TRACING HEPATIC GLUCONEOGENESIS RELATIVE TO CITRIC-ACID CYCLE ACTIVITY IN-VITRO AND IN-VIVO - COMPARISONS IN THE USE OF [3-C-13]LACTATE, [2-C-13]ACETATE, AND ALPHA-KETO[3-C-13]ISOCAPROATE

The validity of the use of a carbon tracer for investigating liver int ermediary metabolism in vivo requires that the labeling pattern of liv er metabolites not be influenced by metabolism of the tracer in other tissues. To identify such specific tracer, livers from 48-h starved ra ts were perfused with recirculating buffer containing [3-C-13]lactate, [2-C-13]acetate, or alpha-keto[3-C-13]isocaproate. Conscious 48-h sta rved rats were infused with the same tracers for 5 h. The labeling pat terns of liver glutamate and extracellular glucose were assayed by gas chromatography-mass spectrometry. In vivo data were corrected for (CO 2)-C-13 reincorporation into C-1 of glutamate and C-3 and C-4 of gluco se, using data from control rats infused with (NaHCO3)-C-13. With [3-C -13]lactate the labeling pattern of liver glutamate was the same in pe rfused organs and in vivo. In contrast, with [2-C-13]acetate and alpha -keto[3-C-13]isocaproate the labeling pattern of liver glutamate in vi vo was clearly influenced by the expected labeling pattern of citric a cid cycle intermediates formed in non gluconeogenic organs, presumably glutamine made in muscle. Indeed, the labeling pattern of plasma glut amine and liver glutamate were similar in experiments with [3-C-13]lac tate but different in experiments with [2-C-13]acetate and alpha-keto[ 3-C-13]isocaproate. Similar conclusions were drawn from the labeling p atterns of glucose. Therefore, labeled lactate appears as the best tra cer for studies of liver intermediary metabolism in vivo. Our data als o show that a substantial fraction of alpha-ketoisocaproate metabolism occurs in peripheral tissues.