Chronic Treatment with the AMP-Kinase Activator AICAR Increases Glycogen Storage and Fatty Acid Oxidation in Skeletal Muscles but Does Not Reduce Hyperglucagonemia and Hyperglycemia in Insulin Deficient Rats

Abstract

This study tested whether the glycogen-accumulating effect of chronic in vivo pharmacological 5′AMP-activated protein kinase (AMPK) activation could improve glycemic control under conditions of insulin deficiency. Male Wistar rats were rendered diabetic through the administration of streptozotocin (STZ) and then treated for 7 consecutive days with the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR). Subsequently, glycogen content and synthesis, glucose oxidation, and fatty acid oxidation (FAO) were determined in oxidative and glycolytic skeletal muscles. Glycemia, insulinemia, glucagonemia, and circulating triglycerides (TG) and non-esterified fatty acids (NEFAs) were measured after AICAR treatment. Insulin was almost undetectable in STZ rats and these animals were severely hyperglycemic. Glycogen content was markedly low mainly in glycolytic muscles of STZ rats and AICAR treatment restored it to control values. No differences were found among all muscles studied with regards to the content and phosphorylation of Akt/protein kinase B and glycogen synthase kinase 3. Even though glycogen synthase content was reduced in all muscles from STZ rats, insulin-induced dephosphorylation/activation of this enzyme was preserved and unaffected by AICAR treatment. Glucagon and NEFAS were 2- and 7.4-fold fold higher in STZ rats than controls, respectively. AICAR did not affect hyperglycemia and hyperglucagonemia in STZ rats; however, it normalized circulating NEFAs and significantly increased FAO in glycolytic muscles. In conclusion, even though AICAR-induced AMPK activation enhanced glycogen accumulation in glycolytic muscles and normalized circulating NEFAs and TG levels, the hyperglycemic effects of glucagon likely offset the potentially glucose-lowering effects of AICAR, resulting in no improvement of glycemic control in insulin-deficient rats.

Fatty acid metabolism, energy expenditure and insulin resistance in muscle

“…Subsequent studies using independently generated Acc2-knockout mice did not reproduce these effects, reporting that although these mice exhibited increased FA oxidation at the whole-body and isolated muscle level, there was no measurable difference in energy expenditure, fat mass or food intake (Hoehn et al 2010). However, there was an increased glycogen content in muscle, an effect of AMPK activation noted previously (Winder et al 2000, Buhl et al 2001, which is consistent with AMPK activation and ACC2 inhibition promoting FA oxidation and channelling glucose taken up by muscle into storage as glycogen (Vitzel et al 2013). Another study using independently generated genetically manipulated mice has reported no difference in body weight, food intake or fat mass in global or muscle-specific Acc2 gene-deleted mice (Olson et al 2010), adding to the conclusion that altering FA oxidation in the absence of any change in energy expenditure or energy intake is insufficient to have a significant impact on whole-body fat mass.…”

Thermogenic Capacity Is Antagonistically Regulated in Classical Brown and White Subcutaneous Fat Depots by High Fat Diet and Endurance Training in Rats

“…After the 1-h incubation period, the media were acidified with 0.2 ml of H 2 SO 4 (5 N), and the flasks were maintained sealed at 37°C for an additional 1 h for collection of the released 14 CO 2 . Subsequently, the filter papers were carefully removed and transferred to scintillation vials for radioactivity counting (20).…”

Iron restriction increases myoglobin gene and protein expression in Soleus muscle of rats

“…The nitrocellulose membranes were Ponceau stained for a better analyses of the equal loading of the gels (Romero-Calvo et al 2010, Tang et al 2011, Vitzel et al 2013, Fortes et al 2016. Blot densitometry was evaluated using Image J Software (National Institutes of Health, Bethesda, MD, USA).…”