Changes-induced in liver and muscle glycogen and glycogen enzymes by 24-hour fasting in the rat.

Liver and muscle glycogen content, and the activities of glycogen synthetase, glycogen phosphorylase and alpha-amylase have been measured in fed and 24 hours fasted rats. Muscle and liver glycogen account for similar amounts of glycosyl residues liberated in this period of food deprivation. With fasting, liver glycogen synthetase activity decreases somewhat, increasing the I versus total ratio; the liver phosphorylase activity decreases considerably and amylases remain constant. In muscle, total synthetase remains constant with fasting, but the I versus total ratio decreases; phosphorylase activity decreases very considerably and alpha-amylase activity increases to almost thrice the values found in fed animals. These changes reflect a tendency towards conservation of glycogen primers with a preparation to increase glycogen synthesis when glucose would be available. In muscle also, the amylase increase in activity would be tentatively explained as a partial replacement of phosphorylase activity as a means of glucose mobilization from glycogen.