Molecular Mechanisms of the Inhibitory Effects of Propofol and Thiamylal on Sarcolemmal Adenosine Triphosphate-sensitive Potassium Channels

Background: Both propofol and thiamylal inhibit adenosine triphosphate-sensitive potassium (KATP) channels. In the current study, the authors investigated the effects of these anesthetics on the activity of recombinant sarcolemmal KATP channels encoded by inwardly rectifying potassium channel (Kir6.1 or Kir6.2) genes and sulfonylurea receptor (SUR1, SUR2A, or SUR2B) genes.

Methods: The authors used inside-out patch clamp configurations to investigate the effects of propofol and thiamylal on the activity of recombinant KATP channels using COS-7 cells transfected with various types of KATP channel subunits.

Results: Propofol inhibited the activities of the SUR1/Kir6.2 (EC50 = 77 [mu]m), SUR2A/Kir6.2 (EC50 = 72 [mu]m), and SUR2B/Kir6.2 (EC50 = 71 [mu]m) channels but had no significant effects on the SUR2B/Kir6.1 channels. Propofol inhibited the truncated isoform of Kir6.2 (Kir6.2[DELTA]C36) channels (EC50 = 78 [mu]m) that can form functional KATP channels in the absence of SUR molecules. Furthermore, the authors identified two distinct mutations R31E (arginine residue at position 31 to glutamic acid) and K185Q (lysine residue at position 185 to glutamine) of the Kir6.2[DELTA]C36 channel that significantly reduce the inhibition of propofol. In contrast, thiamylal inhibited the SUR1/Kir6.2 (EC50 = 541 [mu]m), SUR2A/Kir6.2 (EC50 = 248 [mu]m), SUR2B/Kir6.2 (EC50 = 183 [mu]m), SUR2B/Kir6.1 (EC50 = 170 [mu]m), and Kir6.2[DELTA]C36 channels (EC50 = 719 [mu]m). None of the mutants significantly affects the sensitivity of thiamylal.     

Molecular Mechanisms Underlying Ketamine-mediated Inhibition of Sarcolemmal Adenosine Triphosphate-sensitive Potassium Channels

Background: Ketamine inhibits adenosine triphosphate-sensitive potassium (KATP) channels, which results in the blocking of ischemic preconditioning in the heart and inhibition of vasorelaxation induced by KATP channel openers. In the current study, the authors investigated the molecular mechanisms of ketamine's actions on sarcolemmal KATP channels that are reassociated by expressed subunits, inwardly rectifying potassium channels (Kir6.1 or Kir6.2) and sulfonylurea receptors (SUR1, SUR2A, or SUR2B).

Methods: The authors used inside-out patch clamp configurations to investigate the effects of ketamine on the activities of reassociated Kir6.0/SUR channels containing wild-type, mutant, or chimeric SURs expressed in COS-7 cells.

Results: Ketamine racemate inhibited the activities of the reassociated KATP channels in a SUR subtype-dependent manner: SUR2A/Kir6.2 (IC50 = 83 [mu]m), SUR2B/Kir6.1 (IC50 = 77 [mu]m), SUR2B/Kir6.2 (IC50 = 89 [mu]m), and SUR1/Kir6.2 (IC50 = 1487 [mu]m). S-(+)-ketamine was significantly less potent than ketamine racemate in blocking all types of reassociated KATP channels. The ketamine racemate and S-(+)-ketamine both inhibited channel currents of the truncated isoform of Kir6.2 (Kir6.2[DELTA]C36) with very low affinity. Application of 100 [mu]m magnesium adenosine diphosphate significantly enhanced the inhibitory potency of ketamine racemate. The last transmembrane domain of SUR2 was essential for the full inhibitory effect of ketamine racemate.     

Distinct Roles for Sarcolemmal and Mitochondrial Adenosine Triphosphate-sensitive Potassium Channels in Isoflurane-induced Protection against Oxidative Stress

Background: Cardiac preconditioning, including that induced by halogenated anesthetics, is an innate protective mechanism against ischemia-reperfusion injury. The adenosine triphosphate-sensitive potassium (KATP) channels are considered essential in preconditioning mechanism. However, it is unclear whether KATP channels are triggers initiating the preconditioning signaling, and/or effectors responsible for the cardioprotective memory and activated during ischemia-reperfusion.

Methods: Adult rat cardiomyocytes were exposed to oxidative stress with 200 [mu]m H2O2 and 100 [mu]m FeSO4. Myocyte survival was determined based on morphologic characteristics and trypan blue exclusion. To induce preconditioning, the myocytes were pretreated with isoflurane. The involvement of sarcolemmal and mitochondrial KATP channels was investigated using specific inhibitors HMR-1098 and 5-hydroxydecanoic acid. Data are expressed as mean +/- SD.

Results: Oxidative stress induced cell death in 47 +/- 14% of myocytes. Pretreatment with isoflurane attenuated this effect to 26 +/- 8%. Blockade of the sarcolemmal KATP channels abolished the protection by isoflurane pretreatment when HMR-1098 was applied throughout the experiment (50 +/- 21%) or only during oxidative stress (50 +/- 12%), but not when applied during isoflurane pretreatment (29 +/- 13%). Inhibition of the mitochondrial KATP channels abolished cardioprotection irrespective of the timing of 5-hydroxydecanoic acid application. Cell death was 42 +/- 23, 45 +/- 23, and 46 +/- 22% when 5-hydroxydecanoic acid was applied throughout the experiment, only during isoflurane pretreatment, or only during oxidative stress, respectively.     

Mechanism of Cardiac Sarcolemmal Adenosine Triphosphate-sensitive Potassium Channel Activation by Isoflurane in a Heterologous Expression System

Background: Activation of the cardiac sarcolemmal adenosine triphosphate-sensitive potassium (KATP) channel during metabolic stress initiates cellular events that preserve cardiac performance. Previous studies showed that halogenated anesthetics prime KATP channels under whole cell voltage clamp and act in intracellular pH (pHi)-dependent manner on KATP channels in excised membrane patches. However, it is not known how halogenated anesthetics interact with these channels.

Methods: The authors evaluated the effect of pHi and isoflurane on the KATP channel subunits, the pore-forming inward rectifier Kir6.2, and the regulatory sulfonylurea receptor SUR2A, using HEK293 cells as a heterologous expression system. Single channel activity was recorded in the inside-out patch configuration.

Results: At pHi 7.4, isoflurane had negligible effect on activity of wild-type Kir6.2/SUR2A, but at pHi 6.8, the channel open probability was increased by isoflurane (0.177 +/- 0.077 to 0.364 +/- 0.164). By contrast, the open probability of truncated Kir6.2[DELTA]C26, which forms a functional channel without SUR2A, was attenuated by isoflurane at both pHi 7.4 and pHi 6.8. Coexpression of Kir6.2[DELTA]C26 with SUR2A restored pHi sensitivity of channel activation by isoflurane. Site-directed mutagenesis within the Walker motifs of SUR2A abolished isoflurane activation of KATP channel at pHi 6.8. In addition, the pancreatic-type channels expressing sulfonylurea receptor SUR1 could not be activated by isoflurane.     

Isoflurane Activates Sarcolemmal Adenosine Triphosphate-sensitive Potassium Channels in Vascular Smooth Muscle Cells: A Role for Protein Kinase A

Background: Recent evidence indicates that vascular adenosine triphosphate-sensitive potassium (KATP) channels in vascular smooth muscle cells are critical in the regulation of vascular tonus under both physiologic and pathophysiologic conditions. Studies of the interaction of volatile anesthetics with vascular KATP channels have been limited. In the current study, the authors investigated the molecular mechanism of isoflurane's action on vascular KATP channels.

Methods: Electrophysiologic experiments were performed using cell-attached and inside-out patch clamp techniques to monitor native vascular KATP channels, and recombinant KATP channels comprised of inwardly rectifying potassium channel subunits (Kir6.1) and the sulfonylurea receptor (SUR2B). Isometric tension experiments were performed in rat thoracic aortic rings without endothelium.

Results: Application of isoflurane (0.5 mm) to the bath solution during cell-attached recordings induced a significant increase in KATP channel activity, which was greatly reduced by pretreatment with a selective inhibitor of protein kinase A (PKA), Rp-cAMPS (100 [mu]m). In inside-out patches, isoflurane did not activate KATP channels. Isoflurane significantly activated wild-type recombinant SUR2B/Kir6.1 in cell-attached patches. Isoflurane-induced activation of wild-type channels was diminished in the PKA-insensitive mutant SUR2B-T633A/Kir6.1, SUR2B-S1465A/Kir6.1, and SUR2B/Kir6.1-S385A. In addition, the authors demonstrated that isoflurane-induced PKA activation was associated with isoflurane-induced decreases in isometric tension in the rat aorta.     

Blockade of Adenosine Triphosphate-sensitive Potassium Channels by Thiamylal in Rat Ventricular Myocytes

Background: The adenosine triphosphate (ATP)-sensitive potassium (KATP) channels protect myocytes during ischemia and reperfusion. This study investigated the effects of thiamylal on the activities of KATP channels in isolated rat ventricular myocytes during simulated ischemia.

Methods: Male Wistar rats were anesthetized with ether. Single, quiescent ventricular myocytes were dispersed enzymatically. Membrane currents were recorded using patch-clamp techniques. In the cell-attached configuration, KATP channel currents were assessed before and during activation of these channels by 2,4-dinitrophenol and after administration of 25, 50, and 100 mg/l thiamylal. The open probability was determined from current-amplitude histograms. In the inside-out configuration, the current-voltage relation was obtained before and after the application of thiamylal (50 mg/l).

Results: In the cell-attached configuration, 2,4-dinitrophenol caused frequent channel opening. 2,4-Dinitrophenol-induced channel activities were reduced significantly by glibenclamide, suggesting that the channels studied were KATP channels. Open probability of KATP channels was reduced by thiamylal in a concentration-dependent manner. KATP channels could be activated in the inside-out configuration because of the absence of ATP. Thiamylal inhibited KATP channel activity without changing the single-channel conductance.     

The Placental Transfer and Fetal Effects of Levobupivacaine, Racemic Bupivacaine, and Ropivacaine

Background: The purposes of this study were to assess the effects of levobupivacaine on uterine blood flow and fetal well-being and to compare its placental transfer with that of bupivacaine and ropivacaine.

Methods: After a control period, pregnant ewes that were fitted with instruments for long-term monitoring were randomized to receive a two-step intravenous infusion of levobupivacaine, bupivacaine, or ropivacaine, in a blinded manner, for 1 h. Maternal and fetal hemodynamics were monitored during the study. Arterial blood samples were drawn at 30 and 60 min of infusion from the mother and fetus to determine the acid-base status (60 min only) and serum drug concentrations. The fetal brain, heart, liver, lungs, adrenal glands, and kidneys were obtained to measure tissue drug levels.

Results: Maternal blood pressure, central venous and intraamniotic pressures, acid-base status and uterine blood flow were unaffected by any drug infusion. In contrast to the other two local anesthetics, the infusion of bupivacaine was associated with a small but significant decrease in the ewe's heart rate. At the end of the study, the heart rate in the bupivacaine-treated animals was significantly less than in the animals treated with the other two drugs. All fetuses were in good condition at the start of study, and none of the local anesthetics affected fetal well-being. No significant differences were found among the three drugs in the maternal serum, fetal serum, fetal tissue concentrations, and tissue:serum concentration ratios.     

Role of Adenosine Triphosphate-sensitive Potassium Channels in Coronary Vasodilation by Halothane, Isoflurane, and Enflurane

Background: Halothane, isoflurane, and enflurane cause coronary vasodilation and cardiac depression. This study was performed to assess the role of adenosine triphosphate (ATP)-sensitive potassium channels (K (ATP) channels) in these effects.

Methods: Twenty-five thoracotomized dogs were anesthetized with fentanyl and midazolam. The left anterior descending coronary artery was perfused via either of two pressurized (80 mmHg) reservoirs. One reservoir was supplied with arterial blood free of a volatile anesthetic, and the second reservoir was supplied with arterial blood equilibrated in an oxygenator with a 1 minimum alveolar concentration of either halothane (0.9%, n = 10), isoflurane (1.4%, n = 28), or enflurane (2.2%, n = 7). Coronary blood flow (CBF) was measured using a Doppler flow transducer, and segmental shortening (SS) was measured with ultrasonic crystals. Responses to the volatile anesthetics were assessed under control conditions, during intracoronary infusion of the KATP channel inhibitor glibenclamide (100 micro gram/min), and after cessation of glibenclamide (recovery). The effectiveness of glibenclamide was verified from inhibition of coronary vasodilator responses to the KATP channel opener cromakalim without effect on those to the KATP channel-independent vasodilators, sodium nitroprusside and acetylcholine.

Results: Under control conditions, the volatile anesthetics caused pronounced increases in CBF (isoflurane > halothane = enflurane), and decreases in SS (enflurane > halothane = isoflurane). Glibenclamide blunted significantly (and reversibly) the increases in CBF, but it had no effect on the decreases in SS.     

Effects of CNS Site-directed Carotid Arterial Infusions of Bupivacaine, Levobupivacaine, and Ropivacaine in Sheep

Background: Previous preclinical safety studies in ewes have found intravenous levobupivacaine and ropivacaine to be less potent toward causing central nervous system (CNS) and cardiac toxicity than bupivacaine. Analogous cardiotoxicity has been demonstrated directly in various cardiac preparations ex vivo. Moreover, drug-related arrhythmogenicity has been demonstrated from direct CNS injection of local anesthetic agents in vivo, suggesting CNS-related cardiotoxicity. This study investigated whether CNS site-directed blood-borne drug administration (with minimal systemic recirculation) would demonstrate drug-related cardiotoxicity.

Methods: Direct CNS effects and indirect cardiotoxic sequelae were determined after bilateral carotid arterial infusions of levobupivacaine, bupivacaine, or ropivacaine in ewes. After pilot studies to validate the procedures, equimolar doses (24-96 [mu]mol, [almost equal to]7.5-30 mg) were infused over 3 min using a crossover design. Behavioral CNS signs, quantitative electroencephalographic (EEG), cardiovascular, and electrocardiographic effects were recorded. Drug blood concentrations in superior sagittal sinus and aorta were measured serially.

Results: Blood drug concentrations in the superior sagittal sinus were 5-10 times those concurrently in the aorta, confirming highly selective CNS delivery with minimal systemic recirculation. Dose-dependent CNS excitatory behavior and EEG changes, with increased mean arterial blood pressure, heart rate, cardiac output, and myocardial contractility, were found, consistent with sympathetic nervous system stimulation. The overall rank order of potency for these effects was ropivacaine < levobupivacaine < bupivacaine. Nonfatal cardiac arrhythmias were observed, but the type or frequency did not differ between drugs.     

Sarcolemmal and Mitochondrial Adenosine Triphosphate- dependent Potassium Channels: Mechanism of Desflurane-induced Cardioprotection

Background: Volatile anesthetic-induced preconditioning is mediated by adenosine triphosphate-dependent potassium (KATP) channels; however, the subcellular location of these channels is unknown. The authors tested the hypothesis that desflurane reduces experimental myocardial infarct size by activation of specific sarcolemmal and mitochondrial KATP channels.

Methods: Barbiturate-anesthetized dogs (n = 88) were acutely instrumented for measurement of aortic and left ventricular pressures. All dogs were subjected to a 60-min left anterior descending coronary artery occlusion followed by 3-h reperfusion. In four separate groups, dogs received vehicle (0.9% saline) or the nonselective KATP channel antagonist glyburide (0.1 mg/kg intravenously) in the presence or absence of 1 minimum alveolar concentration desflurane. In four additional groups, dogs received 45-min intracoronary infusions of the selective sarcolemmal (HMR 1098; 1 [mu]g [middle dot] kg-1 [middle dot] min-1) or mitochondrial (5-hydroxydecanoate [5-HD]; 150 [mu]g [middle dot] kg-1 [middle dot] min-1) KATP channel antagonists in the presence or absence of desflurane. Myocardial perfusion and infarct size were measured with radioactive microspheres and triphenyltetrazolium staining, respectively.

Results: Desflurane significantly (P < 0.05) decreased infarct size to 10 +/- 2% (mean +/- SEM) of the area at risk as compared with control experiments (25 +/- 3% of area at risk). This beneficial effect of desflurane was abolished by glyburide (25 +/- 2% of area at risk). Glyburide (24 +/- 2%), HMR 1098 (21 +/- 4%), and 5-HD (24 +/- 2% of area at risk) alone had no effects on myocardial infarct size. HMR 1098 and 5-HD abolished the protective effects of desflurane (19 +/- 3% and 22 +/- 2% of area at risk, respectively).     

Differential Effects of Etomidate and Midazolam on Vascular Adenosine Triphosphate-sensitive Potassium Channels: Isometric Tension and Patch Clamp Studies

Background: The aim of this study was to investigate the effects of two imidazoline-derived intravenous anesthetics, etomidate and midazolam, on vascular adenosine triphosphate-sensitive potassium (KATP) channel activity.

Methods: In isolated rat aorta, isometric tension was recorded to examine the anesthetic effects on vasodilator response to levcromakalim, a selective KATP channel opener. Using the patch clamp method, the anesthetic effects were also examined on the currents through (1) native vascular KATP channels, (2) recombinant KATP channels with different combinations of various types of inwardly rectifying potassium channel (Kir6.0 family: Kir6.1, 6.2) and sulfonylurea receptor (SUR1, 2A, 2B) subunits, (3) SUR-deficient channels derived from a truncated isoform of Kir6.2 subunit (Kir6.2[DELTA]C36 channels), and (4) mutant Kir6.2[DELTA]C36 channels with reduced sensitivity to adenosine triphosphate (Kir6.2[DELTA]C36-K185Q channels).

Results: Etomidate (>= 10-6 m), but not midazolam (up to 10-6 m), inhibited the levcromakalim-induced vasodilation, which was sensitive to glibenclamide (IC50: 7.21 x 10-8 m; maximum inhibitory concentration: 1.22 x 10-4 m). Etomidate (>= 3 x 10-6 m), but not midazolam (up to 10-4 m), inhibited the native KATP channel activity in both cell-attached and inside-out configurations with IC50 values of 1.68 x 10-5 m and 1.52 x 10-5 m, respectively. Etomidate (10-5 m) also inhibited the activity of various types of recombinant SUR/Kir6.0KATP channels, Kir6.2[DELTA]C36 channels, and Kir6.2[DELTA]C36-K185Q channels with equivalent potency.     

Clinically Relevant Concentrations of Propofol Have No Effect on Adenosine Triphosphate-sensitive Potassium Channels in Rat Ventricular Myocytes

Background: Activation of adenosine triphosphate-sensitive potassium (KATP) channels produces cardioprotective effects during ischemia. Because propofol is often used in patients who have coronary artery disease undergoing a wide variety of surgical procedures, it is important to evaluate the direct effects of propofol on KATP channel activities in ventricular myocardium during ischemia.

Methods: The effects of propofol (0.4-60.1 [mu]g/ml) on both sarcolemmal and mitochondrial KATP channel activities were investigated in single, quiescent rat ventricular myocytes. Membrane currents were recorded using cell-attached and inside-out patch clamp configurations. Flavoprotein fluorescence was measured to evaluate mitochondrial oxidation mediated by mitochondrial KATP channels.

Results: In the cell-attached configuration, open probability of KATP channels was reduced by propofol in a concentration-dependent manner (EC50 = 14.2 [mu]g/ml). In the inside-out configurations, propofol inhibited KATP channel activities without changing the single-channel conductance (EC50 = 11.4 [mu]g/ml). Propofol reduced mitochondrial oxidation in a concentration-dependent manner with an EC50 of 14.6 [mu]g/ml.     

Ketamine Preconditions Isolated Human Right Atrial Myocardium: Roles of Adenosine Triphosphate-sensitive Potassium Channels and Adrenoceptors

Background: The authors examined the effect of ketamine and its S(+) isomer on isolated human myocardium submitted to hypoxia-reoxygenation in vitro.

Methods: The authors studied isometric contraction of human right atrial trabeculae suspended in an oxygenated Tyrode's modified solution at 34[degrees]C. Ten minutes before a 30-min hypoxic period followed by a 60-min reoxygenation, muscles were exposed for 15 min to racemic ketamine and its S(+) isomer at 10-6, 10-5, and 10-4 m alone or in the presence of 8.10-4 m 5-hydroxydecanoate, 10-5 m HMR 1098 (sarcolemmal adenosine triphosphate-sensitive potassium channel antagonist), 10-6 m phentolamine ([alpha]-adrenoceptor antagonist), and 10-6 m propranolol ([beta]-adrenoceptor antagonist). Force of contraction at the end of the 60-min reoxygenation period was compared between groups (mean +/- SD).

Results: Ketamine (10-6 m: 85 +/- 4%; 10-5 m: 95 +/- 10%; 10-4 m: 94 +/- 14% of baseline) and S(+)-ketamine (10-6 m: 85 +/- 4%; 10-5 m: 91 +/- 16%; 10-4 m: 93 +/- 14% of baseline) enhanced recovery of force of contraction at the end of the reoxygenation period as compared with the control group (47 +/- 10% of baseline; P < 0.001). Ketamine-induced preconditioning at 10-4 m was inhibited by 5-hydroxydecanoate (60 +/- 16%; P < 0.001), HMR 1098 (60 +/- 14%; P < 0.001), phentolamine (56 +/- 12%; P < 0.001), and propranolol (60 +/- 7%; P < 0.001).     

Halothane and Enflurane Attenuate Pulmonary Vasodilation Mediated by Adenosine Triphosphate-sensitive Potassium Channels Compared to the Conscious State

Background: Adenosine triphosphate (ATP)-sensitive potassium (K sup +ATP) channels play an important role in pulmonary vasoregulation. However, the effects of volatile anesthetics on K sup +ATP channel-mediated pulmonary vasoregulation have not been elucidated. The purpose of the present study was to investigate the effects of halothane and enflurane anesthesia on the pulmonary vasodilator response to the selective K sup +ATP channel agonist lemakalim (BRL38227) compared with that measured in the conscious state. The authors also investigated the extent to which endogenous neurohumoral vasoconstrictor mechanisms modulate the vasodilator response to K sup +ATP channel activation.

Method: Nineteen conditioned, male mongrel dogs were chronically instrumented to measure the left pulmonary vascular pressure-flow (LPQ) relationship. LPQ plots were generated by continuously measuring the pulmonary vascular pressure gradient (pulmonary arterial pressure-left atrial pressure) and left pulmonary blood flow during gradual (approximately 1 min) inflation of a hydraulic occluder implanted around the right main pulmonary artery. After preconstriction with the thromboxane analog, U46619 (9,11-dideoxy-11alpha, 9alpha -epoxymethano-prostaglandin F2alpha), the pulmonary vascular dose-response relationship for the K sup +ATP agonist lemakalim was assessed in the conscious and halothane-anesthetized states and also in the conscious and enflurane-anesthetized states. This protocol was repeated in conscious and halothane-anesthetized dogs after combined neurohumoral block with antagonists of sympathetic alpha1 adrenoreceptors, arginine vasopressin V1 -receptors, and angiotensin II receptors. The effect of the K sup +ATP antagonist glybenclamide on the baseline LPQ relationship and on the lemakalim dose-response relationship also was assessed in conscious dogs.

Results: Compared with the conscious state, halothane, enflurane and glybenclamide had no net effect on the baseline LPQ relationship. In contrast, halothane and enflurane attenuated (P < 0.05) the pulmonary vasodilator response to lemakalim compared with the conscious state. Glybenclamide also caused a rightward shift (P < 0.05) in the lemakalim dose-response relationship. Combined neurohumoral block did not modulate the vasodilator response to lemakalim in the conscious state. The halothane-induced attenuation of the vasodilator response to lemakalim was apparent after combined neurohumoral block.     

Age-related Attenuation of Isoflurane Preconditioning in Human Atrial Cardiomyocytes: Roles for Mitochondrial Respiration and Sarcolemmal Adenosine Triphosphate-sensitive Potassium Channel Activity

Background: Clinical trials suggest that anesthetic-induced preconditioning (APC) produces cardioprotection in humans, but the mechanisms of APC and significance of aging for APC in humans are not well understood. Here, the impact of age on the role of two major effectors of APC, mitochondria and sarcolemmal adenosine triphosphate-sensitive potassium (sarcKATP) channels, in preconditioning of the human atrial myocardium were investigated.

Methods: Right atrial appendages were obtained from adult patients undergoing cardiac surgery and assigned to mid-aged (MA) and old-aged (OA) groups. APC was induced by isoflurane in isolated myocardium and isolated cardiomyocytes. Mitochondrial oxygen consumption measurements, myocyte survival testing, and patch clamp techniques were used to investigate mitochondrial respiratory function and sarcKATP channel activity.

Results: After in vitro APC with isoflurane, the respiratory function of isolated mitochondria was better preserved after hypoxia-reoxygenation stress in MA than in OA. In isolated intact myocytes, APC significantly decreased oxidative stress-induced cell death in MA but not in OA, and isoflurane protection from cell death was attenuated by the sarcKATP channel inhibitor HMR-1098. Further, the properties of single sarcKATP channels were similar in MA and OA, and isoflurane sensitivity of pinacidil-activated whole cell KATP current was no different between MA and OA myocytes.     

Comparative Ventricular Electrophysiologic Effect of Racemic Bupivacaine, Levobupivacaine, and Ropivacaine on the Isolated Rabbit Heart

Background: Numerous local anesthetics have an asymmetric tetrahedron carbon, which confers stereoselective differences between the isomers. The authors attempted to quantify the depressant effect of racemic bupivacaine, levobupivacaine, and ropivacaine on myocardial ventricular conduction and on myocardial contractility.

Methods: The authors studied the pharmacokinetics (outflow concentration) and pharmacodynamics (QRS widening) of the three drugs infused in an isolated rabbit heart preparation. All data were fitted simultaneously with use of mixed-effect modeling, thus allowing precise statistical comparison between the three drug parameters. The rate dependence of QRS widening was fitted separately.

Results: Racemic bupivacaine, levobupivacaine, and ropivacaine induced a calculated maximum increase in QRS duration in the ratio 1:0.4:0.3. Css50, the dose which caused half the maximum increase in QRS duration at steady state, was similar for all three drugs (22 [mu]m free concentration). A rate dependence of QRS widening was observed, which was in the ratio 1:0.5:0.25 for racemic bupivacaine, levobupivacaine, and ropivacaine, respectively.     

Minimum Local Analgesic Doses of Ropivacaine, Levobupivacaine, and Bupivacaine for Intrathecal Labor Analgesia

Background: Doses for intrathecal opioid-local anesthetic mixtures have been arbitrarily chosen. The aim of this study was to compare the analgesic efficacies of intrathecal ropivacaine, levobupivacaine, and bupivacaine for labor analgesia and to determine the analgesic potency ratios for these three drugs. For this purpose, the authors used the up-down sequential allocation model, which estimates the minimum local analgesic dose for intrathecal local anesthetic.

Methods: Ninety-seven nulliparous term parturients in spontaneous labor, requesting combined spinal-epidural analgesia, were randomly allocated to one of three groups to receive 0.25% spinal ropivacaine, levobupivacaine, or bupivacaine. The initial dose of the local anesthetic drug was chosen to be 2.5 mg, and the testing interval was set at 0.25 mg. The subsequent doses were determined by the response of the previous parturient. Efficacy was accepted if the visual analog pain score decreased to 10 mm or less on a 100-mm scale within 30 min. The minimum local analgesic dose was calculated using the method of Dixon and Massey.

Results: The intrathecal minimum local analgesic dose was 3.64 mg (95% confidence interval, 3.33-3.96 mg) for ropivacaine, 2.94 (2.73-3.16) mg for levobupivacaine, and 2.37 (2.17-2.58) mg for bupivacaine. The relative analgesic potency ratios were 0.65 (0.56-0.76) for ropivacaine:bupivacaine, 0.80 (0.70-0.92) for ropivacaine:levobupivacaine, and 0.81 (0.69-0.94) for levobupivacaine:bupivacaine. There were significant trends (P <= 0.021) for greater motor block with bupivacaine and levobupivacaine.     

Comparison of the Effects of Racemic Bupivacaine, Levobupivacaine, and Ropivacaine on Ventricular Conduction, Refractoriness, and Wavelength: An Epicardial Mapping Study

Background: The study was designed to compare the effects of equimolar concentrations of racemic bupivacaine, levobupivacaine, and ropivacaine on ventricular conduction, anisotropy, duration and homogeneity of refractoriness, and wavelengths, and to provide a potency ratio for effects on conduction velocity.

Methods: Isolated frozen rabbit hearts (which leave a thin layer of surviving epicardial muscle) were treated with 0.1, 1, and 10 [mu]m racemic bupivacaine, levobupivacaine, or ropivacaine. Left ventricular longitudinal and transverse conduction velocities, anisotropic ratio, minimum pacing cycle length, use dependency, duration and dispersion of ventricular effective refractory period, and wavelengths were studied. A high-resolution mapping system was used for data acquisition. In addition to two-way analysis of variance for repeated measures, data for conduction velocities were fitted simultaneously using a nonlinear mixed-effect modeling program to allow intergroup comparison.

Results: Each agent induced a concentration- and use-dependent slowing of conduction velocities, with no change of the anisotropic ratio. The use-dependent effect of levobupivacaine is similar to that of racemic bupivacaine concerning longitudinal conduction velocity. Fitting of conduction velocities provided a racemic bupivacaine to levobupivacaine and to ropivacaine ratio of 1:1.38 for concentration effect at 1,000-ms pacing cycle length, and 1:0.74 for use-dependent effect at 600-ms pacing cycle length. Racemic bupivacaine and levobupivacaine prolonged the ventricular effective refractory period, whereas ropivacaine did not. No dispersion in ventricular effective refractory period values occurred. All three agents induced significant decreases in wavelengths. This effect was not different among groups.     

Prevention of Isoflurane-induced Preconditioning by 5-Hydroxydecanoate and Gadolinium: Possible Involvement of Mitochondrial Adenosine Triphosphate-sensitive Potassium and Stretch-activated Channels

Background: Both mitochondrial adenosine triphosphate-sensitive potassium (MKATP) channels (selectively blocked by 5-hydroxydecanoate) and stretch-activated channels (blocked by gadolinium) have been involved in the mechanism of ischemic preconditioning. Isoflurane can reproduce the protection afforded by ischemic preconditioning. We sought to determine whether isoflurane-induced preconditioning may involve MKATP and stretch-activated channels.

Methods: Anesthetized open-chest rabbits underwent 30 min of coronary occlusion followed by 3 h of reperfusion. Before this, rabbits were randomized into one of six groups and underwent a treatment period consisting of either no intervention for 40 min (control group; n = 9) or 15 min of isoflurane inhalation (1.1% end tidal) followed by a 15-min washout period (isoflurane group; n = 9). The two groups received an intravenous bolus dose of either 5-hydroxydecanoate (5 mg/kg) or gadolinium (40 [mu]mol/kg) before coronary occlusion and reperfusion (5-hydroxydecanoate, n = 9; gadolinium, n = 7). Two additional groups received 5-hydroxydecanoate or gadolinium before isoflurane exposure (isoflurane-5-hydroxydecanoate, n = 10; isoflurane-gadolinium, n = 8). Area at risk and infarct size were assessed by blue dye injection and tetrazolium chloride staining.

Results: Area at risk was comparable among the six groups (29 +/- 7, 30 +/- 5, 27 +/- 6, 35 +/- 7, 31 +/- 7, and 27 +/- 4% of the left ventricle in the control, isoflurane, isoflurane-5-hydroxydecanoate, 5-hydroxydecanoate, isoflurane-gadolinium, and gadolinium groups, respectively). Infarct size averaged 60 +/- 20% (SD) in untreated controls versus 54 +/- 27 and 65 +/- 15% of the risk zone in 5-hydroxydecanoate- and gadolinium-treated controls (P = nonsignificant). In contrast, infarct size in the isoflurane group was significantly reduced to 26 +/- 11% of the risk zone (P < 0.05 vs. control). Both 5-hydroxydecanoate and gadolinium prevented this attenuation: infarct size averaged 68 +/- 23 and 56 +/- 21% of risk zone in the isoflurane-5-hydroxydecanoate and isoflurane-gadolinium groups, respectively (P = nonsignificant vs. control).