Diurnal Variations in Tissue Glycogen and Liver Weight of Meal-fed Rats

Abstract

The studies reported were designed 1) to describe the diurnal variations in tissue glycogen levels in meal-fed rats (animals having access to food for a single, daily, 2-hour meal), and 2) to compare the tissue glycogen storage capacity of meal-fed and nibbling (ad libitum-fed) rats. Rats trained to a meal-eating schedule and nibbling rats were maintained without food for 22 hours and then allowed access to food for 2 hours. The rats were killed at various times after the initiation of the meal, and liver, diaphragm and adipose tissue glycogen content was determined. Fasting liver glycogen levels were higher in meal-fed rats, but glycogen accumulated at the same rate in livers of meal-fed and nibbling rats for up to 8 hours after the start of the meal. Liver glycogen then decreased linearly from 8 to 22 hours. Meal-fed rats had a greater capacity to accumulate glycogen in diaphragm muscle and adipose tissue than nibbling rats. In meal-fed animals glycogen accumulated in diaphragm and adipose tissue for 8 hours, fell sharply between 8 and 10 hours after the initiation of the meal, and decreased more slowly from 10 to 22 hours. Liver weight increased significantly upon meal ingestion in both meal-fed and nibbling rats. Water accounted for about 66% of the increase and glycogen was responsible for the remainder. It was estimated that about 30% of the calories stored during and following the daily meal in meal-fed rats were in the form of glycogen. The data are discussed in terms of the importance of glycogen as a storage form of energy for the meal-fed animal.