Mechanical and electrophysiological effects of mepivacaine on direct myocardial depression in vitro

The effects of various concentrations (20, 50, and 100 microgramsmol litre-1) of mepivacaine were studied in isolated guinea pig and rat right ventricular papillary muscles by measuring the effects on myocardial contractility and electrophysiological parameters. Mepivacaine produced dose-dependent depression of peak force during 0.5 to 3 Hz stimulation rates in guinea pig papillary muscles. Conduction block was frequently noted, especially at higher stimulation rates (2 and 3 Hz) with mepivacaine 50 and 100 microgramsmol litre-1. In rat papillary muscle experiments, about 20% depression of peak force was shown at rested state contraction. Shortening of action potential (AP) duration (APD50: about 10%, APD90: about 10%) and rate-dependent depression of dV/dt max was observed with mepivacaine 100 microgramsmol litre-1. In 26 mmol litre-1 K+ Tyrode's solution, mepivacaine 50 and 100 microgramsmol litre-1 produced a dose- dependent depression of early (50 microgramsmol litre-1: about 20%, 100 microgramsmol litre-1: about 30%) and late (50 microgramsmol litre-1: about 30%, 100 microgramsmol litre-1: about 50%) force development. In slow APs, neither shortening of AP duration nor changes of dV/dt max were shown by mepivacaine 100 microgramsmol litre-1. An approximate 30% depression of contracture induced by rapid cooling after 2 Hz stimulation rates was observed with mepivacaine 100 microgramsmol litre-1. It may be concluded that the direct myocardial depressant effect of mepivacaine is likely to be caused by inhibition of Ca2+ release from the sarcoplasmic reticulum. The Na+ channel blocking action may contribute indirectly to the depression of contractility.      

Comparative actions of propofol and thiopentone on cell membranes of isolated guineapig ventricular myocytes

We have examined the effects of propofol and thiopentone on membrane potentials and currents of isolated guineapig ventricular myocytes using the whole-cell patch-clamp technique. After current clamping, propofol at concentrations greater than 0.5 microgramsmol litre-1 shortened the plateau and action potential duration (APD) (P < 0.05). Thiopentone 10 microgramsmol litre-1 prolonged APD (P < 0.05), whereas concentrations of 50 microgramsmol litre-1 or higher decreased plateau height (P < 0.05) and resting membrane potential (RMP) (P < 0.05) with abbreviation of the prolonged APD. With voltage clamping, propofol 1 microgramsmol litre-1 decreased the L- type Ca2+ current (ICa,L) to 88.4% of control (P < 0.01) without affecting the delayed rectifier K+ current (IK) and propofol 10 microgramsmol litre-1 decreased ICa,L and IK to 75.0% (P < 0.01) and 78.4% (P < 0.01), respectively, with no effect on the inward rectifier K+ current (IK1). Thiopentone 10 microgramsmol litre-1 decreased ICa,L to 88.5% (P < 0.01) and IK to 78.3% (P < 0.05), while thiopentone 100 microgramsmol litre-1 depressed ICa,L to 82.8% (P < 0.01), IK to 27.0% (P < 0.01) and IK1 to 67.3% (P < 0.05). These results indicated that propofol, at concentrations greater than those that are clinically relevant, shortened APD mainly by suppression of ICa,L, and the biphasic effects on APD by thiopentone were caused by depression of IK, and concomitant suppression of ICa,L and IK1 at higher concentrations. The distinct cardiodepressant effects of propofol and thiopentone may be, at least in part, attributed to different actions on membrane Ca2+ and K+ currents.      

Thiopental and propofol impair relaxation produced by ATP-sensitive potassium channel openers in the rat aorta

ATP-sensitive potassium channel openers are used as vasodilators in the treatment of cardiovascular disorders. The effects of i.v. anaesthetics on arterial relaxation induced by ATP-sensitive potassium channel openers have not been studied. Therefore, in this study, we have examined if thiopental (thiopentone) and propofol affect the vascular response to the ATP-sensitive potassium channel openers, cromakalim and pinacidil, in the isolated rat aorta. Rings of rat thoracic aortas without endothelium were suspended for isometric force recording. Concentration-response curves were obtained in a cumulative manner. During submaximal contractions with phenylephrine 0.3 microgramsmol litre-1, relaxation after cromakalim 0.1-30 microgramsmol litre-1, pinacidil 0.1-30 microgramsmol litre-1 and papaverine 0.1-300 microgramsmol litre-1 was demonstrated. Thiopental 30-300 microgramsmol litre-1, propofol 10-100 microgramsmol litre-1, 10% Intralipid 45 microliters or glibenclamide 5 microgramsmol litre-1 were applied 15 min before addition of phenylephrine. During contractions with phenylephrine, cromakalim and pinacidil induced concentration-dependent relaxation. A selective ATP-sensitive potassium channel antagonist, glibenclamide 5 microgramsmol litre-1, abolished this relaxation, whereas it did not affect relaxation produced by papaverine. Thiopental concentrations > 30 microgramsmol litre-1 significantly impaired relaxation produced by cromakalim or pinacidil. Propofol concentrations > 10 microgramsmol litre-1 also significantly reduced relaxation produced by cromakalim or pinacidil, whereas Intralipid was ineffective. Thiopental 300 microgramsmol litre-1 and propofol 100 microgramsmol litre-1 did not alter relaxation produced by papaverine. These results suggest that the i.v. anaesthetics, thiopental and propofol, impaired vasodilatation mediated by ATP-sensitive potassium channels in vascular smooth muscle cells.      

Differential inhibitory effects of thiopental, thiamylal and phenobarbital on both voltage-gated calcium channels and NMDA receptors in rat hippocampal slices

Although it is known that there are some pharmacological differences between the structurally similar barbiturates, the underlying mechanism of action remains unclear. We have compared the effects of thiopental, thiamylal and phenobarbital on both voltage-gated calcium channels (VGCC) and N-methyl-D-aspartate (NMDA) receptors in rat hippocampal slices by determining changes in intracellular calcium ([Ca2+]i). Experiments were performed in adult rat hippocampal slices perfused with Krebs solution (37 degrees C). Concentrations of [Ca2+]i in the pyramidal cell layer of the CA1 region were measured using a calcium indicator dye, fura-2. To activate VGCC and NMDA receptors, slices were exposed to K+ 60 mmol litre-1 (< or = 60 s) and NMDA 100 microgramsmol litre-1 (30 s), respectively. Thiopental, thiamylal and phenobarbital were present 5 min before, during and 1 min after high K+ or NMDA application. Both thiamylal and thiopental (50-600 microgramsmol litre-1) attenuated the increases in [Ca2+]i produced by high K+ or NMDA in a concentration-dependent manner, while phenobarbital 50-1000 microgramsmol litre- 1 only slightly attenuated the [Ca2+]i increase produced by high K+ at concentrations of more than 200 microgramsmol litre-1 and was ineffective on the [Ca2+]i response produced by NMDA. Although the increases in [Ca2+]i caused by membrane depolarization with high K+ were reduced equally with thiamylal and thiopental, thiamylal was more effective in attenuating the increase in [Ca2+]i produced by NMDA receptor activation than thiopental. We conclude that the depressant effects of barbiturates on both VGCC and NMDA receptors varied between agents. Differential inhibition of both VGCC and NMDA receptors may determine the pharmacological properties of barbiturates and their ability to protect neurones against ischaemia.      

Lidocaine Increases Intracellular Sodium Concentration through Voltage-dependent Sodium Channels in an Identified Lymnaea Neuron

Background: The local anesthetic lidocaine affects neuronal excitability in the central nervous system; however, the mechanisms of such action remain unclear. The intracellular sodium concentration ([Na+]i) and sodium currents (INa) are related to membrane potential and excitability. Using an identifiable respiratory pacemaker neuron from Lymnaea stagnalis, the authors sought to determine whether lidocaine changes [Na+]i and membrane potential and whether INa is related to these changes.

Methods: Intracellular recording and sodium imaging were used simultaneously to measure membrane potentials and [Na+]i, respectively. Measurements for [Na+]i were made in normal, high-Na+, and Na+-free salines, with membrane hyperpolarization, and with tetrodotoxin pretreatment trials. Furthermore, changes of INa were measured by whole cell patch clamp configuration.

Results: Lidocaine increased [Na+]i in a dose-dependent manner concurrent with a depolarization of the membrane potential. In the presence of high-Na+ saline, [Na+]i increased and the membrane potential was depolarized; the addition of lidocaine further increased [Na+]i, and the membrane potential was further depolarized. In Na+-free saline or in the presence of tetrodotoxin, lidocaine did not change [Na+]i. Similarly, hyperpolarization of the membrane by current injections also prevented the lidocaine-induced increase of [Na+]i. In the patch clamp configuration, membrane depolarization by lidocaine led to an inward sodium influx. A persistent reduction in membrane potential, resulting from lidocaine, brings the cell within the window current of INa where sodium channel activation occurs.     

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 microgramsmol litre-1, similar to that reported for lidocaine (43 microgramsmol 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.      

In vitro electrophysiologic effects of morphine in rabbit ventricular myocytes

BACKGROUND: Morphine is widely used in patients undergoing surgical operations and is also reported to mediate cardioprotection of preconditioning. The current study determined effects of morphine at therapeutic to pharmacologic concentrations on cardiac action potential, L-type Ca2+ current (ICa.L), delayed rectifier K+ current (IK), and inward rectifier K+ current (IK1) in isolated rabbit ventricular myocytes. METHODS: Ventricular myocytes were enzymatically isolated from rabbit hearts. Action potential and membrane currents were recorded in current and voltage clamp modes. RESULTS: Morphine at concentrations from 0.01 to 1 microM significantly prolonged cardiac action potential, and at 0.1 and 1 microM slightly but significantly hyperpolarized the resting membrane potential. In addition, morphine at 0.1 microM significantly augmented ICa.L (at +10 mV) from 5.9 +/- 1.9 to 7.3 +/- 1.7 pA/pF (by 23%; P < 0.05 vs. control) and increased IK1 (at -60 mV) from 2.8 +/- 1.0 to 3.5 +/- 0.9 pA/pF (by 27%; P < 0.05 vs. control). Five microM naltrindole (a selective delta-opioid receptor antagonist) or 5 microM norbinaltorphimine (a selective kappa-opioid receptor antagonist) prevented the increase in ICa.L induced by morphine, but 5 microM CTOP (a selective mu-opioid receptor antagonist) did not. The three types of opioid antagonists did not affect the augmentation of IK1 by morphine. Morphine had no effect on IK. CONCLUSIONS: These results indicate that morphine prolongs action potential duration by increasing ICa.L, an effect mediated by delta- and kappa-opioid receptors. It also hyperpolarizes cardiac resting membrane potential by increasing IK1, which is not mediated by opioid receptors.     

In Vitro Electrophysiologic Effects of Morphine in Rabbit Ventricular Myocytes

Background: Morphine is widely used in patients undergoing surgical operations and is also reported to mediate cardioprotection of preconditioning. The current study determined effects of morphine at therapeutic to pharmacologic concentrations on cardiac action potential, L-type Ca2+ current (ICa.L), delayed rectifier K+ current (IK), and inward rectifier K+ current (IK1) in isolated rabbit ventricular myocytes.

Methods: Ventricular myocytes were enzymatically isolated from rabbit hearts. Action potential and membrane currents were recorded in current and voltage clamp modes.

Results: Morphine at concentrations from 0.01 to 1 [mu]m significantly prolonged cardiac action potential, and at 0.1 and 1 [mu]m slightly but significantly hyperpolarized the resting membrane potential. In addition, morphine at 0.1 [mu]m significantly augmented ICa.L (at +10 mV) from 5.9 +/- 1.9 to 7.3 +/- 1.7 pA/pF (by 23%; P < 0.05 vs. control) and increased IK1 (at -60 mV) from 2.8 +/- 1.0 to 3.5 +/- 0.9 pA/pF (by 27%; P < 0.05 vs. control). Five [mu]m naltrindole (a selective [delta]-opioid receptor antagonist) or 5 [mu]m norbinaltorphimine (a selective [kappa]-opioid receptor antagonist) prevented the increase in ICa.L induced by morphine, but 5 [mu]m CTOP (a selective [mu]-opioid receptor antagonist) did not. The three types of opioid antagonists did not affect the augmentation of IK1 by morphine. Morphine had no effect on IK.     

Modulation of Cardiac Sodium Current by alpha sub 1 -stimulation and Volatile Anesthetics

Background: alpha1 -adrenoceptor stimulation is known to produce electrophysiologic changes in cardiac tissues, which may involve modulations of the fast inward Na sup + current (INa). A direct prodysrhythmic alpha1 -mediated interaction between catecholamines and halothane has been demonstrated, supporting the hypothesis that generation of halothane-epinephrine dysrhythmias may involve slowed conduction, leading to reentry. In this study, we examined the effects of a selective alpha1 -adrenergic receptor agonist, methoxamine, on cardiac INa in the absence and presence of equianesthetic concentrations of halothane and isoflurane in single ventricular myocytes from adult guinea pig hearts.

Methods: INa was recorded using the standard whole-cell configuration of the patch-clamp technique. Voltage clamp protocols initiated from two different holding potentials (VH) were applied to examine state-dependent effects of methoxamine in the presence of anesthetics. Steady state activation and inactivation and recovery from inactivation were characterized using standard protocols.

Results: Methoxamine decreased INa in a concentration- and voltage-dependent manner, being more potent at the depolarized VH. Halothane and isoflurane interacted synergistically with methoxamine to suppress INa near the physiologic cardiac resting potential of -80 mV. The effect of methoxamine with anesthetics appeared to be additive when using a VH of -110 mV, a potential where no Na sup + channels are in the inactivated state. Methoxamine in the absence and presence of anesthetics significantly shifted the half maximal inactivation voltage in the hyperpolarizing direction but had no effect on steady-state activation.     

Antioxidant effects of propofol in human hepatic microsomes: concentration effects and clinical relevance

Propofol is known to possess antioxidant properties. There is controversy regarding the mechanisms by which the drug produces its antioxidant effects and the significance of these effects in relation to plasma concentrations of propofol in clinical practice. We studied the effects of increasing concentrations of Intralipid, propofol, butylated hydroxytoluene (BHT) and a vitamin E analogue (Trolox C) in 0.9% saline on non-enzymic and enzymic lipid peroxidation in human hepatic microsomes, and on concentrations of antioxidant enzymes in a Hep G2 cell line. Propofol showed significant inhibition of lipid peroxidation, but was less potent than BHT or Trolox C. IC50 values for non-enzymic and enzymic lipid peroxidation were mean 9.47 (SD 0.86) and 7.39 (0.84) microgramsmol litre-1 for propofol, 1.30 (0.57) and 0.32 (0.02) microgramsmol litre-1 for BHT and 2.34 (0.68) and 0.35 (0.04) microgramsmol litre-1 for Trolox C, respectively. The antioxidant activities of propofol were substantially retained in the presence of up to 30 g litre-1 of human serum albumin. Propofol at concentrations of up to 100 microgramsmol litre-1 had no significant effect on the activities of antioxidant enzymes. Clinically relevant concentrations of propofol produced significant inhibition of both enzymic and non-enzymic lipid peroxidation in hepatic microsomal preparations, possibly as a result of accumulation in lipophilic environments. Measurement of antioxidant effects of drugs in aqueous media may have little relevance to their effects in protecting against lipid peroxidation in biological systems.      

Mechanisms underlying the inotropic action of halothane on intact rat ventricular myocytes

The mechanisms contributing to the negative inotropic effect of halothane were studied in isolated rate ventricular myocytes. Contraction and intracellular Ca2+ transients were measured optically in these cells. The initial application of halothane (2% or 0.5 mmol litre-1) led to short-lived increases in the Ca2+ transient and contraction, which were abolished by ryanodine. Continued application of halothane led to a sustained decrease in contraction: this resulted from: (i) a decrease in myofilament Ca2+ sensitivity; (ii) a decrease in the Ca2+ transient; and (iii) a decrease in the Ca2+ content of the sarcoplasmic reticulum. Although halothane reduced action potential duration, the sustained negative inotropic effect was similar when action potentials or voltage clamp pulses of constant duration were used to trigger contractions. In cells exposed to nifedipine 0.5 microgramsmol litre-1 (which decreases the L-type Ca2+ current, ICa), Ca2+ transients, sarcoplasmic reticulum Ca2+ content and fractional release (the fraction of sarcoplasmic reticulum Ca2+ content released during each stimulus) were reduced. Halothane 0.5 mmol litre-1 (which also decreases ICa) decreased Ca2+ transients to a lesser extent and reduced sarcoplasmic reticulum Ca2+ content to a greater extent than nifedipine, whereas fractional release was unchanged compared with control. These data suggest that halothane sensitizes Ca(2+)-induced Ca2+ release from the sarcoplasmic reticulum in addition to reducing ICa.      

Effects of isoflurane on receptor-operated Ca2+ channels in rat aortic smooth muscle

We have investigated the effects of isoflurane on receptor-operated Ca2+ channels (ROC) in vascular smooth muscle. In isolated rat thoracic aortic rings denuded of endothelium, the effects of isoflurane on phenylephrine-induced contraction and Ca2+ influx were evaluated in the presence of supramaximal doses of nifedipine or verapamil. Under isometric tension recording, the aortic rings were precontracted by phenylephrine 300 nmol litre-1 and exposed to 1.2%, 2.3% or 3.5% isoflurane. Phenylephrine-induced precontraction was enhanced with 2.3% isoflurane by mean 8.1 (SD 9.3)% (P < 0.05 vs 0% isoflurane). The constrictor effect of 2.3% isoflurane was not inhibited by depletion of intracellular Ca2+ stores with ryanodine 20 microgramsmol litre-1, but was abolished in a Ca(2+)-free solution or by SK&F 96,365 30 microgramsmol litre-1, an ROC blocker. Isoflurane-induced contraction was accompanied by increased intracellular free Ca2+ concentration, monitored using fura PE3. Unidirectional 45Ca2+ influx measurement in phenylephrine- stimulated aortic strips revealed that the mean amount of Ca2+ influx was significantly (P < 0.05) enhanced by 1.2% and 2.3% isoflurane, which were 117.1% and 119.7% of control values, respectively. Our results strongly suggest that isoflurane enhanced Ca2+ influx through ROC that had been submaximally activated by phenylephrine.      

Ketamine inhibits nitric oxide production in mouse-activated macrophage- like cells

We have investigated the effects of ketamine on nitric oxide produced by activated macrophages using a murine macrophage-like cell line, J774. Cells were incubated for 18 h under stimulation with lipopolysaccharide and interferon-gamma or lipoteichoic acid and interferon-gamma, with various concentrations of ketamine (6-600 microgramsmol litre-1). Nitric oxide production was assessed by measuring nitrite, a stable by-product of nitric oxide breakdown, in the medium. Other N- methyl-D-aspartate receptor antagonists, MK-801 (150 microgramsmol litre-1) and dextromethorphan (150 microgramsmol litre-1) were also tested. In addition, we studied the effects of ketamine on production of tumour necrosis factor- alpha by activated macrophages. Ketamine inhibited nitrite production dose-dependently with both lipopolysaccharide- and lipoteichoic acid- activated macrophages by up to approximately 65% at the highest ketamine concentration (600 microgramsmol litre-1). Neither MK-801 nor dextromethorphan had an inhibitory effect. Ketamine also suppressed production of tumour necrosis factor-alpha. The data show that ketamine inhibited nitric oxide production by activated macrophages probably, in part, via inhibition of production of tumour necrosis factor-alpha, an autocrine stimulatory factor for nitric oxide production, but not via the NMDA receptor pathway, which is involved in neuronal nitric oxide production.      

Stereospecific effects of ketamine on dopamine efflux and uptake in the rat nucleus accumbens

In addition to being a general anaesthetic, ketamine is a recognized drug of abuse. Many, if not all, drugs of abuse have been shown to increase dopamine efflux in the nucleus accumbens (NAc). As ketamine is optically active, we examined if its actions on dopamine efflux in the NAc were stereoselective. Slices of rat NAc were superfused with artificial CSF at 32 degrees C. Dopamine efflux was evoked by electrical stimulation (1 or 20 pulses, 100 Hz) and measured using fast cyclic voltammetry. (+/-)-Ketamine 100 microgramsmol litre-1 increased dopamine efflux (to mean 174 (SEM 17)% of control, P < 0.05) and slowed dopamine uptake half-time (T1/2) to 164 (17)% of control, as did (+)-ketamine 100 microgramsmol litre-1 (efflux 236 (16)% (P < 0.001); uptake T1/2 177 (25)% (P < 0.05)). The (-)-isomer was inactive. The effect of (+)-ketamine on dopamine efflux did not correlate with its action on dopamine uptake. (+)-Ketamine increased dopamine efflux on single pulse stimulation but to a lesser extent than on 20 pulse trains (P < 0.05). (+)-Ketamine was unable to block the inhibitory effect of quinpirole on single pulse dopamine efflux. Neither MK 801 10 microgramsmol litre-1 nor metoclopramide 1 microgramsmol litre-1 had any effect on dopamine release after short train stimuli (20 pulses, 100 Hz). We conclude that the (+)-isomer is the active form of ketamine and increases NAc dopamine efflux not by block of dopamine uptake; autoreceptors or NMDA receptors, but by mobilization of the dopamine storage pool to releasable sites.      

Role of prostaglandins and nitric oxide on halothane-induced arteriolar dilatation in rat diaphragm

The effects of anaesthetics on the microcirculation of the diaphragm are incompletely understood. Therefore, we assessed by in vivo intravital microscopy in rats the action of halothane on diaphragmatic arteriolar diameter and the role of nitric oxide and prostaglandins on halothane-induced diaphragmatic arteriolar diameter. We studied 54 rats anaesthetized with thiopentone. Dose-response curves to topically applied Krebs' solution saturated with halothane at increasing concentrations of 0%, 1%, 3% and 5% were carried out in the presence of an inhibitor of nitric oxide synthesis (N omega-nitro-L-arginine (LNA), 300 microgramsmol litre-1) or inhibitors of prostaglandin synthesis (mefenamic acid 20 microgramsmol litre-1 or indomethacin 20 mol litre-1) or in the absence of any inhibitor. We found dose-dependent arteriolar dilatation which was abolished by mefenamic acid and indomethacin. In contrast, the effect of halothane was not modified by LNA. These data demonstrated that halothane-induced arteriolar dilatation in the diaphragm of the rat was mediated by prostaglandins but not by nitric oxide.      

Lidocaine increases intracellular sodium concentration through voltage-dependent sodium channels in an identified lymnaea neuron

BACKGROUND: The local anesthetic lidocaine affects neuronal excitability in the central nervous system; however, the mechanisms of such action remain unclear. The intracellular sodium concentration ([Na]i) and sodium currents (INa) are related to membrane potential and excitability. Using an identifiable respiratory pacemaker neuron from Lymnaea stagnalis, the authors sought to determine whether lidocaine changes [Na]i and membrane potential and whether INa is related to these changes. METHODS: Intracellular recording and sodium imaging were used simultaneously to measure membrane potentials and [Na]i, respectively. Measurements for [Na]i were made in normal, high-Na, and Na-free salines, with membrane hyperpolarization, and with tetrodotoxin pretreatment trials. Furthermore, changes of INa were measured by whole cell patch clamp configuration. RESULTS: Lidocaine increased [Na]i in a dose-dependent manner concurrent with a depolarization of the membrane potential. In the presence of high-Na saline, [Na]i increased and the membrane potential was depolarized; the addition of lidocaine further increased [Na]i, and the membrane potential was further depolarized. In Na-free saline or in the presence of tetrodotoxin, lidocaine did not change [Na]i. Similarly, hyperpolarization of the membrane by current injections also prevented the lidocaine-induced increase of [Na]i. In the patch clamp configuration, membrane depolarization by lidocaine led to an inward sodium influx. A persistent reduction in membrane potential, resulting from lidocaine, brings the cell within the window current of INa where sodium channel activation occurs. CONCLUSION: Lidocaine increases intracellular sodium concentration and promotes excitation through voltage-dependent sodium channels by altering membrane potential in the respiratory pacemaker neuron.     

Thiopental attenuates relaxation and cyclic GMP production in vascular smooth muscle of endotoxin-treated rat aorta, independent of nitric oxide production

As thiopental (thiopentone) suppresses cyclic GMP (cGMP) formation produced by nitric oxide donor drugs, we have tested if it suppresses cGMP formation and increases vascular tone after induction of calcium- calmodulin-independent nitric oxide synthase (iNOS). Rat aortic rings were treated with Escherichia coli lipopolysaccharide (LPS) 1 microgram ml-1 for 4 h, and the effects of thiopental on tension, cGMP concentrations and nitrite accumulation were determined. Thiopental 0.3 mmol litre-1 reduced the tension of phenylephrine-precontracted aortic rings before LPS treatment, but caused no significant effects on tension in the presence of L-arginine 10 microgramsmol litre-1 after LPS treatment. L-Arginine 1 microgramsmol litre-1 to 1 mmol litre-1 increased concentrations of cGMP in LPS-treated aorta in a concentration- dependent manner. This was reduced by thiopental 0.3-1 mmol litre-1. Treatment with L-arginine 1 mmol litre-1 increased concentrations of nitrite, the end product of nitric oxide; this was not affected by thiopental 1 mmol litre-1. We conclude that thiopental suppressed cGMP formation in iNOS-induced vascular smooth muscle without affecting nitric oxide production.      

Effects of tramadol stereoisomers on norepinephrine efflux and uptake in the rat locus coeruleus measured by real time voltammetry

Despite its structural similarity to codeine, tramadol is an unusual analgesic whose antinociceptive efficacy is not solely a result of opioid actions but also of its apparent capacity to block monoamine uptake. Tramadol is a mixture of stereoisomers. In this study, we have examined the actions of racemic, (+)- and (-)-tramadol, in addition to O-desmethyltramadol (the main human metabolite), on electrically evoked norepinephrine efflux and uptake in the locus coeruleus brain slice, measured by fast cyclic voltammetry. Racemic tramadol and its (+)- and (-)-enantiomers (all at 5 microgramsmol litre-1) significantly increased stimulated norepinephrine efflux (P < 0.01) by mean 66 (SEM 10)%, 57 (7)% and 64 (13)%, respectively. However, only (-)-tramadol blocked norepinephrine reuptake (P < 0.01), increasing the reuptake half-time to 499 (63)% of pre-drug values. The metabolite O-desmethyl tramadol was inactive at the concentration tested (5 microgramsmol litre-1). In the case of (-)-tramadol, the effect on norepinephrine efflux was directly proportional to, but significantly smaller than, the effect on norepinephrine uptake (P < 0.01). This appeared to be a result of compensatory alpha 2A autoreceptor tone as the selective alpha 2A autoreceptor antagonist BRL 44408 (1 microgramsmol litre-1) eliminated this difference when its own effects on norepinephrine reuptake were taken into account. The efficacy of (-)-tramadol on norepinephrine uptake, at clinically relevant concentrations, may contribute to its antinociceptive efficacy.      

Fentanyl inhibits metabolism of midazolam: competitive inhibition of CYP3A4 in vitro

Fentanyl decreases clearance of midazolam administered i.v., but the mechanism remains unclear. To elucidate this mechanism, we have investigated the effect of fentanyl on metabolism of midazolam using human hepatic microsomes and recombinant cytochrome P450 isoforms (n = 6). Midazolam was metabolized to l'-hydroxymidazolam (l'-OH MDZ) by human hepatic microsomes, with a Michaelis-Menten constant (K(m)) of 5.0 (SD 2.7) microgramsmol litre-1. Fentanyl competitively inhibited metabolism of midazolam in human hepatic microsomes, with an inhibition constant (Ki) of 26.8 (12.4) microgramsmol litre-1. Of the seven representative human hepatic P450 isoforms, CYP1A2, 2A6, 2C9, 2C19, 2D6, 2E1 and 3A4, only CYP3A4 catalysed hydroxylation of midazolam, with a K(m) of 3.6 (0.8) microgramsmol liter-1. Fentanyl competitively inhibited metabolism of midazolam to l'-OH MDZ by CYP3A4, with a Ki of 24.2 (6.8) microgramsmol litre-1, comparable with the Ki obtained in human hepatic microsomes. These findings indicate that fentanyl competitively inhibits metabolism of midazolam by CYP3A4.      

Conformational State-dependent Effects of Halothane on Cardiac Na sup + Current

Background: The Na sup + channel is voltage gated and characterized by three distinct states: closed, open, and inactivated. To identify the effects of halothane on the cardiac Na sup + current (INa) at various membrane potentials, the effects of 1.2 mm halothane at different holding potentials (VH) on INa were examined in single, enzymatically isolated guinea pig ventricular myocytes.

Methods: The INa was recorded using the whole-cell configuration of the patch-clamp technique. Currents were generated from resting VH s of -110, -80, or -65 mV. State-dependent block was characterized by monitoring frequency dependence, tonic block, and removal of inactivation by veratridine.

Results: Halothane produced significant (P < 0.05) VH -dependent depressions of peak INa (mean +/- SEM): 24.4 +/- 4.1% (VH = -110 mV), 42.1 +/- 3.4% (VH = -80 mV), and 75.2 +/- 1.5% (VH = -65 mV). Recovery from inactivation was significantly increased when cells were held at -80 mV (control, tau = 6.0 +/- 0.3 ms; halothane, tau = 7.1 +/- 0.4 ms), but not at -110 mV. When using a VH of -80 mV, halothane exhibited a use-dependent block, with block of INa increasing from 8.6 +/- 1.4% to 30.7 +/- 3.5% at test pulse rates of 2 and 11 Hz, respectively. Use-dependent inhibition was not apparent at VH of -110 mV. When inactivation of INa was removed by exposure to 100 micro Meter veratridine, no significant difference was observed in the depressant effect of halothane at both VH s: 26.6 +/- 4.5% (VH = -80 mV) and 26.4 +/- 5.6% (VH = -110 mV).