title: Sites of infusion and sampling for measurement of rates of production in steady state.
abstract: 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.
authors.affiliation: Department of Physiology, University of Toronto, Ontario, Canada.
authors.forename: K H
authors.initials: KH
authors.lastname: Norwich
mesh_terms: D000818:Animals; D005990:Glycerol / Q000493:pharmacokinetics; D006706:Homeostasis; D006801:Humans; D007773:Lactates / Q000378:metabolism*; D019344:Lactic Acid; D008954:Models, Biological
chemical_list: D007773:Lactates; D019344:Lactic Acid; D005990:Glycerol