Interpretation of Isotopic Data from Blood — Borne Compounds

Abstract

In a large number of reported in vivo tracer studies, labeled hormones are either infused or injected intravenously and concentrations or specific activities of the labeled compounds isolated from blood or plasma are measured. Typically, the isotopic data from these experiments are steady-state concentration values (c*, in dpm/ml), obtained during infusion of the tracer, or values of concentrations determined in samples drawn at various intervals of time after injection of the tracer. Endogenous concentrations (c, in nmole/ml) of the blood-borne compounds are usually measured before beginning the ad-ministration of tracers or, more properly, in each individual sample taken during the experiment. Specific activities (σ, in dpm/nmole) are then calculated by dividing these two quantities (σ = c*/c). It should be pointed out that specific activities can also be measured directly (e.g., by double isotope derivative procedures (Ulick et al., 1958), without determining the actual concentration of the compound in blood or plasma. The applicability of the concepts presented in previous chapters to the interpretation of data obtained from this type of experiments is now discussed.