Myocardial Protection from Either Ischaemic Preconditioning or Nicorandil Is Not Blocked by Gliclazide

OBJECTIVE: We compared the effects of two sulphonylureas, glibenclamide and gliclazide, on ischaemic preconditioning (IPC) and nicorandil-induced protection in the in-vivo rat. We also studied the effects of these agents on the membrane potential of isolated rat mitochondria. METHODS: Anaesthetised male Sprague-Dawley rats were used in an open chest model of myocardial infarction. Animals were randomly assigned to receive one of the following drugs: (1) saline control, (2) glibenclamide, 0.3 mg/kg, or (3) gliclazide, 1 mg/kg i.v. bolus. Each was then further randomised to one of the following treatments: (a) control, (b) IPC (consisting of 2 x 5 mins of regional ischaemia and 5 minutes reperfusion) or (c) nicorandil (50 ug/kg/min i.v). infusion. Each group then underwent 25 mins regional ischaemia and 2 hrs reperfusion. Infarct to risk zone ratio (%) was calculated by computerised planimetry of tetrazolium stained heart slices. The membrane potential of mitochondria isolated from rat ventricles was measured using flow cytometry. Comparisons were made between groups in control medium, nicorandil alone, and nicorandil with either glibenclamide or gliclazide. RESULTS: Infarct size was significantly reduced with IPC (15.0 +/- 1.1%,) and nicorandil (25.5 +/- 4.2%), versus control (44.1 +/- 3.2%), p < 0.005. Glibenclamide abolished IPC (40.8 +/- 4.6%) and nicorandil-induced protection completely (39.5 +/- 5.1%). Gliclazide had no adverse effect on IPC (20.4 +/- 1.9%) or nicorandil-induced protection (23.6 +/- 2.2%), p < 0.005. Nicorandil caused a partial depolarisation of the mitochondrial membrane potential (-14.92 +/- 2.34%), which was abolished by glibenclamide (+2.03 +/- 0.53%), but not gliclazide (-16.47 +/- 3.36%), p < 0.01. CONCLUSION: Both IPC and nicorandil-induced protection are abolished by glibenclamide but not gliclazide in-vivo. These results may have important clinical implications in type II diabetic patients at risk of acute coronary syndromes.     

Infarct Size as Estimated from Peak Creatine Kinase and Lactate Dehydrogenase Is Probably Reduced in Patients Using Calcium Antagonists at the Onset of Symptoms

In animal models, calcium antagonists (Ca-A) administered before ischemia and reperfusion reduced myocardial necrosis, attenuated postischemic contractile dysfunction, and reduced tissue calcium. In 753 patients with acute myocardial infarction (AMI), we examined if use of Ca-A at the onset of symptoms (n = 127 patients) reduced infarct size as estimated from peak creatine kinase (CK_max) and lactate dehydrogenase (LD_max) activities. The study had an observational exposed/nonexposed design, and both crude and adjusted effects were investigated. Crude effects: In the restricted cohort of patients not receiving thrombolytic treatment (thr− pts; n = 411 patients), CK_max and LD_max were lower in Ca-A+ patients than in Ca-A− patients, being 643 versus 887 U/l (2 p = 0.004) and 708 versus 867 U/l (2 p = 0.005), respectively. When using log (CK_max) and log (LK_max) as outcomes, the same results were found (2 p = 0.002). More of the restricted cohort of thr-pts used Ca-A in the lower quartiles of CK_max and LD_max (p for linear trend = 0.005 and 0.004 for CK_max and LD_max, respectively). Adjusted effects: Thrombolysis was an effect modifier of the association between Ca-A and peak enzyme levels. In thr-pts, the coefficients of Ca-A were negative and borderline significant for log (CK_max; 2 p = 0.088) and negative and highly significant for log (LD_max; 2 p = 0.010) when adjusting for confounders. The present observational study indicates that the use of a Ca-A at the onset of AMI reduces infarct size, as estimated from CK_max and LD_max activities. This revised version was published online in July 2006 with corrections to the Cover Date.     

Angiotensin II Receptor Antagonist EXP 3174 Reduces Infarct Size Comparable with Enalaprilat and Augments Preconditioning in the Pig Heart

There is no agreement on the effect of angiotensin II receptor blockade in the setting of ischemic reperfusion. Our aim was to assess the acute effects of angiotensin-converting enzyme (ACE) inhibition and angiotensin II AT1-subtype receptor blockade in pig heart. Five groups of open-chest pigs received 1 hour of left anterior descending (LAD) coronary artery occlusion and 2 hours of reperfusion. Left ventricular pressure was monitored by an intraventricular catheter, and regional segment shortening (%SS) in the LAD-supplied territory was measured by ultrasonic crystals implanted in the subendocardium. Group 1 (n = 6) served as the control; groups 2 (n = 6) and 3 (n = 6) received the angiotensin II receptor blocker, EXP 3174 (C₂₂H₂₁Cl₁ N₆O₂), and the ACE inhibitor, enalaprilat, respectively, prior to LAD occlusion; group 4 (n = 6) was preconditioned with two cycles of 10 minutes of coronary occlusion and 30 minutes of reperfusion; and group 5 (n = 6) underwent preconditioning with additional administration of EXP 3174 prior to the 60-minute occlusion period. Infarct sizes were measured by p-nitrobluetetrazolium staining and were expressed in percent of the ischemic area of risk. The angiotensin II receptor blocker EXP 3174 and enalaprilat reduced infarct sizes significantly (35.3 ± 17.1% and 40.1 ± 15.1%, respectively) compared with controls (71.2± 12.8%, P < 0.05), and EXP 3174 augmented the infarct size–limiting effects of preconditioning by ischemia (10.5 ± 6% vs. 28.6 ± 5.3%, P < 0.05). Regional contractile dysfunction during reperfusion demonstrated no changes after angiotensin II receptor blockade. Angiotensin II receptor blockade reduced infarct size comparable with that obtained with angiotensin converting-enzyme inhibition. The infarct size–limiting effects of ischemic preconditioning were augmented by administration of the angiotensin II receptor antagonist EXP3174. These data support the concept that blockade or inhibition of angiotensin II before coronary occlusion is protective in a swine model of acute ischemia and reperfusion. This revised version was published online in July 2006 with corrections to the Cover Date.     

Potent and selective activation of the pancreatic beta-cell type KATP channel by two novel diazoxide analogues

Aims/hypothesis We investigated the pharmacological properties of two novel ATP sensitive potassium (K_ATP) channel openers, 6-Chloro-3-isopropylamino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide (NNC 55-0118) and 6-chloro-3-(1-methylcyclopropyl)amino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide (NN414), on the cloned cardiac (Kir6.2/SUR2A), smooth muscle (Kir6.2/SUR2B) and pancreatic beta cell (Kir6.2/SUR1) types of K_ATP channel.Methods We studied the effects of these compounds on whole-cell currents through cloned K_ATP channels expressed in Xenopus oocytes or mammalian cells (HEK293). We also used inside-out macropatches excised from Xenopus oocytes.Results In HEK 293 cells, NNC 55-0118 and NN414 activated Kir6.2/SUR1 currents with EC₅₀ values of 0.33 µmol/l and 0.45 µmol/l, respectively, compared with that of 31 µmol/l for diazoxide. Neither compound activated Kir6.2/SUR2A or Kir6.2/SUR2B channels expressed in oocytes, nor did they activate Kir6.2 expressed in the absence of SUR. Current activation was dependent on the presence of intracellular MgATP, but was not supported by MgADP.Conclusion/interpretation Both NNC 55-0118 and NN414 selectively stimulate the pancreatic beta-cell type of K_ATP channel with a higher potency than diazoxide, by interaction with the SUR1 subunit. The high selectivity and efficacy of the compounds could prove useful for treatment of disease states where inhibition of insulin secretion is beneficial.Abbreviations K_ATP channel ATP-sensitive potassium channel - SUR sulphonylurea receptor - KCO K⁺ channel opener - Kir inwardly rectifying K⁺ channel - TEVC two electrode voltage clamp - HEK293 cell Human Embryonic Kidney 293 cell     

Myocardial Ischemic Tolerance Following Heat Stress Is Abolished by ATP-Sensitive Potassium Channel Blockade

Heat stress is known to confer protection against ischemia, but the mechanisms involved are yet to be elucidated. Opening of ATP-sensitive potassium (K_ATP) channels has been demonstrated to be involved in other endogenous forms of cardioprotection, in particular“classic” ischemic preconditioning and delayed preconditioning following treatment with the endotoxin derivative, monophosphoryl lipid A. We therefore speculated that there may be a role for K_ATPchannels in delayed heat stress–induced cardioprotection. This hypothesis was investigated in an in vivo rabbit model of acute myocardial infarction using two structurally dissimilar K_ATP channel blockers, glibenclamide and sodium 5-hydroxydecanoate. Sodium pentobarbitone–anesthetized rabbits were subjected to either transient heat stress at 42 ± 0.2°C for 15 minutes or sham anesthesia. Twenty-four hours later, animals were reanesthetized (“Hypnorm” and sodium pentobarbitone) and a midline sternotomy and pericardiotomy were performed. An anterolateral branch of the circumflex coronary artery was occluded for 30 minutes and reperfused for 2 hours. The infarct-to-risk ratio was significantly limited in vehicle-treated rabbits from 41.3 ± 4.0% in controls (n = 10) to 24.1 ± 5.0% (n = 9; P = 0.014 by one-factor ANOVA) in heat-stressed hearts. This limitation in infarct size was abolished by 0.3 mg/kg iv glibenclamide or 5 mg/kg iv5-hydroxydecanoate when administered 10 minutes prior to coronary occlusion (45.2 ± 6.4%; n = 9 and 41.5 ± 5.0%; n = 5, respectively.) The same doses of glibenclamide and 5-hydroxydecanoate in sham-anesthetized hearts had no effect (42.3 ±5.1%; n = 10 and 51.9 ± 2.2%; n = 6, respectively). The adequacy of the heat stress protocol was confirmed by Western blot analysis of the inducible 72-kD heat stress protein. It is concluded, therefore, that K_ATP channels appear to play a role in the heat stress response. The underlying mechanisms involved are, however, unclear. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.