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The curve of decline of specific activity after an intravenous injection of C 14 glucose in rats consists of at least three components. Because extrapolation of the third portion of the curve (subsequent to 1 hr.) gives a value for glucose space which appears to be spuriously high, the rate of glucose disposal calculated therefrom is also likely to be in error. Assumption that the total curve is formed from the composite effect of mixing, recycling via nonglucose carbon, and irreversible glucose disposal permits construction of a compatible physiological model. Glucose pool size and rate of irreversible disposal calculated by mathematical treatment of this model may then be approximated. Calculations indicate that glucose space in the rat is approximately 20% of body weight. Irreversible disposal (oxidation plus synthesis to fat, etc.) is in the vicinity of 0.6 mg/min. for fasted rats and 1 mg/min. for postabsorptive rats. Total turnover time of plasma glucose is approximately 2–3 minutes. Combined glucogenesis and gluconeogenesis seem to proceed more rapidly in the postabsorptive state than during fasting.
Baker et al. (Sat,) studied this question.