Blockade of myocardial ATP-sensitive potassium channels by ketamine

Targeting Affective Mood Disorders With Ketamine to Prevent Chronic Postsurgical Pain

The phencyclidine-derivative ketamine [2-(2-chlorophenyl)-2-(methylamino)cyclohexan-1-one] was added to the World Health Organization's Model List of Essential Medicines in 1985 and is also on the Model List of Essential Medicines for Children due to its efficacy and safety as an intravenous anesthetic. In sub-anesthetic doses, ketamine is an effective analgesic for the treatment of acute pain (such as may occur in the perioperative setting). Additionally, ketamine may have efficacy in relieving some forms of chronic pain. In 2019, Janssen Pharmaceuticals received regulatory-approval in both the United States and Europe for use of the S-enantiomer of ketamine in adults living with treatment-resistant major depressive disorder. Pre-existing anxiety/depression and the severity of postoperative pain are risk factors for development of chronic postsurgical pain. An important question is whether short-term administration of ketamine can prevent the conversion of acute postsurgical pain to chronic postsurgical pain. Here, we have reviewed ketamine's effects on the biopsychological processes underlying pain perception and affective mood disorders, focusing on non-NMDA receptor-mediated effects, with an emphasis on results from human trials where available.

Ketamine attenuates the Na+-dependent Ca2+ overload in rabbit ventricular myocytes in vitro by inhibiting late Na+ and L-type Ca2+ currents

AIM

Intracellular Ca(2+) ([Ca(2+)]i) overload occurs in myocardial ischemia. An increase in the late sodium current (INaL) causes intracellular Na(+) overload and subsequently [Ca(2+)]i overload via the reverse-mode sodium-calcium exchanger (NCX). Thus, inhibition of INaL is a potential therapeutic target for cardiac diseases associated with [Ca(2+)]i overload. The aim of this study was to investigate the effects of ketamine on Na(+)-dependent Ca(2+) overload in ventricular myocytes in vitro.

METHODS

Ventricular myocytes were enzymatically isolated from hearts of rabbits. INaL, NCX current (INCX) and L-type Ca(2+) current (ICaL) were recorded using whole-cell patch-clamp technique. Myocyte shortening and [Ca(2+)]i transients were measured simultaneously using a video-based edge detection and dual excitation fluorescence photomultiplier system.

RESULTS

Ketamine (20, 40, 80 μmol/L) inhibited INaL in a concentration-dependent manner. In the presence of sea anemone toxin II (ATX, 30 nmol/L), INaL was augmented by more than 3-fold, while ketamine concentration-dependently suppressed the ATX-augmented INaL. Ketamine (40 μmol/L) also significantly suppressed hypoxia or H2O2-induced enhancement of INaL. Furthermore, ketamine concentration-dependently attenuated ATX-induced enhancement of reverse-mode INCX. In addition, ketamine (40 μmol/L) inhibited ICaL by 33.4%. In the presence of ATX (3 nmol/L), the rate and amplitude of cell shortening and relaxation, the diastolic [Ca(2+)]i, and the rate and amplitude of [Ca(2+)]i rise and decay were significantly increased, which were reverted to control levels by tetrodotoxin (TTX, 2 μmol/L) or by ketamine (40 μmol/L).

CONCLUSION

Ketamine protects isolated rabbit ventricular myocytes against [Ca(2+)]i overload by inhibiting INaL and ICaL.

Vasorelaxant mechanisms of ketamine in rabbit renal artery

Background

Ketamine is a non-barbiturate anesthetic agent which has various effects on the cardiovascular system. Among them, ketamine is known for its hypotensive properties. The hypotension is thought to be mediated by a direct effect on vascular smooth muscles. This study is designed to examine the effects of ketamine on KCl- and histamine-induced contraction in isolated rabbit renal arteries.

Methods

Endothelium-intact or -denuded smooth muscle rings were prepared and mounted in myographs for isometric tension measurements. The inhibitory effect of ketamine were investigated in smooth muscle rings precontracted with either 50 mM KCl- or 10 µM histamine.

Results

Ketamine (0.1-100 µg/ml) produced similar concentration-dependent inhibition of contractile responses induced by either 50 mM KCl or 10 µM histamine. The respective IC₅₀ values measured for ketamine following precontractions by 50 mM KCl and 10 µM histamine were 28.9 µg/ml (105.5 µM) and 26.7 µg/ml (97.5 µM). The inhibitory effect of 30 µg/ml ketamine were similarly observed after removal of endothelium or pretreatment with N^G-Nitroarginine Methyl Ester (0.1 mM). The inhibitory effect of 30 µg/ml ketamine on histamine-evoked contraction was reduced by either tetraethylammonium (10 mM) or iberiotoxin, a large conductance Ca²⁺-activated K⁺ channel blocker. However, depletion of intracellular Ca²⁺ stores by ryanodine (10 µM) or thapsigargin (10 µM) showed no significant effect on 30 µg/ml ketamine-induced relaxation. Pre-incubation with 30 µg/ml ketamine significantly inhibited CaCl₂-induced contraction at almost all ranges of concentration.

Conclusions

Ketamine-induced relaxation of rabbit renal arteries is mediated by both the activation of large conductance Ca²⁺-activated K⁺ channel and the inhibition of Ca²⁺ influx.

Propofol with N-acetylcysteine reduces global myocardial ischemic reperfusion injury more than ketamine in a rat model

OBJECTIVE

We examined the cardioprotective effects of propofol and ketamine with and without N-acetylcysteine (NAC).

METHODS

60 rats were divided into six groups of 10 rats each. Anesthesia induction was produced with an intraperitonal injection of ketamine in Groups 1-3 and propofol in Groups 4-6. NAC (200 mg kg(- 1)) was given intraperitonally during anesthesia induction in Groups 3 and 6. Groups 2, 3, 5, and 6 were subjected to 90 s of myocardial ischemia by clamping the ascending aorta, and then reperfusion was begun by unclamping the ascending aorta. After 60 min of reperfusion, blood samples were taken from the ascending aorta for biochemical analyses, and heart tissue samples were taken for biochemical and histopathological analyses.

RESULTS

Creatine kinase (CK), myocardial band of creatine kinase (CK-MB), and troponin-I (Tn-I) levels were significantly higher in the ischemia-reperfusion groups (2, 3, 5, 6) compared to the nonischemic groups (1, 4). CK, CK-MB, and Tn-I levels did not differ significantly between the ketamine groups (1-3) and the propofol groups (4-6) p > .05). Malondialdehyde levels were significantly higher in Groups 2 and 3 than in Group 1 and were significantly lower in Groups 4 and 6 than in Group 5 (p < .05). Malondialdehyde levels in the propofol groups (4-6) were significantly lower than in the ketamine groups (1-3; p < .05). Catalase levels in propofol groups were higher than ketamine groups. Superoxide dismutase levels were significantly higher in Group 6 than in Group 3 (p < .05).

CONCLUSIONS

In this rat model of global cardiac ischemia, propofol with NAC attenuates myocardial injury more than ketamine (with or without NAC).

Anaesthesia and myocardial ischaemia/reperfusion injury

Anaesthetists are confronted on a daily basis with patients with coronary artery disease, myocardial ischaemia, or both during the perioperative period. Therefore, prevention and ultimately adequate therapy of perioperative myocardial ischaemia and its consequences are the major challenges in current anaesthetic practice. This review will focus on the translation of the laboratory evidence of anaesthetic-induced cardioprotection into daily clinical practice.

Pioglitazone protects the myocardium against ischemia-reperfusion injury in eNOS and iNOS knockout mice

Endothelial nitric oxide synthase (eNOS) activation with subsequent inducible NOS (iNOS), cytosolic phospholipase A2 (cPLA2), and cyclooxygenase-2 (COX2) activation is essential to statin inhibition of myocardial infarct size (IS). In the rat, the peroxisome proliferator-activated receptor-gamma agonist pioglitazone (Pio) limits IS, upregulates and activates cPLA2 and COX2, and increases myocardial 6-keto-PGF1alpha levels without activating eNOS and iNOS. We asked whether Pio also limits IS in eNOS-/- and iNOS-/- mice. Male C57BL/6 wild-type (WT), eNOS-/-, and iNOS-/- mice received 10 mg.kg(-1).day(-1) Pio (Pio+) or water alone (Pio-) for 3 days. Mice underwent 30 min coronary artery occlusion and 4 h reperfusion, or hearts were harvested and subjected to ELISA and immunoblotting. As a result, Pio reduced IS in the WT (15.4+/-1.4% vs. 39.0+/-1.1%; P<0.001), as well as in the eNOS-/- (32.0+/-1.6% vs. 44.2+/-1.9%; P<0.001) and iNOS-/- (18.0+/-1.2% vs. 45.5+/-2.3%; P<0.001) mice. The protective effect of Pio in eNOS-/- mice was smaller than in the WT (P<0.001) and iNOS-/- (P<0.001) mice. Pio increased myocardial Ser633 and Ser1177 phosphorylated eNOS levels in the WT and iNOS-/- mice. iNOS was undetectable in all six groups. Pio increased cPLA2, COX2, and PGI2 synthase levels in the WT, as well as in the eNOS-/- and iNOS-/-, mice. Pio increased the myocardial 6-keto-PGF1alpha levels and cPLA2 and COX2 activity in the WT, eNOS-/-, and iNOS-/- mice. In conclusion, the myocardial protective effect of Pio is iNOS independent and may be only partially dependent on eNOS. Because eNOS activity decreases with age, diabetes, and advanced atherosclerosis, this effect may be relevant in a clinical setting and should be further characterized.

Preconditioning, anesthetics, and perioperative medication

Activation of endogenous signal transduction pathways, by a variety of stimuli including ischemic and anesthetic pre- and post-conditioning, protects myocardium against ischemia and reperfusion injury. Experimental evidence suggests that adenosine-regulated potassium channels, cyclooxygenase-2, intracellular kinases, endothelial nitric oxide synthase, and membrane bound receptors play critical roles in signal transduction, and that intracellular signaling pathways ultimately converge on mitochondria to produce cardioprotection. Disease states, and perioperative medications such as sulfonylureas and COX-2 antagonists, could have adverse effects on cardioprotection by impairing activation of ion channels and proteins that are important in cell signaling. Insights gained from animal and clinical studies are reviewed and recommendations given for the use of perioperative anesthetics and medications.

The electrophysiological effects of racemic ketamine and etomidate in an in vitro model of "border zone" between normal and ischemic/reperfused guinea pig myocardium

BACKGROUND

Etomidate and ketamine are used during induction of anesthesia in high-risk patients. However, their effects on action potential (AP) variables and ischemia/reperfusion-induced arrhythmias and conduction blocks are unknown.

METHODS

Guinea pig right ventricular muscle strips were mounted in a 5-mL double chamber bath with the strips separated into two zones by an impermeable latex membrane. One-half (normal zone) was exposed to normal perfusate while the other half (altered zone) was exposed to hypoxia, hyperkalemia, acidosis, and lack of glucose. AP variables were recorded continuously in the normal and altered zones. Spontaneous arrhythmias and conduction blocks were noted. Etomidate (10(-7), 10(-6), and 10(-5) M) and ketamine (10(-6), 10(-5), and 10(-4) M) were superfused into the bath throughout the experiment and the electrophysiologic effects compared with the control group.

RESULTS

We found that under control conditions, etomidate and ketamine did not modify resting membrane potential, maximal upstroke velocity, AP amplitude, or AP duration at 90% of repolarization (APD90). Ketamine (10(-4) M), but not weaker concentrations and none of the concentration of etomidate, reversed the ischemia-induced shortening of APD90 and APD dispersion. Etomidate and ketamine did not modify the occurrence of conduction block during simulated ischemia. In contrast, ketamine (25% at 10(-6) M, 13% at 10(-5) M, and 13% at 10(-4) M vs 90% in the control group, P < 0.05) but not etomidate (38% at 10(-7) M, 63% at 10(-6) M, and 63% at 10(-5) M vs 90% in the control group, NS) decreased the incidence of reperfusion-induced spontaneous arrhythmias.

CONCLUSIONS

In guinea pig myocardium, our data suggest that ketamine, in clinically relevant concentrations, decreases ischemia-induced AP shortening and spontaneous reperfusion-induced ventricular arrhythmias. Further study is required to precisely determine the effect of etomidate on reperfusion-induced arrhythmias.

Inflammatory response and cardioprotection during open-heart surgery: the importance of anaesthetics

Open-heart surgery triggers an inflammatory response that is largely the result of surgical trauma, cardiopulmonary bypass, and organ reperfusion injury (e.g. heart). The heart sustains injury triggered by ischaemia and reperfusion and also as a result of the effects of systemic inflammatory mediators. In addition, the heart itself is a source of inflammatory mediators and reactive oxygen species that are likely to contribute to the impairment of cardiac pump function. Formulating strategies to protect the heart during open heart surgery by attenuating reperfusion injury and systemic inflammatory response is essential to reduce morbidity. Although many anaesthetic drugs have cardioprotective actions, the diversity of the proposed mechanisms for protection (e.g. attenuating Ca(2+) overload, anti-inflammatory and antioxidant effects, pre- and post-conditioning-like protection) may have contributed to the slow adoption of anaesthetics as cardioprotective agents during open heart surgery. Clinical trials have suggested at least some cardioprotective effects of volatile anaesthetics. Whether these benefits are relevant in terms of morbidity and mortality is unclear and needs further investigation. This review describes the main mediators of myocardial injury during open heart surgery, explores available evidence of anaesthetics induced cardioprotection and addresses the efforts made to translate bench work into clinical practice.

Ketamine-induced cardiac depression is associated with increase in [Mg2+]i and activation of p38 MAP kinase and ERK 1/2 in guinea pig

This study investigated the signaling pathways responsible for ketamine-induced cardiac depression in guinea pigs. The left ventricular development pressure (LVDP), velocity of the change in pressure (dP/dt), and heart rate (HR) accompanied with the total magnesium efflux ([Mg]e) were measured simultaneously in perfused hearts. The level of activation of the extracellular signal-regulated kinases 1/2 (ERK 1/2) and p38 mitogen-activated protein (MAP) kinase. The intracellular ionized magnesium concentration ([Mg2+]i) was measured using Mag-fura 2 AM in a single cardiomyocyte. Ketamine produced reversible decreases in the LVDP, dP/dt, and HR accompanied by increases in the [Mg]e. Ketamine also produced significant activation of p38 MAP kinase and ERK 1/2, and produced a dose-dependent increase in the [Mg2+]i, which was inhibited SB203580 and PD98059. These results suggest that ketamine-induced cardiac depression can be partly responsible for the increase in [Mg2+]i and [Mg]e, accompanied by the activation of p38 MAP kinase and ERK 1/2 in guinea pigs.

Glutamate cysteine ligase catalysis: dependence on ATP and modifier subunit for regulation of tissue glutathione levels

Glutamate cysteine ligase (GCL), which synthesizes gamma-glutamyl-cysteine (gamma-GC), is the rate-limiting enzyme in GSH biosynthesis. gamma-GC may be produced by the catalytic subunit GCLC or by the holoenzyme (GCLholo), which comprises GCLC and the modifier subunit GCLM. The Gclm(-/-) knock-out mouse shows tissue levels of GSH that are between 9 and 40% of the Gclm(+/+) wild-type mouse. In the present study, we used recombinant GCLC and GCLM and Gclm(-/-) mice to examine the role of GCLM on gamma-GC synthesis by GCLholo. GCLM decreased the Km for ATP by approximately 6-fold and, similar to other species, decreased the Km for glutamate and increased the Ki for feedback inhibition by GSH. Furthermore, GCLM increased by 4.4-fold the Kcat for gamma-GC synthesis; this difference in catalytic efficiency of GCLholo versus GCLC allowed us to derive a mathematical relationship for gamma-GC production and to determine the relative levels of GCLholo and GCLC; in homogenates of brain, liver, and lung, the ratio of GCLC to GCLholo was 7.0, 2.0, and 3.5, respectively. In kidney, however, the relationship between GCLC and GCLholo was complicated. Kidney contains GCLholo, free GCLC, and free GCLM, and free GCLC in kidney cannot interact with GCLM. Taken together, we conclude that, in most tissues, GCLM is limiting, suggesting that an increase in GCLM alone would increase gamma-GC synthesis. On the other hand, our results from kidney suggest that gamma-GC synthesis may be controlled post-translationally.

Effects of isoflurane and ketamine on ATP-sensitive K channels in rat substantia nigra

Whole cell recordings were made using midbrain slices to examine the effects of two different anaesthetics on ATP-sensitive K (K(ATP)) channels in principle neurons of rat substantia nigra pars compacta. When neurons were dialyzed with an ATP-free pipette solution during perfusion with a glucose-free external solution, a hyperpolarization and an outward current developed slowly in a tolbutamide-inhibitable manner. The volatile anaesthetic 3% isoflurane slightly depolarised the neurons in the presence of ATP in the pipette solution and glucose in the external solution, but it did not affect the hyperpolarization or outward current in response to omission of ATP and glucose. Ketamine, an intravenous anaesthetic, did not change the membrane potential when ATP and glucose were included; however, it reversibly inhibited the hyperpolarization and outward current induced by intracellular ATP depletion in a dose-dependent manner. These effects of ketamine were not mimicked by AP-5, an NMDA receptor antagonist, or indatraline, an inhibitor of catecholamine uptake. These findings suggest that these anaesthetics have no stimulatory action on K(ATP) channels in these neurons when intracellular ATP is preserved and that ketamine but not isoflurane inhibits K(ATP) channels when the channels were activated by low intracellular ATP.

Anaesthetics and cardiac preconditioning. Part II. Clinical implications

There is compelling evidence that preconditioning occurs in humans. Experimental studies with potential clinical implications as well as clinical studies evaluating ischaemic, pharmacological and anaesthetic cardiac preconditioning in the perioperative setting are reviewed. These studies reveal promising results. However, there are conflicting reports on the efficacy of preconditioning in the diseased and aged myocardium. In addition, many anaesthetics and a significant number of perioperatively administered drugs affect the activity of cardiac sarcolemmal and mitochondrial K(ATP) channels, the end-effectors of cardiac preconditioning, and thereby markedly modulate preconditioning effects in myocardial tissue. Although these modulatory effects on K(ATP) channels have been investigated almost exclusively in laboratory investigations, they may have potential implications in clinical medicine. Important questions regarding the clinical utility and applicability of perioperative cardiac preconditioning remain unresolved and need more experimental work and randomized controlled clinical trials.

Ketamine blocks Ca2+-activated K+ channels in rabbit cerebral arterial smooth muscle cells

Although ketamine and Ca2+-activated K+ (KCa) channels have been implicated in the contractile activity regulation of cerebral arteries, no studies have addressed the specific interactions between ketamine and the KCa channels in cerebral arteries. The purpose of this study was to examine the direct effects of ketamine on KCa channel activities using the patch-clamp technique in single-cell preparations of rabbit middle cerebral arterial smooth muscle. We tested the hypothesis that ketamine modulates the KCa channel activity of the cerebral arterial smooth muscle cells of the rabbit. Vascular myocytes were isolated from rabbit middle cerebral arteries using enzymatic dissociation. Single KCa channel activities of smooth muscle cells from rabbit cerebral arteries were recorded using the patch-clamp technique. In the inside-out patches, ketamine in the micromolar range inhibited channel activity with a half-maximal inhibition of the ketamine concentration value of 83.8 +/- 12.9 microM. The Hill coefficient was 1.2 +/- 0.3. The slope conductance of the current-voltage relationship was 320.1 +/- 2.0 pS between 0 and +60 mV in the presence of ketamine and symmetrical 145 mM K+. Ketamine had little effect on either the voltage-dependency or open- and closed-time histograms of KCa channel. The present study clearly demonstrates that ketamine inhibits KCa channel activities in rabbit middle cerebral arterial smooth muscle cells. This inhibition of KCa channels may represent a mechanism for ketamine-induced cerebral vasoconstriction.

Effects of ketamine on isoflurane- and sevoflurane-induced cerebral vasodilation in rabbits

Although ketamine has been reported to have little effect on the cerebral circulation when used with other anesthetics, its effect on the cerebral vascular response to volatile anesthetics, which increase cerebral blood flow in a concentration-dependent manner, remains obscure. A closed cranial window was prepared in 15 pentobarbital-anesthetized adult rabbits. The cerebral pial arteriolar alteration induced by either isoflurane (n = 8) or sevoflurane (n = 7) at 0 (before volatile anesthetic), 0.33, 0.67, and 1.0 minimum alveolar concentration (MAC) was measured under three consecutive conditions: intravenous infusion with saline, with ketamine, and with ketamine plus l-arginine. Ketamine reduced the vasodilation induced by 0.67 (120 +/- 9% versus 113 +/- 9%; P <.05) and 1.0 MAC isoflurane (136 +/- 11% versus 118 +/- 10%; P <.05), but l-arginine did not restore the isoflurane-induced cerebral vasodilation. In rabbits inhaling sevoflurane, the degree of cerebral vasodilator response was smaller than that by isoflurane, and the cerebral vasodilation was comparable whether in the presence or absence of ketamine (with or without l-arginine). In conclusion, ketamine reduces isoflurane-induced cerebral vasodilation, apparently independently of nitric oxide formation, while sevoflurane-induced cerebral vasodilation is not significantly affected by ketamine.

Myocardial protection by anesthetic agents against ischemia-reperfusion injury: an update for anesthesiologists

PURPOSE

The aim of this review of the literature was to evaluate the effectiveness of anesthetics in protecting the heart against myocardial ischemia-reperfusion injury.

SOURCE

Articles were obtained from the Medline database (1980-, search terms included heart, myocardium, coronary, ischemia, reperfusion injury, infarction, stunning, halothane, enflurane, desflurane, isoflurane, sevoflurane, opioid, morphine, fentanyl, alfentanil sufentanil, pentazocine, buprenorphine, barbiturate, thiopental, ketamine, propofol, preconditioning, neutrophil adhesion, free radical, antioxidant and calcium).

PRINCIPAL FINDINGS

Protection by volatile anesthetics, morphine and propofol is relatively well investigated. It is generally agreed that these agents reduce the myocardial damage caused by ischemia and reperfusion. Other anesthetics which are often used in clinical practice, such as fentanyl, ketamine, barbiturates and benzodiazepines have been much less studied, and their potential as cardioprotectors is currently unknown. There are some proposed mechanisms for protection by anesthetic agents: ischemic preconditioning-like effect, interference in the neutrophil/platelet-endothelium interaction, blockade of Ca2+ overload to the cytosolic space and antioxidant-like effect. Different anesthetics appear to have different mechanisms by which protection is exerted. Clinical applicability of anesthetic agent-induced protection has yet to be explored.

CONCLUSION

There is increasing evidence of anesthetic agent-induced protection. At present, isoflurane, sevoflurane and morphine appear to be most promising as preconditioning-inducing agents. After the onset of ischemia, propofol could be selected to reduce ischemia-reperfusion injury. Future clinical application depends on the full elucidation of the underlying mechanisms and on clinical outcome trials.

Ketamine abolishes ischemic preconditioning through inhibition of K(ATP) channels in rabbit hearts

Although ketamine inhibits ATP-sensitive K (K(ATP)) channels in rat ventricular myocytes and abolishes the cardioprotective effect of ischemic preconditioning in isolated rat hearts and in rabbits in in vivo, no studies to date specifically address the precise mechanism of this prevention of ischemic preconditioning by ketamine. This study investigated the mechanism of the blockade of ischemic preconditioning by ketamine in rabbit ventricular myocytes using patch-clamp techniques and in rabbit heart slices model for simulated ischemia and preconditioning. In cell-attached and inside-out patches, ketamine inhibited sarcolemmal K(ATP) channel activities in a concentration-dependent manner. Ketamine decreased the burst duration and increased the interburst duration without a change in the single-channel conductance. In the heart slice model of preconditioning, heart slices preconditioned with a single 5-min anoxia, pinacidil, or diazoxide, followed by 15-min reoxygenation, were protected against subsequent 30-min anoxia and 1-h reoxygenation, and the cardioprotection was blocked by the concomitant presence of ketamine. These data are consistent with the notion that inhibition of sarcolemmal or mitochondrial K(ATP) channels may contribute, at least in part, to the mechanism of the blockade of ischemic preconditioning by ketamine.

L-arginine and nitroglycerin restore hypercapnia-induced cerebral vasodilation in rabbits after its attenuation by ketamine

Although it has been reported that ketamine attenuates hypercapnia-induced cerebral vasodilation, the mechanism remains unknown. Because nitric oxide is involved in cerebral CO2 reactivity, we studied the effects of L-arginine and nitroglycerin on ketamine-mediated attenuation of vascular responses to hypercapnia. Under pentobarbital anesthesia, 16 rabbits underwent closed cranial window preparation. Hypercapnic challenges were repeated after IV saline, ketamine (10 mg/kg, followed by 20 mg x kg(-1) x h(-1)), or ketamine plus either L-arginine (150 mg/kg, followed by 100 mg x kg(-1) x h(-1); n = 8) or nitroglycerin (5 microg x kg(-1) x min(-1) infusion; n = 8). Ketamine reduced hypercapnia-induced cerebral vasodilation (1.27%/mm Hg +/- 0.45%/mm Hg [saline] versus 0.82%/mm Hg +/- 0.53%/mm Hg [ketamine]: P < 0.05), but L-arginine restored reactivity (1.28%/mm Hg +/- 0.73%/mm Hg: P < 0.05 versus ketamine), as did nitroglycerin (1.14%/mm Hg +/- 0.73%/mm Hg [saline] versus 0.56%/mm Hg +/- 0.63%/mm Hg [ketamine]: P < 0.05, and 1.15%/mm Hg +/- 0.74%/mm Hg [ketamine plus nitroglycerin]: P < 0.05 versus ketamine). This indicates that ketamine attenuates cerebral CO2 reactivity, at least in part, via suppression of nitric oxide-cyclic guanosine monophosphate mechanisms in the cerebral vasculature.

IMPLICATIONS

The attenuation of cerebral vasodilation to hypercapnia seen under ketamine anesthesia is reversed by L-arginine or nitroglycerin infusion.

Involvement of potassium channels in spinal antinociceptions induced by fentanyl, clonidine and bethanechol in rats

In the central nervous systems, intracellular and extracellular movement of potassium ions plays an important role in regulating neuronal excitability and the release of neurotransmitters. The purpose of our study was to determine whether nicorandil (adenosine triphosphate-sensitive K+ channel opener) exerts antinociceptive effects by itself or in combination with fentanyl, clonidine and bethanechol and whether glibenclamide (adenosine triphosphate-sensitive K+ channel blocker) and charybdotoxin (Ca2+-activated K+ channel blocker) may antagonize the antinociceptive action of fentanyl, clonidine and bethanechol. Antinociceptive effects were assessed using the tail-flick test in rats. Nicorandil (100 microg) and antinociceptively ineffective doses of fentanyl (1 microg), clonidine (2.5 microg) or bethanechol (10,ug) were coadministered intrathecally (i.t.). Glibenclamide (100 microg) or charybdotoxin (2.5 ng) were administered i.t. at 5 min before each effective dose of fentanyl (2.5 microg), clonidine (10 microg) or bethanechol (40 microg). The present findings demonstrated that i.t. administration of nicorandil alone exerted no influence on the tail-flick latency. However, concomitant administrations of antinociceptively inactive doses of fentanyl, clonidine or bethanechol with nicorandil elicited significant suppression of the thermonociceptive response. Also, each antinociception induced by fentanyl, clonidine or bethanechol was partially antagonized by both glibenclamide and charybdotoxin. These findings showed that activation of the K+ channel might enhance the antinociceptive effects of fentanyl, clonidine and bethanechol.

Thiopentone does not block ischemic preconditioning in the isolated rat heart

PURPOSE

Ischemic preconditioning protects the heart against subsequent prolonged ischemia by opening of adenosine triphosphate-sensitive potassium (K(ATP)) channels. Thiopentone blocks K(ATP) channels in isolated cells. Therefore, we investigated the effects of thiopentone on ischemic preconditioning.

METHODS

Isolated rat hearts (n=56) were subjected to 30 min of global no-flow ischemia, followed by 60 min of reperfusion. Thirteen hearts underwent the protocol without intervention (control, CON) and in 11 hearts (preconditioning, PC), ischemic preconditioning was elicited by two five-minute periods of ischemia. In three additional groups, hearts received 1 (Thio 1, n=11), 10 (Thio 10, n=11) or 100 microg x mL(-1) (Thio 100, n=10) thiopentone for five minutes before preconditioning. Left ventricular (LV) developed pressure and creatine kinase (CK) release were measured as variables of myocardial performance and cellular injury, respectively.

RESULTS

Recovery of LV developed pressure was improved by ischemic preconditioning (after 60 min of reperfusion, mean +/- SD: PC, 40 +/- 19% of baseline) compared with the control group (5 +/- 6%, P <0.01) and this improvement of myocardial function was not altered by administration of thiopentone (Thio 1, 37 +/- 15%; Thio 10, 36 +/- 16%; Thio 100, 38 +/- 16%, P=0.87-0.99 vs PC). Total CK release over 60 min of reperfusion was reduced by preconditioning (PC, 202 +/- 82 U x g(-1) dry weight) compared with controls (CON, 383 +/- 147 U x g(-1), P <0.01) and this reduction was not affected by thiopentone (Thio 1, 213 +/- 69 U x g(-1); Thio 10, 211 +/- 98 U x g(-1); Thio 100, 258 +/- 128 U x g(-1), P=0.62-1.0 vs PC).

CONCLUSION

These results indicate that thiopentone does not block the cardioprotective effects of ischemic preconditioning in an isolated rat heart preparation.

Late preconditioning is blocked by racemic ketamine, but not by S(+)-ketamine

Racemic ketamine blocks K(ATP) channels in isolated cells and abolishes short-term cardioprotection against prolonged ischemia. We investigated the effects of racemic ketamine and S(+)-ketamine on ischemic late preconditioning (LPC) in the rabbit heart in vivo. A coronary occluder was chronically implanted in 36 rabbits. After recovery, the rabbits divided into four groups (each n = 9). LPC was induced in conscious rabbits by a 5-min coronary occlusion. Twenty-four hours later, the animals were instrumented for measurement of left ventricular systolic pressure (LVSP, tip manometer), cardiac output (CO, ultrasonic flowprobe) and myocardial infarct size (triphenyltetrazolium staining). All rabbits were then subjected to 30-min coronary occlusion and 2 h reperfusion. Controls underwent the ischemia-reperfusion program without LPC. To test whether racemic ketamine or S(+)-ketamine blocks the cardioprotection induced by LPC, the drugs (10 mg/kg) were given 10 min before the 30-min ischemia. Hemodynamic values were not significantly different between groups during the experiments (baseline: LVSP, 94 +/- 3 mm Hg [mean +/- SEM] and CO, 243 +/- 9 mL/min; coronary occlusion: LVSP, 93% +/- 4% of baseline and CO, 84% +/- 4%; after 2 h of reperfusion: LVSP, 85% +/- 4% and CO, 83% +/- 4%). LPC reduced infarct size from 44% +/- 3% of the area at risk in controls to 22% +/- 3% (P = 0.002). Administration of racemic ketamine abolished the cardioprotective effects of LPC (44 +/- 4%, P = 0.002). S(+)-ketamine did not affect the infarct size reduction induced by LPC (26 +/- 6%, P = 0.88).

IMPLICATIONS

Racemic ketamine, but not S(+)-ketamine, blocks the cardioprotection induced by ischemic late preconditioning in rabbit hearts in vivo. Thus, the influence of ketamine on ischemic late preconditioning is most likely enantiomer specific, and the use of S(+)-ketamine may be preferable in patients with coronary artery disease.

Analysis of ketamine responses in the pulmonary vascular bed of the cat

OBJECTIVE

To test the hypothesis that pulmonary vasodilator responses of ketamine are dependent on activation of L-type calcium channels, independent of synthesis of nitric oxide from L-arginine, activation of adenosine triphosphate-sensitive potassium channels, and the release of cyclooxygenase products.

DESIGN

Prospective study.

SETTING

Research laboratory.

SUBJECTS

Isolated lobar lung preparation, mongrel cats.

INTERVENTIONS

In separate experiments, the effects of nicardipine; N omega-I-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase; glibenclamide, an adenosine triphosphate-sensitive potassium channel antagonist; and meclofenamate, a cyclooxygenase blocker, were investigated in the pulmonary vascular bed of the cat. The effects of these agents were evaluated on the pulmonary arterial responses of ketamine, acetylcholine, and isoproterenol during elevated tone conditions induced by the thromboxane A2 mimic, U46619 (Upjohn, Kalamazoo, MI).

MEASUREMENTS

Lobar arterial perfusion pressure, systemic pressure, and left atrial pressure were continuously monitored, electronically averaged, and permanently recorded.

MAIN RESULTS

Under elevated tone conditions in the isolated left lower lobe vascular bed of the cat, N omega-I-nitro-L-arginine methyl ester, glibenclamide, and meclofenamate had no significant effect on the vasodilator responses to ketamine. Nicardipine, in a dose that reduced significantly vasopressor effects to BAY K 8644, a calcium-channel opener, attenuated significantly vasodilator responses to ketamine, whereas the L-type calcium-channel blocker had no significant effects on responses to acetylcholine and to isoproterenol.

CONCLUSIONS

These data show that ketamine has significant vasodilator activity in the pulmonary vascular bed of the cat. The present data also suggest that responses to ketamine during elevated tone conditions may in part be mediated by the activation of L-type calcium channels.

Patch-clamp analysis of anesthetic interactions with recombinant SK2 subtype neuronal calcium-activated potassium channels

Small conductance calcium-activated potassium channels (SK) mediate spike frequency adaptation and underlie the slow afterhyperpolarization in central neurons. We tested the actions of several anesthetics on the SK2 subtype of recombinant SK channels, cloned from rat brain and functionally expressed in a mammalian cell line. Butanol, ethanol, ketamine, lidocaine, and methohexital blocked recombinant SK2 channel currents, measured in the whole-cell patch clamp recording mode. The block was reversible, dose-dependent, and of variable efficacy. The inhaled anesthetics chloroform, desflurane, enflurane, halothane, isoflurane, and sevoflurane produced little or no block when applied at 1 minimum alveolar anesthetic concentration; varying degrees of modulation were observed at very large concentrations (10 minimum alveolar concentration). The extent of block by inhaled anesthetics did not appear to depend on concentration or membrane voltage.

IMPLICATIONS

We describe differential effects of anesthetics on cloned small conductance calcium-activated potassium channels from brain that may play a role in generating the effects or side effects of anesthetics.

Pulmonary vasodilation by ketamine is mediated in part by L-type calcium channels

We studied the effects of ketamine in the isolated rat lung under conditions of increased pulmonary arterial pressure using the thromboxane A2 mimic, U46619, and in response to ventilatory hypoxia. Ketamine caused dose-dependent vasodilation, and possible mechanisms were evaluated using verapamil, meclofenamate, N(omega)-L-nitro-L-arginine benzyl ester (an inhibitor of nitric oxide synthase), and U-38883A (an ATP-sensitive potassium channel antagonist) in the isolated blood-perfused rat lung. Under increased tone conditions, N(omega)-L-nitro-L-arginine benzyl ester, meclofenamate, and U-38883A had no significant effect in attenuating ketamine-induced vasodilator responses. In a final series of experiments, verapamil significantly attenuated ketamine-induced vasodilator responses. These data suggest that ketamine has significant vasodilator activity in the pulmonary vascular bed of the rat, which seems to be mediated by an L-type calcium channel-sensitive pathway. These responses are not mediated or modulated by the release of nitric oxide, the activation of K+ ATP channels, or the release of vasodilator cyclooxygenase products.

IMPLICATIONS

In this study, we examined the mechanism of the vasodilator effects of ketamine in the blood-perfused rat lung. The results of the present study suggest that ketamine-induced vasodilator responses are mediated by an L-type calcium channel-sensitive pathway.

Long QT and ventricular arrhythmias in transgenic mice expressing the N terminus and first transmembrane segment of a voltage-gated potassium channel

Voltage-gated potassium channels control cardiac repolarization, and mutations of K+ channel genes recently have been shown to cause arrhythmias and sudden death in families with the congenital long QT syndrome. The precise mechanism by which the mutations lead to QT prolongation and arrhythmias is uncertain, however. We have shown previously that an N-terminal fragment including the first transmembrane segment of the rat delayed rectifier K+ channel Kv1.1 (Kv1.1N206Tag) coassembles with other K+ channels of the Kv1 subfamily in vitro, inhibits the currents encoded by Kv1.5 in a dominant-negative manner when coexpressed in Xenopus oocytes, and traps Kv1.5 polypeptide in the endoplasmic reticulum of GH3 cells. Here we report that transgenic mice overexpressing Kv1.1N206Tag in the heart have a prolonged QT interval and ventricular tachycardia. Cardiac myocytes from these mice have action potential prolongation caused by a significant reduction in the density of a rapidly activating, slowly inactivating, 4-aminopyridine sensitive outward K+ current. These changes correlate with a marked decrease in the level of Kv1.5 polypeptide. Thus, overexpression of a truncated K+ channel in the heart alters native K+ channel expression and has profound effects on cardiac excitability.