AJP - Endocrinology and Metabolism, Vol 263, Issue 5 E817-E822, Copyright © 1992 by American Physiological Society
Sites of infusion and sampling for measurement of rates of production in steady state
K. H. Norwich
Department of Physiology, University of Toronto, Ontario, Canada.
When labeled and unlabeled (endogenously produced) substances enter an
organism through different ports of entry, the anatomical location of these
entry sites must be taken into account explicitly in calculating rates of
appearance. All traditional formulas that are used for calculating rates of
appearance are based on the assumption that labeled and unlabeled
substances enter by means of the same port. These formulas are, therefore,
not in general valid for metabolites such as lactate, where the entry ports
differ. In such cases, specific activity will not be uniform throughout the
organism even when the labeled and unlabeled substances are both in steady
state. One cannot speak of the (unique) specific activity, because none
exists. It is useful in these cases to define a distributed specific
activity, which is the ratio of the concentration of labeled substance at
one anatomical site to the concentration of unlabeled substance at another.
We can then show that the rate of appearance for the double steady state
(steady tracer infusion method) is given, approximately, as the ratio of
the rate of infusion of the labeled substance to a particular distributed
specific activity.
