ATP-dependent Potassium Research Articles

Functioning of the mitochondrial ATP-dependent potassium channel in rats varying in their resistance to hypoxia. Involvement of the channel in the process of animal’s adaptation to hypoxia

The mechanism of tissue protection from ischemic damage by activation of the mitochondrial ATP-dependent K(+) channel (mitoK(ATP)) remains unexplored. In this work, we have measured, using various approaches, the ATP-dependent mitochondrial K(+) transport in rats that differed in their resistance to hypoxia. The transport was found to be faster in the hypoxia-resistant rats as compared to that in the hypoxia-sensitive animals. Adaptation of animals to the intermittent normobaric hypoxia increased the rate of transport. At the same time, the intramitochondrial concentration of K(+) in the hypoxia-sensitive rats was higher than that in the resistant and adapted animals. This indicates that adaptation to hypoxia stimulates not only the influx of potassium into mitochondria, but also K(+)/H(+) exchange. When mitoK(ATP) was blocked, the rate of the mitochondrial H(2)O(2) production was found to be significantly higher in the hypoxia-resistant rats than that in the hypoxia-sensitive animals. The natural flavonoid-containing adaptogen Extralife, which has an evident antihypoxic effect, increased the rate of the mitochondrial ATP-dependent K(+) transport in vitro and increased the in vivo tolerance of hypoxia-sensitive rats to acute hypoxia 5-fold. The involvement of the mitochondrial K(+) transport in the mechanism of cell adaptation to hypoxia is discussed.

Lactate levels in the brain are elevated upon exposure to volatile anesthetics: A microdialysis study

Anesthetic agents have well-defined pharmacological targets but their effects on energy metabolism in the brain are poorly understood. In this study, we examined the effects of different anesthetics on extracellular lactate and glucose levels in blood, CSF and brain of the mouse. In vivo-microdialysis was used to monitor extracellular energy metabolites in the brain of awake mice and during anesthesia with seven different anesthetic drugs. In separate groups, lactate and glucose concentrations in blood and CSF were measured for each anesthetic. We found that anesthesia with isoflurane caused a large increase of extracellular lactate levels in mouse striatum and hippocampus (300–400%). Pyruvate levels also increased while glucose and glutamate levels were unchanged. This effect was dose-dependent and was mimicked by other gaseous anesthetics such as halothane and sevoflurane but not by intravenous anesthetics. Ketamine/xylazine and chloral hydrate caused 2-fold increases of glucose levels in mouse blood and brain while lactate levels were only moderately increased. Propofol caused a minor increase of extracellular glucose levels while pentobarbital had no effect on either lactate or glucose. Volatile anesthetics also increased lactate levels in blood and CSF by 2–3-fold but had no effect on plasma glucose. Further experiments demonstrated that lactate formation by isoflurane in mouse brain was independent of neuronal impulse flow and did not involve ATP-dependent potassium channels. We conclude that volatile anesthetics, but not intravenous anesthetics, cause a specific, dose-dependent increase in extracellular lactate levels in mouse brain. This effect occurs in the absence of ischemia, is independent of peripheral actions and is reflected in strongly increased CSF lactate levels.

Localization in the cell of the protein forming ATP-dependent potassium-selective channels in a bilayer lipid membrane. An ultrastructural study

The localization in the cell of the protein forming the ATP-dependent potassium-selective channels in the bilayer lipid membrane has been studied. The electron microscope investigation of rat liver and heart tissue sections after their incubation with Abs against this protein and the visualization of the protein with secondary Abs conjugated with colloid gold were carried out. Colloid gold particles were observed both in mitochondrial membranes and in membranes of endoplasmic and sarcoplasmic reticulum. In heart mitochondria, these particles were significantly greater than in liver mitochondria. The localization of the channel protein both in mitochondria and reticulum, as well as the structural similarity between the mitochondrial channel and the precursor of calreticulin suggest that the channel protein belongs to the family of calreticulins. The possible function of the protein as a channel subunit of the mitochondrial ATP-dependent potassium channel is discussed.

Mitochondria in Postconditioning

Several signal transduction pathways are activated by cardioprotective stimuli, including ischemic or pharmacological postconditioning. These pathways converge on a common target, the mitochondria, and cardioprotection by postconditioning is associated with preserved mitochondrial function after ischemia/reperfusion. The present review discusses the role of mitochondria in cardioprotection, especially the involvement of ATP-dependent potassium channels, reactive oxygen species, and the mitochondrial permeability transition pore, and focuses on the effects of postconditioning on mitochondrial function (i.e., their oxygen consumption and calcium retention capacity). The contribution of mitochondria to loss of protection by postconditioning in diseased or aged myocardium is also addressed.

Levosimendan improves cardiac function and survival in rats with angiotensin II-induced hypertensive heart failure

Calcium-sensitizing agents improve cardiac function in acute heart failure; however, their long-term effects on cardiovascular mortality are unknown. We tested the hypothesis that levosimendan, an inodilator that acts through calcium sensitization, opening of ATP-dependent potassium channels and phosphodiesterase III inhibition, improves cardiac function and survival in double transgenic rats harboring human renin and angiotensinogen genes (dTGRs), a model of angiotensin II (Ang II)-induced hypertensive heart failure. Levosimendan (1 mg kg(-1)) was administered orally to 4-week-old dTGRs and normotensive Sprague-Dawley rats for 4 weeks. Untreated dTGRs developed severe hypertension, cardiac hypertrophy, heart failure with impaired diastolic relaxation, and exhibited a high mortality rate at the age of 8 weeks. Levosimendan did not decrease blood pressure and did not prevent cardiac hypertrophy. However, levosimendan improved systolic function, decreased cardiac atrial natriuretic peptide mRNA expression, ameliorated Ang II-induced cardiac damage and decreased mortality. Levosimendan did not correct Ang II-induced diastolic dysfunction and did not influence heart rate. In a separate survival study, levosimendan increased dTGR survival by 58% and median survival time by 27% (P=0.004). Our findings suggest that levosimendan ameliorates Ang II-induced hypertensive heart failure and reduces mortality. The results also support the notion that the effects of levosimendan in dTGRs are mediated by blood pressure-independent mechanisms and include improved systolic function and amelioration of Ang II-induced coronary and cardiomyocyte damage.

The Atrial Natriuretic Peptide and Guanylyl Cyclase-A System Modulates Pancreatic β-Cell Function

Atrial natriuretic peptide (ANP) and its guanylyl cyclase-A (GC-A) receptor are being involved in metabolism, although their role in the endocrine pancreas is still greatly unknown. The aim of this work is to study a possible role for the ANP/GC-A system in modulating pancreatic beta-cell function. The results presented here show a direct effect of the GC-A receptor in regulating glucose-stimulated insulin secretion (GSIS) and beta-cell mass. GC-A activation by its natural ligand, ANP, rapidly blocked ATP-dependent potassium (K(ATP)) channel activity, increased glucose-elicited Ca(2+) signals, and enhanced GSIS in islets of Langerhans. The effect in GSIS was inhibited in islets from GC-A knockout (KO) mice. Pancreatic islets from GC-A KO mice responded to increasing glucose concentrations with enhanced insulin secretion compared with wild type (WT). Remarkably, islets from GC-A KO mice were smaller, presented lower beta-cell mass and decreased insulin content. However, glucose-induced Ca(2+) response was more vigorous in GC-A KO islets, and basal K(ATP) channel activity in GC-A KO beta-cells was greatly diminished compared with WT. When protein levels of the two K(ATP) channel constitutive subunits sulfonylurea receptor 1 and Inward rectifier potassium channel 6.2 were measured, both were diminished in GC-A KO islets. These alterations on beta-cell function were not associated with disruption of glucose tolerance or insulin sensitivity in vivo. Glucose and insulin tolerance tests were similar in WT and GC-A KO mice. Our data suggest that the ANP/GC-A system may have a modulating effect on beta-cell function.

Combination therapy for patients with Type 2 diabetes: repaglinide in combination with metformin

In patients with Type 2 diabetes mellitus (T2DM), adequate glycemic control is a critical factor in reducing long-term micro- and macro-vascular complications. Traditionally, the approach is to initiate monotherapy first, followed by combination therapy that targets two main defects in T2DM. Repaglinide, a rapidly acting insulin secretagog, stimulates insulin secretion via closure of ATP-dependent potassium channels on the cell membrane of β-cells. Repaglinide is ideally used at mealtime to reduce postprandial glucose levels, thus lowering the 24-h blood glucose profile and improving HbA1c levels. Metformin is an insulin sensitizer that effectively acts against insulin resistance, one of the predominant metabolic defects in T2DM. A combination of repaglinide and metformin is indicated as an adjunct to diet and exercise to improve glycemic control in adults with T2DM. When monotherapy with oral antidiabetic agents fails, combination therapy with repaglinide plus metformin has been demonstrated to be safe and effective in the treatment of T2DM.

The Mitochondrial 2-Oxoglutarate Carrier Is Part of a Metabolic Pathway That Mediates Glucose- and Glutamine-stimulated Insulin Secretion

Glucose-stimulated insulin secretion from pancreatic islet beta-cells is dependent in part on pyruvate cycling through the pyruvate/isocitrate pathway, which generates cytosolic alpha-ketoglutarate, also known as 2-oxoglutarate (2OG). Here, we have investigated if mitochondrial transport of 2OG through the 2-oxoglutarate carrier (OGC) participates in control of nutrient-stimulated insulin secretion. Suppression of OGC in clonal pancreatic beta-cells (832/13 cells) and isolated rat islets by adenovirus-mediated delivery of small interfering RNA significantly decreased glucose-stimulated insulin secretion. OGC suppression also reduced insulin secretion in response to glutamine plus the glutamate dehydrogenase activator 2-amino-2-norbornane carboxylic acid. Nutrient-stimulated increases in glucose usage, glucose oxidation, glutamine oxidation, or ATP:ADP ratio were not affected by OGC knockdown, whereas suppression of OGC resulted in a significant decrease in the NADPH:NADP(+) ratio during stimulation with glucose but not glutamine + 2-amino-2-norbornane carboxylic acid. Finally, OGC suppression reduced insulin secretion in response to a membrane-permeant 2OG analog, dimethyl-2OG. These data reveal that the OGC is part of a mechanism of fuel-stimulated insulin secretion that is common to glucose, amino acid, and organic acid secretagogues, involving flux through the pyruvate/isocitrate cycling pathway. Although the components of this pathway must remain intact for appropriate stimulus-secretion coupling, production of NADPH does not appear to be the universal second messenger signal generated by these reactions.

Oxytocin protects rat heart against ischemia–reperfusion injury via pathway involving mitochondrial ATP-dependent potassium channel

Cardiac preconditioning represents the most potent and consistently reproducible method of rescuing heart tissue from undergoing irreversible ischemic damage. One of the major goals of the current cardiovascular research is to identify a reliable cardioprotective intervention that can salvage ischemic myocardium. The aim of the present study is to evaluate the oxytocin (OT)-induced cardioprotection and the signaling pathway involved with mitochondrial ATP-dependent potassium (mitoKATP) channel in the anesthetized rat heart. Animals were divided into six groups ( n = 6): (1) IR; hearts were subjected to 25 min ischemia and 120 min reperfusion, (2) OT; oxytocin was administered (0.03 μg/kg i.p.) 25 min prior to ischemia, (3) ATO+OT; atosiban (ATO) was used as an OT-selective receptor antagonist (1.5 μg/kg i.p.) 10 min prior to OT administration, (4) ATO; atosiban was used 35 min prior to ischemia, (5) 5HD + OT; 5-hydroxydecanoic acid (5HD) was used as a specific inhibitor of mitoKATP channel (10 mg/kg i.v.) 10 min prior to OT administration, (6) 5HD; 5HD was used 35 min prior to ischemia. Then infarct size, ventricular arrhythmia and creatine kinase-MB isoenzyme (CK-MB) plasma level were measured. Hemodynamic parameters were recorded throughout the experiment. OT administration significantly decreased infarct size, CK-MB plasma level, severity and incidence of ventricular arrhythmia as compared to IR group. Administration of atosiban and 5HD abolished the cardiopreconditioning effect of OT. This study demonstrates that cardioprotective effects of OT are mediated through opening the mitoKATP channels.

Abstract 1057: BRMS1 restoration of gap junctional intercellular communication involves modulation of KATP channels and downregulation of phosphoinositide 3-kinase (PI3K)

Abstract Metastatic disease remains the most lethal aspect of cancer, accounting for over 90% of cancer related mortality. Breast Cancer Metastasis Suppressor 1 (BRMS1) specifically inhibits the ability of tumor cells to metastasize to secondary sites while having little or no affect on primary tumor growth. Clinically, a loss of BRMS1 mRNA expression in breast cancer patients is correlated with poor prognosis and a decrease in BRMS1 protein levels predicts reduced disease free survival. The mechanisms of BRMS1 remain unclear; however expression of BRMS1 significantly decreases the PI3K signaling pathway, a pathway that is often over activated in cancer cells. PI3K is a lipid kinase that phosphorylates phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P2) to PtdIns(3,4,5)P3, leading to downstream signaling. BRMS1 acts to reduce the substrate of PI3K, PtdIns(4,5)P2, by >90% when expressed in metastatic cells. Re-expression of BRMS1 in cancer cell lines also restores gap junctional intercellular communication (GJIC), a process that mediates cellular homeostasis and is often lost during neoplastic progression. GJIC has been shown to be affected by PI3K; thus we hypothesized that inhibition of PI3K signaling by BRMS1 might be responsible for the restoration of GJIC. In support of this, treatment with the PI3K inhibitor LY294002 (10µM) in vector control and parental cell lines from multiple cancer cell types, mimicked the effects of BRMS1 on GJIC. Interestingly, inhibition of signaling molecules further downstream of PI3K (AKT, mTOR) did not produce the same effect, suggesting the mechanism of action may directly involve the ratio of PtdIns(4,5)P2/PtdIns(3,4,5)P3. We show that alterations in an ATP dependent potassium channel, KATP, regulated by phosphoinositides, is responsible for the changes in GJIC observed by BRMS1 expression. Delineation of this mechanism will allow for in vivo analysis to determine if changes in GJIC are causative for the BRMS1 mediated suppression of metastasis. Taken together, our data suggest an inhibitory mechanism on gap junctional communication mediated by PI3K, that is relieved by restoration of BRMS1 expression. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1057.

Antidiabetic Sulfonylureas Modulate Farnesoid X Receptor Activation and Target Gene Transcription

The sulfonylureas glibenclamide and glimepiride are oral antidiabetic drugs that stimulate insulin secretion by closing pancreatic ATP-dependent potassium channels. The farnesoid X receptor (FXR) is a ligand-activated transcription factor that regulates the expression of several target genes involved in bile acid metabolism and lipid and glucose homeostasis. In this study we investigated the potential effects of sulfonylureas on the signaling of FXR using a reporter-gene assay, real-time qPCR and computational methods such as molecular docking and molecular dynamic simulations. We demonstrate that glibenclamide and glimepiride modulate FXR activation in a reporter-gene assay and induce FXR target genes in HepG2 cells. Within the docking experiments and molecular dynamics simulation, we found glibenclamide interacting with the ligand-binding domain of FXR and with helix 12. Glibenclamide and glimepiride are potential ligands of FXR and modulate activation and signaling.

A mathematical model of β‐cells in an islet of Langerhans sensing a glucose gradient

Pancreatic β-cells release insulin in response to increased glucose levels. Compared to isolated β-cells, β-cells embedded within the islets of Langerhans network exhibit a coordinated and greater insulin secretion response to glucose. This coordinated activity is considered to rely on gap-junctions. We investigated the β-cell electrophysiology and the calcium dynamics in islets in response to glucose gradients. While at constant glucose the network of β-cells fires in a correlated fashion, a glucose gradient induces a sharp division into an active and an inactive part. We hypothesized that this sharp transition is mediated by the specific properties of the gap-junctions. We used a mathematical model of the β-cell electrophysiology in islets to discuss possible origins of this sharp transition in electrical activity. In silico, gap-junctions were required for such a transition. However, the small width of transition was only found when a stochastic variability of the expression of key transmembrane proteins, such as the ATP-dependent potassium channel, was included. The agreement with experimental data was further improved by assuming a delay of gap-junction currents, which points to a role of spatial constraints in the β-cell. This result clearly demonstrates the power of mathematical modeling in disentangling causal relationships in complex systems.

Perioperative Organprotektion – Organprotektion durch Konditionierung

Recent demographic developments challenge anaesthesiologists with an increasing number of elderly patients with cardiovascular comorbidities undergoing major surgery. Interventions that are capable to increase tissue tolerance against ischemia are of paramount importance. In this context, conditioning is defined as a mechanism that fosters tissue by specific adaptive processes to develop tolerance against a subsequent ischaemia. Dependent upon the temporal relationship between the intervention and the index ischaemia, preconditioning (intervention before ischaemia) is differentiated from postconditioning (intervention upon reperfusion). Ischaemia induced in tissue remote from the target organ is called remote preconditioning. Both brief periods of ischaemia as well as volatile anaesthetics and opioids are able to trigger conditioning. On a cellular level, ATP-dependent potassium channels and the mitochondrial permeability transition pore are thought to be key effectors. Effective conditioning has been unequivocal demonstrated for various tissues in animal experiments. Clinical trials in patients undergoing cardiac surgery have provided evidence for organ protection by conditioning. Large scale multicenter randomised trials, however, are still needed.

In vivo stimulation of AMP-activated protein kinase enhanced tubuloglomerular feedback but reduced tubular sodium transport during high dietary NaCl intake

AMP-activated protein kinase (AMPK) is expressed in the apical membrane of cortical thick ascending limb, distal, and collecting tubules as well as macula densa cells of the kidneys. AMPK is an active modulator of epithelial Na(+) channels, Na(+)-2Cl(-)-K(+) cotransporter, and the ATP-dependent potassium channel. The present experiments explored whether AMPK participates in the regulation of tubuloglomerular feedback (TGF) and renal tubular sodium handling. To this end, renal clearance and micropuncture experiments were performed in anesthetized rats. Under normal NaCl diet, neither TGF response nor renal fluid and sodium excretion were altered by pharmacological activation of AMPK in vivo. However, under high NaCl diet, the TGF response was significantly enhanced after intravenous or intratubular application of the AMPK activator AICAR. Moreover, AICAR application significantly increased fractional delivery of fluid and sodium to the end of the proximal tubule. High dietary NaCl intake increased the renal transcript levels encoding the AMPK-alpha1 subunit, while it decreased the expression of AMPK-beta1 and AMPK-gamma2 subunits. Immunoblots revealed that high dietary NaCl intake reduced renal expression of activated AMPK by about three times compared to normal NaCl diet whereas additional AICAR application increased AMPK activity. Our results suggest that AMPK regulates tubuloglomerular balance as well as tubular transport upon change of renal work load.

Endothelium-Derived Nitric Oxide Inhibits the Relaxation of the Porcine Coronary Artery to Natriuretic Peptides by Desensitizing Big Conductance Calcium-Activated Potassium Channels of Vascular Smooth Muscle

The present experiments investigated whether endothelium-derived mediators modulate the effect of natriuretic peptides in porcine coronary arteries. Rings with and without endothelium were suspended in organ chambers for isometric tension recording. Concentration-relaxation curves to C-type natriuretic peptide (CNP) and atrial natriuretic peptide (ANP) were obtained during contractions to endothelin-1. Removal of the endothelium potentiated relaxations to both CNP and ANP. N(omega)-nitro-L-arginine methyl ester potentiated relaxations to natriuretic peptides only in arteries with endothelium. Sodium nitroprusside (SNP) inhibited the response to the natriuretic peptides only in the absence of the endothelium. In rings with endothelium, 1H-[1,2,4]oxadiazolo [4,3-a]quinoxalin-1-one (ODQ) and 4H-8-bromo-1,2,4-oxadiazolo[3,4-d]benz[b][1,4]oxazin-1-one (NS2028) potentiated CNP-mediated relaxations. Iberiotoxin (IBTX) reduced the response only in rings without endothelium. Glybenclamide inhibited the relaxations in both the presence and absence of endothelium. CNP-induced relaxations were reduced by 8-bromoguanosine 3',5'-cGMP (8-bromo-cGMP) to the same extent in rings with and without endothelium. There was no significant difference between the increased cGMP content caused by CNP in porcine coronary arteries with or without endothelium. In patch-clamp studies in porcine coronary arterial smooth muscle cells, the natriuretic peptide-mediated enhancement of the IBTX-sensitive big conductance calcium-activated potassium channel (BK(Ca)) amplitude was reversed by SNP and 8-bromo-cGMP. These findings demonstrate that, in the porcine coronary artery, the opening of BK(Ca) and ATP-dependent potassium channels of the vascular smooth muscle contributes to CNP-mediated relaxations. Endothelium-derived and exogenous NO inhibit the direct relaxing effect of natriuretic peptides by desensitizing the response of the BK(Ca)s of the vascular smooth muscle to the generation of cGMP.

Glucose-Induced Intestinal Vasodilation Via Adenosine A1 Receptors Requires Nitric Oxide but Not K +ATP Channels

Both nitric oxide (NO) and adenosine A1 receptor activation mediate microvascular vasodilation during intestinal glucose absorption. Our overall hypothesis is that adenosine triphosphate (ATP) utilization during glucose absorption would increase adenosine metabolite release, which acts on adenosine A1 receptors to alter endothelial production of NO and/or activate ATP-dependent potassium channels (K(+)(ATP)) to dilate intestinal microvessels. Intravital videomicroscopy of the rat jejunum was used to record the vascular responses of inflow (termed 1A) arterioles, proximal (p3A), and distal (d3A) premucosal arterioles during exposure to isotonic glucose or mannitol solutions alone or in the presence of the selective nitric oxide synthase (NOS) inhibitor (L-NMMA), an adenosine A1 receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine (DPCPX)), or a K(+)(ATP) channel inhibitor (glibenclamide). As expected, glucose exposure caused rapid dilation of both p3A and d3A arterioles, while mannitol exposure had no effect on microvascular diameters. Adenosine A1 receptor blockade completely prevented glucose-induced dilation of the premucosal arterioles. NOS inhibition significantly blunted the glucose-induced vasodilation of the premucosal arterioles, but had little effect in the mannitol group. Simultaneous application of both the NOS inhibitor and the adenosine A1 receptor antagonist gave the same reduction in glucose-induced dilation of the premucosal arterioles as the adenosine A1 receptor antagonist alone. Blockade of K(+)(ATP) channels with glibenclamide did not attenuate glucose-induced vasodilation of the premucosal arterioles. These data suggest that glucose-induced vasodilation of premucosal jejunal arterioles is mediated through adenosine A1 receptors, and NO at least partially mediates the adenosine A1 receptor-induced vasodilation. In addition, K(+)(ATP) channels are not involved in premucosal arteriolar vasodilation during intestinal glucose exposure.

Effect of Potassium Channel Modulators on Morphine Withdrawal in Mice

The present study was conducted to investigate the effect of potassium channel openers and blockers on morphine withdrawal syndrome. Mice were rendered dependent on morphine by subcutaneous injection of morphine; four hours later, withdrawal was induced by using an opioid antagonist, naloxone. Mice were observed for 30 minutes for the withdrawal signs ie, the characteristic jumping, hyperactivity, urination and diarrhea. ATP-dependent potassium (K+ATP) channel modulators were injected intraperitoneally (i.p.) 30 minutes before the naloxone. It was found that a K+ATP channel opener, minoxidil (12.5–50 mg/kg i.p.), suppressed the morphine withdrawal significantly. On the other hand, the K+ATP channel blocker glibenclamide (12.5–50 mg/kg i.p.) caused a significant facilitation of the withdrawal. Glibenclamide was also found to abolish the minoxidil’s inhibitory effect on morphine withdrawal. The study concludes that K+ATP channels play an important role in the genesis of morphine withdrawal and K+ATP channel openers could be useful in the management of opioid withdrawal. As morphine opens K+ATP channels in neurons, the channel openers possibly act by mimicking the effects of morphine on neuronal K+ currents.

Endothelium-Independent Vasorelaxation by Ligusticum wallichii in Isolated Rat Aorta: Comparison of a Butanolic Fraction and Tetramethylpyrazine, the Main Active Component of Ligusticum wallichii

Ligusticum wallichii is an herb widely used to treat vascular disorders in Asian countries, and tetramethylpyrazine (TMP) has been identified as one of its vasorelaxant active components. This study was performed to examine the endothelium-independent relaxation produced by the butanol-soluble fraction of L. wallichii extract (LwBt) and its possible mechanisms of action in isolated rat aortic rings. The effects were compared with those of TMP. LwBt produced vasorelaxation that increased gradually after 2-3 min of LwBt administration and reached a maximum within 30 min. LwBt-induced relaxation was significantly attenuated by pretreatment with 4-aminopyridine and apamin. Additionally, LwBt attenuated CaCl(2)-induced vasoconstriction in high-potassium depolarized medium. Thus, LwBt-induced vasorelaxation apparently involved inhibition of calcium influx, mediated by the opening of voltage-dependent and/or Ca(2+)-activated potassium channels. On the other hand, the effect of TMP was significantly attenuated by pretreatment with glibenclamide, and 4-aminopyridine had no effect. In conclusion, LwBt-induced endothelium-independent vasorelaxation was mediated by the opening of voltage-dependent potassium channels, while TMP-induced relaxation was mediated by the opening of ATP-dependent potassium channels. These effects of LwBt may be due to a substance other than TMP.

Drug-induced QT interval shortening: potential harbinger of proarrhythmia and regulatory perspectives.

ATP-dependent potassium channel openers such as pinacidil and levcromakalim have long been known to shorten action potential duration and to be profibrillatory in non-clinical models, raising concerns on the clinical safety of drugs that shorten QT interval. Routine non-clinical evaluation of new drugs for their potential to affect cardiac repolarization has revealed that drugs may also shorten QT interval. The description of congenital short QT syndrome in 2000, together with the associated arrhythmias, suggests that drug-induced short QT interval may be proarrhythmic, and an uncanny parallel is evolving between our appreciation of the short and the long QT intervals. Epidemiological studies report an over-representation of short QT interval values in patients with idiopathic ventricular fibrillation. Therefore, as new compounds that shorten QT interval are progressed further into clinical development, questions will inevitably arise on their safety. Arising from the current risk-averse clinical and regulatory environment and concerns on proarrhythmic safety of drugs, together with our lack of a better understanding of the clinical significance of short QT interval, new drugs that substantially shorten QT interval will likely receive an unfavourable regulatory review unless these drugs fulfil an unmet clinical need. This review provides estimates of parameters of QT shortening that may be of potential clinical significance. Rufinamide, a recently approved anticonvulsant, illustrates the current regulatory approach to drugs that shorten QT interval. However, to further substantiate or confirm the safety of these drugs, their approval may well be conditional upon large-scale post-marketing studies with a focus on cardiac safety.

Adaptation of Animals to Different Types of Oxidative Stress: The Role of Mitochondrial Potassium Transport Systems

We studied parameters of the ATP-dependent influx of potassium into mitochondria, which were isolated from rats varying in their resistance to ischemia and from hypoxia-adapted animals. It has been found that in the heart and liver mitochondria, the rates of the ATP-dependent potassium influx and H2O2 production (in case of ATP-inhibited transport) are higher in the hypoxia-resistant rats, as compared to those in the hypoxia-sensitive animals. When adapted to low oxygen, the hypoxia-sensitive rats demonstrated rates of the both processes increasing to the levels observed in the hypoxia-resistant animals. However, the concentration of potassium in the mitochondria of hypoxia-resistant and adapted animals decreased. This indicates that adaptation to hypoxia stimulates not only the influx of potassium into mitochondria, but also K+/H+ exchange. The activation of such a potassium cycle can lower the production of ROS, which plays a crucial role in the lethal cell injury associated with cardiac ischemia and reperfusion. It has been further found that uridine and UMP ( precursors of UDP, a metabolic activator of mitoKATP) greatly decreased the index of ischemic alteration upon 60-min acute ischemia, as well as the size of infarction zone under ischemia-reperfusion conditions. The inhibitors of KATP channels (glibenclamide and 5-HD) reversed the anti-ischemic effect of uridine and UMP. These agents also exerted an anti-arrhythmic effect, which was completely abolished by glibenclamide but not 5-HD. It should be noted that uridine and UMP recovered the levels of ATP, phosphocreatin and glycogen, which were decreased during ischemia, while glibenclamide and 5-HD eliminated these effects. Also demonstrated was the effectiveness of uridine in the reduction of lipopolysaccharide-induced inflammation (another model of oxidative stress).