APPLIED PHYSIOLOGY JOURNALOF Vol. 39, No. 5, November 1975. Printed

An automatic, apparatus

in U.S.A.

closed-circuit

oxygen

consumption

for small animals

MICHAEL Department

J. STOCK of Physiology,

Queen

Elizabeth

College,

London,

STOCK, MICHAEL J. An automatic, closed-circuit oxvgen consumption apparatus for small animals. J. Appl. Physiol. 39(5) : 8494350. 1975. -An apparatus suitable for the continuous measurement of oxygen consumption of rats and mice is described. The system uses a motorized syringe dispenser to deliver fixed volumes of oxygen to a closed animal chamber. The dispenser is controlled by a microdifferential pressure switch to maintain chamber pressure slightly above ambient. The rate of oxygen consumption is determined by timing the interval between successive operations of the dispenser. The system has proved suitable for a range of experimental conditions and treatments.

metabolic

rate;

respirometer

SEEMINGLY SIMPLE MEASUREMENT of respiratory exchange in small animals such as the rat and mouse has provoked the development of innumerable methods and devices. The reasons for such multiplicity are probably twofold. First, the experimental requirements of individual workers differ according to duration of experiments, required accuracy, ease of operation, and demands on observer time. Second, with improvements in instrumentation, established or even defunct methods can be redesigned. Although burdening the literature still further, the present system represents an example of an old principle being applied with simple instrumentation with resultant advantages that suit a greater range of experimental requirements. THE

England

end, which has a removable cover plate. This cover, as well as allowing access to the chamber interior, also holds the connectors for the oxygen delivery line and the pressure line. For experiments involving injections, infusions and blood sampling, catheters are passed through, and sealed into, rubber bungs which are then forced into holes in the cover plate. A rubber gasket forms an airtight seal between the cover and the chamber. Within the chamber, the animal is supported on a wire grid over a layer of self-indicating soda lime (Carbosorb, British Drug Houses) and silica gel. A major determinant of sensitivity in this system is the dead space of the chamber. The chambers used here have internal dimensions of 20 x 10 x 10 cm and are suitable for animals such as mice and young rats up to about 250 g body weight. Fixed volumes of oxygen are introduced into the chamber by an automatic syringe dispenser (Fisons Scientific) which draws pure oxygen from a spirometer through a drying tube filled with silica gel. When chamber pressure exceeds atmospheric by about 3 mmHzO, the microdifferential pressure switch (KDG Instruments Ltd.) inactivates the dispenser. The dispenser is reactivated when the pressure differential drops below this threshold value. The volume of oxygen dispensed is adjusted to the smallest volume that, with a single action of the syringe, will return chamber pressure to above the threshold value. With the present system this volume is about 0.8 ml. If smaller volumes are used, two strokes of the syringe are required to exceed threshold pressure and with larger volumes sensitivity is lost due to a longer pump interval. The particular dispenser used in this system has the advantages of being a) gas tight and b) when activated will complete its pump cycle even if the chamber pressure exceeds the threshold value in

PRINCIPLE

The pressure inside a closed animal chamber is raised slightly above ambient by introducing a fixed volume of oxygen. Expired carbon dioxide and water are absorbed and thus the time taken for chamber pressure to return to its initial value indicates the rate of oxygen consumption by the animal. Smothers (1) described a system based on this principle which involved manually injecting a fixed volume of water from a syringe pipette to displace an equal volume of oxygen into the animal chamber. The rise in chamber pressure was indicated on a manometer. The major disadvantage with this system was the need for constant attention, since both the timing and the maintenance of the oxygen supply were dependent on observing the manometer and injecting more oxygen each time chamber pressure returned to ambient. The system described here automates both of these processes by using a motorized syringe controlled by a microdifferential pressure switch and counting the pumping rate of the syringe.

Thermostated water bath

O2 delivery

Automatic dispenser

line I

Micro-switch To recorder

-differential switch

DESCRIPTION

A diagram of the apparatus animal chamber is surrounded

is shown in Fig. by a water jacket

1. The except

Perspex for one

FIG.

shown

Oxygen consumption apparatus. in black. Not drawn to scale.

Nylon

tubing

connections

849

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850 midcycle. In this way, a discrete fixed volume of oxygen is delivered each time it is activated and to obtain the rate of oxygen consumption it is merely necessary to record the pump rate. This is done using the travel of the syringe piston to close a microswitch connected to an event recorder. Calibration of the dispensing volume is carried out by slowly withdrawing 50-ml aliquots of air from the chamber with an oiled glass syringe and recording the total number of pumps required to maintain chamber pressure. The absence of leaks in the system is easily verified by running the apparatus without an animal in the chamber at a steady temperature and confirming that the dispenser remains inactive. The apparatus in this, its simplest, form has been used extensively for measuring the influence of a variety of thermogenic and other agents on oxygen consumption in rats and mice. Periods of measurement have ranged from as little as 30 min up to as long as 6 h. Because the oxygen concentration of the chamber air is maintained constant by replacing oxygen as it is consumed, it is possible

M.

J. STOCK

to study the influence of hypoxic and hyperoxic mixtures simply by flushing the chamber with the appropriate gas mixture before connecting the oxygen supply line. For 24-h measurements, the chamber is modified by absorbing water and carbon dioxide in an external circuit using a diaphragm pump to circulate the chamber air. This allows greater amounts of the absorbants to be used and in this form the chamber is particularly useful for energy balance experiments. Measurement of carbon dixode (or [14C]carbon dioxide) production could be achieved by replacing the soda lime in the external circuit with potassium hydroxide solutions and taking serial samples, but this has yet to be tested. Received

for publication

27 May

1975.

REFERENCE 1. SMOTHERS, J. L. A volumetric respirometer AppZ. Physiol. 2 1: 1117-l 118, 1966.

for small

animals.

J.

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