Activation of AMP-activated protein kinase (AMPK) results in the upregulation of several intracellular systems which help to prepare a cell for a high energy challenge. The magnitude of the AMPK response to a 10 min bout of exercise has been found to decrease in red quadriceps (RQ) following training, while putative AMPK roles seem to be maintained; specifically, the biogenesis of mitochondria and higher levels of hexokinase II and glucose transporter 4 (GLUT4). If the AMPK response to exercise is responsible in part for these adaptations, how can they be maintained if the AMPK response is attenuated? The purpose of this study was to determine whether phosphorylation of AMPK in RQ increases during 2-hr training bouts after rats have trained for 8 wks. Male Sprague-Dawley rats ran up to 30 m/min up a 15% grade, 2 hr/day for 8 wks. On the final bout of exercise, trained rats ran for 0 (TRC), 30 (TR1), or 120 min (TR2) up a 15% grade at 30 m/min. Red quadriceps (RQ), soleus, and white quadriceps (WQ) were immediately collected and frozen for analysis. Citrate synthase activity increased in RQ (79 ± 3 vs. 37 ± 4 µmol/g/min) and soleus (64 ± 4 vs. 35 ± 2 µmol/g/min) but not in WQ compared to non-trained controls. In trained rats, maximal increases in T-172 phosphorylation of AMPK occurred after 30 min of exercise (relative values = 1.29 ± 0.06 vs. 1.00 ± 0.06). AMPK phosphorylation did not change significantly in trained rats that ran for 2 hrs (1.31 ± 0.09) compared to rats that ran for 30 min. Similarly, maximal increases in AMPK activity in trained rats occurred after 30 min of exercise (pmoles/min/mg = 2.67 ± .05 vs. 1.09 ± .41) and AMPK activity did not change significantly in trained rats that ran for 2 hrs (2.79 ± .17) compared to rats that ran for 30 min. Previous studies demonstrated a 2−3 fold increase in AMPK activity in non-trained rats after 30 min of exercise at lower work rates. These results demonstrate that the AMPK response to exercise is attenuated even after two-hr bouts of exercise. This implies that the increase in mitochondrial oxidative enzymes, GLUT4, and hexokinase II may be maintained by signals other than the AMPK signaling system. The CREB signaling pathway is one such system. Western analysis of phospho-CREB (Ser133) showed a statistically significant increase in phospho-CREB content in trained rats relative to control. No change in phospho-CREB protein expression was observed between TRC, TR1, and TR2 rats. Significant increases of muscle phospho-CREB content in TRC relative to untrained rats suggest that CREB remains phosphorylated in trained rats even after 24 hrs of rest. Accordingly, chronically increased phospho-CREB in muscle of trained rats relative to controls may explain in part how increased levels of mitochondria are maintained in the face of reduced AMPK response. Alternatively, the attenuated AMPK response may still be above the threshold required for inducing adaptations to endurance training.



College and Department

Life Sciences; Physiology and Developmental Biology



Date Submitted


Document Type





AMPK, GLUT4, hexokinase II, mitochondria, CREB