Upregulation of 5'-AMP-activated protein kinase is responsible for the increase in myocardial fatty acid oxidation rates following birth in the newborn rabbit.

In newborn rabbits, fatty acid oxidation rates in the heart significantly increase between 1 and 7 days after birth. This is due in part to a decrease in malonyl coenzyme A (CoA) production by acetyl CoA carboxylase (ACC). In other tissues, 5'-AMP-activated protein kinase (AMPK) can phosphorylate and inhibit ACC activity. In this study, we show that 1- and 7-day-old rabbit hearts have a high AMPK activity, with AMPK expression and activity being greatest in 7-day-old hearts. Hearts were also perfused in the Langendorff mode with Krebs-Henseleit buffer containing 0.4 mmol/L [14C]palmitate and 11 mmol/L glucose +/- 100 microU/mL insulin. In the absence of insulin, fatty acid oxidation rates were significantly higher in 7-day-old hearts compared with 1-day-old hearts. AMPK activity was also greater in 7-day-old hearts compared with 1-day-old hearts (909 +/- 60 and 585 +/- 75 pmol.min-1.mg protein-1, respectively; P < .05). In 1-day-old hearts, the presence of insulin resulted in a significant decrease in AMPK activity, an increase in ACC activity, and a decrease in fatty acid oxidation rates. In 7-day-old hearts, AMPK activity was also decreased by insulin, although ACC activity remained low and fatty acid oxidation rates remained high. Stimulation of AMPK in 7-day-old hearts with 200 mumol/L 5-amino 4-imidazolecarboxamide ribotide resulted in a further decrease in ACC activity and an increase in fatty acid oxidation rates. These data suggest that AMPK, ACC, and fatty acid oxidation are sensitive to insulin in 1-day-old rabbit hearts and that the decrease in circulating insulin levels seen after birth leads to an increased activity of AMPK. This can then lead to a phosphorylation and inhibition of ACC activity, with a resultant increase in fatty acid oxidation rates.