Lactate and pyruvate isotopic enrichments in plasma and tissues of postabsorptive and starved rats

It has been proposed that plasma pyruvate isotopic enrichment (IE) during infusion of labeled lactate could be used to estimate the intracellular IE of lactate and pyruvate and thus to calculate their turnover rate. We determined the relations of plasma and tissue IE of lactate and pyruvate in anesthetized rats infused with [3-13C]lactate in an artery and sampled from a vein (A-V mode) or infused in a vein and sampled from an artery (V-A mode). In both groups of rats, the ratio of tissue to plasma lactate IE was < 1 with large differences between tissues: the highest ratio was observed in heart and the lowest in soleus. With the exception of liver, this ratio was higher in the A-V than in the V-A mode. Pyruvate IE was lower than lactate IE in tissues, with a few exceptions, and in plasma. This ratio of pyruvate to lactate IE was approximately 0.70 in plasma in A-V and V-A modes. Moreover pyruvate IE was also always higher in plasma than in tissues. This seemingly surprising result could be explained by the production of labeled pyruvate from labeled lactate inside the circulation by erythrocytes, because we observed a rapid isotopic equilibrium between lactate and pyruvate in blood "in vitro." Apparent lactate turnover was higher in the A-V than in the V-A mode when it was calculated using lactate as well as pyruvate IE. Therefore plasma pyruvate IE cannot be used in rats to estimate tissue IE and did not reconcile turnover rates measured using the A-V or V-A mode.(ABSTRACT TRUNCATED AT 250 WORDS)