AICAR inhibits the Na+/H+ exchanger in rat hearts—possible contribution to cardioprotection

Abstract

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.

AMP-Activated Protein Kinase Phosphorylates Cardiac Troponin I and Alters Contractility of Murine Ventricular Myocytes

“…In LV myocytes in the absence of Fura-2, AICAR significantly prolonged relaxation but did not induce significant increase in myocyte cell shortening. It is known that AMPK activation exerts various functions in cardiac myocytes; for example, it can inhibit the Na ϩ -H ϩ exchanger 43 and therefore the negligible effect of AICAR on myocyte shortening in Fura-2 unloaded conditions may represent combined effects on myocyte function. Furthermore, AICAR is known to have AMPK-independent effects: for example, in skeletal muscle AICAR can increase both phosphorylation of acetyl-coA carboxylase 2 and rates of fatty acid oxidation and reduce malonyl-CoA levels independent of AMPK 44 and can activate glycogen phosphorylase directly in fibroblasts.…”

AMP-activated protein kinase activator A-769662 is an inhibitor of the Na+-K+-ATPase

“…In rat cardiomyocytes, AICAR inhibits the Na ϩ /H ϩ exchanger independently from activation of AMPK (38,46 (54). However, the mechanism of whole body AICAR-induced potassium clearance and whether AMPK activation affects K ϩ fluxes and Na ϩ -K ϩ -ATPase activity in skeletal muscle remains to be determined.…”

“…AICAR is taken up into the cell and converted by adenosine kinase to the monophosphorylated nucleotide, ZMP, which mimics the effect of AMP on AMPK activation (14). AICAR may also have AMPK-independent side effects (21,33,38,46).…”

2′‐O‐Alkyl Derivatives and 5′‐Analogues of 5‐Aminoimidazole‐4‐carboxamide‐1‐β‐D‐ribofuranoside (AICAR) as Potential Hsp90 Inhibitors

“…[5] Indeed, it is now recognized that AICAR alters glucose uptake differently in different tissues and cells, and some of the biological effects are independent of AMPK activation. Furthermore, AICAR enhances extracellular adenosine levels under the conditions of net ATP breakdown and, therefore, in light of the cardioand neuroprotective properties of adenosine, it may have therapeutic potential for neuropsychiatric symptoms generally associated with chronically low levels of adenosine.…”