In vivo characterization of the mitochondrial selective K(ATP) opener (3R)-trans-4-((4-chlorophenyl)-N-(1H-imidazol-2-ylmethyl)dimethyl-2H-1-benzopyran-6-carbonitril monohydrochloride (BMS-191095): cardioprotective, hemodynamic, and electrophysiological effects

Electrophysiology Methods for Assessing of Neurodegenerative and Post-Traumatic Processes as Applied to Translational Research

Electrophysiological studies have long established themselves as reliable methods for assessing the functional state of the brain and spinal cord, the degree of neurodegeneration, and evaluating the effectiveness of therapy. In addition, they can be used to diagnose, predict functional outcomes, and test the effectiveness of therapeutic and rehabilitation programs not only in clinical settings, but also at the preclinical level. Considering the urgent need to develop potential stimulators of neuroregeneration, it seems relevant to obtain objective data when modeling neurological diseases in animals. Thus, in the context of the application of electrophysiological methods, not only the comparison of the basic characteristics of bioelectrical activity of the brain and spinal cord in humans and animals, but also their changes against the background of neurodegenerative and post-traumatic processes are of particular importance. In light of the above, this review will contribute to a better understanding of the results of electrophysiological assessment in neurodegenerative and post-traumatic processes as well as the possibility of translating these methods from model animals to humans.

KATP channels are regulators of programmed cell death and targets for the creation of novel drugs against ischemia/reperfusion cardiac injury

BACKGROUND

The use of percutaneous coronary intervention (PCI) in patients with ST-segment elevation myocardial infarction (STEMI) is associated with a mortality rate of 5%-7%. It is clear that there is an urgent need to develop new drugs that can effectively prevent cardiac reperfusion injury. ATP-sensitive K+ (KATP ) channel openers (KCOs) can be classified as such drugs.

RESULTS

KCOs prevent irreversible ischemia and reperfusion injury of the heart. KATP channel opening promotes inhibition of apoptosis, necroptosis, pyroptosis, and stimulation of autophagy. KCOs prevent the development of cardiac adverse remodeling and improve cardiac contractility in reperfusion. KCOs exhibit antiarrhythmic properties and prevent the appearance of the no-reflow phenomenon in animals with coronary artery occlusion and reperfusion. Diabetes mellitus and a cholesterol-enriched diet abolish the cardioprotective effect of KCOs. Nicorandil, a KCO, attenuates major adverse cardiovascular event and the no-reflow phenomenon, reduces infarct size, and decreases the incidence of ventricular arrhythmias in patients with acute myocardial infarction.

CONCLUSION

The cardioprotective effect of KCOs is mediated by the opening of mitochondrial KATP (mitoKATP ) and sarcolemmal KATP (sarcKATP ) channels, triggered free radicals' production, and kinase activation.

ATP-sensitive potassium channels: A double-edged sword in neurodegenerative diseases

ATP-sensitive potassium channels (K_ATP channels), a group of vital channels that link the electrical activity of the cell membrane with cell metabolism, were discovered on the ventricular myocytes of guinea pigs by Noma using the patch-clamp technique in 1983. Subsequently, K_ATP channels have been found to be expressed in pancreatic β cells, cardiomyocytes, skeletal muscle cells, and nerve cells in the substantia nigra (SN), hippocampus, cortex, and basal ganglia. K_ATP channel openers (KCOs) diazoxide, nicorandil, minoxidil, and the K_ATP channel inhibitor glibenclamide have been shown to have anti-hypertensive, anti-myocardial ischemia, and insulin-releasing regulatory effects. Increasing evidence has suggested that K_ATP channels also play roles in Alzheimer's disease (AD), Parkinson's disease (PD), vascular dementia (VD), Huntington's disease (HD) and other neurodegenerative diseases. KCOs and K_ATP channel inhibitors protect neurons from injury by regulating neuronal excitability and neurotransmitter release, inhibiting abnormal protein aggregation and Ca²⁺ overload, reducing reactive oxygen species (ROS) production and microglia activation. However, K_ATP channels have dual effects in some cases. In this review, we focus on the roles of K_ATP channels and their related openers and inhibitors in neurodegenerative diseases. This will enable us to precisely take advantage of the K_ATP channels and provide new ideas for the treatment of neurodegenerative diseases.

Mitochondrial K+ Transport: Modulation and Functional Consequences

The existence of a K⁺ cycle in mitochondria has been predicted since the development of the chemiosmotic theory and has been shown to be crucial for several cellular phenomena, including regulation of mitochondrial volume and redox state. One of the pathways known to participate in K⁺ cycling is the ATP-sensitive K⁺ channel, MitoK_ATP. This channel was vastly studied for promoting protection against ischemia reperfusion when pharmacologically activated, although its molecular identity remained unknown for decades. The recent molecular characterization of MitoK_ATP has opened new possibilities for modulation of this channel as a mechanism to control cellular processes. Here, we discuss different strategies to control MitoK_ATP activity and consider how these could be used as tools to regulate metabolism and cellular events.

Antiarrhythmic activity of a new spiro-cyclic benzopyran activator of the cardiac mitochondrial ATP dependent potassium channels

'Compound A' (4(ı)-(N-(4-acetamidobenzyl))-2,2-dimethyl-2,3-dihydro-5(ı)H-spiro[chromene-4,2(ı)-[1,4]oxazinan]-5(ı)-one) is a new spiro-cyclic benzopyran activator of the mitochondrial ATP-dependent potassium channels (mitoKATP). We researched the effect of compound A on ischemia/reperfusion (I/R)-induced ventricular arrhythmias. We also tested the hypothesis that the application of the activation of mitoKATP in combination with the inhibition of sarcolemmal ATP-dependent potassium channels (sarcKATP) may produce a stronger antiarrhythmic effect. In anesthetized rats, myocardial ischemia was performed by ligating the left main coronary artery followed by reperfusion. At a dose of 10 mg/kg, compound A significantly decreased arrhythmia scores and the total length of arrhythmias, whereas this was found to be ineffective at a dose of 3 mg/kg. Pre-treatment with 5-HD, a selective mitoKATP blocker, abolished the antiarrhythmic effect of compound A. Both diazoxide, a selective mitoKATP opener and HMR 1098, a selective sarcKATP blocker, significantly decreased the total length of arrhythmias. However, the combination of neither diazoxide nor compound A with HMR 1098 showed no additional therapeutic benefit. These results reveal that compound A may have a dose-dependent antiarrythmic effect, which is more pronounced than the antiarrhythmic effect of diazoxide. The antiarrhythmic effect of compound A may possibly depend on mitoKATP activation.

Modulation of mitochondrial respiratory function and ROS production by novel benzopyran analogues

A substantial body of evidence indicates that pharmacological activation of mitochondrial ATP-sensitive potassium channels (mKATP) in the heart is protective in conditions associated with ischemia/reperfusion injury. Several mechanisms have been postulated to be responsible for cardioprotection, including the modulation of mitochondrial respiratory function. The aim of the present study was to characterize the dose-dependent effects of novel synthetic benzopyran analogues, derived from a BMS-191095, a selective mKATP opener, on mitochondrial respiration and reactive oxygen species (ROS) production in isolated rat heart mitochondria. Mitochondrial respiratory function was assessed by high-resolution respirometry, and H2O2 production was measured by the Amplex Red fluorescence assay. Four compounds, namely KL-1487, KL-1492, KL-1495, and KL-1507, applied in increasing concentrations (50, 75, 100, and 150 μmol/L, respectively) were investigated. When added in the last two concentrations, all compounds significantly increased State 2 and 4 respiratory rates, an effect that was not abolished by 5-hydroxydecanoate (5-HD, 100 μmol/L), the classic mKATP inhibitor. The highest concentration also elicited an important decrease of the oxidative phosphorylation in a K+ independent manner. Both concentrations of 100 and 150 μmol/L for KL-1487, KL-1492, and KL-1495, and the concentration of 150 μmol/L for KL-1507, respectively, mitigated the mitochondrial H2O2 release. In isolated rat heart mitochondria, the novel benzopyran analogues act as protonophoric uncouplers of oxidative phosphorylation and decrease the generation of reactive oxygen species in a dose-dependent manner.

Diversity of mitochondria-dependent dilator mechanisms in vascular smooth muscle of cerebral arteries from normal and insulin-resistant rats

Mitochondrial depolarization following ATP-sensitive potassium (mitoKATP) channel activation has been shown to induce cerebral vasodilation by generation of mitochondrial reactive oxygen species (ROS), which sequentially promotes frequency of calcium sparks and activation of large conductance calcium-activated potassium channels (BKCa) in vascular smooth muscle (VSM). We previously demonstrated that cerebrovascular insulin resistance accompanies aging and obesity. It is unclear whether mitochondrial depolarization without the ROS generation enhances calcium sparks and vasodilation in phenotypically normal [Sprague Dawley (SD); Zucker lean (ZL)] and insulin-resistant [Zucker obese (ZO)] rats. We compared the mechanisms underlying the vasodilation to ROS-dependent (diazoxide) and ROS-independent [BMS-191095 (BMS)] mitoKATP channel activators in normal and ZO rats. Arterial diameter studies from SD, ZL, and ZO rats showed that BMS as well as diazoxide induced vasodilation in endothelium-denuded cerebral arteries. In normal rats, BMS-induced vasodilation was mediated by mitochondrial depolarization and calcium sparks generation in VSM and was reduced by inhibition of BKCa channels. However, unlike diazoxide-induced vasodilation, scavenging of ROS had no effect on BMS-induced vasodilation. Electron spin resonance spectroscopy confirmed that diazoxide but not BMS promoted vascular ROS generation. BMS- as well as diazoxide-induced vasodilation, mitochondrial depolarization, and calcium spark generation were diminished in cerebral arteries from ZO rats. Thus pharmacological depolarization of VSM mitochondria by BMS promotes ROS-independent vasodilation via generation of calcium sparks and activation of BKCa channels. Diminished generation of calcium sparks and reduced vasodilation in ZO arteries in response to BMS and diazoxide provide new insights into mechanisms of cerebrovascular dysfunction in insulin resistance.

Depolarization of mitochondria in endothelial cells promotes cerebral artery vasodilation by activation of nitric oxide synthase

OBJECTIVE

Mitochondrial depolarization after ATP-sensitive potassium channel activation has been shown to induce cerebral vasodilation by the generation of calcium sparks in smooth muscle. It is unclear, however, whether mitochondrial depolarization in endothelial cells is capable of promoting vasodilation by releasing vasoactive factors. Therefore, we studied the effect of endothelial mitochondrial depolarization by mitochondrial ATP-sensitive potassium channel activators, BMS-191095 (BMS) and diazoxide, on endothelium-dependent vasodilation.

APPROACH AND RESULTS

Diameter studies in isolated rat cerebral arteries showed BMS- and diazoxide-induced vasodilations that were diminished by endothelial denudation. Mitochondrial depolarization-induced vasodilation was reduced by inhibition of mitochondrial ATP-sensitive potassium channels, phosphoinositide-3 kinase, or nitric oxide synthase. Scavenging of reactive oxygen species, however, diminished vasodilation induced by diazoxide, but not by BMS. Fluorescence studies in cultured rat brain microvascular endothelial cells showed that BMS elicited mitochondrial depolarization and enhanced nitric oxide production; diazoxide exhibited largely similar effects, but unlike BMS, increased mitochondrial reactive oxygen species production. Measurements of intracellular calcium ([Ca(2+)]i) in cultured rat brain microvascular endothelial cells and arteries showed that both diazoxide and BMS increased endothelial [Ca(2+)]i. Western blot analyses revealed increased phosphorylation of protein kinase B and endothelial nitric oxide synthase (eNOS) by BMS and diazoxide. Increased phosphorylation of eNOS by diazoxide was abolished by phosphoinositide-3 kinase inhibition. Electron spin resonance spectroscopy confirmed vascular nitric oxide generation in response to diazoxide and BMS.

CONCLUSIONS

Pharmacological depolarization of endothelial mitochondria promotes activation of eNOS by dual pathways involving increased [Ca(2+)]i as well as by phosphoinositide-3 kinase-protein kinase B-induced eNOS phosphorylation. Both mitochondrial reactive oxygen species-dependent and -independent mechanisms mediate activation of eNOS by endothelial mitochondrial depolarization.

Single channel studies of the ATP-regulated potassium channel in brain mitochondria

Mitochondrial potassium channels in the brain have been suggested to have an important role in neuroprotection. The single channel activity of mitochondrial potassium channels was measured after reconstitution of the purified inner membrane from rat brain mitochondria into a planar lipid bilayer. In addition to a large conductance potassium channel that was described previously, we identified a potassium channel that has a mean conductance of 219 +/- 15 pS. The activity of this channel was inhibited by ATP/Mg(2+) and activated by the potassium channel opener BMS191095. Channel activity was not influenced either by 5-hydroxydecanoic acid, an inhibitor of mitochondrial ATP-regulated potassium channels, or by the plasma membrane ATP-regulated potassium channel blocker HMR1098. Likewise, this mitochondrial potassium channel was unaffected by the large conductance potassium channel inhibitor iberiotoxin or by the voltage-dependent potassium channel inhibitor margatoxin. The amplitude of the conductance was lowered by magnesium ions, but the opening ability was unaffected. Immunological studies identified the Kir6.1 channel subunit in the inner membrane from rat brain mitochondria. Taken together, our results demonstrate for the first time the single channel activity and properties of an ATP-regulated potassium channel from rat brain mitochondria.

GSK3β inhibition and KATP channel opening mediate acute opioid-induced cardioprotection at reperfusion

Both glycogen synthase kinase 3beta (GSK3beta) and the ATP-dependant potassium channel (K(ATP)) mediate opioid-induced cardioprotection (OIC). However, whether direct K(ATP) channel openers induce cardioprotection prior to reperfusion and their signaling cascade position with respect to GSK3beta inhibition is unknown. Therefore, we investigated the role of K(ATP) channel opening at reperfusion in OIC, and the interaction between the GSK signaling axis and K(ATP) channels in cardioprotection.Male Sprague-Dawley rats underwent 30 minutes ischemia with 2 hours of reperfusion and infarct size was determined. Rats given the nonselective opioid agonist, morphine (0.3 mg/kg), or the selective delta opioid agonist, BW373U86 (1.0 mg/kg), 5 minutes prior to reperfusion reduced infarct size (40.3+/-1.6*, 39.7+/-1.9* versus 60.0+/-1.1%, respectively, * P<0.001%). This protection was abrogated with prior administration of the putative sarcolemmal K(ATP) antagonist, HMR-1098 (6 mg/kg), or the putative mitochondrial K(ATP) antagonist, 5-HD (10 mg/kg). The putative sK(ATP) channel opener, P-1075 (1microg/kg) or the putative mK(ATP) channel opener, BMS-191095 (1 mg/kg) given 5 minutes prior to reperfusion also reduced infarct size (41.8+/-2.4*, 43.4+/-1.4*) and protection was abrogated by prior administration of the PI3k inhibitor wortmannin (60.0+/-1.7, 64.0+/-2.6%, respectively, * P<0.001). Cardioprotection afforded by the GSK inhibitor SB216763 (0.6 mg/kg) given 5 minutes prior to reperfusion was also partially blocked by either HMR or 5-HD and completely blocked when HMR and 5-HD were given in combination (40.8+/-1.6*, 50.4+/-1.6;; 49.4+/-1.7;, 61.6+/-1.6%, respectively, * or ; P<0.001). These data indicate that both the sK(ATP) and mK(ATP) channel are involved in acute OIC and the GSK signaling axis regulates cardioprotection via K(ATP) channel opening.

The mitochondrial K(ATP) channel opener BMS-191095 reduces neuronal damage after transient focal cerebral ischemia in rats

Activation of mitochondrial ATP-sensitive potassium (mitoK(ATP)) channels protects the brain against ischemic or chemical challenge. Unfortunately, the prototype mitoK(ATP) channel opener, diazoxide, has mitoK(ATP) channel-independent actions. We examined the effects of BMS-191095, a novel selective mitoK(ATP) channel opener, on transient ischemia induced by middle cerebral artery occlusion (MCAO) in rats. Male Wister rats were subjected to 90 mins of MCAO. BMS-191095 (25 microg; estimated brain concentration of 40 micromol/L) or vehicle was infused intraventricularly before the onset of ischemia. In addition, the effects of BMS-191095 on plasma and mitochondrial membrane potentials and reactive oxygen species (ROS) production in cultured neurons were examined. Finally, we determined the effects of BMS-191095 on cerebral blood flow (CBF) and potassium currents in cerebrovascular myocytes. Treatment with BMS-191095 24 h before the onset of ischemia reduced total infarct volume by 32% and cortical infarct volume by 38%. However, BMS-191095 administered 30 or 60 mins before MCAO had no effect. The protective effects of BMS-191095 were prevented by co-treatment with 5-hydroxydecanoate (5-HD), a mitoK(ATP) channel antagonist. In cultured neurons, BMS-191095 (40 micromol/L) depolarized the mitochondria without affecting ROS levels, and this effect was inhibited by 5-HD. BMS-191095, similar to the vehicle, caused an unexplained but modest reduction in the CBF. Importantly, BMS-191095 did not affect either the potassium currents in cerebrovascular myocytes or the plasma membrane potential of neurons. Thus, BMS-191095 afforded protection against cerebral ischemia by delayed preconditioning via selective opening of mitoK(ATP) channels and without ROS generation.

DNA damage is an early event in doxorubicin-induced cardiac myocyte death

Anthracyclines are antitumor agents the main clinical limitation of which is cardiac toxicity. The mechanism of this cardiotoxicity is thought to be related to generation of oxidative stress, causing lethal injury to cardiac myocytes. Although protein and lipid oxidation have been documented in anthracycline-treated cardiac myocytes, DNA damage has not been directly demonstrated. This study was undertaken to determine whether anthracyclines induce cardiac myocyte DNA damage and whether this damage is linked to a signaling pathway culminating in cell death. H9c2 cardiac myocytes were treated with the anthracycline doxorubicin at clinically relevant concentrations, and DNA damage was assessed using the alkaline comet assay. Doxorubicin induced DNA damage, as shown by a significant increase in the mean tail moment above control, an effect ameliorated by inclusion of a free radical scavenger. Repair of DNA damage was incomplete after doxorubicin treatment in contrast to the complete repair observed in H2O2-treated myocytes after removal of the agent. Immunoblot analysis revealed that p53 activation occurred subsequent in time to DNA damage. By a fluorescent assay, doxorubicin induced loss of mitochondrial membrane potential after p53 activation. Chemical inhibition of p53 prevented doxorubicin-induced cell death and loss of mitochondrial membrane potential without preventing DNA damage, indicating that DNA damage was proximal in the events leading from doxorubicin treatment to cardiac myocyte death. Specific doxorubicin-induced DNA lesions included oxidized pyrimidines and 8-hydroxyguanine. DNA damage therefore appears to play an important early role in anthracycline-induced lethal cardiac myocyte injury through a pathway involving p53 and the mitochondria.

Mitochondrial potassium channels: from pharmacology to function

Mitochondrial potassium channels, such as ATP-regulated or large conductance Ca2+ -activated and voltage gated channels were implicated in cytoprotective phenomenon in different tissues. Basic effects of these channels activity include changes in mitochondrial matrix volume, mitochondrial respiration and membrane potential, and generation of reactive oxygen species. In this paper, we describe the pharmacological properties of mitochondrial potassium channels and their modulation by channel inhibitors and potassium channel openers. We also discuss potential side effects of these substances.

Characterization of human cardiac mitochondrial ATP-sensitive potassium channel and its regulation by phorbol ester in vitro

Activation of the mitochondrial ATP-sensitive K+ channel (mitoKATP) and its regulation by PKC are critical events in preconditioning induced by ischemia or pharmaceutical agents in animals and humans. The properties of the human cardiac mitoKATP channel are unknown. Furthermore, there is no evidence that cytosolic PKC can directly regulate the mitoKATP channel located in the inner mitochondrial membrane (IMM) due to the physical barrier of the outer mitochondrial membrane. In the present study, we characterized the human cardiac mitoKATP channel and its potential regulation by PKC associated with the IMM. IMM fractions isolated from human left ventricles were fused into lipid bilayers in symmetrical potassium glutamate (150 mM). The conductance of native mitoKATP channels was usually below 80 pS ( approximately 70%), which was reduced by ATP and 5-hydroxydecanoic acid (5-HD) in a dose- and time-dependent manner. The native mitoKATP channel is activated by diazoxide and inhibited by ATP and 5-HD. The PKC activator phorbol 12-myristate 13-acetate (2 microM) increased the cumulative open probability of the mitoKATP channel previously inhibited by ATP (P < 0.05), but its inactive analog 4alpha-phorbol 12,13-didecanoate had no effect. Western blot analysis detected an inward rectifying K+ channel (Kir6.2) immunoreactive protein at 56 kDa and PKC-delta in the IMM. These data provide the first characterization of the human cardiac mitoKATP channel and its regulation by PKC(s) in IMM. This local PKC control mechanism may represent an alternative pathway to that proposed previously for cytosolic PKC during ischemic/pharmacological preconditioning.

BMS-191095, a cardioselective mitochondrial K(ATP) opener, inhibits human platelet aggregation by opening mitochondrial K(ATP) channels

We evaluated the antiplatelet effects of two classes of ATP-sensitive potassium channel openers (K(ATP) openers) on washed human platelets, and the study's emphasis was on the role of mitochondrial K(ATP) in platelet aggregation. Collagen-induced platelet aggregation was inhibited in a dose dependent manner by lemakalim and SKP-450, which are potent cardio-nonselective K(ATP) openers, and also by cardioselective BMS-180448 and BMS-191095 (IC50: 1,130, >1,500, 305.3 and 63.9 microM, respectively), but a significantly greater potency was noted for the cardioselective K(ATP) openers. The latter two K(ATP) openers also inhibited platelet aggregation induced by thrombin, another important blood-borne platelet activator, with similar rank order of potency (IC50: 498.0 and 104.8 microM for BMS-180448 and BMS-191095, respectively). The inhibitory effects of BMS-191095 on collagen-induced platelet aggregation were significantly blocked by a 30-min pretreatment of platelets with glyburide (1 microM) or sodium 5-hydroxydecanoate (5-HD, 100 microM), a nonselective and selective mitochondrial K(ATP) antagonist, respectively, at similar magnitudes; this indicates the role of mitochondrial K(ATP) in the antiplatelet activity of BMS-191095. However, glyburide and 5-HD had no effect when they were added to the platelet cuvette immediately prior to the addition of BMS-191095. These findings indicate that cardioselective mitochondrial K(ATP) openers like BMS-191095 are able to exert cardioprotective effects in cardiac ischemia/reperfusion injury via dual mechanisms directed at the inhibition of platelet aggregation and the protection of cardiomyocytes, and both these mechanisms are mediated by mitochondrial K(ATP).

Mitochondrial KATPchannels in hindlimb remote ischemic preconditioning of skeletal muscle against infarction

We previously demonstrated in the pig that instigation of three cycles of 10 min of occlusion and reperfusion in a hindlimb by tourniquet application (∼300 mmHg) elicited protection against ischemia-reperfusion injury (infarction) in multiple distant skeletal muscles subsequently subjected to 4 h of ischemia and 48 h of reperfusion, but the mechanism was not studied. The aim of this project was to test our hypothesis that mitochondrial ATP-sensitive potassium (KATP) (mKATP) channels play a central role in the trigger and mediator mechanisms of hindlimb remote ischemic preconditioning (IPC) of skeletal muscle against infarction in the pig. We observed in the pig that hindlimb remote IPC reduced the infarct size of latissimus dorsi (LD) muscle flaps (8 × 13 cm) from 45 ± 2% to 22 ± 3% ( n = 10; P < 0.05). The nonselective KATPchannel inhibitor glibenclamide (0.3 mg/kg) or the selective mKATPchannel inhibitor 5-hydroxydecanoate (5-HD, 5 mg/kg), but not the selective sarcolemmal KATP(sKATP) channel inhibitor HMR-1098 (3 mg/kg), abolished the infarct-protective effect of hindlimb remote IPC in LD muscle flaps ( n = 10, P < 0.05) when these drugs were injected intravenously at 10 min before remote IPC. In addition, intravenous bolus injection of glibenclamide (1 mg/kg) or 5-HD (10 mg/kg) at the end of hindlimb remote IPC also abolished the infarct protection in LD muscle flaps ( n = 10; P < 0.05). Furthermore, intravenous injection of the specific mKATPchannel opener BMS-191095 (2 mg/kg) at 10 min before 4 h of ischemia protected the LD muscle flap against infarction to a similar extent as hindlimb remote IPC, and this infarct-protective effect of BMS-191095 was abolished by intravenous bolus injection of 5-HD (5 mg/kg) at 10 min before or after intravenous injection of BMS-191095 ( n = 10; P < 0.05). The infarct protective effect of BMS-191095 was associated with a higher muscle content of ATP at the end of 4 h of ischemia and a decrease in muscle neutrophilic myeloperoxidase activity at the end of 1.5 h of reperfusion compared with the time-matched control ( n = 10, P < 0.05). These observations led us to conclude that mKATPchannels play a central role in the trigger and mediator mechanisms of hindlimb remote IPC of skeletal muscle against infarction in the pig, and the opening of mKATPchannels in ischemic skeletal muscle is associated with an ATP-sparing effect during sustained ischemia and attenuation of neutrophil accumulation during reperfusion.

The impact of ATP-sensitive K+ channel subtype selectivity of insulin secretagogues for the coronary vasculature and the myocardium

Insulin secretagogues (sulfonylureas and glinides) increase insulin secretion by closing the ATP-sensitive K+ channel (KATP channel) in the pancreatic beta-cell membrane. KATP channels subserve important functions also in the heart. First, KATP channels in coronary myocytes contribute to the control of coronary blood flow at rest and in hypoxia. Second, KATP channels in the sarcolemma of cardiomyocytes (sarcKATP channels) are required for adaptation of the heart to stress. In addition, the opening of sarcKATP channels and of KATP channels in the inner membrane of mitochondria (mitoKATP channels) plays a central role in ischemic preconditioning. Opening of sarcKATP channels also underlies the ST-segment elevation of the electrocardiogram, the primary diagnostic tool for initiation of lysis therapy in acute myocardial infarction. Therefore, inhibition of cardiovascular KATP channels by insulin secretagogues is considered to increase cardiovascular risk. Electrophysiological experiments have shown that the secretagogues differ in their selectivity for the pancreatic over the cardiovascular KATP channels, being either highly selective (approximately 1,000x; short sulfonylureas such as nateglinide and mitiglinide), moderately selective (10-20x; long sulfonylureas such as glibenclamide [glyburide]), or essentially nonselective (<2x; repaglinide). New binding studies presented here give broadly similar results. In clinical studies, these differences are not yet taken into account. The hypothesis that the in vitro selectivity of the insulin secretagogues is of importance for the cardiovascular outcome of diabetic patients with coronary artery disease needs to be tested.

KC 12291: an atypical sodium channel blocker with myocardial antiischemic properties

KC 12291 was designed as a voltage-gated sodium channel (VGSC) blocker with cardioprotective properties. KC 12291 has moderate inhibitory effects on peak (or rapid) Na+ current, and markedly reduces sustained (or slowly or non-inactivating) Na+ current. This distinguishes KC 12291 from conventional VGSC blockers such as local anesthetics or antiarrhythmics, which have little or no cardioprotective properties. Since VGSCs represent the main pathway for ischemic Na+ loading by failing to inactivate fully, KC 12291 exerts pronounced antiischemic activity principally by reducing the amplitude of sustained Na+ current. In isolated atria and Langendorff-perfused hearts, KC 12291 inhibits diastolic contracture, renowned for its resistance to pharmacological inhibition, reduces ischemic Na+ loading and preserves cardiac energy status. KC 12291 exerts oral antiischemic activity in vivo in the absence of major hemodynamic effects. Cardiac VGSC blockers such as KC 12291, which block cardiac VGSCs in atypical fashion by effectively inhibiting the sustained component of Na+ current, represent, therefore, promising potential antiischemic and cardioprotective drugs.

KATP channel openers: structure-activity relationships and therapeutic potential

ATP-sensitive potassium channels (K(ATP) channels) are heteromeric complexes of pore-forming inwardly rectifying potassium channel subunits and regulatory sulfonylurea receptor subunits. K(ATP) channels were identified in a variety of tissues including muscle cells, pancreatic beta-cells, and various neurons. They are regulated by the intracellular ATP/ADP ratio; ATP induces channel inhibition and MgADP induces channel opening. Functionally, K(ATP) channels provide a means of linking the electrical activity of a cell to its metabolic state. Shortening of the cardiac action potential, smooth muscle relaxation, inhibition of both insulin secretion, and neurotransmitter release are mediated via K(ATP) channels. Given their many physiological functions, K(ATP) channels represent promising drug targets. Sulfonylureas like glibenclamide block K(ATP) channels; they are used in the therapy of type 2 diabetes. Openers of K(ATP) channels (KCOs), for example, relax smooth muscle and induce hypotension. KCOs are chemically heterogeneous and include as different classes as the benzopyrans, cyanoguanidines, thioformamides, thiadiazines, and pyridyl nitrates. Examples for new chemical entities more recently developed as KCOs include cyclobutenediones, dihydropyridine related structures, and tertiary carbinols.

Small molecule mitochondrial F1F0 ATPase hydrolase inhibitors as cardioprotective agents. Identification of 4-(N-arylimidazole)-substituted benzopyran derivatives as selective hydrolase inhibitors

In this paper we show that 4-aryl-CH2-imidazole-substituted benzopyran compounds with 3S,4R-stereochemistry are cardioprotective by inhibiting the F1F0 mitochondrial ATP hydrolase. Compounds (e.g., 13) with 3R,4S-stereochemistry act as mitochondrial KATP openers. This resulted from an inversion of stereochemistry for the F1F0 mitochondrial ATP hydrolase vs mitochondrial KATP. Structure-activity relationships for the inhibition of mitochondrial ATP hydrolase are also delineated. It is not clear how 13 (3R,4S) can selectively inhibit the hydrolytic activity of the F1F0 mitochondrial enzyme without interfering with the synthase activity.

Protective effect of mitochondrial KATP activation in an isolated gracilis model of ischemia and reperfusion in dogs

Adenosine triphosphate-sensitive potassium channel (KATP) openers protect ischemic myocardium by direct protection of cardiac myocytes, which is thought to be a result of activation of mitochondrial KATP (mKATP). KATP is expressed in skeletal muscle, and the purpose of this study was to determine the effect of the mKATP opener BMS-191095 on infarct size in an isolated gracilis model of ischemia and reperfusion in dogs. The right and left gracilis muscles were isolated in anesthetized dogs except for the artery and vein supplying these muscles (pedicle). BMS-191095 (0.4 mg) or vehicle were infused directly into the artery supplying each gracilis muscle (each animal had one drug-treated and one vehicle-treated muscle). The pedicle was completely occluded for 5 hours followed by 48 hours of reperfusion, after which infarct size was determined. In the vehicle-treated gracilis muscles, significant necrosis was observed (82% +/- 3% of gracilis muscle). BMS-191095 significantly reduced the infarct size in the contralateral gracilis muscle (55% +/- 6%). Reflow into the gracilis muscle was significantly greater in BMS-191095-treated muscles. BMS-191095 appears to reduce damage in ischemic/reperfused skeletal muscle, suggesting that mKATP activation is an important protective mechanism in this tissue.

SNC-80-induced preconditioning: selective activation of the mitochondrial adenosine triphosphate-gated potassium channel

Pharmacologic preconditioning by delta-opioid agonists occurs via activation of an adenosine triphosphate (ATP)-gated potassium channel (I(KATP)). Opening of mitochondrial I(KATP) confers pharmacologic preconditioning whereas opening the sarcolemmal I(KATP) shortens action potential duration and is proarrhythmic. This study investigated whether SNC-80, a selective delta-opioid agonist, is associated with development of ventricular arrhythmia due to activation of I(KATP). Rabbit isolated hearts were subjected to 12 min of hypoxia and 40 min of reoxygenation after pretreatment with SNC-80 (1 microM, n = 6), pinacidil (1.25 microM, n = 12), or BMS-191095 (6.0 microM, n = 4). Nine additional hearts served as controls. The cytoprotective effects of SNC-80 at a concentration of 1 microM were confirmed using 30 min of regional ischemia followed by 120 min of reperfusion. Ventricular fibrillation (VF) developed in 11 of 12 pinacidil-treated hearts whereas none of the SNC-80-treated (zero of six) hearts developed VF (P < 0.001 compared with pinacidil pretreatment) and zero of four BMS-191095-pretreated hearts developed VF. Similarly, zero of nine control hearts developed VF. SNC-80 reduced infarct size expressed as a percentage of the area at risk from 33 +/- 4% to 14 +/- 3% (P = 0.004) compared with control. SNC-80, which selectively activates the delta-opioid receptor, provided cytoprotection but did not induce VF after hypoxia reoxygenation. The results indicate that pinacidil-induced nonselective activation of I(KATP) results in proarrhythmia that is dependent on activation of the sarcolemmal I(KATP). Selectivity for the mitochondrial I(KATP) is necessary to prevent induction of a proarrhythmic state.

Binding and effect of K ATP channel openers in the absence of Mg2+

1 Openers of ATP-sensitive K(+) channels (K(ATP) channels) are thought to act by enhancing the ATPase activity of sulphonylurea receptors (SURs), the regulatory channel subunits. At higher concentrations, some openers activate K(ATP) channels also in the absence of MgATP. Here, we describe binding and effect of structurally diverse openers in the absence of Mg(2+) and presence of EDTA. 2 Binding of openers to SUR2B was measured using a mutant with high affinity for [(3)H]glibenclamide ([(3)H]GBC). In the absence of Mg(2+), 'typical' openers (benzopyrans, cyanoguanidines and aprikalim) inhibited [(3)H]GBC binding with K(i) values approximately 200 x higher than in the presence of MgATP. Minoxidil sulphate and nicorandil were inactive, whereas binding of diazoxide was unaffected by MgATP. 3 In the absence/presence of MgATP, N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine (P1075) activated the Kir6.2/SUR2B channel in inside-out patches with EC(50)=2000/67nM and E(max)=32/134%. In the absence of Mg(2+), responses were variable with only a small part of the variability being explained by a decrease in channel responsiveness with time after patch excision and to differences in the ATP sensitivity between patches. 4 The rank order of efficacy of the openers was P1075>rilmakalim approximately nicorandil>diazoxide>minoxidil sulphate. 5 The data show that structurally diverse openers are able to bind to, and to activate the Kir6.2/SUR2B channel by a pathway independent of ATP hydrolysis. These effects are observed at concentrations used to define the biochemical mechanism of the openers in the presence of MgATP and allow the openers to be classified into 'typical' and 'atypical' KCOs with diazoxide standing apart.