Control of urea cycle enzymes in rat liver by glucagon

A single injection of glucagon increased the levels of all five urea cycle enzymes in 28-day-old rats, but had no significant effect in 35-day-old animals. Rats weaned onto diets varying in protein content showed rapid adaptation in the levels of carbamoylphosphate synthetase, which were complete 4 days after weaning. Induction of the enzyme by glucagon was independent of the diet. In animals subjected to a controlled feeding regime, no circadian variation in the levels of urea cycle enzymes could be observed. However, the inducibility of the enzyme by glucagon was greatest if the hormone was given near the beginning of the period of darkness and feeding.