Modulation of the Cardiac Sodium Current by Inhalational Anesthetics in the Absence and Presence of beta-Stimulation

Background: Cardiac dysrhythmias during inhalational anesthesia in association with catecholamines are well known, and halothane is more "sensitizing" than isoflurane. However, the underlying mechanisms of action of volatile anesthetics with or without catecholamines on cardiac Na channels are poorly understood. In this study, the authors investigated the effects of halothane and isoflurane in the absence and presence of beta-stimulation (isoproterenol) on the cardiac Na sup + current (INa) in ventricular myocytes enzymatically isolated from adult guinea pig hearts.

Methods: A standard whole-cell patch-clamp technique was used. The INa was elicited by depolarizing test pulses from a holding potential of -80 mV in reduced Na sup + solution (10 mM).

Results: Isoproterenol alone depressed peak INa significantly by 14.6 +/- 1.7% (means +/- SEM). Halothane (1.2 mM) and isoflurane (1.0 mM) also depressed peak INa significantly by 42.1 +/- 3.4% and 21.3 +/- 1.9%, respectively. In the presence of halothane, the effect of isoproterenol (1 micro Meter) was potentiated, further decreasing peak I sub Na by 34.7 +/- 4.1%. The halothane effect was less, although significant, in the presence of a G-protein inhibitor (GDP beta S) or a specific protein kinase A inhibitor [PKI-(6-22)-amide], reducing peak I sub Na by 24.2 +/- 3.3% and 24 +/- 2.4%, respectively. In combination with isoflurane, the effect of isoproterenol on INa inhibition was less pronounced, but significant, decreasing current by 12.6 +/- 3.9%. GDP beta S also reduced the inhibitory effect of isoflurane. In contrast, PKI-(6-22)-amide had no effect on isoflurane INa inhibition.     

Sensitization of the Cardiac Na Channel to alpha sub 1 -Adrenergic Stimulation by Inhalation Anesthetics: Evidence for Distinct Modulatory Pathways

Background: Alpha1 -Adrenergic receptor stimulation has been shown to inhibit cardiac Na sup + current (INa). Furthermore, some form of synergistic interaction of alpha1 -adrenergic effects on I sub Na in combination with volatile anesthetics has been reported. In this study, the authors investigated the possible role of G proteins and protein kinase C in the effects of halothane and isoflurane in the absence and presence of alpha1 -adrenergic stimulation on the cardiac INa.

Methods: The standard whole-cell configuration of the patch-clamp technique was used. INa was elicited by depolarizing test pulses from a holding potential of -80 mV in reduced Na sup + on ventricular myocytes enzymatically isolated from adult guinea pig hearts.

Results: The inhibitory effect of halothane (1.2 mM) and isoflurane (1 mM) on peak INa was significantly diminished in the presence of guanosine 5'-O-[2-thiodiphosphate (GDP beta S). In myocytes pretreated with pertussis toxin (PTX), the potency of halothane was significantly enhanced, but the isoflurane effect was unchanged. In the presence of the protein kinase C (PKC) inhibitor bisindolylmaleimide (BIS), the effect of halothane was unchanged. In contrast, the effect of isoflurane on INa in the presence of BIS was significantly enhanced. The positive interaction between methoxamine and halothane was evident in the presence of G protein and PKC inhibitors. In contrast, the effect of methoxamine with isoflurane was additive in the presence of GDP beta S or BIS.     

Influence of Volatile Anesthetics on Thromboxane A sub 2 Signaling

Background: Thromboxane A2 (TXA2) is a member of the prostaglandin family; activation of its receptor induces several important effects, including platelet aggregation and smooth muscle contraction. Because volatile anesthetics interfere with aggregation and contraction, the authors investigated effects of halothane, isoflurane, and sevoflurane on TXA2 signaling in an isolated receptor model.

Methods: mRNA encoding TXA2 receptors was prepared in vitro and expressed in Xenopus oocytes. The effects of halothane, isoflurane, and sevoflurane on Ca2+ -activated Cl sup - currents induced by the TXA2 agonist U-46619 and on those induced by intracellular injection of inositol 1-4-5 trisphosphate or guanosine 5'-O-(2-thiodiphosphate) were measured using the voltage-clamp technique.

Results: Expressed TXA2 receptors were functional (half maximal effect concentration [EC50], 3.2 x 10 sup -7 +/- 1.1 x 10 sup -7 M; Hill coefficient (h), 0.8 +/- 0.2). Halothane and isoflurane inhibition of TXA2 signaling was reversible and concentration dependent (halothane half maximal inhibitory concentration [IC50], 0.46 +/- 0.04 mM; h, 1.6 +/- 0.21; isoflurane IC50, 0.69 +/- 0.12 mM; h, 1.3 +/- 0.27). 0.56 mM halothane (1%) right-shifted the U-46619 concentration-response relationship by two orders of magnitude (EC50, 1 x 10 sup -5 M). That h and maximal effect (Emax) were unchanged indicates that halothane acts in a competitive manner. In contrast, isoflurane acted noncompetitively, decreasing Emax by 30% (h and EC50 were unchanged). Both halothane and isoflurane had no effect on intracellular signaling pathways. Sevoflurane (0-1.3 mM) did not affect TXA2 signaling.     

Inhibition of Sodium/Calcium Exchange and Calcium Channels of Heart Cells by Volatile Anesthetics

Background: Volatile anesthetics exert profound effects on the heart, probably through their effect on Calcium2+ movements during the cardiac cycle. Calcium2+ movements across the sarcolemma are thought to involve mainly Calcium2+ channels and the Sodium sup +/Calcium2+ exchanger. We have therefore investigated the action of halothane, isoflurane, and enflurane on Sodium sup +/Calcium2+ exchange and Calcium2+ channel activity to assess the contribution of these pathways to the observed effect of the anesthetics on the myocardium.

Methods: Sarcolemmal ion fluxes were investigated using radioisotope uptake by isolated adult rat heart cells in suspension. Sodium sup +/Calcium2+ exchange activity was measured from45 Calcium2+ uptake by Sodium sup + -loaded cells. Calcium2+ channel activity was measured from verapamil-sensitive trace54 Manganese2+ uptake during electric stimulation.

Results: Halothane, isoflurane, and enflurane inhibited Sodium sup +/Calcium2+ exchange completely, with similar potency when concentrations were expressed in millimolar units in aqueous medium but not when expressed as minimum alveolar concentration (MAC). The inhibition by enflurane was particularly strong, > 50%, at 2 MAC. In contrast, the three anesthetics inhibited Calcium2+ channels with similar potency when concentrations were expressed as MAC but not when expressed in millimolar units in aqueous medium. Hill plots of pooled data with all three anesthetics showed a slope of 3.87 plus/minus 0.50 for inhibition of Sodium sup +/Calcium2+ exchange and 1.73 plus/minus 0.19 for inhibition of Calcium2+ channels.     

吸入麻醉药的心肌保护机制

日渐增多的证据表明,吸入麻醉药可保护心肌免受可复性或不可复性缺血性损伤。探讨吸入麻醉药介导抗缺血性效应的机制近来备受关注,由于吸入麻醉药可明显影响心血管功能,如降低动脉及冠脉灌注压、剂量依赖性抑制心肌收缩、舒张冠脉、影响电生理、不同程度调节自主神经系统活性等,使得机制的探讨较为困难。因此心肌氧供需关系的适度     

Mechanisms Underlying Greater Sensitivity of Neonatal Cardiac Muscle to Volatile Anesthetics

Background: In neonatal heart, plasma membrane Na+-Ca2+ exchange (NCX) and Ca2+ influx channels play greater roles in intracellular Ca2+ concentration [Ca2+]i regulation compared with the sarcoplasmic reticulum (SR). In neonatal (aged 0-3 days) and adult (aged 84 days) rat cardiac myocytes, we determined the mechanisms underlying greater sensitivity of the neonatal myocardium to inhibition by volatile anesthetics.

Methods: The effects of 1 and 2 minimum alveolar concentration halothane and sevoflurane on Ca2+ influx during electrical stimulation in the presence or blockade of NCX and the Ca2+ channel agonist BayK8644 were examined. [Ca2+]i responses to caffeine were used to examine anesthetic effects on SR Ca2+ release (via ryanodine receptor channels) and reuptake (via SR Ca2+ adenosine triphosphatase). Ca2+ influx via NCX was examined during rapid activation in the presence of the reversible SR Ca2+ adenosine triphosphatase inhibitor cyclopiazonic acid and ryanodine to inhibit the SR. Efflux mode NCX was examined during activation by extracellular Na+ in the absence of SR reuptake.

Results: Intracellular Ca2+ concentration transients during electrical stimulation were inhibited to a greater extent in neonates by halothane (80%) and sevoflurane (50%). Potentiation of [Ca2+]i responses by BayK8644 (160 and 120% control in neonates and adults, respectively) was also blunted by anesthetics to a greater extent in neonates. [Ca2+]i responses to caffeine in neonates (~30% adult responses) were inhibited to a lesser extent compared with adults (35 vs. 60% by halothane). Both anesthetics inhibited Ca2+ reuptake at 2 minimum alveolar concentration, again to a greater extent in adults. Reduction in NCX-mediated influx was more pronounced in neonates (90%) compared with adults (65%) but was comparable between anesthetics. Both anesthetics also reduced NCX-mediated efflux to a greater extent in neonates. Potentiation of NCX-mediated Ca2+ efflux by extracellular Na+ and NCX-mediated Ca2+ influx by intracellular Na+ were both prevented by halothane, especially in neonates.     

Neutrophils Pretreated with Volatile Anesthetics Lose Ability to Cause Cardiac Dysfunction

Background: Volatile anesthetics can precondition the myocardium against functional depression and infarction following ischemia-reperfusion. Neutrophil activation, adherence, and release of superoxide play major roles in reperfusion injury. The authors tested the hypothesis that pretreatment of neutrophils with a volatile anesthetic, i.e., simulated preconditioning, can blunt their ability to cause cardiac dysfunction.

Methods: Studies were performed in 60 buffer-perfused and paced isolated rat hearts. Left ventricular developed pressure served as an index of myocardial contractility. Polymorphonuclear neutrophils and/or drugs were added to coronary perfusate for 10 min, followed by 30 min of recovery. Platelet-activating factor was used to stimulate neutrophils. Pretreatment of neutrophils consisted of incubation with 1.0 minimum alveolar concentration (MAC) isoflurane or sevoflurane for 15 min, followed by washout. Additional studies were performed with 0.25 MAC isoflurane. Effects of superoxide dismutase were compared to those of volatile anesthetics. Superoxide production was measured by spectrophotometry. Neutrophil adherence to coronary vascular endothelium was estimated from the difference between neutrophils administered and recovered in coronary venous effluent.

Results: Activated neutrophils caused marked, persistent reduction (> 50%) in left ventricular developed pressure. Isoflurane and sevoflurane at 1.0 MAC and superoxide dismutase abolished this effect. Isoflurane and sevoflurane reduced superoxide production of activated neutrophils by 29% and 33%, respectively, and completely prevented the platelet-activating factor-induced increases in neutrophil adherence. Isoflurane at 0.25 MAC blunted, but did not abolish, the neutrophil-induced decreases in left ventricular developed pressure.     

Differential Inhibition of Lysophosphatidate Signaling by Volatile Anesthetics

Background: Volatile anesthetics have been found to interfere with the functioning of several G protein-coupled receptors, effects that may be relevant to the mechanism of anesthetic action. Lysophosphatidate (1-acyl-2-sn-glycero-3-phosphate; LP) is the simplest natural phospholipid. It has pronounced biological effects and signals through a specific G protein-coupled receptor. Because of its lipophilicity, the LP receptor is a feasible site of anesthetic interaction. Therefore, the authors investigated the effects of halothane and isoflurane on LP signaling using Xenopus oocytes.

Methods: Mature oocytes were harvested from Xenopus frogs, isolated, and defolliculated manually. Lysophosphatidate receptors are endogenously present in these cells. Angiotensin receptors were expressed recombinantly to study anesthetic effects on intracellular signaling. Oocytes were studied individually with a two-electrode voltage clamp at room temperature. Integrated Ca2+ -activated Cl sup - currents (ICl(Ca)) were used to evaluate the effects of anesthetics on changes in intracellular Ca2+ concentration in response to receptor agonists (10 sup -7 M LP or 10 sup -7 M angiotensin II) or intracellular inositoltrisphosphate (IP3) injection.

Results: Halothane depressed LP signaling in a concentration-dependent manner, with half-maximal inhibition at 0.23 mM and virtually complete inhibition at 0.34 mM. Responses could be recovered after an anesthetic-free wash. Oocyte injection with heparin, an IP3 receptor antagonist, completely blocked LP and angiotensin signaling, indicating similar IP3 -dependent pathways. However, ICl(Ca) induced by angiotensin receptor activation or intracellular IP3 injection were not inhibited by halothane. Isoflurane, at comparable concentrations, did not depress LP responses in oocytes significantly.     

Central Nervous System Sodium Channels Are Significantly Suppressed at Clinical Concentrations of Volatile Anesthetics

Background: Although voltage-dependent sodium channels have been proposed as possible molecular sites of anesthetic action, they generally are considered too insensitive to be likely molecular targets. However, most previous molecular studies have used peripheral sodium channels as models. To examine the interactions of volatile anesthetics with mammalian central nervous system voltage-gated sodium channels, rat brain IIA sodium channels were expressed in a stably transfected Chinese hamster ovary cell line, and their modification by volatile anesthetics was examined.

Methods: Sodium currents were measured using whole cell patch clamp recordings. Test solutions were equilibrated with the test anesthetics and perfused externally on the cells. Anesthetic concentrations in the perfusion solution were determined by gas chromatography.

Results: All anesthetics significantly suppressed sodium currents at clinical concentrations. This suppression occurred through at least two mechanisms: (1) a potential-independent suppression of resting or open sodium channels, and (2) a hyperpolarizing shift in the voltage-dependence of channel inactivation resulting in a potential-dependent suppression of sodium currents. The voltage-dependent interaction results in IC50 values for anesthetic suppression of sodium channels that are close to clinical concentrations at potentials near the resting membrane potential.     

Use of Volatile Anesthetics in Extracorporeal Circulation

Background and objectivesThe use of volatile anesthetics in cardiac surgery is not recent. Since the introduction of halothane in clinical practice, several cardiac surgery centers started to use these anesthetics constantly.ContentIn the last years a great number of studies have shown that the volatile anesthetics have a protecting effect against myocardial ischemic dysfunction. Experimental evidences have shown that the halogenated anesthetics have cardioprotective effects that cannot be only explained by coronary flow alterations or by the balance between myocardium available and consumed oxygen. In addition to that, the use of volatile anesthetics during extracorporeal circulation (ECC) in cardiac surgery plays an important role. Recent studies have proven that these agents have cardioprotective properties and produce better results when the volatile anesthetic is used during the whole surgery procedure, including ECC. The use of halogenated anesthetics through calibrated vaporizers adapted to the ECC circuit via oxygenator membranes has become popular. Therefore, the professionals involved such as anesthesiologists and perfusionists should learn specifcs in order to solve possible doubts.     

注射挥发性麻醉药对动物的效应

研究了注射挥发性麻醉药对动物的效应，在小鼠，乙醚的催眠ＥＤ５０是５．３５（ｓｃ）和１．６２（ｉｐ）ｍｌ／ｋｇ；ＬＤ５０是１３．７（ｓｃ）和２．３２（ｉｐ）ｍｌ／ｋｇ。甲氧氟烷的ＥＤ５０为０．７５（ｉｐ）和４．１２（ｉｍ）ｍｌ／ｋｇ；ＬＤ５０为１．２３（ｉｐ）５．８１（ｉｍ）ｍｌ／ｋｇ，ｉｐ氟烷的ＥＤ５０和ＬＤ５０分别是１．２０和１．９４ｍｌ／ｋｇ，ｉｐ安氟醚的ＥＤ５０和ＬＤ５０分别是３．０１和７．     

Behavioral Effects of Volatile Anesthetics in Caenorhabditis elegans

Background: The nematode Caenorhabditis elegans offers many advantages as a model organism for studying volatile anesthetic action: It has a simple, well-understood nervous system; it allows the researcher to do forward genetics; and its genome will soon be completely sequenced. C. elegans is immobilized by volatile anesthetics only at high concentrations and with an unusually slow time course. Here other behavioral dysfunctions are considered as anesthetic endpoints in C. elegans.

Methods: The potency of halothane for disrupting eight different behaviors was determined by logistic regression of concentration and response data. Other volatile anesthetics were also tested for some behaviors. Established protocols were used for behavioral endpoints that, except for pharyngeal pumping, were set as complete disruption of the behavior. Time courses were measured for rapid behaviors. Recovery from exposure to 1 or 4 vol% halothane was determined for mating, chemotaxis, and gross movement. All experiments were performed at 20 to 22 degrees Celsius.

Results: The median effective concentration values for halothane inhibition of mating (0.30 vol% - 0.21 mM), chemotaxis (0.34 vol% - 0.24 mM), and coordinated movement (0.32 vol% - 0.23 mM) were similar to the human minimum alveolar concentration (MAC; 0.21 mM). In contrast, halothane produced immobility with a median effective concentration of 3.65 vol% (2.6 mM). Other behaviors had intermediate sensitivities. Halothane's effects reached steady-state in 10 min for all behaviors tested except immobility, which required 2 h. Recovery was complete after exposure to 1 vol% halothane but was significantly reduced after exposure to immobilizing concentrations.