Isolation and measurement of ¹⁵N₂ from respiratory gases of animals administered ¹⁵N‐labeled substances

Abstract

A method is described for collection of metabolic ¹⁵N₂ from in vitro preparations or intact rats administered ¹⁵N‐containing compounds. The method enables routine collection and mass spectrometric measurement of as little as 10 μmol ¹⁵N₂ respired by a rat over a 24‐h period. A device is described that includes either an animal chamber or a tissue reaction vessel in a closed recycling atmosphere, with automatic O₂ replenishment and removal of CO₂ and water. It is capable of sustaining moderate vacuum and is coupled to a high‐vacuum manifold designed to process the contained atmosphere and respiratory gases. The starting atmosphere is an 80:20 mix of sulfur hexafluoride and O₂. Recovery of ¹⁵N₂ gas from the system without an animal present was 101.3 ± 5. 75%. When ¹⁵N₂ gas was very slowly infused iv into an animal, recovery was 89.1 ± 5.38%. Use of the method in studies of the fate of [¹⁵N]hydrazine in rats indicated that about 15% of the administered hydrazine is rapidly converted to ¹⁵N₂, followed by slower conversion of an additional 7–10% over the next several hours.