When rats are fed a semisynthetic diet containing 10% glycine and then stressed (cold water swim), the animals maintain liver glycogen at a much higher level than do animals treated similarly but fed the diet without the added glycine. During a 3-hour recovery following the stress, the glycine-fed animals synthesize considerable liver glycogen while the control-fed rats show no net deposition of liver glycogen. These findings constitute the "glycine effect." When these diets supplemented with 1% creatine are used, the rats do not exhibit the glycine effect. Moreover, the nonstressed levels of liver glycogen are less than half the normal values. No explanation is available to explain this "blocking" of the glycine effect.
Muscle creatine levels were determined before stress, after stress and following recovery from stress in animals prefed creatine-control diet and creatine-glycine diet. The only significant change observed in free creatine was a decrease in the stressed animals (either diet) compared to nonstressed rats prefed the creatine-control ration.
The activity of the enzymes phosphorylase and glycogen synthetase was determined at the three times in the stress procedure in muscle from animals prefed four diets: the control, the glycine, the creatine-control and the creatine-glycine rations. Active phosphorylase activity was found to be increased in animals on all of the diets as a result of stress and remained elevated at the end of the recovery period. Glycogen synthetase activity of muscle was lowered at all times in the procedure as a result of feeding diets supplemented with glycine, creatine, or both. The variations in muscle enzyme activity did not conform to the observed changes in liver glycogen and thus do little to help explain the mechanism of the glycine effect.