Figure 1. Natural nitrogen isotopic abundances of metabolic pools in rats under standard conditions.

Isotopic abundances, measured in the protein (P) and free amino acid (AA) fractions of tissues and blood, in plasma urea, in urinary urea and ammonia (NH₄), and in the total nitrogen (N) of intestinal contents, are expressed as the difference between the measured natural ¹⁵N enrichment of the pool and that of the diet (Δ¹⁵N_pool = δ¹⁵N_tissue−δ¹⁵N_diet). RBC, Red blood cells; TA, Tibialis anterior; EDL, Extensor digitorum longus. Asterisks indicate significant Δ¹⁵N differences between two N fractions (P<0.01, independent Student t-tests).