Effect of high FFA on glycogenolysis in oxidative rat hindlimb muscles during twitch stimulation

The effect of elevated free fatty acids (FFA) on carbohydrate (CHO) utilization in the oxidative muscles of the isolated hindlimb was determined using twitch contraction paradigms evoking a wide range of O2 uptakes and glycogenolysis. The hindlimb was perfused with either 0 or 1.8 mM FFA for 10 min at rest and then subjected to 20 min of stimulation at 0.4, 0.7, 1, 2, 3, or 4 Hz. Soleus (Sol), plantaris (Pl), and red gastrocnemius (RG) were sampled after rest perfusion or stimulation. FFA had little effect on glycogenolysis during stimulation, although glycogen sparing occurred with one of the lesser intensity protocols in each muscle (Sol, 0.4 Hz; RG, 0.7 Hz; Pl, 1 Hz). Muscle citrate and acetyl-CoA were elevated in Sol during several stimulation protocols with high FFA, but this effect was inconsistent in Pl and RG. The sparing of glycogen, when it did occur, was generally unrelated to increases in either citrate or acetyl-CoA content. Furthermore, protocols in which citrate or acetyl-CoA were elevated in the presence of elevated FFA did not demonstrate glycogen sparing. Hindlimb lactate efflux at rest was reduced with FFA but unaffected during stimulation. Glucose uptake was unaffected by FFA at rest and during stimulation protocols, except 3 Hz. The present study does not support the classically proposed roles of citrate and acetyl-CoA in the FFA-induced downregulation of CHO utilization in electrically stimulated rat skeletal muscle.