title: Cardioprotective-mimetics reduce myocardial infarct size in animals resistant to ischemic preconditioning.
abstract: BACKGROUND
  Ischemic preconditioning (IPC) elicits two distinct windows of cardioprotection, an early phase that lasts for 1-2 h and a delayed phase that lasts for 24-72 h. However, there is conflicting data as to how long the heart is resistant to IPC-induced cardioprotection after the initial protection wanes, leading to the demonstration of IPC-resistance. This resistance to IPC appears to be dependent on the timing of the next IPC stimulus, the species of animals used and the model studied. Furthermore, the mechanisms responsible IPC-resistance are unknown. It is also important to demonstrate therapeutic interventions that will produce cardioprotection during this period of IPC-resistance.
  
  
  RESULTS
  To examine potential mechanisms responsible for acute IPC-induced resistance, the NHE-1 inhibitor EMD 85131 (2-methyl-5-methylsulfonyl-1-(1-pyrrollyl)-benzoylguanidine), which exerts its effects via mechanisms distinct from IPC, and the K(ATP) channel opener bimakalim, which bypasses the signaling mechanisms of IPC to directly open K(ATP) channels, were examined in a canine model of IPC-resistance. One 10 min. IPC stimulus followed by 10 min. of reperfusion produced a significant reduction in IS/AAR compared to Control (7.1 +/- 2.6% versus 26.0 +/- 6.2%; P < 0.05). However, IPC did not significantly protect the myocardium if a 2 h reperfusion period occurred between the initial IPC stimulus and the subsequent prolonged (60 min) ischemic challenge (IS/AAR: 22.5 +/- 4.8%: P > 0.05). Furthermore, hearts treated with IPC followed by 2 h of reperfusion were resistant to an additional IPC stimulus administered just prior to the subsequent 60 min. occlusion period (IS/AAR: 22.9 +/- 3.2%: P > 0.05). In contrast, administration of the NHE-1 inhibitor EMD 85131 (IS/AAR: 7.4 +/- 2.5%: P < 0.05) or the K(ATP) channel opener bimakalim (IS/AAR: 11.8 +/- 2.4%: P < 0.05) both afforded significant cardioprotection when administered at 2 h of reperfusion in previously preconditioned canine hearts resistant to IPC.
  
  
  CONCLUSIONS
  IPC resistance occurs in this canine model of ischemia-reperfusion injury. However, in spite of IPC resistance, hearts can still be pharmacologically protected by direct application of the K(ATP) channel opener bimakalim or the NHE inhibitor EMD 85131.
authors.affiliation: Interventional Cardiology Fellow, Division of Cardiovascular Medicine, Department of Internal Medicine, Mayo Clinic and Foundation, Rochester, MN, USA.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Gumina
authors.forename: Jo
authors.initials: J
authors.lastname: El Schultz
authors.forename: Jeanine
authors.initials: J
authors.lastname: Moore
authors.forename: Norbert
authors.initials: N
authors.lastname: Beier
authors.forename: Pierre
authors.initials: P
authors.lastname: Schelling
authors.forename: Garrett J
authors.initials: GJ
authors.lastname: Gross
mesh_terms: D000818:Animals; D001549:Benzamides / Q000494:pharmacology / Q000627:therapeutic use; D001578:Benzopyrans / Q000494:pharmacology / Q000627:therapeutic use; D002316:Cardiotonic Agents / Q000494:pharmacology / Q000627:therapeutic use*; D004095:Dihydropyridines / Q000494:pharmacology / Q000627:therapeutic use; D004195:Disease Models, Animal; D004285:Dogs; D006321:Heart / Q000187:drug effects; D019157:Ischemic Preconditioning, Myocardial; D009203:Myocardial Infarction / Q000473:pathology / Q000517:prevention & control*; D015221:Potassium Channels / Q000819:agonists; D011758:Pyrroles / Q000494:pharmacology / Q000627:therapeutic use; D015427:Reperfusion Injury / Q000473:pathology / Q000517:prevention & control*; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors; D013997:Time Factors
chemical_list: D001549:Benzamides; D001578:Benzopyrans; D002316:Cardiotonic Agents; D004095:Dihydropyridines; C113375:N-diaminomethylene-2-methyl-5-methylsulfonyl-4-pyrrolobenzamide, hydrochloride; D015221:Potassium Channels; D011758:Pyrroles; D017923:Sodium-Hydrogen Exchangers; C069139:growth factor-activatable Na-H exchanger NHE-1; C061050:bimakalim