CHARACTERIZATION OF 5'AMP-ACTIVATED PROTEIN-KINASE ACTIVITY IN THE HEART AND ITS ROLE IN INHIBITING ACETYL-COA CARBOXYLASE DURING REPERFUSION FOLLOWING ISCHEMIA

Despite the high expression of 5'AMP activated protein kinase (AMPK) i n heart, the activity and function of this enzyme in heart muscle has not been characterized. We demonstrate that rat hearts have a high AMP K activity, comparable to that found in liver, which could be stimulat ed up to 3-fold by 5'AMP. Cardiac AMPK is also under phosphorylation c ontrol, since in vitro incubation of cardiac AMPK with protein phospha tase 2A completely abolished activity, while incubation with ATP/Mg2resulted in over a 2-fold increase in activity. To investigate the fun ction of AMPK in heart muscle, isolated working rat hearts were subjec ted to 30 min of global no-flow ischemia, followed by 60 min of aerobi c reperfusion. AMPK activity was increased in heart at the end of repe rfusion compared to aerobic controls (379 +/- 53 (n = 5) vs. 139 +/- 1 9 (n = 5) pmol . min(-1). mg protein(-1), P < 0.05, respectively). Tre atment of AMPK in vitro with protein phosphatase 2A reversed this acti vation. Since AMPK can phosphorylate and inactivate acetyl-CoA carboxy lase (ACC) in other tissues, and heart ACC has an important role in re gulating fatty acid oxidation, we measured ACC activity in hearts repe rfused post-ischemia. ACC activity was decreased at the end of reperfu sion compared to aerobic controls (3.64 +/- 0.36 (n = 9) vs. 10.93 +/- 0.60 (n = 11) nmol . min(-1). 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 isch emic hearts. Low ACC activity could be reversed if ACC was extracted f rom hearts in the absence of phosphatase inhibitors, suggesting that p hosphorylation of ACC decreased enzyme activity. This suggests that fo llowing ischemia AMPK is phosphorylated and activated (possibly by an AMPK kinase). AMPK then phosphorylates and inactivates ACC. The result ant decrease in malonyl-CoA levels could explain the acceleration of f atty acid oxidation that is observed during reperfusion of ischemic he arts.