High-affinity blockade of voltage-operated skeletal muscle and neuronal sodium channels by halogenated propofol analogues.

BACKGROUND AND PURPOSE: Voltage-operated sodium channels constitute major target sites for local anaesthetic-like action. The clinical use of local anaesthetics is still limited by severe side effects, in particular, arrhythmias and convulsions. These side effects render the search for new local anaesthetics a matter of high interest. EXPERIMENTAL APPROACH: We have investigated the effects of three halogenated structural analogues of propofol on voltage-operated human skeletal muscle sodium channels (Na(V)1.4) and the effect of one compound (4-chloropropofol) on neuronal sodium channels (Na(V)1.2) heterologously expressed in human embryonic kidney cell line 293. KEY RESULTS: 4-Iodo-, 4-bromo- and 4-chloropropofol reversibly suppressed depolarization-induced whole-cell sodium inward currents with high potency. The IC(50) for block of resting channels at -150 mV was 2.3, 3.9 and 11.3 microM in Na(V)1.4, respectively, and 29.2 microM for 4-chloropropofol in Na(V)1.2. Membrane depolarization inducing inactivation strongly increased the blocking potency of all compounds. Estimated affinities for the fast-inactivated channel state were 81 nM, 312 nM and 227 nM for 4-iodopropofol, 4-bromopropofol and 4-chloropropofol in Na(V)1.4, and 450 nM for 4-chloropropofol in Na(V)1.2. Recovery from fast inactivation was prolonged in the presence of drug leading to an accumulation of block during repetitive stimulation at high frequencies (100 Hz). CONCLUSIONS AND IMPLICATIONS: Halogenated propofol analogues constitute a novel class of sodium channel-blocking drugs possessing almost 100-fold higher potency compared with the local anaesthetic and anti-arrhythmic drug lidocaine. Preferential drug binding to inactivated channel states suggests that halogenated propofol analogues might be especially effective in suppressing ectopic discharges in a variety of pathological conditions.

Patent blue sentinel node mapping in cervical cancer patients may lead to decreased pulse oximeter readings and positive methaemoglobin results.

BACKGROUND AND OBJECTIVE: Patent blue (4-[(4-diethylaminophenyl)-(4-diethylazaniumylidencyclohexa-2,5-dienyliden) methyl]-6-hydroxy-3-sulfo-benzolsulfonate, sodium salt) is a contrast dye used for the intraoperative detection of the primary lymphatic nodes draining the area of tumour infiltration. The dye is known to interact with pulse oximeter readings. However, the degree of alteration seems to be moderate and predictable when patent blue is injected into the perimammilar region during breast surgery. METHODS: Here we report severe interference with the anaesthetic monitoring when patent blue was injected into the cervix prior to laparoscopy-assisted radical vaginal hysterectomy for cervical cancer. RESULTS: Injection of patent blue into the cervix induced a rapid (within 14 +/- 9 min after the injection) and severe (from 98% to 89 +/- 2%) decrease in pulse oximeter readings, accompanied by positive methaemoglobin values of 7.3 +/- 2.5% (arterial co-oximetry, Bayer Rapidlab 865 blood gas analyser; Bayer, Fernwald, Germany). Control of these values by a different device (Radiometer ABL co-oximeter blood gas analyser; Radiometer, Willich, Germany) yielded negative methaemoglobin results (<1.7%, mean 0.9 +/- 0.6%). The arterial PO2 was normal in all patients throughout the procedure. CONCLUSION: Injection of patent blue into the cervix uteri interferes dramatically with pulse oximeter readings. This situation is further complicated by device-dependent arterial co-oximetry methaemoglobin results. For the time being it is recommendable to monitor adequate oxygenation of the patient in the presence of patent blue by regular control of the arterial PO2. Clearly, the unresolved issue of reliable methaemoglobin determination in the presence of patent blue remains a matter of clinical concern for anaesthetists.

High-affinity blockade of voltage-operated skeletal muscle sodium channels by 2,6-dimethyl-4-chlorophenol.

BACKGROUND AND OBJECTIVE: The aromatic alcohol most closely resembling the aromatic tail of lidocaine is 2,6-dimethylphenol. This agent is as potent as lidocaine in blocking voltage-operated sodium channels. The aim of this study was to show the effect of halogenation in the para-position on the potency of this compound to block voltage-operated sodium channels. METHODS: Insertion of the halogen chloride into the para-position of the molecule 2,6-dimethylphenol yielded 2,6-dimethyl-4-chlorophenol. Block of sodium currents by this compound was studied using heterologously expressed voltage-operated rat neuronal (rat IIa) sodium channels. RESULTS: 2,6-dimethyl-4-chlorophenol reversibly suppressed depolarization-induced whole-cell sodium inward currents. The ECR50 for block of resting channels at a hyperpolarized holding potential (-150 mV) was 127 micromol, the Hill coefficient nH 1.7. Membrane depolarization inducing either fast or slow-inactivation strongly increased the blocking potency. This is an important feature of a local-anaesthetic-like action. The estimated half-maximum effect concentration for the fast-inactivated channel state ECI50 was 28 micromol, the Hill coefficient nH 3.8. When 20-30% of channels were slow-inactivated using long (2.5 s) prepulses, followed by a 10 ms repolarization period to allow recovery from fast inactivation, the IC50 at -100 mV holding potential was reduced to 53 micromol. CONCLUSION: These results, which show that 2,6-dimethyl-4-chlorophenol blocks voltage-operated sodium channels in a lidocaine-like manner while having a several fold higher potency than the non-halogenated parent compound, highlight a potentially meaningful principle of increasing the sodium channel blocking potency of phenol derivatives.

[Cricothyroidotomy training on cadavers - experiences in the education of medical students, anaesthetists, and emergency physicians]. / Koniotomieübungen an der Leiche - Erfahrungen in der Ausbildung mit Medizinstudenten, Anästhesisten und Notärzten.

OBJECTIVE: Should the technique of surgical cricothyroidotomy be practiced on cadavers and should it be a compulsory part of the teaching curriculum? Is it wise to use a speculum for the insertion of the endotracheal tube? What is the optimum size of the tube? METHODS: A surgical cricothyroidotomy with a speculum was carried out on 30 cadavers from the Institute of Legal Medicine, Medical School Hannover. This took place as part of a official and voluntary course for students of advanced semesters, anaesthetists and emergency doctors with the subjects "cricothyroidotomy, chest drainage and venous cut-down". The surgical cricothyroidotomy without the use of a speculum was carried out on 5 cadavers by two clinicians well practiced in this technique. The elapsed time between skin incision and the insertion of the endotracheal tube was measured on all five subjects. After the course the participants were asked if they were able to carry out a cricothyroidotomy in an emergency. They were also asked whether this course should be a compulsory part of their curriculum and whether practical sessions should take place. During autopsies at the Institute of Legal Medicine the length of the ligamentum conicum was measured on 40 corpses with reclined and non-reclined heads. RESULTS: The average time of storage of the cadavers was 4.2 days +/- 1.9 days. The cricothyroidotomy was possible on all 35 cadavers. In one case (3,3 %) the result was a complete rupture of the cricoid cartilage. In 5 cases (16.7 %) the horizontal incision was torn due to prising with the speculum. Difficult situations always occured when the skin incision was not exactly in the midline. The average time to place the endotracheal tube into the trachea by the surgical procedure of cricothyroidotomy was 22.4 seconds +/- 3.1 seconds (minimum 18 seconds, maximum 26 seconds). 10 % of the medical students and 50 % of the anaesthetists and emergency doctors felt they would be prepared to carry out a cricothyroidotomy in an emergency. 90 % of the students and respectively 80 % of the anaesthetists and emergency doctors stated that they would like to practice the technique on a cadaver again. Almost all participants were of the opinion that the course should be integrated as a compulsory course in a future educational curriculum. The average distance between the thyroid cartilage and the cricoid cartilage was 9.5 mm +/- 1.9 mm with non-reclined head (minimum 6 mm, maximum 14 mm) and 11.9 mm +/- 2.5 mm with reclined head (minimum 7 mm, maximum 18 mm). The average difference of distances was 2.4 mm +/- 1.2 mm (minimum 1 mm, maximum 6 mm) in reclined and non-reclined heads. CONCLUSIONS: In our opinion it is highly recommended that the technique of cricothyroidotomy should be practiced on cadavers and that the course should become a compulsory part in a future educational curriculum. In addition the incision of the ligamentum conicum using dilators or a speculum is not to be recommended from the point of view of this study. The tracheal tube used in this study (reinforced wire tube, ID 6.0) was best suited for surgical cricothyroidotomy.

High-affinity block of voltage-operated rat IIA neuronal sodium channels by 2,6 di-tert-butylphenol, a propofol analogue.

BACKGROUND AND OBJECTIVE: Propofol is a phenol derivative (2,6 di-isopropylphenol) with a unique effect profile including activating effects on GABA(A) and blocking effects on voltage-operated sodium channels. If the substituents in the 2- and the 6-positions are replaced by tert-butyl groups, the resulting phenol derivative, 2,6 di-tert-butylphenol, despite being a close structural propofol analogue, completely lacks GABA(A) receptor effects. The aim of this in vitro study was to investigate the effects of 2,6 di-tert-butylphenol on voltage-operated neuronal sodium channels in order to determine whether and, if so, how these structural changes alter the sodium channel-blocking effect seen with propofol. METHODS: Whole-cell sodium inward currents through heterologously expressed rat type IIA sodium channels were recorded in the absence and presence of definite concentrations of 2,6 di-tert-butylphenol and propofol. RESULTS: When applied at concentrations > or = 30 micromol, 2,6 di-tert-butylphenol completely and irreversibly blocked sodium inward currents. The blockade equilibrium time was about 2 min. A partial washout was possible only if the application was stopped before the equilibrium of the blockade was achieved. CONCLUSIONS: 2,6 Di-tert-butylphenol exerts a high-affinity block of neuronal sodium channels. Apparently, the slight structural differences of 2,6 di-tert-butylphenol in comparison with propofol--which account for the lack of GABA(A) receptor effects--enhance its voltage-operated sodium channel-blocking effects. As 2,6 di-tert-butylphenol is much more potent than most sodium channel blockers in clinical use, it might be of interest in the development of local anaesthetics.

Voltage-dependent block of neuronal and skeletal muscle sodium channels by thymol and menthol.

BACKGROUND AND OBJECTIVE: Thymol is a naturally occurring phenol derivative used in anaesthetic practice as a stabilizer and preservative of halothane, usually at a concentration of 0.01%. Although analgesic effects have long been described for thymol and its structural homologue menthol, a molecular basis for these effects is still lacking. We studied the blocking effects of thymol and menthol on voltage-activated sodium currents in vitro as possible molecular target sites. METHODS: Whole cell sodium inward currents via heterologously (HEK293 cells) expressed rat neuronal (rat type IIA) and human skeletal muscle (hSkM1) sodium channels were recorded in the absence and presence of definite concentrations of either thymol or menthol. RESULTS: When depolarizing pulses to 0 mV were started from a holding potential of -70 mV, half-maximum blocking concentrations (IC50) for the skeletal muscle and the neuronal sodium channel were 104 and 149 mumol for thymol and 376 and 571 mumol for menthol. The blocking potency of both compounds increased at depolarized holding potentials with the fraction of inactivated channels. The estimated dissociation constant Kd for thymol and menthol from the inactivated state was 22 and 106 mumol for the neuronal and 23 and 97 mumol for the skeletal muscle sodium channel, respectively. CONCLUSIONS: The results suggest that antinociceptive and local anaesthetic effects of thymol and menthol might be mediated via blockade of voltage-operated sodium channels with the phenol derivative thymol being as potent as the local anaesthetic lidocaine.

Structural requirements of phenol derivatives for direct activation of chloride currents via GABA(A) receptors.

Propofol directly activates gamma-aminobutyric acid (GABA(A)) receptors in the absence of the natural agonist. This mechanism is supposed to contribute to its sedative-hypnotic actions. We studied the effects of seven structurally related phenol derivatives on chloride inward currents via rat alpha1beta2gamma2 GABA(A) receptors, heterologously expressed in HEK 293 cells in order to find structural determinants for this direct agonistic action. Only compounds with the phenolic hydroxyl attached directly to the benzene ring and with aliphatic substituents in ortho position to the phenolic hydroxyl activated chloride currents in the absence of GABA. Concentrations required for half-maximum effect were 980 microM for 2-methylphenol, 230 microM for 2,6-dimethylphenol, 200 microM for thymol, and 23 microM for propofol. Drug-induced chloride currents showed no desensitisation during the 2-s application. These results show that the position of the aliphatic substituents with respect to the phenolic hydroxyl group is the crucial structural feature for direct GABA(A) activation by phenol derivatives.

The anesthetic propofol modulates gating in paramyotonia congenita mutant muscle sodium channels.

We examined the effects of propofol on a paramyotonia congenita mutant skeletal muscle sodium channel in vitro, because life-threatening complications resulting from severe muscle rigidity during induction of anesthesia have been observed using other anesthetics in patients with hereditary sodium channel myopathies. Our hypothesis was that propofol might interact directly with mutant channels, causing enhanced muscle excitability in affected patients. Whole-cell voltage-clamp experiments were performed on HEK 293 cells expressing R1448H mutant sodium channels. Propofol blocked sodium inward current at clinical concentrations (5 micromol/L) when depolarizing pulses were started from holding potentials close to the physiological resting potential (-70 mV). Higher propofol concentrations (>/=25 micromol/L) accelerated pathologically delayed inactivation kinetics and delayed pathologically enhanced recovery from inactivation. Our in vitro results show that inactivation-deficient sodium channels are specifically targeted and blocked by propofol. This might reduce enhanced muscle excitability experienced by affected patients in vivo.

Structural requirements for voltage-dependent block of muscle sodium channels by phenol derivatives.

We have studied the effects of four different phenol derivatives, with methyl and halogen substituents, on heterologously expressed human skeletal muscle sodium channels, in order to find structural determinants of blocking potency. All compounds blocked skeletal muscle sodium channels in a concentration-dependent manner. The methylated phenol 3-methylphenol and the halogenated phenol 4-chlorophenol blocked sodium currents on depolarization from -100 mV to 0 mV with IC(50) values of 2161 and 666 microM respectively. Methylation of the halogenated compound further increased potency, reducing the IC(50) to 268 microM in 2-methyl-4-chlorophenol and to 150 microM in 3,5-dimethyl-4-chlorophenol. Membrane depolarization before the test depolarization increased sodium channel blockade. When depolarizations were started from -70 mV or when a 2.5 s prepulse was introduced before the test pulse inducing slow inactivation, the IC(50) was reduced more than 3 fold in all compounds. The values of K(D) for the fast-inactivated state derived from drug-induced shifts in steady-state availability curves were 14 microM for 3,5-dimethyl-4-chlorophenol, 19 microM for 2-methyl-4-chlorophenol, 26 microM for 4-chlorophenol and 115 microM for 3-methylphenol. All compounds accelerated the current decay during depolarization and slowed recovery from fast inactivation. No relevant frequency-dependent block after depolarizing pulses applied at 10, 50 and 100 Hz was detected for any of the compounds. All the phenol derivatives that we examined are effective blockers of skeletal muscle sodium channels, especially in conditions that are associated with membrane depolarization. Blocking potency is increased by halogenation and by methylation with increasing numbers of methyl groups.

Phenol derivatives accelerate inactivation kinetics in one inactivation-deficient mutant human skeletal muscle Na(+) channel.

Altered inactivation kinetics in skeletal muscle Na(+) channels due to mutations in the encoding gene are causal for the alterations in muscle excitability in nondystrophic myotonia. Na(+) channel blockers like lidocaine and mexiletine, suggested for therapy of myotonia, do not reconstitute inactivation in channels with defective inactivation in vitro. We examined the effects of four methylated and/or halogenated phenol derivatives on one heterologously expressed inactivation-deficient Paramyotonia congenita-mutant (R1448H) muscle Na(+) channel in vitro. All these compounds accelerated delayed inactivation of R1448H-whole-cell currents during a depolarization and delayed accelerated recovery from inactivation. The potency of these effects paralleled the potency of the drugs to block the peak current amplitude. We conclude that the investigated phenol derivatives affect inactivation-deficient Na(+) channels more specifically than lidocaine and mexiletine. However, for all compounds, the effect on inactivation was accompanied by a substantial block of the peak current amplitude.

Propofol blocks human skeletal muscle sodium channels in a voltage-dependent manner.

UNLABELLED: Propofol decreases muscle tone in the absence of neuromuscular blocking drugs. This effect probably cannot be attributed solely to central nervous depression. We studied the effects of propofol on heterologously expressed skeletal muscle sodium channels. Our hypothesis was that the decrease in muscle tone may partly be attributed to an interaction of propofol with sarcolemmal sodium channels. Cells were voltage clamped and whole-cell sodium inward currents were recorded in the absence and presence of propofol. When depolarizing pulses to 0 mV were started from a holding potential close to the normal resting potential of muscle (-70 mV), or when a 2.5-s prepulse inducing slow inactivation was applied before the test pulse at -100 mV, a significant reduction in the peak current amplitude was achieved by 10 and 5 microM propofol, respectively (P < 0.001). Half-maximum blocking concentrations with these protocols were 23 and 22 microM. Blocking potency increased at depolarized membrane potentials with the fraction of inactivated channels; the estimated dissociation constant K(d) from the inactivated state was 4.6 microM. These results suggest that propofol significantly blocks sarcolemmal sodium channels at clinically relevant concentrations while maintaining potentials close to the physiological resting potential. IMPLICATIONS: Voltage-gated sodium channels mediate the initiation and propagation of action potentials along the sarcolemma. Results from our study show that those channels are targeted and blocked in a concentration- and voltage-dependent manner by propofol. This mechanism may contribute to the reduction in muscle excitability.

Anaesthesia with midazolam and S-(+)-ketamine in spontaneously breathing paediatric patients during magnetic resonance imaging.

We evaluated safety and efficacy of a sedation technique based on rectal and intravenous S-(+)-ketamine and midazolam to achieve immobilization during Magnetic Resonance Imaging (MRI). Thirty-four paediatric patients were randomly assigned to undergo either the sedation protocol (study group) or general anaesthesia (control group). Imaging was successfully completed in all children. Children in the study group received a rectal bolus (0.5 mg x kg(-1) midazolam and 5 mg x kg(-1) S-(+)-ketamine) and required additional i.v. supplementation (20+/-10 microg x kg(-1) x min(-1) S-(+)-ketamine and 4+/-2 microg x kg(-1) x min(-1) midazolam), spontaneous ventilation was maintained. Transient desaturation occurred once during sedation and four times in the control group (P=0.34). PECO2 was 5.3+/-0.5 kPa (40+/-4 mm Hg) in the study group and 4.1+/-0.6 kPa (31+/-5 mm Hg) in the control group (P<0.001). Induction and discharge times were shorter in the study group (P<0.001), recovery times did not differ significantly between the groups. Our study confirms that a combination of rectal and supplemental intravenous S-(+)-ketamine plus midazolam is a safe and useful alternative to general anaesthesia for MRI in selected paediatric patients.

Voltage-dependent blockade of normal and mutant muscle sodium channels by benzylalcohol.

1. We studied the effects of benzylalcohol on heterologously expressed wild type (WT), paramyotonia congenita (R1448H) and hyperkalaemic periodic paralysis (M1360V) mutant alpha-subunits of human skeletal muscle sodium channels. 2. Benzylalcohol blocked rested channels at -150 mV membrane potential, with an ECR(50) of 5.3 mM in wild type, 5.1 mM in R1448H, and 6.2 mM in M1360V. When blockade was assessed at -100 mV, the ECR(50) was reduced in R1448H (2 mM) compared with both wild type (4.3 mM; P<0.01) and M1360V (4.3 mM). 3. Membrane depolarization before the test depolarization significantly promoted benzylalcohol-induced sodium channel blockade. The values of K(D) for the fast-inactivated state derived from benzylalcohol-induced shifts in steady-state availability curves were 0.66 mM in wild type and 0.58 mM in R1448H. In the presence of slow inactivation induced by 2.5 s depolarizing prepulses, the ECI(50) for benzylalcohol-induced current inhibition was 0.59 mM in wild type and 0.53 mM in R1448H. 4. Recovery from fast inactivation was prolonged in the presence of drug in all clones. 5. Benzylalcohol induced significant frequency-dependent block at stimulating frequencies of 10, 50, and 100 Hz in all clones. 6. Our results clearly show that benzylalcohol is an effective blocker of muscle sodium channels in conditions that are associated with membrane depolarization. Mutants that enter voltage-dependent inactivation at more hyperpolarized membrane potentials compared with wild type are more sensitive to inhibitory effects at the normal resting potential.

Succinylcholine metabolite succinic acid alters steady state activation in muscle sodium channels.

BACKGROUND: Animal experiments revealed that succinylcholine produced masseter muscle rigidity and activated myotonic discharges despite neuromuscular blockade with a nondepolarizing blocker. These results suggest that either succinylcholine or its metabolites might interfere directly with voltage-operated ion channels of the sarcolemma. The aim of this study was to examine effects of one product of succinylcholine hydrolysis, succinic acid, on voltage-gated muscle sodium (Na+) channels. METHODS: Alpha subunits of human muscle sodium channels were heterologously expressed in HEK293 cells. Activation of Na+ currents was examined applying standard whole-cell voltage-clamp protocols in the absence (control and washout) and presence of succinic acid in different concentrations (0.05-10 mm). RESULTS: Succinic acid shifted the midpoints of steady state activation plots in the direction of more negative test potentials, indicating that channels open during smaller depolarizations in the presence of the drug. The maximum amount of the negative shift in 10 mm succinic acid was -6.3 +/- 1.7 mV; the EC50 for this effect was 0.39 mm. In addition, succinic acid (10 mm) significantly enhanced maximum currents after depolarizations with respect to a series of control experiments. CONCLUSION: Succinic acid facilitates voltage-dependent activation in muscle sodium channels in vitro. This might lead to muscle hyperexcitability in vivo.

Voltage-dependent block of normal and mutant muscle sodium channels by 4-Chloro-m-Cresol.

1 The effects of 4-Chloro-m-Cresol (4-CmC) were examined on heterologously expressed wild type (WT), Paramyotonia Congenita (R1448H) and Hyperkalemic Periodic Paralysis (M1360V) mutant alpha-subunits of human muscle sodium channels. 2 Block of rested sodium channels caused by 4-CmC was concentration-dependent with an ECR50 of 0.40 mM in WT, 0.45 mM in R1448H and 0.49 mM in M1360V. 3 Inactivation significantly promoted 4-CmC-induced sodium channel block in all clones indicated by 4-CmC-induced shifts of steady-state availability curves, reflecting a higher proportion of channel block at depolarized membrane potentials. Channel block was almost complete (>90%) at concentrations close to the ECR50 (0.5 mM) on application of an inactivating prepulse before the test pulse. 4 4-CmC accelerated the current decay following depolarization and prolonged recovery from inactivation in all clones. Of these, R1448H, the mutant which displayed severely impaired inactivation in the controls, responded to 4-CmC with the most pronounced acceleration of inactivation. Control experiments revealed enhanced recovery from inactivation in the mutants, which was restored to normal in 0.1 mM 4-CmC. 5 4-CmC induced no additional frequency-dependent block. 6 Our results clearly demonstrate that 4-CmC is as effective as lidocaine (Fan et al., 1996) in blocking muscle sodium channels. Low concentrations of the compound (

Prevention and emergency first-aid treatment for sports-related dentofacial injuries.

Every day, thousands of athletes participate in sports that are conducive to injuries of the head and face structures. Health-care professionals, coaches, trainers, and the players themselves should be prepared to provide emergency care. More importantly these individuals along with sports administrators, should contribute to the prevention of sports injuries by providing, mandating, and motivating the use of devices that are designed to protect the athlete.