Stable isotope research utilizing small animals, such as mice and rats has been limited by sample volume requirements. In particular, measurements of substrate oxidation, total body water, and energy expenditure, have been unattainable in small animals without sacrificing the animals at each measurement point. For the first time, advances in stable isotope analysis, technology and techniques have made these experiments available.
As part of understanding total substrate utilization, it is often of interest to measure the percentage of any carbon-13 labeled dose that is metabolized to carbon dioxide. This is done by stable isotope analysis of expired breath for the appearance of 13CO2 over time. These experiments are common in humans and other large animals (e.g., dogs and horses). However, small animals have been largely excluded from this research due to their small CO2 output. Recently, a respiratory chamber equipped with a CO2 probe, plus the measurement of 13CO2 in a small sample of chamber air, allows for the estimation of substrate oxidation. Chandler and Venditti have used this technique to study organic acidemia in mice. This stable isotope analysis of breath 13CO2 was performed at Metabolic Solutions.
Total Body Water and Energy Expenditure
The critical measurements in the field of energetics require relatively large sample volumes (0.5 – 1 ml) for D2O and H218O measurements via isotope ratio mass spectrometry (IRMS). Metabolic Solutions has developed and validated both measurements in biological samples as small as 15µl using cavity ring-down spectrometry. This allows researchers to determine total body water and/or energy expenditure in small animals.