Method for kinetic study of in vitro conversion of a C¹⁴-labeled substrate to CO₂

¹ Department of Physiology, University of California, Berkeley, California

A method is described for the determination of CO₂ and C¹⁴O₂ production rates by tissue slices incubated in the presence of a constant concentration and constant specific activity of a C¹⁴-labeled substrate. The constant conditions were maintained by periodic replacement of the incubation medium and gas phase. It is pointed out that, in order to determine the true rates of conversion to CO₂ of an exogenous labeled substrate and of an endogenous, nonequilibrating CO₂ precursor, measurements must be made after isotopic equilibration is attained. In the case of whole brain slices, such equilibration was observed after 3–4 hours of incubation. With glucose as the exogenous substrate, 30–40% of the CO₂ produced by rat whole cerebrum slices was derived from endogenous sources for as long as 12 hours.