Arteriovenous differences and tissue concentrations of branched-chain ketoacids

According to previous reports, branched chain ketoacids (BCKAs) are undetectable in liver and brain. A procedure is described for HPLC analysis of tissue concentrations of BCKA that achieves 80% recovery of labeled 2-ketoisocaproate (KIC) added to frozen powdered liver. In normal overnight-fasted rats, substantial hepatic uptake was demonstrable in the order KIC > 2-ketoisovalerate (KIV) congruent to 2-keto-3-methylvalerate (KMV). All three BCKAs were very low in freeze-clamped liver. Liver concentrations were only 15% to 22% of the mean of portal vein, hepatic vein, and hepatic artery (aorta) concentrations. In view of the probable contribution to total liver content of extracellular BCKAs, the intracellular concentrations in liver tissue must be vanishingly low. Somewhat higher amounts were present in freeze-clamped kidney and heart tissue, and significant uptake of KIC and KMV by kidney was found. Brain released KIC. Muscle contained concentrations of BCKA that were 35% to 72% of concentrations in plasma, and it released all three BCKAs. Similar results were obtained from tissues in overnight-fasted dogs, except that even lower levels were found in freeze-clamped brain (sampled before circulation ceased) than were found in the liver. Plasma levels in dogs were similar to those of rats. In both species the tissue-to-plasma ratios were higher for KIV than for KMV or KIC. Remarkably low BCKA levels in liver and brain are unexplained, particularly in light of evidence that BCKAs gain access to cells in both organs.