Measurement of carbon dioxide production in sheep by isotope dilution.

Abstract

Sheep were given continuous subcutaneous infusions of NaH14CO3 at a rate of 0·03 ml./min, 90–150 nCi/min, for periods of 8 to 36 hr and the rate of entry of metabolic CO2 into the body pool of CO2 was calculated from the relationship between the rate of infusion of radioactivity and the specific activity (SA) of CO2 in blood or urine samples. In preliminary experiments, the variability from hour to hour in the SA of CO2 was found to be two to three times greater for blood than for urine samples and, in urine, was two times greater under a twice‐daily feeding regime than under continuous feeding conditions. An infusion period of about 8 hr was required for the initial equilibration of the infused activity with the body pool of CO2.A close relationship was found to exist between CO2 entry rates derived from the SA of CO2 in samples of urine or expired air collected over 24 hr, and either total CO2 production or total heat production as measured in a closed‐circuit respiration chamber. On average, entry rate values calculated from urine samples overestimated total CO2 production by 3·7 per cent and those calculated from expired air overestimated by 2·2 per cent. The regression equations describing the relationships between CO2 entry rate and CO2 production or heat production had residual standard deviations ranging from 4·3 to 7·5 per cent of the mean values.In sheep given two meals per day there was a reduction in the SA of CO2 in urine immediately after eating. Since feeding had no apparent effect on the rate of transfer of 14CO2 from rumen to urine it was concluded that the depression in SA resulted from a general increase in metabolic activity consequent upon the absorption of nutrients from the gut rather than from dilution with unlabelled CO2 produced in rumen fermentation. In certain sheep the postprandial depression in SA was very marked and appeared to be associated with a reduction in urine pH. Attempts to recreate this condition in penned sheep and to explain it by reference to changes in the acid‐base status of the blood were not entirely successful.