Effects of cardioselective KATP channel antagonism on basal, stimulated, and ischaemic myocardial function in in vivo failing canine heart

Mechanical effects on KATP channel gating in rat ventricular myocytes

Cardiac K_ATP channels link metabolism with electrical activity. They are implicated in arrhythmias, secretion of atrial natriuretic peptide and protection of the heart from hypertrophy and failure. These processes may involve mechanosensitivity. K_ATP channels can be activated by mechanical stimulation and disrupting the cortical actin increases the activity. We propose that K_ATP channels are modulated by local bilayer tension and this tension is affected by cortical F-actin. Here we measured K_ATP background activity and stretch sensitivity with inside-out patches of rat ventricular myocytes before and after disrupting F-actin. Disrupting F-actin potentiated background activity but did not influence the slope sensitivity in the semilog relationship of NP_o vs. suction that is a measure of the change in dimensions between closed and open states. Thus actin alters prestress on the channel probably by parallel elastic sharing of mean cortical tension with the bilayer.

Fibroblast growth factor-2-induced cardioprotection against myocardial infarction occurs via the interplay between nitric oxide, protein kinase signaling, and ATP-sensitive potassium channels

Fibroblast growth factor-2 (FGF2) protects the heart from ischemia-reperfusion (I-R) injury via a vast network of protein kinases. In the heart, downstream effectors of these FGF2-triggered signals have not yet been identified. It is hypothesized that nitric oxide (NO) signaling and ATP-sensitive potassium (K(ATP)) channel activity are key effectors of protein kinases activated by FGF2-mediated cardioprotection. Hearts with a cardiac-specific overexpression of FGF2 (FGF2 Tg) were subjected to I-R injury in the absence or the presence of selective inhibitors of NO synthase (NOS) isoforms or sarcolemmal (sarcK(ATP)) and mitochondrial (mitoK(ATP)) K(ATP) channels. Multiple NOS isoforms are necessary for FGF2-mediated cardioprotection, and nitrite levels are significantly reduced in FGF2 Tg hearts upon inhibition of protein kinase C or mitogen-activated protein kinases. Likewise, sarcK(ATP) and mitoK(ATP) channels are important for cardioprotection elicited by endogenous FGF2. These findings suggest that FGF2-induced cardioprotection occurs via protein kinase-NOS pathways as well as K(ATP) channel activity.

Activation of SUR2B/Kir6.1 subtype of adenosine triphosphate-sensitive potassium channel improves pressure overload-induced cardiac remodeling via protecting endothelial function

We sought to explore new strategies targeting SUR2B/Kir6.1, a subtype of adenosine triphosphate (ATP)-sensitive potassium channels (KATP), against pressure overload-induced heart failure. The effects of natakalim, a SUR2B/Kir6.1 selective channel opener, on progression of cardiac remodeling were investigated. Pressure overload-induced heart failure was induced in Wistar rats by abdominal aortic banding. The effects of natakalim (1, 3, and 9 mg·kg⁻¹·d⁻¹ for 10 weeks) on myocardial hypertrophy and heart failure, cardiac histology, vasoactive compounds, and gene expression were assessed. Ten weeks after the onset of pressure overload, natakalim treatment potently inhibited cardiac hypertrophy and prevented heart failure. Natakalim remarkably inhibited the changes of left ventricular hemodynamic parameters and reversed the increase of heart mass index, left ventricular weight index, and lung weight index. Histological examination demonstrated that there was no significant hypertrophy or fibrosis in pressure-overloaded hearts of natakalim-treated rats. Ultrastructural examination of hearts revealed well-organized myofibrils with mitochondria grouped along the periphery of longitudinally oriented fibers in rats from the natakalim group. The content of serum nitric oxide and plasma prostacyclin was increased, whereas that of plasma endothelin-1 and cardiac tissue hydroxyproline and atrial and B-type natriuretic peptide messenger RNA was downregulated in natakalim-treated rats. Natakalim at 0.01-100 µM had no effects on isolated working hearts derived from Wistar rats; however, natakalim had endothelium-dependent vasodilatory effects on the isolated tail artery helical strips precontracted with norepinephrine. These results indicate that natakalim reduces heart failure caused by pressure overloading by activating the SUR2B/Kir6.1 KATP channel subtype and protecting against endothelial dysfunction.

Endogenous activation of mitochondrial KATP channels protects human failing myocardium from hydroxyl radical-induced stunning

RATIONALE

During reperfusion of ischemic myocardium, a burst of hydroxyl radicals (OH) induces contractile dysfunction ("myocardial stunning"), and OH in the plasma of patients after myocardial infarction predict the development of heart failure. The effects of OH on myocardial function in patients with heart failure; however, have never been assessed. Furthermore, although ATP-dependent K+ channels (K(ATP) channels) are implicated in myocardial protection during ischemia/reperfusion ("ischemic preconditioning"), their role in heart failure has hardly been elucidated.

OBJECTIVE

To investigate the effects of OH on cardiac contractile function in human failing myocardium, and to clarify the role of K(ATP) channels during this response.

METHODS AND RESULTS

In isolated left ventricular trabeculae of nonfailing hearts, OH (produced by Fe3+-nitrilotriacetic acid and H2O2) induced substantial systolic and diastolic dysfunction, whereas in failing myocardium, stunning was virtually absent. Although in failing myocardium, protein expression of sarcolemmal K(ATP) channels (Kir6.2/SUR2) was approximately 2-fold upregulated, their blockade with HMR-1098 did not impair contractile function in the presence of OH. In contrast, when blocking mitochondrial K(ATP) channels during OH exposure (with 5-HD), failing myocardium developed contractile dysfunction to a degree that was comparable to H-induced stunning in nonfailing myocardium without K(ATP) channel blockade.

CONCLUSIONS

Human failing left ventricular myocardium is resistant to OH-induced stunning, and this resistance is related to endogenous activation of putative mitochondrial K(ATP) channels. Given that certain sulfonylurea drugs that also block mitochondrial K(ATP) channels (eg, glibenclamide) are frequently used for the treatment of diabetes, our results imply that in patients with heart failure and diabetes, these drugs may impair left ventricular function during ischemia/reperfusion.

M-LDH physically associated with sarcolemmal K ATP channels mediates cytoprotection in heart embryonic H9C2 cells

Muscle form of lactate dehydrogenase (M-LDH) physically associate with K_ATP channel subunits, Kir6.2 and SUR2A, and is an integral part of the ATP-sensitive K⁺ (K_ATP) channel protein complex in the heart. Here, we have shown that concomitant introduction of viral constructs containing truncated and mutated forms of M-LDH (ΔM-LDH) and 193gly-M-LDH respectively, generate a phenotype of rat heart embryonic H9C2 cells that do not contain functional M-LDH as a part of the K_ATP channel protein complex. The K⁺ current was increased in wild type cells, but not in cells expressing ΔM-LDH/193gly-M-LDH, when they were exposed to chemical hypoxia induced by 2,4 dinitrophenol (DNP; 10 mM). At the same time, the outcome of chemical hypoxia was much worse in ΔM-LDH/193gly-M-LDH phenotype than in the control one, and that was associated with increased loss of intracellular ATP in cells infected with ΔM-LDH/193gly-M-LDH. On the other hand, cells expressing Kir6.2AFA, a Kir6.2 mutant that abolishes K_ATP channel conductance without affecting intracellular ATP levels, survived chemical hypoxia much better than cells expressing ΔM-LDH/193gly-M-LDH. Based on the obtained results, we conclude that M-LDH physically associated with Kir6.2/SUR2A regulates the activity of sarcolemmal K_ATP channels as well as an intracellular ATP production during metabolic stress, both of which are important for cell survival.

A dual mechanism of cytoprotection afforded by M-LDH in embryonic heart H9C2 cells

Muscle form of lactate dehydrogenase (M-LDH), a minor LDH form in cardiomyocytes, physically interacts with ATP-sensitive K+ (K ATP) channel-forming subunits. Here, we have shown that expression of 193gly-M-LDH, an inactive mutant of M-LDH, inhibit regulation of the K ATP channels activity by LDH substrates in embryonic rat heart H9C2 cells. In cells expressing 193gly-M-LDH chemical hypoxia has failed to activate K ATP channels. The similar results were obtained in H9C2 cells expressing Kir6.2AFA, a mutant form of Kir6.2 with largely decreased K+ conductance. Kir6.2AFA has slightly, but significantly, reduced cellular survival under chemical hypoxia while the deleterious effect of 193gly-M-LDH was significantly more pronounced. The levels of total and subsarcolemmal ATP in H9C2 cells were not affected by Kir6.2AFA, but the expression of 193gly-M-LDH led to lower levels of subsarcolemmal ATP during chemical hypoxia. We conclude that M-LDH regulates both the channel activity and the levels of subsarcolemmal ATP and that both mechanism contribute to the M-LDH-mediated cytoprotection.

Persistent beneficial effect of postconditioning against infarct size: role of mitochondrial K(ATP) channels during reperfusion

This study tested the hypothesis that inhibition of myocardial injury and modulation of mitochondrial dysfunction by postconditioning (Postcon) after 24 h of reperfusion is associated with activation of K(ATP) channels. Thirty dogs undergoing 60 min of ischemia and 24 h of reperfusion (R) were randomly divided into four groups:

CONTROL

no intervention at R; Postcon: three cycles of 30 s R alternating with 30 s re-occlusion were applied at R; 5-hydroxydecanoate (5-HD): the mitochondrial K(ATP) channel blocker was infused 5 min before Postcon; HMR1098: the sarcolemmal K(ATP) channel blocker was administered 5 min before Postcon. After 24 h of R, infarct size was smaller in Postcon relative to CONTROL (27 +/- 4%* Vs. 39 +/- 2% of area at risk), consistent with a reduction in CK activity (66 +/- 7* Vs. 105 +/- 7 IU/g). The infarct-sparing effect of Postcon was blocked by 5-HD (48 +/- 5%(dagger)), but was not altered by HMR1098 (29 +/- 3%*), consistent with the change in CK activity (102 +/- 8(dagger) in 5-HD and 71 +/- 6* IU/g in HMR1098). In H9c2 cells exposed to 8 h hypoxia and 3 h of reoxygenation, Postcon up-regulated expression of mito-K(ATP) channel Kir6.1 protein, maintained mitochondrial membrane potential and inhibited mitochondrial permeability transition pore (mPTP) opening evidenced by preserved fluorescent TMRE and calcein staining. The protective effects were blocked by 5-HD, but not by HMR1098. These data suggest that in a clinically relevant model of ischemia-reperfusion (1) Postcon reduces infarct size and decreases CK activity after prolonged reperfusion; (2) protection by Postcon is achieved by opening mitochondrial K(ATP) channels and inhibiting mPTP opening. *P < 0.05 Vs. CONTROL; P < 0.05 Vs. Postcon.

Thiazolidinedione drugs block cardiac KATP channels and may increase propensity for ischaemic ventricular fibrillation in pigs

AIMS/HYPOTHESIS

Opening of ATP-sensitive potassium (K(ATP)) channels during myocardial ischaemia shortens action potential duration and is believed to be an adaptive, energy-sparing response. Thiazolidinedione drugs block K(ATP) channels in non-cardiac cells in vitro. This study determined whether thiazolidinedione drugs block cardiac K(ATP) channels in vivo.

METHODS

Experiments in 68 anaesthetised pigs determined: (1) effects of inert vehicle, troglitazone (10 mg/kg i.v.) or rosiglitazone (0.1 or 1.0 mg/kg i.v.) on epicardial monophasic action potential (MAP) during 90 min low-flow ischaemia; (2) effects of troglitazone, rosiglitazone or pioglitazone (1 mg/kg i.v.) on response of MAP to intracoronary infusion of a K(ATP) channel opener, levcromakalim; and (3) effects of inert vehicle, rosiglitazone (1 mg/kg i.v.) or the sarcolemmal K(ATP) blocker HMR-1098 on time to onset of ventricular fibrillation following complete coronary occlusion.

RESULTS

With vehicle, epicardial MAP shortened by 44+/-9 ms during ischaemia. This effect was attenuated to 12+/-8 ms with troglitazone and 6+/-6 ms with rosiglitazone (p<0.01 for both vs vehicle), suggesting K(ATP) blockade. Intracoronary levcromakalim shortened MAP by 38+/-10 ms, an effect attenuated to 12+/-8, 13+/-4 and 9+/-5 ms during co-treatment with troglitazone, rosiglitazone or pioglitazone (p<0.05 for each), confirming K(ATP) blockade. During coronary occlusion, median time to ventricular fibrillation was 29 min in pigs treated with vehicle and 6 min in pigs treated with rosiglitazone or HMR-1098 (p<0.05 for both vs vehicle), indicating that K(ATP) blockade promotes ischaemic ventricular fibrillation in this model.

CONCLUSIONS/INTERPRETATION

Thiazolidinedione drugs block cardiac K(ATP) channels at clinically relevant doses and promote onset of ventricular fibrillation during severe ischaemia.

Negative regulation of beta-adrenergic function by hydrogen sulphide in the rat hearts

Beta-adrenoceptor is over-stimulated during myocardial ischemia, in which hydrogen sulphide (H2S) concentration was found to be lowered. The present study attempted to investigate if H2S modulates beta-adrenoceptor function and the underlying mechanism. We examined the effect of NaHS (a H2S donor) on myocyte contraction and electrically-induced (EI) intracellular calcium ([Ca2+](i)) transients upon beta-adrenergic stimulation in rat ventricular myocytes with a video edge tracker method and a spectrofluorometric method using fura-2/AM as a calcium indicator, respectively. We found that isoproterenol (ISO, 10(-9)-10(-6) M), a beta-adrenoceptor agonist, concentration-dependently increased the twitch amplitude of ventricular myocytes, which was attenuated by NaHS (10(-5)-10(-3) M) in a dose-dependent manner. The amplitudes and maximal velocities (+/-dl/dt) of myocyte twitch and EI-[Ca2+](i) transient amplitudes were enhanced by ISO, forskolin (an adenylyl cyclase activator), 8-bromoadenosine-3',5'-cyclic monophosphate (an activator of protein kinase A) and Bay K-8644 (a selective L-type Ca2+ channel agonist). Administration of NaHS (100 microM) only significantly attenuated the effects of ISO and forskolin. Moreover, NaHS reversed ISO-induced cAMP elevation and forskolin-stimulated adenylyl cyclase activity. In addition, stimulation of beta-adrenoceptor by ISO significantly decreased endogenous H2S production in rat ventricular myocytes. In conclusion, H2S may negatively modulate beta-adrenoceptor function via inhibiting adenylyl cyclase activity. Impairment of this negative modulation during ischemia may induce cardiac arrhythmias. Our study may provide a novel mechanism for ischemia-induced cardiac injury.

Effect of the sarcolemmal K(ATP) channel blocker HMR1098 on arrhythmias induced by programmed electrical stimulation in canine old myocardial infarction model: comparison with glibenclamide

The blockade of myocardial K(ATP) channels may be antiarrhythmic for ischemic arrhythmias. A new sulfonylthiourea, HMR1098 (1-[5-[2-(5-chloro-o-anisamido)ethyl]-2-methoxyphenylsulfonyl]-3-methylthiourea, sodium salt), was demonstrated to be a cardioselective K(ATP)-channel antagonist and to suppress arrhythmias during acute ischemia. We investigated effects of HMR1098 on the arrhythmias induced by programmed electrical stimulation (PES) in a canine old myocardial infarction model. HMR1098 (3 mg/kg, i.v.) significantly improved the scores of PES-induced ventricular arrhythmias, without changing the blood glucose concentrations. A classical sulfonylurea, glibenclamide (1 mg/kg, i.v.), had no significant effects on these arrhythmias, but reduced the blood glucose and increased the plasma insulin concentrations.

Cellular remodeling in heart failure disrupts K(ATP) channel-dependent stress tolerance

ATP-sensitive potassium (K(ATP)) channels are required for maintenance of homeostasis during the metabolically demanding adaptive response to stress. However, in disease, the effect of cellular remodeling on K(ATP) channel behavior and associated tolerance to metabolic insult is unknown. Here, transgenic expression of tumor necrosis factor alpha induced heart failure with typical cardiac structural and energetic alterations. In this paradigm of disease remodeling, K(ATP) channels responded aberrantly to metabolic signals despite intact intrinsic channel properties, implicating defects proximal to the channel. Indeed, cardiomyocytes from failing hearts exhibited mitochondrial and creatine kinase deficits, and thus a reduced potential for metabolic signal generation and transmission. Consequently, K(ATP) channels failed to properly translate cellular distress under metabolic challenge into a protective membrane response. Failing hearts were excessively vulnerable to metabolic insult, demonstrating cardiomyocyte calcium loading and myofibrillar contraction banding, with tolerance improved by K(ATP) channel openers. Thus, disease-induced K(ATP) channel metabolic dysregulation is a contributor to the pathobiology of heart failure, illustrating a mechanism for acquired channelopathy.