Application of 15N-leucine in the study of the contribution rate of diet and each tissue to nitrogen excretion

Experimental diets (protein-free diet and 2% or 10% lactoalbumin diet) were given to male SD rats, and the urinary N content from diet was obtained from the amount of urine 15N 24 hours after the oral administration of 15N-leucine. N excretion from each tissue was obtained from the contents of tissue-synthesized protein N and tissue-increased N. The synthetic protein N content was obtained from the recovery rate 24 hours after 15N-leucine administration, and the tissue-increased N content from the change in the quantity of N in each tissue on the 14th and 21st days of experimental diet. Body weight increased in the 10% diet group, decreased in the 0% diet group, and showed no change in the 2% group. The mean daily urinary N excretion was inhibited more in the 2% group than in the 0% group. The tissue 15N level was high in the urine, followed by the order of digestive tract, liver and feces, and low in the skeletal muscle and skin. N excretion from tissues was greatest for the skeletal muscle; the ratio to total N excretion was high for the skeletal muscle and low for visceral organs in the 0% diet group, compared with the 10% diet group. With the 2% diet, most tissues showed intermediate values, and excretion from the skeletal muscle was lowest among the 3 groups, accounting for the greater inhibition of urinary N excretion than that in the 0% group. (Chiba, N.)