Glycogen turnover in skeletal muscle is stimulated along with glucose uptake in vivo during contraction

Abstract

We have shown previously that glycogen synthesis in the heart can be stimulated in vivo by epinephrine. Our aim in this study was to determine whether glycogen synthesis in skeletal muscle can be similarly affected during increased energy expenditure. Left sciatic nerves of anesthetized fasted rats were electrically stimulated to allow left hindlimb muscles to contract for 5, 10, and 20 min. Glycogen contents in the contracting muscles at the end of electrical stimulation were found to be ~40% less than resting muscles in the right hindlimbs in all three groups of rats. Accompanying the enhanced glycogenolysis was increased incorporation of the intravenously infused [3- 3H]-glucose into glycogen. The rate of tritium incorporation into glycogen in the contracting muscle was found to be 34-fold greater than resting muscles. Glucose utilization was determined by the phosphorylation of the intravenously injected [ 14C]-2-deoxyglucose in the skeletal muscle. The rate of accumulation of [ 14C]-2-deoxyglucose-6-phosphate in the contracting muscles was found to be 28-fold greater than resting muscles. Glycogen synthesis and glucose uptake indexes, calculated by dividing the radioactivity in [ 3H]-glycogen and [ 14C]-2-DGP by the mean specific activity of their respective precursors in the plasma, were not found to be significantly different in the contracting muscle. In conclusion, our data indicate that: (i) glycogenesis and glycogenolysis can be stimulated concurrently in the skeletal muscle; and (ii) glucose utilization in the skeletal muscle during contraction may be mediated through glycogen turnover.