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.