Measurement of the turnover of substrates of carbohydrate and protein metabolism using radioactive isotopes.

The rate of turnover of glucose or leucine can be measured in the steady state by using the radioactive isotopes 14C or 3H to 'trace' these metabolites. The tracer can either be injected or infused intravenously and its metabolism followed by monitoring levels of the tracer in blood, breath and urine. If a 14C labelled metabolite is used the oxidation rate may sometimes be determined by measuring the expiry rate of 14CO2. If the single injection technique is used the specific activity curve is fitted with an exponential function and the area under the curve calculated. Turnover rate is calculated from dose of isotope injected/area under the specific activity time curve. This technique can be used to construct a compartmental model to describe the behaviour of the metabolite. If the constant infusion technique is used, tracer is infused until a steady state level of tracer is achieved. This plateau is used to calculate the turnover of the metabolite from infusion rate of tracer/specific activity at steady state. While this latter method provides the simplest method of calculating turnover it cannot be used to model the data. The infusion technique can be used to measure changes in turnover and oxidation in the non-steady state. Tracer is infused to a steady state level and the system is then perturbed by for example, the infusion of a hormone. The changes in Ra and Rd can be determined either by using the Steele equation which assumes a single pool or by using compartmental analysis.