Glycogen content and repletion rates in beef muscle, effect of feeding and fasting.

The effect of type of diet and fasting on resting muscle glycogen concentration and on rate of recovery after glycogen depletion was studied in beef M. logissimus dorsi. Hereford heifers were fed either barley or hay, or were fasted for 9 days. Resting muscle glycogen concentration was significantly increased by feeding barley and decreased by fasting, relative to the value in cattle on a maintenance diet of hay. Rates of repletion of muscle glycogen following depletion by adrenalin injection averaged 7.6 mumol/g per day on barley, 6.1 mumol/g per day on hay and 1.5 mumol/g per day in fasted heifers. On refeeding the fasted group with hay the rate of glycogen reported for nonruminant species. The specific activity of muscle glycogen synthase (I + D) was 2.13 mumol/minute per gram of wet tissue, and normal conversion to the active (I) form was observed in glycogen-depleted beef muscles. Determination of blood glucose, plasma insulin and muscle glucose 6-phosphate concentrations provided some evidence for a decreased availability of glycogen precursors during the period of muscle glycogen resynthesis. It was concluded that a lower glucose availability in cattle compared with nonruminant species caused a slower rate of muscle glycogen repletion. Different repletion rates between the experimental groups reflected different levels of energy intake and corresponding changes in glucose utilization rates.