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Influence of Periodicity of Eating on the Activity of Adipose Tissue and Muscle Glycogen Synthesizing Enzymes in the Rat¹

Division of Nutritional Biochemistry, Department of Animal Science, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801

Activities of enzymes involved in the conversion of glucose to glycogen were measured in adipose tissue and muscle of nibbling (ad libitum fed) and meal-fed (access to food for a single daily 2-hour period) rats immediately before and after the daily meal. Adipose tissue hexokinase, phosphoglucomutase, UDPG-pyrophosphorylase and glycogen synthetase and muscle hexokinase activities were significantly higher in meal-fed than in nibbling animals. These increases in enzyme activity are discussed in relation to the increased rate of glycogen accumulation produced by meal-feeding. Adipose tissue glycogen synthetase activity, assayed in the presence or absence of glucose-6-phophate, was the only enzyme activity which increased as a result of meal ingestion. Glycogen synthetase activity increased slightly after the daily 2-hour meal and had more than doubled 5 hours after meal initiation. Adipose tissue glycogen synthetase activity also increased as a result of refeeding fasted nibbling animals, but the increase was less than observed in meal-fed animals. Puromycin injection only partially blocked the increase in glycogen synthetase activity produced by meal ingestion, indicating that de novo protein synthesis could account for no more than a part of the increased activity. Possible relations between the increase in glycogen synthetase activity and the interconversions of the various forms of the enzyme are discussed.

Manuscript received 5 August 1969.