SUR, ABC proteins targeted by KATP channel openers

The sulfonylurea receptor SUR is an ATP binding cassette (ABC) protein of the ABCC/MRP family. Unlike other ABC proteins, it has no intrinsic transport function, neither active nor passive, but associates with the potassium channel proteins Kir6.1 or Kir6.2 to form the ATP-sensitive potassium (K(ATP)) channel. Within the channel complex SUR serves as a regulatory subunit which fine-tunes the gating of Kir6.x in response to alterations in cellular metabolism. It constitutes a major pharmaceutical target as it binds numerous drugs, K(ATP) channel openers and blockers, capable of up- or down-regulating channel activity. We here review current knowledge on the molecular basis of the interaction of classical K(ATP) channel openers (cromakalim, pinacidil, diazoxide) with SUR.     

Effects of KATP channel openers diazoxide and pinacidil in coronary-perfused atria and ventricles from failing and non-failing human hearts

This study compared the effects of ATP-regulated potassium channel (K_ATP) openers, diazoxide and pinacidil, on diseased and normal human atria and ventricles. We optically mapped the endocardium of coronary-perfused right (n = 11) or left (n = 2) posterior atrial-ventricular free wall preparations from human hearts with congestive heart failure (CHF, n = 8) and non-failing human hearts without (NF, n = 3) or with (INF, n = 2) infarction. We also analyzed the mRNA expression of the K_ATP targets K_ir6.1, K_ir6.2, SUR1, and SUR2 in the left atria and ventricles of NF (n = 8) and CHF (n = 4) hearts. In both CHF and INF hearts, diazoxide significantly decreased action potential durations (APDs) in atria (by − 21 ± 3% and − 27 ± 13%, p < 0.01) and ventricles (by − 28 ± 7% and − 28 ± 4%, p < 0.01). Diazoxide did not change APD (0 ± 5%) in NF atria. Pinacidil significantly decreased APDs in both atria (− 46 to −80%, p < 0.01) and ventricles (− 65 to − 93%, p < 0.01) in all hearts studied. The effect of pinacidil on APD was significantly higher than that of diazoxide in both atria and ventricles of all groups (p < 0.05). During pinacidil perfusion, burst pacing induced flutter/fibrillation in all atrial and ventricular preparations with dominant frequencies of 14.4 ± 6.1 Hz and 17.5 ± 5.1 Hz, respectively. Glibenclamide (10 μM) terminated these arrhythmias and restored APDs to control values. Relative mRNA expression levels of K_ATP targets were correlated to functional observations. Remodeling in response to CHF and/or previous infarct potentiated diazoxide-induced APD shortening. The activation of atrial and ventricular K_ATP channels enhances arrhythmogenicity, suggesting that such activation may contribute to reentrant arrhythmias in ischemic hearts.     

K+ transport and energetics in Kir6.2(-/-) mouse hearts assessed by 87Rb and 31P magnetic resonance and optical spectroscopy

Cardiac sarcolemmal K(ATP) channels are crucial in adaptation to stress caused by metabolic inhibition and moderate exercise, which requires not only down-regulation of energy spending, but also up-regulation of mitochondrial ATP synthesis. To investigate sarcolemmal and mitochondrial effects of a Kir6.2 (K(+) ion-selective subunit of the channel) knockout, we used non-invasive techniques ((87)Rb, (31)P NMR and optical spectroscopy) to study (1) K(+) fluxes, (2) high-energy phosphates, (3) the cytochrome c oxidase redox state, (4) myoglobin deoxygenation, and (5) contractile function at the baseline and in response to metabolic uncoupling with 2,4-dintrophenol (DNP) and stimulation with isoproterenol in Langendorff-perfused mouse hearts. Comparison with control C57BL6 hearts demonstrated that the Kir6.2 knockout resulted in: (a) a lack of stimulation of the unidirectional potassium efflux from the hearts when K(ATP) channels were activated metabolically by DNP (50 muM, 20 min); (b) a decrease in ATP, but not phosphocreatine, at the baseline, that became even more pronounced when the hearts were subjected to stress due to metabolic inhibition or increased workload caused by isoproterenol infusion (0.1 microM, 20 min); (c) significantly higher reduction of cytochrome c oxidase in response to DNP uncoupling; (d) a blunted response to isoproterenol stimulation. Thus Kir6.2 knockout is associated with decreased tolerance of mouse hearts to metabolic inhibition and catecholamine stress.     

Role of K+ATP Channels in Ischemic Preconditioning and Cardioprotection

Summary. Since the phenomenon of ischemic preconditioning was first described some 15 years ago, interest in strategies aimed at reducing infarct size has increased. During the past 10 years, investigations into the mechanism of ischemic preconditioning have clearly demonstrated the cardioprotective effect of K⁺ _ATP channel opening. Thus, K⁺ _ATP channel activation has been shown to be involved in this protection by a variety of stimuli, including a brief period of complete ischemia (classic ischemic preconditioning) or a partial coronary artery occlusion. In addition, ischemia in remote organs and nonischemic stimuli in the heart such as ventricular pacing, stretch, and heat stress also confer protection via K⁺ _ATP channel activation. Pharmacological agents that open K⁺ _ATP channels reduce infarct size, but K⁺ _ATP channel opening must occur prior to or early during the sustained infarct-producing coronary artery occlusion, while the degree and memory of cardioprotection are less than those produced by classic ischemic preconditioning. Although the exact mechanism by which K⁺ _ATP channel activation protects is still incompletely understood, recent studies indicate a role for the mitochondrial K⁺ _ATP channels. Before K⁺ _ATP channel opening can be employed in patients at increased risk of developing myocardial infarction (e.g., unstable angina), it is mandatory to determine whether tolerance (tachyphylaxia) occurs with repeated administration of K⁺ _ATP channel openers in a fashion similar to what occurs with ischemic preconditioning.     

Early reperfusion levels of Na(+) and Ca(2+) are strongly associated with postischemic functional recovery but are disassociated from K(ATP) channel-induced cardioprotection

We previously demonstrated that pinacidil does not affect Na(+)(i) accumulation, cellular energy depletion, or acidosis during myocardial ischemia, but dramatically improves the cationic/energetic status during reperfusion. We investigated the role of this latter effect in K(ATP) channel-induced cardioprotection. Employing (23)Na and (31)P nuclear magnetic resonance spectroscopy with perfused rat hearts, reperfusion Na(+)(i) was altered with brief infusions of ouabain and/or RbCl to transiently decrease or increase Na(+)/K(+) ATPase activity. The increases and decreases in functional recovery (%LVDP-R) with pinacidil or ouabain, respectively, were largely unaltered by each other's presence. Early reperfusion Na(+)(i) and cellular energy were greatly altered by ouabain and indicated linear relationships with %LVDP-R. Pinacidil shifted these relationships to higher %LVDP-R. Increasing early reperfusion Na(+)(i) decreased %LVDP-R but did not diminish pinacidil's capacity to improve %LVDP-R. Approximately 75% and 45% of the pinacidil-induced improvements in %LVDP-R, could be disassociated from early reperfusion Na(+)(i) and cellular energy, respectively. Both pinacidil and RbCl infusion blunted ouabain's elevation of reperfusion Na(+)(i), but RbCl did not improve %LVDP-R. Atomic absorption tissue Ca(2+) measurements indicated that pinacidil reduced late reperfusion Ca(2+) uptake, but did not reduce early reperfusion Ca(2+), and its beneficial effects were resistant to ouabain-induced early reperfusion Ca(2+) increases. In conclusion, K(ATP) channel-induced cardioprotection does not require moderation of Na(+)(i) accumulation, cellular energy depletion, or acidosis during ischemia. K(ATP) channel-induced cardioprotection is largely independent of the accelerated reperfusion Na(+)(i) recovery it induces and does not require early reperfusion reductions of tissue Ca(2+). A larger role for early reperfusion cellular energy cannot be excluded.     

Phenylephrine preconditioning involves modulation of cardiac sarcolemmal K(ATP) current by PKC delta, AMPK and p38 MAPK

Preconditioning of hearts with the α₁-adrenoceptor agonist phenylephrine decreases infarct size and increases the functional recovery of the heart following ischaemia-reperfusion. However, the cellular mechanisms responsible for this protection are not known. We investigated the role of protein kinase C ε and δ (PKCε and PKCδ), AMP-activated protein kinase (AMPK), p38 MAPK (p38) and sarcolemmal ATP-sensitive potassium (sarcK_ATP) channels in phenylephrine preconditioning using isolated rat ventricular myocytes. Preconditioning of ventricular myocytes with phenylephrine increased the recovery of contractile activity following metabolic inhibition and re-energisation from 30.1 ± 1.9% to 66.5 ± 5.2% (P < 0.01) and increased the peak sarcK_ATP current activated during metabolic inhibition from 32.1 ± 1.8 pA/pF to 46.0 ± 5.0 pA/pF (P < 0.05), which was required for protection. Phenylephrine preconditioning resulted in a sustained activation of PKCε and PKCδ, and transient activation of AMPK, which was dependent upon activation of PKCδ but not PKCε. P38 was also activated by phenylephrine preconditioning and this was blocked by inhibitors of PKCε, PKCδ or AMPK. Inhibition of PKCδ, AMPK or p38 was sufficient to prevent the increase in current, suggesting that these kinases are involved in modulation of sarcK_ATP channel current by phenylephrine preconditioning. However, whilst inhibition of AMPK and p38 prevented the protection from phenylephrine preconditioning, PKCδ inhibition paradoxically had no effect. The increase in sarcK_ATP current induced by phenylephrine preconditioning requires PKCδ, AMPK and p38 and may contribute to the observed improvement in contractile recovery.     

Energy metabolism,intracellular Na+ and contractile function in isolated pig and rat hearts during cardioplegic ischemia and reperfusion:23Na- and31P-NMR studies

The purpose of the study was to compare the role of Na ions in the damage caused by cardioplegic ischemia in fast (rat) and slow (pig) hearts. Changes in intracellular Na⁺ (Na⁺ _i), high energy phosphates, and contractile function were assessed during ischemia (36°C) and reperfusion in KCl-arrested perfused hearts using³¹P-NMR and shift reagent (DyTTHA^3–)-aided²³Na-NMR spectroscopy. In the pig hearts the rates of decrease of phosphocreatine (PCr), ATP and intracellular pH (pHi) were 3–4 times slower than the rates observed in the rat hearts. In the pig hearts PCr was observable (10%) during first 80 min of the ischemic period (90 min). Comparable decreases in ATP (32.0±6 vs. 38±15% of initial) and pH_i, (to 6.14±0.06 vs. 6.10±0.15) observed after 90 and 20 min ischemia in pig and rat hearts, respectively, were associated with a smaller Na⁺ _i increase in the pig hearts (to 175±31%) than in the rat hearts (to 368±62%). This Na⁺ increase in the rat hearts preceded development of ischemic contracture (41±6 mmHg at 23.6±0.7 min) while no contracture was observed in pig hearts. Reperfusion of the rat hearts (at 30 min ischemia) was followed by partial recovery of PCr (44±3%) and Na⁺ _i (234±69%) and poorer recovery of the pressure-rate product (PRP, 9±4%) and end-diastolic pressure (EDP, 72±5 mm Hg) compared to the pig hearts (PCr, 106±25%; Na⁺ _i, 82±17%; PRP, 24±3%; EDP, 4.6±2.5 mmHg). The loss of function in the pig hearts was reversed by increasing Ca⁺⁺ in the perfusate from 1 to 2.3 mM and resulted in a rise in both PRP and oxygen consumption rate, V(O₂), from 24±3.3 to 64.5±5.8% and from 55±10 to 74±10% of the control levels, respectively. The PRP/V(O₂) ratio was halved in the post-ischemic pig hearts and returned to the pre-ischemic level following Ca⁺⁺ stimulation. It is suggested that the higher stability of Na⁺ homeostasis to ischemic stress in the pig heart may result from: 1) a lower ratio of the rate of ATP hydrolysis to glycolytic ATP production; 2) differences in the kinetic properties of the Na⁺ transporters. Reduced Na⁺ accumulation during ischemia and reperfusion is benefical for metabolic and functional preservation of cardiomyocytes.     

Acetylcholine-induced production of reactive oxygen species in adult rabbit ventricular myocytes is dependent on phosphatidylinositol 3- and Src-kinase activation and mitochondrial K(ATP) channel opening

Acetylcholine (ACh), like ischemic preconditioning (PC), protects against infarction and is dependent on generation of reactive oxygen species (ROS). To investigate the mechanism by which ACh causes ROS production, isolated adult rabbit cardiomyocytes underwent a timed incubation in reduced MitoTracker Red, which is oxidized to a fluorescent form after exposure to ROS. The mitochondrial ATP-sensitive potassium (mK(ATP)) channel opener diazoxide (50 microM) increased fluorescence by 47 +/- 9% (P = 0.007), indicating that opening of mK(ATP) leads to ROS generation, and that increase was blocked by the mK(ATP) blocker 5-hydroxydecanoate (5HD, 1 mM); 250 microM ACh caused a similar increase in ROS generation (+45 +/- 6% for all experiments, P < 0.001). ACh-induced ROS production was prevented by (1) blockade of muscarinic surface receptors with 100 microM atropine (-6 +/- 2%, P = n.s.) or 250 nM 4-DAMP (+5 +/- 13%, P = n.s.), indicating that ACh's effect was receptor mediated; (2) closing K(ATP) channels with either the non-selective channel closer glibenclamide (50 microM) (-1.2 +/- 17%, P = n.s.) or the selective mK(ATP) closer 5HD (-1.8 +/- 9%, P = n.s.), indicating that increased ROS production involved opening of mK(ATP); (3) blockade of mitochondrial electron transport chain with 200 nM myxothiazol (-4 +/- 9%, P = n.s.), indicating ROS came from the mitochondria; (4) addition of 100 nM wortmannin (-13 +/- 12%, P = n.s.), indicating that phosphatidylinositol 3-(PI3)-kinase was involved; and (5) blockade of Src-kinase with 1 microM PP2 (-2 +/- 5%, P = n.s.), indicating the involvement of an Src-kinase. These results support the hypothesis that occupation of muscarinic surface receptors by ACh causes activation of PI3- and Src-kinases that then open mK(ATP) resulting in mitochondrial ROS generation and triggering of the preconditioned state.     

Effects of adult-onset streptozotocin-induced diabetes on the rat brain antioxidant status and the activities of acetylcholinesterase, (Na+,K+)- and Mg2+-ATPase: modulation by L-cysteine

Uncontrolled diabetes is known to affect the nervous system. The aim of this study was to investigate the effect of the antioxidant L-cysteine (Cys) on the changes caused by adult-onset streptozotocin (STZ)-induced diabetes on the rat brain total antioxidant status (TAS) and the activities of acetylcholinesterase (AChE), (Na⁺,K⁺)-ATPase and Mg²⁺-ATPase. Thirty-eight male Wistar rats were divided into six groups: C_A (8-week-control), C_B (8-week-control + 1-week-saline-treated), C + Cys (8-week-control + 1-week-Cys-treated), D_A (8-week-diabetic), D_B (8-week-diabetic + 1-week-saline-treated) and D + Cys (8-week-diabetic + 1-week-Cys-treated). All diabetic rats were once treated with an intraperitoneal (i.p.) STZ injection (50 mg/kg body weight) at the beginning of the experiment, while all Cys-treated groups received i.p. injections of Cys 7 mg/kg body weight (daily, for 1-week, during the 9th-week). Whole rat brain parameters were measured spectrophotometrically. In vitro incubation with 0.83 mM of Cys or 10 mM of STZ for 3 h was performed on brain homogenate samples from groups C_B and D_B, in order to study the enzymes’ activities. Diabetic rats exhibited a statistically significant reduction in brain TAS (−28%, D_A vs C_A;−30%, D_B vs C_B) that was reversed after 1-week-Cys-administration into basal levels. Diabetes caused a significant increase in AChE activity (+27%, D_A vs C_A; +15%, D_B vs C_B), that was further enhanced by Cys-administration (+57%, D + Cys vs C_B). The C + Cys group exhibited no significant difference compared to the C_B group in TAS (+2%), but showed a significantly increased AChE activity (+66%, C + Cys vs C_B). Diabetic rats exhibited a significant reduction in the activity of Na⁺,K⁺-ATPase (−36%, D_A vs C_A;−48%, D_B vs C_B) that was not reversed after 1-week Cys administration. However, in vitro incubation with Cys partially reversed the diabetes-induced Na⁺,K⁺-ATPase inhibition. Mg²⁺-ATPase activity was not affected by STZ-induced diabetes, while Cys caused a significant inhibition of the enzyme, both in vivo (−14%, C + Cys vs C_B;−17%, D + Cys vs C_B) and in vitro (−16%, D_B + in vitro Cys vs C_B). In vitro incubation with STZ had no effect on the studied enzymes. The present data revealed a protective role for Cys towards the oxidative effect of diabetes on the adult rat brain. Moreover, an increase in whole brain AChE activity due to diabetes was recorded (not repeatedly established in the literature, since contradictory findings exist), that was further increased by Cys. The inhibition of Na⁺,K⁺-ATPase reflects a possible mechanism through which untreated diabetes could affect neuronal excitability, metabolic energy production and certain systems of neurotransmission. As concerns the use of Cys as a neuroprotective agent against diabetes, our in vitro findings could be indicative of a possible protective role of Cys under different in vivo experimental conditions.     

Sarcoplasmic ATP-sensitive potassium channel blocker HMR1098 protects the ischemic heart: implication of calcium, complex I, reactive oxygen species and mitochondrial ATP-sensitive potassium channel

The aim of this study was to investigate the effects of HMR1098, a selective blocker of sarcolemmal ATP-sensitive potassium channel (sarcK(ATP)), in Langendorff-perfused rat hearts submitted to ischemia and reperfusion. The recovery of heart hemodynamic and mitochondrial function, studied on skinned fibers, was analyzed after 30-min global ischemia followed by 20-min reperfusion. Infarct size was quantified on a regional ischemia model after 2-h reperfusion. We report that the perfusion of 10 microM HMR1098 before ischemia, delays the onset of ischemic contracture, improves recovery of cardiac function upon reperfusion, preserves the mitochondrial architecture, and finally decreases infarct size. This HMR1098-induced cardioprotection is prevented by 1 mM 2-mercaptopropionylglycine, an antioxidant, and by 100 nM nifedipine, an L-type calcium channel blocker. Concomitantly, it is shown that HMR1098 perfusion induces (i) a transient and specific inhibition of the respiratory chain complex I and, (ii) an increase in the averaged intracellular calcium concentration probed by the in situ measurement of indo-1 fluorescence. Finally, all the beneficial effects of HMR1098 were strongly inhibited by 5-hydroxydecanoate and abolished by glibenclamide, two mitoK(ATP) blockers. This study demonstrates that the HMR1098-induced cardioprotection occurs indirectly through extracellular calcium influx, respiratory chain complex inhibition, reactive oxygen species production and mitoK(ATP) opening. Taken together, these data suggest that a functional interaction between sarcK(ATP) and mitoK(ATP) exists in isolated rat heart ischemia model, which is mediated by extracellular calcium influx.     

Lysophosphatidylcholine enhances I(Ks) currents in cardiac myocytes through activation of G protein, PKC and Rho signaling pathways

Lysophosphatidylcholine (LPC) is a bioactive phospholipid that accumulates rapidly in the ischemic myocardium. In recent years, it has been shown that some of the actions of LPC are mediated through the activation of the membrane G proteins. However, the precise mechanism(s) responsible for the LPC-related intracellular signaling in the regulation of cardiac ion channels are still poorly understood. The present study was undertaken to examine whether LPC regulates the slow component of the delayed rectifier K⁺ current (I_Ks) and, if so, what intracellular signals are important for this process. Isolated guinea pig cardiac myocytes were voltage-clamped using the whole-cell configuration of the patch-clamp method. The bath application of 1-palmitoyl-lysophosphatidylcholine (LPC-16) concentration-dependently (EC₅₀ = 0.7 μM) and reversibly increased I_Ks in atrial cells, but failed to potentiate I_Ks in ventricular myocytes. In contrast, 1-oleoyl-lysophosphatidylcholine (LPC-18:1) only produced a slight I_Ks increase, and 1-caproyl-lysophosphatidylcholine (LPC-6) or the LPC-16 precursor (phosphatidylcholine) had no effect on I_Ks. Pretreatment of atrial cells with an antibody against the N-terminus of the G2A receptor significantly reduced the LPC-16-induced potentiation of I_Ks. The inhibition of heterotrimeric G protein, phospholipase C (PLC) and protein kinase C (PKC) significantly reduced LPC-16-induced enhancement of I_Ks. Moreover, the blockade of Rho and Rho-kinase by specific inhibitors also inhibited the activity of LPC-16. Immunohistochemical studies demonstrated that G2A was densely distributed in the plasma membrane of atrial myocytes. Therefore, the present study suggests that the activation of a G protein (probably Gα_q) by LPC-16 potentiates I_Ks currents through the PLC-PKC and Rho-kinase pathways.