VALIDATION OF THE LABELED BICARBONATE TECHNIQUE FOR MEASUREMENT OF SHORT-TERM ENERGY-EXPENDITURE IN THE MOUSE

The energy expenditure of free-living animals has been studied extensi vely by the doubly-labeled water (DLW) technique. This method provides a reasonably accurate estimate of daily energy needs. However, there is considerable interest in the energy demands of animals over much sh orter timescales, for which the DLW technique is less useful. We exami ned the possibility of measuring the expenditure of small animals over these shorter timescales from the washout kinetics of a bolus dose of C-13 labeled bicarbonate. The study involved 19 laboratory mice which were injected either IP or SC with 0.2 ml of C-13 labeled bicarbonate in water. Mice were placed in a standard respirometry system, maintai ned at different temperatures to precipitate a 3 fold variation in met abolism. Samples of breath were collected from the chamber into vacuta iners at one minute intervals for approximately 40 minutes to an hour. Samples were analyzed by admission to a mass spectrometer (VG Optima) via a GC interface which identified and admitted the CO2 peak. The lo g converted isotope elimination was linear (r(2) > 98% in all cases) i ndicating a single pool was involved. We evaluated the pool size from a dilution series of the injectate in equilibrium with CO2 gas. Conven tional compartmental analysis produced an estimate which on average ac ross the 19 individuals provided a reasonable estimate of the CO2 prod uction. Individual estimates were however imprecise and the overall co rrelation between isotope and calorimeter estimates had an r(2) of onl y 15%. Reasons for this discrepancy are unclear. Nevertheless an empir ical model, using the elimination gradient, pool size and route of iso tope administration as predictors explained 86% of the variation in CO 2 production. Elimination of a bolus dose of C-13 labeled bicarbonate provides a useful tool for estimating the energy metabolism of mice ov er intervals between 15 and 40 minutes.