Metabolic contribution of CO2 to the "group effect"

Metabolic rates (VCO2) were determined in 22 mice. Subsequently 11 of the mice were placed into separate metabolic chambers. Each chamber was connected to one other metabolic chamber containing one mouse. The metabolic rates of the 11 "donor" and the 11 "'recipient" mice were measured. Afterwards the direction of airflow was reversed so that the previous donor became the recipient mouse and the previous recipient became the donor. Again VCO2 was measured. When the mice were donors, VCO2 = 2.75 +/- 1.0 ml CO2 . g-1 . h-1, whereas when recipients, VCO2 = 1.9 +/- 1.0 ml CO2 . g-1 . h-1 (P less than 0.01). Eleven of the mice were then made anosmic using an intranasal instillation of 5% ZnSO4. Anosmia was confirmed behaviorally. The metabolic rates of the anosmic and control mice were again determined when each was a donor and a recipient. VCO2 dropped from 3.1 to 1.8 ml CO2 . g-1 . h-1 from donor status to recipient status. Finally, VCO2 was determined when all 22 mice were exposed to 0.05% CO2 or 0.2% CO2 in air (the level of CO2 in the exhaust air of donor mice). VCO2 dropped from 3.0 +/- 0.38 on 0.05% CO2 to 1.7 +/- 1.01 on 1.01 ml CO2 . g-1 . h-1 on 0.2% CO2 in the anosmic group and from 2.9 +/- 0.29 to 1.91 +/- 0.51 ml CO2 . g-1 . h-1 in the control group when exposed to the same gases. These experiments suggest that the airborne factor emanating from the donor chambers was acting systematically and not via the olfactory system. This factor may be a low level of CO2.