Glycogen Metabolism in Meal-fed Rats and Chicks and the Time Sequence of Lipogenic and Enzymatic Adaptive Changes

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Abstract
The time course of the lipogenic and enzymatic adaptations to meal-feeding (access to food for a single daily 2-hour period) in rat adipose tissue and in rat and chick liver were investigated. The incorporation of acetate-1-14C into fatty acid had increased in rat adipose tissue after 5 to 7 days of meal-feeding, and in chick liver after 7 days. The activities of glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and NADP-malic dehydrogenase did not increase in rat adipose until after 9 days of meal-feeding and did not increase over control values in liver of meal-fed chicks. The data are interpreted as demonstrating that the hyperlipogenesis induced by meal-feeding is not dependent upon increased activities of the dehydrogenase enzymes studied. These data are discussed in relation to the NADPH requirements for the support of lipogenesis. Glycogen metabolism was studied in liver of meal-fed rats and chicks and in rat diaphragm and adipose tissue. In both rats and chicks, the level of glycogen in liver of meal-fed animals was depleted to a lesser extent by 22 hours of fasting than that in liver of nibbling (ad libitum-fed) animals. In chicks, liver glycogen increased to twice normal levels in meal-fed animals killed after the daily 2-hour meal. In meal-fed rats, liver glycogen was not increased by the 2-hour meal, and values remained at about 50% of those found in fed control animals. However, four hours after the initiation of the meal period, hepatic glycogen levels in meal-fed animals were equivalent to those observed in livers of fed control rats. Glycogen in diaphragm and adipose tissue of meal-fed or nibbling rats fasted for 22 hours was similar, but glycogen increased more rapidly and reached a higher level in tissues of meal-fed rats in a 6-hour period following refeeding. The possible physiological significance of these observations is discussed.