The energy value of food intake and excreta, primarily feces, is determined with a calorimeter system (IKA C7000, Staufen, Germany). After drying, food and feces samples are homogenised, squeezed to a pill and placed inside a decomposition vessel (bomb) and combusted with O2. The temperature measurement takes place directly in the bomb and caloric value is calculated from the heat released during the combustion process.
To measure O2 consumption and CO2 production mice are placed inside an open respiratory system (Fig. 1). Gas concentrations are measured by pumping compressed air through the metabolic chambers with a known flow rate and measured by electronic mass flow meters. The animal air is dried by cooling traps and lead through a pneumatic switch unit containing filters, pumps and magnetic valves. The magnetic valve system allows the measurement of up to four mice simultaneously. A fifth channel is used for zero reference checks. A computer triggers the valves at adjustable intervals via interface. The O2 and CO2 content of separate animal chambers is analysed with an O2 and CO2 analyser, respectively. The measurement principle of the oxygen analyser is based on the paramagnetic properties of oxygen. The CO2 analyser is a two channel device which compares two measurement cells from reference air and animal chamber, whereas the O2 analyser shows the deviation from reference air caused by oxygen consumption. The metabolic rate is calculated according to the equation by Heldmaier and Steinlechner (1981):
MR [ml O2 h-1] = (DVol% O2) * Flow rate [l h-1] * 10.
Analog outputs of gas analysers are continuously interfaced and data are stored on computer disk. This set-up enables determination of basal and resting metabolic rate of mice inside the metabolic chambers. It further allows prolonged stays when food and water are offered to measure daily energy expenditure. The calculation of respiratory exchange ratio from the quotient of CO2 produced and O2 consumed gives information about the preferred fuel used in energy metabolism. Ambient temperature is controlled by thermocouples and registered on the computer.
Upgrading the metabolic system with a telemetrical system enables a parallel recording of body temperature and activity pattern. By implanting a temperature sensitive transmitter into the abdominal cavity, prolonged measurements in unrestrained mice are possible. The metabolic chamber is placed on top of a receiver which directly records the transmitter signals and transfers them to an interface/computer unit.
For the analysis of body composition, carcasses are dried to constant weight at 65°C and fat content is determined by extraction of lipids with petroleum ether using a Soxhlet apparatus. Fat mass is calculated as the difference between dry weight before and after petroleum ether extraction. Lean body mass is carcass weight minus fat mass. Water content of the carcasses is calculated by subtracting the dried carcasses weight from the original weight before drying procedure.
Heldmaier G and Steinlechner S (1981) Seasonal pattern and energetics of short daily torpor in the djungarian hamster, Phodopus sungorus. Oecologia 48: 265-270.