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An approach toward the elucidation of the mechanism of action of gluoagon in promoting protein catabolism has been made by comparing the metabolic effects of glucagon and hydrocortisone using female rats fasted five days. The possible relationship of the level of liver glycogen to protein catabolism has also been investigated by comparing the action of glucagon and epinephrine. Both glucagon and hydrocortisone-treated animals lost more weight and excreted more urinary nitrogen, phosphorus and creatinine than controls. The rate of nitrogen and phosphorus excretion of glucagon-treated rats declined progressively whereas that of hydrocortisone-treated rats increased progressively throughout the course of the experiment. Although the animals in both groups lost similar amounts of urinary nitrogen the glucagon-treated rats lost less body weight, tissue protein and urinary phosphorus. Furthermore, hydrocortisone caused an elevation in the level of blood a amino nitrogen whereas glucagon caused blood a amino nitrogen to be depressed. These results suggest that glucagon and hydrocortisone cause protein catabolism by different mechanisms. It is proposed that glucagon acts directly on the liver to facilitate the catabolism of amino acids in contrast to the postulated action of hydrocortisone in mobilizing amino acids from extrahepatic tissues. Epinephrine and glucagon depressed liver glycogen but the glycogenolytic effect of epinephrine was of borderline significance. However, epinephrinetreated rats did not differ from controls in respect to any of the above mentioned parameters of protein metabolism. This suggests that the protein catabolic action of glucagon is not directly a consequence of reduced hepatic glycogen.
Izzo et al. (Wed,) studied this question.