The sulphonylurea glibenclamide inhibits voltage dependent potassium currents in human atrial and ventricular myocytes

Inhibition of Cardiac Kv4.3/KChIP2 Channels by Sulfonylurea Drug Gliquidone

The Kv4.3 channel features fast N-type inactivation and also undergoes a slow C-type inactivation. The gain-of-function mutations of Kv4.3 channels cause an inherited disease called Brugada syndrome (BrS), characterized by a shortened duration of cardiac action potential repolarization and ventricular arrhythmia. The sulfonylurea drug gliquidone, an ATP-dependent K+ channel antagonist, is widely used for the treatment of type 2 diabetes. Here, we report a novel role of gliquidone in inhibiting Kv4.3 and Kv4.3/KChIP2 channels that encode the cardiac transient outward K+ currents responsible for the initial phase of action potential repolarization. Gliquidone results in concentration-dependent inhibition of both Kv4.3 and Kv4.3/KChIP2 fast or steady-state inactivation currents with an IC50 of approximately 8 μM. Gliquidone also accelerates Kv4.3 channel inactivation and shifts the steady-state activation to a more depolarizing direction. Site-directed mutagenesis and molecular docking reveal that the residues S301 in the S4 and Y312A and L321A in the S4-S5 linker are critical for gliquidone-mediated inhibition of Kv4.3 currents, as mutating those residues to alanine significantly reduces the potency for gliquidone-mediated inhibition. Furthermore, gliquidone also inhibits a gain-of-function Kv4.3 V392I mutant identified in BrS patients in voltage- and concentration-dependent manner. Taken together, our findings demonstrate that gliquidone inhibits Kv4.3 channels by acting on the residues in the S4 and the S4-S5 linker. Therefore, gliquidone may hold repurposing potential for the therapy of Brugada syndrome. SIGNIFICANCE STATEMENT: We describe a novel role of gliquidone in inhibiting cardiac Kv4.3 currents and the channel gain-of-function mutation identified from patients with Brugada syndrome, suggesting its repurposing potential for therapy for the heart disease.

Pharmacological Modulation and (Patho)Physiological Roles of TRPM4 Channel—Part 1: Modulation of TRPM4

Transient receptor potential melastatin 4 is a unique member of the TRPM protein family and, similarly to TRPM5, is Ca²⁺-sensitive and permeable to monovalent but not divalent cations. It is widely expressed in many organs and is involved in several functions by regulating the membrane potential and Ca²⁺ homeostasis in both excitable and non-excitable cells. This part of the review discusses the pharmacological modulation of TRPM4 by listing, comparing, and describing both endogenous and exogenous activators and inhibitors of the ion channel. Moreover, other strategies used to study TRPM4 functions are listed and described. These strategies include siRNA-mediated silencing of TRPM4, dominant-negative TRPM4 variants, and anti-TRPM4 antibodies. TRPM4 is receiving more and more attention and is likely to be the topic of research in the future.

Large-conductance calcium-activated K+ channels, rather than KATP channels, mediate the inhibitory effects of nitric oxide on mouse lymphatic pumping

BACKGROUND AND PURPOSE

K_ATP channels are negative regulators of lymphatic vessel excitability and contractility and are proposed to be targets for immune cell products that inhibit lymph transport. Previous studies in rat and guinea pig mesenteric lymphatics found that NO-mediated inhibition of lymphatic contraction was prevented or reversed by the K_ATP channel inhibitor, glibenclamide. We revisited this hypothesis using mouse lymphatic vessels and K_ATP channel knockout mice.

EXPERIMENTAL APPROACH

Mouse popliteal lymphatics were isolated, and contractility was assessed using pressure myography. K⁺ channel expression was determined by PCR analysis of FACS-purified lymphatic smooth muscle cells.

KEY RESULTS

The NO-producing agonist, ACh, and the NO donor, NONOate, both produced dose-dependent inhibition of spontaneous lymphatic contractions that were blocked by the soluble GC inhibitor, ODQ, or the PKG inhibitor, Rp-8-Br-PET-cGMPS. Surprisingly, the inhibitory effects of both were preserved in K_ir 6.1^-/- vessels, suggesting that K_ATP channels did not mediate NO-induced responses. We hypothesized a role for BK channels, given their prominence in arterial smooth muscle. Indeed, BK channels were expressed in mouse lymphatic smooth muscle and NS11021 (a BK channel activator) caused dilation and reduced contraction frequency, whereas iberiotoxin and penitrem A (BK channel inhibitors) produced right-ward shifts in NONOate concentration-response curves.

CONCLUSION AND IMPLICATIONS

Inhibition of mouse lymphatic contractions by NO primarily involves activation of BK channels, rather than K_ATP channels. Thus, BK channels are a potential target for therapeutic reversal of lymph pump inhibition by NO generated by immune cell activation of iNOS in chronic lymphoedema.

Age-dependent decrease in TRPM4 channel expression but not trafficking alters urinary bladder smooth muscle contractility

During development, maturation, or aging, the expression and function of urinary bladder smooth muscle (UBSM) ion channels can change, thus affecting micturition. Increasing evidence supports a novel role of transient receptor potential melastatin‐4 (TRPM4) channels in UBSM physiology. However, it remains unknown whether the functional expression of these key regulatory channels fluctuates in UBSM over different life stages. Here, we examined TRPM4 channel protein expression (Western blot) and the effects of TRPM4 channel inhibitors, 9‐phenanthrol and glibenclamide, on phasic contractions of UBSM isolated strips obtained from juvenile (UBSM‐J, 5–9 weeks old) and adult (UBSM‐A, 6–18 months old) male guinea pigs. Compared to UBSM‐J, UBSM‐A displayed a 50–70% reduction in total TRPM4 protein expression, while the surface‐to‐intracellular expression ratio (channel trafficking) remained the same in both age groups. Consistent with the reduced total TRPM4 protein expression in UBSM‐A, 9‐phenanthrol showed lower potencies and/or maximum efficacies in UBSM‐A than UBSM‐J for inhibiting amplitude and muscle force of spontaneous and 20 mM KCl‐induced phasic contractions. Compared to 9‐phenanthrol, glibenclamide also attenuated both spontaneous and KCl‐induced contractions, but with less pronounced differential effects in UBSM‐A and UBSM‐J. In both age groups, regardless of the overall reduced total TRPM4 protein expression in UBSM‐A, cell surface TRPM4 protein expression (~80%) predominated over its intracellular fraction (~20%), revealing preserved channel trafficking mechanisms toward the cell membrane. Collectively, this study reports novel findings illuminating a fundamental physiological role for TRPM4 channels in UBSM function that fluctuates with age.

TRPM4 channel inhibitors 9-phenanthrol and glibenclamide differentially decrease guinea pig detrusor smooth muscle whole-cell cation currents and phasic contractions

Nonselective cation channels, consistent with transient receptor potential melastatin-4 (TRPM4), regulate detrusor smooth muscle (DSM) function. TRPM4 channels can exist as homomers or assemble with sulfonylurea receptors (SURs) as complexes. We evaluated contributions of TRPM4/SUR-TRPM4 channels to DSM excitability and contractility by examining the effects of TRPM4/SUR-TRPM4 channel modulators 9-phenanthrol, glibenclamide, and diazoxide on freshly-isolated guinea pig DSM cells (amphotericin-B perforated patch-clamp electrophysiology) and mucosa-free DSM strips (isometric tension recordings). In DSM cells, complete removal of extracellular Na⁺ decreased voltage-step-induced cation (non-K⁺ selective) currents. At high positive membrane potentials, 9-phenanthrol at 100 μM attenuated voltage step-induced currents more effectively than at 30 μM, revealing concentration-dependent, voltage-sensitive inhibition. In comparison to 9-phenanthrol, glibenclamide (100 μM) displayed lower inhibition of cation currents. In the presence of glibenclamide (100 μM), 9-phenanthrol (100 μM) further decreased the currents. The SUR-TRPM4 complex activator diazoxide (100-300 μM) weakly inhibited the currents. 9-Phenanthrol, but not glibenclamide or diazoxide, increased cell capacitance (a cell surface area indicator). In contractility studies, glibenclamide displayed lower potencies than 9-phenanthrol attenuating spontaneous and 20 mM KCl-induced DSM phasic contractions. While both compounds showed similar maximum inhibitions on DSM spontaneous phasic contractions, glibenclamide was generally less efficacious on 20 mM KCl-induced phasic contractions. In summary, the observed differential effects of 9-phenanthrol and glibenclamide on DSM excitability and contractility support unique mechanisms for the two compounds. The data suggest that SUR-TRPM4 complexes do not contribute to DSM function. This study advances our understanding of pharmacological effects of glibenclamide and 9-phenanthrol on DSM cell cation currents.

Pro- and Antiarrhythmic Actions of Sulfonylureas: Mechanistic and Clinical Evidence

Sulfonylureas are the most commonly used second-line drug class for treating type 2 diabetes mellitus (T2DM). While the cardiovascular safety of sulfonylureas has been examined in several trials and nonrandomized studies, little is known of their specific effects on sudden cardiac arrest (SCA) and related serious arrhythmic outcomes. This knowledge gap is striking, because persons with DM are at increased risk of SCA. In this review, we explore the influence of sulfonylureas on the risk of serious arrhythmias, with specific foci on ischemic preconditioning, cardiac excitability, and serious hypoglycemia as putative mechanisms. Elucidating the relationship between individual sulfonylureas and serious arrhythmias is critical, especially as the diabetes epidemic intensifies and SCA incidence increases in persons with diabetes.

A critical role for cystathionine-β-synthase in hydrogen sulfide-mediated hypoxic relaxation of the coronary artery

Hypoxia-induced coronary artery vasodilatation protects the heart by increasing blood flow under ischemic conditions, however its mechanism is not fully elucidated. Hydrogen sulfide (H₂S) is reported to be an oxygen sensor/transducer in the vasculature. The present study aimed to identify and characterise the role of H₂S in the hypoxic response of the coronary artery, and to define the H₂S synthetic enzymes involved. Immunoblotting and immunohistochemistry showed expression of all three H₂S-producing enzymes, cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (MPST), in porcine coronary artery. Artery segments were mounted for isometric tension recording; hypoxia caused a transient endothelium-dependent contraction followed by prolonged endothelium-independent relaxation. The CBS inhibitor amino-oxyacetate (AOAA) reduced both phases of the hypoxic response. The CSE inhibitor dl-propargylglycine (PPG) and aspartate (limits MPST) had no effect alone, but when applied together with AOAA the hypoxic relaxation response was further reduced. Exogenous H₂S (Na₂S and NaHS) produced concentration-dependent contraction followed by prolonged relaxation. Responses to both hypoxia and exogenous H₂S were dependent on the endothelium, NO, cGMP, K⁺ channels and Cl^-/HCO₃^- exchange. H₂S production in coronary arteries was blocked by CBS inhibition (AOAA), but not by CSE inhibition (PPG). These data show that H₂S is an endogenous mediator of the hypoxic response in coronary arteries. Of the three H₂S-producing enzymes, CBS, expressed in the vascular smooth muscle, appears to be the most important for H₂S generated during hypoxic relaxation of the coronary artery. A contribution from other H₂S-producing enzymes only becomes apparent when CBS activity is inhibited.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Activation of glibenclamide-sensitive ATP-sensitive K+ channels during β-adrenergically induced metabolic stress produces a substrate for atrial tachyarrhythmia

BACKGROUND

Cardiac ATP-sensitive K(+) channels have been suggested to contribute to the adaptive physiological response to metabolic challenge after β-adrenoceptor stimulation. However, an increased atrial K(+)-conductance might be expected to be proarrhythmic. We investigated the effect of ATP-sensitive K(+) channel blockade on the electrophysiological responses to β-adrenoceptor-induced metabolic challenge in intact atria.

METHODS AND RESULTS

Atrial electrograms were recorded from the left atrial epicardial surface of Langendorff-perfused rat hearts using a 5×5 electrode array. Atrial effective refractory period and conduction velocity were measured using an S(1)-S(2) protocol. The proportion of hearts in which atrial tachyarrhythmia was produced by burst-pacing was used as an index of atrial tachyarrhythmia-inducibility. Atrial nucleotide concentrations were measured by high performance liquid chromatography. Perfusion with ≥10(-9) mol/L of the β-adrenoceptor agonist, isoproterenol (ISO), resulted in a concentration-dependent reduction of atrial effective refractory period and conduction velocity. The ISO-induced changes produced a proarrhythmic substrate such that atrial tachyarrhythmia could be induced by burst-pacing. Atrial [ATP] was significantly reduced by ISO (10(-6) mol/L). Perfusion with either of the ATP-sensitive K(+) channel blockers, glibenclamide (10(-5) mol/L) or tolbutamide (10(-3) mol/L), in the absence of ISO had no effect on basal atrial electrophysiology. On the other hand, the proarrhythmic substrate induced by 10(-6) mol/L ISO was abolished by either of the sulfonylureas, which prevented induction of atrial tachyarrhythmia.

CONCLUSIONS

Atrial ATP-sensitive K(+) channels activate in response to β-adrenergic metabolic stress in Langendorff-perfused rat hearts, resulting in a proarrhythmic substrate.

Role of KATP channel in electrical depression and asystole during long-duration ventricular fibrillation in ex vivo canine heart

Long-duration ventricular fibrillation (LDVF) in the globally ischemic heart is characterized by transmurally heterogeneous decline in ventricular fibrillation rate (VFR), emergence of inexcitable regions, and eventual global asystole. Rapid loss of both local and global excitability is detrimental to successful defibrillation and resuscitation during cardiac arrest. We sought to assess the role of the ATP-sensitive potassium current (I(KATP)) in the timing and spatial pattern of electrical depression during LDVF in a structurally normal canine heart. We analyzed endo-, mid-, and epicardial unipolar electrograms and epicardial optical recordings in the left ventricle of isolated canine hearts during 10 min of LDVF in the absence (control) and presence of an I(KATP) blocker glybenclamide (60 μM). In all myocardial layers, average VFR was the same or higher in glybenclamide-treated than in control hearts. The difference increased with time of LDVF and was overall significant in all layers (P < 0.05). However, glybenclamide did not significantly affect the transmural VFR gradient. In epicardial optical recordings, glybenclamide shortened diastolic intervals, prolonged action potential duration, and decreased the percentage of inexcitable area (all differences P < 0.001). During 10 min of LDVF, asystole occurred in 55.6% of control and none of glybenclamide-treated hearts (P < 0.05). In three hearts paced after the onset of asystole, there was no response to LV epicardial or atrial pacing. In structurally normal canine hearts, I(KATP) opening during LDVF is a major factor in the onset of local and global inexcitability, whereas it has a limited role in overall deceleration of VFR and the transmural VFR gradient.

Endothelium-independent vasodilation induced by kolaviron, a biflavonoid complex from Garcinia kola seeds, in rat superior mesenteric arteries

Previous studies have established the hepatoprotective, gastroprotective, hypolipidemic and hypoglycemic effects of kolaviron (KV), a biflavonoid complex from Garcinia kola seeds. In this study, we investigated the mechanisms involved in the vasorelaxant effects of KV in isolated superior mesenteric arteries from normotensive rats. KV (1, 10, 30, 100, 300, 500 and 1,000 microg/ml) concentration-dependently inhibited the contractions induced by phenylephrine (PHE) (10 microM) and KCl (80 mM) in both endothelium-intact (E(max) = 58.3 +/- 1.7% and 51.4 +/- 1.3%, respectively) and -denuded rings (E(max) = 59.3 +/- 5.5% and 64.3 +/- 2.4%, respectively). Furthermore, KV reduced CaCl(2)-induced contraction in Ca(2+)-free medium containing KCl 60 mM, thus acting as a Ca(2+)-antagonist. In addition, KV inhibited the transient contraction by PHE in Ca(2+)-free medium containing EGTA, suggesting a possible action on the release of intracellular Ca(2+) via the inositol-1,4,5-triphosphate (IP(3)) pathway. KV is not a specific alpha-adrenoceptor blocker, since it also caused a concentration-dependent inhibition of contractile responses to KCl, suggesting that KV also blocks the L-type Ca(2+)-channel. As a Ca(2+) antagonist, KV (100 microg/ml) potentiates the relaxant effects of nifedipine in denuded rings (E(max) = 97.6 +/- 1.2%; control = 75.1 +/- 3.0%, P<0.05). Also, the vasorelaxation induced by KV was significantly inhibited after pre-treatment of the denuded rings with 4-aminopyridine (4-AP) 1 mM, a selective blocker of voltage-dependent K(+) (K(v)) channels and, tetraethylammonium (TEA) 1 mM or charybdotoxin (ChTX) 0.1 microM, non-selective blockers of large and intermediate conductance Ca(2+)-activated K(+) (BK(Ca)) channels. In contrast, neither glibenclamide (10 microM), BaCl2 (1 mM) nor apamin (0.1 microM), blockers of K(ATP), K(IR) and SK(Ca) channels, respectively affected the KV-induced vasorelaxation. In conclusion, our results provide functional evidence that the vasorelaxant effects by KV involve extracellular Ca(2+) influx blockade, inhibition of intracellular Ca(2+) release and the opening of K(+) channels sensitive to 4-AP and ChTX with a resultant membrane hyperpolarization/ repolarization.

Common variation in the NOS1AP gene is associated with reduced glucose-lowering effect and with increased mortality in users of sulfonylurea

OBJECTIVE

The single nucleotide polymorphism rs10494366 in the nitric oxide synthase 1 adaptor protein (NOS1AP) gene is associated with QTc prolongation, through an effect on the intracellular Ca levels. As sulfonylurea stimulate insulin secretion by an increased influx of Ca, we hypothesized that this polymorphism is associated with the glucose-lowering effect and mortality risk in sulfonylurea users.

METHODS

Associations between the NOS1AP polymorphism, prescribed doses, and mortality rates in sulfonylurea, metformin, and insulin users were assessed in the Rotterdam Study, a population-based cohort study of 7983 elderly people.

RESULTS

We identified 619 participants who were prescribed oral antidiabetic drugs during follow-up. In glibenclamide users carrying the TG genotype, the prescribed doses were higher compared with the glibenclamide users carrying the TT genotype [0.38 defined daily dose units, 95% confidence interval (CI) 0.14-0.63]. Glibenclamide users with the TG or GG genotype had an increased mortality risk compared with glibenclamide users with the TT genotype [hazard ratio (HR) 2.80, 95% CI: 1.09-7.22]. Tolbutamide users with the TG or GG genotype (HR: 0.30, 95% CI: 0.14-0.63) and glimepiride users with the TG or GG genotype (HR: 0.18, 95% CI: 0.04-0.74) had a decreased mortality risk compared with tolbutamide and glimepiride users with the TT genotype.

CONCLUSION

In participants with the TG or GG genotype at rs10494366 in the NOS1AP gene, glibenclamide is less effective in reducing glucose levels and mortality rates were higher compared with glibenclamide users with the TT genotype. In tolbutamide and glimepiride users, the TG and GG genotype were associated with a reduced mortality rate.

Selective cardiac plasma-membrane KATP channel inhibition is defibrillatory and improves survival during acute myocardial ischemia and reperfusion

ATP-dependent potassium channels (K(ATP)) have been implicated in cardioprotection both during myocardial ischemia and reperfusion. We compared the effect of a non-selective K(ATP) inhibitor glibenclamide, a selective mitochondrial K(ATP) inhibitor 5-hydroxy-decanoate (5-HD) and a selective sarcolemmal K(ATP) blocker HMR 1883, on survival and incidence of arrhythmias during myocardial ischemia in conscious, and during ischemia-reperfusion in pentobarbitone anesthetized rats. Glibenclamide (5 mg/kg i.p.) or HMR 1883 (3 mg/kg i.v.) reduced ischemia-induced irreversible ventricular fibrillation and improved survival during myocardial ischemia (64% and 61% vs. 23% in controls, respectively). 5-HD (5 mg/kg i.v.) did not influence survival and the incidence of ventricular arrhythmias. The incidence of reperfusion-induced arrhythmias was reduced by both glibenclamide and HMR 1883 (3 or 10 mg/kg) resulting in improved survival during reperfusion (81%, 82% and 96% vs. 24% in controls, respectively) in anesthetized rats. 5-HD did not reduce the incidence of lethal reperfusion arrhythmias. Glibenclamide and HMR 1883 prolonged (89+/-4.6 and 89+/-4.9 ms vs. 60+/-2.4 ms in controls), while 5-HD did not change the QT interval. In conclusion, inhibition of sarcolemmal K(ATP) reduces the incidence of lethal ventricular arrhythmias and improves survival both during acute myocardial ischemia and reperfusion in rats. This beneficial effect correlates with the prolongation of repolarization. Inhibition of mitochondrial K(ATP) does not improve survival or reduce the occurrence of ischemia and/or reperfusion-induced arrhythmias and does not prolong the QT interval. The present results also suggest that the antiarrhythmic effect of K(ATP) inhibitors is not influenced by pentobarbitone anesthesia.

Potassium channel opener pinacidil induces relaxation of the isolated human radial artery

Taking into consideration that the search for drugs capable of modifying blood flow through human radial artery (RA) is warranted, the present study was designed to examine the vasodilatatory effects of the potassium channel opener, pinacidil on the RA and to define the contribution of different K+ -channel subtypes in the endothelium-independent pinacidil action on this blood vessel. Pinacidil relaxed the RA rings with endothelium and without endothelium with comparable potency. N-nitro-L-arginine methyl ester (L-NAME) and methylene blue did not affect the pinacidil-induced vasorelaxation in rings with endothelium. In the rings without endothelium, the K+ -channel blockers glibenclamide and tetraethylammonium (TEA) moderately antagonized the pinacidil-induced relaxation, while charybdotoxin and 4-aminopiridine did not. In endothelium-denuded rings, precontracted with 100 mM K+, the relaxant responses to pinacidil were highly significantly shifted to the right compared to those obtained in RA precontracted with phenylephrine, but pinacidil-induced maximal relaxation was not affected. Addition of nifedipine did not but addition of nifedipine and nickel (Na+ -Ca2+ exchanger inhibitor) did cause a statistically significant rightward shift of the pinacidil concentration-relaxation curve, although the effect 0.1 mM pinacidil was preserved. Thus, pinacidil induces relaxation of the human RA in endothelium-independent manner, and glibenclamide- and TEA-sensitive vascular smooth muscle K+ channels are probably involved. Its ability to completely relax the RA precontracted with K+ -rich solution suggests that pinacidil has additional K+ channel-independent mechanism(s) of action. It seems that stimulation of the forward mode of the Na+ -Ca2+ exchanger plays a part in this K+ channel-independent effect of pinacidil.

Endogenous hydrogen peroxide regulates the excitability of midbrain dopamine neurons via ATP-sensitive potassium channels

ATP-sensitive K+ (K(ATP)) channels link metabolic state to cell excitability. Here, we examined regulation of K(ATP) channels in substantia nigra dopamine neurons by hydrogen peroxide (H2O2), which is produced in all cells during aerobic metabolism. Blockade of K(ATP) channels by glibenclamide (100 nM) or depletion of intracellular H2O2 by including catalase, a peroxidase enzyme, in the patch pipette increased the spontaneous firing rate of all dopamine neurons tested in guinea pig midbrain slices. Using fluorescence imaging with dichlorofluorescein to visualize intracellular H2O2, we found that moderate increases in H2O2 during partial inhibition of glutathione (GSH) peroxidase by mercaptosuccinate (0.1-0.3 mM) had no effect on dopamine neuron firing rate. However, with greater GSH inhibition (1 mM mercaptosuccinate) or application of exogenous H2O2, 50% of recorded cells showed K(ATP) channel-dependent hyperpolarization. Responsive cells also hyperpolarized with diazoxide, a selective opener for K(ATP) channels containing sulfonylurea receptor SUR1 subunits, but not with cromakalim, a selective opener for SUR2-based channels, indicating that SUR1-based K(ATP) channels conveyed enhanced sensitivity to elevated H2O2. In contrast, when endogenous H2O2 levels were increased after inhibition of catalase, the predominant peroxidase in the substantia nigra, with 3-amino-1,2,4-triazole (1 mM), all dopamine neurons responded with glibenclamide-reversible hyperpolarization. Fluorescence imaging of H2O2 indicated that catalase inhibition rapidly amplified intracellular H2O2, whereas inhibition of GSH peroxidase, a predominantly glial enzyme, caused a slower, smaller increase, especially in nonresponsive cells. Thus, endogenous H2O2 modulates neuronal activity via K(ATP) channel opening, thereby enhancing the reciprocal relationship between metabolism and excitability.

Partial mitochondrial inhibition causes striatal dopamine release suppression and medium spiny neuron depolarization via H2O2 elevation, not ATP depletion

Mitochondrial dysfunction is a potential causal factor in Parkinson's disease. We show here that acute exposure to the mitochondrial complex I inhibitor rotenone (30-100 nM; 30 min) causes concentration-dependent suppression of single-pulse evoked dopamine (DA) release monitored in real time with carbon-fiber microelectrodes in guinea pig striatal slices, with no effect on DA content. Suppression of DA release was prevented by the sulfonylurea glibenclamide, implicating ATP-sensitive K+ (KATP) channels; however, tissue ATP was unaltered. Because KATP channels can be activated by hydrogen peroxide (H2O2), as well as by low ATP, we examined the involvement of rotenone-enhanced H2O2 generation. Confirming an essential role for H2O2, the inhibition of DA release by rotenone was prevented by catalase, a peroxide-scavenging enzyme. Striatal H2O2 generation during rotenone exposure was examined in individual medium spiny neurons using fluorescence imaging with dichlorofluorescein (DCF). An increase in intracellular H2O2 levels followed a similar time course to that of DA release suppression and was accompanied by cell membrane depolarization, decreased input resistance, and increased excitability. Extracellular catalase markedly attenuated the increase in DCF fluorescence and prevented rotenone-induced effects on membrane properties; membrane changes were also largely prevented by flufenamic acid, a blocker of transient receptor potential (TRP) channels. Thus, partial mitochondrial inhibition can cause functional DA denervation via H2O2 and KATP channels, without DA or ATP depletion. Furthermore, amplified H2O2 levels and TRP channel activation in striatal spiny neurons indicate potential sources of damage in these cells. Overall, these novel factors could contribute to parkinsonian motor deficits and neuronal degeneration caused by mitochondrial dysfunction.

Clarithromycin reducesIsusandItocurrents in human atrial myocytes with minor repercussions on action potential duration

The macrolide antibacterial agent clarithromycin has been shown to cause QT interval prolongation on the electrocardiogram. In rabbit heart preparations clarithromycin (concentration dependently) lengthened the action potential duration and blocked the delayed rectifier current. The aim of the present study was to investigate the clarithromycin effects: (i) on the Ca2+ L-type and the main K+ repolarizing currents on human atrial myocytes, using whole-cell patch clamp recordings and (ii) on action potentials recorded from human atrial and ventricular myocardium using conventional microelectrodes. It has been found that (i) 10-30 microM clarithromycin reduced the sustained current Isus significantly and that a 100 microM concentration was needed to cause a significant reduction in the transient outward current Ito, whereas clarithomycin did not affect the calcium current and (ii) clarithromycin (10-100 microM) prolonged the action potential duration in atrial preparations but did not alter the different parameters of the ventricular action potential. It is concluded that clarithromycin exerts direct cardiac electrophysiological effects that may contribute to pro-arrythmic potential.

NADH supplementation decreases pinacidil-primed I K ATP in ventricular cardiomyocytes by increasing intracellular ATP

1 The aim of this study was to investigate the effect of nicotinamide-adenine dinucleotide (NADH) supplementation on the metabolic condition of isolated guinea-pig ventricular cardiomyocytes. The pinacidil-primed ATP-dependent potassium current I(K(ATP)) was used as an indicator of subsarcolemmal ATP concentration and intracellular adenine nucleotide contents were measured. 2 Membrane currents were studied using the patch-clamp technique in the whole-cell recording mode at 36-37 degrees C. Adenine nucleotides were determined by HPLC. 3 Under physiological conditions (4.3 mM ATP in the pipette solution, ATP(i)) I(K(ATP)) did not contribute to basal electrical activity. 4 The ATP-dependent potassium (K((ATP))) channel opener pinacidil activated I(K(ATP)) dependent on [ATP](i) showing a significantly more pronounced activation at lower (1 mM) [ATP](i). 5 Supplementation of cardiomyocytes with 300 micro g ml(-1) NADH (4-6 h) resulted in a significantly reduced I(K(ATP)) activation by pinacidil compared to control cells. The current density was 13.8+/-3.78 (n=6) versus 28.9+/-3.38 pA pF(-1) (n=19; P<0.05). 6 Equimolar amounts of the related compounds nicotinamide and NAD(+) did not achieve a similar effect like NADH. 7 Measurement of adenine nucleotides by HPLC revealed a significant increase in intracellular ATP (NADH supplementation: 45.6+/-1.88 nmol mg(-1) protein versus control: 35.4+/-2.57 nmol mg(-1) protein, P<0.000005). 8 These data show that supplementation of guinea-pig ventricular cardiomyocytes with NADH results in a decreased activation of I(K(ATP)) by pinacidil compared to control myocytes, indicating a higher subsarcolemmal ATP concentration. 9 Analysis of intracellular adenine nucleotides by HPLC confirmed the significant increase in ATP.

Effect of insulinotropic agent nateglinide on Kv and Ca(2+) channels in pancreatic beta-cell

Novel insulinotropic agent nateglinide stimulates insulin via binding to sulfonylurea receptor and closing the ATP-dependent K+ (K(ATP)) channels in pancreatic beta-cells, leading to an increase in [Ca(2+)](i) for exocytosis. The voltage-dependent Ca(2+) channel and the delayed rectifier K+ (Kv) channels are also present in beta-cells and their activities determine the configuration of action potential and hence contribute to the regulation of [Ca(2+)](i) and insulin secretion. This study, by using the patch-clamp method in whole cell configuration, comparatively characterized the direct effects of sulfonylurea receptor ligands including nateglinide, glyburide, and repaglinide on Kv and Ca(2+) channels. Each agent inhibited Kv currents in a concentration-dependent manner with effective concentration range two to three orders higher than that for blocking K(ATP) channels. A marginal stimulation of Ca(2+) current was observed with all drugs, while repaglinide at concentration greater than 300 nM inhibited Ca(2+) current. The direct effects of these antidiabetic agents on Kv and Ca(2+) channels may act concertedly with their primary action on K(ATP) channels in regulating [Ca(2+)](i) and the stimulus-secretion coupling.