Recovery of 15N in the Body, Urine, and Gas Phase of Piglets Infused Intravenously with 15N l-Alanine from 12–72 Hours of Age

Previous studies of nitrogen metabolism provided evidence suggesting that nitrogen excretory product(s) not measured by standard methods of analysis escape detection. To determine whether (15)N could be recovered quantitatively in the body, urine, or expired gas, newborn piglets (n = 16; 1.47 +/- 0.27 kg) were infused intravenously with (15)N L-alanine from 12 to 72 h of age at a rate providing 25% of the piglets' resting energy expenditure and a (15)N abundance of 2.3 (n = 4), 2.8 (n = 10), or 3.3 (n = 2) atom percent. To investigate the possibility of gaseous nitrogen excretion, 4 piglets infused with (15)N L-alanine were housed in a closed circuit respiration system initially flushed with an 80% argon:20% O(2) mixture. The gas composition of the system was monitored at 12-h intervals throughout the experiment. Mean total recovery of (15)N was 93.3 +/- 2.8% and was significantly different from 100% (P < 0.001). To determine whether (15)N recovery was altered by metabolism, 2 piglets (1.34 +/- 0.13 kg) were killed 6 min after a bolus i.v. infusion of (15)N L-alanine (97.96 +/- 1.13 atom percent). Mean recovery of (15)N in the bodies of these piglets was 101.5 +/- 1.6% and was not different from 100%. No change in chamber gas (28)N(2) (P = 0.0969) or (29)N(2) (P = 0.08565) over 72 h was evident. The inability to recover 6.7 +/- 2.8% of infused (15)N suggests that a nitrogen-containing excretory product or metabolite may be escaping detection, but the discrepancy cannot be explained by gaseous nitrogen ((28)N(2), (29)N(2), or (30)N(2)) excretion.