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Possible Intervention of Insulin, Cyclic AMP, and Glucocorticoids in Protein-sparing Action of Dietary Carbohydrate in Rats

Laboratory of Nutritional Biochemistry, Department of Agricultural Chemistry, Nagoya University, Chikusa, Nagoya, Japan

The present study was carried out to examine the possible intervention of some hormones in the "protein-sparing" actions of dietary carbohydrate and fat. Administration of either a carbohydrate meal or a fat meal to fasted rats caused a reduction in the urinary output of urea and total nitrogen with a concomitant decrease in rate of urogenesis in liver as well as the activities of some amino acid-catabolizing enzymes in liver. The sparing action of carbohydrate but not that of fat was abolished in alloxan-diabetic rats. Feeding rats a carbohydrate meal caused a marked reduction in the amount of cyclic AMP in liver. The change was coincident with a reduction in the level of plasma urea and the urinary output of urea and total nitrogen. Administration of dibutyryl cyclic AMP abolished the carbohydrate-induced depression of urinary output of urea and total nitrogen as well as partially the activity of serine dehydratase in liver. Feeding a carbohydrate meal resulted in a significant reduction in the level of corticosterone in plasma. However, the possible intervention of glucocorticoids in the protein-sparing action of carbohydrate was ruled out inasmuch as the action of carbohydrate was also observed in the adrenalectomized rats. The overall results suggest that the protein-sparing action of dietary carbohydrate may be exerted in a different fashion from that of fat, that is, through depression of cyclic AMP in liver and thus the reduction of the degradation of amino acids in liver.

KEY WORDS: • protein-sparing action • carbohydrate • cyclic AMP

Manuscript received 3 December 1974.