorded in the presence of different concentrations of the local anaesthetics. In current-clamp mode the resting membrane potential and the voltage response of DRG neurones to injected current pulses were investigated. (3) I(h) was reversibly blocked by bupivacaine, lidocaine and mepivacaine applied externally in clinically relevant concentrations. Concentration-response curves gave half-maximum inhibiting concentrations of 55, 99 and 190 micro M, respectively. Bupivacaine block of the I(h) current was not stereoselective. No significant effect was observed when QX-314 was applied to the external surface of the membrane. (4) In current-clamp experiments 60 micro M bupivacaine slightly hyperpolarized the membrane. The membrane stimulation by low-amplitude current pulses in the presence of bupivacaine showed an increase of the hyperpolarizing responses. (5) Our findings suggest an important role of the I(h)-block by local anaesthetics in the complex mechanism of drug action during epidural and spinal anaesthesia.

Block of neuronal tetrodotoxin-resistant Na+ currents by stereoisomers of piperidine local anesthetics

Tetrodotoxin (TTX)-sensitive Na(+) channels in the peripheral nervous system are the major targets for local anesthetics. In the peripheral nociceptive system, a Na(+) channel subtype resistant to TTX and with distinct electrophysiological properties seems to be of importance for impulse generation and conduction. A current through TTX-resistant Na(+) channels displays slower activation and inactivation kinetics and has an increased activation threshold compared with TTX-sensitive Na(+) currents and may have different pharmacological properties. We studied the effects of stereoisomers of piperidine local anesthetics on neuronal TTX-resistant Na(+) currents recorded with the whole-cell configuration of the patch clamp method in enzymatically dissociated dorsal root ganglion neurons of adult rats. Stereoisomers of mepivacaine, ropivacaine, and bupivacaine reversibly inhibited TTX-resistant Na(+) currents in a concentration and use-dependent manner. All drugs accelerated time course of inactivation. Half-maximal blocking concentrations were determined from concentration-inhibition relationships. Potencies for tonic and for use-dependent block increased with rising lipid solubilities of the drugs. Stereoselective action was not observed. We conclude that block of TTX-resistant Na(+) currents may lead to blockade of TTX-resistant action potentials in nociceptive fibers and consequently may be responsible for pain suppression during local anesthesia.

Tonic blocking action of meperidine on Na+ and K+ channels in amphibian peripheral nerves

BACKGROUND

Among opioids, meperidine (pethidine) also shows local anesthetic activity when applied locally to peripheral nerve fibers and has been used for this effect in the clinical setting for regional anesthesia. This study investigated the blocking effects of meperidine on different ion channels in peripheral nerves.

METHODS

Experiments were conducted using the outside-out configuration of the patch-clamp method applied to enzymatically prepared peripheral nerve fibers of Xenopus laevis. Half-maximal inhibiting concentrations were determined for Na+ channels and different K+ channels by nonlinear least-squares fitting of concentration-inhibition curves, assuming a one-to-one reaction.

RESULTS

Externally applied meperidine reversibly blocked all investigated channels in a concentration-dependent manner, i.e., voltage-activated Na+ channel (half-maximal inhibiting concentration, 164 microM), delayed rectifier K+ channels (half-maximal inhibiting concentration, 194 microM), the calcium-activated K+ channel (half-maximal inhibiting concentration, 161 microM), and the voltage-independent flicker K+ channel (half-maximal inhibiting concentration, 139 microM). Maximal block in high concentrations of meperidine reached 83% for delayed rectifier K+ channels and 100% for all other channels. Meperidine blocks the Na+ channel in the same concentration range as the local anesthetic agent lidocaine (half-maximal inhibiting concentration, 172 microM) but did not compete for the same binding site as evaluated by competition experiments. Low concentrations of meperidine (1 nM to 1 microM) showed no effects on Na+ channels. The blockade of Na+ and delayed rectifier K+ channels could not be antagonized by the addition of naloxone.

CONCLUSIONS

It is concluded that meperidine has a nonselective inhibitory action on Na+ and K+ channels of amphibian peripheral nerve. For tonic Na+ channel block, neither an opioid receptor nor the the local anesthetic agent binding site is the target site for meperidine block.

Effect of bupivacaine on ATP-dependent potassium channels in rat cardiomyocytes

Bupivacaine induces fatal arrhythmia when accidentally injected i.v. or overdosed, whereas lidocaine is used as an anti-arrhythmic agent. We have suggested recently that the anti-arrhythmic effect of lidocaine may be explained by suppression of ATP-sensitive potassium (KATP) channels. Therefore, it could be argued that different sensitivities of KATP channels to both drugs could be a reason for their different arrhythmic and anti-arrhythmic properties. In this study, we have investigated the direct action of bupivacaine on KATP channels in cardiomyocytes. The effects of bupivacaine on the cardiac KATP channel were investigated using the patch-clamp technique on enzymatically dissociated cardiomyocytes of adult rats. Bupivacaine was applied to the outer side of excised membrane patches using a multiple-barrel perfusion system. Concentration-response curves indicated that bupivacaine blocked the mean current of the KATP channels at a half-maximum inhibiting concentration (IC50) of 29 mumol litre-1, similar to that reported for lidocaine (43 mumol litre-1). Binding of bupivacaine influenced the gating of this channel, but did not reduce the conductance of the open channel. Bupivacaine and lidocaine were equipotent in blocking KATP channels. However, because of its excessive block of the sodium channel in the inactivated state, block of KATP channels by bupivacaine will only enhance its cardiotoxicity.

Local anaesthetic effects on tetrodotoxin-resistant Na+ currents in rat dorsal root ganglion neurones

Besides the fast tetrodotoxin-sensitive Na+ current, small dorsal root ganglion neurones of rats also possess a slower tetrodotoxin-resistant Na+ current. The blocking effect of commonly used local anaesthetics upon the tetrodotoxin-resistant Na+ current was investigated in the present paper. Dorsal root ganglia were dissected from adult rats and cells were enzymatically isolated. The whole-cell patch clamp technique was then used to measure inward Na+ currents of small dorsal root ganglion neurones. Externally applied local anaesthetics reversibly blocked the tetrodotoxin-resistant Na+ current in a dose-dependent manner. Half-maximal blocking concentrations for tonic block were: lignocaine, 326 microM; prilocaine, 253 microM; mepivacaine, 166 microM; etidocaine, 196 microM bupivacaine, 57 microM procaine, 518 microM benzocaine, 489 microM; tetracaine, 21 microM; and dibucaine, 23 microM. Blocking of the current by lignocaine was independent of temperature. The quaternary lignocaine derivative OX-314 did not have any effect upon the tetrodotoxin-resistant Na+ current when applied externally. High concentrations of tetrodotoxin also blocked the tetrodotoxin-resistant Na+ current with a half-maximal blocking concentration of 115 microM. The block by high tetrodotoxin concentrations did not compete with the lignocaine block, suggesting that there were two independent blocking mechanisms for the two substances. The tetrodotoxin-resistant Na+ currents also showed a marked sensitivity to phasic (use-dependent) block by local anaesthetics.

Fundamental properties of local anesthetics: half-maximal blocking concentrations for tonic block of Na+ and K+ channels in peripheral nerve

Local anesthetics suppress excitability by interfering with ion channel function. Ensheathment of peripheral nerve fibers, however, impedes diffusion of drugs to the ion channels and may influence the evaluation of local anesthetic potencies. Investigating ion channels in excised membrane patches avoids these diffusion barriers. We investigated the effect of local anesthetics with voltage-dependent Na+ and K+ channels in enzymatically dissociated sciatic nerve fibers of Xenopus laevis using the patch clamp method. The outside-out configuration was chosen to apply drugs to the external face of the membrane. Local anesthetics reversibly blocked the transient Na+ inward current, as well as the steady-state K+ outward current. Half-maximal tonic inhibiting concentrations (IC50), as obtained from concentration-effect curves for Na+ current block were: tetracaine 0.7 microM, etidocaine 18 microM, bupivacaine 27 microM, procaine 60 microM, mepivacaine 149 microM, and lidocaine 204 microM. The values for voltage-dependent K+ current block were: bupivacaine 92 microM, etidocaine 176 microM, tetracaine 946 microM, lidocaine 1118 microM, mepivacaine 2305 microM, and procaine 6302 microM. Correlation of potencies with octanol:buffer partition coefficients (logP0) revealed that ester-bound local anesthetics were more potent in blocking Na+ channels than amide drugs. Within these groups, lipophilicity governed local anesthetic potency. We conclude that local anesthetic action on peripheral nerve ion channels is mediated via lipophilic drug-channel interactions.

IMPLICATIONS

Half-maximal blocking concentrations of commonly used local anesthetics for Na+ and K+ channel block were determined on small membrane patches of peripheral nerve fibers. Because drugs can directly diffuse to the ion channel in this model, these data result from direct interactions of the drugs with ion channels.

Basic electrical properties of in situ endothelial cells of small pulmonary arteries during postnatal development

Small pulmonary arteries are the major determinants of pulmonary artery pressure and vascular resistance. Their endothelium modulates pulmonary resistance, remodeling, and blood fluidity. We developed a method that provides access to the luminal surface of small pulmonary arteries of rat and allows the patch-clamp study of electrical properties of in situ endothelium. At birth, the membrane was predominantly permeable for K(+), showing a resting potential of -70 mV. This conductance was not voltage-dependent and was insensitive to standard blockers of K(+) channels such as tetraethylammonium, charybdotoxin, and 4-aminopyridine. The first 22 d of development were accompanied by an additional expression of a Cl(-) conductance, increasing membrane potential to -45 mV. Acidosis reduced K(+) conductance and depolarized the membrane, whereas alkalosis resulted in hyperpolarization. Two-electrode recordings revealed tight electrical coupling (83%) between neighboring cells in the circumferential direction of the artery. The electrotonic length constant for endothelium was 13.3 microm, indicating that most cells in one cross section of a small artery are well coupled. Thus, the resting membrane conductances in small pulmonary artery endothelial cells change with postnatal development and are modulated by pH.

Increased seroprevalence of parvovirus B 19 IgG in complex regional pain syndrome is not associated with antiendothelial autoimmunity

The etiology of complex regional pain syndrome (CRPS) is unclear yet. Recently autoantibodies and antecedent viral infections have been discussed to be involved in the pathogenesis of CRPS. We investigated sera from 39 CRPS patients and healthy controls for parvovirus B19 IgG and the occurrence of antiendothelial autoantibodies (AECA). CRPS patients showed a higher seroprevalence of parvovirus B19 IgG than controls (p < 0.01). All CRPS 2 patients were positive. 10.2% of the CRPS patients and 10.0% of the controls had AECA (n.s.) and AECA were not associated with parvovirus B19 seropositivity. Our findings suggest the involvement of parvovirus B19, but not autoantibody-mediated endothelial cell damage, in the pathogenesis of CRPS.

Differential block of fast and slow inactivating tetrodotoxin-sensitive sodium channels by droperidol in spinal dorsal horn neurons

BACKGROUND

Dorsal horn neurons of the spinal cord participate in neuronal pain transmission. During spinal and epidural anesthesia, dorsal horn neurons are exposed to local anesthetics and opioids. Droperidol is usually given with opioids to avoid nausea and vomiting. A recently developed method of "entire soma isolation" has made it possible to study directly the action of droperidol on different components of Na+ current in dorsal horn neurons.

METHODS

Using a combination of the whole-cell patch-clamp recording from spinal cord slices and the entire soma isolation method, we studied the direct action of droperidol on two types of Na+ currents in dorsal horn neurons of young rats.

RESULTS

The tetrodotoxin-sensitive Na+ current in isolated somata consisted of a fast inactivating (tauF, 0.5-2 ms; 80-90% of the total amplitude) and a slow inactivating (tauS, 6-20 ms; 10-20% of the total amplitude) component. Droperidol, at concentrations relevant for spinal and epidural anesthesia, selectively and reversibly suppressed the fast component with a half-maximum inhibiting concentration (IC50) of 8.3 microm. The slow inactivating component was much less sensitive to droperidol; the estimated IC50 value was 809 microm.

CONCLUSIONS

Droperidol selectively blocks fast Na+ channels, the fast and slow components of the Na+ current in dorsal horn neurons are carried through pharmacologically distinct types of Na+ channels, and the effects of droperidol differ from those of local anesthetics and tetrodotoxin, which equipotently suppress both components. Droperidol may be suggested as a pharmacologic tool for separation of different types of inactivating tetrodotoxin-sensitive Na+ channel.

Enhancement of delayed-rectifier potassium conductance by low concentrations of local anaesthetics in spinal sensory neurones

Combining the patch-clamp recordings in slice preparation with the 'entire soma isolation' method we studied action of several local anaesthetics on delayed-rectifier K(+) currents in spinal dorsal horn neurones. Bupivacaine, lidocaine and mepivacaine at low concentrations (1 - 100 microM) enhanced delayed-rectifier K(+) current in intact neurones within the spinal cord slice, while exhibiting a partial blocking effect at higher concentrations (>100 microM). In isolated somata 0.1 - 10 microM bupivacaine enhanced delayed-rectifier K(+) current by shifting its steady-state activation characteristic and the voltage-dependence of the activation time constant to more negative potentials by 10 - 20 mV. Detailed analysis has revealed that bupivacaine also increased the maximum delayed-rectifier K(+) conductance by changing the open probability, rather than the unitary conductance, of the channel. It is concluded that local anaesthetics show a dual effect on delayed-rectifier K(+) currents by potentiating them at low concentrations and partially suppressing at high concentrations. The phenomenon observed demonstrated the complex action of local anaesthetics during spinal and epidural anaesthesia, which is not restricted to a suppression of Na(+) conductance only.

Lidocaine and octanol have different modes of action at tetrodotoxin-resistant Na(+) channels of peripheral nerves

Local anesthetics and alcohols block impulse conduction in peripheral nerves by inhibiting Na(+) currents. In small peripheral nerve fibers, tetrodotoxin-resistant (TTX-r) Na(+) channels play an important role in impulse generation. We investigated the effects of lidocaine and the alcohol octanol on TTX-r Na(+) channels. Currents were recorded with the whole-cell patch-clamp method from enzymatically isolated rat dorsal root ganglion cells (data evaluation: nonlinear least-squares fitting). Lidocaine and octanol blocked the TTX-r Na(+) current in a reversible and concentration-dependent manner (50% inhibitory concentration values: 177 +/- 25 and 455 +/- 25 microM, respectively). Lidocaine additionally produced a strong use-dependent block. Both drugs showed a strong dynamic block (i.e., block developed during the time course of current activation and inactivation). Double-pulse protocols showed a slow dissociation of lidocaine from the channel during repolarization (time constant: 1763 +/- 63 ms; 300 microM). The dissociation of octanol was too quick to be distinguished from normal current repriming kinetics of 2.2 ms. Lidocaine and octanol acted noncompetitively in the Na(+) channel. Lidocaine and octanol have different blocking properties on the TTX-r Na(+) current and bind to different channel sites.

IMPLICATIONS

Lidocaine and octanol have different inhibitory effects on the function of tetrodotoxin-resistant Na(+) channels in rat dorsal root ganglion cells, as well as noncompetitive modes of action, as investigated by the whole-cell patch-clamp method, and therefore are likely to have different binding sites on the channel.