Compartmental and semicompartmental approaches for measuring glucose carbon flux to fatty acids and other products in vivo.

We have attempted to estimate the flux of glucose carbon to total body fatty acids and to other metabolic end products in Bar Harbor 129/J mice fasted 5-8 hr. Tracer [U-(14)C]glucose was injected intravenously, and the following data were obtained at various times up to 180 min: plasma glucose C specific activity, plasma glucose concentration, total body glycogen, and (14)C in total body fatty acid, total body lipid, unsaponifiable lipid, expired CO(2), and in hepatic and extrahepatic glycogen. The data were analyzed by three techniques, namely, multicompartmental, semicompartmental, and noncompartmental analyses. All three methods yielded comparable rates of glucose C conversion to total body fatty acids (2-3 micro g of glucose C/min/20 g of body weight). Although the semicompartmental approach is extremely simple (it only requires analyses of plasma glucose specific activity as a function of time and (14)C-labeled fatty acid at one point in time), it gives an apparently valid approximation for the flux of glucose C to fatty acids. Other quantitative aspects of glucose metabolism in postabsorptive mice are also considered.