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Validity of a Tracer-Injection Method for Studying Glucose Turnover in Normal Dogs

University of Toronto, Toronto, Ontario, Canada

Correspondence: For reprints contact: G. Hetenyi, Dept. of Physiology, Univ. of Toronto, Medical Sciences Building, Toronto 5, Ontario, Canada.

ABSTRACT

A method is described for testing the validity of absolute appearance rates of a given substance determined in vivo in plasma by isotopic labels. In twelve experiments on unanesthetized dogs the method of successive measured injections of tracer (SMIT) was tested for glucose, six times in near dynamic steady-state conditions and three times each with linearly increasing and decreasing appearance rates. In these tests a known rate of appearance (i.v. infusion) of 1-¹⁴C-glucose was compared with its calculated rate using a single injection method with 6-¹⁴C-glucose as tracer. Data on recycling of ¹⁴C into endogenously produced glucose are also presented, as well as some observations on intermixing. The absolute appearance rate determined by the SMIT method was found to be valid within 10% for glucose in normal dogs near dynamic steady states. With a rapid linear increase in the rate of appearance of 1-¹⁴C-glucose the tracer-determined rate was 31–41% higher than the known value at time of injection, while with a similar linear decrease in the appearance rate, the tracer-determined rate did not vary significantly from that determined in the near dynamic steady state cases.