Glycogen Content Regulates Peroxisome Proliferator Activated Receptor-∂ (PPAR-∂) Activity in Rat Skeletal Muscle

Performing exercise in a glycogen depleted state increases skeletal muscle lipid utilization and the transcription of genes regulating mitochondrial β-oxidation. Potential candidates for glycogen-mediated metabolic adaptation are the peroxisome proliferator activated receptor (PPAR) coactivator-1α (PGC-1α) and the transcription factor/nuclear receptor PPAR-∂. It was therefore the aim of the present study to examine whether acute exercise with or without glycogen manipulation affects PGC-1α and PPAR-∂ function in rodent skeletal muscle. Twenty female Wistar rats were randomly assigned to 5 experimental groups (n = 4): control [CON]; normal glycogen control [NG-C]; normal glycogen exercise [NG-E]; low glycogen control [LG-C]; and low glycogen exercise [LG-E]). Gastrocnemius (GTN) muscles were collected immediately following exercise and analyzed for glycogen content, PPAR-∂ activity via chromatin immunoprecipitation (ChIP) assays, AMPK α1/α2 kinase activity, and the localization of AMPK and PGC-1α. Exercise reduced muscle glycogen by 47 and 75% relative to CON in the NG-E and LG-E groups, respectively. Exercise that started with low glycogen (LG-E) finished with higher AMPK-α2 activity (147%, p<0.05), nuclear AMPK-α2 and PGC-1α, but no difference in AMPK-α1 activity compared to CON. In addition, PPAR-∂ binding to the CPT1 promoter was significantly increased only in the LG-E group. Finally, cell reporter studies in contracting C2C12 myotubes indicated that PPAR-∂ activity following contraction is sensitive to glucose availability, providing mechanistic insight into the association between PPAR-∂ and glycogen content/substrate availability. The present study is the first to examine PPAR-∂ activity in skeletal muscle in response to an acute bout of endurance exercise. Our data would suggest that a factor associated with muscle contraction and/or glycogen depletion activates PPAR-∂ and initiates AMPK translocation in skeletal muscle in response to exercise

The adult heart responds to increased workload with physiologic hypertrophy, cardiac stem cell activation, and new myocyte formation

AIMS: It is a dogma of cardiovascular pathophysiology that the increased cardiac mass in response to increased workload is produced by the hypertrophy of the pre-existing myocytes. The role, if any, of adult-resident endogenous cardiac stem/progenitor cells (eCSCs) and new cardiomyocyte formation in physiological cardiac remodelling remains unexplored. METHODS AND RESULTS: In response to regular, intensity-controlled exercise training, adult rats respond with hypertrophy of the pre-existing myocytes. In addition, a significant number (∼7%) of smaller newly formed BrdU-positive cardiomyocytes are produced by the exercised animals. Capillary density significantly increased in exercised animals, balancing cardiomyogenesis with neo-angiogenesis. c-kit(pos) eCSCs increased their number and activated state in exercising vs. sedentary animals. c-kit(pos) eCSCs in exercised hearts showed an increased expression of transcription factors, indicative of their commitment to either the cardiomyocyte (Nkx2.5(pos)) or capillary (Ets-1(pos)) lineages. These adaptations were dependent on exercise duration and intensity. Insulin-like growth factor-1, transforming growth factor-β1, neuregulin-1, bone morphogenetic protein-10, and periostin were significantly up-regulated in cardiomyocytes of exercised vs. sedentary animals. These factors differentially stimulated c-kit(pos) eCSC proliferation and commitment in vitro, pointing to a similar role in vivo. CONCLUSION: Intensity-controlled exercise training initiates myocardial remodelling through increased cardiomyocyte growth factor expression leading to cardiomyocyte hypertrophy and to activation and ensuing differentiation of c-kit(pos) eCSCs. This leads to the generation of new myocardial cells. These findings highlight the endogenous regenerative capacity of the adult heart, represented by the eCSCs, and the fact that the physiological cardiac adaptation to exercise stress is a combination of cardiomyocyte hypertrophy and hyperplasia (cardiomyocytes and capillaries)

PPAR-∂ activity in response to exercise in a normal or low glycogen environment.

<p>Post-exercise skeletal muscle activity of PPAR-∂ was determined by chromatin immunoprecipitation (ChIP) assay followed by qRT-PCR using primers spanning the PPAR-∂ binding region in the CPT-1 promoter. * indicates significantly different than control (p<0.05; n = 4).</p

Schematic of the experimental design.

<p>Skeletal muscle glycogen content was manipulated using pre-exercise and varying periods of recovery, prior to a main 45 min exercise trial. Five groups performed 3-days acclimatization to treadmill running. Following which, the CON group was sacrificed 24 h after the final acclimatization trial, the normal glycogen control (NG-C) ran for 1 hour and then rested 23 h before sacrifice, the normal glycogen exercise (NG-E) performed a subsequent 45 min run 23 h after the initial 1 hour run, the low glycogen control (LG-C) was collected 1 h after the initial 1 hour run, and the low glycogen exercise group (LG-E) was collected immediately following a 45 min run commenced 1 hour after the initial 1 hour trial. All experiments took place in the morning to avoid significant alterations in diet and diurnal variation in metabolic responses.</p

Glycogen levels in the 5 experimental groups.

<p>Glycogen was determined after hydrolysis of 30* indicates significantly different than CON and † indicates significantly different than NG-E and LG-C (p<0.05; n = 4).</p

AMPK activity following exercise in a normal or low glycogen environment.

<p>The activity of (A) α1-AMPK and (B) α2-AMPK was determined in each of the 5 groups by IP kinase activity assay. * indicates significantly different than control (p<0.05; n = 4).</p