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

The Efficacy and Safety of Ultrasound-Guided, Bi-Level, Erector Spinae Plane Block With Different Doses of Dexmedetomidine for Patients Undergoing Video-Assisted Thoracic Surgery: A Randomized Controlled Trial

Background: The anesthetic characteristics of ultrasound-guided bi-level erector spinae plane block (ESPB) plus dexmedetomidine (Dex) remain unclear. We compared the efficacy and safety of ultrasound-guided bi-level ESPB plus different doses of Dex in patients undergoing video-assisted thoracic surgery (VATS).

Methods: One-hundred eight patients undergoing VATS were randomized into three groups: R group (n = 38, 15 ml of 0.375% ropivacaine with 0.1 mg/kg dexamethasone), RD1 group (n = 38, 15 ml of 0.375% ropivacaine plus 0.5 μg/kg DEX with 0.1 mg/kg dexamethasone) and RD2 group (n = 38, 15 ml of 0.375% ropivacaine plus 1.0 μg/kg DEX with 0.1 mg/kg dexamethasone). The primary outcome was the pain 12 h after surgery. Secondary outcomes included the Prince Henry Hospital Pain Score; hemodynamics; consumption of sufentanil; anesthetized dermatomal distribution; recovery time; rescue analgesia; satisfaction scores of patients and surgeon; quick recovery index; adverse effects; the prevalence of chronic pain and quality of recovery.

Results: The visual analog scale (VAS) and the Prince Henry pain score were significantly lower in both the RD1 and RD2 groups during the first 24 h after surgery (P < 0.05). Both VAS with coughing and the Prince Henry pain score were significantly lower in the RD2 group than in the RD1 group 8–24 h after surgery (P < 0.05). Both heart rate and mean arterial pressure were significantly different from T2 to T6 in the RD1 and RD2 groups (P < 0.05). The receipt of remifentanil, propofol, Dex, and recovery time was significantly reduced in the RD2 group (P < 0.05). The requirement for sufentanil during the 8–72 h after surgery, less rescue medication, and total press times were significantly lower in the RD2 group (P < 0.05). The time to the first dose of rescue ketorolac was significantly longer in the RD2 group (P < 0.05). Further, anal exhaust, removal of chest tubes, and ambulation were significantly shorter in the RD2 group (P < 0.05). The incidence of tachycardia, post-operative nausea and vomiting, and chronic pain was significantly reduced in the RD2 group, while the QoR-40 score was significantly higher in the RD2 group (P < 0.05).

Conclusions: Pre-operative bi-level, single-injection ESPB plus 1 μg/kg DEX provided superior pain relief and long-term post-operative recovery for patients undergoing VATS.

Clinical Trial Registration:http://www.chictr.org.cn/searchproj.aspx.

Chiral Aspects of Local Anesthetics

Thanks to the progress made in chemical technology (particularly in the methodologies of stereoselective syntheses and analyses) along with regulatory measures, the number of new chiral drugs registered in the form of pure enantiomers has increased over the past decade. In addition, the pharmacological and pharmacokinetic properties of the individual enantiomers of already-introduced racemic drugs are being re-examined. The use of the pure enantiomer of a drug that has been used to date in the form of a racemate is called a "chiral switch". A re-examination of the properties of the pure enantiomers of racemates has taken place for local anesthetics, which represent a group of drugs which have long been used. Differences in (R) and (S)-enantiomers were found in terms of pharmacodynamic and pharmacokinetic activity as well as in toxicity. Levobupivacaine and robivacaine were introduced into practice as pure (S)-(-)-enantiomers, exhibiting more favorable properties than their (R)-(+)-stereoisomers or racemates. This overview focuses on the influence of chirality on the pharmacological and toxicological activity of local anesthetics as well as on individual HPLC and capillary electrophoresis (CE) methods used for enantioseparation and the pharmacokinetic study of individual local anesthetics with a chiral center.

[Stocktaking of local anesthetics 2020]

For decades local anesthetics have proven to be safe and effective drugs in the clinical practice, crucially promoting the enormous achievements in regional anesthesia. Meanwhile, it is a well-known fact that local anesthetics are much more than just "simple" sodium channel blockers. They also interact with numerous other ion channels and subcellular structures, enhancing nerve blockade and resulting in systemic "alternative" effects, which can sometimes even be clinically used. By the simultaneous administration of various adjuvants (e.g., opioids, corticosteroids and α₂-receptor agonists) attempts are made to prolong the time of action of local anesthetics after a single administration in order to achieve the best possible improvement in postoperative analgesia. In this context, ultralong-acting local anesthetics, such as liposomal bupivacaine, which at least theoretically can provide a sensory nerve block for several days, have been developed and clinically introduced. The coming years will show whether these approaches will develop into genuine alternatives to the personnel and cost-intensive continuous nerve blockades.Local anesthetic-induced systemic toxicity is meanwhile rare but still a potentially life-threatening event, frequently resulting from accidental intravascular injection or extensive systemic resorption. Consequently, slow and fractional application of these agents with intermittent aspiration helps to prevent toxic sequelae. If toxic symptoms occur, however, the intravenous infusion of 20% lipid solutions in addition to basic treatment measures can enhance the success of treatment.

Bupivacaine Induces Apoptosis via ROS in the Schwann Cell Line

Local anesthetics have been generally accepted as being safe. However, recent clinical trials and basic studies have provided strong evidence for the neurotoxicity of local anesthetics, especially through apoptosis. We hypothesized that local anesthetics cause neural complications through Schwann cell apoptosis. Among local anesthetics tested on the Schwann cell line, RT4-D6P2T, bupivacaine significantly induced cell death, measured by the methyl tetrazolium (MTT) assay, in a dose- (LD50 = 476 μM) and time-dependent manner. The bupivacaine-induced generation of reactive oxygen species (ROS), which was initiated within 5 hrs and preceded the activation of caspase-3 and poly ADP-ribose polymerase (PARP) degradation, was suggested to trigger apoptosis, exhibited by Hoechst 33258 nuclear staining and DNA fragmentation. Furthermore, concomitant block of ROS by anti-oxidants significantly inhibited bupivacaine-induced apoptosis. Among the local anesthetics for peripheral neural blocks, bupivacaine induced apoptosis in the Schwann cell line, which may be associated with ROS production.

Development and validation of a thallium flux-based functional assay for the sodium channel NaV1.7 and its utility for lead discovery and compound profiling

Ion channels are critical for life, and they are targets of numerous drugs. The sequencing of the human genome has revealed the existence of hundreds of different ion channel subunits capable of forming thousands of ion channels. In the face of this diversity, we only have a few selective small-molecule tools to aid in our understanding of the role specific ion channels in physiology which may in turn help illuminate their therapeutic potential. Although the advent of automated electrophysiology has increased the rate at which we can screen for and characterize ion channel modulators, the technique's high per-measurement cost and moderate throughput compared to other high-throughput screening approaches limit its utility for large-scale high-throughput screening. Therefore, lower cost, more rapid techniques are needed. While ion channel types capable of fluxing calcium are well-served by low cost, very high-throughput fluorescence-based assays, other channel types such as sodium channels remain underserved by present functional assay techniques. In order to address this shortcoming, we have developed a thallium flux-based assay for sodium channels using the NaV1.7 channel as a model target. We show that the assay is able to rapidly and cost-effectively identify NaV1.7 inhibitors thus providing a new method useful for the discovery and profiling of sodium channel modulators.

Actions of bupivacaine, a widely used local anesthetic, on NMDA receptor responses

NMDA receptors mediate excitatory neurotransmission in brain and spinal cord and play a pivotal role in the neurological disease state of chronic pain, which is caused by central sensitization. Bupivacaine is the indicated local anesthetic in caudal, epidural, and spinal anesthesia and is widely used clinically to manage acute and chronic pain. In addition to blocking Na(+) channels, bupivacaine affects the activity of many other channels, including NMDA receptors. Importantly, bupivacaine inhibits NMDA receptor-mediated synaptic transmission in the dorsal horn of the spinal cord, an area critically involved in central sensitization. We used recombinant NMDA receptors expressed in HEK293 cells and found that increasing concentrations of bupivacaine decreased channel open probability in GluN2 subunit- and pH-independent manner by increasing the mean duration of closures and decreasing the mean duration of openings. Using kinetic modeling of one-channel currents, we attributed the observed current decrease to two main mechanisms: a voltage-dependent "foot-in-the-door" pore block and an allosteric gating effect. Further, the inhibition was state-independent because it occurred to the same degree whether the drug was applied before or after glutamate stimulation and was mediated by extracellular and intracellular inhibitory sites, via hydrophilic and hydrophobic pathways. These results predict that clinical doses of bupivacaine would decrease the peak and accelerate the decay of synaptic NMDA receptor currents during normal synaptic transmission. These quantitative predictions inform possible applications of bupivacaine as preventative and therapeutic approaches in chronic pain.

Two-pore domain potassium channels enable action potential generation in the absence of voltage-gated potassium channels

In this study, we explored the possibility that two-pore domain potassium (K2P) channels are sufficient to support action potential (AP) generation in the absence of conventional voltage-gated potassium (KV) channels. Hodgkin-Huxley parameters were used to mimic the presence of voltage-gated sodium (NaV) channels in HEK-293 cells. Recombinant expression of either TREK-1 or TASK-3 channels was then used to generate a hyperpolarised resting membrane potential (RMP) leading to the characteristic non-linear current-voltage relationship expected of a K2P-mediated conductance. During conductance simulation experiments, both TASK-3 and TREK-1 channels were able to repolarise the membrane once AP threshold was reached, and at physiologically relevant current densities, this K2P-mediated conductance supported sustained AP firing. Moreover, the magnitude of the conductance correlated with the speed of the AP rise in a manner predicted from our computational studies. We discuss the physiological impact of axonal K2P channels and speculate on the possible clinical relevance of K2P channel modulation when considering the actions of general and local anaesthetics.

[Contribution to post-caesarean analgesia of ultrasound-guided transversus abdominis plane block]

OBJECTIVE

The aim of this study was to evaluate the contribution made by ultrasound-guided transversus abdominis plane block (TAP) to the quality of the analgesia with intrathecal opioids obtained in patients undergoing elective caesarean delivery.

MATERIAL AND METHODS

A prospective, randomized study in patients submitted to elective caesarean section with spinal anaesthesia with 0.5% hyperbaric bupivacaine. The patients were randomized into 3 groups according to the added complementary drug for analgesia: group A morphine 0.1mg; group B fentanyl 10 μg; group C 10 μg fentanyl+bilateral TAP block. The TAP block with 20 ml of 0.5% levobupivacaine on each side, after surgery. Groups A and B, were injected with 20 ml of saline. Postoperative analgesia was performed with morphine bolus through a system of patient-controlled analgesia (PCA). We studied the pain on a visual analogue scale at 12 and 24h at rest and movement, the time elapsed to require the first bolus, and morphine bolus in 24h. Secondary effects such as nausea, vomiting, pruritus, and drowsiness, were also evaluated. The level of patient satisfaction was also recorded.

RESULTS

A total of 90 patients were included. At rest the 12/24h VAS score was: group A, at 12h 2.1 ± 1.2, at 24h 4.7 ± 1.6; group B at 12h 4.3 ± 2.9, at 24h 4.8 ± 2; group C at 12h 1.9 ± 1.09, at 24h 2.3 ± 1.2 (P<.05). Walking improved analgesia more in group C (P ≤.02). The time of asking for the first bolus was lower in group B: group A 9.3 ± 4.9h (P=.02 compared to group C), in group B 2 ± 1.8h (P<.001 compared to group C) and group C 13.2 ± 2.1h. The number of bolus in 24h in group B was 38 ± 5, in group A 10 ± 2 (P<.05), group C 5 ± 2 (P<.001). Delayed nausea was increased in group B (36.6%) and pruritus was greater in group A (36.6%).

CONCLUSIONS

Ultrasound (US)-guided TAP block improves spinal opioid analgesia, with a decrease in VAS scores in the first 24h, and reduces opioid requirement and secondary effects after caesarean delivery.

Membrane interactivity of charged local anesthetic derivative and stereoselectivity in membrane interaction of local anesthetic enantiomers

With respect to the membrane lipid theory as a molecular mechanism for local anesthetics, two critical subjects, the negligible effects of charged drugs when applied extracellularly and the stereoselective effects of enantiomers, were verified by paying particular attention to membrane components, phospholipids with the anionic property, and cholesterol with several chiral carbons. The membrane interactivities of structurally-different anesthetics were determined by their induced fluidity changes of liposomal membranes. Lidocaine (3.0 μmol/mL) fluidized phosphatidylcholine membranes, but not its quaternary derivative QX-314 (3.0 μmol/mL). Similarly to the mother molecule lidocaine, however, QX-314 fluidized phosphatidylserine-containing nerve cell model membranes and acidic phospholipids-constituting membranes depending on the acidity of membrane lipids. Positively charged local anesthetics are able to act on lipid bilayers by ion-pairing with anionic (acidic) phospholipids. Bupivacaine (0.75 mol/mL) and ropivacaine (0.75 and 1.0 μmol/mL) fluidized nerve cell model membranes with the potency being S(−)-enantiomer < racemate < R(+)-enantiomer (P < 0.01, vs antipode and racemate) and cardiac cell model membranes with the potency being S(−)-ropivacaine < S(−)-bupivacaine < R(+)-bupivacaine (P < 0.01). However, their membrane effects were not different when removing cholesterol from the model membranes. Stereoselectivity is producible by cholesterol which increases the chirality of lipid bilayers and enables to discriminate anesthetic enantiomers. The membrane lipid interaction should be reevaluated as the mode of action of local anesthetics.

The effects of lipid infusion on myocardial function and bioenergetics in l-bupivacaine toxicity in the isolated rat heart

BACKGROUND

It is unclear whether improved metabolism or a "lipid sink" effect of lipid infusion is responsible for the positive effects in local anesthetic-induced myocardial depression.

METHODS

We used an isolated rat heart, constant-pressure perfused, nonrecirculating Langendorff preparation and exposed hearts to 5 mug/mL l-bupivacaine and 9 microL/mL lipid emulsion. Hearts were freeze-clamped and energy was charge measured by HPLC. In a second experiment the effects of pacing hearts was evaluated. The effects of lipid addition on local anesthetic concentrations in Krebs-Henseleit buffer and human plasma were examined by using a mass spectrometer.

RESULTS

With spontaneously beating hearts l-bupivacaine led to a significant decrease in heart rate (to 74% +/- 7% of baseline), +dP/dt (69% +/- 7%), systolic pressure (78% +/- 6%), coronary flow (61% +/- 8%), and to an increase in PR (177% +/- 52%) and QRS intervals (166% +/- 36%). Lipid infusion exerted a positive inotropic effect, significantly augmenting +dP/dt and systolic pressure back to 94% +/- 11% and 102% +/- 16% of baseline in l-bupivacaine-treated hearts. Heart rate, coronary flow, PR, and QRS intervals remained unchanged after lipid intervention. Lipid infusion in paced hearts had a significant effect on +dP/dt, systolic pressure, and Mvo2. Neither l-bupivacaine nor lipids had an effect on energy charge. A lipid concentration of 500 muL/mL plasma was necessary to effect changes in the plasma concentration of local anesthetics.

CONCLUSION

Lipid application in l-bupivacaine-induced cardiac depression had a significant positive inotropic effect, which we would attribute to a direct inotropic effect. However, in an isolated heart model, indirect, local anesthetic plasma-binding effect of lipids cannot be excluded.

Interactions of anesthetics with their targets: non-specific, specific or both?

What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.

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.

Amide local anesthetics potently inhibit the human tandem pore domain background K+ channel TASK-2 (KCNK5)

Blockade of voltage-gated sodium (Na+) channels by local anesthetics represents the main mechanism for inhibition of impulse propagation. Local anesthetic-induced potassium (K+) channel inhibition is also known to influence transmission of sensory impulses and to potentiate inhibition. K+ channels involved in this mechanism may belong to the emerging family of background tandem pore domain K+ channels (2P K+ channels). To determine more precisely the effects of local anesthetics on members of this ion channel family, we heterologously expressed the 2P K+ channels TASK-2 (KCNK5), TASK-1 (KCNK3), and chimeric TASK-1/TASK-2 channels in oocytes of Xenopus laevis. TASK-2 cDNA-transfected HEK 293 cells were used for single-channel recordings. Local anesthetic inhibition of TASK-2 was dose-dependent, agent-specific, and stereoselective. The IC50 values for R-(+)-bupivacaine and S-(-)-bupivacaine were 17 and 43 micro M and for R-(+)-ropivacaine and S-(-)-ropivacaine, 85 and 236 micro M. Lidocaine (1 mM) inhibited TASK-2 currents by 55 +/- 4%, whereas its quaternary positively charged analog N-ethyl lidocaine (QX314) had no effect. Bupivacaine (100 micro M) decreased channel open probability from 20.8 +/- 1.6% to 5.6 +/- 2.2%. Local anesthetics [300 micro M R-(+)-bupivacaine] caused significantly greater depolarization of the resting membrane potential of TASK-2-expressing oocytes compared with water-injected control oocytes (15.8 +/- 2.5 mV versus 0.1 +/- 0.05 mV; p < 0.001). Chimeric TASK-1/TASK-2 2P K+ channel subunits that retained pH sensitivity demonstrated that the carboxy domain of TASK-2 mediates the greater local anesthetic sensitivity of TASK-2. These results show that clinically achievable concentrations of local anesthetics inhibit background K+ channel function and may thereby enhance conduction blockade.

Inhibition of human TREK-1 channels by bupivacaine

Human TWIK-related K(+) channels (TREK-1) stabilize the membrane potential (mp) of neurons and have a major role in the regulation of membrane excitability. In view of their physiological significance, interaction of bupivacaine with TREK-1 channels may be clinically important. Our aim was to characterize with the patch-clamp technique the properties of human TREK-1 channels and the effects of bupivacaine on these channels expressed in Chinese hamster ovary (CHO) cells. Transfection of CHO cells with TREK-1 channels (CHO(TREK-1) cells) hyperpolarized the mp from -33 +/- 13 to -78 +/- 4 mV. The channels were stimulated by intracellular acidosis. Inhibition of TREK-1 channels by bupivacaine was reversible, concentration-dependent, voltage-independent, and increased with intracellular acidosis. Bupivacaine depolarized the mp of CHO(TREK-1) cells in a reversible and concentration-dependent manner. Concentrations for channel inhibition and membrane depolarization were not linearly related (50% inhibitory concentration value for channel inhibition 370 +/- 20 micro M, Hill coefficient 1.8 +/- 0.1, n = 51; 50% inhibitory concentration value for membrane depolarization 856 +/- 14 micro M, Hill coefficient 2.4 +/- 0.1, mean +/- SEM, n = 27). The results suggest that protonated bupivacaine elicits the observed effects via a site of interaction accessible from the intracellular space. Inhibition of TREK-1 channels and consecutive depolarization of the cell membrane by bupivacaine may contribute to blockade of neuronal signal conduction during regional anesthesia.

IMPLICATIONS

The interaction of bupivacaine with human TREK-1 channels was studied with the patch-clamp technique. Bupivacaine inhibited TREK-1 channels and depolarized the membrane potential of cells expressing TREK-1 channels in a concentration-dependent and reversible manner. Both effects may contribute to conductance block caused by bupivacaine.

Seizure after levobupivacaine for interscalene brachial plexus block

IMPLICATIONS

This case report describes a patient who demonstrated generalized seizure activity after an injection of 30 mL of levobupivacaine 0.5% for interscalene brachial plexus block. No evidence of cardiovascular toxicity was noted.

Differential effects of bupivacaine enantiomers, ropivacaine and lidocaine on up-regulation of cell surface voltage-dependent sodium channels in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, (+/-)-bupivacaine inhibited veratridine-induced 22Na(+) influx (IC(50) 6.8 microM). The IC(50) of (+)-bupivacaine (2.8 microM) was 6.2-, 7.4-, and 17.1-fold lower than those of (-)-bupivacaine (17.3 microM), (-)-ropivacaine (20.6 microM), and lidocaine (47.8 microM). Chronic (i.e. 3-h) treatment of cells with (+/-)-bupivacaine increased cell surface [3H]saxitoxin ([3H]STX) binding capacity by 48% (EC(50) of 233 microM; t(1/2)=7.4 h), without changing the K(d) value. Treatment for 24 h with either (+)- or (-)-bupivacaine, or (-)-ropivacaine elevated [3H]STX binding, whereas 24-h treatment with lidocaine had no effect. The rise of [3H]STX binding by (+/-)-bupivacaine was prevented by cycloheximide, an inhibitor of protein synthesis, or brefeldin A, an inhibitor of cell surface vesicular exit from the trans-Golgi network; however, (+/-)-bupivacaine did not increase Na(+) channel alpha- and beta(1)-subunit mRNA levels. In cells subjected to (+/-)-bupivacaine treatment (1 mM for 24 h) followed by 3-h washout, veratridine-induced 22Na(+) influx was enhanced, even when measured in the presence of ouabain, an inhibitor of Na(+),K(+)-ATPase. Ptychodiscus brevis toxin-3 potentiated veratridine-induced 22Na(+) influx by 2.3-fold in the (+/-)-bupivacaine-treated cells, as in non-treated cells. These results suggest that lipophilic bupivacaine enantiomers or (-)-ropivacaine acutely inhibit Na(+) channel gating, whereas its chronic treatment up-regulates cell surface expression of Na(+) channels via translational and externalization events.

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.

Bupivacaine effects on hKv1.5 channels are dependent on extracellular pH

1. Bupivacaine-induced cardiotoxicity increases in hypoxic and acidotic conditions. We have analysed the effects of R(+)bupivacaine on hKv1.5 channels stably expressed in Ltk(-) cells using the whole-cell patch-clamp technique, at three different extracellular pH (pH(o)), 6.5, 7.4 and 10.0. 2. Acidification of the pH(o) from 7.4 to 6.5 decreased 4 fold the potency of R(+)bupivacaine to block hKv1.5 channels. At pH(o) 10.0, the potency of the drug increased approximately 2.5 fold. 3. Block induced by R(+)bupivacaine at pH(o) 6.5, 7.4 and 10.0, was voltage- and time-dependent in a manner consistent with an open state block of hKv1.5 channels. 4. At pH(o) 6.5, but not at pH(o) 7.4 or 10.0, R(+)bupivacaine increased by 95+/-3 % (n=6; P<0.05) the hKv1.5 current recorded at -10 mV, likely due to a drug-induced shift of the midpoint of activation (DeltaV=-8.5+/-1.4 mV; n=7). 5. R(+)bupivacaine development of block exhibited an 'instantaneous' component of block at the beginning of the depolarizing pulse, which averaged 12.5+/-1.8% (n=5) and 4.6+/-1.6% (n=6), at pH(o) 6.5 and 7.4, respectively, and that was not observed at pH(o) 10.0. 6. It is concluded that: (a) alkalinization of the pH(o) increases the potency of block of R(+)bupivacaine, and (b) at pH(o) 6.5, R(+)bupivacaine induces an 'agonist effect' of hKv1.5 current when recorded at negative membrane potentials.

Cardiotoxicity with modern local anaesthetics: is there a safer choice?

The recognition that long-acting local anaesthetics, particularly bupivacaine the de facto standard long-acting local anaesthetic, were disproportionately more cardiotoxic than their shorter-acting counterparts stimulated the development of the bupivacaine congeners, ropivacaine and levobupivacaine. These agents, like all local anaesthetics, can produce cardiotoxic sequelae by direct and indirect mechanisms that derive from their mode of local anaesthetic actions, i.e. inhibition of voltage-gated ion channels. While all local anaesthetics can cause direct negative inotropic effects, ropivacaine and levobupivacaine are less cardiotoxic than bupivacaine judging by the larger doses tolerated in laboratory animal preparations before the onset of serious cardiotoxicity (particularly electro-mechanical dissociation or malignant ventricular arrhythmias). Additionally, they are less toxic to the CNS than bupivacaine judging by the larger doses tolerated before the onset of seizures. This may be clinically important because CNS effects may be involved in the production of serious cardiotoxicity. Preclinical studies in humans are a 'blunt instrument' in their ability to distinguish significant differences between these drugs because of the relatively small doses that can be used. Nevertheless, available evidence from human studies corroborates the preclinical laboratory animal studies. Because clinically significant differences between these drugs are more quantitative than qualitative, i.e. toleration of a larger dose before manifestation of toxicity, we have concluded that these newer agents have a lower risk of causing serious cardiotoxicity than bupivacaine. Thus, compared with bupivacaine, the newer agents may be seen as 'safer', but they must not be regarded as 'safe'.

Direct cardiac effects of intracoronary bupivacaine, levobupivacaine and ropivacaine in the sheep

1. The racemic local anaesthetic agent bupivacaine is widely used clinically for its long duration of action. Levobupivacaine and ropivacaine are bupivacaine enantiopure congeners, developed to improve upon the clinical safety of bupivacaine, especially the risk of fatal arrhythmogenesis. 2. In previous preclinical studies of the safety of these drugs with intravenous administration in conscious ewes over a wide dose range, we found that central nervous system (CNS) excito-toxicity reversed the cardiac depressant effects when doses approached the convulsant threshold and thus precluded accurate comparison of their cardiovascular system (CVS) effects. 3. To study CVS effects over a wide range of doses with minimal CNS and other influences, brief (3 min) infusions of bupivacaine, levobupivacaine or ropivacaine were administered into the left main coronary arteries of previously instrumented conscious ewes (approximately 50 Kg body weight). After dose-ranging studies, the drugs were compared in a randomized, blinded, parallel group design. Equimolar doses were increased from 8 micromol (approximately 2.5 mg) in 8 micromol increments, to either a fatal outcome or a 40 micromol (approximately 12.5 mg) maximum. 4. All three drugs produced tachycardia, decreased myocardial contractility and stroke volume and widening of electrocardiographic QRS complexes. Thirteen of 19 animals died of ventricular fibrillation: four of six with bupivacaine (mean+/-s.e.mean actual fatal dose: 21.8+/-6.4 micromol), five of seven with levobupivacaine (22.9+/-3.5 micromol), four of six with ropivacaine (22.9+/-5.9 micromol). No significant differences in survival or in fatal doses between these drugs were found. 5. The findings suggest that ropivacaine, levobupivacaine and bupivacaine have similar intrinsic ability to cause direct fatal cardiac toxicity when administered by left intracoronary arterial infusion in conscious sheep and do not explain the differences between the drugs found with intravenous dosage.