AJP - Endocrinology and Metabolism, Vol 251, Issue 6 715-E720, Copyright © 1986 by American Physiological Society
Calculation of metabolic fluxes with anatomically separated sources of tracer and tracee
K. H. Norwich and D. H. Wasserman
Traditional calculations of metabolic fluxes using isotope dilution are
based on the assumption that tracer and tracee enter the distribution space
through effectively identical ports. If the tracer infusion site is not
identical with the site of endogenous release of the tracee, the
traditional equations for calculating rate of appearance (Ra) of a
metabolite may give rise to appreciable errors due to the presence of
gradients in specific activity. When tracer and tracee enter by means of
anatomically disparate sites, such as may be encountered in the study of
metabolite (e.g., lactate, alanine, and glycerol) or free fatty acid
turnover, one must employ a modification of the traditional specific
activity. This modified specific activity is obtained as the ratio of
tracer concentration "near" (in the same compartment as) the source of
tracee to the concentration of tracee near the source of tracer infusion.
This concept is employed to derive equations for calculating metabolic
turnover in both steady- and non-steady-state conditions when entry sites
of tracer and tracee are dissimilar.
