AICAR inhibits the Na+/H+ exchanger in rat hearts--possible contribution to cardioprotection.
AICAR (5-amino-1-beta-D: -ribofuranosyl-imidazole-4-carboxamide) is an adenosine analog which improves the recovery of the heart after ischemia. In some tissues AICAR enters cells and stimulates AMP-activated protein kinase (AMPK). We explored the mechanism of cardioprotection in isolated rat hearts. We confirmed that AICAR (0.5 mM) applied 10 min prior to a 30-min period of ischemia and present throughout ischemia and reperfusion caused a substantial improvement in the recovery of developed pressure on reperfusion. However, adenosine (100 microM) produced no improvement, suggesting that the mechanism of action of AICAR was not increased endogenous adenosine production. Measurements of intracellular sodium concentration ([Na(+)](i)) showed that AICAR prevented the rapid rise of [Na(+)](i), which normally occurs on reperfusion. Inhibitors of the cardiac sodium-hydrogen exchanger (NHE1) also protect the heart from ischemic damage and also prevent the rapid rise of [Na(+)](i) on reperfusion, suggesting that AICAR might cause the inhibition of NHE1. We tested this possibility on isolated rat ventricular myocytes in which the recovery of pH(i) after NH(4)Cl exposure provides a measure of NHE1 activity. AICAR (0.5 micromM) inhibited NHE1 activity in response to an acid load by about 80%. To test whether the AICAR-induced inhibition of NHE1 arose through adenosine, we used the adenosine receptor blocker 8-sulfophenyltheophylline (8-SPT) and found that it had no measureable effect. To test whether the AICAR-induced inhibition of NHE1 might occur through the activation of AMPK, we measured the activity of two isoforms of AMPK. Surprisingly, activity was reduced, whereas in many other tissues AICAR increases AMPK activity. Furthermore, this effect of AMPK was blocked by 8-SPT, suggesting that the inhibition of AMPK arose through an adenosine-receptor-related pathway. We conclude that AICAR inhibits NHE1 through an unidentified pathway. This inhibition may make a contribution to the cardioprotective effects of AICAR.
Moopanar
Terence R
TR
Department of Physiology and Institute for Biomedical Research, University of Sydney, Sydney, NSW 2006, Australia.
Xiao
Xiao-Hui
XH
Jiang
Lele
L
Chen
Zhi-Ping
ZP
Kemp
Bruce E
BE
Allen
David G
DG
D055372:AMP-Activated Protein Kinases; D000620:Aminoimidazole Carboxamide / Q000031:analogs & derivatives* / Q000494:pharmacology / Q000627:therapeutic use; D000818:Animals; D002316:Cardiotonic Agents / Q000494:pharmacology; D005260:Female; D006321:Heart / Q000187:drug effects*; D006352:Heart Ventricles / Q000166:cytology; D007004:Hypoglycemic Agents / Q000494:pharmacology* / Q000627:therapeutic use; D066298:In Vitro Techniques; D009097:Multienzyme Complexes / Q000187:drug effects; D015428:Myocardial Reperfusion Injury / Q000188:drug therapy*; D032383:Myocytes, Cardiac / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D012265:Ribonucleotides / Q000494:pharmacology* / Q000627:therapeutic use; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors*
D002316:Cardiotonic Agents; D007004:Hypoglycemic Agents; D009097:Multienzyme Complexes; D012265:Ribonucleotides; D017923:Sodium-Hydrogen Exchangers; D000620:Aminoimidazole Carboxamide; D017346:Protein Serine-Threonine Kinases; D055372:AMP-Activated Protein Kinases; C031143:AICA ribonucleotide