Measurement of gluconeogenesis and mass isotopomer analysis based on [U-C-13]glucose

Two methods of measuring rates of gluconeogenesis based on label redistribu tion after the introduction of [U-C-13]glucose into the whole body are exam ined. These methods are compared with methods previously derived for carbon -14 tracers. It is shown that the three approaches (stoichiometric, dilutio n, and combinatorial) are equivalent, provided the same set of assumptions are used. Barring a factor of two [see Am. J. Physiol. 270 (Endocrinol. Met ab. 33): E799-E717, 1996], the differences (similar to 10-15%) in the carbo n-based dilutional and the molecule-based estimates of the rate of gluconeo genesis from published isotopomer data likely arise from small differences in the assumptions that concern the relative rate of label loss from the di fferent isotopomers. The production of unlabeled substrate for glucose synt hesis (phosphoenolpyruvate) from the different isotopomers of lactate is sh own to be a potential source of error in these methods. This error is estim ated using models of the interaction of the gluconeogenetic pathway and the tricarboxylic acid (TCA) cycle and is shown to vary from negligible to 30% depending on the relative flux of the two pathways through the oxaloacetat e pool. Because the estimates obtained by both methods considered are lower than is physiologically expected, some of the assumptions made may not hol d. Future work will exploit the rich information content of isotopomer data to yield improved estimates.