Despite the high expression of
5'AMP activated protein kinase (AMPK) in heart, the activity and function of this
enzyme in heart muscle has not been characterized. We demonstrate that rat hearts have a high AMPK activity, comparable to that found in liver, which could be stimulated up to 3-fold by 5'
AMP. Cardiac AMPK is also under phosphorylation control, since in vitro incubation of cardiac AMPK with
protein phosphatase 2A completely abolished activity, while incubation with
ATP/Mg(2+) resulted in over a 2-fold increase in activity. To investigate the function of AMPK in heart muscle, isolated working rat hearts were subjected to 30 min of global no-flow
ischemia, followed by 60 min of aerobic reperfusion. AMPK activity was increased in heart at the end of reperfusion compared to aerobic controls (379 +/- 53 (n=5) vs. 139 +/- 19 (n=5) pmol x min(-1) x mg
protein(-1), P<0.05, respectively). Treatment of AMPK in vitro with
protein phosphatase 2A reversed this activation. Since AMPK can phosphorylate and inactivate
acetyl-CoA carboxylase (ACC) in other tissues, and heart ACC has an important role in regulating
fatty acid oxidation, we measured ACC activity in hearts reperfused post-
ischemia. ACC activity was decreased at the end of reperfusion compared to aerobic controls (3.64 +/- 0.36 (n=9) vs. 10.93 +/- 0.60 (n=11) nmol x min(-1) x mg
protein(-1), respectively, P<0.05). A significant negative correlation (r= -0.78) was observed between AMPK activity and ACC activity measured in aerobic and reperfused ischemic hearts. Low ACC activity could be reversed if ACC was extracted from hearts in the absence of
phosphatase inhibitors, suggesting that phosphorylation of ACC decreased
enzyme activity. This suggests that following
ischemia AMPK is phosphorylated and activated (possibly by an
AMPK kinase). AMPK then phosphorylates and inactivates ACC. The resultant decrease in
malonyl-CoA levels could explain the acceleration of
fatty acid oxidation that is observed during reperfusion of ischemic hearts.