Basic studies on metabolic steady state. Linearity of a metabolic tracer system

In dogs the changes in the plasma concentration following the injection and/or infusion of labeled glucose reveal the “labeled glucose system” to be a linear system. Based on a simple chemical model it was shown that the rate of removal from the system of any tracer injected as single impulse can be described by a first order chemical reaction, even in systems from which the tracee is removed by a process of a higher order, provided the tracee is in the steady state and the concentration of tracer in the plasma is low compared to that of the tracee. The validity of the commonly used formulae for the calculations of the rates of disappearance from systems in a steady state is based on this first order process.